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In this study, we investigated the triple‐dip La Niña during 2020–2022 by comparing it with the previous (1973–1975 and 1998–2000) La Niña events. We found that the cold sea surface temperature (SST) in the eastern equatorial Pacific was moderate during the study period; however, the accompanying near‐surface easterly wind anomaly was unusually stronger during its lifecycle than during the previous two events. The maintenance of 2020–2022 La Niña appeared to be attributable to the strong zonal SST gradient. The strong zonal SST gradient resulted from the La‐Niña‐associated interannual SST anomaly, which was further enhanced by a warming trend in the western equatorial Pacific (165°E−160°W, 5°S–5°N) and the interdecadal oscillation of the Pacific‐Decadal‐Oscillation‐associated cold SST in the eastern tropical Pacific. The warming trend in the western equatorial Pacific, with a faster warming speed than global warming, also modified the La‐Niña‐associated Pacific–North American teleconnection to shifted eastward. Plain Language Summary: The La Niña during 2020–2022 was the first triple‐dip event in the 21st century. It induced several major disasters worldwide, such as the record flooding in Pakistan caused 30 million people to lose their homes and took the lives of more than 1,000 individuals. However, how this long‐duration La Niña was maintained remained unclear. The physical processes related to the 2020–2022 La Niña were investigated by comparing them with those of the previous 3‐year La Niña events (1973–1975 and 1998–2000). The major distinctive feature of the 2020–2022 La Niña was the moderate negative sea surface temperature (SST) anomaly in the eastern equatorial Pacific, which was accompanied by an unusually strong near‐surface easterly wind anomaly in the central equatorial Pacific. Our study revealed the maintenance of the triple‐dip La Niña during 2020–2022 was attributable to the strong zonal SST gradient between the western and eastern equatorial Pacific, with the western waters being dominated by a warming trend and the eastern waters being influenced by the cold phase of the Pacific Decadal Oscillation. The warming trend in the western equatorial Pacific provided a favorable background for a triple‐dip La Niña event and also modified the La‐Niña‐associated Pacific–North American teleconnection so that it shifted eastward. Key Points: The La‐Niña‐associated cold SST was moderate during 2020–2022, but the accompanying easterly wind anomaly reached an unusual highThe strong easterly was in response to the zonal SST gradient between the warm western tropical Pacific and cold eastern tropical PacificThe western equatorial Pacific SST, with a faster warming speed than global warming, modulated the PNA teleconnection to shift eastward [ABSTRACT FROM AUTHOR]

Observational analyses demonstrate that the Ural persistent positive height anomaly event (PAE) experienced a decadal increase around the year 2000, exhibiting a southward displacement afterward. These decadal variations are related to a large-scale circulation shift over the Eurasian Continent. The effects of underlying sea ice and sea surface temperature (SST) anomalies on the Ural PAE and the related atmospheric circulation were explored by Atmospheric Model Intercomparison Project (AMIP) experiments from the Coupled Model Intercomparison Project Phase 6 and by sensitivity experiments using the Atmospheric General Circulation Model (AGCM). The AMIP experiment results suggest that the underlying sea ice and SST anomalies play important roles. The individual contributions of sea ice loss in the Barents-Kara Seas and the SST anomalies linked to the phase transition of the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) are further investigated by AGCM sensitivity experiments isolating the respective forcings. The sea ice decline in Barents-Kara Seas triggers an atmospheric wave train over the Eurasian mid-to-high latitudes with positive anomalies over the Urals, favoring the occurrence of Ural PAEs. The shift in the PDO to its negative phase triggers a wave train propagating downstream from the North Pacific. One positive anomaly lobe of the wave train is located over the Ural Mountains and increases the PAE there. The negative-to-positive transition of the AMO phase since the late-1990s causes positive 500-hPa height anomalies south of the Ural Mountains, which promote a southward shift of Ural PAE. [ABSTRACT FROM AUTHOR]

The North Pacific Central Mode Water (CMW) is examined from the viewpoint of its volume variations. The volume of the CMW layers thicker than 150 m is used as an index of CMW variations, which successfully represents the year-to-year and decadal variations in the CMW volume. The CMW index shows the variation close to that of the Pacific Decadal Oscillation (PDO). The CMW variation is strongly tied with large-scale, dominant variations in sea surface temperature (SST), surface dynamic height (SDH), and sea surface height (SSH) anomalies in the North Pacific, with a significant correlation with the PDO. Year-to-year and decadal variations of CMW volume in May to July are significantly correlated with the wintertime Aleutian Low and 500 hPa geopotential height variations, which indicates that the Aleutian Low induces the eastward extension/retreat of the strong winter westerlies and resultant net surface heat flux anomaly over the CMW distribution region. Thus, the CMW volume variation can be regarded as a significant manifestation of the PDO. The SST, SDH, and SSH anomalies are associated with the surface cooling in the northern sector of the CMW distribution region. On the other hand, the SDH and SSH anomalies throughout a year and the SST anomaly in the cold season in the southeastern sector of the CMW region are formed due to the heaving of isopycnal surfaces in the subsurface layer above the CMW in response to its volume variations. [ABSTRACT FROM AUTHOR]

The upper reaches of the Yangtze River Basin (UYRB) are famous for their hydropower generation and water resources in China, relying greatly on precipitation. The UYRB summer precipitation has decreased since the early 2000s and has been exposed to a particularly dry period in the most recent two decades. By analyzing the long‐term variability from the beginning of the 20th Century, we found that the precipitation exhibits a profound periodic interdecadal variation, with a significant cycle of 30–50 years. The interdecadal variability of precipitation is shown to be significantly modulated by both the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO), exerting their impact through related circulation anomalies. Moreover, the periodical nature of the UYRB precipitation variation and its correlation with AMO/PDO suggest that the UYRB might enter a wet period in the forthcoming decade, i.e. rich in hydroelectric resources but a high risk of flood. [ABSTRACT FROM AUTHOR]

As the major sink of anthropogenic heat, the Southern Ocean has shown quasi-symmetric, deep-reaching warming since the mid-twentieth century. In comparison, the shorter-term heat storage pattern of the Southern Ocean is more complex and has notable impacts on regional climate and marine ecosystems. By analyzing observational datasets and climate model simulations, this study reveals that the Southern Ocean exhibits prominent decadal (>8 years) variability extending to ∼700-m depth and is characterized by out-of-phase changes in the Pacific and Atlantic–Indian Ocean sectors. Changes in the Pacific sector are larger in magnitude than those in the Atlantic–Indian Ocean sectors and dominate the total heat storage of the Southern Ocean on decadal time scales. Instead of heat uptake through surface heat fluxes, these asymmetric variations arise primarily from wind-driven heat redistribution. Pacemaker and preindustrial simulations of the Community Earth System Model version 1 (CESM1) suggest that these variations in Southern Ocean winds arise primarily from natural variability of the tropical Pacific, as represented by the interdecadal Pacific oscillation (IPO). Through atmospheric teleconnection, the positive phase of the IPO gives rise to higher-than-normal sea level pressure and anticyclonic wind anomalies in the 50°–70°S band of the Pacific sector. These winds lead to warming of 0–700 m by driving the convergence of warm water. The opposite processes, involving cyclonic winds and upper-layer divergence, occur in the Atlantic–Indian Ocean sector. These findings aid our understanding of the time-varying heat storage of the Southern Ocean and provide useful implications on initialized decadal climate prediction. [ABSTRACT FROM AUTHOR]

Changes in body size have been documented across taxa in response to human activities and climate change. Body size influences many aspects of an individual's physiology, behavior, and ecology, ultimately affecting life history performance and resilience to stressors. In this study, we developed an analytical approach to model individual growth patterns using aerial imagery collected via drones, which can be used to investigate shifts in body size in a population and the associated drivers. We applied the method to a large morphological dataset of gray whales (Eschrichtius robustus) using a distinct foraging ground along the NE Pacific coast, and found that the asymptotic length of these whales has declined since around the year 2000 at an average rate of 0.05–0.12 m/y. The decline has been stronger in females, which are estimated to be now comparable in size to males, minimizing sexual dimorphism. We show that the decline in asymptotic length is correlated with two oceanographic metrics acting as proxies of habitat quality at different scales: the mean Pacific Decadal Oscillation index, and the mean ratio between upwelling intensity in a season and the number of relaxation events. These results suggest that the decline in gray whale body size may represent a plastic response to changing environmental conditions. Decreasing body size could have cascading effects on the population's demography, ability to adjust to environmental changes, and ecological influence on the structure of their community. This finding adds to the mounting evidence that body size is shrinking in several marine populations in association with climate change and other anthropogenic stressors. Our modeling approach is broadly applicable across multiple systems where morphological data on megafauna are collected using drones. [ABSTRACT FROM AUTHOR]

Statistical change-point analysis was used to examine a significant change in the average longitude locations of tropical cyclone (TC) genesis from June to October between 1979 and 2020. It was observed that since 1998, the TC genesis location has shifted westward. This study investigates the causes of this interdecadal shift in the western North Pacific (WNP). During the recent period (1998–2020), a westward shift was noted in precipitation, outgoing longwave radiation, precipitable water, and total cloud cover. The primary driver for this shift is the negative phase of the Pacific Decadal Oscillation during the second period. A positive sea surface temperature (SST) anomaly enhances convection over the South China Sea and adjacent waters west of 140° E, which retreats the monsoon trough toward the west. Furthermore, the Rossby wave response to a negative SST anomaly over the central and eastern Pacific causes the WNP subtropical high in the lower troposphere to move further west, pushing the monsoon trough in the same direction. The westward shifts in convection and circulation are maintained by a strengthened Walker circulation. [ABSTRACT FROM AUTHOR]

Frequent occurrences of extreme heats in recent years pose a threat to the local society, economy, and ecosystem in Southern China. Understanding the process and predictability of these heat events is crucial for scientific research and policy-making. The long-term variation characteristics and the corresponding driving factors of high-temperature days (HTDs) and surface air temperature (SAT) in midsummer in Southern China were analyzed by observed data of meteorological stations and atmospheric reanalysis data. The results reveal an abrupt rise in HTDs around the early 2000s during the period of 1979–2021, rather than a sustained increase. Prior to and after this mutation, no clear trend is observed. The changes in SAT align closely with the variations in HTDs. The abrupt increase in solar radiation, resulting from a decrease in total cloud cover, is identified as the direct cause of the SAT change. In the decade following the abrupt change point, significant anomalous descent motion and low-level divergent winds over Southern China are observed, creating favorable climatic conditions for the rise in SAT. The primary driving factor behind these circulation changes is identified as the Pacific interdecadal oscillation, which shifted from a positive to a negative phase during the same period. This shift led to the strengthening of the Walker circulation and the formation of an anomalous anticyclone over the northwest Pacific. It is worth noting that current mainstream climate dynamical models failed to capture the abrupt rise in SAT, instead exhibiting a sustained increasing trend that compromised the accuracy of the forecast. This discrepancy may be attributed to the models' slow response to rapid changes in sea surface temperature. [ABSTRACT FROM AUTHOR]

Climate network (CN) analysis has shown great potential in detecting early warning signals for major climate events, but it is still challenging to reveal the underlying mechanisms. One possible reason for this issue is related to the ubiquitous climate memory, which may affect the calculations of links in climate networks, and further hinder us from a clear judgment on the sources of the early warning signal. Here in this study, we aim at identifying the climate memory impacts on the CN analysis. Combining with the Fractional Integral Statistical Model (FISM), we proposed a new approach named as CN-FISM. With FISM, one can extract the climate memory component and modify the considered time series into new series with a given length of memory preserved. By repeating the CN analysis, one thus can quantify the impacts of climate memory. We employed this approach to a recent CN analysis on the Pacific Decadal Oscillation (PDO) phase change. By comparing the CN results based on data with different memory lengths preserved, we found the climate memory within timescale of 2 years plays an important role in the arising of the early warning signal for the PDO phase change. This finding suggests that some physical processes on timescale of 2 years may be crucial for the PDO phase change, according to which one may better understand its underlying mechanisms. Compared with the current Pearson correlation-based CN approach, the CN-FISM offers the potential of improved interpretability of the CN results. [ABSTRACT FROM AUTHOR]

The current reconstructions of the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO) primarily rely on marine sediment proxies. However, the limited resolution of these records makes it difficult to precisely understand the short‐term dynamics of the PDO and AMO, and consequently, their influence on global climate. This study unveils two new Holocene reconstructions of AMO and PDO derived from a large‐scale compilation of speleothem isotope records (δ18Os) from Chinese monsoon region, in which spatial patterns of summer rainfall are generally accepted as good indicators of internal variability. Principal component analysis applied to Chinese δ18Os records identifies the first principal component as representative of Holocene Asian monsoon (AM) variation. This data‐driven approach reveals a significant shift in hydroclimatic conditions after 8.2 ka, potentially providing valuable insights into the underlying forcing mechanisms driving these changes. The remanent components, which show spatial rainfall patterns, are effectively validated through independent records of AMO and PDO derived from the previous paleo‐proxy based reconstructions and model simulations. The present reconstructions suggest a significantly enhanced stability of the AMO and PDO in the middle Holocene, which may explain the concurrent more stable and optimum climate observed in the AM region. These findings also imply that the Holocene AM intensity recorded in Chinese cave records is largely controlled by external forcing, whereas the regionally heterogenous rainfall is regulated by internal variability. The successful attempt on the Holocene AMO and PDO reconstructions indicates an effective isolation between different modes of climate variability from paleoclimate records. Key Points: Different modes of climate variability are isolated from a large‐scale compilation of Chinese speleothem δ18O recordsHolocene high‐resolution Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation variations are reconstructed based on Chinese speleothem δ18O recordsHigh stability of internal variability in the middle Holocene may explain the Holocene climate optimum [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs) lead to huge economic losses and casualties in coastal regions, but multi‐decadal variation of TC remains poorly understood due to the limited length of observational records and low‐resolution reconstructions. Porites corals provide continuous and monthly resolved records that allow for the determination of weather‐timescale (weekly to monthly) extreme events, including TC, to overcome the above limitations. Here we synthesize five coral Δδ18O records from the South China Sea (SCS) and develop a coral TC frequency index, in good agreement with post‐1980 instrumental records and sediment‐based TC reconstructions over the past century. Our results indicate that TC frequency in the SCS exhibits pronounced decadal variations, with active TCs corresponding to the negative phase of the Pacific Decadal Oscillation. Our findings here provide novel evidence for past TC changes in the SCS and a constraint for predicting future TC changes. Plain Language Summary: The impact of tropical cyclones (TCs) on coastal regions results in significant economic losses and casualties. Due to the relatively short observations or low‐resolution records, assessing long‐term changes of TCs has suffered from large uncertainties. Porites corals are sensitive to environmental changes and can provide century‐long, monthly resolved records, which could serve as a potential archive for reconstructing past TCs. Here we present five coral Δδ18O records from the South China Sea (SCS) and develop two TC frequency indexes. Both the TCs generated in the SCS and those entering the SCS from the Western North Pacific are reflected in our coral‐based TCs reconstruction. In addition, our reconstruction suggests that the TCs in the SCS were increased in 1878–1888, 1894–1904, 1934–1952, 1971–1978, and 1995–2004, which corresponded to the negative phase of the Pacific Decadal Oscillation over the past century. Our findings provide new evidence for historical changes in TCs in the SCS and a valuable constraint for predicting future TC variations. Key Points: The low residual oxygen isotope values of coral strongly respond to tropical cyclones in the South China SeaExcluding temperature, we reconstruct tropical cyclone activities in the South China Sea over the past century using coral oxygen isotopesPacific Decadal Oscillation modulates tropical cyclones in the South China Sea over the past century [ABSTRACT FROM AUTHOR]

(1) Background: Global climate change is expected to significantly alter growing conditions along mountain gradients. Landscape ecological patterns are likely to shift significantly as species attempt to adapt to these changes. We evaluated the extent to which spatial (elevation and canopy cover) and temporal (decadal trend and El Niño–Southern Oscillation/Pacific Decadal Oscillation) factors impact seasonal snowmelt and forest community dynamics in the Western Hemlock–True Fir ecotone region of the Oregon Western Cascades, USA. (2) Methods: Tsuga heterophylla and Abies amabilis seedling locations were mapped three times over 20 years (2002–2022) on five sample transects strategically placed to cross the ecotone. Additionally, daily ground temperature readings were collected over 10 years for the five transects using 123 data loggers to estimate below-canopy snow metrics. (3) Results: Based on validation using time-lapse cameras, the data loggers proved highly reliable for estimating snow cover. The method reported fewer days of snow cover as compared to meteorological station-based snow products for the region, emphasizing the importance of direct under-canopy field observations of snow. Snow season variability was most significantly impacted temporally by cyclical ENSO/PDO climate patterns and spatially by differences in canopy cover within the ecotone. The associated seedling analysis identified clear sorting of species by elevation within the ecotone but reflected a lack of a long-term trend, as species dominance in the seedling strata did not significantly shift along the elevation gradient over the 20-year study. (4) Conclusions: The data logger-based approach provided estimates of snow cover at ecologically significant locations and fine enough spatial resolutions to allow for the study of forest regeneration dynamics. The results highlight the importance of long-term, understory snow measurements and the influence of climatic oscillations in understanding the vulnerability of mountain areas to the changing climate. [ABSTRACT FROM AUTHOR]

Over the past two decades, there has been a significant shift in tropical cyclone (TC) activity in the western North Pacific (WNP) basin during the boreal summer. Our analysis of data spanning from 1979 to 2021 reveals significant shifts in the WNP TC characteristics and rainfall pattern variation. To deepen our understanding of TC-related precipitation dynamics, we expressly address the difference between TC-related core precipitation (TCP) and remote precipitation (TRP). The results show that TRP significantly impacts the East Asian (EA) continent, especially on the Korean Peninsula. Notably, TCP exhibits decadal variability, with a pronounced negative correlation identified between it and the Pacific decadal oscillation (PDO) following a strong climate shift. This pivotal shift was marked by the PDO first transitioning to its negative phase in 1997, a notable change since 1979, resulting in a marked increase in TC-related extreme rainfall over the EA area. Concurrently, the rising sea surface temperatures (SSTs) over the WNP have intensified the western Pacific subtropical high (WPSH) circulation. The easterly steering flow associated with the WPSH then strengthened, leading to the continental migration of TC trajectories, thereby intensifying TC-related extreme precipitation.

Abstract In this study, we investigated the triple‐dip La Niña during 2020–2022 by comparing it with the previous (1973–1975 and 1998–2000) La Niña events. We found that the cold sea surface temperature (SST) in the eastern equatorial Pacific was moderate during the study period; however, the accompanying near‐surface easterly wind anomaly was unusually stronger during its lifecycle than during the previous two events. The maintenance of 2020–2022 La Niña appeared to be attributable to the strong zonal SST gradient. The strong zonal SST gradient resulted from the La‐Niña‐associated interannual SST anomaly, which was further enhanced by a warming trend in the western equatorial Pacific (165°E−160°W, 5°S–5°N) and the interdecadal oscillation of the Pacific‐Decadal‐Oscillation‐associated cold SST in the eastern tropical Pacific. The warming trend in the western equatorial Pacific, with a faster warming speed than global warming, also modified the La‐Niña‐associated Pacific–North American teleconnection to shifted eastward.

Abstract Primary production in the Sea of Okhotsk is largely supported by dissolved iron (dFe) transported by the Amur river, indicating the importance of dFe discharge from terrestrial environments. However, little is known about the mechanisms of dFe discharge into the Amur river, especially in terms of long-term change in dFe concentration. In the Amur river, extreme increase in dFe concentration was observed between 1995 and 1997, the cause of which remains unclear. As a cause of this iron anomaly, we considered the impact of permafrost degradation. To link the permafrost degradation to long-term variation in dFe concentration, we examined the changes in annual air temperature (Ta), accumulated temperature (AT), and net precipitation for three regions (northeast, south, and northwest) of the basin between 1960 and 2006. Ta and AT were relatively high in one out of every few years, and were especially high during 1988–1990 continuously. Net precipitation in late summer (July to September) has increased since 1977 and has stayed positive until 2006 throughout the basin. Most importantly, we found significant correlations between Ta and late summer dFe concentration with a 7-year lag (r = 0.54–0.69, p

We used observational data and a long‐term piControl simulation from the Community Earth System Model Version 2 to investigate the influence of the Pacific Decadal Oscillation (PDO) on the winter climate over the Tibetan Plateau. The results showed that changes in the phase of the PDO have a significant effect on winter temperatures and precipitation over the southern Tibetan Plateau. Changes in the sea surface temperature (SST) during the positive PDO can weaken the Walker circulation and increase the SST in the Indian Ocean. Our analyses of the moist static energy showed that warming of the tropical troposphere over the Indian Ocean caused by the increased SST has resulted in the horizontal advection of anomalous moist enthalpy by the climatological zonal winds, which was responsible for anomalous ascending motion over the Tibetan Plateau. The additional moisture budget suggests that enhanced vertical motion contributes to the increase in winter precipitation and related total cloud cover over the Tibetan Plateau, leading to the increase of snow depth. The increased total cloud cover and snow depth, in turn, reduces net surface shortwave radiation. The surface air temperature of the Tibetan Plateau is then decreased as a result of the reduction in the net surface shortwave radiation. The PDO therefore has an important modulating role in the interdecadal variability of the winter climate over the Tibetan Plateau. We therefore need to focus on changes in the PDO in research related to the decadal prediction of the climate over the Tibetan Plateau. Plain Language Summary: Under the background of global change, the climate of the Tibetan Plateau has undergone significant changes, and it has an important impact on the regional and global climate. Therefore, it is very important to find out the mechanisms of climate variabilities on the Tibetan Plateau. In this study, we aim to investigate the influences of the Pacific Decadal Oscillation (PDO) on the winter climate of the Tibetan Plateau and the related physical mechanisms by using observational data and a long‐term piControl simulation from the Community Earth System Model Version 2. Results suggest that the PDO has an important modulating role in the interdecadal variability of the winter climate over the Tibetan Plateau. The positive PDO phase can increase winter precipitation and decrease the surface air temperature of the Tibetan Plateau. The related mechanisms are also investigated in this study. Key Points: The positive Pacific Decadal Oscillation (PDO) can increase the precipitation and thereby decreasing the temperature in winter over the southern Tibetan PlateauThe horizontal advection of anomalous moist enthalpy is responsible for anomalous ascending motion and increased precipitationThe anomalous advection of zonal warm air is caused by warming of the tropical troposphere over the Indian Ocean due to the positive PDO [ABSTRACT FROM AUTHOR]

Primary production in the Sea of Okhotsk is largely supported by dissolved iron (dFe) transported by the Amur river, indicating the importance of dFe discharge from terrestrial environments. However, little is known about the mechanisms of dFe discharge into the Amur river, especially in terms of long-term change in dFe concentration. In the Amur river, extreme increase in dFe concentration was observed between 1995 and 1997, the cause of which remains unclear. As a cause of this iron anomaly, we considered the impact of permafrost degradation. To link the permafrost degradation to long-term variation in dFe concentration, we examined the changes in annual air temperature (Ta), accumulated temperature (AT), and net precipitation for three regions (northeast, south, and northwest) of the basin between 1960 and 2006. Ta and AT were relatively high in one out of every few years, and were especially high during 1988–1990 continuously. Net precipitation in late summer (July to September) has increased since 1977 and has stayed positive until 2006 throughout the basin. Most importantly, we found significant correlations between Ta and late summer dFe concentration with a 7-year lag (r = 0.54–0.69, p < 0.01), which indicate a close relationship between high Ta in year Y and increased late summer dFe concentration in year Y + 7. This correlation was the strongest in northeastern Amur basin where permafrost coverage is the highest. Similar 7-year lag correlation was also found between AT in the northeastern basin and late summer dFe concentration (r = 0.51, p < 0.01). Based on our findings, we propose the following hypothesis as a cause of iron anomaly. (1) Increased net precipitation since 1977 has increased soil moisture, which created suitable conditions for microbial dFe generation; (2) permafrost degradation during the warm years of 1988–1990 promoted iron bioavailability and led to the intensive dFe generation in the deeper part of the active layer; and (3) dFe took approximately 7 years to reach the rivers and extremely increased dFe concentration during 1995–1997. This is the first study to suggest the time-lagged impact of permafrost degradation on iron biogeochemistry in the Amur river basin. [ABSTRACT FROM AUTHOR]

Using typhoon data from the Shanghai Typhoon Institute of the China Meteorological Administration, the Japan Meteorological Agency's annual Pacific decadal oscillation (PDO) index, and NCEP/NCAR reanalysis data from 1951 to 2021, correlation and composite analyses were carried out to study the relationship between the variability among tropical cyclones of different magnitudes affecting South China and the PDO. The results show that there is an obvious out-of-phase relationship between the proportion of tropical cyclones reaching a typhoon-level intensity or above in South China and the PDO index. When the PDO is in a cold (warm) phase, the sea surface temperature in the eastern and central equatorial Pacific is cold (warm), similar to the eastern Pacific La Niña (El Niño) phenomenon, and the SST in the eastern and western tropical Pacific Ocean shows a negative (positive) gradient; the subtropical high in the western Pacific Ocean is weaker (stronger) than normal, with the western ridge point to the east (west), and the 500 hPa geopotential height in the South China Sea and the area east of the Philippines is weaker (stronger), which is conducive to (unfavorable to) the formation of a monsoon trough; and the westerly (easterly) winds at high altitudes and the southwesterly (northeasterly) winds at low altitudes from the South China Sea to the Philippines are abnormally strong, and a positive (negative) vorticity at low altitudes, a low (high) sea level pressure, and strong (weak) convection are shown. These conditions are favorable (unfavorable) for the intensification of typhoons affecting South China, and as a result, the number of tropical cyclones reaching the level of typhoons or above account for a greater (smaller) proportion of those affecting South China. [ABSTRACT FROM AUTHOR]

The zonal sea surface temperature (SST) gradient across the tropical Pacific is a pacemaker of the variable rates of global warming. In both historical simulations and future projections, the current state-of-the-art climate models show evident spread in the changes of zonal SST gradient, but the reasons remain unknown. Here, we quantify the contributions of internal variability and model spread to the uncertainty of zonal SST gradient changes by analyzing 342 realizations from CMIP5 and CMIP6 models and several sets of large-ensemble simulations. We found that the internal variability dominates the total uncertainty at multidecadal time scales (∼31-yr trends). Although the ratio of internal uncertainty to the total uncertainty declines along with higher emission of greenhouse gases under global warming, it is still over 80% at the multidecadal time scales in the future. The Pacific decadal oscillation is identified as the key internal mode responsible for the multidecadal uncertainty. For the future projections at centurial time scales, the uncertainty of zonal SST gradient changes is mainly from the intermodel spread in response to external forcing, accounting for about 70% of the uncertainty based on the difference between 2070–99 and 1970–99. The model spread in the cloud–shortwave radiation–SST feedback over the tropical Pacific is important in the uncertainty of zonal SST gradient changes. In particular, the intensity of negative convective cloud feedback in the western Pacific dominates the spread in CMIP5 models, while the intensity of stratocumulus cloud feedback over the southeastern Pacific is the primary process influencing the uncertainty in CMIP6 models. [ABSTRACT FROM AUTHOR]

Using the observational and reanalysis datasets during the period of 1979–2018, this study examines the modulation effects of the Atlantic multidecadal oscillation (AMO) and Pacific decadal oscillation (PDO) on the interdecadal change in summer surface air temperature (SAT) over the eastern Tibetan Plateau (TP). The results show that the eastern TP has experienced a remarkable warming in boreal summer since 1997. In addition, the warming is associated with simultaneously enhanced westward water vapor transport and the northward shift of the subtropical westerly jet (SWJ) over the northeast of the TP. Further research indicates that both water vapor transport and the SWJ are modulated by the PDO and AMO on interdecadal time scales. Since the late 1990s, the PDO has changed from the positive to negative phase, while the AMO has changed from the negative to positive phase. The change in the out-of-phase combination of the PDO and AMO has resulted in a northward shift of the SWJ at 200 hPa near the northeast of the TP through the Rossby wave train originating from the northwestern Atlantic. In addition, the PDO and AMO could both affect the Rossby wave train. From the northwestern Atlantic to Eurasia, the wave train may be dominated by the PDO, while the wave train near the TP is dominated by the AMO. The anomalous anticyclone over the northeastern TP is part of the Rossby wave train, causing the northward shift of the SWJ, favoring the intrusion of the water vapor from the eastern China into the eastern TP, thus leading to the TP warming. Significance Statement: It is understood that the Tibetan Plateau (TP) may be among the most sensitive regions to the global climate change. In addition, this region has warmed very rapidly in the latter half of the twentieth century. Based on the statistical analysis and dynamic diagnostics, the influences of interdecadal variations of atmospheric circulations on summer surface air temperature (SAT) over the eastern TP were analyzed for the years 1979 to 2018. We found that the eastern TP experienced an increase in summer SAT. In addition, the interdecadal change in SAT over the eastern TP is strongly affected by the water vapor transport and the subtropical westerly jet. Further analyses show that the PDO and AMO were the key factors influencing summer SAT over the eastern TP on the interdecadal time scales. The PDO and AMO could both affect the Rossby wave train. From the northwest Atlantic to Eurasia, the wave train may be dominated by the PDO, while the wave train near the TP is dominated by the AMO. In addition, the northwest Atlantic SSTs, which are dominated by the AMO, could also affect the SAT over the eastern TP by the Rossby wave train. [ABSTRACT FROM AUTHOR]

During the pre-monsoon season, the Northwest Himalayas region receives a significantly larger proportion of precipitation than the pan Indian average. This heightened precipitation makes the region vulnerable to the repercussions of extreme weather events. Therefore, this study delves into the variations in precipitation during the pre-monsoon period over the Northwest Himalayas spanning from 1979 to 2018. It has been observed that since 1998, the mean baseline for precipitation has shifted downward, signifying a bi-decadal transition. Notably, the seasonal average precipitation has dropped to 50% of the levels recorded before 1998, with March emerging as the primary contributor to this overall decline. Subsequent analysis revealed that western disturbances (WDs) predominantly account for the precipitation in this region during this season. This dominance arises due to the lack of conducive local atmospheric conditions for any other significant source of moisture and precipitation. However, the strength of zonal moisture transport and WDs aided vorticities are seen to be weakening post 1998 which also interestingly coincides with the strongest El Nino phase on record and the start of a negative Pacific decadal oscillation (PDO) cycle. As a result, the role of global teleconnections is investigated, which concluded that negative PDO conditions after 1998 are responsible for changes in the atmospheric circulation pattern. Such changes resulted in air subsidence over the study region which increased the atmospheric stability while weakening the lower-level convergence and the westerly component of moisture transport. Consequently, the pre-monsoon precipitation has experienced a steep decline in the recent decades. To further examine this hypothesis, a threefold process is employed, which encompasses the study of changes in circulation patterns, cause-effect cluster analysis, and case study comparisons. [ABSTRACT FROM AUTHOR]

Major hurricanes (MHs) in the eastern North Pacific (ENP) in 1970–2020 are clustered into 3 categories with different quantity, intensity, lifetime, and track. MHs in all three clusters are more active in the Pacific Decadal Oscillation (PDO) warm than cold phases. However, only the relationship between MHs in the western part of ENP (cluster A) and El Niño Southern Oscillation (ENSO) is significantly modulated by the PDO. This cluster is more active during El Niño than La Niña years in the PDO cold phases, which results from the local sea surface temperature (SST) warm anomalies caused by the combined influences of ENSO and the PDO. Warmer SST can make a stronger ascending flow, and strengthen the local activity of MHs by leading to anomalous atmospheric circulation. In the PDO warm phases, however, there is no distinct local SST anomalies between two ENSO phases. Therefore, the modulation of PDO on ENSO and cluster A only occurs in the PDO negative phases. In the region of the eastern part of ENP where two other clusters are located, the PDO hardly modulates the relationship between ENSO and MHs activity as the PDO exerts little influences on the ENSO-related SST patterns in both the positive and negative phases. The conclusion is also supported by first mode of empirical orthogonal functions analysis for interannual MHs activity. Therefore, the PDO modulation cannot be ignored when predicting the activity of tropical cyclones in the ENP, especially for MHs with strong wind and rainstorm. [ABSTRACT FROM AUTHOR]

Key message: Phase transitions of the AMO and PDO promote shifts in non-summer hydrothermal conditions, which further affect tree growth in north subtropical China. Based on tree-ring samples collected from Shiyan, Hubei Province, a region with superior hydrothermal conditions during summer, we produced a 188-year standardized tree-ring width (STD) chronology, and calculated the accumulated anomalies of the STD chronology (STDA) to obtain more significant, low-frequency periodic signals. The response analysis showed that the restrictive effect of climate on tree growth was mainly manifested through the positive influence of non-summer (previous September to current May) precipitation and the negative influence of the maximum temperature during the previous autumn and winter (September‒December). Moisture conditions in the current April‒May were particularly critical for early tree growth. The different growth stages represented in the STD chronology indicated that a wet–cold non-summer climate was more conducive to rapid tree growth. Power spectrum analysis results indicated that hydrothermal variations in this area may be influenced by the El Niño–Southern Oscillation (ENSO) and sunspot activity at high frequencies and by Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO) at low frequencies. Further analyses yielded that phase transitions of AMO and PDO promoted shifts in non-summer hydrothermal conditions, which further drove tree growth in the study area. At the same time, the partial correlation results emphasized that AMO mainly regulated precipitation variability, whereas PDO had a more pronounced effect on temperature variability. These insights will improve future decisions related to tree growth to cope with climate forcings based on regional hydrothermal evolution. [ABSTRACT FROM AUTHOR]

The relationship between Arctic Oscillation (AO) and surface air temperature (SAT) over the Tibetan Plateau (TP) has been long debated, particularly in boreal summer. Based on the statistical analysis, this study reveals a significant interdecadal shift in the relationship between summer AO and the first mode of the summer SAT (SAT_EOF1) over the eastern TP in recent decades. Specifically, during 1979–1998, the correlation between summer AO and SAT_EOF1 was weak and insignificant, but during 1999–2018, it was strong and significant. Changes in the AO-related regional atmospheric circulations are responsible for this enhanced AO-SAT_EOF1 connection. During the latter period, the anomalous anticyclone related to AO has extended further westward from eastern China to the eastern TP, leading to the increased water vapor transport and prevalent descending over the eastern TP. As a result, there has been a decrease in cloud cover over the eastern TP, allowing more solar radiation to reach the ground surface and thus contributing to the enhanced AO-SAT_EOF1 linkage. Further analysis indicates that the phase transition of the Pacific decadal oscillation (PDO) is a crucial factor in regulating the AO-SAT_EOF1 linkage. Since the late 1990s, the PDO has transitioned from a positive to a negative phase, playing a role in the westward extension of the AO-related anomalous anticyclone over the eastern side of the TP through the Pacific-East Asian (PEA) teleconnection. Finally, the model simulation results provide conclusive evidence that the phase transition of the PDO contributes to the unstable AO-SAT_EOF1 connection. [ABSTRACT FROM AUTHOR]

Fully understanding the past characteristics of climate and patterns of climate change can contribute to future climate prediction. Tree-ring stable oxygen isotope ratio (δ18O) is crucial for high-resolution research of past climate changes and their driving mechanisms. A tree-ring δ18O chronology from 1896 to 2019 was established using Pinus tabulaeformis Carr. from the Yimeng Mountains (YMMs) in the central part of eastern China. We found that precipitation from the 41st pentad (five days) of the previous year to the 40th pentad of the current year (P41–40) was the main factor influencing the YMMs tree-ring δ18O change. We then created a transfer function between P41–40 and tree-ring δ18O. The reconstructed P41–40 explained 39% of the variance in the observed precipitation during the common period of 1960–2016. Over the past 124 years, the YMMs experienced 19 dry years and 20 wet years. The spatial correlation results indicate that the reconstructed precipitation could, to some extent, represent the precipitation changes in Shandong Province, and even the central part of eastern China, from the early 20th century to the present. In addition, it was found that the trends in YMMs tree-ring δ18O were similar at both high frequency and low frequency to those in tree-ring δ18O series from Mt. Tianmu in eastern China and from Jirisan National Park in southern South Korea. However, the YMMs tree-ring δ18O was only correlated at low frequency with the tree-ring δ18O of the Ordos Plateau in northwestern China and that of Nagano and Shiga in central Japan, which are far from the YMMs. The changes in precipitation and tree-ring δ18O in the YMMs were, to some extent, influenced by the Pacific decadal oscillation. [ABSTRACT FROM AUTHOR]

This study identifies that cold surges over the South China Sea (SCS) have experienced a significant change on decadal time scales. The results indicate that cold surges occur more frequently after the early 2000s than before and are at least partially explained by changes in circulation patterns. Both the negative phase of the Scandinavian (SCA) pattern and the cold phase of the interdecadal Pacific oscillation (IPO) can induce increased cold surges and the IPO effect dominates in recent decades. When the IPO shifts to its cold phase, low-level cyclones are induced over the western North Pacific through a Gill response. The northeasterlies along the northwest flank of the cyclones further lead to intensified cold surges over the SCS. The above processes can be reproduced in coupled models, suggesting a robust connection between the IPO and cold surges. The present findings highlight the role of tropical forcing and bring insight into understanding of the future climate variability and change over East Asia during boreal winter. [ABSTRACT FROM AUTHOR]

In this study, we investigated the possible relation between the Indian summer monsoon and the combination of the different phases of Pacific Decadal Oscillation (PDO) and El Niño Southern Oscillation (ENSO) before and after the climate shift in 1976. This study is carried out using IMD's rainfall dataset, HadISST v1.1 dataset and twentieth century reanalysis dataset by comparing anomalies of the respective parameters from 1901 to 2020. It is found that when positive (negative) phases of PDO and El Niño (La Niña) co-occur, deficit (surplus) rainfall are likely to occur over entire India. The SST signatures of both the PDO and ENSO are showing their respective spatial structures. However, when negative (positive) PDO and El Niño (La Niña) co-occur, the signal is mixed and it is unlikely that either surplus or deficit rainfall conditions will occur over entire India, and the SST signatures are not capturing their proper spatial pattern. In other words, when ENSO and PDO are in (out of) phase they enhance (counteract) the conventional monsoon-ENSO relationship. After confirming the climate shift in 1976, study periods were further divided into pre and post climate shift periods based on Niño 3.4 index and PDO index and analysed their impact on the Indian summer monsoon rainfall. In the pre-shift example, in-phase conditions exhibit similar qualities to those described above. Rainfall patterns are more indicative of ENSO than PDO. In the post-shift situation, the positive anomaly of SST in the PDO and Niño region is significantly stronger than in the pre-shift phase. When compared to the pre-shift example, positive rainfall anomalies are amplified during positive PDO and El Niño, while negative PDO and La Niña show a weakening of positive rainfall anomalies. The out-of-phase condition has a balancing effect due to the counteracting impact, but with an increased positive anomaly of SST. In that combination, rainfall patterns with PDO characteristics rather than ENSO characteristics emerge. Circulation features at 850 hPa during the pre-shift and post shift periods show considerable changes as an indication of the climate shift. During the pre-shift of positive PDO and La Niña, convergence at low level enhances over Indian subcontinent and resulting enhanced rainfall; however, in the post shift period the strength of convergence reduces, and it leads to reduced rainfall. The patterns of stream function and velocity potential are also consistent with rainfall during the pre and post shift periods. [ABSTRACT FROM AUTHOR]

From 1980 to 2020, drought events accounted for only 11.4% of the billion-dollar disasters in the United States (U.S.), yet caused the second-highest total amount in damages, at USD 236.6 billion. With the average cost of a drought being upwards of USD 9.5 billion, these natural disasters can create serious problems in agriculture. Drought is defined as a period of below-average precipitation that causes damage to agriculture and water supplies. Previous research has linked drought events in the U.S. Great Plains to oceanic teleconnections in the Pacific and Atlantic basins, indicating the influence of El Niño—Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO). This study looks to identify areas of the Great Plains where drought, as measured by PDSI, has the strongest relationship to ENSO, PDO, and AMO from 1950 to 2019. The states studied are Iowa, Illinois, Minnesota, Texas, Nebraska, and Kansas because these rank as the second through seventh most agriculturally productive states in terms of crop and livestock production. Results show that most of this region displays a relationship between drought and the ENSO and PDO, with less of the region displaying a relationship with the AMO. [ABSTRACT FROM AUTHOR]

During the Last Glacial Maximum (approximately 21 ka BP) the presence of large ice sheets over the Northern Hemisphere (NH) caused significant changes in the ocean–atmosphere interaction. Remarkable changes are noticed in NH both topography and atmospheric CO2 levels. This paper investigates the impact of the most recent paleotopography (ICE6G) in the Earth's climate and the oceanic carbonate system, based on a series of experiments conducted with the oceanic‐atmosphere‐vegetation‐ice‐carbon model, UVic ESCM. Results indicate enhanced cooling in northern North America in the ICE6G compared to the ICE4G simulation due to the lapse rate effect. The decrease of −24°C in the surface temperature in the ICE6G relative to the present day (PD) led to a modification of the atmospheric circulation in the Atlantic and North Pacific regions. Positive and negative anomalies vary widely for the E–P (evaporation–precipitation) flux pattern, but colder and drier atmosphere leads to a reduction in precipitation in the ICE6G experiment. Changes in wind stress between ICE6G and PD induce low temperatures in the Northern Hemisphere. These features are related changes of Ekman's transport and evaporative cooling resulting in positive anomalies of SST. Thus, changes in sea surface temperature and salinity (SST, SSS), and E–P flux led to modifications in the oceanic carbonate system, resulting in an overall increase of total alkalinity and a reduction in the concentrations of total carbon dioxide (TCO2) in the ICE6G with respect to the PD values. This is a consequence of the low concentration of glacial CO2 and increment in CO23− concentrations. The total alkalinity (TA) and TCO2 do not show a similar response to the SST and SSS, in the sense that larger departures from the PD are found in the Pacific equatorial region, which are affected by changes in water dilution as a result of precipitation and evaporation processes. [ABSTRACT FROM AUTHOR]

During the period 1970–2020, evident interdecadal variability of meridional tropical cyclone (TC) genesis occurs in the western North Pacific (WNP) from September through October (SO). From the periods 1970–1996 (IP1) to 1998–2020 (IP2), TC genesis during SO is enhanced (suppressed) in the region north (south) of 20°N. The large‐scale modulating processes feature a general warming of sea surface temperature (SST) over the WNP and systematic northward displacement in atmospheric circulations. The northward‐shifted Pacific subtropical high and monsoon trough result in an anomalous anticyclone across the subtropical region and an anomalous cyclone across the South China Sea and the northern WNP east of Taiwan. This anomalous cyclone facilitates TC genesis to increase in the northern WNP. Meanwhile, the southeastward‐extending monsoon trough retreats northwestward, leading to an anomalous anticyclone over the tropical eastern WNP and a decrease in TC genesis over the southern region. Correlation analyses indicate that the Pacific Decadal Oscillation (PDO) is more effective than global warming in modulating interdecadal increases of TC genesis in the northern region due to inclusions of both trend and oscillation features. Interdecadal decreases of TC genesis in the southern region exhibit a major oscillation feature and are thus mainly affected by interdecadal oscillation components of PDO. More TCs forming in the oceans east of Taiwan are steered by the overlying anomalous cyclone to recurve northward toward oceans to the northeast and north of Taiwan. TC‐induced rainfall over Taiwan thus increases by 25% from IP1 to IP2, which accounts for about 82% of increased total rainfall in Taiwan, showing the notable impact of interdecadal TC genesis on local climate in the WNP. [ABSTRACT FROM AUTHOR]

We present oxygen isotope and charcoal accumulation records from two lakes in eastern Washington that have sufficient temporal resolution to quantitatively compare with tree‐ring records and meteorological data. Hydroclimate reconstructions from tree‐rings and lake sediments show close correspondence after accounting for seasonal‐ to centennial‐ scale temporal sensitivities. Carbonate δ18O measurements from Castor and Round lakes reveal that the Medieval Climate Anomaly (MCA) experienced wetter November‐March conditions than the Little Ice Age (LIA). Charcoal records from Castor, Round, and nearby lakes show elevated fire activity during the LIA compared to the MCA. Increased multidecadal hydroclimate variability after 1250 CE is evident in proxy records throughout western North America. In the Upper Columbia River Basin, multidecadal wet periods during the LIA may have enhanced fuel loads that burned in subsequent dry periods. A notable decline in biomass burning occurred with Euro‐American settlement in the late nineteenth century. Plain Language Summary: Paleoclimate reconstructions are based on proxy records that are sensitive to particular aspects of the climate system. We compare high‐resolution lake sediment records with nearby tree‐ring chronologies and instrumental data to explore hypotheses about climate variability. Here, we confirm that lake‐sediment oxygen isotope ratios in the Upper Columbia River Basin are primarily sensitive to cool‐season precipitation, whereas many tree‐ring records are primarily sensitive to warm‐season conditions. When lake records are compared with winter‐sensitive tree‐ring width chronologies, the reconstructions are similar. Oxygen isotopes indicate wet and stable winter conditions during the Medieval Climate Anomaly (900–1250 CE) and variable but generally dry conditions during the Little Ice Age (1450–1850 CE). Wet periods of fuel buildup, followed by drought, may have driven greater fire activity during the LIA. An abrupt decline in fire begins occurred Euro‐American settlement, after the 1850s, and is attributed to loss of fuel connectivity and less ignition. Key Points: Sediment δ18O records show heightened hydroclimate variability during the Little Ice AgeDiscrepancies between tree rings and lake sediments from the Pacific Northwest stem from different seasonal and temporal sensitivitiesCharcoal records in the region indicate enhanced fire activity during periods of higher multi‐decadal hydroclimate variability [ABSTRACT FROM AUTHOR]

This work explores the modulation of the Pacific decadal oscillation (PDO) on the relationship between the occurrence position of rapid intensification (RI) events of tropical cyclones (TCs) over the western North Pacific (WNP) in boreal autumn and El Niño–Southern Oscillation (ENSO). From the warm to cold phase of the PDO, the occurrence position of WNP RI events experiences a significant westward shift of 5.5° in El Niño years and a significant northward shift of 4.5° in La Niña years. The strengthening of thermodynamic conditions west of 160°N plays a dominant role in the westward shift of RI events in El Niño years, and the northward shift in La Niña years is associated with the expansion of areas with warm sea surface temperature, high tropical cyclone heat potential and midlevel relative humidity, strengthening of relative vorticity north of 20°N, and weakening of dynamic conditions within 10°–20°N. During the PDO cold phase, the descending branch of the Walker circulation over the western Pacific is weak and shifts west of 140°E in El Niño years, whereas it is much stronger in La Niña years. In addition, the Hadley circulation over the WNP shows little change during El Niño, but the ascending branch around 10°N expands to 20°N during La Niña. These trends reflect the changing responses of the WNP environment to ENSO variation and are consistent with the changing distribution of WNP RI events. Moreover, during the PDO cold phases, SST over the north Indian Ocean is much warmer, and anomalous anticyclonic circulation occurs in the WNP in boreal spring (summer and autumn) during the developing phase of El Niño (La Niña) years, which may also contribute to strengthening the thermodynamic conditions over the WNP. [ABSTRACT FROM AUTHOR]

Based on the ERA5 reanalysis since the 1950s, this study investigates the coherent inter‐decadal variations of the winter continental surface air temperature (SAT) between the Eurasian (EUA) and the North American continents in the past 70 years. The SAT variation over the two continents can be characterized with five typical periods successively from a 'both‐warmer' (1950s–1960s), to a 'both‐colder' (1960s–1980s), and then a 'EUA‐warmer' (1980s–1990s) followed by another 'both‐warmer' (1990s–2000s) and a 'EUA‐colder' (2000s–2010s) period. Such periodic variation of the SAT is closely coupled with changes of the low isentropic‐level meridional mass transport (IMMT), as manifested by the coupling relationship between the midlatitude SAT and the net IMMT out of the polar circle, in terms of their zonal patterns and geographical patterns. The SAT variation can be well reconstructed with the first two leading maximum covariance analysis (MCA) modes between the SAT and IMMT. The first leading MCA mode (MCA1) mainly dominates a 'EUA‐warmer/colder' pattern, while the second (MCA2) dominates the 'both‐warmer/colder' pattern. There exists a significant lead–lag correlation between the MCA1 and MCA2, which determines the changing SAT pattern in the five successive periods. Furthermore, the MCA1 and MCA2, especially their lead–lag coupling after the 1975 is significantly associated with the Pacific decadal oscillation (PDO) which dominates the much‐intensified MCA1 and the related 'EUA‐warmer/colder' SAT pattern since then. The Atlantic multi‐decadal oscillation mainly dominates the third leading MCA mode with it's effects on the SAT over Europe coincidently to amplify the PDO‐related SAT changes. [ABSTRACT FROM AUTHOR]

Research on the evolution and driving mechanisms of East Asian monsoon (EAM) precipitation during the past millennium has received considerable attention. Previous studies have focused on comparisons between relative mean dry/wet conditions and those linked to the Pacific sea surface temperature (SST) anomalies during the Medieval Warm Period (MWP) and the Little Ice Age (LIA); however, uncertainties remain regarding the relationship between precipitation changes and Pacific SST anomalies under sub-cool internal conditions within the LIA. Herein, we present a composite high-resolution monsoon precipitation record (from 1400 CE to the present) based on chroma and median grain size data from Lake Nvshan sediments in the Jiang-Huai (JH) region, central eastern China. The results indicate that more (less) precipitation in the JH region could be associated with the warm (cold) Pacific Decadal Oscillation (PDO) phase on inter-decadal scales. We also observed an enhanced modulating effect of the PDO on JH precipitation during the late LIA. These changes in the modulating effect were likely a response to enhanced PDO amplitudes induced by increased effective radiative forcing. [ABSTRACT FROM AUTHOR]

Generating plausible future climate time series is needed for bottom-up climate impact modeling, as well as downscaling climate model output for hydrological applications. A novel method for generating multisite daily stochastic climate series is developed based on 1) linear regression between climate teleconnection time series (e.g., IPO/SOI) and annual rainfall, 2) clustered method of fragments for subannual disaggregation, and 3) a regression-based approach to daily potential evapotranspiration (PET) for hydrological modeling. We demonstrate that bias (i.e., oversampling) occurs with the standard method of fragments disaggregation in the multisite context and show that selection of an analog year from clustered rainfall amounts provides better sampling properties than the standard method of fragments. Using hydrological data for southeastern Australia, we model runoff from observed and simulated rainfall and PET using the GR4J (Génie Rural à 4 paramètres Journalier) model. Simulated annual and daily rainfall and runoff characteristics from the new method are similar to existing methods, with improvements demonstrated in wet–wet transition probabilities and spatial (between-site) correlations. Significance Statement: In this paper we develop a novel method for generating multisite daily stochastic climate series regressing climate teleconnections (e.g., ENSO, IPO) to annual site rainfall, and disaggregation using a clustered version of the method of fragments. The modular nature of the method also allows for the possibility of the generation of replicates from GCM output of climate teleconnections, as well as perturbed climate futures for scenario-neutral modeling. Results demonstrate that rainfall and runoff metrics of interest for water resource modeling are reproduced well using the model. [ABSTRACT FROM AUTHOR]

The water disponibility of Lake Titicaca is important for local ecosystems, domestic water, industry, fishing, agriculture, and tourism in Peru and Bolivia. However, the water level variability in Lake Titicaca (LTWL) still needs to be understood. The fluctuations of LTWL during the 1921–2018 period are investigated using continuous wavelet techniques on high- and low-pass filters of monthly time series, ERA-20C reanalysis, sea surface temperature (SST), and water level. We also built multiple linear regression (MLR) models based on SST indices to identify the main drivers of the LTWL variability. LTWL features annual (12 months), biennial (22–28 months), interannual (80–108 months), decadal (12.75–14.06 years), interdecadal (24.83–26.50 years), and multidecadal (30–65 years) signals. The high- and low-frequency components of the LTWL are triggered by the humidity transport from the lowland toward the Lake Titicaca basin, although different forcings could cause it. The biennial band is associated with SST anomalies over the southeastern tropical Atlantic Ocean that strengthen the Bolivian High-Nordeste Low system. The interannual band is associated with the southern South Atlantic SST anomalies, which modulate the position of the Bolivian High. According to the MLR models, the decadal and interdecadal components of the LTWL can be explained by the linear combination of the decadal and interdecadal variability of the Pacific and Atlantic SST anomalies (r > 0.83, p < 0.05). In contrast, the multidecadal component of the LTWL is driven by the multidecadal component of the North Atlantic SST anomalies (AMO) and the southern South Atlantic SST anomalies. Moreover, the monthly time series of LTWL exhibits four breakpoints. The signs of the first four trends follow the change of phases of the multidecadal component of LTWL, while the fifth trend is zero attributable to the diminished amplitude of the interdecadal component of LTWL.

Abstract We present oxygen isotope and charcoal accumulation records from two lakes in eastern Washington that have sufficient temporal resolution to quantitatively compare with tree‐ring records and meteorological data. Hydroclimate reconstructions from tree‐rings and lake sediments show close correspondence after accounting for seasonal‐ to centennial‐ scale temporal sensitivities. Carbonate δ18O measurements from Castor and Round lakes reveal that the Medieval Climate Anomaly (MCA) experienced wetter November‐March conditions than the Little Ice Age (LIA). Charcoal records from Castor, Round, and nearby lakes show elevated fire activity during the LIA compared to the MCA. Increased multidecadal hydroclimate variability after 1250 CE is evident in proxy records throughout western North America. In the Upper Columbia River Basin, multidecadal wet periods during the LIA may have enhanced fuel loads that burned in subsequent dry periods. A notable decline in biomass burning occurred with Euro‐American settlement in the late nineteenth century.

The diversity of summer precipitation regimes over eastern China has raised concerns. Its variations during different warming periods and the relevant mechanisms remain unclear. In this study, the decadal precipitation changes and the associated atmospheric circulations, ocean conditions, and warming over Northern Hemisphere were investigated. The results indicated a quadrupole precipitation pattern over eastern China in the boreal summer, with a significant shift around 1998. From 1985–98 to 1999–2014, two wet anomalies were over North China (NC) and South China (SC), while two dry anomalies were over the Yangtze–Huaihe River basin (YHRB) and northeastern China (NEC). This quadrupole pattern is associated with the circumglobal teleconnection (CGT), the negative phase of the interdecadal Pacific oscillation ("−IPO"), the warming over the high latitudes of North Atlantic ("+HNA"), and the warming rate over the Northern Hemisphere (NWR). The CGT would induce the weakened polar front jet (PJ), southward shift of the PJ, and northward shift of the entire subtropical jet (SJ), which decreased the meridional distance between the PJ and SJ, and therefore resulted in the NC-wet–YHRB-dry–SC-wet variation. As well, the dry NEC was attributed to the enhanced PJ, weakened Iranian Plateau subtropical jet (IPSJ), and northward movement of the PJ and Tibetan Plateau subtropical jet (TPSJ), which were accompanied by −IPO and +HNA. Moreover, the NWR led to the weakened PJ and SJ, as well as the southward shift of PJ, also contributed to the dry YHRB, and wet anomalies over NC and SC. Particularly, the +HNA plays an important role in this quadrupole precipitation variation. Our findings have highlighted the role of jet streams in complex precipitation patterns, which will contribute to understanding the diverse variations in precipitation regimes over eastern China under global warming. Significance Statement: The purpose of this study is to understand the regime shifts of the East Asian summer monsoon precipitation in different warming periods. We revealed a new precipitation pattern over eastern China as a "+ − + −" precipitation pattern from south to north. The circumglobal teleconnection, the negative phase of the interdecadal Pacific oscillation, the warming over the high-latitude North Atlantic, and the warming over Northern Hemisphere can be used to explain the quadrupole precipitation pattern. These findings could highlight the role of jet streams linking the quadrupole pattern to the teleconnections, dominant decadal ocean variabilities and the warming over Northern Hemisphere. Future work will focus on the role of Arctic rapid warming in summer precipitation over eastern China. [ABSTRACT FROM AUTHOR]

The multidecadal decrease of precipitation in North China since the 1960s has caused a significant impact on the ecological environment and social development. It has become a key concern for meteorologists and national decision-makers to understand what is driving this. To better understand the oceanic impact on the interdecadal variation of summer precipitation in North China, we analyze two 1200-yr global climate simulations from state-of-the-art coupled models with an explainable convolutional neural network (CNN). The accuracy of the CNN model in correctly diagnosing the increased and decreased summer precipitation anomaly in North China reaches 98% and 96.6% in two CMIP6 models, respectively. We find that a sea surface temperature anomaly (SSTA) in the northern Pacific, associated with the Pacific decadal oscillation (PDO), and an SSTA in the northern Atlantic, associated with the Atlantic multidecadal oscillation (AMO), play essential roles in the interdecadal variation of summer precipitation in North China. The modulations of the PDO and AMO on summer precipitation in North China are consistent with the results derived from limited observations and simulations, which means these mechanisms are robust. In particular, when the PDO is in a negative phase and the AMO is in a positive phase, the positive anomaly of precipitation and the associated circulation anomalies are intensified. This study also confirms that with the explainable deep learning method, a convolutional neural network can be used to identify multidecadal variability in North China summer precipitation, although limited datasets such as observational records make it more challenging to apply deep learning methods. [ABSTRACT FROM AUTHOR]

Against the backdrop of global warming, the Eurasian continent and its surrounding areas have experienced ununiform warming in recent decades. Variations in the meridional thermal contrast (MTC) in East Asia and its adjacent regions have a critical influence on the East Asian summer monsoon (EASM); therefore, its forcings were analyzed in this study. The results show that the MTC in East Asia and the surrounding regions showed significant decadal changes during the interval 1960–2014, with a decadal strengthening in the late 1970s and a decadal weakening in the late 1990s. Further investigation showed that internal variability was the dominant control on these two decadal changes of MTC, accounting for 135% and 78.5% of the two observed changes in the 1970s and 1990s, respectively. Based on pacemaker experiments and a method that reconstructs decadal changes in the interdecadal Pacific oscillation (IPO), we found that a positive phase of the IPO tends to cause cooling in the low latitudes and warming in the mid–high latitudes and that a negative phase of the IPO tends to cause warming in the low latitudes and cooling in the mid–high latitudes, which in turn can explain 54.4% and 35.8% of the two internally generated decadal changes in the MTC, respectively. The Atlantic multidecadal oscillation (AMO), when in the same phase as the IPO, exhibits an opposing pattern of surface air temperature. This only had a significant influence in the late 1990s, where it explained 37.9% of the internally generated decadal changes in the MTC. Our findings highlight that internal variability, especially that relating to the IPO and the AMO, should be considered when predicting future changes in thermal conditions in East Asia and its adjacent regions. [ABSTRACT FROM AUTHOR]

The Pacific decadal oscillation (PDO) is the most dominant decadal climate variability over the North Pacific and has substantial global impacts. However, the interannual and decadal PDO prediction skills are not satisfactory, which may result from the failure of appropriately including the North Pacific midlatitude air–sea interaction (ASI) in the initialization for climate predictions. Here, we present a novel initialization method with a climate model to crack this nutshell and achieve successful PDO index predictions up to 10 years in advance. This approach incorporates oceanic observations under the constraint of ASI, thus obtaining atmospheric initial conditions (ICs) consistent with oceanic ICs. During predictions, positive atmospheric feedback to sea surface temperature changes and time-delayed negative ocean circulation feedback to the atmosphere over the North Pacific play essential roles in the high PDO index prediction skills. Our findings highlight a great potential of ASI constraints during initialization for skillful PDO predictions. Significance Statement: The Pacific decadal oscillation is a prominent decadal climate variability over the North Pacific. However, accurately predicting the Pacific decadal oscillation remains a challenge. In this study, we use an advanced initialization method where the oceanic observations are incorporated into a climate model constrained by air–sea interactions. We can successfully predict the Pacific decadal oscillation up to 10 years in advance, which is hardly achieved by the state-of-the-art climate prediction systems. Our results suggest that the constraint of air–sea interaction during initialization is important to skillful predictions of the climate variability on decadal time scales. [ABSTRACT FROM AUTHOR]

Reference crop evapotranspiration (ET0 ) can fully represent the evapotranspiration capacity in areas, such as efficient water-saving irrigation in agriculture. Most studies have typically conducted sensitivity analyses of changing ET0 in the local meteorological factors, without considering the remotely correlated effects of large-scale climate variability on ET0 . Large-scale climate variability generated by the ocean is a significant driver of global and regional climate change, indicating the nonnegligible impact on the evolution pattern of ET0 . However, it is still lacking in the influence of large-scale climate variability on ET0, leading to the inaccurate attribution of ET0 changes. Therefore, this study aims to explore the relationships between the large-scale climate variability of El Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) with the ET0 variability characteristics in Xinjiang region. The daily meteorological data and climate variability indices were collected from 84 meteorological stations using multiple linear regression and Cramer's mutation test. The results indicate that the ET0 showed a decreasing trend from 1960 to 2020, with an average decreasing rate of 0.75 mm/a; the year 1998 was the mutation of the ET0, and the ET0 showed an obvious decreasing trend from 1960 to 1997, with an average decreasing rate of 2.50 mm/a, and then turned to an outstandingly increasing trend from 1998 to 2020, with an average increasing rate of 3.18 mm/a. The multivariate linear correlation results show that the ET0 was negatively correlated with the RH with a regression coefficients of −0.46, and positively correlated with the U, T, and Rn with regression coefficients of 0.95, 0.36, and 0.20, where the changes of ET0 were mainly dominated by the changes of U. On the seasonal scale, the interannual variation trend of ET0 in the four seasons was more consistent with the annual cumulative ET0 variation trend, which decreased at the rate of 0.01, 0.54, 0.23, and 0.01 mm/a in spring, summer, autumn, and winter, respectively, whereas, the trend change in the spring, summer, and autumn seasons occurred in the mid1990s, respectively. PDO was the main variable affecting the trend shift of ET0, where the regression coefficients between the two was -0.34. There was also a significant negative correlation of U with PDO and AMO, with regression coefficients of −0.31 and −0.34, respectively. The PDO was shifted from a positive to a negative phase in 1998, resulting in the wind speed shifting from a downward to an upward trend, which in turn led to the trend shift of ET0 in the Xinjiang region in 1998. The shift of AMO from a negative to a positive phase strengthened the trend change of wind speed, while the effects of ENSO and IOD on wind speed were relatively weak. The negative phase of PDO was achieved in the spatial regression of sea surface temperature (SST) with ET0, which further verified the influence of PDO on the trend shift of ET0 in the Xinjiang region of western China. [ABSTRACT FROM AUTHOR]

We evaluate causal dependencies between thirteen indices that represent large-scale climate patterns (El Nino/Southern Oscillation, the North Atlantic Oscillation, the Pacific Decadal Oscillation, etc.) using a recently proposed approach based on a linear approximation of the transfer entropy. We demonstrate that this methodology identifies causal relations that are well-known, as well as it uncovers some relations which, to the best of our knowledge, have not yet been reported in the literature. We also identify significant changes in causal dependencies that have occurred in the last three decades. [ABSTRACT FROM AUTHOR]

(1) Background: Global climate change is expected to significantly alter growing conditions along mountain gradients. Landscape ecological patterns are likely to shift significantly as species attempt to adapt to these changes. We evaluated the extent to which spatial (elevation and canopy cover) and temporal (decadal trend and El Niño–Southern Oscillation/Pacific Decadal Oscillation) factors impact seasonal snowmelt and forest community dynamics in the Western Hemlock–True Fir ecotone region of the Oregon Western Cascades, USA. (2) Methods: Tsuga heterophylla and Abies amabilis seedling locations were mapped three times over 20 years (2002–2022) on five sample transects strategically placed to cross the ecotone. Additionally, daily ground temperature readings were collected over 10 years for the five transects using 123 data loggers to estimate below-canopy snow metrics. (3) Results: Based on validation using time-lapse cameras, the data loggers proved highly reliable for estimating snow cover. The method reported fewer days of snow cover as compared to meteorological station-based snow products for the region, emphasizing the importance of direct under-canopy field observations of snow. Snow season variability was most significantly impacted temporally by cyclical ENSO/PDO climate patterns and spatially by differences in canopy cover within the ecotone. The associated seedling analysis identified clear sorting of species by elevation within the ecotone but reflected a lack of a long-term trend, as species dominance in the seedling strata did not significantly shift along the elevation gradient over the 20-year study. (4) Conclusions: The data logger-based approach provided estimates of snow cover at ecologically significant locations and fine enough spatial resolutions to allow for the study of forest regeneration dynamics. The results highlight the importance of long-term, understory snow measurements and the influence of climatic oscillations in understanding the vulnerability of mountain areas to the changing climate.

Using typhoon data from the Shanghai Typhoon Institute of the China Meteorological Administration, the Japan Meteorological Agency’s annual Pacific decadal oscillation (PDO) index, and NCEP/NCAR reanalysis data from 1951 to 2021, correlation and composite analyses were carried out to study the relationship between the variability among tropical cyclones of different magnitudes affecting South China and the PDO. The results show that there is an obvious out-of-phase relationship between the proportion of tropical cyclones reaching a typhoon-level intensity or above in South China and the PDO index. When the PDO is in a cold (warm) phase, the sea surface temperature in the eastern and central equatorial Pacific is cold (warm), similar to the eastern Pacific La Niña (El Niño) phenomenon, and the SST in the eastern and western tropical Pacific Ocean shows a negative (positive) gradient; the subtropical high in the western Pacific Ocean is weaker (stronger) than normal, with the western ridge point to the east (west), and the 500 hPa geopotential height in the South China Sea and the area east of the Philippines is weaker (stronger), which is conducive to (unfavorable to) the formation of a monsoon trough; and the westerly (easterly) winds at high altitudes and the southwesterly (northeasterly) winds at low altitudes from the South China Sea to the Philippines are abnormally strong, and a positive (negative) vorticity at low altitudes, a low (high) sea level pressure, and strong (weak) convection are shown. These conditions are favorable (unfavorable) for the intensification of typhoons affecting South China, and as a result, the number of tropical cyclones reaching the level of typhoons or above account for a greater (smaller) proportion of those affecting South China.

Fully understanding the past characteristics of climate and patterns of climate change can contribute to future climate prediction. Tree-ring stable oxygen isotope ratio (δ18O) is crucial for high-resolution research of past climate changes and their driving mechanisms. A tree-ring δ18O chronology from 1896 to 2019 was established using Pinus tabulaeformis Carr. from the Yimeng Mountains (YMMs) in the central part of eastern China. We found that precipitation from the 41st pentad (five days) of the previous year to the 40th pentad of the current year (P41–40) was the main factor influencing the YMMs tree-ring δ18O change. We then created a transfer function between P41–40 and tree-ring δ18O. The reconstructed P41–40 explained 39% of the variance in the observed precipitation during the common period of 1960–2016. Over the past 124 years, the YMMs experienced 19 dry years and 20 wet years. The spatial correlation results indicate that the reconstructed precipitation could, to some extent, represent the precipitation changes in Shandong Province, and even the central part of eastern China, from the early 20th century to the present. In addition, it was found that the trends in YMMs tree-ring δ18O were similar at both high frequency and low frequency to those in tree-ring δ18O series from Mt. Tianmu in eastern China and from Jirisan National Park in southern South Korea. However, the YMMs tree-ring δ18O was only correlated at low frequency with the tree-ring δ18O of the Ordos Plateau in northwestern China and that of Nagano and Shiga in central Japan, which are far from the YMMs. The changes in precipitation and tree-ring δ18O in the YMMs were, to some extent, influenced by the Pacific decadal oscillation.

The Pacific Decadal Oscillation (PDO) and Pacific Meridional Mode (PMM) are prominent climate modes in the North Pacific with well-established impacts on tropical cyclone (TC) genesis in the western North Pacific (WNP) basin. While previous research has primarily focused on the roles of the PDO and PMM in regulating TC genesis through the modification of large-scale environmental factors, this study investigates the evolving influence of the PDO on WNP TC genesis since the 1950s. Remarkably, our analysis reveals a shift in the PDO-TC genesis relationship, transitioning from a significant negative correlation to a significant positive correlation since the 1990s. This shift is attributed to variations in the specific large-scale factors through which the PDO affects TC genesis. Furthermore, this study suggests that these changes appear to be linked to the PMM strengthening on the interdecadal timescale in recent decades. The linkage of the PMM strengthening to the PDO-related atmospheric circulation is further confirmed by the results of a 500 year pre-industrial numerical experiment, suggesting that the PMM strengthening may result from natural internal variability. The results underscore the non-stationary relationship between PDO and WNP TC genesis, with the PMM intensity probably influencing their relationship.

We conducted an investigation into the characteristics of marine heatwaves (MHWs) in Indonesian waters, aiming to understand the underlying mechanisms responsible for their formation, particularly the ones generated by net surface heat flux. To accomplish this, we utilized remote sensing data from the National Oceanic and Atmospheric Administration and the European Centre for Medium-Range Weather Forecasts. The dataset covered a 40-year period (1982–2021) encompassing both warm (1982–2007) and cold (2008–2021) phases of the Pacific Decadal Oscillation (PDO). Statistical analysis methods were employed to process the data. Our study reveals significant findings regarding MHWs in Indonesian waters. We observed the highest average frequency and maximum intensity of MHWs, occurring approximately 2–3 times a year and exceeding 1.5 °C, respectively, during the warm phase of PDO (with El Niño events occurring more frequently than La Niña). Conversely, the longest durations of MHWs in Indonesian waters were recorded during the cold phase of PDO (with La Niña events occurring more frequently than El Niño), spanning approximately 7–15 days. We identified local forcing in the form of net surface heat flux as the primary driver of MHW frequency and maximum intensity. During the warm phase of PDO, the net surface heat flux was notably higher, primarily due to increased shortwave radiation (heat gain) and reduced latent heat flux (heat loss) through the evaporation process. These factors collectively contributed to maintaining warmer ocean temperatures. Moreover, our study provides valuable insights into the interannual variability of MHWs through the application of composite calculations. We discovered a strong correlation between the occurrence of El Niño and positive Indian Ocean Dipole (IOD) events during the warm PDO phase and the highest frequency and maximum intensity of MHWs, with approximately 2.52 events and 1.54 °C, respectively. In contrast, we found that MHWs with the longest durations were closely linked to La Niña and negative IOD events during the cold PDO phase, lasting approximately 10.90 days. These findings highlight the complex interplay between climate phenomena and MHW characteristics, further deepening our understanding of their dynamics. [ABSTRACT FROM AUTHOR]