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The present study investigates the subsurface temperature trend in the tropical Pacific in historical and SSP2‐4.5 simulation by 14 models from Coupled Model Intercomparison Project Phase 6. Robust cooling trends exist between 100 and 200 m in the off‐equatorial region (2°–8°N/S) of both the North and South Pacific during 1950–2100. Both spiciness variation and thermocline heave are important in driving this off‐equatorial subsurface cooling at a fixed level. The cooling spiciness trends in the off‐equatorial region originates from the eastern subtropical Pacific along isopycnal streamlines. Besides, the contribution of the thermocline heave to the cooling is governed by the large wind stress curl trend. Furthermore, the maximum negative spiciness trends occur in the eastern subtropics of both the North and South Pacific. The consistently poleward migration of outcropping lines makes cooler subducting water at higher latitudes directly flow into the subtropical subsurface, which induces the negative spiciness trends here. The generation of the negative spiciness trends is also partly attributed to the anomalous advection process on distinct isopycnal surfaces. These two processes both contribute to the generation of spiciness variation in both the North and South Pacific. The generation region is partly covered by the outcropping region, thus, the strong vertical mixing within the mixed layer may weaken the effect of the subduction and advection. This study mainly highlights the significance of the spiciness variation in the projection of the subsurface temperature trend on a longer time scale under global warming. Plain Language Summary: Temperature change in the tropical Pacific has crucial impacts on changes in regional or global climate variables, such as precipitation, large‐scale circulation, and even climate feedback. Besides the sea surface temperature that most studies focus on, the subsurface temperature can affect the upper ocean thermal structure, which further modulates the air‐sea interactions through vertical processes, so it is also vital to investigate the subsurface temperature change in a warming climate. In this study, we find two robust symmetric subsurface cooling trends in the tropical Pacific off‐equatorial region and elucidate two important impact factors of these cooling trends: spiciness variation and thermocline heave. The former is the temperature/salinity anomaly along the isopycnal surface, and the latter is the vertical displacement of the mean thermocline. Negative spiciness trends originated from the eastern subtropics and local thermocline shallowing induce the subsurface cooling in the off‐equatorial Pacific. We also find that both the direct subduction and anomalous advection processes can induce the maximum cooling trends in the eastern subtropics. The present study shows that spiciness variation is the same significance as thermocline heave, while most previous studies neglect the former's contribution. Key Points: Robust long‐term cooling trends exist between 100 and 200 m in the Pacific off‐equatorial region in historical and SSP2‐4.5 simulationSpiciness variation and thermocline heave are both important in driving the off‐equatorial subsurface cooling at a fixed levelThe maximum cooling spiciness trends occur in the eastern subtropics, mainly due to direct subduction and anomalous advection processes [ABSTRACT FROM AUTHOR]

The mechanisms regulating spring phenology have been extensively studied in angiosperm species. However, given that gymnosperms and angiosperms diverged 300 million years ago, phenology may be triggered by different cues in gymnosperm species. The regulatory mechanisms of phenology in subtropical regions remain largely unknown. In combination, it remains untested whether subtropical gymnosperm species have chilling requirements and are photosensitive. We conducted a climate chamber experiment with three chilling and three photoperiod treatments to investigate budburst during an 8-week forcing period. We tested whether budburst of eight gymnosperms species (Cryptomeria japonica , Cunninghamia lanceolata , Cupressus funebris , Ginkgo biloba , Metasequoia glyptostroboides , Pinus massoniana , Pseudolarix amabilis and Podocarpus macrophyllus) was photoperiod sensitive or has strong chilling requirements and whether photoperiod or chilling was more important for advancing budburst. Chilling advanced budburst and increased the percentage of budburst for gymnosperm species. Gymnosperm species required moderate chilling days to advance budburst. Interestingly, the forcing requirement for gymnosperm species was higher than that for angiosperms in the same forest, suggesting that gymnosperms may need more cumulative forcing to initiate budburst than do angiosperms. Compared with temperate gymnosperm species in Germany (194–600 °C days), the subtropical species studied here had a much higher forcing requirement (814–1150 °C days). The effects of photoperiod were minor, suggesting that chilling outweighs photoperiod in advancing budburst of gymnosperm species in this subtropical region. These results reveal that increased winter temperatures with continued global warming may impact not only angiosperms but also gymnosperms, leading to their delayed spring budburst. [ABSTRACT FROM AUTHOR]

As an important ecotone, the alpine timberline is the boundary between closed-canopy montane forest and alpine vegetation, and is highly sensitive to global and regional climate changes. We provided a way to identify and extract the alpine timberline in Yarlung Zangpo Grand Canyon Nature Reserve by using remote sensing data and spatial analysis based on land use/land cover classification and NDVI distribution characteristics. Combining DEM data, the influence of slope and aspect on the distribution of alpine timberline was explored. The results showed that the alpine timberline in Yarlung Zangpo Grand Canyon is transitional timberline, with the upper boundary approximately distributed at the elevation of 3422–4373 m, the lower boundary at approximately 3270–4164 m, with a width of about 110–280 m. Alpine timberline was mainly distributed on steep and very steep slopes ranging from 25° to 45°. The maximum elevation of both the upper and lower boundaries occurred on steep slopes. The distribution of alpine timberline varies with aspects, with sunny slopes having a higher boundary than shady slopes. [ABSTRACT FROM AUTHOR]

The Subantarctic Mode Water (SAMW) plays an essential role in the global heat, freshwater, carbon, and nutrient budgets. In this study, decadal changes in the SAMW properties in the southern Indian Ocean (SIO) and associated thermodynamic and dynamic processes are investigated during the Argo era. Both temperature and salinity of the SAMW in the SIO show increasing trends during 2004–18. A two-layer structure of the SAMW trend, with more warm and salty light SAMW but less cool and fresh dense SAMW, is identified. The heaving and spiciness processes are important but have opposite contributions to the temperature and salinity trends of the SAMW. A significant deepening of isopycnals (heaving), peaking at σθ = 26.7–26.8 kg m−3 in the middle layer of the SAMW, expands the warm and salty light SAMW and compresses the cool and fresh dense SAMW corresponding to the change in subduction rate during 2004–18. The change in the SAMW subduction rate is dominated by the change in the mixed layer depth, controlled by the changes in wind stress curl and surface buoyancy fluxes. An increase in the mixed layer temperature due to weakening northward Ekman transport of cool water leads to a lighter surface density in the SAMW formation region. Consequently, density outcropping lines in the SAMW formation region shift southward and favor the intrusion and entrainment of the cooler and fresher Antarctic surface water from the south, contributing to the cooling/freshening trend of isopycnals (spiciness). Subsequently, the cooler and fresher SAMW spiciness anomalies spread in the SIO via the subtropical gyre. Significance Statement: Subantarctic Mode Water is a distinct water mass with vertically uniform properties in the Southern Hemisphere's subtropical gyres. Climate change is imprinted in the SAMW through the ventilation at the base of the winter mixed layer. The ocean modulation associated with wind-forced large-scale waves and circulation also plays an essential role in heat, salinity, and water mass redistribution. A net increase in volume-weighted potential temperature and salinity of the SAMW is found during the Argo era since 2004, resulting from a combination of climate change and ocean modulation through opposite heaving and spiciness processes. This study improves our understanding of the dynamics and thermodynamics involved in the SAMW formation during rapid climate change. [ABSTRACT FROM AUTHOR]

Using an eastern tropical Pacific pacemaker experiment called the Pacific Ocean–Global Atmosphere (POGA) run, this study investigated the internal variability in sea surface salinity (SSS) and its impacts on the assessment of long-term trends. By constraining the eastern tropical Pacific sea surface temperature variability with observations, the POGA experiment successfully simulated the observed variability of SSS. The long-term trend in POGA SSS shows a general pattern of salty regions becoming saltier (e.g., the northern Atlantic) and fresh regions becoming fresher, which agrees with previous studies. The 1950–2012 long-term trend in SSS is modulated by the internal variability associated with the interdecadal Pacific oscillation (IPO). Due to this variability, there are some regional discrepancies in the SSS 1950–2012 long-term change between POGA and the free-running simulation forced with historical radiative forcing, especially for the western tropical Pacific and southeastern Indian Ocean. Our analysis shows that the tropical Pacific cooling and intensified Walker circulation caused the SSS to increase in the western tropical Pacific and decrease in the southeastern Indian Ocean during the 20-yr period of 1993–2012. This decadal variability has led to large uncertainties in the estimation of radiative-forced trends on a regional scale. For the 63-yr period of 1950–2012, the IPO caused an offset of ~40% in the radiative-forced SSS trend in the western tropical Pacific and ~170% enhancement in the trend in the southeastern Indian Ocean. Understanding and quantifying the contribution of internal variability to SSS trends helps improve the skill for estimates and prediction of salinity/water cycle changes. [ABSTRACT FROM AUTHOR]

The Indo-Pacific Convergence Zone (IPCZ) has a complex ocean dynamical system. All scale processes are active and interplay from small-scale turbulent mixing to basin-scale circulation. The IPCZ acts as an "oceanic bridge" for the inter-basin mass transports and basin-scale planetary waves, closely linking basin-scale circulations in the Pacific and Indian Oceans. Numerous straits in the Indonesian Seas provide oceanic channels for planetary waves propagating between the tropical Pacific and the southeast Indian Ocean. On a large scale, the inter-basin mass transports and planetary waves change the ocean thermal structure, triggering strong air-sea interactions, and further regulating the variability of Walker and Hadley Circulations. The latter form an "atmospheric bridge", which significantly affects the Asian and Australian monsoon systems and a series of global climate events like El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole, and Indian Ocean Basinwide warming. On meso- and small- scales, eddy, submesoscale processes, and turbulent motions mix the water mass from different sources, dramatically stirring the upper ocean in the IPCZ. Ocean dynamics have essential impacts on the ecological system in the IPCZ. On the one hand, these processes directly affect biodiversity by controlling the genetic exchanges and species dispersals between basins. On the other hand, the ocean dynamical processes influence biogeochemical cycling by controlling the trophic transfer, which further impacts primary productivity. With the intensified climate variations under global warming, e.g., marine heatwaves and extreme events, the ocean dynamical processes turn more active and enhance the influence on the ecosystem. This work overviews a series of studies focusing on the multi-scale ocean dynamical and environmental processes in the IPCZ, including the climatic signatures in coral records and the ecological response. [ABSTRACT FROM AUTHOR]

The Indo-western Pacific Ocean capacitor (IPOC) effect induces coherent climate variations over the Indo-western Pacific Ocean with significant sea-surface temperature (SST) anomalies over the South China Sea (SCS). Most IPOC events are related to El Ni n ∼ o-Southern Oscillation (ENSO), occurring in the post-ENSO years. The coral δ18O and Sr/Ca precisely recorded climate variations as proxies for SST. This study investigated the IPOC via a reduced major axis (RMA) regression analysis based on coral records in the SCS. The coral records of Yongxing Island show a double SST peak during El Niño events after the 1960s, one in the mature phase and the other in the post-El Ni n ∼ o summer. Tropical Indian Ocean warming persists into summer and anchors an anomalous anticyclonic circulation over the western North Pacific, extending the ENSO effect on the SCS SST through the IPOC effect, despite the decay of El Ni n ∼ o. The corals from Yongle Atoll, Dongsha Atoll, and the region off central Vietnam well recorded the IPOC impacts associated with two strong El Ni n ∼ o events (2009–2010 and 1997–1998) on SST anomalies in summer. According to coral records off central Vietnam, the 1959 IPOC effect occurred without an antecedent El Ni n ∼ o event. The results show that coral proxies could capture the IPOC events in the SCS and prove that IPOC could act as an intrinsic climate mode in the Indo-western Pacific Ocean. • Coral proxies from Yongxing Island record the IPOC events since 1960. • Most IPOC events recorded by corals are associated with the El Ni n ∼ o events. • Some IPOC events, e.g., recorded off central Vietnam in 1959, occur without antecedent El Ni n ∼ o events. [ABSTRACT FROM AUTHOR]

Climate models consistently project reduced surface warming over the eastern equatorial Indian Ocean (IO) under increased greenhouse gas (GHG) forcing. This IO dipole (IOD)-like warming pattern, regarded as robust based on consistency among models by the new Intergovernmental Panel on Climate Change (IPCC) report, results in a large increase in the frequency of extreme positive IOD (pIOD) events, elevating the risk of climate and weather disasters in the future over IO rim countries. These projections, however, do not consider large model biases in both the mean state and interannual IOD variance. In particular, a 'present-future relationship' is identified between the historical simulations and representative concentration pathway (RCP) 8.5 experiments from phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel ensemble: models with an excessive IOD amplitude bias tend to project a strong IOD-like warming pattern in the mean and a large increase in extreme pIOD occurrences under increased GHG forcing. This relationship links the present simulation errors to future climate projections, and is also consistent with our understanding of Bjerknes ocean-atmosphere feedback. This study calibrates regional climate projections by using this present-future relationship and observed IOD amplitude. The results show that the projected IOD-like pattern of mean changes and frequency increase of extreme pIOD events are largely artifacts of model errors and unlikely to emerge in the future. These results illustrate that a robust projection may still be biased and it is important to consider the model bias effect. [ABSTRACT FROM AUTHOR]

Interannual-to-decadal variability and trends of sea level in the South China Sea (SCS) are studied using altimetric data during 1993-2012 and reconstructed sea level data from 1950-2009. The interannual variability shows a strong seasonality. Surface wind anomalies associated with El Niño-Southern Oscillation explain the sea-level anomaly pattern in the interior SCS, while Rossby waves radiated from the eastern boundary dominate the sea-level variability in the eastern SCS. Decadal variability of sea level in the SCS follows that in the western tropical Pacific, with large variance found west of Luzon Island. Local atmospheric forcing makes a negative contribution to decadal variability in the central SCS, and Rossby waves radiated from the eastern boundary appear to be important. During 1993-2012, decadal sea level averaged in the SCS is significantly correlated with the Pacific Decadal Oscillation (PDO) ( r = −0.96). The decadal variability associated with the PDO accounts for most part of sea-level trends in the SCS in the last two decades. [ABSTRACT FROM AUTHOR]

The South Asian high (SAH) is a huge anticyclone in the upper troposphere. It influences the climate and the distribution of trace constituents and pollutants. The present study documents the change in the SAH and precipitation under global warming, as well as the possible link between the changes, based on 17 Coupled Model Intercomparison Project Phase 5 (CMIP5) model simulations. The CMIP5 historical simulation reproduces reasonably the tropospheric circulation (including the SAH), precipitation, and moisture. Under global warming, more than 75 % of the CMIP5 models project a southward shift of the SAH. The southward shift is more significant in the models with stronger anticyclonic circulation in the south part of the climatological SAH. The precipitation response displays a contrasting feature: negative over the southeastern equatorial Indian Ocean (IO) and positive over the tropical northern IO, the Bay of Bengal, and the equatorial western Pacific. The results of a linear baroclinic model (LBM) show that the regional rainfall changes over the Bay of Bengal and the equatorial western Pacific have a main contribution to the southward shift of the SAH. In addition, the precipitation and the surface wind responses over the Indo-Pacific region are well coupled. On one hand, the surface wind anomaly affects the rainfall response through altering the SST and moisture. On the other hand, the condensational heating released by regional rainfall changes sustains the surface wind response. [ABSTRACT FROM AUTHOR]

Variation in intermediate water salinity in the South China Sea (SCS) between the 1960s and 1980s was studied using historical hydrographic data. The results demonstrate that the water was significantly fresher in the 1980s than in the 1960s, indicating that vertical mixing at intermediate water depth was reduced in the 1980s. This was partially because of the change of the SCS meridional overturning circulation (MOC) connecting local intermediate water with deep water. Data assimilation showed a 0.5 Sv (1 Sv=10 m /s) reduction in the strength of the MOC, which is about one third of the mean SCS MOC. Because the SCS MOC is linked to the Pacific Ocean, such an interdecadal variation in the intermediate water SCS may reflect anthropogenic climate change in the world ocean. [ABSTRACT FROM AUTHOR]

A contrasting trend pattern of sea surface salinity (SSS) between the western tropical Pacific (WTP) and the southeastern tropical Indian Ocean (SETIO) is observed during 2004-2013, with significant salinity increase in the WTP and freshening in the SETIO. In this study, we show that increased precipitation around the Maritime Continent (MC), decreased precipitation in the western-central tropical Pacific, and ocean advection processes contribute to the salinity trends in the region. From a longer historical record, these salinity trends started in the mid-1990s, a few years before the Global Warming Hiatus from 1998 to present. The salinity trends are associated a strengthening trend of the Walker Circulation over the tropical Indo-Pacific, which have reversed the long-term salinity changes in the tropical Indo-Pacific as a consequence of global warming. Understanding decadal variations of SSS in the tropical Indo-Pacific will better inform on how the tropical hydrological cycle will be affected by the natural variability and a warming climate. [ABSTRACT FROM AUTHOR]

Abstract: Satellite imagery of SeaWiFS from October 1997 to November 2007 is used to investigate the dominant seasonal and interannual variations of the surface light absorption due to Colored Dissolved Organic Materials (CDOM) in the South China Sea (SCS). Results show that the spatial distribution of CDOM mimics the major features of the SCS basin-scale circulation. High values of CDOM are found in upwelling regions like southeast of Vietnam in summer and northwest of Luzon in winter. At a basin scale, CDOM is high in winter when upwelling is strong, solar shortwave radiation and stratification weak, and vertical mixing intense. Opposite conditions exist in spring and summer. Interannual variability of the basin-wide CDOM is characterized by abnormal troughs during the El Niño events. A strong relationship exists between the time series of the first EOF mode (for both winter and summer) and Niño 3.4 Index. Associations of these events with climatic and hydrographic properties (i.e. wind forcing, solar shortwave radiation, Ekman pumping, vertical mixing, sea surface height and temperature) are discussed. [Copyright &y& Elsevier]