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The climatological connection between tropical cyclones (TCs) and boreal summer intraseasonal oscillation (BSISO) over the western North Pacific (WNP) is known, but here we additionally find that the influences of May–October BSISO on TC genesis and intensification at the interannual timescale reveal distinctive regional features. The strengthened BSISO variability over the subtropical WNP (7.5°–27.5°N, 145°E–165°W) contributes positively to the basin-total TC genesis count, while the BSISO amplitude over the central-western equatorial Pacific (5°S–5°N, 140°E–160°W) is significantly correlated with the average intensity that TCs can reach. The physical processes causing the differences in TC behavior associated with interannual changes in BSISO are quantified from the viewpoint of eddy energetics. During summers with enhanced BSISO over the subtropical WNP, more TCs form there. Composite diagnostic analysis results for TC genesis days show that local synoptic eddies (or TC embryos) gain more kinetic energy to grow into TCs from both the seasonal-mean and BSISO anomalies via barotropic energy conversion. Eddy self-augmentation processes related to baroclinic conversion and eddy geopotential flux also aid the generation of TCs. During summers with enhanced central-western equatorial BSISO, TCs stay longer over the open ocean before they reach maximum intensity. Further diagnosis along the TC tracks shows that these eddies may grow faster (with a larger kinetic energy tendency) and reach a higher intensity with the help of enhanced barotropic and baroclinic energy conversion processes. [ABSTRACT FROM AUTHOR]

Reanalysis data are extensively employed in investigation of the tropical large-scale climate variability. In this study, we attempt to use the Advanced Microwave Sounding unit-A (AMSU-A) window channel brightness temperature (TB) observations to examine their relationship with the sea surface temperature (SST). A close relationship is found between the AMSU-A window channel TB and SST in the tropical oceans. A TB Niño Index (TNI) of AMSU-A window (sounding) channels is found to sometimes reach its maximum for about 2 months ahead of Oceanic Niño Index (ONI) for half of El Niño cases from 1998 to 2020. The AMSU-A sounding channels TNI had minimum values about 1–3 months behind ONI peak month. Spatial correlation maps of AMSU-A window channel TB anomalies with TNI are similar to those of SST anomalies with ONI, featuring a horseshoe-shaped pattern in the tropical Pacific and a dipole-shaped pattern in the tropical Indian Ocean. The AMSU-A retrieved liquid water path (LWP) is also highly correlated with the ONI, with larger values in El Niño years. An active development of tropical convective disturbances and their movement associated with westerly wind bursts may be the reason for the peak TNI to occur 1–3 months earlier than the peak ONI. The potential for AMSU-A TB observations to reveal El Niño signals adds additional satellite data source for El Niño research. [ABSTRACT FROM AUTHOR]

Westerly wind bursts (WWBs), aperiodic burst-like atmospheric disturbances in the tropics, play an important role during the development stage of El Niño–Southern Oscillation (ENSO) events and in turn can be strongly modulated by ENSO itself. Previous work argued that WWBs exhibit a pronounced seasonality at the equator during an El Niño evolution, i.e., being more frequent and vigorous during the developing boreal autumn compared to the decaying spring. Here we show that this seasonal difference in WWB activity at the equator is a segmentary manifestation of a meridional migration of WWB activity, which is closely linked to the southward displacement of the El Niño-related deep convection from the developing autumn to decaying spring. The highest climatological SSTs over the western-central Pacific are located mainly to the north of the equator in boreal autumn and south of it during boreal spring; hence the equatorially quasi-symmetric El Niño-related SST anomalies tend to induce deep convection and generate more WWBs in the Northern Hemisphere during the developing autumn and more in the Southern Hemisphere during the decaying spring. Despite some differences in the zonal distribution and intensity, there appears a close resemblance between the seasonal-varying features of WWBs during eastern Pacific (EP) and central Pacific (CP) El Niño events regarding the north–south displacement, due to similar meridional shifts of deep convection anomalies. This study provides a more comprehensive picture of the spatiotemporal characteristics of WWBs during El Niño events and improves our understanding of the relationship between ENSO and high-frequency atmospheric variability. [ABSTRACT FROM AUTHOR]

As the dominant form of mesoscale variability in the equatorial eastern Pacific, Tropical Instability Waves (TIWs) are known to interact with the El Niño and Southern Oscillation (ENSO) in complex ways. TIWs activity is modulated by the ENSO state and also provide significant feedback on ENSO via nonlinear dynamic heating (NDH), acting as a source of asymmetry between the El Niño and La Niña phases. In this work, we show that the interannual variability of TIWs-induced heat flux and NDH can be approximately expressed in terms of the mean meridional temperature gradient as TIWs tend to transport heat downgradient of the temperature anomalies along the Sea Surface Temperature (SST) front. The TIWs-induced NDH can be quantified as an asymmetric negative feedback on ENSO by a nonlinear thermal eddy diffusivity which depends on the background TIWs pattern and the ENSO-related linear and nonlinear processes. This proposed parameterization scheme can capture well the direct ENSO modulation on TIWs activity, the combination effect arising from the nonlinear interaction between ENSO and the cold tongue annual cycle, and associated ENSO nonlinearity. This parameterization scheme is effectively tested using four ocean reanalysis datasets with different horizontal resolutions that exhibit contrasted patterns of TIWs activity. This scheme may be useful for assessing the TIWs-induced feedback on ENSO in mechanistic ENSO models to better understand the dynamics of ENSO complexity. [ABSTRACT FROM AUTHOR]

The Arctic warming around the Barents and Kara Sea frequently coincides with the Eurasian cooling winters, which is referred to as the warm Arctic-cold Eurasia (WACE) pattern. We here show that the WACE pattern during late winter (January-March) is closely associated with the tropical El Niño-Southern Oscillation but with a striking asymmetric linkage. In general, La Niña events are accompanied with an anomalous high-pressure system over Eurasia, facilitating the formation of the positive WACE pattern. However, a weak connection is detected between the El Niño and WACE pattern. This asymmetric influence of El Niño-Southern Oscillation (ENSO) on Eurasian circulation and surface air temperature can be attributed to the nonlinearity in its modulation on planetary waves and the spatial structure of stratospheric polar vortex during the warm and cold phases. During La Niña winters, the height anomalies over Eurasia tend to prevail in the troposphere due to an enhancement in the wave-1 planetary wave in the middle and low layers, in contrast to El Niño years with height anomalies more confined to the upper stratosphere. The identified linkage between La Niña and the WACE pattern provides a potential predictability source of cold winters over Eurasia on seasonal-to-interannual timescales. [ABSTRACT FROM AUTHOR]

The three strongest positive Indian Ocean dipole (pIOD) events over the past four decades occurred in 1994, 1997, and 2019 and caused severe social-economic impacts in the Indian Ocean (IO) rim countries. These three strongest positive pIOD events exhibited similar oceanic and atmospheric anomaly patterns during their mature phase, though with different physical origins. The extreme amplitudes of these pIOD events were closely related to concurrent El Niño or Madden–Julian oscillation (MJO) activity. For the 1997 extreme pIOD case, the concurrent extreme El Niño in the Pacific triggered strong easterly wind anomalies over the equatorial IO and therefore favored the rapid growth of the pIOD via intense upwelling along the Sumatra-Java coast. In contrast, the long stalled MJO convective anomalies over the western IO exerted an important role in the 2019 extreme pIOD event especially during September–October by causing easterly anomalies over the equatorial IO. During that year, the Pacific moderate warming had likely a small impact since no strong associated convection was present over the western and central equatorial Pacific due to the small amplitude of the Pacific sea surface temperature anomalies. Different from the 1997 and 2019 cases, both El Niño conditions in the Pacific as well as MJO activity over the western IO contributed to the 1994 extreme pIOD event. Our results emphasize that MJO activity should be considered as one physical drivers for the extreme pIOD event in addition to the traditional El Niño-associated forcing. [ABSTRACT FROM AUTHOR]

The present work investigates leading modes of winter fog days over eastern China and possible association with sea surface temperature (SST) variability. Based on the Empirical Orthogonal Function (EOF) analysis, two dominant interannual modes of winter fog days are identified: a monopole mode and a meridional dipole mode. The monopole mode displays a significantly positive relationship with El Niño-Southern Oscillation (ENSO). The meridional dipole mode is closely linked to North Atlantic tripole (NAT) SST anomalies. A quasi-barotropic Rossby wave train can be excited by the NAT SST anomalies and propagates eastwards to East Asia. During winters with the positive phase of the NAT SST anomalies, there appear cyclonic and anticyclonic circulation anomalies over northern and southern China, favoring less and more winter fog frequency, respectively. These observed impacts from the ENSO and NAT SST anomalies can be further reproduced by atmospheric general circulation model (AGCM) experiments. Consideration of the NAT SST anomalies can apparently improve the explained variance of winter fog days over eastern China on the basis of the common-used ENSO signal. When ENSO and NAT occur as in-phase combinations and out-of-phase combinations, winter fog days anomalies appear over southern and northern China, respectively. These results have important implications on the seasonal-to-interannual predictability of the winter fog frequency over eastern China. [ABSTRACT FROM AUTHOR]

How much and fast the Earth is warming in response to increasing greenhouse gas concentrations is one of the fundamental questions in climate science. Here we investigate the role that different modes of climate variability play in modulating the temperature response. We show evidence for a robust statistical relationship between global warming rate variations and Atlantic Multidecadal Variability (AMV) across multiple observational datasets since 1850. The correlation between AMV and the global warming rate is maximized—with a correlation coefficient of about − 0.8—at ~ 10 to 20 years lead-time. In contrast, such a relation between global warming rate and the Interdecadal Pacific Oscillation (IPO) is far less coherent, showing negative correlation before the 1920s and positive correlation after that. Similar statistical relationships between global warming rate variations and the AMV/IPO can also be seen in the majority of the models from the Phase 5 of Coupled Models Inter-comparison Project. Further, a targeted model experiment is conducted to demonstrate the dominant control of the AMV on the unforced fraction of the global warming rate (compared to the IPO). [ABSTRACT FROM AUTHOR]

This study investigates the joint impact of the boreal late winter–early spring North Pacific Victoria mode (VM) and South Pacific Quadrapole (SPQ) mode on the following summer Pacific Intertropical Convergence Zone (ITCZ) precipitation. Here, both observational data and historical simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) models show that the precipitation anomalies are enhanced when the VM and the SPQ are in phase but are weakened when they are out of phase. The strength of the anomalous vertical velocity (ω) and outgoing longwave radiation also tend to be stronger (weaker) in same-sign (opposite-sign) events. One possible reason for this is that when the VM and the SPQ are in (out of) phase, westerly anomalies over the western–central tropical Pacific are greatly increased (reduced) due to the interactions between preceding North and South Pacific sea surface temperature anomalies, thereby enhancing (reducing) convective upward motion over the western and central Pacific. In addition, an empirical linear regression model is also developed to forecast Pacific ITCZ summer precipitation based on both the late winter–early spring VM and SPQ indices. The combination of the two precursors in North and South Pacific show higher prediction skills than the single VM or SPQ model, which means it is necessary to consider their joint impact when we focus on the Pacific ITCZ precipitation. [ABSTRACT FROM AUTHOR]

The winter fog frequency over eastern China displays remarkable interannual variability, which has a linear relationship with El Niño-Southern Oscillation (ENSO). Eastern China usually experiences more (less) frequent fog during El Niño (La Niña) winters. During El Niño winters, an anomalous anticyclone tends to appear over the western North Pacific (WNP), which can weaken the climatological winter northerly winds and enhance water vapor supply from oceans, conducive to the formation of foggy weather. Roughly opposite anomalies of fog frequency are displayed during La Niña winters. However, this linear relationship is mainly contributed by the La Niña and partial El Niño events, since the El Niño events exhibit diversity in impacts on the winter fog frequency due to their different types. Increased winter fog frequency can be significantly detected during eastern-Pacific (EP) El Niño, while this signal is not observed during central-Pacific (CP) El Niño. It is found that the winter fog frequency during the CP El Niño seems to be dependent on its zonal locations, associated with different WNP atmospheric circulation and local difference between air temperature and dew point temperature (T − Td) anomalies. The further eastward CP El Niño largely coincides with more frequent fog weather similar to the EP El Niño, while the further westward CP El Niño is usually accompanied with less frequent fog weather. This relationship has important implications for seasonal prediction of winter fog frequency and places a high requirement on consideration of zonal location of the CP El Niño. [ABSTRACT FROM AUTHOR]

The tropical cyclone (TC) genesis number in the western North Pacific (WNP) exhibits a pronounced decadal decrease around the mid-1990s, with prominent seasonal and spatial inhomogeneity. This decadal shift of TC activity is mostly confined to the southeastern part of the WNP and occurs mainly during the second half of the calendar year. Accordingly, westward and northeastward TC recurving movements strongly decreased in recent decades after 1995 compared with TC tracks in the earlier period (1979–1994). We find that this TC activity decadal change is associated with tropical Pacific decadal variability, which is measured here by a low-pass filtered Niño3.4 index. In contrast to the earlier period, the anomalous cold mean state in the tropical Pacific during recent decades favored the enhancement of zonal vertical wind shear (UVWS) and suppressed TC activity. This tropical Pacific mean state change is possibly related to decadal changes of El Niño–Southern Oscillation (ENSO) properties (i.e., more La Niña events occurred during recent decades). This relationship between tropical Pacific mean state change and the UVWS in the southeastern WNP on decadal timescales is further validated based on longer observations (1951–2017) and control simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The statistical relationships between TC activity and the Pacific Decadal Oscillation (PDO) or Atlantic Multidecadal Oscillation (AMO) are weaker and insignificant, both for the observations and for simulations. Our results imply that decadal variations of the tropical Pacific mean state should be taken into account when predicting WNP TC activities on decadal timescales. [ABSTRACT FROM AUTHOR]

While significant improvements have been made in understanding how the El Niño-Southern Oscillation (ENSO) impacts both North American and Asian climate, its relationship with the North Atlantic Oscillation (NAO) remains less clear. Observations indicate that ENSO exhibits a highly complex relationship with the NAO-associated atmospheric circulation. One critical contribution to this ambiguous ENSO/NAO relationship originates from ENSO's diversity in its spatial structure. In general, both eastern (EP) and central Pacific (CP) El Niño events tend to be accompanied by a negative NAO-like atmospheric response. However, for two different types of La Niña the NAO response is almost opposite. Thus, the NAO responses for the CP ENSO are mostly linear, while nonlinear NAO responses dominate for the EP ENSO. These contrasting extra-tropical atmospheric responses are mainly attributed to nonlinear air-sea interactions in the tropical eastern Pacific. The local atmospheric response to the CP ENSO sea surface temperature (SST) anomalies is highly linear since the air-sea action center is located within the Pacific warm pool, characterized by relatively high climatological SSTs. In contrast, the EP ENSO SST anomalies are located in an area of relatively low climatological SSTs in the eastern equatorial Pacific. Here only sufficiently high positive SST anomalies during EP El Niño events are able to overcome the SST threshold for deep convection, while hardly any anomalous convection is associated with EP La Niña SSTs that are below this threshold. This ENSO/NAO relationship has important implications for NAO seasonal prediction and places a higher requirement on models in reproducing the full diversity of ENSO. [ABSTRACT FROM AUTHOR]

This work investigates the decadal modulation of the El Niño-Southern Oscillation (ENSO)-East Asian winter monsoon (EAWM) relationship by the Atlantic Multidecadal Oscillation (AMO). A stable ENSO-EAWM relationship is found during the positive AMO phase but not during the negative phase. While the impact of El Niño events on the EAWM does not depend on the AMO phase, a different picture is observed for La Niña events. The La Niña boreal winter season coincides with a strengthened EAWM during a positive AMO phase and a weakened EAWM during a negative AMO phase. We suggest that the AMO's modulating effect mainly comprises two pathways that influence ENSO's impact on the EAWM. On one hand, when La Niña coincides with a positive AMO, the warm SST anomalies over the western North Pacific (WNP) are amplified both in intensity and spatial extent, which favors strengthened WNP cyclonic anomalies and an enhanced EAWM. During La Niña with a negative AMO, only very weak SST anomalies occur over the WNP with reduced WNP cyclonic anomalies that are confined to the tropics, thus having little effect on the EAWM. On the other hand, an eastward-propagating Rossby wavetrain across the mid-high latitudes of Eurasia during a warm AMO phase strengthens the Siberian high and thus leads to a strengthened EAWM, while during a cold AMO phase the Siberian high is weakened, leading to a reduced EAWM. In contrast, El Niño and its associated atmospheric responses are relatively strong and stable, independent of the AMO phase. These results carry important implications to the seasonal-to-interannual predictability associated with ENSO. [ABSTRACT FROM AUTHOR]

The wintertime Arctic Oscillation (AO) manifests a high degree of uncertainty in its spatial structure, which was suggested to be associated with some other extratropical atmospheric variabilities (e.g., the Aleutian-Icelandic Low seesaw and the stratospheric polar vortex). Based on reanalysis and model simulation data in 1950–2019, the present work investigates the possible effects of El Niño-Southern Oscillation (ENSO) on the AO spatial structure. It is found that there exist obvious differences in AO structure between early (November–December) and late (January–February) winter. In early winter, the AO is manifested by its traditional pattern with three action centers, one of which is over the Arctic and the other two are with the opposite sign over the North Atlantic and North Pacific. However, the AO pattern in late winter changes remarkably, resembling a regional North Atlantic Oscillation (NAO) pattern with the absence of its significant Pacific center. The noteworthy sub-seasonal variation in the AO spatial pattern can be largely ascribed to the observed fact that ENSO atmospheric teleconnections to the Northern Hemisphere are significantly enhanced in late winter compared with early winter. The enhanced ENSO-associated teleconnection during late winter can significantly weaken the teleconnectivity of the Arctic center with the Pacific center, resulting in the absence of the AO’s significant Pacific center. The understanding of ENSO impacts on the AO structure has some significance for the AO-associated climate prediction around the North Pacific region. [ABSTRACT FROM AUTHOR]

The present work identifies two types of La Niña based on the spatial distribution of sea surface temperature (SST) anomaly. In contrast to the eastern Pacific (EP) La Niña event, a new type of La Niña (central Pacific, or CP La Niña) is featured by the SST cooling center over the CP. These two types of La Niña exhibit a fundamental difference in SST anomaly evolution: the EP La Niña shows a westward propagation feature while the CP La Niña exhibits a standing feature over the CP. The two types of La Niña can give rise to a significantly different teleconnection around the globe. As a response to the EP La Niña, the North Atlantic (NA)-Western European (WE) region experiences the atmospheric anomaly resembling a negative North Atlantic Oscillation (NAO) pattern accompanied by a weakening Atlantic jet. It leads to a cooler and drier than normal winter over Western Europe. However, the CP La Niña has a roughly opposing impact on the NA-WE climate. A positive NAO-like climate anomaly is observed with a strengthening Atlantic jet, and there appears a warmer and wetter than normal winter over Western Europe. Modeling experiments indicate that the above contrasting atmospheric anomalies are mainly attributed to the different SST cooling patterns for the two types of La Niña. Mixing up their signals would lead to difficulty in seasonal prediction of regional climate. Since the La Niña-related SST anomaly is clearly observed during the developing autumn, the associated winter climate anomalies over Western Europe could be predicted a season in advance. [ABSTRACT FROM AUTHOR]