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This paper investigates the effects of advancing deforestation in the southwestern Amazon on local hydroclimate. For this purpose, a high-resolution mesoscale model, coupled with a land surface model and a hydrologic routing model, is integrated for five years in two experiments, one control without changes in land use and another changing land use annually. During the rainy season, simulated higher precipitation volumes are reduced with the expansion of pasture lands. In the dry periods, mainly in August, there is an increase in precipitation over some deforested areas, which can be associated with mesoscale circulations induced by the heterogeneous surface. In general, land-use changes reduce evapotranspiration and the net radiation at the surface and increase the sensible heat flux during the dry season. In the rainy period, there is an increase in evapotranspiration with the intensification of deforestation, which modifies the other components of energy balance. Results suggest that the impact of LUCC on the local climate is more affected by conditions that generate precipitation extremes (El Niño/La Niña events) than by the abrupt increase in the annual deforestation rate. Land-use changes reduce the streamflow in all sub-basins of Ji-Parana during the dry season. In the rainy season, LUCC increases the streamflow during periods with normal precipitation conditions and La Niña in almost all sub-basins, except in the sub-basin with more than 20% deforestation. [ABSTRACT FROM AUTHOR]

Nepal experienced record-breaking summer monsoon precipitation in 2020, becoming the wettest year of the last four decades (1980–2020). This paper explores the role of large-scale atmospheric circulation and sea surface temperature patterns in tipping an otherwise typical wet summer into an extreme one in 2020. The unusually high precipitation was fuelled by the Western North Pacific Anticyclone (WNPAC), concomitant with the strong presence of moist static energy and enhanced vertical velocity in and near Nepal. Sea surface temperature anomalies showed an intense Indian Ocean warming and features of a La Niña, succeeding a weak El Niño event that had sustained the WNPAC. The southwesterly winds on the northern flank of then WNPAC contributed excess moisture transport and convergence to Nepal, consequently resulting in elevated precipitation. This feature is consistent with previous wet summer years, but the presence of WNPAC in those years followed the El Niño event. Therefore, it appears that the Indo-western Pacific Ocean Capacitor (IPOC) effect exerts the above-normal precipitation for the summer of 2020 without involving the El Niño event, while anthropogenic warming may have further tipped the balance. [ABSTRACT FROM AUTHOR]

Previous studies have revealed that extratropical forcing can lead to changes in the intertropical convergence zone (ITCZ). Based on reanalysis data from 1979 to 2019, this paper discusses the influence of the South Pacific Ocean Dipole (SPOD, a dominant mode between the subtropics and extratropics in the South Pacific) and its coordination with ENSO in the ITCZ. Results show that SPOD has an effect on the position and intensity of the ITCZ, especially in the region around Niño4. From the latitudes of ITCZ axes and precipitation averaged over 160°E–160°W, positive (negative) SPOD events force a northward (southward) shift in the ITCZ in boreal winter (DJF), while the position moves in the opposite direction in boreal summer (JJA). In addition, ENSO can obviously enhance the influence of SPOD on the ITCZ. It is evident that there is a wider range of northward (southward) shift in the ITCZ under both positive SPOD and El Niño (negative SPOD and La Niña) conditions during DJF. Similar to DJF, both positive SPOD and El Niño (negative SPOD and La Niña) events still cause the ITCZ to move toward (away from) the equator during JJA, except that the magnitude of the movement in JJA is less than that in DJF. Precipitation averaged over 160°E–160°W is at a maximum under both positive SPOD and El Niño events, which have the greatest influence on ITCZ intensity, followed by both negative SPOD and La Niña in DJF, while in JJA, ITCZ intensity is most sensitive to negative SPOD events. [ABSTRACT FROM AUTHOR]

This paper investigates the causes for the interdecadal change in the relationships between early and late summer precipitation over South China (SC). It is found that the correlations of the precipitation over SC between June and August shift from weak positive in 1979–1995 to significantly negative in 1996–2019. Further analysis demonstrates that the distinct evolution of sea surface temperature (SST) pattern between the two periods accounts for the interdecadal variations of their relationships. Although the warming of the tropical western Indian Ocean (WIO) in June favors increasing precipitation over the SC via enhancing the Northwest Pacific Subtropical High (NWPSH) during the whole period, the associated SST anomalies in August are rather different between the two periods. Specifically, the WIO warming in June corresponds to slightly positive anomalies over the tropical central-eastern Pacific in August during the 1979–1995, which has weak impact on the NWPSH and results in a weak precipitation correlation between June and August. However, the WIO warming in June corresponds to La Niña's rapid development in August during the 1996–2019, which favors the enhancement of the NWPSH via increasing the regional Hadley circulation. Due to the climatologically northward movement of NWPSH from June to August, the enhanced NWPSH in August acts to decrease the precipitation over SC, causing a significantly negative correlation between precipitation in June and August. Overall, the distinct evolution of tropical SST pattern is the key factor inducing the change of the relationships between June and August precipitation in the two periods. [ABSTRACT FROM AUTHOR]

This paper studies the impacts of El Niño-Southern Oscillation (ENSO) on wintertime extreme precipitation in China from 1961 to 2017, and possibly different influences of two El Niño types are also examined. We find that ENSO poses profound influences on extreme precipitation in many portions of China. El Niño (La Niña) intensifies (weakens) the winter precipitation extremes in southeastern China (SEC) and slightly weakens (intensifies) those in central-north China, mainly by changing the frequency rather than the intensity of extreme precipitation. Further analysis suggests that the East Asian winter monsoon tends to be weaker during El Niño winters, suppressing the southward invasion of cold dry air. Via the weakened Walker circulation, El Niño also triggers anomalously descending motion and anticyclone over the western North Pacific (WNP), which exhibits southwesterly anomalies over SEC and is thus conducive to the transport of sufficient moisture into that area. The changes over WNP are also accompanied by ascending motions over East Asia via a local meridional circulation alike the Pacific–Japan pattern. Meanwhile, El Niño induces a southward-displaced East Asian jet stream, as characterized by intensified westerly over southern China, corresponding to anomalously ascending motion there. Together, these changes facilitate the updrafts of circulations and the condensation of water vapor, thus increasing the occurrence of extreme precipitation events in SEC. Moreover, we find that different Niño types exhibit distinct subregional influences. In particular, the canonical eastern Pacific El Niño significantly increases the frequency of extreme precipitation in South China, whereas the Modoki central Pacific El Niño mainly impacts that in East China. [ABSTRACT FROM AUTHOR]

Ocean chlorophyll (Chl)-induced heating can affect the climate system through the penetration of solar radiation in the upper ocean. Currently, the ocean biology-induced heating (OBH) feedback effects on the climate in the tropical Pacific are still not well understood, and the mechanisms regarding how SST is modulated remain elusive. In this paper, chlorophyll (Chl) data from satellites are combined with physical fields from Argo profiles to estimate OBH-related fields, including the penetration depth (Hp) and the ocean mixed-layer (ML) depth (Hm). In addition, some directly related heating terms with Hm and Hp are diagnosed, including the absorbed solar radiation component within the ML (denoted as Qabs), the rate of ML temperature changes that are directly induced by Qabs (denoted as Rsr = Qabs/(ρ0CpHm)), and the portion of solar radiation that penetrates through the bottom of the ML (denoted as Qpen). The structural relationships between these related fields are examined to illustrate how these heating terms are affected by Hp and Hm. The extent to which Rsr and Qpen are modulated by Hp is strikingly different during ENSO cycles. In the western-central equatorial Pacific, inter-annual variations in Hp tend to be out of phase with those in Hm. A decrease (increase) in Qabs from a positive (negative) Hp anomaly during El Niño (La Niña) tends to be offset by a negative (positive) Hm anomaly. Thus, Rsr is not closely related with Hp, even though Qabs is highly correlated with Hp, indicating that the direct thermal effect through Qabs is not a dominant factor that affects the SST. In contrast, the inter-annual variability of Qpen in the region is significantly enhanced by that of Hp, with their high positive correlation. The Hp-induced differential heating in the ML and subsurface layers from the Qpen and Qabs terms modifies the thermal contrast, stratification and vertical mixing, which represent a dominant indirect ocean dynamical effect on the SST. The revealed relationships between these related fields provide an observational basis for gaining structural insights into the OBH feedback effects and validating model simulations in the tropical Pacific. [ABSTRACT FROM AUTHOR]

In this paper, mechanisms for the formation of quasi-stationary extratropical wave trains associated with four distinct winter patterns of seasonal mean tropical Pacific convection anomalies, including the western CP (W-CP) pattern, La Niña (LN) pattern, CP El Niño (CPEN) pattern and EP El Niño (EPEN) pattern, are investigated by utilizing observational and six Atmospheric Model Intercomparison Project Phase 5 (AMIP5) high-skilled models (ACCESS1.0, CanAM4, CCSM4, CMCC-CM, HadGEM2-A, MPI-ESM-LR) datasets. The observational results show that tropical Pacific forcing induces upper-level anomalous Rossby wave source that force vorticity over the central North Pacific through modulating the local Hadley circulation, then excite eastward propagation of quasi-stationary planetary waves penetrating into the North Atlantic, and finally establish the associated teleconnections in mid- and high-latitudes by local synoptic eddy–mean flow interaction. Different structures of wave trains may be explained by evident differences in intensity and scale of the extratropical vorticity forcing originating from distinct convection anomalies over the tropical Pacific. The MME results further confirm the possible physical processes obtained from observations, but there still exist some significant differences. These cover the North Pacific and North America and can possibly be attributed to the models' simulation ability of extratropical vorticity forcing and local storm track intensity. In addition, the stratosphere–troposphere interaction plays an important role for extratropical atmospheric circulation anomalies. The stratospheric polar vortex not only modulates the underlying wave train, especially for the North Atlantic/Europe sector, but is also affected by the upper-tropospheric disturbance in high-latitudes through the upward wave activity flux. [ABSTRACT FROM AUTHOR]

This paper investigates the impacts of the El Niño Southern Oscillation (ENSO) on various aspects of heat waves (HWs), including frequency, duration, and magnitude, over China during 1961-2014. Results show that El Niño (La Niña) significantly amplifies (weakens) the HW activities in most areas of China. Such influences are particularly strong in southern China. HW activities are enhanced (weakened) during the summers following mature El Niño (La Niña) episodes, with increased (reduced) occurrence of HW events and number of HW days, as well as prolonged (shortened) duration and elevated (decreased) amplitude of such events. These amplifying effects are substantially stronger for severe HWs (i.e., hottest and longest episodes) than for those events of average duration and intensity. Diagnosis by compositing reanalysis data over ENSO events indicates that the atmospheric circulation during the summers following El Niño is characterized by anomalous rising motion over the tropical central Pacific and Indian Oceans, and sinking motion over the western North Pacific (WNP). The anomalous subsidence over the WNP is accompanied by reduced precipitation and condensational heating, as well as anomalous anticyclonic flows in the lower troposphere. El Niño also induces a stronger South Asian high, which can strengthen the WNP anticyclone and subtropical high. These changes provide a favorable environment for the occurrence and sustenance of HWs in China. Analogous arguments apply to changes with the opposite polarity in association with La Niña events. [ABSTRACT FROM AUTHOR]

In this paper, distinct patterns of boreal winter convection anomalies over the tropical Pacific and associated wave trains in the extratropics are addressed. The first leading mode (EOF1) of convection anomalies as measured by outgoing longwave radiation demonstrates an east-west oscillation of deep convection with centers over the equatorial central Pacific (CP) and over the tropical western North Pacific and the Maritime Continent. The second leading mode (EOF2) is also a dipole pattern with opposite centers straddling 170°W, possibly modifying EOF1 to some extent. Combining the first two leading modes, five major categories of tropical convection anomalies can be identified for the period 1979/80-2012/13. The comparison between these five categories and the corresponding SST anomaly patterns indicates a nonlinear relationship between convection and SST. The combination of EOF1 and EOF2 with in-phase PCs exhibits an east-west dipole pattern with opposite signs over west of the dateline and the Maritime Continent. The negative phase of the two PCs, named La Niña pattern, induces a negative Pacific/North American—positive North Atlantic Oscillation teleconnection in the extratropics. Approximately opposite responses can be detected in its positive phase, named CP El Niño pattern. The negative PC2 superposing positive PC1, named EP El Niño pattern, shows the strongest convection anomalies with enhanced (depressed) convection over the eastern (western) Pacific and leads to a Tropical/Northern Hemisphere-like teleconnection pattern and an anomalous anticyclone extending from the North Pacific to the North Atlantic. The positive PC2 with neutral PC1, named western CP pattern, shows weakly enhanced convection to the west of the dateline as a response to local SST warming around the dateline. This convection anomaly pattern, although weak, is important and excites a northeastward wave train from the tropics to Greenland, resulting in surface air temperature cooling covering the northeastern North America and warmer and wetter conditions over Western Europe. [ABSTRACT FROM AUTHOR]

The influence of strong Indian Ocean Dipole (IOD) events on the evolution of the following year's La Niña events is investigated using the National Marine Environmental Forecasting Center (NMEFC) operational seasonal forecasting system. The observation results show that when the strong IOD occurs, the tropical Pacific can be in different sea surface temperature states. As such prediction system can well reproduce the air-sea evolution of the 1998/1999 and 2020/2021 La Niña events, the ocean temperature initializations in December during above events were perturbed with the system to assess the role of the oceanic channel and atmospheric bridge across the maritime continent in the forecasting of the La Niña events 1 year later. In the case of the neutral state of the tropical Pacific at the peak of the 2019 positive IOD, pacemaker experiments have demonstrated that the Indian Ocean subsurface temperature initialization in December 2019 is critically important for the successful prediction of the 2020/2021 La Niña. Experiments of sea surface temperature initialization in December 2019, with only the Indo-Pacific atmospheric bridge at work, failed to predict the 2020/2021 La Niña 1 year in advance. The comparison underlined the dominant role of the oceanic channel dynamics in the evolution of the 2020/2021 La Niña. Forecasting experiments beginning from the 1997/98 El Niño with and without the IOD initializations suggest that the delayed feedback of the tropical Pacific coupled system is more important in the forecasting of the 1998/1999 La Niña, with the Indo-Pacific oceanic channel dynamics playing a secondary yet very important role. Our study has underscored the importance of the Indo-Pacific oceanic channel dynamics in ENSO prediction and suggested the effectiveness of IOD subsurface temperature initialization in La Niña predictions at the 1-year lead time. [ABSTRACT FROM AUTHOR]

The El Niño–Southern Oscillation (ENSO) has a quasi-biennial band with a period of two to three years. This quasi-biennial (QB) component of ENSO is one of the main factors influencing the spring persistence barrier of ENSO, but its root cause remains unclear. We presented a quasi-biennial cyclical process between the tropics and extratropics, which suggests that the North Pacific Victoria mode (VM) has a key role in the quasi-biennial variability of ENSO. The negative North Pacific Oscillation (NPO) during winter induces a positive VM-associated sea surface temperature pattern in the subsequent spring, which, in turn, forces an El Niño pattern in the subsequent winter through surface air–sea coupling in the subtropical/tropical Pacific. Meanwhile, associated with warming in the central-eastern equatorial Pacific, a positive SLP anomaly occurs near the Hawaiian region, and a negative SLP anomaly occurs in high latitudes near the Aleutian Islands, which eventually leads to the emergence of a positive NPO over the North Pacific during the El Niño mature winter. The positive NPO can then produce a negative VM in the spring of the next year, which further forces cooling in the equatorial Pacific, initiating a La Niña event in the following winter. Our findings suggest that the impact of the VM should be considered when we focus on the quasi-biennial-type ENSO. [ABSTRACT FROM AUTHOR]

In this study, based on the ECMWF 20-year (1997 ~ 2016) hindcasts, the subseasonal prediction of the weekly summer rainfall anomaly over the middle and lower reaches of the Yangtze River (YR) were studied. The skill at 2-week lead time exhibits prominent interannual variation, which is significantly correlated with the preceding winter ENSO. That is, rainfall anomaly can be better predicted in El Niño decaying summer than in La Niña decaying summer. Observation analyses show that in El Niño decaying summer the intraseasonal variation of YR summer rainfall is featured by strong low-frequency (> 30 days) intraseasonal oscillation (ISO) partly associated with the first boreal summer ISO mode (BSISO1) activity, in comparison with La Niña decaying summer. This is possibly because El Niño-induced mean state western North Pacific (WNP) anti-cyclone blocks the northward propagation of convection over the WNP, resulting in BSISO1 stagnation in phases 3–4. The phase stagnation could force stable atmospheric teleconnection, which is favorable to sufficient moisture transportation to the YR and persistent rainfall formation. Finally, prediction verification showed that more accurate prediction for the middle-low-level circulation contribute to the better prediction of rainfall anomaly in El Niño decaying summer than in La Niña decaying summer. [ABSTRACT FROM AUTHOR]

The role of the El Niño-Southern Oscillation (ENSO) in modulating the mean sea surface temperature (SST) in the tropical eastern Pacific is investigated. A strategy is developed to separate the observational record from 1958 to 2010 into two groups, ENSO and non-ENSO periods. A simple analytical framework is constructed to quantitatively delineate the contributions of oceanic dynamic heating (ODH) and surface heat fluxes to rectifying the mean SST. It is found that the differences in the mean SST between the two periods are evident, though minor, despite distinctive interannual SST variabilities. Both linear and nonlinear ODH, as well as surface heat fluxes, contribute to this slight mean SST difference. Idealized oceanic model experiments in the presence and absence of ENSO are conducted. The experiments confirm that ENSO moderately impacts the mean SST in the equatorial eastern Pacific. Although the amplitude of the linear ODH associated with ENSO is large, its impact on the long-term mean SST is small because the sign of its contribution changes between El Niño and La Niña phases. The nonlinear ODH, on the other hand, has the same sign during the warm and cold episodes. However, its accumulated effect on the mean SST is small due to its weak amplitude. [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 effects of El Niño-Southern Oscillation (ENSO) on winter precipitation in South China are considerably asymmetric. A positive correlation is significant between the sea surface temperature anomalies (SSTA) in the equatorial middle-eastern Pacific and the precipitation in South China during El Niño winters, whereas no significant relationship instead of a negative correlation between them during La Niña winters. The possible causes of abnormally positive precipitation in South China during La Niña winters remain a puzzle. The analysis indicates that positive precipitation anomalies in South China during La Niña winters are closely related to the strengthening of southerly winds over the South China Sea and convective activities over South China. Moreover, during La Niña winters, the low-level southerly wind anomalies over the South China Sea are modulated by an anomalous positive local zonal sea surface temperature gradient (SSTG) between the South China Sea and the tropical western Pacific, which strengthens in late autumn and maintains through the following winter. This positive local zonal SSTG is independent of La Niña and leads to the weakening of Walker circulation. The strengthened downward motion over the tropical western Pacific further promotes the weakening of local Hadley circulation. Weakened local Hadley circulation, with strengthened southerlies over the South China Sea in the lower troposphere, is conducive to the enhancement of water vapor transport from the South China Sea to South China. Besides, the weakened local Hadley circulation also shows strengthened upward motion over South China, which provides more advantageous dynamic conditions for precipitation. Therefore, the positive local zonal SSTG simultaneously promotes the two most important conditions affecting precipitation, resulting in positive precipitation anomalies over South China during La Niña winters. These results advance the understanding of prediction of winter precipitation in south China. [ABSTRACT FROM AUTHOR]

In 2022, South China suffered severe flooding due to unexpected persistent heavy rainfall in the pre-flood season. The in situ cumulative precipitation amount during May–June in South China broke the record established in 1981. The present study ascribed this record-breaking event to extreme warm sea surface temperature anomalies (SSTAs) in the Northeast Pacific (NP) and the Maritime Continent (MC) region. In May, the NP warming associated with La Niña and the negative phase of the Pacific Decadal Oscillation intensified extratropical diabatic heating and induced a cyclonic anomaly over Northeast Asia, which shifted the East Asian trough (EAT) southward. Subsequently, the westerly wind accelerated to the south of the anomalous EAT and strengthened the western North Pacific subtropical high, which led to increase in rainfall over South China. In June, the rainfall over South China remained above normal owing to anomalies of the meridional monsoon circulation manifested as a response to the development of a warm SSTA in the MC region. The anomalous warming in the deep tropics suppressed the Asian summer monsoon convection through anomalous atmospheric descent over South Asia, accompanied by enhanced atmospheric ascent and increased water vapor convergence over South China. Numerical sensitivity experiments validated the above processes and indicated that historic warm SSTAs in the NP and MC regions could increase the probability of extreme persistent above-normal pre-flood rainfall over South China. The results support improved predictability of pre-flood rainfall over South China on the interannual timescale outside the equatorial central–eastern Pacific. [ABSTRACT FROM AUTHOR]

Most tropical cyclones (TCs) generated over the eastern North Pacific (ENP) do not make landfall. Consequently, TCs in this basin have received less attention, especially those that occur away from the mainland. Furthermore, there have been few studies of the climatic effects of ENP TCs. This study explores the feedback relationship between ENP TCs and the intensity of the El Niño–Southern Oscillation (ENSO), including El Niño and La Niña events, from the perspective of accumulated cyclone energy (ACE). Observational and modeling results indicate that the ENP ACE 3 months earlier can still affect the intensity of El Niño and La Niña events, although the SST persistence is main contributor. Thereinto, the impact of ENP TCs on El Niño appears to be approximately equal to that on La Niña. Moreover, this impact is independent of the persistence of the sea surface temperature (SST) in the Niño 3.4 region and the Madden–Julian Oscillation. Generally, the greater the ENP ACE, the stronger the El Niño, and the smaller the ENP ACE, the stronger the La Niña; this is especially the case for those TCs that develop over the July‒September period. In addition, results show that the ENP TCs modulate ENSO intensity by changing anomalous zonal wind at the low-level atmospheric layer. And the joint impacts of the low-level zonal wind anomalies on the Walker circulation and the east–west thermocline gradient lead to the time characteristics that ENP TCs lead ENSO intensity by about 3 months. [ABSTRACT FROM AUTHOR]

Hot extremes impose severe effects on human health and the ecosystem, especially when high-temperature extremes sequentially occur in both daytime and nighttime within 1 day, known as Compound Hot Extremes (CHEs). Although a number of studies have focused on independent hot extremes, not enough work is devoted to compound ones, not to mention the coupling strength in covariations between the two variables (daytime and nighttime extreme temperature: Tmax and Tmin) over a given region. The instantaneous coupling strength can be derived by Dynamical System (DS) approach from covariations between Tmax and Tmin over a given region, and used to classify CHEs into coupled and decoupled types. Results show that more frequent coupled CHEs tend to be more intense with prolonged duration and extensive spatial extent compared with decoupled CHEs. Also, the mechanisms behind these two types of CHEs are largely different. Coupled CHEs are accompanied by a significant intensification and westward extension of the western North Pacific subtropical high (WNPSH), and the extremely high-temperature is mainly caused by receiving more solar radiation under the corresponding anticyclone. It is found that barotropic structure, weak jet stream and developing La Niña are conducive to the enhancement and persistence of WNPSH, in favor of the occurrence of long-lasting CHEs. Decoupled CHEs are associated with strong sea-land breeze (SLB), whose diurnal cycle could weaken the persistent large-scale circulation and suppress covariations between Tmax and Tmin. This kind of decoupled hot extremes are attributed to the combined effect of receiving more solar radiation during the day and trapping more long-wave radiation at night, where moisture and cloud cover play an important role. [ABSTRACT FROM AUTHOR]

The characteristics of tropical cyclones (TCs) in sub-seasonal forecasting with the Global Seasonal Forecast System 5 (GloSea5) of the Korea Meteorological Administration (KMA) were assessed for June–September (JJAS) from 1991 to 2010 over the western North Pacific (WNP). The performance of GloSea5 was examined for its ability to reproduce observed TC climatology as well as changes in TC genesis with the El Niño-Southern Oscillation (ENSO) and a 1998/1999 climate regime shift (e.g., frequency, genesis spatial distribution). GloSea5 showed skillful performance in predicting the frequency and genesis spatial distribution of TCs in climatology and both ENSO phases; this performance was best during periods of La Niña. Environmental fields related to TC genesis (e.g., sea surface temperature [SST], vertical wind shear [VWS], 850-hPa wind and relative vorticity) were also reasonably captured, despite some systematic biases in SST, low-level circulation, relative vorticity, and VWS. GloSea5 performed well in terms of characteristic of changes in TC genesis before and after the regime shift. However, there were biases in TC frequency before the regime shift and changes in TC-related environmental fields. Our results imply that GloSea5 with a high predictive skill for TC genesis over the WNP can be used as an operational model for sub-seasonal TC forecasting, although it requires continuous improvements to reduce systematic errors. [ABSTRACT FROM AUTHOR]

This study examines the modulation by ENSO of the diurnal cycle of precipitation in boreal winter for the period 1998–2016 over the maritime continent (MC), using the Tropical Rainfall Measuring Mission data and the weather research and forecasting (WRF) model. The diurnal cycles are composited for the ENSO warm (El Niño) and cold (La Niña) phases. It is found that there are statistically significant differences in the diurnal cycle amplitude between El Niño and La Niña, with a non-uniform spatial pattern over the MC. The amplitude of precipitation diurnal cycle is larger over the western MC but smaller over the eastern MC during El Niño, and vice versa for La Niña. A diagnosis of the moisture budget shows that the difference in the diurnal cycle amplitude is primarily attributed to the vertically integrated convergence of the interannual background moisture by diurnal winds. A further analysis indicates that the ENSO-related background moisture plays a key role in causing the difference in diurnal wind activity between El Niño and La Niña, which leads to the difference in the precipitation diurnal cycle. [ABSTRACT FROM AUTHOR]

The validity of the long-held understanding or assumption that El Niño-Southern Oscillation (ENSO) has a remote influence on the North Atlantic Oscillation (NAO) in the January–February–March (JFM) months has been questioned recently. We examine this claim further using atmospheric data filtered to separate the variability orthogonal and parallel to NAO. This decomposition of the atmospheric fields is based on the Principal Component/Empirical Orthogonal Function method whereby the leading mode of the sea-level pressure in the North Atlantic sector is recognised as the NAO, while the remaining variability is orthogonal (unrelated) to NAO. Composite analyses indicate that ENSO has statistically significant links with both the non-NAO and NAO variability at various atmospheric levels. Additional bootstrap tests carried out to quantify the uncertainty and statistical significance confirm these relationships. Consistent with previous studies, we find that an ENSO teleconnection in the NAO-related variability is characterised by lower-stratospheric eddy heat flux anomalies (related to the vertical propagation of planetary waves) which appear in November–December and strengthen through JFM. Under El Niño (La Niña), there is constructive (destructive) interference of anomalous eddy heat flux with the climatological pattern, enhancing (reducing) fluxes over the northern Pacific and Barents Sea areas. We further show that the teleconnection of extreme El Niño is essentially a non-NAO phenomenon. Some non-linearity of the teleconnections is suggested, with El Niño including more NAO-related variability than La Niña, but the statistical significance is degraded due to weaker signals and smaller sample sizes after the partitioning. Our findings have implications for the general understanding of the nature of ENSO teleconnections over the North Atlantic, as well as for refining methods to characterise and evaluate them in models. [ABSTRACT FROM AUTHOR]

In 2021, the Yangtze–Huaihe River Basin (YHRB) of China underwent normal precipitation during the classical Meiyu period in June–July but suffered extreme precipitation and severe floods in August. The present study revealed that the extreme heavy precipitation anomalies over the YHRB region were closely related to the combined effect of the strong western North Pacific anomalous anticyclone (WNPAC) and East Asian middle–high latitude circulation anomalies in August 2021. Moreover, the background of sea surface temperature (SST) anomalies was a strong eastern-Pacific La Niña event that rapidly decayed in spring and a second La Niña emerged in late summer, accompanied by a weak tropical Indian Ocean warming and a strong tropical Atlantic warming in summer. The rapidly decaying La Niña event and the weak tropical Indian Ocean warming alone seem to be insufficient to induce the strong WNPAC in August 2021. Observational evidence and numerical experiments suggest that the rapid tropical Atlantic warming from late spring to summer observably contributed to the enhancement and westward shift of the WNPAC, and the cold SST anomalies in the eastern tropical Pacific associated with the second La Niña event also provided favorable conditions for a strong WNPAC in August 2021. Overall, rapid tropical Atlantic warming and the development of the second La Niña contributed to the strong WNPAC in August 2021, while the maintenance mechanism of the East Asian middle–high latitude circulation anomalies that controlled cold air activity remains unclear. [ABSTRACT FROM AUTHOR]

During late July 2022, the western North Pacific subtropical high (WNPSH) and the Iran high merge persistently, leading to concurrent extreme climate events over multiple regions. Such extraordinary circulation results from two extreme anticyclonic anomalies centered over the WNP–South Asia and Central Asia, respectively. The WNP–South Asia anticyclonic anomaly is related to La Niña SSTA pattern and the enhanced convection over the tropical eastern Indian Ocean (TEIO). SSTA serves as a seasonal-mean background, with abnormally cold tropical central Pacific (TCP) favoring the anticyclonic anomaly through Rossby wave response. The TEIO convection is extremely strong during late July and induces the anticyclonic anomaly via Kelvin wave response and a local Hadley circulation. The TEIO convection anomaly during late July results from different time scales, with the seasonal mean accounting for 30% and the 25–90-day intraseasonal oscillation (ISO) for 50%. The seasonal mean TEIO convection anomaly is related to La Niña, triggered by not only the local positive SSTA but also the negative TCP SSTA through the Walker circulation. The 25–90-day ISO features a northward-propagating convection dipole over the Indian Ocean. An enhanced ISO convection center develops over the TEIO during late July, which moves northward and is gradually replaced by suppressed convection. On the other hand, the Central Asia anticyclonic anomaly is associated with the Silk Road pattern propagating eastward along the westerly jet. During late July, a remarkable anticyclonic node moves across Central Asia, cooperating with the enhanced TEIO convection to facilitate the merging of the WNPSH and the Iran high. [ABSTRACT FROM AUTHOR]

This study investigates the impact of El Niño Southern Oscillation (ENSO) and ENSO Modoki on rainfall variability over Southeast Asia (SEA) with a focus on its twenty sub-regions. For the period 1979–2019, seven El Niño (EN), six La Niña (LN), five El Niño Modoki (EM), and five La Niña Modoki (LM) events were identified. In the boreal summer (JJA) and winter (DJF) of EN events, rainfall reduction occurred over the Maritime Continent and the Philippines except for West Philippines in JJA and West Kalimantan in DJF. Similar patterns but with enhanced drier/wetter conditions were detected during EM. During LN, rainfall increased in most sub-regions except for West Philippines and parts of mainland Indochina in JJA, and in DJF in some southern areas such as South Sumatra, West Kalimantan, and Papua. Compared to LN, LM generally exhibited less wet/drier conditions in JJA over most sub-regions and wetter conditions in DJF over a major part of the Maritime Continent. The decrease (increase) in rainfall over SEA during Modoki events compared to the canonical ENSO events was explained by a reduced (enhanced) moisture transport into the region and a weakening (strengthening) of the ascending branch of the Walker circulation. [ABSTRACT FROM AUTHOR]

Southeastern South America (SESA)'s precipitation is thought to be influenced by both El Niño Southern Oscillation (ENSO) and Antarctic Oscillation (AAO), especially in austral spring. Previous studies conclude AAO can modulate ENSO's impact on precipitation over the SESA region without differentiating between El Niño and La Niña events. In the present study, we use composite analysis to further explore the combined impact of AAO and ENSO on austral spring precipitation over Southeastern South America (SESA) to answer this question and explain the dynamic mechanisms. We found that different AAO phases can influence La Niña's impact on SESA austral spring precipitation considerably, while this does not apply for El Niño events. From our analysis, we found that AAO exerts more impact on austral spring precipitation over SESA compared to ENSO during La Niña years by influencing northerly wind and southward water vapor flux, which contributes most of the moisture into the SESA region, due to the strengthening of South Atlantic subtropical anticyclone and stronger meridional gradient in low-level pressure. Besides, there is an upper-level trough (ridge) over subtropical South America indicating advection of cyclonic (anticyclonic) vorticity inducing anomalous increase (decrease) of precipitation over that region during La Niña/AAO− (La Niña/AAO+). We do not see this opposite difference within El Niño groups combined with different phases of AAO. [ABSTRACT FROM AUTHOR]

In the first half of winter 2020/2021, several unprecedented cold events occurred in most parts of China and caused record-breaking low temperatures in many cities. However, seasonal predictions related to the onset and evolution of extreme cold events are still challenging. In this study, we first evaluated the short-term climate prediction skills in winter of the CAS-ESM-c global coupled model and revealed the key dynamic processes of the onset and development of 2020/2021 extreme cold events based on the ensemble forecasts starting on October 1st, 2020. Under the background provided by the synergistic effect of the warm Arctic and the cold tropical Pacific (La Niña), the model captured the abnormal meridional atmospheric pattern well, including the intensification of the Ural Blocking High and the negative phase of the Arctic Oscillation, forecasted the negative geopotential height anomalies over the Eastern Asian region, and finally, successfully predicted the outbreak of cold waves in December 2020 two months in advance. Moreover, the dominant differences in the initial ocean fields between the best and the worst forecast members were further compared to isolate the triggering factor for predicting cold events by CAS-ESM-c. The initial warm sea surface temperature over the Barents Sea can gradually form a meridional pattern between the warm Arctic and the cold Asian continent, which could lead to a reduction in the large-scale meridional temperature gradient at mid-high latitudes and weaken the atmospheric baroclinicity with more conducive to the southward outbreak of cold waves, highlighting that realistic Arctic ocean conditions in autumn can be reasonably assimilated into coupled models to stimulate the successful prediction of extreme cold events in the 2020/2021 winter. [ABSTRACT FROM AUTHOR]

The interactions between the tropical Indian Ocean and Pacific Ocean via the Walker circulation and the Indonesian Throughflow (ITF) play an important role in the inter-basin exchange and climate variations. The effects of the eastern Indian Ocean subsurface temperature conditions during the 2019 Indian Ocean dipole (IOD) event on the development of the 2020/2021 La Niña event are investigated of the National Marine Environmental Forecasting Center of China. Results of pacemaker experiments comparing the effects of the atmospheric bridge and the oceanic channel suggest that the subsurface temperature anomalies in the tropical Indian Ocean dominates the inter-basin forcing leading to the outburst of the 2020/2021 La Niña event. The strong subsurface cooling is found to propagate to the eastern equatorial Pacific Ocean through the Indonesian seas and induce a strong La Niña event at the end of 2020, showing the long memory of the oceanic channel dynamics and the amplification by the ocean-atmospheric coupling. In comparison, experiments forced by surface temperature anomalies over the tropical Indian Ocean show that the atmospheric bridge alone is unable to induce the onset of the 2020 La Niña. The results suggest the importance of the Indo-Pacific oceanic channel dynamics in ENSO forecasts. [ABSTRACT FROM AUTHOR]

Compared with well documented and frequent occurrence of multi-year La Niña, double-year El Niño is less frequent and has not been well investigated. Both of them are a discrepancy from the cyclic behavior of the El Niño-Southern Oscillation and deserve investigation. Here, we demonstrate the diversity of single- and double-year El Niño events in their strengths, flavors, as well as associations with the recharge/discharge processes. The possible different climate impacts are also discussed. During 1950–2021, 75% of El Niño events persist for one year, and 25% of them last for two years. Both central and eastern Pacific type El Niños occur in the single-year and double-year El Niños with various strengths. On average, there is no relationship between the initial time and duration of an El Niño event. Compared with the single-year El Niños, the averaged warm water volume (WWV) is larger in the peak and declines slower for the double-year El Niños, suggesting that a persistently recharged heat condition of the equatorial Pacific is a precondition for the emergence of a second-year El Niño. The faster decline of WWV in the single-year El Niños is associated with the in-phase decrease of its intraseasonal-interseasonal and interannual components, while the slower decline of WWV in the double-year El Niños is determined by the interannual component. In addition, the single-year and double-year El Niño may have different impacts on regional climate. [ABSTRACT FROM AUTHOR]

Based on 12 years (2007–2018) of salinity data from the Array for Real-Time Geostrophic Oceanography (Argo) dataset, we found significant positive salinity anomalies in the upper layer of the central tropical Indian Ocean from autumn 2010 to spring 2011 and from autumn 2016 to spring 2017. We used wind, precipitation, outgoing longwave radiation and ocean current data from satellites and reanalysis datasets to analyze the atmospheric conditions, ocean dynamic processes and salinity budget associated with these high salinity events. Our results suggest that surface buoyancy fluxes are not the dominant factor affecting the positive salinity anomalies and that ocean dynamic processes have a more important role. Under the influence of the La Niña events and strong negative Indian Ocean dipole in 2010 and 2016, positive salinity anomalies appeared in the eastern Indian Ocean at the end of 2010 and 2016 as a result of strong westerlies and positive zonal currents. However, because the La Niña event in 2010 was stronger than that in 2016, the salinity anomalies in 2010 were also stronger and the decrease in the following year was both stronger and lasted longer, meaning that the salinity anomalies weakened only gradually. The maximum value of the salinity anomalies in 2011 therefore appeared in January, whereas in 2017 the salinity anomalies first decreased and then increased, with the largest anomalies in March. Salinity budget analyses showed that ocean advection was the main factor leading to the variations in the salinity anomaly during these two periods and that the changes in the zonal velocity in the zonal advection anomalies had the greatest impact. Zonal advection was positive and strongest at the end of 2010 and negative in early 2011, but weakly positive at the end of 2016. In early 2017, the zonal advection was first negative, then became positive and strengthened in spring, so the salinity anomalies in spring 2017 were higher than those in 2011. The freshwater flux had a small, negative effect on the positive salinity anomalies for these two events. The mutual effects of the horizontal advection and the freshwater flux led to high salinity anomalies. The high salinity anomalies reflect the response of the upper ocean to climate events and may influence regional air–sea interactions and large-scale processes. [ABSTRACT FROM AUTHOR]

The connection between the meridional structure of tropical sea surface temperature (SST) and the Hadley circulation (HC) under the effect of ENSO (El Niño Southern Oscillation) from 1950 to 1977 is studied. We decompose the HC and zonal mean SST into equatorially symmetric (HES for HC, SES for SST) and asymmetric variations (HEA for HC, SEA for SST) to discuss the modulation of their connection by ENSO. During El Niño events from 1950 to 1977, the HC is less sensitive to the different SST meridional structures and expressed by response ratio. The ratio in La Niña and neutral events is around 4, which is equivalent to the result in the climatology. The reason for the decreased ratio during El Niño events is explored. The interdecadal variation in the linkage between the HC and tropical SST is due to a clear interdecadal shift in the impacts of ENSO on the tropical Indian Ocean (TIO) SST. For the period 1950–1977, when El Niño events occur, larger SST warming amplitude is observed over the northern TIO (0°–15°N, 50°–100°E). However, the southern TIO (15°S–0°, 50°–100°E) shows greater warming amplitude during 1980–2016. The anomalous SST variation over the TIO linked to El Niño events alters the meridional SST distribution, inducing anomalies in the meridional circulation. These results can help us understand the interdecadal modulation by ENSO on the relationship between tropical SST and HC. [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated that the tropical Indian Ocean basin mode (IOB) favors the phase transition of ENSO; however, they have not differentiated the two types of ENSO. This study reports that the boreal winter-spring IOB appears more likely to induce the central-Pacific (CP) ENSO than the eastern-Pacific ENSO one year later based on observation data and CMIP5 simulations. We find a strong asymmetry in the forcing of the IOB on the CP ENSO. The impact of IOB warming on CP La Niña events is more significant than the impact of IOB cooling on CP El Niño events. IOB warming during late winter to early spring produces easterly wind anomalies over the western equatorial Pacific and an anomalous descending motion of the Walker circulation near the dateline, inducing cooling in the central Pacific during the subsequent summer to winter. This suggests that interbasin interactions with the tropical Indian Ocean, in addition to the northern tropical Atlantic Ocean, are capable of generating CP La Niña events. Most CMIP5 models generally capture the negative IOB-CP ENSO lagged correlation. However, the cooling center and sinking branch of the anomalous Walker circulation in the Pacific Ocean forced by the IOB warming are weaker and shifted eastward in the multimodel ensemble than in the observations. Additionally, the observed asymmetry of the IOB impact on the CP ENSO is not captured by the CMIP5 models. This study indicates that improving the simulation of the IOB as a contributor to ENSO diversity remains a challenge. [ABSTRACT FROM AUTHOR]

This study investigates the dominant characteristics of winter Arctic tropospheric thickness (1000–200 hPa), the variations of winter atmospheric circulation in the Northern Hemisphere, and the related winter North Pacific storm track (NPST) variabilities during 1979–2018 under the combined effects of the La Niña events with different periods of Arctic tropospheric thermal conditions. Results show that the leading mode (42.7%) exhibits prominent warm anomalies centered on Greenland and Baffin Bay. The winter Arctic tropospheric thickness experienced a phase shift from a cold period of the Arctic tropospheric temperature in 1979–1999 to the warm period after 2000. During the La Niña events with Arctic tropospheric warm anomalies, a wave train is shown in the mid-high latitudes with alternative anticyclonic, cyclonic, and anticyclonic anomalies over the Ural Mountains, Lake Baikal, and North Pacific, respectively. This atmospheric circulation pattern not only intensifies the linkage between the Arctic and mid-low latitudes but also induces the winter NPST shifting poleward. The possible physical mechanism is attributed to the large-scale circulation change and the local baroclinic energy conversion (BCEC). The enhanced anticyclonic anomaly in the North Pacific alters the climatological mean flow, further influencing the local BCEC through the interaction between the mean flow and eddies. The significantly robust BCEC over the North Pacific possibly induces the poleward shift of winter NPST during the La Niña events under the warm period. [ABSTRACT FROM AUTHOR]

A prominent change occurred in the relationship between August Indochina Peninsula (ICP) precipitation and tropical Pacific sea surface temperature (SST) around 1980. More August ICP precipitation falls in La Niña developing years during 1959–1979, but in El Niño developing years during 1983–2003. The relation change is associated with a change in the evolution of equatorial Pacific SST anomalies. During 1959–1979, negative SST anomalies develop in the equatorial eastern Pacific (EEP) in early spring and extend westward. They cause anomalous southwesterly and reduce surface latent heat flux (LHF) over the tropical western North Pacific (WNP), inducing positive SST anomalies there in early summer. Negative SST anomalies form in the tropical South Indian Ocean (SIO) due to LHF increase attributed to a local air-sea interaction process in preceding winter and a remote forcing of negative EEP SST anomalies in late spring-early summer. More August ICP precipitation is due to anomalous ascent induced by a Rossby wave response to positive WNP SST anomalies and a cross-equatorial vertical circulation due to negative SIO SST anomalies. During 1983–2003, positive SST anomalies form in the equatorial central Pacific (ECP) in late spring and extend eastward. Negative SST anomalies develop in the tropical southeastern Indian Ocean (SEIO) due to sustained LHF increase induced by both preceding winter negative SST anomalies and late spring-early summer positive SST anomalies in the EEP. More August ICP precipitation is induced by a Rossby wave response to positive ECP SST anomalies and a cross-equatorial vertical circulation due to negative SEIO SST anomalies. [ABSTRACT FROM AUTHOR]

The Victoria mode (VM), as a basin-scale sea surface temperature (SST) pattern over the North Pacific, is suggested to facilitate subsequent development of El Niño–Southern Oscillation (ENSO) through both the seasonal footprinting mechanism (SFM) and the trade wind charging (TWC) mechanism. The present study aims at investigating the distinct roles and relative contributions to ENSO of the SFM and the TWC mechanism associated with the VM using atmospheric and oceanic reanalysis data as well as modeling simulations. Our results reveal that the positive SST anomalies (SSTAs) over the subtropical northeast Pacific (SNP) related to the VM effectively trigger the initiation of ENSO via the SFM, which emphasizes an air–sea surface thermodynamic-coupling process. In contrast, the negative SSTAs over the western North Pacific (WNP) associated with the VM primarily induce ENSO via a thermocline–SST feedback process, known as the TWC mechanism. Further analysis indicates that the SFM related to the VM may play a relatively independent role in affecting ENSO and is more closely linked to ENSO than is the TWC mechanism related to the VM, which is shown to be reasonably reproduced by the Community Earth System Model. Additionally, the SFM associated with the positive (negative) SNP SSTAs may induce fewer El Niño events (more La Niña events) than the TWC mechanism related to the positive (negative) WNP SSTAs. Our findings suggest that the SFM and the TWC mechanism associated with the VM both contribute to enhanced predictive skill for ENSO. [ABSTRACT FROM AUTHOR]

Using reanalysis and high-resolution ensemble simulations, we characterize cold season (December−March) North Atlantic (NA) atmospheric river (AR) tracks by grouping them into four distinct clusters; then for each cluster, we link the year-to-year variations in track count to large-scale climate variability and examine the climatological effects of the cluster on extreme precipitation and winds. The four clusters share similar prevailing AR track orientation, but differ in AR genesis locations and dominate over different regions. Cluster 1, with the longest average track of the four clusters, originates near the U.S. East Coast during La Niña and positive North Atlantic Oscillation (NAO) years and produces extreme precipitation and winds primarily over the eastern coast of North America. Cluster 2, which is weak in intensity and short-lived, forms north of 30°N of the open ocean during positive NAO years and contributes to more than 25% of the precipitation and wind extremes along the coasts of Northwestern Europe. Cluster 3, with the strongest intensity and longest duration among the four clusters, is generated surrounding the Gulf of Mexico during El Niño and negative NAO years and produces respectively more than 50% and 40% of the extreme precipitation and wind events over the eastern U.S. Cluster 4, the smallest and weakest among the four clusters, is favored under negative NAO conditions and generates roughly 25% of the extreme precipitation and winds along the coast of the Iberian Peninsula. The similarities and discrepancies between reanalysis and model simulations and among different member simulations are also discussed. [ABSTRACT FROM AUTHOR]

The Madden–Julian oscillation (MJO) is one of the leading modes of tropical intra-seasonal variability, which exerts significant impacts on the weather and climate across the globe, particularly in the tropics. MJO affects the Asian monsoon by producing enhanced and suppressed convection during the active and break periods, respectively. In the recent decades, the heat content of Indo-western Pacific Ocean has increased significantly, which strengthened the MJO activity. Previous studies also have shown that the expansion of Indo-western Pacific warm pool led to the warping of MJO life cycle, which decreases its residence time over the Indian Ocean (IO) and increases over the Pacific Ocean. Here we show that in the boreal summer months, MJO amplitude has strengthened during the global warming hiatus or rapid IO warming period (1999–2015) compared to the previous period (1982–1998). In the later period, MJO exhibits a faster regeneration over the western IO, and its residence time has increased in the western hemisphere and western IO but decreased in the eastern IO and eastern Pacific Ocean. The strengthening of MJO and the readjustment in its residence time are due to the local MJO feedback on the IO and the La Nina like sea surface temperature pattern in the Pacific Ocean. The prolonged MJO activity leads to bursts of rainfall over the Indian subcontinent in Phase 3 and Phase 4, influencing the active spells of the Indian summer monsoon and causing heavy rainfall over central India and East Asia. [ABSTRACT FROM AUTHOR]

The occurrence of cold events over East Asia is modulated by the background mean state associated with the El Niño-Southern Oscillation (ENSO). The present study compares the processes of occurrence of cold events over East Asia between El Niño and La Niña winters. It is found that the difference in atmospheric mean state in El Niño and La Niña winters leads to different processes of occurrence of cold events over East Asia during boreal winter. The cold events during El Niño winters are closely related to negative Arctic Oscillation (AO) and those during La Niña winters are contributed by the Ural blocking. Both negative AO in El Niño winters and the Ural blocking in La Niña winters are followed by a Rossby wave train over Eurasia that strengthens the Siberian high and the East Asian trough, leading to cold anomalies over East Asia. The negative AO during El Niño winters is related to a tropospheric teleconnection that weakens the polar vortex, which is confirmed by numerical experiment with a linear baroclinic model. The Ural blocking is associated with the Arctic sea ice loss in the Barents Sea during La Niña winters through the reduced meridional temperature gradient and decreased zonal mean winds. The Arctic sea ice loss over the Barents Sea during La Niña winters is contributed by the development of anticyclonic anomalies induced by the enhanced stationary waves. [ABSTRACT FROM AUTHOR]

The sea surface temperature (SST)-forced and internal variability in cold-season (December–March) atmospheric river (AR) occurrence frequency during 1951–2010 over the North Atlantic (NA) basin are examined using a 30-member ensemble of high-resolution atmospheric simulations. The first leading mode of the forced variability features a north–south wobbling pattern modulated by an out-of-phase combination of El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). Co-existing El Niño and negative NAO act to shift ARs equatorward, whereas concurrent La Niña and positive NAO tend to displace ARs poleward. The second leading mode is characterized by a meridional concentration and dispersion of AR occurrence at a basin scale and can be linked to the Scandinavian pattern and the SST difference between the central and easternmost tropical Pacific. The third leading mode is dominated by an oscillation of AR occurrence north and south of 40°N in the eastern NA basin, and modulated by an in-phase combination of ENSO and the NAO. Its time series exhibits a significant upward trend, which can be linked to the SST warming in the Indo-western Pacific since the 1970s. The internal variability in cold-season NA AR occurrence frequency is then quantified by means of the signal-to-noise ratio. The calculations show that the internal variability is relatively weak over the Great Antilles and central-to-eastern US but extremely strong over Northwestern Europe, which can be attributed to the strong SST control associated with ENSO and the chaotic variations of the NAO, respectively. [ABSTRACT FROM AUTHOR]

In the present study, heat redistribution in the Tropical Indian Ocean (TIO) associated with the prolonged La Niña events during 1958–2017 is examined using reanalysis/observations. It is found that the prolonged La Niña forcing strengthened the east–west thermocline gradient in the equatorial Indian Ocean and propelled the eastward extension of thermocline ridge of the Indian Ocean (TRIO) from its climatological location of southwestern TIO. The cyclonic winds over the southeastern TIO and the associated upwelling Rossby waves are primarily driving the TRIO intensification and its eastward extension. Anomalous subsurface warming, thermocline deepening, and the associated increase in the upper ocean heat content and sea-level in the eastern equatorial Indian Ocean, southeastern TIO and Bay of Bengal (BoB) are found to be the characteristic features of the prolonged La Niña events. Cross equatorial Sverdrup transport near the eastern boundary during the prolonged La Niña events has increased the heat content of BoB and is found to be a pathway of Pacific water entering the north Indian Ocean. [ABSTRACT FROM AUTHOR]

The impact of El Niño-Southern Oscillation (ENSO) on the late-winter extra-tropical stratosphere (January–March) is assessed in a multi-model framework. Three state-of-the-art atmospheric models are run with prescribed SST anomalies representative of a strong ENSO event, with symmetric patterns for El Niño and La Niña. The well-known temperature perturbation in the lower stratosphere during El Niño is captured by two models, in which the anomalous warming at polar latitudes is accompanied by a positive geopotential height anomaly that extends over the polar cap. In the third model, which shows a lack of temperature anomalies over the pole, the anomalous anticyclone is confined over Canada and does not expand to the polar cap. This anomalous center of action emerges from the large-scale tropospheric Rossby wave train forced by ENSO, and shrinking/stretching around the polar vortex is invoked to link it to the temperature response. No disagreement across models is found in the lower stratosphere for La Niña, whose teleconnection is opposite in sign but weaker. In the middle-upper stratosphere (above 50 hPa) the geopotential height anomalies project on a wavenumber-1 (WN1) pattern for both El Niño and, more weakly, La Niña, and show a westward tilt with height up to the stratopause. It is suggested that this WN1 pattern arises from the high-latitude lower-stratospheric anomalies, and that the ENSO teleconnection to the polar stratosphere can be interpreted in terms of upward propagation of the stationary Rossby wave train and quasi-geostrophic balance, instead of wave breaking. [ABSTRACT FROM AUTHOR]

Intense cyclones have become more frequent over the North Indian Ocean (NIO) region causing significant risk and vulnerability to the coastal regions. Analysis clearly revealed an increasing trend (~ 16.75%) in the intensity of severe cyclonic storms that formed over the NIO during the past 4 decades. Power dissipation index (PDI) showed a correlation of 0.58 with severe cyclonic storms during the study period (1979–2019). Study reveals that the genesis location for post-monsoon cyclones is statistically significant against the genesis potential index (GPI) difference, especially for the intense cyclone categories (> 48 kt). During the La Niña years, increased tropospheric water vapor content initiates over prominent locations in the Andaman Sea and the North China Sea. Also, La Niña periods showed positional shift in the average genesis location of intense tropical cyclones compared to El Niño periods. Cyclogenesis shift is more concentrated towards the west-equatorward directions over the Bay of Bengal basin. Strong mid-level relative humidity (RH600), positive low-level relative vorticity (RV850), weak vertical wind shear (VWS), warm sea surface temperature (SST), and suppressed outgoing longwave radiation (OLR500) are the responsible factors leading to increased tropical cyclone activity in the NIO. Study signifies that RH600, RV850, and OLR500 are distinct during pre-monsoon seasons of the La Niña epoch, favoring the genesis of severe cyclones over this region. Variables such as SST, wind streamlines (W200, W850), vertical velocity (VV500), and specific humidity (SH500) exhibit comparable contributions towards cyclogenesis during both El Niño and La Niña phases. [ABSTRACT FROM AUTHOR]

Predictability of Ethiopian Kiremt rainfall (June to September: JJAS) and forecast skill of the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation seasonal forecast system 5 (SEAS5) is explored during 1981–2019. The first empirical orthogonal function of observed rainfall explains 50.6% of the total variability and is characterized by positive rainfall anomalies largely confined over the northwestern and central-western regions of Ethiopia. Consequently, a Kiremt rainfall index (KRI) is defined for this region. The correlation coefficient (CC) between the observed and predicted KRI is 0.68 and 0.53 for May and April starts, respectively. Composite analysis of sea surface temperature (SST) and lower-level circulation based on excess and deficit years of Kiremt rains shows that the El Niño Southern–Oscillation is the main modulator of the Kiremt rainfall variability. The CC between KRI and Niño3.4 index is − 0.62, indicating that El Niño is accompanied by below-normal Kiremt rainfall, while La Niña is accompanied by above-normal amounts. The fifth generation of ECMWF atmospheric reanalysis (ERA5) shows that excess (deficit) Kiremt rainfall anomalies are associated with an anomalous low (high) pressure centered over northeast Arabian Peninsula and an anomalous in-phase (reverse) low-level Somali Jet. SEAS5 reproduces the spatial and temporal components of observed Kiremt rainfall variability, including the main climatic features associated with excess and deficit Kiremt rainfall in May and April starts. However, certain important observed features like above-normal SSTs in the Gulf of Guinea are not well predicted. Results indicate that Kiremt rains has some potential predictability and SEAS5 shows a moderate forecast skill. Probabilistic analysis shows highest values where predictability and deterministic skill are also highest. [ABSTRACT FROM AUTHOR]

The build-up of decadal timescale upper ocean heat content in the off-equatorial western tropical Pacific can provide necessary conditions for interannual El Niño/Southern Oscillation (ENSO) events to contribute to decadal timescale transitions of tropical Pacific SSTs to the opposite phase of the Interdecadal Pacific Oscillation (IPO). This can be viewed as a corollary to subseasonal westerly wind burst events contributing to El Niño interannual timescale transitions. A long pre-industrial control run with CESM1 is analyzed to show that there is a greater chance of ENSO activity to contribute to an IPO transition when off-equatorial western Pacific Ocean heat content reaches either a maximum (for El Niño to contribute to a transition to positive IPO) or minimum (for La Niña to contribute to a transition to negative IPO) as seen in observations. If above a necessary ocean heat content threshold, the convergence associated with westerly anomaly near-equatorial surface winds associated with El Niño activity can draw that heat content equatorward to sustain anomalously warm western and central Pacific SSTs. These are associated with positive precipitation and convective heating anomalies, a Gill-type response and wind stress curl anomalies that continue to feed warm water into the near-equatorial western Pacific. These conditions then sustain the decadal-timescale transition to positive IPO (with the opposite sign for transition to negative IPO). Associated central equatorial Pacific convective heating anomalies produce SLP and wind stress anomalies in the North and South Pacific that can excite westward-propagating off-equatorial oceanic Rossby waves to contribute to the western Pacific thermocline depth and consequent heat content anomalies. [ABSTRACT FROM AUTHOR]

Deuterium excess (d-excess), as a secondary parameter of water stable isotopes, inherits the meteorological information from moisture source to the site of interest. Understanding the climatic implications of d-excess in ice cores from the Tibetan Plateau (TP) is critical for reconstructing the paleoclimate. Here we present an annually-dated ice core record retrieved from the central TP and analyze the correlations between the ice core d-excess record and local and regional climate variables in the past decades (1948–2011). Our results show poor correlations between d-excess and local meteorological parameters, deemphasizing the local control on the interannual variations in d-excess records. However, significant correlations exist between the d-excess record and the relative humidity over the eastern Arabian Sea and Bay of Bengal. In addition, our results indicate that d-excess is positively correlated with temperature in the Niño 3.4 region, demonstrating that large-scale atmospheric circulation through ENSO cycle affecting the interannual d-excess signal in ice cores from the central TP. Mechanically speaking, in annual scale, the higher (lower) temperature that occurs in the eastern tropical Pacific Ocean in El Niño (La Niña) years is accompanied by the ascending branch of the Walker circulation weakening (enhancing) in the Indian Ocean region, leading to lower (higher) relative humidity value in the moisture source regions of the Arabian Sea and Bay of Bengal, and subsequently producing higher (lower) d-excess value in the central Tibetan ice core records. This finding will enhance the understanding of the climatic significance of ice core d-excess in the Asian monsoon region. [ABSTRACT FROM AUTHOR]

The western North Pacific (WNP) intraseasonal oscillation (ISO) is the strongest over the globe, its prediction is the cornerstone for subseasonal prediction of the Asian summer monsoon. Yet, our understanding of the diversity of the WNP ISO is limited, which challenges our modeling and prediction efforts. We study the diversity of observed WNP ISO by performing cluster analysis on propagation patterns of ISO events, targeting three clusters: westward, northeastward, and northwestward propagations. The westward cluster exists within the WNP, while the other two are related to the northeastward propagating rain band originated from the central equatorial Indian Ocean and to the northwestward propagating dipole across the western Pacific and Indian Ocean, respectively. Moist static energy (MSE) tendency contributing to these different propagations is mainly due to horizontal advection, while radiative heating mainly maintains the ISO's development. Background sea surface temperature (SST) and MSE anomalies partly determine this ISO diversity, especially for those ISOs with large similarity. The westward cluster is related to warm SST anomalies in the western Indian Ocean, as a combination of seasonal cycle and internal interannual-to-interdecadal variability. The northeastward cluster is related to the cold Pacific Meridional Mode and La Niña-like pattern, while the northwestward cluster is related to the opposite warm background, both as the internal variability. Our finding of the background-affected ISO diversity over the WNP can be conducive to both model simulation and subseasonal prediction of the Asian summer monsoon. [ABSTRACT FROM AUTHOR]

This study explores the counterpart of marine heatwave, known as the cold blob, in the Northeast Pacific (NEP) by selecting the long-lived events with the normalized sea surface temperature (SST) anomalies averaged over the blob area (160°–135° W, 40°–50° N) smaller than − 0.75 for at least 5 months. There are 10 summer-peak events with a stronger intensity and 7 winter-peak events. The summer-peak cold blob events are mostly attributed to the vertical entrainment when the mixed layer depth is shoaling. For the winter-peak group, the greater heat loss at surface is the principal factor to stimulate the cold blob, associated with the intense westerly anomalies around the blob area. However, the vertical entrainment generally hinders the amplification of winter-peak cold blobs. Moreover, the advection of colder-than-normal water to the blob area contributes much less for both two groups. We conjecture that the NEP cold blob and tropical La Niña-like state are potentially linked. The summer-peak cold blob event is often followed by a La Niña event in the ensuing winter. On the other hand, the equatorial La Niña-like SST cooling in late summer and autumn in turn enhances the winter-peak cold blobs. We find the subtropical region, particularly near the Baja California, may act as the oceanic bridge to connect the NEP cold blob and equatorial La Niña. [ABSTRACT FROM AUTHOR]

We explore the current (1958–2005 period) and near future (2006–2050 period) teleconnections between El Niño Southern Oscillation (ENSO), Indian Ocean Basin Mode (IOBM), and Indian Ocean Dipole (IOD) as simulated in historical and Representative Concentration Pathway (RCP8.5) simulations of 32 coupled models that participated in the phase five of Coupled Model Intercomparison Project (CMIP5). A set of 16 CMIP5 models out of 32 models, which perform best to simulate tropical climate variability in recent decades, is first selected using a robust method based on the Empirical Orthogonal Function analysis for detailed analysis. Most of these models show modest capability in reproducing the seasonal cycle of ENSO types in the current period. Further, amplitude of Indian Ocean (IO) modes is overestimated by the 16 models along with large inter-model spread. Based on these results, a subset of 9 models is formed, which simulate a realistic seasonal phase-locking of ENSO for a robust assessment of future teleconnections. No significant change in El Niño amplitude is detected in near future. However, the IOBM is projected to be weaker during late spring and early summer. The IOD is projected to be stronger during boreal summer in the future relative to the current period. We also investigate if there are any changes from historical to RCP 8.5 simulations in the strength of the IO negative feedback on ENSO with a multiple linear regression approach. The IO negative feedback strengthens significantly in the RCP8.5 scenario due to the increasing role of IOBM in speeding the transition from El Niño to La Niña, despite its reduction of amplitude. In contrast, IOD loses its predictive value in the future projections. [ABSTRACT FROM AUTHOR]

This study investigates the summer climate variability over Korea that is associated with diverse features of El Niño-Southern Oscillation (ENSO) evolution. Four different ENSO evolution features were analyzed, and the related Korean climate variability during July and August are investigated. The two evolution phases in which El Niño transitions into La Niña and El Niño develops from a neutral state show roughly similar circulation features in affecting the Korean summer climate, but with opposite signs. During the transition of El Niño into La Niña (after El Niño develops from a neutral state), the positive (negative) western North Pacific anticyclonic circulation anomaly directly affects Korea in August, producing a significant warm (cold) surface air temperature (SAT) anomaly in Korea. Notably, the SAT that occurs in July generally exhibits an opposite tendency to that of August in Korea, displaying subseasonal variations. However, the persistent phases of La Niña and El Niño exhibit distinctive features from the two transition phases mentioned. During the persistent La Niña phase, the Pacific-Japan like atmospheric circulation pattern affects Korea, showing a significant warm SAT anomaly over Korea in July, and a roughly similar situation with opposite signs is observed in the persistent El Niño phase. In these cases, the Korean SAT in August exhibits similar anomalies to July. The distinctive variability in the summer climate of Korea during the diverse ENSO evolution phases presented in this study provides a statistical basis for understanding the Korean summer climate in relation to ENSO. [ABSTRACT FROM AUTHOR]

This study investigates the influence of the anomalously warm Indian Ocean state on the unprecedentedly weak Indonesian Throughflow (ITF) and the unexpected evolution of El Niño-Southern Oscillation (ENSO) during 2014–2016. It uses 25-month-long coupled twin forecast experiments with modified Indian Ocean initial conditions sampling observed decadal variations. An unperturbed experiment initialized in Feb 2014 forecasts moderately warm ENSO conditions in year 1 and year 2 and an anomalously weak ITF throughout, which acts to keep tropical Pacific ocean heat content (OHC) anomalously high. Changing only the Indian Ocean to cooler 1997 conditions substantially alters the 2-year forecast of Tropical Pacific conditions. Differences include (i) increased probability of strong El Niño in 2014 and La Niña in 2015, (ii) significantly increased ITF transports and (iii), as a consequence, stronger Pacific ocean heat divergence and thus a reduction of Pacific OHC over the two years. The Indian Ocean's impact in year 1 is via the atmospheric bridge arising from altered Indian Ocean Dipole conditions. Effects of altered ITF and associated ocean heat divergence (oceanic tunnel) become apparent by year 2, including modified ENSO probabilities and Tropical Pacific OHC. A mirrored twin experiment starting from unperturbed 1997 conditions and several sensitivity experiments corroborate these findings. This work demonstrates the importance of the Indian Ocean's decadal variations on ENSO and highlights the previously underappreciated role of the oceanic tunnel. Results also indicate that, given the physical links between year-to-year ENSO variations, 2-year-long forecasts can provide additional guidance for interpretation of forecasted year-1 ENSO probabilities. [ABSTRACT FROM AUTHOR]

We examine the transition processes of El Niño occurring in 1 year after using observations, a multi-century model simulation and atmospheric general circulation model (AGCM) experiments. One type is characterized by a quick transition from an El Niño event into a La Niña event in the following winter, and the other type involves a slow decay to an almost neutral state or a continuous El Niño event. While both the Indian and the Atlantic Oceans contribute to the El Niño's transition processes, we further find that sea surface temperature (SST) conditions in the eastern subtropical North Pacific and the associated surface wind anomalies play important roles in determining the different types of El Niño's transition processes through atmosphere–ocean coupled processes. In particular, northeasterly wind anomalies in the central subtropical North Pacific during the early spring, which is in consequence of atmosphere–ocean coupled processes, contribute to decreasing ocean heat content (OHC) anomalies and strengthening easterly wind anomalies in the central tropical Pacific. These anomalies lead up to a transition into a La Niña event in the following winter. On the contrary, weakening of the northeasterly wind and warm SST anomalies in the eastern subtropical North Pacific during the early spring are conducive to a neutral state or a continuous El Niño event in the subsequent winter. Similar transition processes are also found in a multi-century model simulation. By conducting idealized AGCM experiments, we also show that the anomalous SST in the eastern subtropical North Pacific during El Niño peak season may induce surface wind anomalies in the central tropical Pacific during El Niño onset season. These results provide potential precursors for predicting the occurrence of a La Niña event, a neutral state and an El Niño event 1 year after the occurrence of an El Niño event. [ABSTRACT FROM AUTHOR]