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Precipitation in eastern China exhibits large interannual variability during July with the northward movement of the monsoon rain belt. Thus, eastern China usually experiences severe droughts and floods in July. However, the influences of underlying surface thermal drivers, particularly the land factors, remain poorly understood. This study investigates the leading modes of July precipitation in eastern China and their potential influencing factors. The first and second empirical orthogonal function (EOF) modes show meridional dipole and tripolar precipitation anomalies in eastern China, respectively. The EOF1 mode is found to be closely associated with sea surface temperature (SST) anomalies in the tropical Pacific and North Atlantic Oceans in June, while the EOF2 mode is mainly linked to anomalous Indian Ocean SST and Indochina Peninsula soil moisture in June. During years with a strong El Niño–South Oscillation (ENSO) signal, the EOF1 mode is mainly related to the enhanced Walker and Hadley circulations associated with the cold tropical Pacific SST anomalies. In contrast, during years with a weak ENSO signal, the Eurasian midlatitude wave train and the westward zonal overturning circulation associated with tripole-like North Atlantic SST anomalies play a leading role. The EOF2 mode is mainly influenced by Indian Ocean SST anomalies that alter the Walker circulation and by soil moisture anomalies in the Indochina Peninsula that induce an anomalous regional cyclonic circulation. Numerical experiments further demonstrated that the combined effects of soil moisture and SST exert a more substantial impact than their individual effects. These results emphasize the importance of surface thermal factors in understanding regional climate dynamics. [ABSTRACT FROM AUTHOR]

The influence of El Niño‐Southern Oscillation (ENSO) on the North Tropical Atlantic (NTA) sea surface temperature (SST) is well‐documented in boreal spring, yet it manifests greater complexity in winter. Distinct from El Niño, La Niña has a statistically significant impact on winter NTA SST, which is primarily contributed by the re‐occurring (i.e., second and third years of multi‐year) La Niña events. Despite similar atmospheric forcing, prominent SST anomalies are observed in the NTA during the re‐occurring La Niña winters but not in the first‐year La Niña winters, mainly due to different initial states associated with previous tropical Pacific forcing. Targeted pacemaker experiments well reproduce the robust impacts of re‐occurring La Niña events on the wintertime NTA SST, highlighting the crucial impact of ENSO cycle complexity in its inter‐basin linkages. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) significantly influences the sea surface temperature (SST) in the North Tropical Atlantic (NTA), particularly as ENSO events decay during boreal spring. However, during winter when ENSO reaches its peak, the NTA SST anomalies exhibit strong uncertainty. While no marked SST anomalies are observed in the NTA during El Niño winters, prominent SST anomalies can often be found in the NTA during La Niña winters. The significant La Niña‐related SST anomalies in the NTA are primarily contributed by the second and third years of multi‐year La Niña events, rather than the first‐year La Niña events. The SST difference between the first‐year and re‐occurring La Niña winters can be attributed to the SST initial state in the NTA, which is related to the preceding forcing from the tropical Pacific. By incorporating this initial state, we can more accurately reconstruct the NTA SST anomalies during La Niña winters. Targeted pacemaker experiments corroborate the significant role of re‐occurring La Niña events in influencing wintertime NTA SST. Key Points: La Niña usually has stronger impacts on winter NTA SST than El Niño, primarily contributed by re‐occurring La Niña eventsDifferent NTA SST anomalies during the first‐year and re‐occurring La Niña can be well understood by considering the NTA initial stateTargeted pacemaker experiments confirm the important effect of re‐occurring La Niña on the NTA SST [ABSTRACT FROM AUTHOR]

Atmospheric variability associated with the winter North Pacific Oscillation (NPO), preceding El Niño‐Southern Oscillation (ENSO) events by about 1 year, is driven mainly by the tropical Pacific sea surface temperature (SST) anomalies but may also affect the subsequent ENSO. To isolate the latter, we conduct an idealized hindcast experiment using a climate model MIROC6, in which the NPO‐related surface heat flux and wind stress anomalies are overridden over the North Pacific. The ENSO evolution can be overall reproduced without the NPO forcing, confirming the predominant intrinsic predictability arising from the tropical coupled dynamics. However, the extratropical atmospheric forcing associated with the NPO, when it reaches three standard deviations, can enhance the ENSO development by about 29.6%, contrasting with limited effects of the moderate NPO forcing. La Niña events exhibit much less sensitivity to the NPO forcing compared to El Niño events, possibly due to asymmetric patterns of the simulated NPO. Plain Language Summary: The winter North Pacific Oscillation (NPO), manifesting as a meridional dipole structure in sea level pressure over the North Pacific, is suggested to play a role in the subsequent El Niño‐Southern Oscillation (ENSO) development. Recent studies demonstrate that the preceding winter NPO signal mainly originates from the tropical Pacific SST forcing, making it challenging to isolate its specific impact on ENSO. In this study, we design a series of "overriding" experiments to decouple the tropical and extratropical Pacific processes based on the MIROC6 model. It is found that the ENSO evolution can be reproduced when excluding the NPO forcing, suggesting that the ENSO events is primarily driven by internal dynamics within the tropics. While the moderate NPO forcing has minor effects on the ENSO events, the exceptionally strong NPO forcing can further amplify the ENSO development. This study underscores that while NPO‐related extratropical variabilities do affect ENSO, their impact, especially under moderate conditions, should not overshadow the intrinsic predictability from the tropics. Key Points: The El Niño‐Southern Oscillation (ENSO) development is primarily driven by internal dynamics within tropicsThe NPO forcing of the extreme rather than moderate amplitude can significantly amplify the ENSO developmentDevelopment of El Niño events are more sensitive to the NPO forcing compared to La Niña events [ABSTRACT FROM AUTHOR]

Extreme El Niño events have outsized global impacts and control the El Niño Southern Oscillation (ENSO) warm/cold phases asymmetries. Yet, a consensus regarding the relative contributions of atmospheric and oceanic nonlinearities to their genesis remains elusive. Here, we isolate the contribution of oceanic nonlinearities by conducting paired experiments forced with opposite wind stress anomalies in an oceanic general circulation model, which realistically simulates extreme El Niño events and oceanic nonlinearities thought to contribute to ENSO skewness (Tropical Instability Waves (TIWs), Nonlinear Dynamical Heating (NDH)). Our findings indicate a weak contribution of oceanic nonlinearities to extreme El Niño events in the eastern Pacific, owing to compensatory effects between lateral (NDH and TIWs) and vertical processes. These results hold across different vertical mixing schemes and modifications of the upper‐ocean heat budget mixed layer criterion. Our study reinforces previous research underscoring the pivotal role of atmospheric nonlinearities in shaping extreme El Niño events. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) is the primary driver of year‐to‐year climate variations in the tropics and beyond. Originating from air‐sea interactions in the tropical Pacific, ENSO oscillates between warm (El Niño) and cold (La Niña) phases, modulating sea surface temperature in the central and eastern equatorial Pacific. Occasionally, El Niño events intensify into "super" El Niño events, causing widespread impacts globally. Utilizing a state‐of‐the‐art oceanic model, our research challenges previous results suggesting a strong oceanic contribution to the amplitude difference between "normal" and "super" El Niño events. Instead, our findings reveal that potential oceanic influences on "super" El Niño events tend to offset each other. This is consistent with recent research highlighting the crucial role of atmospheric processes in the transformation from a "normal" to a "super" El Niño. Key Points: A state‐of‐the‐art ocean model reproduces extreme El Niño events and the corresponding nonlinear oceanic processes realisticallyContributions from oceanic nonlinearities are isolated using paired simulations forced by opposite wind stress anomaliesEffects of oceanic nonlinearities on extreme El Niño events are small, due to compensation between lateral and vertical 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]

Sea surface temperature (SST) variability in the East China Sea–Kuroshio (EK) region has important implications for the surrounding weather, climate, and marine ecology. The year-to-year variations of the EK SST are expectedly linked to El Niño–Southern Oscillation (ENSO), the predominant predictability source of seasonal-to-interannual climate variability. Surprisingly, no significant SST signal is observed in the EK region when focusing on the ENSO autumn–winter season with the persistent and pronounced SST anomalies in the tropical Pacific. We find that a remarkable seasonal reversal appears in the ENSO–EK SST connection, shifting from a negative relationship in autumn [Aug(0)–Oct(0)] to a positive relationship in winter [Dec(0)–Feb(1)]. This reversal is mainly attributed to the seasonally varying ENSO-associated western North Pacific (WNP) atmospheric circulation patterns. During ENSO autumns, the anomalous WNP anticyclone is confined south of 20°N, which is accompanied with cyclonic circulation anomalies in the EK region. The associated anomalous northerly wind tends to enhance the background northerly wind, thereby facilitating the local SST cooling mainly via the wind–evaporation–SST effect. In the subsequent winter, the ENSO-related WNP anticyclonic anomalies intensify and extend toward the EK region. Consequently, the weakened background northerly wind induced by southerly wind anomalies leads to the increase of downward latent and sensible heat flux in the EK region, fostering the local SST warming. The observed seasonal reversal of ENSO impacts can be evidenced by the tropical Pacific pacemaker experiments, emphasizing the importance of seasonally modulated ENSO teleconnection and holding implications for the local SST climate prediction. [ABSTRACT FROM AUTHOR]

While the prominent influence of El Niño‐Southern Oscillation (ENSO) on the Indian Ocean Oscillation (IOD) is widely recognized, intricate relationships between them are often invoked that introduce challenges into seasonal predictions. Previous studies have shown that different flavors of El Niño exhibit distinct associations with the IOD. In this study, we demonstrate that La Niña's teleconnection to the IOD is primarily controlled by its longitudinal position. Westward‐displaced La Niña events tend to produce stronger negative convection anomalies in the central Pacific and more pronounced Walk Circulation anomalies, thereby triggering strong negative IOD events. In contrast, eastward‐displaced La Niña events are usually accompanied by feeble convection response due to the excessively cold conditions in the cold tongue, yielding insignificant IOD response. The pivotal role of La Niña's longitudinal position on the IOD's response is realistically reproduced by targeted pacemaker experiments, providing new insights into inter‐basin climate connections. Plain Language Summary: The tropical Indian Ocean usually witnesses a dipolar pattern of sea surface temperature (SST) anomalies, which is commonly referred to as the Indian Ocean Dipole (IOD). The IOD phenomenon has received much attention due to its profound global impacts, yet its seasonal prediction remains a large challenge for the climate community. The year‐to‐year variability of IOD has usually been linked to the El Niño–Southern Oscillation (ENSO), the predominant interannual climate variability in the tropical Pacific. The relationship between El Niño (i.e., warm phase of ENSO) and the intensity of IOD has been demonstrated in previous studies, which is shown to be dependent on the different El Niño flavors. In this study, we show that the longitudinal position of negative SST anomalies during La Niña events (i.e., cold phase of ENSO) in shaping their connection with IOD. Different from westward‐displaced La Niña events that can drive robust negative IOD events, eastward‐displaced La Niña events do not yield significant IOD anomalies due to the excessively cold conditions prevalent in the eastern Pacific cold tongue region. We highlight the importance of La Niña's longitudinal position in its teleconnection to other ocean basins and the associated regional climate anomalies. Key Points: The La Niña's regulation of Indian Ocean Oscillation (IOD) mainly depends on the La Niña longitudinal positionThe westward‐displaced La Niña events produce significant Walker Circulation anomalies, triggering robust negative IOD eventThe importance of La Niña longitudinal position on the IOD's response can be evidenced by the targeted pacemaker experiments [ABSTRACT FROM AUTHOR]

The Pacific Meridional Mode (PMM) has long been associated with extra‐tropical air‐sea coupling processes, which are thought to influence the development of El Niño‐Southern Oscillation (ENSO). Here we show that the PMM on seasonal to interannual timescales is closely associated with a newly proposed tropical mode known as the ENSO Combination mode (C‐mode), which arises from the nonlinear interaction between ENSO and the background annual cycle in the deep tropics. The PMM exhibits a remarkable resemblance with the C‐mode in atmospheric patterns, spectral characteristics, and local impacts. Based on a simple Hasselmann‐type model, we further demonstrate that the C‐mode‐related atmospheric anomalies can effectively drive PMM‐like sea surface temperature anomalies. As the C‐mode captures seasonally modulated ENSO characteristics, the seasonal‐to‐interannual PMM variability could naturally establish a connection with ENSO, thereby offering an alternative explanation for the observed relationship between PMM and ENSO. Plain Language Summary: Previous studies demonstrated that the Pacific Meridional Mode (PMM) is driven by the extratropical climate systems, which can impact the development of El Niño‐Southern Oscillation (ENSO) events in the tropics. In our study, we establish a close connection between the PMM on seasonal‐to‐interannual timescales and a tropical atmospheric mode arising from the nonlinear interaction between ENSO and the background annual cycle (the ENSO Combination mode, C‐mode). The C‐mode closely resembles the PMM in terms of atmospheric patterns and spectral characteristics. We further demonstrate that the C‐mode could drive a PMM‐like SST pattern in the tropical Pacific. Consequently, we suggest that the PMM on seasonal‐to‐interannual timescales has tropical origins, thereby offering an alternative explanation for the relationship between the conventional PMM index and the mature phase of ENSO. Key Points: The spatiotemporal features of Pacific Meridional Mode (PMM) on seasonal‐to‐interannual timescales exhibit a remarkable resemblance with those of the tropical combination modeC‐mode‐related atmospheric anomalies are capable of inducing PMM‐like sea surface temperature changes in the tropical PacificEl Niño‐Southern Oscillation (ENSO)'s interaction with the annual cycle manifests in PMM‐related air‐sea conditions, which explains the observed ENSO‐PMM linkage [ABSTRACT FROM AUTHOR]

El Niño–Southern Oscillation (ENSO) exhibits highly asymmetric temporal evolutions between its warm and cold phases. While El Niño events usually terminate rapidly after their mature phase and show an already established transition into the cold phase by the following summer, many La Niña events tend to persist throughout the second year and even reintensify in the ensuing winter. While many mechanisms were proposed, no consensus has been reached yet and the essential physical processes responsible for the multiyear behavior of La Niña remain to be illustrated. Here, we show that a unique ocean physical process operates during multiyear La Niña events. It is characterized by rapid double reversals of zonal ocean current anomalies in the equatorial Pacific and exhibits a fairly regular near-annual periodicity. Mixed-layer heat budget analyses reveal comparable contributions of the thermocline and zonal advective feedbacks to the SST anomaly growth in the first year of multiyear La Niña events; however, the zonal advective feedback plays a dominant role in the reintensification of La Niña events. Furthermore, the unique ocean process is identified to be closely associated with the preconditioning heat content state in the central to eastern equatorial Pacific before the first year of La Niña, which has been shown in previous studies to play an active role in setting the stage for the future reintensification of La Niña. Despite systematic underestimation, the above oceanic process can be broadly reproduced by state-of-the-art climate models, providing a potential additional source of predictability for the multiyear La Niña events. [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]

El Niño-Southern Oscillation (ENSO) sea surface temperature (SST) anomaly skewness encapsulates the nonlinear processes of strong ENSO events and affects future climate projections. Yet, its response to CO2 forcing remains not well understood. Here, we find ENSO skewness hysteresis in a large ensemble CO2 removal simulation. The positive SST skewness in the central-to-eastern tropical Pacific gradually weakens (most pronounced near the dateline) in response to increasing CO2, but weakens even further once CO2 is ramped down. Further analyses reveal that hysteresis of the Intertropical Convergence Zone migration leads to more active and farther eastward-located strong eastern Pacific El Niño events, thus decreasing central Pacific ENSO skewness by reducing the amplitude of the central Pacific positive SST anomalies and increasing the scaling effect of the eastern Pacific skewness denominator, i.e., ENSO intensity, respectively. The reduction of eastern Pacific El Niño maximum intensity, which is constrained by the SST zonal gradient of the projected background El Niño-like warming pattern, also contributes to a reduction of eastern Pacific SST skewness around the CO2 peak phase. This study highlights the divergent responses of different strong El Niño regimes in response to climate change. [ABSTRACT FROM AUTHOR]

Understanding the interaction between the tropical Pacific and Atlantic Oceans has challenged the climate community for decades. Typically, boreal summer Atlantic Niño events are followed by vigorous Pacific events of opposite sign around two seasons later. However, incorporating the equatorial Atlantic information to variabilities internal to the Pacific lends no significant additional predictive skill for the subsequent El Niño‐Southern Oscillation (ENSO). Here we resolve this conundrum in a physically consistent frame, in which the nascent onset of a Pacific event rapidly induces an opposite‐signed summer equatorial Atlantic event and the lead correlation of Atlantic over Pacific is a statistical artifact of ENSO's autocorrelation. This Pacific‐to‐Atlantic impact is limited to a short window around late spring due to seasonally‐amplified Atlantic atmosphere‐ocean coupling. This new frame reconciles the discrepancies between the observed and multi‐model simulated inter‐basin relationship, providing a major advance in understanding seasonally‐modulated inter‐basin climate connections as well as their predictability. Plain Language Summary: Previous studies interpreted the observed lead/lag relationship between Atlantic Niño/Niña and Pacific Niño/Niña sea surface temperature anomalies as evidence for a precursory role of the equatorial Atlantic on the development of El Niño–Southern Oscillation (ENSO) events. This study clearly demonstrates that this statistical relationship is not related to Atlantic‐to‐Pacific causality, but can rather be explained by seasonally modulated equatorial Atlantic's response to ENSO. We find that Pacific ENSO events drive equatorial Atlantic events rather than vice versa, and reconcile the apparent discrepancies between the observed and multi‐model simulated tropical Pacific/Atlantic relationship. Key Points: The lead correlation of the equatorial Atlantic over the Pacific is not related to Atlantic‐to‐Pacific causalityThe tropical Pacific/Atlantic interaction is consistent with the nascent onset of Pacific events driving the equatorial Atlantic eventsThe discrepancies between the observed and multi‐model simulated tropical Pacific/Atlantic relationship can be reconciled in this new frame [ABSTRACT FROM AUTHOR]

The impact of El Niño–Southern Oscillation (ENSO) on the North Atlantic Oscillation (NAO) has been controversially discussed for several decades, which exhibits prominent seasonality and nonstationarity. During early winter, there appears a positive ENSO-NAO relationship, while this relationship reverses its sign in late winter. Here, we show that this subseasonal variation in the ENSO-NAO relationship could be attributed to the different mechanisms involved in early and late winters. In early winter, the positive linkage between the ENSO and NAO could be simply understood as resulting from the changes in tropical Walker circulation and the associated atmospheric meridional circulation over the North Atlantic. In the following late winter, an opposite NAO-like response appears as the large-scale Pacific–North Atlantic teleconnection pattern fully establishes and evident sea surface temperature anomalies occur over the North Tropical Atlantic (NTA). We further show that the phase shift in NAO during ENSO late winter is largely contributed by the establishment of the ENSO-associated NTA SST anomaly via its excited convection in the subtropical Atlantic. The competing roles of mechanisms explain the subseasonal variation in the ENSO-NAO relationship from early to late winter, providing useful information for seasonal prediction over the North Atlantic–European region. [ABSTRACT FROM AUTHOR]

The interactions between El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are known to have great implications for global climate variability and seasonal climate predictions. Observational analysis suggests that the ENSO–IOD inter‐basin connection is time‐varying and related to the Atlantic Multidecadal Oscillation (AMO) with weakened ENSO–IOD relationship corresponding to AMO warm phases. A suite of Atlantic pacemaker simulations successfully reproduces the decadal fluctuations in ENSO–IOD relationship and its link to the AMO. The warm sea surface temperature (SST) anomalies associated with the AMO drive a series of Indo‐Pacific mean climate changes through tropical‐wide teleconnections, including the La Niña‐like mean SST cooling over the central Pacific and the deepening of mean thermocline depth in the eastern Indian Ocean. By modulating ocean–atmosphere feedback strength, those mean state changes decrease both ENSO amplitude and the Indian Ocean sensitivity to ENSO forcing, therefore decoupling the IOD from ENSO. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are two most important interannual climate signals exciting world‐wide climate and socioeconomic impacts. Their interactions are known to have great implications for global climate variability and seasonal climate predictions. However, the ENSO–IOD relationship is time‐varying and show prominent decadal fluctuations. Such changes in ENSO–IOD relationship are associated with the decadal variation in IOD teleconnections and predictability. The prediction skill of IOD is high when the ENSO–IOD relationship is strong, and vice versa. But it remains unknown what controls the decadal modulation of Indo‐Pacific inter‐basin connection. Our findings suggest that the Atlantic Multidecadal Oscillation (AMO) may modulate the ENSO–IOD inter‐basin connection. The AMO sea surface temperature anomalies drive a series of Indo‐Pacific mean climate changes through tropical‐wide teleconnections. Those mean state changes further influence both ENSO amplitude and the Indian Ocean sensitivity to ENSO forcing, thereby modulating the ENSO–IOD relationship. This study advances our understandings on inter‐basin and cross‐scale interactions in the climate system. Key Points: The inter‐basin connection between El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole is time‐varying and controlled by the remote Atlantic Multidecadal Oscillation (AMO)The AMO sea surface temperature anomalies drive a series of Indo‐Pacific mean climate changes through tropical‐wide teleconnectionsThe AMO induced mean state changes influence both ENSO amplitude and the Indian Ocean sensitivity to ENSO forcing [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]

This study demonstrates the main physical mechanism for the maintenance of the western North Pacific (WNP) anomalous anticyclone (WNPAC) during the El Niño decaying summer by analyzing the respective effects of the cold sea surface temperature (SST) anomalies in the WNP, the warm SST anomalies in the Indian Ocean (IO), and the El Niño and Southern Oscillation (ENSO) combination mode. We find that the WNPAC is usually accompanied by significant cold WNP SST anomalies in the El Niño mature winter and following spring, which almost disappear in the decaying summer and cannot explain the maintenance of the WNPAC in summer. The influence of the IO warm SST anomalies on the WNPAC exhibits conspicuous decadal differences. Before the 2000s, the IO warm SST anomalies play a role in the WNPAC maintenance through the response of the baroclinic atmospheric Kelvin wave; however, this effect cannot be evidently detected after the 2000s. This decadal difference may be related to changes in the decaying speed of ENSO events. In contrast to El Niño events before the 2000s, El Niño events after the 2000s decay more rapidly, and the associated tropical central-eastern Pacific SST features a La Niña-like condition in the El Niño decaying summer. Concomitantly, no significant warm SST anomalies appear over the tropical Indian Ocean, exerting a weak influence on the WNPAC. Relative to the cold WNP SST anomalies and warm IO SST anomalies, the ENSO combination mode, originating from the nonlinear interaction between ENSO and the annual cycle, has a relatively more stable relationship with the WNPAC during the El Niño decaying summer, which exhibits a crucial role in the maintenance of the WNPAC. Considering the persistence of the ENSO combination mode, the WNPAC and associated climate variability during the El Niño decaying summer can be skillfully predicted at least one season in advance based on the ENSO combination mode. [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated that the Atlantic multidecadal oscillation (AMO) could affect El Niño–Southern Oscillation (ENSO) through thermocline adjustment, with a stronger ENSO sea surface temperature (SST) amplitude during a negative AMO phase than during a positive phase. In this study, we find that the ENSO atmospheric anomaly amplitudes in the tropical Pacific during different AMO phases are not necessarily consistent with these ENSO SST changes. For El Niño episodes, the low-level wind and precipitation anomalies over the tropical Pacific in the boreal winter are more pronounced during the negative AMO phase than during the positive phase, corresponding well to the stronger SST anomalies. However, La Niña events during the negative AMO phase are accompanied by weaker atmospheric anomalies in the tropical Pacific, although their SST anomalies are stronger than those during the positive phase. We suggest that this mismatch between La Niña SST and atmospheric anomalies can be largely attributed to AMO decadal modulation. A positive AMO favors intensified trade winds and weakened precipitation in the central tropical Pacific by modifying Walker circulation. Therefore, when La Niña coincides with a positive AMO, the low-level easterly and negative precipitation anomalies are superimposed, which gives rise to stronger atmospheric perturbations. In contrast, under a negative AMO background, the atmospheric anomalies induced by La Niña anomalous SST are partly counteracted by the AMO remote decadal modulation, thereby resulting in weaker anomaly amplitudes. Here, we highlight that AMO decadal forcing needs to be considered when investigating ENSO atmospheric variabilities and related regional climate impacts. [ABSTRACT FROM AUTHOR]

Quasi‐decadal (QD) climate variability is detected in the tropical Pacific based on the recent 70 years of observations. This QD variability is identified in equatorial sea surface temperatures (SSTs), the pattern of which resembles the Central Pacific (CP) El Niño‐Southern Oscillation (ENSO) but extends further meridionally to the northeastern subtropical Pacific. Whereas equatorial upper‐ocean heat content and SSTs are in quadrature for ENSO, these two quantities are almost in phase on the QD timescale. Further analysis shows that nonlinear dynamical heating, primarily originating from strong El Niño events, tends to lead QD SSTs by a quarter of its dominant period (approximately 30 months) and shapes the dominant QD periodicity in observations. Our results suggest that the observed QD variability largely originates from ENSO nonlinearity and thus is fundamentally different from ENSO's oscillatory nature. Plain Language Summary: The climate variability of the tropical Pacific atmosphere‐ocean system profoundly influences the global weather and climate patterns on a wide range of temporal scales. The dynamical understanding of interannual climate variability dominated by the well‐known El Niño‐Southern Oscillation (ENSO) phenomena has established a promising way for the operational seasonal to interannual climate prediction. So far, nevertheless, there is still a lack of consensus for the understanding of tropical Pacific decadal climate variability despite the increasing desire for climatic information on longer timescales. In this article, we illustrate the evolution features and possible governing mechanisms of a prominent quasi‐decadal (QD) variability in the tropical Pacific with the utilization of an advanced spectrum approach and reliable observational data sets. The results show that this QD variability that makes up a substantial portion of tropical Pacific low‐frequency variance is largely equatorial originated from the ENSO nonlinearity, which is dynamically different from the oscillatory nature of the ENSO phenomenon. Our study has important implications for the decadal climate prediction and improving the representations of tropical Pacific decadal variability in climate models. Key Points: Observations show that CP‐ENSO‐like quasi‐decadal variability originates in the equatorial Pacific and extends to the off‐equatorial regionThe equatorial ocean heat content and sea surface temperature are in phase on quasi‐decadal timescales, whereas they are in quadrature for interannual ENSOEquatorial nonlinear dynamical heating is proposed as the dynamical origin of the quasi‐decadal variability [ABSTRACT FROM AUTHOR]

The El Niño–Southern Oscillation (ENSO) is known to influence the North Tropical Atlantic (NTA) sea surface temperature (SST) during its decaying spring, providing an essential source of NTA predictability. However, the positive ENSO–NTA SST relationship is absent during boreal winter, impeding accurate anticipation of wintertime NTA SST fluctuations. We find that this absence is mainly due to different NTA preconditions. Taking into account the previous summer NTA SST state as an initial condition, a robust relationship is established between ENSO and wintertime NTA SST variability. Furthermore, we show that the NTA initial conditions are in part traced to the ENSO preconditions. Besides, the simultaneous local atmospheric conditions play a role in shaping the initial NTA SST variabilities, especially in the developing summer of El Niño events. The understanding of the complicated ENSO–wintertime NTA SST relationship has implications for exploring trans‐basin climate interactions and associated climate predictability. Plain Language Summary: One of the most striking features of the North Tropical Atlantic (NTA) sea surface temperature (SST) variability is its pronounced dependency on the El Niño–Southern Oscillation (ENSO). The ENSO remote effects are felt in the NTA SST a few months after the mature phase. However, an absence of statistically significant linkage between wintertime NTA SST anomalies and ENSO largely limits our understanding of the wintertime climatic variations over the NTA sector. In this study, we show that a robust positive linkage exists between ENSO and NTA SST variability during winter in light of the NTA SST tendency, and the previous summer NTA SST state as an initial condition acts to explain the complicated ENSO–wintertime NTA SST relationship. We also discuss about the possible dynamical origins of the NTA initial conditions, which are attributable to the preceding ENSO conditions as well as the simultaneous local atmospheric conditions. Key Points: A robust linkage between ENSO and wintertime NTA SST variability could be established by considering the previous summer NTA SST stateThe NTA initial state is in part traced to the preceding ENSO conditions, and also partially attributable to the local disturbanceThe understanding of the ENSO–wintertime NTA SST relationship has implications for extending the seasonal predictability for the NTA SST [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]

The El Niño-Southern Oscillation (ENSO), the primary driver of year-to-year global climate variability, is known to influence the North Tropical Atlantic (NTA) sea surface temperature (SST), especially during boreal spring season. Focusing on statistical lead-lag relationships, previous studies have proposed that interannual NTA SST variability can also feed back on ENSO in a predictable manner. However, these studies did not properly account for ENSO's autocorrelation and the fact that the SST in the Atlantic and Pacific, as well as their interaction are seasonally modulated. This can lead to misinterpretations of causality and the spurious identification of Atlantic precursors for ENSO. Revisiting this issue under consideration of seasonality, time-varying ENSO frequency, and greenhouse warming, we demonstrate that the cross-correlation characteristics between NTA SST and ENSO, are consistent with a one-way Pacific to Atlantic forcing, even though the interpretation of lead-lag relationships may suggest otherwise. It has been suggested that sea surface temperatures in the North Tropical Atlantic exert strong influence on the evolution of the El Nino Southern Oscillation (ENSO). Here, the authors argue that observed statistics are fully consistent with ENSO driving climate variations in the Atlantic and not vice versa. [ABSTRACT FROM AUTHOR]

Many previous studies have shown that an Indian Ocean basin warming (IOBW) occurs usually during El Niño–Southern Oscillation (ENSO) decaying spring to summer seasons through modifying the equatorial zonal circulation. Decadal modulation associated with the interdecadal Pacific oscillation (IPO) is further investigated here to understand the nonstationary ENSO–IOBW relationship during ENSO decaying summer (July–September). During the positive IPO phase, significant warm sea surface temperature (SST) anomalies are observed over the tropical Indian Ocean in El Niño decaying summers and vice versa for La Niña events, while these patterns are not well detected in the negative IPO phase. Different decaying speeds of ENSO associated with the IPO phase, largely controlled by both zonal advective and thermocline feedbacks, are suggested to be mainly responsible for these different ENSO–IOBW relationships. In contrast to ENSO events in the negative IPO phase, the ones in the positive IPO phase display a slower decaying speed and delay their transitions both from a warm to a cold state and a cold to a warm state. The slower decay of El Niño and La Niña thereby helps to sustain the teleconnection forcing over the equatorial Indian Ocean and corresponding SST anomalies there can persist into summer. This IPO modulation of the ENSO–IOBW relationship carries important implications for the seasonal prediction of the Indian Ocean SST anomalies and associated summer climate anomalies. [ABSTRACT FROM AUTHOR]

Many previous studies have demonstrated that the boreal winters of super El Niño events are usually accompanied by severely suppressed Madden-Julian oscillation (MJO) activity over the western Pacific due to strong descending motion associated with a weakened Walker Circulation. However, the boreal winter of the 2015/16 super El Niño event is concurrent with enhanced MJO activity over the western Pacific despite its sea surface temperature anomaly (SSTA) magnitude over the Niño 3.4 region being comparable to the SSTA magnitudes of the two former super El Niño events (i.e., 1982/83 and 1997/98). This study suggests that the MJO enhanced over western Pacific during the 2015/16 super El Niño event is mainly related to its distinctive SSTA structure and associated background thermodynamic conditions. In comparison with the previous super El Niño events, the warming SSTA center of the 2015/16 super El Niño is located further westward, and a strong cold SSTA is not detected in the western Pacific. Accordingly, the low-level moisture and air temperature (as well as the moist static energy, MSE) tend to increase in the central-western Pacific. In contrast, the low-level moisture and MSE show negative anomalies over the western Pacific during the previous super El Niño events. As the MJO-related horizontal wind anomalies contribute to the further westward warm SST-induced positive moisture and MSE anomalies over the western tropical Pacific in the boreal winter of 2015/16, stronger moisture convergence and MSE advection are generated over the western Pacific and lead to the enhancement of MJO convection. [ABSTRACT FROM AUTHOR]

Australia suffered a long‐lasting extensive drought in 2019 with catastrophic wildfires creating about $4.4 billion damages, the worst record in the recent four decades. Concurrent with this extreme drought, the tropical Indo‐Pacific oceans exhibited an extraordinary combination of sea surface temperature (SST) anomalies, characterized by a Central‐Pacific (CP) El Niño event with westernmost location and the third strongest positive Indian Ocean Dipole (IOD) event. We here show evidence that this unique combination was responsible for the pan‐Australian drought as a westward located CP El Niño event and a strong positive IOD event can exacerbate precipitation reduction in northeastern and southern Australia, respectively. These continent‐wide extreme droughts could become more frequent over Australia in a warming world, considering projected increases in both CP El Niño and extreme positive IOD events superimposed on secular warming and drying trends. Plain Language Summary: Australia is the driest inhabited continent in the world, 70% of which is either arid or semiarid land. During 2019, Australia was ravaged by the worst drought in decades with devastating wildfires destroying around 3,000 homes and affecting about one billion animals. The physical mechanisms driving this extreme drought remains unclear. Concurrent with this long‐lasting continent‐wide drought, the Indo‐Pacific oceans showed pronounced sea surface temperature anomalies associated with a record westernmost‐located Central Pacific (CP) El Niño event and the third strongest Indian Ocean Dipole (IOD) event co‐occurring. Our observational diagnostics and numerical model experiments show that these two compounding events caused atmospheric circulation anomalies that were responsible for the extreme precipitation deficiencies over Australia. With global warming, both CP El Niño and extreme positive IOD events are projected to become more frequent, which hints to increased risk of the severe Australian droughts in the future. Key Points: Australia suffered a long‐lasting extensive drought in 2019, the worst record in the recent four decadesThis extreme drought can be well explained by combined effects of the westernmost‐located CP El Niño and third strongest positive Indian Ocean Dipole (IOD) eventsSevere Australian droughts could occur more frequently in a warming world with projected increases in both CP El Niño and extreme IOD events [ABSTRACT FROM AUTHOR]

The temperature of the subsurface water entrained into the surface mixed layer plays a key role in controlling the sea surface temperature (SST) and its interannual variability in the equatorial Pacific. In this paper, we combine a hyperbolic tangent function bounded by the warm pool SST and centered at the thermocline depth with a variable sharpness parameter to describe the time‐space evolutions of the subsurface temperature. Under simple approximations of the sharpness parameter, this concise expression becomes remarkably efficient in capturing the observed and climate‐model simulated subsurface temperature variability in terms of anomalies of the thermocline depth and SST of the El Niño‐Southern Oscillation (ENSO) phenomenon. The formulations for the subsurface temperature and thermocline sharpness developed in this work should be useful tools for evaluating and understanding the role of the thermocline feedback in ENSO behaviors in both theoretical and comprehensive climate models. Plain Language Summary: While significant advances have been made in our ability to understand, simulate, and predict the El Niño‐Southern Oscillation (ENSO), its remarkable complexity is still not well understood. The subsurface ocean temperature in the equatorial Pacific is known to play a key role in controlling sea surface temperature and its interannual variations. An accurate description of its relationship with thermocline depth variability is of essential importance to understanding ENSO's complex dynamics in mechanistic studies. In this paper, we develop a concise expression which captures the observed relationship between anomalies of the subsurface temperature and thermocline depth remarkably well. This expression should be useful for ENSO mechanistic model studies and diagnostic investigations into climate models' performances in their ENSO simulations. Key Points: The equatorial Pacific subsurface temperature is expressed as a hyperbolic tangent function of thermocline depth and sharpnessEffective parameterizations for the subsurface temperature are derived by simple approximations of the thermocline sharpnessThese parameterizations can be useful tools for evaluating the impacts of thermocline feedback on ENSO [ABSTRACT FROM AUTHOR]

Recent studies demonstrated the existence of a conspicuous atmospheric combination mode (C-mode) originating from nonlinear interactions between El Niño–Southern Oscillation (ENSO) and the Pacific warm pool annual cycle (AC). Here we find that the C-mode exhibits prominent decadal amplitude variations during the ENSO decaying boreal spring season. It is revealed that the Atlantic multidecadal oscillation (AMO) can largely explain this waxing and waning in amplitude. A robust positive correlation between ENSO and the C-mode is detected during a negative AMO phase but not during a positive phase. Similar results can also be found in the relationship of ENSO with 1) the western North Pacific (WNP) anticyclone and 2) spring precipitation over southern China, both of which are closely associated with the C-mode. We suggest that ENSO property changes due to an AMO modulation play a crucial role in determining these decadal shifts. During a positive AMO phase, ENSO events are distinctly weaker than those in an AMO negative phase. In addition, El Niño events concurrent with a positive AMO phase tend to exhibit a westward-shifted sea surface temperature (SST) anomaly pattern. These SST characteristics during the positive AMO phase are both not conducive to the development of the meridionally asymmetric C-mode atmospheric circulation pattern and thus reduce the ENSO/C-mode correlation on decadal time scales. These observations can be realistically reproduced by a coupled general circulation model (CGCM) experiment in which North Atlantic SSTs are nudged to reproduce a 50-yr sinusoidally varying AMO evolution. Our conclusion carries important implications for understanding seasonally modulated ENSO dynamics and multiscale climate impacts over East Asia. [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]

Previous studies argued that the Pacific Meridional Mode (PMM) impacts tropical cyclone (TC) genesis variability over the southeastern part of the western North Pacific (SE‐WNP). Here, we find that the statistical relationship between PMM and SE‐WNP TC genesis frequency is dominated by their co‐variability on decadal timescales. The decadal component of the PMM exhibits very similar temporal and spatial features to quasi‐decadal tropical Pacific sea surface temperature (SST) variability. The latter can affect SE‐WNP TC activity via changes in both zonal vertical wind shear and low‐level vorticity. In contrast, the interannual component of the PMM exhibits no statistically significant correlation with SE‐WNP TC genesis. Furthermore, observations show that both interannual and decadal variability of SE‐WNP TC activity are well correlated with the commonly used Niño3.4 El Niño‐Southern Oscillation index. Thus, equatorial Pacific SST variability is the dominant source of SE‐WNP TC activity predictability on different timescales. Key Points: PMM and WNP TC variability show a strong relationship on quasi‐decadal timescales but less so on interannual timescalesPMM and central Pacific ENSO‐like SST variability are indistinguishable on quasi‐decadal timescalesEquatorial Pacific SST variability impacts WNP TCs on both interannual and decadal timescales [ABSTRACT FROM AUTHOR]

Here we investigate the response of boreal spring precipitation over southern China (SPSC) to central Pacific (CP) El Niño based on observational datasets. While there is enhanced precipitation over southern China during the decaying boreal spring of eastern Pacific (EP) El Niño events, so far no clear precipitation response has been detected during the same decaying stage for CP El Niño composites. Here we show that around half of the CP El Niño events coincide with enhanced SPSC (wet CP El Niño), while the other half are accompanied by reduced SPSC (dry CP El Niño). These two types of CP El Niño events bear dramatically different evolution features in their respective tropical sea surface temperature anomaly (SSTA) patterns. Wet CP El Niño events are characterized by an SSTA longitudinal position confined to the tropical central-eastern Pacific. In contrast, dry CP El Niño events exhibit a clear westward propagation of SSTAs during their evolution, with maximum SSTAs located to the west of the date line after their mature phase. These different longitudinal positions of positive SSTAs during their decaying phase result in distinct meridional structures of the tropical Pacific convection anomalies as well as the ENSO combination mode (C-mode) response. An anomalous low-level anticyclone is evident over the western North Pacific during wet CP El Niño events during their decaying phase, while an anomalous cyclonic circulation is found for dry CP El Niño events. We emphasize that the impacts of CP El Niño on the SPSC depend crucially on the simultaneous zonal location of warm SSTAs in the tropical Pacific. [ABSTRACT FROM AUTHOR]

The relationship between the equatorial Pacific warm water volume (WWV) and El Niño–Southern Oscillation (ENSO) sea surface temperature (SST) has varied considerably on decadal timescales. These changes are strongly related to the occurrence frequency of central Pacific (CP) ENSO events. While both eastern Pacific (EP) and CP ENSO events show clear signatures of WWV recharge/discharge, their phase‐lag relationships between WWV and Niño3.4 SST are different. The WWV usually leads the Niño3.4 SST by two to three seasons during EP ENSO, while the lead time is reduced to one season during CP ENSO. The different phase‐lag relationships can be explained by distinct periodicities of the two ENSO types. Hence, ENSO regime changes associated with decadal predominance of either EP or CP ENSO events can give rise to decadal variations in the statistical WWV‐ENSO SST relationship. We emphasize the importance of identifying these different ENSO types and potentially different ENSO regimes to assess ENSO predictability. Key Points: Relationship between WWV and Niño3.4 SST varies on decadal timescales, corresponding to the occurrence frequency of central Pacific ENSOBoth eastern and central Pacific ENSO events show recharge/discharge signatures but with different WWV/Niño3.4 phase‐lag relationshipsThis difference can be explained by the existence of two different ENSO types characterized by distinct intrinsic periodicities [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]

Previous studies documented that a distinct southward shift of central Pacific low-level wind anomalies occurring during the ENSO decaying phase is caused by an interaction between the western Pacific annual cycle and El Niño-Southern Oscillation (ENSO) variability. The present study finds that the meridional movement of the central Pacific wind anomalies appears only during traditional eastern Pacific El Niño (EP El Niño) events rather than in central Pacific El Niño (CP El Niño) events in which sea surface temperature (SST) anomalies are confined to the central Pacific. The zonal structure of ENSO-related SST anomalies therefore has an important effect on meridional asymmetry in the associated atmospheric response and its modulation by the annual cycle. In contrast to EP El Niño events, the SST anomalies of CP El Niño events extend farther west toward the warm pool region with its climatological warm SSTs. In the warm pool region, relatively small SST anomalies are thus able to excite convection anomalies on both sides of the equator, even with a meridionally asymmetric SST background state. Therefore, almost meridionally symmetric precipitation and wind anomalies are observed over the central Pacific during the decaying phase of CP El Niño events. The SST anomaly pattern of La Niña events is similar to CP El Niño events with a reversed sign. Accordingly, no distinct southward displacement of the atmospheric response occurs over the central Pacific during the La Niña decaying phase. These results have important implications for ENSO climate impacts over East Asia, since the anomalous low-level anticyclone over the western North Pacific is an integral part of the annual cycle-modulated ENSO response. [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]

A severe drought struck southwest China during autumn 2009, which had a huge impact on productivity and the lives of the affected population. A nonconventional El Niño, the so-called warm pool (WP) El Niño, was supposed to be a principal factor of this strong autumn drought. In sharp contrast to a conventional El Niño, in the 2009 WP El Niño year the maximum sea surface temperature (SST) anomalies are confined to the central equatorial Pacific Ocean. Moreover, this WP El Niño was characterized by the relatively farther westward location and the strongest intensity among the WP El Niño events in the past 60 years. Observations and modeling studies both indicate that the rainfall deficits over southwest China are significantly influenced by the combined effects of the location and intensity of the WP El Niño. That is, the drought over southwest China tends to be more severe when the warming SST anomalies associated with the WP El Niño are located farther westward and are stronger. Therefore, the strong autumn drought over southwest China in 2009 can be largely attributed to the concurrent distinctive WP El Niño, which generates a strongly anomalous cyclone over the west North Pacific and leads to a serious reduction in rainfall over southwest China. The influence of the Indian Ocean warming on autumn rainfall over southwest China was also examined but seems to have little contribution to this drought. [ABSTRACT FROM AUTHOR]

The fidelity of coupled climate models simulating El Niño-Southern Oscillation (ENSO) patterns has been widely examined. Nevertheless, a systematical narrow bias in the simulated meridional width of the sea surface temperature anomaly (SSTA) of ENSO has been largely overlooked. Utilizing the preindustrial control simulations of 11 coupled climate models from phase 3 of the Coupled Model Intercomparison Project (CMIP3), it was shown that the simulated width of the ENSO SSTA is only about two-thirds of what is observed. Through a heat budget analysis based on simulations and ocean reanalysis datasets, it is demonstrated that the SSTA outside of the equatorial strip is predominantly controlled by the anomalous meridional advection by climatological currents and heat-flux damping. The authors thus propose a simple damped-advective conceptual model to describe ENSO width. The simple model indicates that this width is primarily determined by three factors: meridional current, ENSO period, and thermal damping rate. When the meridional current is weak, it spreads the equatorial SSTA away from the equator less effectively and the ENSO width thus tends to be narrow. A short ENSO period allows less time to transport the equatorial SSTA toward the off-equatorial region, and strong damping prevents expansion of the SSTA away from the equator, both of which lead to the meridional width becoming narrow. The narrow bias of the simulated ENSO width is mainly due to a systematical bias in weak trade winds that lead to weak ocean meridional currents, and partly due to a bias toward short ENSO periods. [ABSTRACT FROM AUTHOR]

The persistence barrier of sea surface temperature anomalies (SSTAs) in the North Pacific was investigated and compared with the ENSO spring persistence barrier. The results show that SSTAs in the central western North Pacific (CWNP) have a persistence barrier in summer: the persistence of SSTAs in the CWNP shows a significant decline in summer regardless of the starting month. Mechanisms of the summer persistence barrier in the CWNP are different from those of the spring persistence barrier of SSTAs in the central and eastern equatorial Pacific. The phase locking of SSTAs to the annual cycle does not explain the CWNP summer persistence barrier. Remote ENSO forcing has little linear influence on the CWNP summer persistence barrier, compared with local upper-ocean process and atmospheric forcing in the North Pacific. Starting in wintertime, SSTAs extend down to the deep winter mixed layer then become sequestered beneath the shallow summer mixed layer, which is decoupled from the surface layer. Thus, wintertime SSTAs do not persist through the following summer. Starting in summertime, persistence of summer SSTAs until autumn can be explained by the atmospheric forcing through a positive SSTAs-cloud/radiation feedback mechanism because the shallow summertime mixed layer is decoupled from the temperature anomalies at depth, then the following autumn-winter-spring, SSTAs persist. Thus, summer SSTAs in the CWNP have a long persistence, showing a significant decline in the following summer. In this way, SSTAs in the CWNP show a persistence barrier in summer regardless of the starting month. [ABSTRACT FROM AUTHOR]