Central-eastern Pacific Research Articles

Impact of the Winter Regional Hadley Circulation over Western Pacific on the Frequency of Following Summer Tropical Cyclone Landfalling in China

Abstract The poleward migration of tropical cyclone (TC) activity in recent years has been linked to the expansion of the Hadley circulation (HC). Here, we investigate the impact of the winter regional HC over the western Pacific (WPHC) on the frequency of following summer landfalling TC (LTC) in China. Results show that interannual variation of the LTC frequency has a very close connection with the northern WPHC edge (WPHCE). After removing the El Niño–Southern Oscillation signal, there still exists a significant correlation between them. When the winter WPHCE shifts poleward, the associated lower-level southwesterly (easterly) wind anomalies over the subtropical western Pacific (tropical central-eastern Pacific) induce sea surface temperature (SST) warming (cooling) anomalies therein via suppressing (enhancing) upward surface heat flux. In turn, the SST warming (cooling) excites an anomalous cyclonic (anticyclonic) circulation to its west via a Rossby wave response, thus maintaining the southwesterly (easterly) wind anomalies. In addition, the negative rainfall anomalies over the tropical central-eastern Pacific induced by negative SST anomalies can stimulate an anomalous intensive Walker circulation with anomalous upward motion around the tropical western Pacific. Through this positive air–sea interaction, the winter WPHCE signal would be preserved in the ocean and maintained to the succeeding summer, then favoring LTC genesis landward by decreasing the vertical wind shear and increasing the low-level vorticity and midlevel humidity. Meanwhile, anomalous midtropospheric easterly winds over the subtropics are favorable for steering more LTCs toward China’s coast. This study suggests that the winter WPHCE variation is a potential predictor for the prediction of the following summer LTC activity over China. Significance Statement Tropical cyclone (TC) is one of the most catastrophic high-impact weather events, which may cause great casualties and severe property losses over the coastal areas, particularly when it makes landfall. Previous research studies have related the poleward migration trend of TC locations to the Hadley circulation (HC) expansion. Compared to the long-term trend, the magnitude of the year-to-year change of the HC edge (HCE) is even larger, leading to a stronger impact on the TC activity. A recent study has suggested that the northern HCE over the western Pacific (WPHCE) in boreal winter exhibits a notable interannual variability. In this study, we reveal that the wintertime WPHCE has a very close connection with the landfalling TC (LTC) frequency over China in the following summer. After removing the El Niño–Southern Oscillation (ENSO) signal, there still exists a significant positive correlation between them. Observational evidence and numerical model experiments consistently confirm that this time-lagged association is attributable to the air–sea interaction processes in the tropical Pacific. Thus, the results of this study could provide an additional predictor besides ENSO to improve understanding of the LTC activity in China.

Nonlinear Response of Summertime Synoptic-Scale Disturbance Intensity over the Tropical Western North Pacific to ENSO Amplitude

Abstract El Niño–Southern Oscillation (ENSO) exhibits nonlinearity in its amplitude and impacts. This study investigates the dependence of summertime synoptic-scale disturbance (SSD) intensity over the tropical western North Pacific (TWNP) on the ENSO amplitude. A tendency of nonlinearity exists in the observed response of the TWNP SSD intensity to the amplitude of tropical central-eastern Pacific (CEP) sea surface temperature (SST) anomalies in boreal summer. Numerical experiments are conducted with an atmospheric general circulation model with linearly varying tropical CEP SST anomalies imposed to illustrate the nonlinearity exclusively induced by changes in the ENSO amplitude. A linear increase in the amplitude of El Niño–like SST anomalies results in a nonlinear enhancement of SSD intensity over the TWNP, manifested as the increase in SSD intensity at a rate larger than expected by linear response with an eastward shift. This is attributed to the nonlinear intensification of anomalous ascent over the TWNP induced by tropical convection response to positive tropical CEP SST anomalies and the nonlinear effect of anomalous convection on the synoptic-scale activity. In contrast, as La Niña–like SST anomalies increase linearly, the SSD intensity over the TWNP decreases at a rate slower than expected from a linear response and even reaches saturation with little longitudinal shift. Due to the thermodynamic control on the occurrence of deep convection in the tropics, enhanced negative SST anomalies do not induce additional changes in anomalous descent over the tropical CEP. Thus, the TWNP SSD intensity no longer decreases with further increase in tropical CEP cold SST anomalies. Significance Statement Synoptic-scale disturbances (SSDs) over the tropical western North Pacific (TWNP) play an important role in weather and climate over East and Southeast Asia. Impacts of those SSDs are contingent on their intensity. El Niño–Southern Oscillation (ENSO) has substantial impacts on weather and climate worldwide, including the SSDs over the TWNP. ENSO displays a diversity in amplitude, spatial pattern, and temporal evolution. The present study investigates the response of the TWNP SSD intensity to varying ENSO amplitude during boreal summer and reveals a distinctive nonlinear response of the TWNP SSD intensity to the amplitude of El Niño and La Niña, which has important implication for understanding the impacts of ENSO on climate over the TWNP and Asia.

Forecasting the El Niño type well before the spring predictability barrier

El Niño events represent anomalous episodic warmings, which can peak in the equatorial Central Pacific (CP events) or Eastern Pacific (EP events). The type of an El Niño (CP or EP) has a major influence on its impact and can even lead to either dry or wet conditions in the same areas on the globe. Here we show that the difference of the sea surface temperature anomalies between the equatorial western and central Pacific in December enables an early forecast of the type of an upcoming El Niño (p-value < 10−3). Combined with a previously introduced climate network-based approach that allows to forecast the onset of an El Niño event, both the onset and type of an upcoming El Niño can be efficiently forecasted. The lead time is about 1 year and should allow early mitigation measures. In December 2022, the combined approach forecasted the onset of an EP event in 2023.

Investigation of the consistency of changes in the wavelet phase characteristics of heliocosmic and climatic variables and changes in the components of the word water balance. Part 3

The relevance of research is due to the need to establish the true causes and patterns of changes in the hydrometric characteristics of the global water balance as a whole and its components, natural environments on Earth.&#x0D; Purpose of research: to establish patterns and causes of changes in the components of the global water balance on Earth: trends in the increase in water mass reserves, dynamics of the water balance of the South Seas of Europe, the Black Sea, dynamics of levels of the Indian Ocean, Western and Central Pacific Ocean, Eastern Pacific and Atlantic Oceans, level fluctuations of the World Ocean, changes in sea ice cover, river resources, reservoirs, variability of sea levels in the Arctic Ocean according to observations at coastal stations, changes in the balance of ice masses in glacial areas, changes in interseasonal thawing in the permafrost zones of the continent.&#x0D; Objects of research: time series of heliocosmic, climatic variables, hydrometric characteristics of the components of the global water balance.&#x0D; Methods of research: developed by the author, a method for interaction of observations of a variable or variables with groups of specified heliocosmic and climatic factors in the frequency and time domains, obtained using wavelet transformations of observations with the calculation of phase-frequency and phase-time characteristics in equal time intervals; comparative analysis of changes in the obtained phase characteristics of groups of variables with the calculation of their consistency matrices and the construction of graphs in the frequency and time domains.&#x0D; Main results of research: in the observed time intervals, in the trend of periodic changes in the reserves of water masses (glaciers, groundwater, lakes, reservoirs) in the frequency domain, significant negative unidirectional influences of changes in heliocosmic and multidirectional influences of climatic variables are observed; changes in the reserves of the World Ocean are more influenced by multidirectional changes in climatic factors compared to the unidirectional influences of heliocosmic factors; the frequency bands of changes in water mass reserves are less than the frequency bands of heliocosmic factors, due to the influence of climatic factors; The variability of water mass reserves on Earth is significantly and unidirectionally consistent with changes in evaporation, evaporation and precipitation on land, and in different directions with changes in cloudiness on the continents. Similar patterns are observed in changes in the water balance of the South Seas in Europe, the Black Sea, sea ice cover, river resources, reservoirs on land, ice mass balances in glacial areas, and interseasonal thawing in the permafrost zones of the continent. In the dynamics of the levels of the Indian, Western and Central Pacific, Eastern Pacific and Atlantic, fluctuations in the level of the World Ocean, significant multidirectional changes in levels in the frequency domain are observed on the effects of heliocosmic variables with the greatest influences of solar activity and long-term solar insolation, the Ap-index of geomagnetic activity, the parameter the ratio of plasma pressure to magnetic solar wind, changes in the ozone layer in the ionosphere, changes in the speed of rotation of the Earth, the influence of climate variables in the frequency domain with the greatest influences of solar radiation, precipitation, atmospheric processes, PTV is significant. The variability of sea levels in the Arctic Ocean, as observed at coastal stations, is characterized by significant multidirectional changes in the frequency domain due to the influence of heliocosmic and climatic factors, multidirectional mutual consistency of the component levels of the ocean seas, caused by anomalous changes in the Earths magnetic field in areas of the Northern Hemisphere. In changes in the phase-time characteristics of groups of factors, lattice structures of differently and unidirectional changes in variables are observed, characterizing the cyclical nature of climate change on Earth.

Possible factors responsible for the withdrawal process of the South Asian high during summer-to-winter transition

This study investigates the possible factors responsible for interannual variations in the withdrawal process of the South Asian high (SAH) during its summer-to-winter transition. The SAH is more obviously intensified from July to September and lasts longer over the Tibetan Plateau in late-withdrawal years (L_SAH) than early-withdrawal years (E_SAH). The L_SAH tends to appear during the stage when an El Niño has faded away and a La Niña begins to emerge, with warm SST anomalies occurring over the northern Indian Ocean and tropical northwestern Pacific, and cold anomalies existing across the equatorial central eastern Pacific. The influences of these anomalous SST variations on the SAH's withdrawal process take place primarily through two pathways. On the one hand, warm SST anomalies over the northern Indian Ocean and tropical western Pacific can warm the local tropospheric column above via a Matsunuo-Gill response to moisture adjustment and subsequently lead to an intensified and later withdrawal of the SAH. On the other hand, accompanied by warm SST anomalies, an abnormally intensified anticyclone is observed over the northwestern Pacific that shifts southwestwards and brings abundant rainfall to the region between the Yellow River and Yangtze River in July and August via southwesterly water vapor transport. Consequently, latent heating will generate positive geopotential height anomalies on its northwest side in the upper troposphere and ultimately result in an intensified and later withdrawal of the SAH. In addition, warm surface air temperature over the Tibetan Plateau in September also plays a significant role in lengthening the maintenance of the SAH over the Tibetan Plateau. However, the E_SAH tends be weak and lasts for a shorter period over the Tibetan Plateau. The E_SAH tends to occur together with a developing El Niño event, with cold SST anomalies existing over the northern Indian Ocean and tropical northwestern Pacific. Negative SST anomalies, together with a weak and northeastward-shifting subtropical western Pacific high, result in a weak SAH and quicken its withdrawal process from the Tibetan Plateau via cooling the local tropospheric column and reducing rainfall over East Asia, respectively.

El Nio southern oscillations effects on China

ENSO, known as the dominant interannual variation in the climate system, consists of two components: La Nia and El Nio. Earlier research indicates that ENSO is contributed to by red noise, while other studies attribute it to deterministic oceanic dynamical processes. The Bjerknes feedback amplifies the sea surface temperature anomaly, forming a cycle of 2-7 years through delayed negative feedback. ENSO not only significantly impacts the climate of the eastern equatorial Pacific but also influences mid-to-high latitudes by affecting equatorial atmospheric circulation. Additionally, ENSO has important effects on the western Pacific coastal region. For example, sea surface temperature anomalies during ENSO affect East Asian monsoon activity in subsequent years, thereby impacting the Chinese monsoon. Moreover, ENSO influences winter synoptic-scale temperature variability in China by affecting synoptic-scale transient vortices over Siberia. Furthermore, we discuss the similarities and discrepancies in the influence of Central Pacific and Eastern Pacific ENSO events on East Asia.

The mechanism of boreal summer SSTA phase-locking in the far eastern Pacific

In observations, the boreal winter El Niño—Southern Oscillation (ENSO) phase-locking phenomenon is evident in the central-eastern Pacific. In the far eastern equatorial Pacific (FEP) and South American coastal regions, however, the peak of sea surface temperature anomalies (SSTA) tends to occur in the boreal summer, with fewer winter peak events. By separating the direct ENSO forcing from the FEP SSTA, we found that the summer peak preference is contributed by the residual SSTA component, while the ENSO forcing provides only a small probability of winter peak. The dynamics of FEP SSTA phase-locking in observations and its biases in the climate models are investigated by adopting a linear stochastic-dynamical model. In observations, the summer phase-locking of FEP SSTA is controlled by the seasonal modulation of the SSTA damping process. In contrast, in the climate models the strength of FEP SSTA phase-locking is much smaller than observed due to the overly negative SSTA damping rate.

Varying Relationship between La Niña and SST Anomalies in the Kuroshio and Adjacent Regions during Boreal Winter: Role of the East Asian Winter Monsoon

Abstract The winter sea surface temperature (SST) anomalies in the Kuroshio and adjacent regions (KAR), which greatly influence the East Asian–North Pacific–North American climate, are closely related to El Niño–Southern Oscillation (ENSO). This SST relationship between the KAR and the equatorial eastern-central Pacific is widely assumed to be symmetric between El Niño and La Niña. Compared to previous studies indicating the significant and strong KAR warming during El Niño winters, this study indicates weakly negative KAR SST anomalies in the composite analysis for all La Niña events. Positive winter KAR SST anomalies unexpectedly appear in approximately half of La Niña events, which counteract negative SST anomalies in the rest of La Niña events. Further analysis suggests that the impact of La Niña on KAR SST anomalies is modulated by the East Asian winter monsoon (EAWM) during early winter. The weaker-than-normal EAWM offsets the anomalous northeasterly winds in the KAR induced by La Niña and then reinforces the KAR warming through warm oceanic advection. As for strong EAWM, it enhances the northeasterly winds to the west of an anomalous Philippine Sea cyclone associated with La Niña, leading to KAR cooling with more latent heat flux loss from the ocean and anomalous cold oceanic advection. Additionally, when the EAWM is independent of ENSO and is associated with the western Pacific pattern, it also can exhibit a pronounced influence on the KAR SST anomalies via the major processes of surface latent flux and horizontal heat advection in the ocean, accompanied by a change in Kuroshio transport. Significance Statement The interannual variability of SST from Philippine Sea to the Kuroshio and adjacent regions is considered an important forecast factor of the eastern China climate. Previous studies have reported a dipole SST pattern with Philippine Sea cooling and warming in the Kuroshio and adjacent regions during El Niño winter is dominant, and simply regarded that the effect of La Niña on western North Pacific SST anomalies was a mirror image of that of El Niño events. Here, we have found two distinctive SST patterns in the western North Pacific during La Niña winter. One type is the dipole SST pattern, characterized by Philippine Sea warming and by cooling in the Kuroshio and adjacent regions. The other type is the monopole SST pattern with uniform warming expanding from the Philippine Sea to the Kuroshio and adjacent regions. The dipole (monopole) SST pattern in western North Pacific is modulated by La Niña and a strong (weak) EAWM. Analysis shows that the above two SST patterns during La Niña winter are equally important during 1950–2021. This study has identified the influence of the EAWM independent of ENSO on the SST anomalies in the Kuroshio and adjacent regions.

A Trans-Season Out-of-Phase Relationship of Tropical Cyclogenesis between the Western North Pacific and South China Sea

Abstract The present study investigates the relationship of tropical cyclone (TC) genesis between the western North Pacific (WNP) and South China Sea (SCS) from 1979 to 2020. A significantly out-of-phase variation is found between spring [March–May (MAM)] TC genesis over the WNP and the following summer–fall [June–November (JJASON)] TC genesis over the SCS. More TCs over the WNP in MAM are followed by fewer TCs over the SCS in the succeeding JJASON. Composite analysis and numerical model experiments show that negative sea surface temperature (SST) anomalies during MAM in the tropical central-eastern Pacific (CEP) and southeastern Indian Ocean work together to induce a lower-level cyclonic circulation over the WNP, with the latter more important. The positive specific humidity and ascending motion favor the TC genesis over the WNP in MAM. In the following JJASON, the SST anomalies are reversed in the tropical CEP. The positive precipitation anomalies over the western-central Pacific induced by positive SST anomalies further stimulate an anomalous zonal overturning circulation with anomalous descending motion and boundary layer divergence over the SCS. In addition, the persistent negative SST anomalies around the Maritime Continent (MC) induce an anomalous anticyclone to the west. Both processes lead to negative genesis potential index (GPI) anomalies and thus inhibit the TC genesis over the SCS. This out-of-phase relationship of TC genesis between the WNP and SCS also exists when the El Niño–Southern Oscillation (ENSO) transition years are removed. This finding may be helpful to improve the seasonal prediction of the SCS TC activity over the peak TC season.

Impacts of the SSTs over Equatorial Central–Eastern Pacific and Southeastern Indian Ocean on the Cold and Rainy/Snowy/Icy Weather in Southern China

Impacts of the SSTs over Equatorial Central–Eastern Pacific and Southeastern Indian Ocean on the Cold and Rainy/Snowy/Icy Weather in Southern China

Dynamical Predictability of Leading Interannual Variability Modes of the Asian-Australian Monsoon in Climate Models

The dynamical prediction of the Asian-Australian monsoon (AAM) has been an important and long-standing issue in climate science. In this study, the predictability of the first two leading modes of the AAM is studied using retrospective prediction datasets from the seasonal forecasting models in four operational centers worldwide. Results show that the model predictability of the leading AAM modes is sensitive to how they are defined in different seasonal sequences, especially for the second mode. The first AAM mode, from various seasonal sequences, coincides with the El Niño phase transition in the eastern-central Pacific. The second mode, initialized from boreal summer and autumn, leads El Niño by about one year but can exist during the decay phase of El Niño when initialized from boreal winter and spring. Our findings hint that ENSO, as an early signal, is conducive to better performance of model predictions in capturing the spatiotemporal variations of the leading AAM modes. Still, the persistence barrier of ENSO in spring leads to poor forecasting skills of spatial features. The multimodel ensemble (MME) mean shows some advantage in capturing the spatiotemporal variations of the AAM modes but does not provide a significant improvement in predicting its temporal features compared to the best individual models in predicting its temporal features. The BCC_CSM1.1M shows promising skill in predicting the two AAM indices associated with two leading AAM modes. The predictability demonstrated in this study is potentially useful for AAM prediction in operational and climate services.

Impact of Westerly Wind Bursts (WWBs) on ENSO based on a Hybrid Coupled Model: Part II – ENSO Prediction

ABSTRACT With a westerly wind burst (WWB) parameterization scheme introduced into a hybrid coupled model (HCM), we investigated in Part 1 of this study the impact of WWBs on El Nino – Southern Oscillation (ENSO) simulation features, including asymmetry, phase locking, and diversity. In the second part, we investigate the impact of WWBs on ENSO prediction skills. To achieve this, two ensemble experiments, one with WWBs and one without WWBs, are performed to evaluate the predictions of sea surface temperature (SST) anomalies. The results show that both experiments can predict the SST anomalies in the equatorial central and eastern Pacific up to lead times of 12 months. The correlation coefficient between the model and observations shows that the WWB experiment produces better prediction skills than the experiment without WWBs, especially at lead times longer than four months during El Niño events. This result is consistent with the expectation that the WWB parameterization scheme plays an important role in describing physical processes, which is indicated in Part 1. We also presented a predictability analysis for Central Pacific (CP) and Eastern Pacific (EP) El Niño events. The prediction of both types of El Niño events is also improved by the WWB parameterization scheme at long lead times.

Weakened Impacts of the East Asia-Pacific Teleconnection on the Interannual Variability of Summertime Precipitation over South China since the Mid-2000s

The current study concentrates on the interdecadal shift in the interannual variability of summertime precipitation (IVSP) over South China (SC). Possible causes for the interdecadal shift are explored. The IVSP on a decadal time scale presents a significant weakening after the mid-2000s. The results show that the variances of the interannual precipitation variability over the SC region between 1993 and 2004 (hereafter S1) and 2005 and 2020 (hereafter S2) are 1.40 mm d−1 and 0.58 mm d−1, respectively. The variance of the IVSP has decreased by 58.6% since the mid-2000s. The current study reveals that the reduction in the IVSP over SC after the mid-2000s is prominently attributed to the weakened impact of the East Asia-Pacific (EAP) teleconnection. Before the mid-2000s, the interannual variation of the east-west movement of the western Pacific subtropical high was more significant. The warming over the tropical central-eastern Pacific (CEP) and cooling over the western Pacific (WP) suppress the Walker cell in the tropical Pacific and induce anomalous Hadley cell with its descending branch over the WP in the wet years. The anomalies of SST and atmospheric circulation show opposite phases in the dry years. This SSTA pattern enhances the northward propagation of the EAP teleconnection through a Rossby-wave-type response, which triggers an ascending/descending branch with active/suppressed convection over the northwestern Pacific in the wet/dry years. Therefore, the cooling WP and El Niño in its developing phase provide an ideal condition for more precipitation over SC. However, the above ocean–atmosphere interactions changed after the mid-2000s. The significant SST changes in the tropical CEP and the WP weaken the EAP teleconnection and atmospheric circulation anomalies over SC, leading to a significant interdecadal reduction in the IVSP over SC after the mid-2000s.

Approaches for estimating natural mortality in tuna stock assessments: Application to global yellowfin tuna stocks

The values used for natural mortality (M) are very influential in stock assessment models, affecting model outcomes and management advice. Natural mortality is one of the most difficult demographic parameters to estimate, and there is often limited information about the true levels. Here, we summarise the evidence used to estimate natural mortality at age for the four main stocks of yellowfin tuna (Indian, Western and Central Pacific, Eastern Pacific, and Atlantic Oceans), including catch curves, tagging experiments, and maximum observed age. We identify important issues for estimating M such as variation with age linked to size, maturity state or senescence, and highlight information gaps. We describe the history of natural mortality values used in stock assessments by the tuna Regional Fisheries Management Organisations responsible for managing each stock and assess the evidence supporting these values. In June 2021, an online meeting was held by the Center for the Advancement of Population Assessment Methodology (CAPAM), to provide advice and guidance on practices for modelling natural mortality in fishery assessments. Based on approaches presented and discussed at the meeting, we develop a range of yellowfin tuna natural mortality estimates for each stock. We also recommend future research to improve these estimates of natural mortality.

Recent Decadal Weakening of the Summertime Rainfall Interannual Variability Over Yellow-Huaihe River Valley Attributable to the Western Pacific Cooling

This study focuses on the interdecadal transition of summer rainfall interannual variability over the Yellow-Huaihe River valley (YHRV). It is found that the interannual variability of summer rainfall over the YHRV becomes significantly weakened after the late 2000s. In the decade before the late 2000s (hereafter P1), the variance is 1.28 mm d−1 and the mean value of interannual variability is 1.49 mm d−1. In the decade after the late 2000s (P2), the variance is 0.35 mm d−1 and the mean value of interannual variability is 0.62 mm d−1. The variance and mean value of summer rainfall interannual variability have considerably decreased by 72.7% and 58.4% since the late 2000s, respectively. The reasons for the aforementioned interdecadal transition are explored. The results show that the interdecadal shift of the East Asian jet stream axis and the western Pacific subtropical high (WPSH) are two major factors leading to the interdecadal transition of summer rainfall interannual variability over the YHRV. The northward shift of the jet stream axis, which resembles the Pacific-Japan (PJ) pattern, suppresses the development of summer rainfall over the YHRV. The interannual oscillation of WPSH in P1 is more significant than that in P2, which is consistent with the weakened interannual variability of summer rainfall in P2. Further analysis reveals that the modulation factors responsible for the above changes are El Niño in its decaying phase in the equatorial central eastern Pacific and the warming in the Maritime continent and the western Pacific (WP) in P1. While in P2, the primary modulation factor is La Niña in its developing phase in the equatorial central eastern Pacific. As for the mechanism of the growth of summer rainfall in P1 positive years, the WP warm pool and El Niño in its decaying phase strengthen the Walker circulation in the tropical Pacific and induce Hadley circulation, whose ascending branch over the YHRV provides a favorable condition for the development of rainfall. At the same time, the northward propagation of Rossby waves in response to the above SSTA pattern suppresses convective activities over the Northwestern Pacific (NWP), which indirectly intensifies the WPSH. As a result, the PJ pattern is triggered and provides a favorable large-scale circulation condition for the growth of rainfall.

Comparison of East Asian Summer Monsoon Simulation between an Atmospheric Model and a Coupled Model: An Example from CAS-ESM

In this study, the Chinese Academy of Sciences’ Earth System Model Version 2 (CAS-ESM2) and its atmospheric component were evaluated for the ability to simulate the East Asian summer monsoon (EASM), in terms of climatology and composites in El Niño decaying years (EN) and La Niña years (LN). The results show that the model can realistically simulate the El Niño Southern Oscillation (ENSO) annual cycle, the interannual variation, the evolution process, and the prerequisites of ENSO, but the trend of developing and decaying is faster than that of the observations. With regard to the climatological mean state in the EASM, the coupled model run can largely improve the precipitation and 850 hPa wind simulated in the atmospheric model. Moreover, the coupled run can also reduce the mid-latitude bias in the atmospheric model simulation. Composite methods were then adopted to examine performance in different phases of the ENSO, from a mature winter to a decaying summer. The atmospheric model can well reproduce the Western North Pacific Anomalous Anticyclone (WNPAC)/Western North Pacific Anomalous Cyclone (WNPC) during EN/LN well, but the westerly/easterly anomalies and the associated precipitation anomalies over the equatorial Central Eastern Pacific are somewhat overestimated. Compared with the atmospheric model, these anomalies are all underestimated in the coupled model, which may be related to the ENSO-related SST bias appearing in the Eastern Indian Ocean. Due to the ENSO and ITCZ bias in the historical simulations, the simulated ENSO-related SST and the precipitation anomaly are too equator-trapped in comparison with the observations, and the cold tongue overly extends westward. This limits the ability of the model to simulate ENSO-related EASM variability. For the subseasonal simulations, though atmospheric model simulations can reproduce the westward extension of the Western Pacific subtropic high (WPSH) in EN decaying summers, the eastward retreat of the WPSH in LN is weak. The historical simulations show limited improvement, indicating that the subseasonal variation in the EASM is still a considerable challenge for current generation models.

Wind-Driven Mechanisms for the Variations of the Pacific Equatorial Undercurrent Based on Adjoint Sensitivity Analysis

The maintenance and variation of the Pacific Equatorial Undercurrent (EUC) are thought to be controlled by the zonal pressure gradient force (ZPGF). However, a recent study found that a large-scale circulation associated with Rossby waves can also lead to EUC variation, implying that the structures and timing of the influential winds and the underlying wind-driven mechanisms need to be revisited. Here, we use the adjoint-sensitivity method to obtain the crucial winds that can most efficiently influence EUC variations. The obtained adjoint sensitivities (denoting the sensitive winds) are confined to 15°S–15°N and exhibit a funnel-shaped pattern with high symmetry about the equator. The remote winds, which occur 4 to 11 months prior, can lead to EUC variations at the basin scale; in contrast, the near-term winds (occurring not earlier than 4 months) lead only to local EUC variations. Accordingly, we find that wind-initiated equatorial Kelvin waves, equatorial Rossby waves, and the reflected waves at both the western and eastern boundaries superimpose onto each other to result in EUC variations. Specifically, when the travel time is longer than 4 months, the waves can form a negative-positive-negative sea surface height anomaly (SSHA) pattern between 15°S and 15°N in the central-eastern tropical Pacific, indicating the joint effect of both types of waves; they also form a positive SSHA in the western equatorial Pacific, indicating the dominance of the Kelvin wave therein. These mechanisms are complementary to the canonical ZPGF mechanism, which provide a clear theoretical basis for EUC variation studies.

Toward Understanding El Niño Southern-Oscillation’s Spatiotemporal Pattern Diversity

The El Niño Southern Oscillation (ENSO) phenomenon, manifested by the great swings of large-scale sea surface temperature (SST) anomalies over the equatorial central to eastern Pacific oceans, is a major source of interannual global shifts in climate patterns and weather activities. ENSO’s SST anomalies exhibit remarkable spatiotemporal pattern diversity (STPD), with their spatial pattern diversity dominated by Central Pacific (CP) and Eastern Pacific (EP) El Niño events and their temporal diversity marked by different timescales and intermittency in these types of events. By affecting various Earth system components, ENSO and its STPD yield significant environmental, ecological, economic, and societal impacts over the globe. The basic dynamics of ENSO as a canonical oscillator generated by coupled ocean–atmosphere interactions in the tropical Pacific have been largely understood. A minimal simple conceptual model such as the recharge oscillator paradigm provides means for quantifying the linear and nonlinear seasonally modulated growth rate and frequency together with ENSO’s state-dependent noise forcing for understanding ENSO’s amplitude and periodicity, boreal winter-time phase locking, and warm/cold phase asymmetry. However, the dynamical mechanisms explaining the key features of ENSO STPD associated with CP and EP events remain to be better understood. This article provides a summary of the recent active research on the dynamics of ENSO STPD together with discussions on challenges and outlooks for theoretical, diagnostic, and numerical modeling approaches to advance our understanding and modeling of ENSO, its STPD, and their broad impacts.

A tripole winter precipitation change pattern around the Tibetan Plateau in the late 1990s

Classical monsoon dynamics considers the winter/spring snow amount on the Tibetan Plateau (TP) as a major factor driving the East Asian summer monsoon (EASM) for its direct influence on the land–sea thermal contrast. Actually, the TP snow increased and decreased after the late 1970s and 1990s, respectively, accompanying the two major interdecadal changes in the EASM. Although studies have explored the possible mechanisms of the EASM interdecadal variations, and change in TP snow is considered as one of the major drivers, few studies have illustrated the underlying mechanisms of the interdecadal changes in the winter TP snow. This study reveals a tripole pattern of change, with decreased winter precipitation over the TP and an increase to its north and south after the late 1990s. Further analyses through numerical experiments demonstrate that the tropical Pacific SST changes in the late 1990s can robustly affect the winter TP precipitation through regulating the Walker and regional Hadley circulation. The cooling over the tropical central-eastern Pacific can enhance the Walker circulation cell over the Pacific and induce ascending motion anomalies over the Indo-Pacific region. These anomalies further drive descending motion anomalies over the TP and ascending motion anomalies to the north through regulating the regional Hadley circulation. Therefore, the positive–negative–positive winter precipitation anomalies around the TP are formed. This study improves the previously poor understanding of TP climate variation at interdecadal timescales.摘要在20世纪70年代和90年代末, 伴随着东亚夏季风的两次主要年代际变化, 高原积雪分别显著增加和减少. 尽管很多学者研究了东亚夏季风年代际变化的可能机制, 高原积雪变化也被认为是主要因素之一, 但是关于高原冬季积雪本身发生年代际变化的潜在机制尚鲜有研究. 本文揭示了20世纪90年代末高原及周边冬季降水的三极子变化特征: 高原主体上空主要为降水减少, 其南北两侧区域降水增加. 数值试验结果表明, 热带太平洋海温变化可以通过调节沃克环流和局地哈德莱环流, 对上述三极子降水变化型态产生显著影响.

ENSO Diversity Simulated in a Revised Cane-Zebiak Model

The El Niño-Southern Oscillation (ENSO) phenomenon features rich sea surface temperature (SST) spatial pattern variations dominated by the Central Pacific (CP) and Eastern Pacific (EP) patterns during its warm phase. Understanding such ENSO pattern diversity has been a subject under extensive research activity. To provide a framework for unveiling the fundamental dynamics of ENSO diversity, an intermediate coupled model based on the Cane-Zebiak-type framework, named RCZ, is established in this study. Compared with the original Cane-Zebiak model, RCZ consists of revised model formulation and well-tuned parameterization schemes. All model components are carefully validated against the observations via the standalone mode, in which the observed anomalous SST (wind stress) forcing is prescribed to drive the atmospheric (oceanic) component. The superiority of RCZ’s model components over those in the original Cane-Zebiak model is evidenced by their better performance in simulating the observations. Coupled simulation with RCZ satisfactorily reproduces aspects of the observed ENSO characteristics, including the spatial pattern, phase-locking, amplitude asymmetry, and, particularly, ENSO diversity/bi-modality. RCZ serves as a promising tool for studying dynamics of ENSO diversity as it resolves most of the relevant processes proposed in the literature, including atmospheric nonlinear convective heating, oceanic nonlinear dynamical heating, and the ENSO/westerly wind burst interaction.