Pattern Of Sea Surface Temperature Anomalies Research Articles

Climatic influence of the Antarctic ozone hole on the East Asian winter precipitation

The Antarctic ozone hole exerts a substantial impact on the climate of the Southern Hemisphere, yet research exploring its potential influence on the Northern Hemisphere climate is limited. This study unveils a significant positive relationship between interannual variations of Antarctic total column ozone (TCO) during September–October and East Asian precipitation in the subsequent boreal winter. Specifically, ~10% of the East Asian winter precipitation variability is attributed to Antarctic TCO during September–October. Both observational data and model results indicate that the increased Antarctic TCO during September–October triggers a simultaneous meridional southern Indian Ocean tripole sea surface temperature anomalies (SSTA) through the negative phase of the Southern Annular Mode. This SSTA pattern persists from September–November through the boreal winter, subsequently weakening the local-scale zonal-vertical circulation anomalies in the Indian Ocean. The process leads to positive outgoing longwave radiation (OLR) anomalies over the southern Marine Continent. As a result, the linear response of wind anomalies at 850 hPa over East Asia to the OLR-induced diabatic heating anomalies exhibits southwesterlies, as demonstrated by a linear baroclinic model. These anomalous winds facilitate the transport of abundant moisture from the tropics to East Asia, favoring the formation of winter precipitation. We employ the Specified-Chemistry version of the Whole Atmosphere Community Climate Model to validate that the increase of September–October Antarctic ozone substantially enhances East Asian precipitation during boreal winter. Importantly, the relationship between Antarctic ozone and East Asian winter precipitation is found to be independent of El Niño-Southern Oscillation and the Indian Ocean Dipole Mode. Our findings provide a fresh insight into the prediction of the East Asian winter precipitation.

Contribution of El Niño Southern Oscillation (ENSO) Diversity to Low‐Frequency Changes in ENSO Variance

AbstractEl Niño Southern Oscillation (ENSO) diversity is characterized based on the longitudinal location of maximum sea surface temperature anomalies (SSTA) and amplitude in the tropical Pacific, as Central Pacific events are typically weaker than Eastern Pacific events. SSTA pattern and intensity undergo low‐frequency modulations, affecting ENSO prediction skill and remote impacts, and resulting in low‐frequency changes in ENSO variance. Yet, how different ENSO types contribute to these decadal variance changes remains unclear. Here, we decompose the low‐frequency changes of ENSO variance into contributions from ENSO diversity categories. We propose a fuzzy clustering of monthly SSTA to allow for non‐binary event category memberships, where each event can belong to different clusters. Our approach identifies two La Niña and three El Niño categories and shows that the major shift of ENSO variance in the mid‐1970s was associated with an increasing likelihood of strong La Niña and extreme El Niño events.

Relationship between the AMOC and the Multidecadal Variability of the Midlatitude Southern Indian Ocean

Abstract Air–sea coupling system in the southwestern Indian Ocean (SWIO; 35°–55°S, 40°–75°E) exhibits predominant multidecadal variability that is the strongest during austral summer. It is characterized by an equivalent barotropic atmospheric high (low) pressure over warm (cold) sea surface temperature (SST) anomalies and a poleward (equatorward) shift of the westerlies during the positive (negative) phase. In this study, physical processes of this multidecadal variability are investigated by using observations/reanalysis and CMIP6 model simulations. Results suggest that the multidecadal fluctuation can be explained by the modulation of the Atlantic meridional overturning circulation (AMOC) and the local air–sea positive feedback in the SWIO. In both observations/reanalysis and CMIP6 model simulations, the AMOC fluctuation presents a significantly negative correlation with the multidecadal SST variation in the SWIO when the AMOC is leading by about a decade. The mechanisms are that the preceding AMOC variation can cause an interhemispheric dipolar pattern of SST anomalies in the Atlantic Ocean. Subsequently, the SST anomalies in the midlatitudes of the South Atlantic can propagate to the SWIO by the oceanic Rossby wave under the influence of the Antarctic Circumpolar Current (ACC). Once the SST anomalies reach the SWIO, these SST anomalies in the oceanic front can affect the baroclinicity in the lower troposphere to influence the synoptic transient eddy and then cause the atmospheric circulation anomaly via the eddy–mean flow interaction. Subsequently, the anomalous atmospheric circulation over the SWIO can significantly strengthen the SST anomalies through modifying the oceanic meridional temperature advection and latent and sensible heat flux.

Quantifying the dynamic and synergistic effects of low and mid-latitudes signals on regional extreme climate events

Unlike previous studies solely focused on the influences of low or mid-latitudes signals on regional extreme events, this paper used the three-pattern decomposition of global atmospheric circulation (3P-DGAC), taking the 2018 hot summer in East Asia for example, to quantitatively reveal the dynamic and synergistic effects of low and mid-latitudes signals on regional extreme events. By analyzing the anomalous three-dimensional structure of atmospheric circulations in 2018 hot summer, we discovered that the local meridional circulation at low latitudes and zonal circulation at mid-latitudes both produced abnormal downdrafts in East Asia, which were consistent with the anomalous anticyclone in this region. Then, we investigated the relative vorticity advection combined with 3P-DGAC to uncover the dominant circulation patterns, and found the horizontal circulation at mid-latitudes and local meridional circulation at low latitudes were the major contributor to the anomalous anticyclone, accounting for 56% and 44% respectively. This result indicated the 2018 hot summer might be influenced by the synergistic effect of low and mid-latitudes signals. Finally, by investigating sea surface temperature anomaly (SSTA) and mid-latitudes wave train, we considered the tripole-like SSTA pattern in the North Atlantic and positive SSTA in Kuroshio and its extension area (K-KE) as the main causes of horizontal circulation anomalies, and the anomalous meridional circulation was related to the extremely late onset of South China Sea summer monsoon (SCSSM). This study provided a novel way to quantitatively study synergistic effects of low and mid-latitudes signals on regional extreme events.

Hot and dry compound events in South America: present climate and future projections, and their association with the Pacific Ocean

Compound hot and dry events can cause greater impacts than those generated by individual extreme events. Understanding the physical mechanisms that lead to their development is particularly important for an early warning. The aim of this study is to assess the ability of global climate models (GCMs) to simulate hot/dry compound events in South America (SA) during the historical period 1979–2014, in comparison with observational and reanalysis datasets. Additionally, this work seeks to investigate the potential changes in these events under two future climate scenarios for the period 2065–2100. Furthermore, we analyze the spatial patterns of sea surface temperature anomalies (SSTA) in the Pacific Ocean associated with these events in tropical and extratropical SA. In the historical period, reanalysis tends to overestimate the number of hot/dry events, while the ensemble median of GCMs performs better than the individual ones. The future projections under the high emissions scenario show longer heat waves, but a low model agreement about the number of compound events in tropical SA. For southern SA, an increase in the annual frequency of compound events is projected, and more than two hot/dry events per year are expected to occur relative to the 1979–2014 baseline. Finally, we find that compound events in tropical SA are favored during the El Niño phase, even though two other SSTA patterns have gained prominence in recent years. In southern SA, hot/dry events are associated with the negative phase of the Pacific Decadal Oscillation and the La Niña phase.

Southern Annular Mode: A New Factor Impacts the East African Short Rains after the Early 1990s

Abstract It is well known that the Indian Ocean dipole (IOD) and El Niño–Southern Oscillation (ENSO) are closely related to rainfall variability over East Africa. This study found that the preceding August–October (ASO) Southern Annular Mode (SAM) can influence East African rainfall from October to December [i.e., East African short rains (EASR)], with a strong and significant relationship during the period of 1992–2019 than 1971–91. This is likely a result of the changes in SAM variations. It is further revealed that, during 1992–2019, the ASO SAM, via air–sea interactions, can excite meridional dipole sea surface temperature anomalies (SSTA) and warm SSTA over the southern Indian and Atlantic Oceans, respectively. In contrast, during 1971–91, the SAM induces warm SSTA over the southern Indian Ocean. Further analysis indicated that these ASO SAM-related SSTA patterns persist through the following October–December and exert a contrasting influence on the EASR. Based on the possible mechanism, the enhanced influence of SAM on EASR appears to be achieved through significant meridional circulation in the Indian Ocean basin, with anomalous sinking motion in the extratropics and the rising motion in the tropical Maritime Continent (MC) region. Correspondingly, strong convection is enhanced over the MC, altering zonal vertical circulation. This process results in anomalous westerlies in the tropical Indian Ocean, which reduces the atmospheric moisture, weakens convection, and suppresses rainfall over the central-western Indian Ocean, including the study region. Further analysis suggests that the prediction skill of the EASR related to the SAM, ENSO, and IOD was enhanced during 1992–2019. Significance Statement Several studies have investigated the influence of tropical climate anomalies, such as El Niño–Southern Oscillation and the Indian Ocean dipole, on the rainfall variability over East Africa. The region continues to be plagued by numerous climate-related disasters, including flooding and drought, and the associated varying impacts have been detrimental. As such, there is a need to understand further the factors and associated physical processes that cause such events and improve forecasting accuracy. This research emphasizes the role of the Southern Annular Mode (SAM), finding a strengthened lagged relationship with East African rainfall in recent decades. This implies that changes in the SAM should be considered in understanding present and future rainfall variability in the region.

Impacts of the extratropical North Pacific on boreal summer Arctic circulation

A distinct characteristic of the Arctic summer atmospheric circulation is the anomalous anticyclonic circulation centered over the Arctic Ocean associated with significant Arctic warming. Previous studies have related the underlying mechanisms to the earlier spring Eurasian snowmelt and the tropical Pacific forcing. Here, the authors show that the Arctic summer anomalous anticyclonic circulation is significantly related to the extratropical North Pacific sea surface temperature anomalies (SSTAs) in spring, indicating a teleconnection from the extratropical North Pacific to the Arctic. The SSTA pattern is characterized by warm anomalies in the midlatitudes of the extratropical North Pacific surrounded by significant cold anomalies, resembling the negative phase of the Pacific Decadal Oscillation (PDO) but without significant signals in the tropics (referred to as the negative PDO-like pattern). This negative PDO-like pattern in May can stimulate a Rossby wave propagating from the Bering Sea to the Arctic and lead to an anomalous Arctic anticyclone which can be sustained during summer. Meanwhile, the negative PDO-like pattern can persist to summer and induce an anomalous surface low pressure over the Bering Sea in summer. This anomalous surface low causes anomalous rising motions and induces upper-level divergence anomalies which further intensify the summer Arctic anticyclone. The upper-level tropospheric Arctic anticyclone can force anomalous adiabatic descent over the Arctic and sub-Arctic, leading to significant adiabatic heating in the Arctic. As a result, a significant warming emerges over the Arctic with the center located in the middle troposphere. The connection between the negative PDO-like SSTAs and the summer Arctic anticyclone has been confirmed by numerical experiments.

Effects of Equatorial Ocean Current Bias on Simulated El Niño Pattern in CMIP6 Models

AbstractThis study utilized the Coupled Model Intercomparison Project Phase 6 (CMIP6) models to examine the simulations of equatorial ocean currents and explore their substantial influences on the systematic bias of westward‐extended sea surface temperature anomalies (SSTA) pattern during El Niño. The results show that models simulate an excessive westward ocean current field over the equatorial central Pacific in the mean state. It tends to suppress the equatorial eastward ocean current anomalies with their maximum centering over the equatorial western Pacific in the El Niño developing phase. As a consequence, an overestimated zonal advective feedback toward the maritime continent exists, subsequently inducing the biased westward extension of SSTA pattern. Our results show that the mean‐state performance of equatorial ocean currents plays a key role on simulations of El Niño evolution in CMIP6 models.

Formation Mechanism of the ENSO-Independent Summer Western North Pacific Anomalous Anticyclone

Abstract The western North Pacific anomalous anticyclone (WNPAC) is the key circulation modulating the East Asian summer climate. In this study, the formation mechanism of the summer WNPAC that is independent of El Niño–Southern Oscillation (ENSO) is investigated. Although ENSO has a significant relationship with WNPAC, except for the super El Niño years, the WNPAC index remains almost unchanged after removing ENSO’s impact, suggesting the possibility of other origins of the WNPAC apart from ENSO. An Atlantic-to-Pacific two-step mechanism is proposed for the formation of ENSO-independent summer WNPAC. In boreal spring, diabatic heating induced by the positive sea surface temperature anomalies (SSTAs) over the tropical Atlantic could stimulate a stationary equivalent barotropic Rossby wave train that travels across the Eurasian continent and ends in the tropical North Pacific. At the end of the Rossby wave train, the lower-level anomalous anticyclone advects negative moist enthalpy into the equator, which suppresses the local convection over the tropical North Pacific and equatorial central Pacific, and thus triggers the lower-level equatorial easterly anomaly to its west. During boreal summer, the lower-level easterly anomaly leads to the zonal dipole SSTA pattern with a negative center in the tropical central Pacific and a positive one in the Maritime Continent. Then, this dipole SSTA pattern over the Pacific exerts a relaying effect that further reinforces and westward shifts the dipole convection anomaly pattern, generating the WNPAC as a Gill-type response. This study underpins the independent role of Atlantic oceanic forcing through the extratropical route in the formation of ENSO-independent summer WNPAC. Significance Statement By linearly removing ENSO’s impact on summer monthly sea surface temperature anomalies (SSTAs), this study explores the sources of summer western North Pacific anomalous anticyclone (WNPAC) apart from ENSO. It is found that the high correlation between WNPAC and ENSO largely depends on the super El Niño events, and the origin of the ENSO-independent WNPAC is rooted in the tropical Atlantic oceanic forcing. The positive tropical Atlantic SSTAs in spring can independently induce atmospheric teleconnection from the North Atlantic to the North Pacific through the extratropical route, which triggers anomalous easterlies over the equatorial Pacific via suppressed local convection caused by the dry air advection over the North Pacific. The equatorial easterlies enhance the dipole anomalous SST and convection pattern over the Maritime Continent and tropical western/central Pacific in summer, which finally stimulates the WNPAC. The result emphasizes the independent role of tropical Atlantic SSTAs in the formation of summer WNPAC, thus providing a new perspective for the seasonal prediction of East Asian summer climate.

The Influences of Atlantic Sea Surface Temperature Anomalies on the ENSO-Independent Interannual Variability of East Asian Summer Monsoon Rainfall

Abstract As the most dominant tropical climate mode on the interannual time scale, El Niño–Southern Oscillation (ENSO) is suggested to significantly influence the interannual variation of East Asian summer monsoon rainfall (IEASMR). However, the leading mode of IEASMR remains almost untouched when the impacts of preceding ENSO events are linearly removed, suggesting the existence of alternative impact factors and predictability sources of IEASMR. After removing the impact of ENSO, the sea surface temperature anomalies (SSTAs) over both the tropical Atlantic and extratropical North Atlantic are found to be related to IEASMR through atmospheric teleconnections. Positive SSTA over the tropical Atlantic could induce tropical diabatic heating, which triggers an equivalent barotropic Rossby wave train emanating from the Atlantic, going across the Eurasian continent, and ending with a cyclonic anomaly over northeast Asia. The tropical diabatic heating could also induce western North Pacific anomalous anticyclone via tropical routes. The dipole SSTA pattern with cooling in the west and warming in the east over the extratropical North Atlantic induces local circulation anomalies through heat flux exchange, which could further perturb a Rossby wave train with a cyclonic anomaly over northeast Asia, thus modulating IEASMR. Numerical experiments with prescribed atmospheric heating associated with Atlantic SSTAs could realistically reproduce these teleconnections toward IEASMR. By adding the predictability sources of Atlantic SSTAs, the seasonal hindcast skills of IEASMR could be significantly improved over both the tropical western North Pacific and subtropical land regions such as central China and Japan. Significance Statement The purpose of this article is to identify the alternative impact factors of the interannual variation of East Asian summer monsoon rainfall (IEASMR), after removing the impact of ENSO, considering the limited contribution of ENSO to the variances of IEASMR. Here, we find that the Atlantic sea surface temperature anomalies (SSTAs) play a considerable role in driving IEASMR. The impact of ENSO on IEASMR is mainly confined to the tropical western North Pacific, while the Atlantic SSTAs influence rainfall over subtropical East Asia and the tropical western North Pacific through both tropical and extratropical routes. The results unravel the important roles of Atlantic SSTAs in driving ENSO-independent IEASMR, which will have a large implication for the seasonal prediction of East Asian summer climate.

Distinctive Rainfall Evolutions in East Asia between Super and Regular El Niño Events during Their Decaying Summers

Abstract While enhanced rainbands progressed northward in East Asia from June to August during the regular El Niño decaying summer, strengthened rainbands were only observed in the earlier summer and disappeared in August in the super El Niño composite. The cause of this distinctive feature is investigated through a combined observational and modeling study. The relative roles of the mean state and anomalous heating in causing the northward progression in the regular El Niño group are assessed through idealized numerical experiments. The result shows that the monthly evolving mean state is more important, while the anomalous forcing also plays a role. The distinctive rainfall feature in the super El Niño composite was primarily contributed by the 1982/83 and 2015/16 events, whereas the rainband evolution in 1998 resembled the regular El Niño composite. The cause of the different rainfall pattern in August among the super El Niño events is further investigated. A marked difference exists in the tropical sea surface temperature anomaly (SSTA) and associated anomalous precipitation patterns. A low-level cyclonic (anticyclonic) anomaly appeared south of Japan in August 1983 and 2016 (1998), inducing northerly (southerly) anomalies and thus suppressed (enhanced) rainfall in eastern China. Whereas an anomalous anticyclone in the western North Pacific (WNP) is a typical response to an El Niño during its mature and decaying phases, the formation of a cyclonic anomaly in the WNP resulted from anomalous enthalpy advection associated with the eastward retreat of an anomalous anticyclone triggered by a local cold SSTA belt in August 1983 and from a Pacific meridional mode (PMM)-like positive SSTA pattern in August 2016.

A Positive Low Cloud–Sea Surface Temperature Feedback in the East Asian Marginal Seas during El Niño Mature Winters and Their Following Spring

Abstract During the winter and subsequent spring of an El Niño year, the East Asian marginal sea (EAMS) exhibits positive sea surface temperature anomalies (SSTAs) and fewer low clouds, while the western North Pacific experiences negative SSTAs. In this study, we suggest that the positive SSTAs in EAMS are maintained by a positive low cloud–SST feedback. In neutral winters and springs, the EAMS is covered by low clouds, which have a cooling effect on surface temperatures. During an El Niño year, a western North Pacific anomalous anticyclone is established, and along its northwestern flank, there are favorable conditions for convergence of moisture and weaker surface latent heat flux over the EAMS. Once a positive SSTA has been established, a further reduction of turbulent mixing results in less low cloud and enhanced solar heating of the ocean mixed layer; this reinforces and maintains both the positive SSTA and the lack of low cloud via a positive feedback mechanism. The concurrent increase of low cloud–SST feedback and anticyclone circulation strengths is evident in the coupled-model simulations from phase 6 of the Coupled Model Intercomparison Project. Furthermore, sensitivity experiments, performed with the atmospheric components of Community Earth System Model (CESM2), reveal that a positive SSTA helps to maintain the western North Pacific anomalous anticyclone. Four pacemaker-coupled experiments by CESM2, with sea surface temperature in the equatorial Pacific restored to the observational anomalies plus the model climatology and altered low cloud feedback over EAMS, suggest that the low cloud–SST feedback results in more than the maintenance of a positive SSTA over the EAMS: the positive feedback is also a previously overlooked mechanism for the maintenance of the western North Pacific anomalous anticyclone. Significance Statement The East Asian marginal sea (EAMS) and western North Pacific are important areas that bridge El Niño and the climate of East Asia. Unlike the cold sea surface temperature anomaly (SSTA) over the western North Pacific during El Niño, the positive SSTA over EAMS, which is covered by winter low cloud, has received less attention. We suggest that a “low cloud–SST” feedback—namely, one in which decreasing low-level clouds allows more sunlight to strike the ocean surface and favors higher SST—maintains the positive SSTA over EAMS. We also configure a widely used atmospheric model with a set of preset SSTA patterns that mimic different climate patterns. Our experiments with different climate patterns and CMIP6 historical runs show that the low cloud–SST feedback (through the positive SSTA) is a possible supplementary mechanism for reinforcing the WNP anomalous anticyclone.

Interannual Variability of Autumn Precipitation Over the Greater Mekong Subregion

AbstractThis study investigates the interannual variability in autumn precipitation over the Greater Mekong Subregion (GMS) during the period 1981–2019 and its possible causes. Statistical analysis indicates that GMS autumn precipitation showed a seesaw pattern between the central–southern and northeastern GMS over interannual timescales. The tropical sea surface temperature anomalies (SSTAs), reflected in the tropical northeastern Pacific SST index (TNEPI), significantly modulate anomalous GMS autumn precipitation. When the TNEPI is higher than normal, the SSTAs pattern prevails during autumn, with warmer SSTAs in the tropical eastern Pacific and western Indian Ocean, and colder SSTAs in the tropical western Pacific and southeastern Indian Ocean. This SSTAs pattern excites an anomalous anticyclone in the lower troposphere over the central–southern GMS and an anomalous cyclone over the northeastern GMS by the Gill–Matsuno mechanism. Subsequently, the anomalous anticyclone with anomalous descending motion results in below‐normal autumn precipitation over the central–southern GMS via a weakening of the convergence of water vapor from the tropical western Pacific and the Bay of Bengal. The anomalous cyclone on the northern flank of anomalous anticyclone, benefiting from water vapor supply from the Bay of Bengal, leads to above‐normal autumn precipitation over the northeastern GMS. The TNEPI, incorporating signals from the eastern‐Pacific (EP) type of ENSO, Indian Ocean dipole (IOD), and SSTAs associated with the Trans‐Niño index (TNI‐like), shows a stronger relationship with GMS autumn precipitation than individual indexes that describing the EP‐type ENSO, IOD or TNI‐like pattern. Therefore, the TNEPI can be considered an efficient predictor of GMS autumn precipitation.

What caused the interdecadal shift in the El Niño–Southern Oscillation (ENSO) impact on dust mass concentration over northwestern South Asia?

Abstract. Changes in large-scale circulation, especially El Niño–Southern Oscillation (ENSO), have significant impacts on dust activities over the dust source and downwind regions. However, these impacts present an interdecadal pattern, and it remains less clear which factors lead to the interdecadal variability of the ENSO impact on dust activities over northwestern South Asia, although previous studies have discussed the response of interannual dust activities over northwestern South Asia to the ENSO circle. Based on the linear regression model and MERRA-2 atmospheric aerosol reanalysis data, this study investigated the interdecadal variability of the ENSO impact on dust activities as well as the associated possible atmospheric drivers under two different warming phases over northwestern South Asia. Results indicated that the relationship between ENSO and dust column mass density (DUCMASS) experienced an obvious shift from the accelerated global warming period (1982–1996) to the warming hiatus period (2000–2014). The change in Atlantic and Indian Ocean sea surface temperature anomaly (SSTA) patterns weakened the impact of ENSO on dust activities over northwestern South Asia during 1982–1996, while the change in Pacific Decadal Oscillation (PDO) strengthened ENSO's effect when it was in phase with ENSO. Both the Atlantic and Indian Ocean SSTA patterns were modulated by the duration of ENSO events (i.e., continuing and emerging ENSO). This study provides new insights into numerical simulation involving the influence of atmospheric teleconnections on the variability of dust activities and their influence mechanisms.

Simulation, precursor analysis and targeted observation sensitive area identification for two types of ENSO using ENSO-MC v1.0

Abstract. The global impact of an El Niño–Southern Oscillation (ENSO) event can differ greatly depending on whether it is an eastern Pacific (EP)-type event or a central Pacific (CP)-type event. Reliable predictions of the two types of ENSO are therefore of critical importance. Here we construct a deep neural network with multichannel structure for ENSO (named ENSO-MC) to simulate the spatial evolution of sea surface temperature (SST) anomalies for the two types of events. We select SST, heat content and wind stress (i.e., three key ingredients of Bjerknes feedback) to represent coupled ocean–atmosphere dynamics that underpin ENSO, achieving skilful forecasts for the spatial patterns of SST anomalies out to 1 year ahead. Furthermore, it is of great significance to analyse the precursors of EP-type or CP-type events and identify targeted observation sensitive areas for the understanding and prediction of ENSO. Precursors analysis is to determine what type of initial perturbations will develop into EP-type or CP-type events. Sensitive area identification is to determine the regions where initial states tend to have the greatest impacts on the evolution of ENSO. We use the saliency map method to investigate the subsurface precursors and identify the sensitive areas of ENSO. The results show that there are pronounced signals in the equatorial subsurface before EP events, while the precursory signals of CP events are located in the northern Pacific. It indicates that the subtropical precursors seem to favour the generation of the CP-type El Niño and that the EP-type El Niño is more related to the tropical thermocline dynamics. Furthermore, the saliency maps show that the sensitive areas of the surface and the subsurface are located in the equatorial central Pacific and the equatorial western Pacific respectively. The sensitivity experiments imply that additional observations in the identified sensitive areas can improve forecasting skills. Our results of precursors and sensitive areas are consistent with the previous theories of ENSO, demonstrating the potential usage and advantages of the ENSO-MC model in improving the simulation, understanding and observations of the two ENSO types.

Impact of the North Atlantic Sea Surface Temperature Tripole on the Northwestern Pacific Weak Tropical Cyclone Frequency

Abstract Previous studies focused on the intense TCs in the central-southeastern western North Pacific (WNP), whose variability is intimately linked to El Niño–Southern Oscillation and the extratropical sea surface temperature anomaly (SSTA) in the Pacific. Compared with them, weak TCs (WTCs) are more numerous and form farther northwestward. The great number of WTCs and thereby the landfall cases may also cause huge damage to countries in Southeast Asia. However, their modulators are far from fully understood. Our research emphasizes the delayed impact of the early spring North Atlantic tripole SSTA (NAT) on the WTC formation frequency through the “capacitor” effect of sea ice (SIC) and SST in the Barents Sea. Detailed analysis indicates that a positive NAT may modulate an anomalous high in the Barents Sea–North Europe and decrease the local low cloud cover. Thus, more downward solar radiation tends to heat the local SST and decrease the SIC. This warmer Barents Sea could maintain through the typhoon season and excite a significant southeastward wave train, with several centers in the Arctic, central Asia, and East Asia. The abnormal easterly wind to the south of the anticyclone in East Asia facilitates the cyclonic anomaly in the South China Sea, the Philippines, and the subtropical WNP, which reinforces the local monsoon trough and favors the WTC formation there. A physical-based empirical model is developed for the WTC frequency, and hindcast is performed from 1979 to 2018. It shows the early spring NAT effectively improves the prediction skill for the WTC frequency, which can be considered as a crucial source of predictability for WTCs. Significance Statement Previous studies have focused on the western North Pacific intense TCs. The great number of weak TCs (WTCs) and thereby the landfall cases may also cause huge damage. However, modulators for WTCs have not been fully understood. This research emphasizes the potential impact of the early spring North Atlantic tripole SSTA (NAT) pattern on the WTC frequency through the persistent sea ice and sea surface temperature in the Barents Sea. By considering the NAT signal, the seasonal forecasting skill for WTC frequency is effectively improved. Therefore, the NAT signal may help better understand the WTC variability about 1–2 seasons in advance.

Impact of tropical Atlantic SST anomaly on ENSO in the NUIST‐CFS1.0 Hindcasts

AbstractUsing both observations and ensemble hindcasts of Nanjing University of Information Science and Technology Climate Forecast System (NUIST‐CFS1.0) for the period of 1983–2020, the leading modes of sea surface temperature anomalies (SSTAs) over the tropical Atlantic in boreal spring and summer (March–April–May–June–July–August, MAM‐JJA) and their connections with El Niño‐Southern Oscillation (ENSO) are investigated with Empirical Orthogonal Function (EOF), partial‐correlation, and composite analyses. In both observations and hindcasts, the first EOF mode is characterized by a basin‐wide SSTA pattern, and the second mode features a meridional dipole pattern. The meridional dipole mode cannot trigger ENSO, while the basin‐wide SST warming mode contributes to a subsequent La Niña via both north‐tropical and equatorial pathways. The north‐tropical pathway involves a low‐level anomalous anticyclone over North Pacific as a Matsuno‐Gill‐type response to the basin‐wide SST warming, related to a North Pacific Meridional Mode characterized with a cold SSTA and north‐easterly wind coupling over the northeast tropical Pacific, and finally triggers La Niña via the seasonal footprinting mechanism. This pathway is less important in the hindcasts than in observations. The equatorial pathway is tightly linked to the Kelvin wave, characteristic of an eastward propagation of the coupled pattern of SST and low‐level winds and a wedge‐like 500‐hPa air temperature anomaly over the tropical Indian and western equatorial Pacific Oceans, which is more significant in the hindcasts than in observations. Further composite analyses confirm that, even without the effect of the preceding ENSO, a basin‐wide tropical Atlantic SST warming favours the development of La Niña.

Three Types of Positive Indian Ocean Dipoles and Their Relationships with the South Asian Summer Monsoon

Abstract The relationship between the Indian Ocean dipole (IOD) and the South Asian summer monsoon (SASM), which remains a subject of controversy, was investigated using data analyses and numerical experiments. We categorized IOD events according to their sea surface temperature anomaly (SSTA) pattern: type W and type E are associated with stronger SSTA amplitudes in the western and eastern poles of the IOD, respectively, while type C has comparable SSTA amplitudes in both poles during boreal autumn. Type W is associated with a weak SASM from May to summer, which contributes to substantial warming of the western pole in autumn; the east–west SST gradient linked to the warming of the western pole causes weak southeasterly wind anomalies off Sumatra and feeble and cold SSTAs in the eastern pole during the mature phase. Type E is associated with a strong SASM and feeble warming of the western pole; interaction between the strong SASM and cold SSTAs in the eastern pole in summer results in strong southeasterly wind anomalies off Sumatra and substantial cooling of the eastern pole during the mature phase. For type C, warming of the western pole and cooling of the eastern pole develop synchronously without apparent SASM anomalies and reach comparable intensities during the mature phase. Observations and numerical simulation results both indicate the role of disparate SASM anomalies in modulating SSTA patterns during the development of positive IODs. Warming of the tropical Indian Ocean becomes established in the winter and spring following type W and type C IODs but not following type E events.

Defining the Internal Component of Atlantic Multidecadal Variability in a Changing Climate

AbstractThe canonical index of “Atlantic Multidecadal Variability” (AMV) is the low‐pass filtered timeseries of sea surface temperature anomalies (SSTA) averaged over the North Atlantic. This index and its associated SSTA spatial pattern confound externally forced climate change and internally generated climate variability. The internal component of AMV is commonly isolated by either subtracting the global‐mean SSTA or removing the pattern of SSTA associated with the global‐mean. This study evaluates the skill of each method with regard to the spatial pattern of internal AMV, using nine coupled model Large Ensembles over the period 1940–2100 as a testbed in which the true internal AMV is known a priori. The first method aliases the structure of forced climate change onto internal AMV, while the second method is generally robust to climate change. The models simulate realistic patterns of internal AMV, although such an assessment is hampered by the brevity of the observational record.

Spatiotemporal characteristics of spring rainfall over Southwest China and their relationships with sea surface temperature during 1961–2017

The spatiotemporal variability of southwest China (SWC) spring (March–April–May) rainfall (SWCSR) during 1961–2017 is investigated using rainfall data from 75 meteorological stations over SWC in this study. It is found that (1) SWC can be divided into four subregions using the rotated empirical orthogonal function (REOF) method: southern Yunnan province (SY), northwestern Yunnan province (NWY), eastern Guizhou province (EG), and northeastern Sichuan province (NES). (2) All subregions show significant 2–8-year periodicity on the interannual timescale. However, only SY rainfall shows significant interdecadal periodicity. (3) The spring La Nina has a significant impact on the simultaneous rainfall over SY. The sea surface temperature anomalies (SSTAs) in the central North Pacific during late winter can be an important oceanic signal for the prediction of spring rainfall variability over NWY. The southern Indian Ocean dipole (SIOD)–like SSTA pattern during late autumn (early winter) can be an important predictor of rainfall variability over EG (NES) during the following spring. Moreover, the impact of the late autumn SIOD-like pattern on the following spring rainfall over EG is stronger than the impact of the early winter SIOD-like pattern on the NES rainfall during the following spring.