Variability Of Sea Surface Temperature Anomalies Research Articles

Role of the Aleutian Low in Modulating the Connection between the Moderate El Niño and the Following Spring North Tropical Atlantic SST Anomalies

Abstract El Niño events are likely to be followed by significant positive sea surface temperature anomalies (SSTAs) over the north tropical Atlantic (NTA). We find that this is not always true for El Niño with moderate intensity. Nearly half of the moderate El Niño cases are followed by the strong NTA SSTAs, but others are followed by the weak NTA SSTAs, indicating the uncertainty in the connection between the moderate El Niño and the NTA. The differences in the El Niño–related NTA SSTAs are due to the different extratropical teleconnection between El Niño and the NTA, manifesting as a Pacific–North American (PNA) pattern. Further analysis suggests that the deepened Aleutian low (AL) induces the negative North Pacific SSTA, which is associated with a wave train propagating eastward, and, modulates the El Niño–generated PNA pattern. Therefore, El Niños that are accompanied by the strong AL induce a strong PNA pattern that, in turn, leads to the strong NTA SSTAs. The strength of the AL coinciding with El Niño plays a crucial role in modulating the connection between the moderate El Niño and the NTA SSTAs. This observational evidence about the uncertainty in the El Niño–NTA connection and the role of the AL in modulating this connection is further supported by the model simulations that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). These results indicate that the combined effects of El Niño and the AL must be considered to fully understand the NTA SSTA variability. Significance Statement El Niño events often lead to significant warming over the north tropical Atlantic (NTA). We find that the strong El Niño almost always is accompanied by the strong NTA sea surface temperature anomalies (SSTAs), whereas the moderate El Niño can be followed by either the strong or weak NTA SSTAs, indicating the uncertainty in the connection between the moderate El Niño and the NTA SSTAs. Further analysis demonstrates that El Niño, which is accompanied by the strong (weak) Aleutian low leads to the strong (weak) NTA SSTAs, suggesting that the role of the Aleutian low must be considered to fully understand the El Niño–NTA connection.

Enhanced Predictability of Rapidly Intensifying Tropical Cyclones over the Western North Pacific Associated with Snow Depth Changes over the Tibetan Plateau

Abstract This study finds an enhanced relationship in recent years between January–March eastern Tibetan Plateau snow depth (TPSD) and the frequency of rapidly intensifying tropical cyclones (RITCs) over the western North Pacific (WNP) during the following peak TC season (July–November). The correlation between TPSD and RITCs is significant during 2000–14 but was insignificant during 1979–99. During 2000–14, when TPSD increases, there is an enhanced low-level anomalous anticyclone over the subtropical eastern North Pacific mainly due to the combined effect of advection and dynamics of the climatological prevailing westerly jet. Northeasterly wind anomalies are observed on the flank of the anticyclonic circulation anomaly, favoring anomalously cool sea surface temperature (SST). These anomalies lead to an anomalous pattern similar to the Pacific meridional mode (PMM), via a wind–evaporation feedback and cold advection. A Gill-type Rossby response to the PMM-like negative phase results in an anticyclonic circulation anomaly over the WNP, suppressing RITCs during 2000–14. A nearly opposite circulation anomaly occurred when TPSD was lower during 2000–14. There is a weak relationship between TPSD and RITCs, due to the lack of a link between TPSD and the PMM-like pattern from 1979 to 1999. Decadal changes in the relationship between TPSD and RITCs are mainly due to the meridional displacement of the prevailing westerly jet, which may be in response to decadal-to-multidecadal variability of SST anomalies. These changes then result in changes in the relationship between January–March TPSD and the PMM-like pattern. Significance Statement Forecasts of tropical cyclone rapid intensification, typically defined to be when a tropical cyclone intensifies by at least 30 knots (∼15 m s−1) in 24 h, remain extremely challenging. This study finds an enhanced relationship since the start of the twenty-first century between winter–spring Tibetan Plateau snow depth and western North Pacific rapidly intensifying tropical cyclones, while the relationship between snow depth and rapidly intensifying tropical cyclones was weak from 1979 to 1999. Decadal changes in the relationship between Tibetan Plateau snow depth and western North Pacific rapidly intensifying tropical cyclones is mainly due to the north–south displacement of the prevailing westerly jet, which may be in response to a transition in a midlatitude North Pacific climate mode. This study highlights the importance of the synergetic impact of the land, air, and sea on tropical cyclone climate and provides a potential predictor for seasonal-to-decadal prediction of rapidly intensifying tropical cyclones.

Diagnostics of the La Niña events in 1900–2018

Diagnostics of La-Nina events from 1900 to 2018 is done in this paper using COBE SST2 data sets. Two La Niña space-time types were selected on the basis of two methods. The first of them is empirical orthogonal functions method. The second one is hierarchical cluster analysis. Both of methods showed the absolute identity two selected types. Analyzing the spatiotemporal series of the sea surface temperature anomalies (SSTA) in the equatorial Pacific two main EOFs were obtained. The first EOF mode describes ∼46% of the total dispersion SSTA variability in the equatorial Pacific and demonstrated two consecutive La Niña events that arise after the strong canonical El Niño. The second mode accounts for ∼ 21% of the total dispersion SSTA variability and shows a nature of the East Pacific La Niña occurrence.

The covariability of sea surface temperature and MAM rainfall on East Africa using singular value decomposition analysis

The study assesses the covariability of Sea Surface Temperature (SST) and March to May (MAM) rainfall variability on East Africa (EA) from 1981 to 2018. Singular Value Decomposition (SVD) analysis reveals the significant influence of SST anomalies on MAM rainfall, with covariability of 91%, 88.61%, and 82.9% for Indian, Atlantic, and the Pacific Ocean, respectively. The Indian Ocean explains the variability of rainfall to the large extent followed by the Atlantic Ocean and the Pacific Ocean. The rainfall patterns over the EA correspond to SST variability over the western, central, and Eastern Indian Ocean. Likewise, the variability of SST anomalies was observed over the central, south, and North of the Atlantic Ocean while the Pacific Ocean captured the El Nino Modoki (ENSO) like pattern in the SVD1 (SVD2). The heterogeneous correlation of Indian SST anomalies and rainfall over EA of the first (second) principal component (PC) shows a positive correlation over much of the domain (central region). The SST anomalies over the Pacific Ocean show higher correlation values with the rainfall over much of the study domain except over the southwestern highland and southern region of Tanzania. Over the Atlantic Ocean, the correlation result shows the patterns of positive (negative) values over the northern (southern) part for PC1, while PC2 depicts negative correlation values over much of the Ocean. SST anomalies over the Indian (Atlantic) Ocean are highly correlated with MAM rainfall when SST leads by 1(7) month(s). The Pacific Ocean shows a weak (strong) correlation across all (zero) lead seasons.

The signal of La Niña in wave transport

La Niña phenomena have significant impact on extreme weather and marine disasters in China. Large-scale water transport is one of the primary factors affecting the development of La Niña events; however, the contribution of swell transport from the Southern Ocean to the sea surface temperature anomaly (SSTA) in the eastern equatorial Pacific (EEP) has been ignored in previous studies. Through various statistical techniques, this study used seven sets of satellite altimeter and reanalysis data to reveal the relationship between swell transport and La Niña. Based on the characteristics of significant SSTA changes in the Niño 3 region (5°S–5°N, 90°–150°W) following an El Niño event, two cases of La Niña and non-La-Niña were defined. Wave transport in the first case presents an anomaly to the north of the 30°S section, i.e., cold water carried by swell transport to the EEP increases prior to the onset of a La Niña event. However, the characteristics of swell transport in the non-La-Niña scenario are contrary to the first case. Wave transport anomalies in the two considered cases produce sufficient and necessary conditions for the development of La Niña and non-La-Niña events. In addition, the impact of swell transport from the westerlies on La Niña events is confirmed further based on heat flux calculations. Wave transport has considerable effect on SSTA variability in the EEP; therefore, it should not be neglected in future studies.

Comparison of the Pacific Decadal Oscillation in climate model simulations and observations

ABSTRACTThe ability of climate models to simulate the Pacific Decadal Oscillation (PDO) and its global teleconnections is examined using the outputs of 20 Coupled Model Intercomparison Project (CMIP5) models from historical experiments. It is shown that the global warming trend is reproduced by the models, but there is a considerable difference between the models in the PDO region. The standard deviation of detrended sea surface temperature anomalies (SSTAs) in the models is similar to that of the observations, especially in the central and eastern equatorial Pacific and the North Pacific, which are the regions closely associated with the PDO and El Nino‐Southern Oscillation. The globally averaged SSTA during 1900–1940 in the CMIP5 models and observations showed large divergence, which is mainly due to the SSTA in the PDO region. The most significant cycle of the PDO indexes in most of the models had a 60–80 years period, which is different from the observed PDO period (40–60 years). However, the models perform better for the spatial pattern of the PDO, and 90% of the spatial correlations between the models and observations of the spatial pattern of the PDO are above 0.8. In addition, the models can simulate the basic correlation between the PDO and Pacific SSTA variability, but most of them overestimate the relationship in the Indian and Atlantic Oceans. The CMIP5 models are better at simulating the impact of the PDO on winter precipitation in southern North America but are less accurate in simulating summer precipitation in eastern China. This is probably because most of the models are not accurate in simulating the impacts of the PDO on the western Pacific subtropical high and the East Asian summer monsoon.

The Record-Breaking Hot Summer in 2015 over Hawaii and Its Physical Causes

Abstract Hawaiian surface air temperature (HST) during the summer of 2015 (from July to October) was about 1.5°C higher than the climatological mean, which was the hottest since records began in 1948. In the context of record-breaking seasonal-mean high temperature, 98 exceptional local heatwave days occurred during the summer of 2015. Based on diagnoses and simulations, this paper demonstrates that the record-high HST during the summer of 2015 arose mainly from the combined effects of the interannual and interdecadal variability of sea surface temperature anomalies (SSTAs). The interannual variability of SSTAs, with an El Niño–like pattern in the tropics and cold (warm) anomalies over the western (eastern) North Pacific, was the primary contributor to the abnormally high HST in the summer of 2015. This interannual tropical–extratropical SSTA pattern was accompanied by low-level southwesterly anomalies over the central North Pacific, which weakened the climatological northeasterly trade winds and reduced the ventilation effect, warming Hawaii. Numerical experiments further revealed that the SST warming in the subtropical eastern North Pacific was mostly responsible for the weakened trade winds and warming over Hawaii. Interdecadal SST warming in the tropics was a secondary factor. By superimposing the positive SSTAs over the Indo-Pacific warm pool and tropical North Atlantic Ocean upon the climatological-mean maximum SST regions, it was found that these anomalies led to enhanced convection over the Maritime Continent and the oceans around Mexico, causing anomalous subsidence and reduced cloud cover over the tropical central North Pacific. The reduced cloudiness increased the amount of downward solar radiation, thus warming Hawaii.

Ocean atmosphere thermal decoupling in the eastern equatorial Indian ocean

Eastern equatorial Indian ocean (EEIO) is one of the most climatically sensitive regions in the global ocean, which plays a vital role in modulating Indian ocean dipole (IOD) and El Nino southern oscillation (ENSO). Here we present evidences for a paradoxical and perpetual lower co-variability between sea-surface temperature (SST) and air-temperature (Tair) indicating instantaneous thermal decoupling in the same region, where signals of the strongly coupled variability of SST anomalies and zonal winds associated with IOD originate at inter-annual time scale. The correlation minimum between anomalies of Tair and SST occurs in the eastern equatorial Indian ocean warm pool region (≈70°E–100°E, 5°S–5°N), associated with lower wind speeds and lower sensible heat fluxes. At sub-monthly and Madden–Julian oscillation time scales, correlation of both variables becomes very low. In above frequencies, precipitation positively contributes to the low correlation by dropping Tair considerably while leaving SST without any substantial instant impact. Precipitation is led by positive build up of SST and post-facto drop in it. The strong semi-annual response of SST to mixed layer variability and equatorial waves, with the absence of the same in the Tair, contributes further to the weak correlation at the sub-annual scale. The limited correlation found in the EEIO is mainly related to the annual warming of the region and ENSO which is hard to segregate from the impacts of IOD.

Study of relationship between wave transport and sea surface temperature anomaly (SSTA) in the tropical Pacific

Large-scale water transport is one of the key factors that affect sea surface temperature anomaly (SSTA) in the eastern equatorial Pacific (EEP). The relationship between the wave transport in the tropical Pacific and the SSTA in the EEP is examined by different methods, including band-pass filtering, period analysis, correlation analysis, significant analysis, and empirical orthogonal function (EOF) analysis. We have found that the eastward shift of the wave transport anomaly in the tropical Pacific, with a period of 2 a and enhancing the transport of warm waters from the western Pacific warm pool, precedes the increase of sea surface temperature (SST) in the EEP. The wave transport and the SSTA in the EEP have a maximum correlation of 0.65 with a time-lag of 6 months (transport variation precedes the temperature). The major periods (3.7 a and 2.45 a) of the wave transport variability, as revealed by the EOF analysis, appear to be consistent with the SSTA oscillation cycle in the EEP. Based on the first occurrence of a significant SSTA in the Nino 3 region (5°S–5°N, 90°–150°W), two types of warm events are defined. The wave transport anomalies in two types present predominantly the west anomaly in the tropical Pacific, it is that the wave transport continues transport warm water from west to east before the onset of the warm event. The impact of wave-induced water transport on the SSTA in the EEP is confirmed by the heat flux of the wave transport. The wave transport exerts significant effect on the SSTA variability in the EEP and thus is not neglectable in the further studies.

Potential for seasonal prediction of Atlantic sea surface temperatures using the RAPID array at 26 $$^{\circ }$$ ∘ N

The Atlantic meridional overturning circulation (AMOC) plays a critical role in the climate system and is responsible for much of the meridional heat transported by the ocean. In this paper, the potential of using AMOC observations from the 26 $$^{\circ }$$ N RAPID array to predict North Atlantic sea surface temperatures is investigated for the first time. Using spatial correlations and a composite method, the AMOC anomaly is used as a precursor of North Atlantic sea-surface temperature anomalies (SSTAs). The results show that the AMOC leads a dipolar SSTA with maximum correlations between 2 and 5 months. The physical mechanism explaining the link between AMOC and SSTA is described as a seesaw mechanism where a strong AMOC anomaly increases the amount of heat advected north of 26 $$^{\circ }$$ N as well as the SSTA, and decreases the heat content and the SSTA south of this section. In order to further understand the origins of this SSTA dipole, the respective contributions of the heat advected by the AMOC versus the Ekman transport and air–sea fluxes have been assessed. We found that at a 5-month lag, the Ekman component mainly contributes to the southern part of the dipole and cumulative air–sea fluxes only explain a small fraction of the SSTA variability. Given that the southern part of the SSTA dipole encompasses the main development region for Atlantic hurricanes, our results therefore suggest the potential for AMOC observations from 26 $$^\circ$$ N to be used to complement existing seasonal hurricane forecasts in the Atlantic.

Global land vegetation and marine fishery responses to atmospheric and oceanic decadal variability

The decadal variability of sea surface temperature (SST) and sea level pressure (SLP) anomalies, as well as the response of global land vegetation and marine fisheries, are investigated for three periods: 1982–1988, 1989–1998, and 1999–2008, separated by the 1988–89 and 1998–99 regime shifts. The goal is to develop a global-scale ecosystem concept to support an improved understanding of the corresponding changes in atmospheric, oceanic, and biological responses. The analysis is based on global SST, SLP, precipitable water content (PWC), land vegetation condition index (VCI), and the United Nations Food and Agriculture Organization’s (FAO) fish capture data. The results show that SST and SLP displayed significant decadal variability. The decadal variability of sea surface temperature anomalies (SSTA) associated with sea level pressure anomalies (SLPA) has an influence on the land vegetation moisture condition (VCI). Positive SSTA tends to be associated with negative SLPA, and vice versa, in the corresponding ocean areas and most land areas. Consequently, clearly opposing distributions of SSTA and SLPA are observed in the periods 1982–1988 and 1999–2008. With positive SSTA and negative SLPA, VCI tends to increase in value representing more favourable vegetation conditions. Negative SSTA and positive SLPA is generally unfavourable for global vegetation development. The decadal variability of SSTA is closely related to the number of fish species (NFS) doing better or worse based on normalized fish landing data. However, the fishery responses show different yet consistent trends in the three ocean basins. When SSTA is negative, it appears more beneficial for the number of fish species with improved landings in the Atlantic Ocean. However, positive SSTA leads to more fish species with improved landings in the Indian and Pacific Oceans.

Ocean dynamical processes associated with the tropical Pacific cold tongue mode

AbstractThe cold tongue mode (CTM) is the second EOF mode of sea surface temperature anomaly (SSTA) variability over the tropical Pacific and represents the out‐of‐phase relationship in SSTA variability between the Pacific cold tongue region and elsewhere in the tropical Pacific. A positive CTM is characterized by cold SSTA in the Pacific cold tongue region and warm SSTA in the rest of the tropical Pacific, with conditions reversed for a negative CTM. The CTM is a coupled air‐sea mode, and its long‐term variability is most probably induced by ocean dynamical processes in response to global warming [Zhang et al., 2010]. This study focuses on the specific ocean dynamical processes associated with the CTM and its possible relationship with global warming. A heat budget diagnosis of ocean temperature in the eastern equatorial Pacific shows that the net heat flux plays a damping role and the four ocean advection terms ( , , , and ) contribute to the temperature change associated with the CTM. Among them, the vertical advection of the anomalous temperature by the mean upwelling ( ) makes a dominant contribution to the long‐term change in the CTM. The long‐term change of the term is controlled mainly by the decreasing vertical gradient of the ocean temperature anomaly ( ). The other three advection terms make a minor contribution to the long‐term change in the CTM.

Change of the wintertime SSTA variability over the West Pacific after the mid‐1980s: Effect of the increasing El Niño Modoki

AbstractFrom 1949 to 2010, the EOF 1 of sea surface temperature anomalies (SSTA) over the West Pacific (WP) in the boreal winter shows a clear increase during the 1980s, which is associated with the increase in global SSTA. It is found that the characteristics of WP SSTA are different since the mid‐1980s. Before the mid‐1980s, the first EOF presents a dipole pattern associated with El Niño. After the mid‐1980s, EOF1 of WP SSTA captures a uniform SSTA variability over WP and is related to El Niño Modoki. Meanwhile, the dipole SSTA pattern captured by EOF2 is associated with the canonical El Niño. The generation of these two different SSTA patterns might be attributed to the different atmospheric responses to El Niño and El Niño Modoki. The impact of El Niño Modoki over the WP SSTA is significantly enhanced along with the increased occurrences of El Niño Modoki. The increasing intensity of WP SSTA variability has a similar period with that of El Niño Modoki. During El Niño Modoki, the central Pacific SSTA has a leading impact on the development of the WP SSTA by a Rossby wave response. July(0) to October(0) is suggested as a crucial period for the development of WP SSTA during El Niño Modoki. The increased wind speed and evaporation might be responsible for the formation of WP SSTA. After the negative SSTA established, the suppressed convection would counteract the impact of evaporation anomaly and prevent the decreasing of WP SSTA.

Possible causes for the persistence barrier of SSTA in the South China Sea and the vicinity of Indonesia

The persistence barrier refers to the lag correlation of sea surface temperature anomalies (SSTA) showing a rapid and significant decline in a specific season, regardless of the starting month. This implies that there is a decrease in forecast skill for SSTA in this specific season. This paper investigates the possible causes for the persistence barrier of SSTA in the South China Sea (SCS) and its adjacent regions from the perspective of interannual-interdecadal time scales. The results show that the persistence barrier of SSTA exists not only in the SCS, but also in the vicinity of Indonesia south of the equator. The SCS barrier occurs around October-November, while the occurrence of the barrier in the Indonesia region is around November-December. For these two regions, the occurrence of the persistence barrier is closely associated with the interdecadal variability of SSTA, as well as the interannual variability. The persistence barriers in the SCS and the Indonesia region do not exist alone if the interdecadal variability is not considered, because SSTA have a short memory of less than 4 months, regardless of the starting month. Moreover, the influence of the interdecadal variability of SSTA on the persistence barrier of SSTA in the SCS and the Indonesia region may be associated with SSTA in the Indian Ocean and the western Pacific, but is not closely associated with the Pacific Decadal Oscillation. However, compared with the spring persistence barrier (SPB) of ENSO, the close relationship between the persistence barriers in the SCS and the Indonesia region and the interdecadal variability is unique, since the ENSO SPB is not significantly affected by such variability. In addition, although the persistence barriers in both the SCS and the Indonesia region are quite obvious in strong ENSO cases, the interdecadal variability of SSTA also plays a non-negligible role in this relationship.

The Low-Frequency Relationship of the Tropical–North Pacific Sea Surface Temperature Teleconnections

Abstract The low-frequency relationship between interannual tropical and North Pacific sea surface temperature anomalies (SSTAs) in observations and a coupled general circulation model (CGCM) is investigated. The authors use the interactive ensemble CGCM, which advances a new approach for artificially increasing the signal-to-noise ratio, making it easier to detect physical and dynamical links with much reduced interference by atmospheric noise. The results presented here suggest that decadal variations in the relationship between the dominant modes of tropical and North Pacific interannual SSTA variability result from changes of spatial manifestation of North Pacific SSTA, both in the observation and in the model. The authors conjecture that the details of tropical Pacific SST forcing ultimately determine the tropical–North Pacific SST teleconnections, and this conjecture is examined in a much longer time series from a CGCM simulation. There are two patterns of North Pacific interannual SSTA variability in the model. The first pattern is locally forced by noise in the surface air–sea fluxes associated due to internal atmospheric dynamics. The second pattern is remotely forced by tropical SSTA. As the relationship of tropical–North Pacific SST teleconnections varies in the model, the spatial manifestation of the North Pacific SSTA changes from the atmospheric noise-forced pattern to the remotely forced pattern and vice versa. In the model, the amplitude of the tropical Pacific SSTA variance varies on decadal time scales and this largely determines the dominant structure of North Pacific SSTA variability. Furthermore, the change in location of the maximum tropical SST forcing is associated with the changes in the spatial manifestation of North Pacific interannual SSTA variability.

Droughts and Persistent Wet Spells over the United States and Mexico

Abstract Droughts and persistent wet spells over the United States and northwest Mexico have preferred regions of occurrence and persistence. Wet or dry conditions that persist more than 1 yr tend to occur over the interior United States west of 90°–95°W and northwest Mexico. In contrast, events over the eastern United States are less likely to occur and often last less than 6 months. The long persistent drought and wet spells are often modulated by low-frequency sea surface temperature anomalies (SSTAs). The persistent dry or wet conditions over northwest Mexico and the Southwest are associated with decadal variability of SSTAs over the North Pacific. Persistent events over the northwestern mountains are associated with two decadal SSTA modes. One mode has loadings over three southern oceans and another one is an El Niño–Southern Oscillation (ENSO) like decadal mode. Wet and dry conditions over the Pacific Northwest and the Great Plains are often associated with ENSO. The seasonal cycle of precipitation over the central-eastern United States, the East Coast, and the Ohio Valley is weak. Drought and wet spells over these regions are less persistent because the ENSO events have opposite impacts on precipitation for summer and winter.

Local versus non‐local atmospheric weather noise and the North Pacific SST variability

The interactive ensemble coupling strategy has been developed specifically to determine how noise impacts climate variability within context of coupled general circulation model (CGCM). This study examines the impacts of local versus non‐local noise on the North Pacific sea surface temperature anomaly (SSTA) variability using three CGCM simulations. The control run uses the standard coupling strategy. In the first experiment, the interactive ensemble strategy is applied globally thereby reducing the noise at the air‐sea interface at each grid point. In the second experiment, the interactive ensemble strategy is applied locally in the extra‐tropics only. Perhaps as expected, our analysis indicates that the impact of local noise on the North Pacific SSTA variability is much larger than that of non‐local noise. However, non‐local noise can not be neglected; for example, non‐local noise influences decadal SSTA variability in the central North Pacific. The hypothesis put forward here is that the noise due to enhanced tropical internal atmospheric variability causes the modulation of El Niño and Southern Oscillation, which in turn affects the North Pacific SSTA variability through the atmospheric bridge.

Local Coupled Equatorial Variability versus Remote ENSO Forcing in an Intermediate Coupled Model of the Tropical Atlantic

Abstract The relative roles played by the remote El Niño–Southern Oscillation (ENSO) forcing and the local air–sea interactions in the tropical Atlantic are investigated using an intermediate coupled model (ICM) of the tropical Atlantic. The oceanic component of the ICM consists of a six-baroclinic mode ocean model and a simple mixed layer model that has been validated from observations. The atmospheric component is a global atmospheric general circulation model developed at the University of California, Los Angeles (UCLA). In a forced context, the ICM realistically simulates both the sea surface temperature anomaly (SSTA) variability in the equatorial band, and the relaxation of the Atlantic northeast trade winds and the intensification of the equatorial westerlies in boreal spring that usually follows an El Niño event. The results of coupled experiments with or without Pacific ENSO forcing and with or without explicit air–sea interactions in the equatorial Atlantic indicate that the background energy in the equatorial Atlantic is provided by ENSO. However, the time scale of the variability and the magnitude of some peculiar events cannot be explained solely by ENSO remote forcing. It is demonstrated that the peak of SSTA variability in the 1–3-yr band as observed in the equatorial Atlantic is due to the local air–sea interactions and is not a linear response to ENSO. Seasonal phase locking in boreal summer is also the result of the local coupling. The analysis of the intrinsic sustainable modes indicates that the Atlantic El Niño is qualitatively a noise-driven stable system. Such a system can produce coherent interdecadal variability that is not forced by the Pacific or extraequatorial variability. It is shown that when a simple slab mixed layer model is embedded into the system to simulate the northern tropical Atlantic (NTA) SST variability, the warming over NTA following El Niño events have characteristics (location and peak phase) that depend on air–sea interaction in the equatorial Atlantic. In the model, the interaction between the equatorial mode and NTA can produce a dipolelike structure of the SSTA variability that evolves at a decadal time scale. The results herein illustrate the complexity of the tropical Atlantic ocean–atmosphere system, whose predictability jointly depends on ENSO and the connections between the Atlantic modes of variability.

Origin of decadal El Niño–Southern Oscillation–like variability in a coupled general circulation model

The origin of decadal El Niño–Southern Oscillation–like variability (hereinafter referred to as DEV) is analyzed in coupled general circulation model (CGCM) simulations. The CGCM experiments test the null hypothesis for the existence of DEV, that is, that DEV is linear, damped, and stochastically forced by atmospheric internal dynamics. In order to test this hypothesis we employ the interactive ensemble coupling strategy. It is found that the spatially averaged variance of sea surface temperature anomaly (SSTA) and heat content anomaly (HCA) associated with the DEV is linearly related to the amplitude of atmospheric internal dynamics. This result suggests that the DEV is primarily noise forced but that damped coupled feedbacks and/or internal ocean dynamics enhance this variability. The SSTA variability associated with the DEV appears to be of a basin‐type standing mode with a triangular shape. However, the decadal evolution of the HCA associated with the DEV shows a systematic westward propagation from the central North Pacific and the eastern South Pacific. Both a singular vector analysis and noise variance analysis based on the coupled simulation results support the argument that atmospheric noise in the subtropics leads to the excitation of the DEV.

Investigation of tropical Pacific upper-ocean variability using cyclostationary EOFs of assimilated data

Variability of the subsurface temperature, current, and heat content in the tropical Pacific Ocean has been extracted in association with the two dominant modes of the sea surface temperature anomaly (SSTA): the low-frequency mode and the biennial mode. In a recent paper, these two modes were identified as the major modes of El Nino-Southern Oscillation (ENSO). The low-frequency mode, which explains about 36% of the total SSTA variability, represents the dominant component of SSTA variability in the tropical Pacific, and is associated not with a fast physical evolution but with a slow stochastic undulation. The biennial mode, which is the second dominant component and explains about 12% of the total variability exhibits, on the other hand, a strong physical evolution. The space–time patterns of the subsurface variability were derived from an assimilated data set via a cyclostationary empirical orthogonal functions (CSEOF) analysis and the regression of the resulting principal component (PC) time series on the target PC time series of the surface modes. Extracted space–time patterns describe the detailed evolution of the physical changes in the upper ocean of the tropical Pacific that are associated with the corresponding surface modes. Specifically, they clearly show the surface and subsurface connection of the physical changes during ENSO events, and the role of equatorial waves in the manifestation of physical changes at the surface. The derived patterns of heat content, subsurface temperature, and zonal current anomalies realistically depict the detailed temporal changes of those variables and are consistent with our understanding of the physics in the tropical Pacific Ocean. The biennial mode appears to depict faithfully the phase progression of El Nino and La Nina. The propagation of equatorial Kelvin waves along the thermocline is clearly visible during El Nino and La Nina events in the cyclostationary representation of the physical modes in the tropical Pacific Ocean. Although the low-frequency mode explains three times more SSTA variability than the biennial mode, the former does not induce strong equatorial wave activity. This observation is significant considering that both El Nino or La Nina are often viewed simply in terms of a significant SST change in the tropical Pacific. The results of the present study indicate: (1) that the two ENSO modes represent significantly different physical evolutions; (2) that the amount of SST warming or cooling does not dictate the physical evolution of ENSO; and (3) that the two modes play essentially different dynamical roles including the generation of equatorial waves.