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Recently, El Niño diversity has been paid much attention because of its different global impacts. However, most studies have focused on a single warm peak in sea surface temperature anomalies (SSTAs), either in the central Pacific or the eastern Pacific Ocean. Here, we demonstrate from observational analyses that several recent El Niño events show double warm peaks in SSTA—called “double-peaked (DP) El Niño”—that have only been observed since 2000. The DP El Niño has two warm centers, which grow concurrently but separately, in both the central and eastern Pacific. In general, the atmospheric and oceanic patterns of the DP El Niño are similar to those of the warm-pool (WP) El Niño from the development phase, such that the central Pacific peak is developed by the zonal advective feedback and reduced wind speed anomalies. However, a distinctive difference exists in the eastern Pacific where the DP El Niño has a second SSTA peak. In addition, the DP El Niño shows more distinctive anomalous precipitation along the Pacific intertropical convergence zone (ITCZ) when compared with the WP El Niño. We demonstrate that the peculiar precipitation anomalies along the Pacific ITCZ play a critical role in enhancing the equatorial westerly wind stress anomalies, which help to develop the eastern SSTA peak by deepening the thermocline in the eastern Pacific.

The role of the Tibetan Plateau (TP) in El Niño–Southern Oscillation (ENSO) variability is investigated using coupled model experiments with different topography setups. Removing the TP results in weakened trade winds in the tropical Pacific, an eastward shift of atmospheric convection center, a shallower mixed layer in the equatorial Pacific, and a flattened equatorial thermocline, which leads to an El Niño–like sea surface temperature (SST) response. In association with these mean climate changes in the tropical atmosphere–ocean system, the ENSO variability exhibits a much stronger amplitude in the world without the TP. Detailed diagnoses reveal that in the absence of the TP, both thermocline feedback in the eastern equatorial Pacific and Ekman pumping feedback in the central-eastern equatorial Pacific are enhanced substantially, leading to stronger ENSO variability. The changes of these two feedbacks are caused by the eastward shift of the atmospheric convection center and enhanced ocean sensitivity; the latter is due to the shallower mixed layer and flattened thermocline. This study suggests that the presence of the TP may be of fundamental importance for modern-day tropical climate variability; namely, the TP may have played a role in suppressing ENSO variability.

Recent studies have shown that regardless of model configuration, skill in predicting El Niño–Southern Oscillation (ENSO), in terms of target month and forecast lead time, remains largely dependent on the temporal characteristics of the boreal spring predictability barrier. Continuing the 2019 study by O’Kane et al., we compare multiyear ensemble ENSO forecasts from the Climate Analysis Forecast Ensemble (CAFE) to ensemble forecasts from state-of-the-art dynamical coupled models in the North American Multimodel Ensemble (NMME) project. The CAFE initial perturbations are targeted such that they are specific to tropical Pacific thermocline variability. With respect to individual NMME forecasts and multimodel ensemble averages, the CAFE forecasts reveal improvements in skill when predicting ENSO at lead times greater than 6 months, in particular when predictability is most strongly limited by the boreal spring barrier. Initial forecast perturbations generated exclusively as disturbances in the equatorial Pacific thermocline are shown to improve the forecast skill at longer lead times in terms of anomaly correlation and the random walk sign test. Our results indicate that augmenting current initialization methods with initial perturbations targeting instabilities specific to the tropical Pacific thermocline may improve long-range ENSO prediction.

Variability of oceanic salinity, an indicator of the global hydrological cycle, plays an important role in the basin-scale ocean circulation. In this study, interannual to decadal variability of salinity in the upper layer of the Indian Ocean is investigated using Argo observations since 2004 and data assimilating model outputs (1992–2015). The southeastern Indian Ocean shows the strongest interannual to decadal variability of upper-ocean salinity in the Indian Ocean. Westward propagation of salinity anomalies along isopycnal surfaces is detected in the southern Indian Ocean and attributed to zonal salinity advection anomalies associated with the Indonesian Throughflow and the South Equatorial Current. Composite and salinity budget analyses show that horizontal advection is a major contributor to the interannual to decadal salinity variability of the southern Indian Ocean, and the local air–sea freshwater flux plays a secondary role. The Pacific decadal oscillation (PDO) and El Niño–Southern Oscillation (ENSO) modulate the salinity variability in the southeastern Indian Ocean, with low salinity anomalies occurring during the negative phases of the PDO and ENSO and high salinity anomalies during their positive phases. The Indonesian Throughflow plays an essential role in transmitting the PDO- and ENSO-related salinity signals into the Indian Ocean. A statistical model is proposed based on the PDO index, which successfully predicts the southeastern Indian Ocean salinity variability with a lead time of 10 months.

This study investigates the sudden reversal of anomalous zonal equatorial transport above thermocline at the peak phase of ENSO. The oceanic processes associated with zonal transport are separated into low-frequency ENSO cycle and high-frequency oceanic wave processes. Both processes can generate a reversal of equatorial zonal current at the ENSO peak phase, which is a trigger for the rapid termination of ENSO events. For the low-frequency process, zonal transport exhibits slower and basinwide evolution. During the developing phase of El Niño (La Niña), eastward (westward) transport prevails in the central-eastern Pacific, which enhances ENSO. At the peak of ENSO, a basinwide reversal of the zonal transport resulting from the recharge–discharge process occurs and weakens the existing SST anomalies. High-frequency zonal transport presents clear eastward propagation related to Kelvin wave propagation at the equator, reflection at the eastern boundary, and the westward propagating Rossby waves. The major westerly wind bursts (easterly wind surges) occur in late boreal summer and fall with coincident downwelling (upwelling) Kelvin waves for El Niño (La Niña) events. After the peak of El Niño (La Niña), Kelvin waves reach the eastern boundary in boreal winter and reflect as off-equatorial Rossby waves; then, the zonal transport switches from eastward (westward) to westward (eastward). The high-frequency zonal transport can be represented by equatorial wave dynamics captured by the first three EOFs based on the high-pass-filtered equatorial thermocline. The transport anomaly during the decaying phase is dominated by the low-frequency process in El Niño. However, the transport anomaly is caused by both low- and high-frequency processes during La Niña.

Previous studies have indicated that a sea surface temperature anomaly (SSTA) dipole in the subtropical South Pacific (SPSD), which peaks in austral summer (January–March), is dominated by thermodynamic processes. Observational analyses and numerical experiments were used to investigate the influence of SPSD mode on the equatorial Pacific. The model is an atmospheric general circulation model coupled to a reduced-gravity ocean model. An SPSD-like SSTA was imposed on 1 March, after which the model was free to evolve until the end of the year. The coupled model response showed that warm SSTAs extend toward the equator with northwesterly wind anomalies and then grow to El Niño–like anomalies by the end of the year. SPSD forcing weakens southeasterly trade winds and propagates warm SSTAs toward the equator through wind–evaporation–SST (WES) feedback. Meanwhile, relaxation of trade winds in the eastern equatorial Pacific depresses the thermocline and upwelling. Eastward anomalous currents near the equator cause warm horizontal advection in the central Pacific. Further experiments showed that thermodynamic coupling mainly acts on but is not essential for SSTA propagation, either from the subtropics to the equator or westward along the equator, while oceanic dynamic coupling alone also appears to be able to initiate anomalies on the equator and plays a critical role in SSTA growth in the tropical Pacific. This is consistent with observational analyses, which indicated that influence of WES feedback on SSTA propagation associated with the SPSD is limited. Finally, the warm pole close to the equator plays the dominant role in inducing the El Niño–like anomalies.

El Niño–Southern Oscillation (ENSO) is the most active interannual climatic mode, with great global impacts. The state-of-the-art climate models can simulate this dominant mode variability to a large extent. Nevertheless, some of ENSO’s fundamental time–space characteristics still have a large spread in the simulations across the array of recent climate models. For example, the large biases of ENSO periodicity still exist among model simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Based on the recharge oscillator framework, a coupled dynamic index for ENSO periodicity is proposed in this study, referred to as the Wyrtki index, in parallel to the Bjerknes index for ENSO instability. The Wyrtki index provides an approximate dynamic measure for ENSO linear periodicity. It has two main contribution terms: the thermocline and zonal advective feedbacks (or F factor) multiplied by the efficiency factor B of discharging–recharging of the equatorial heat content driven by ENSO wind stress anomalies. It is demonstrated that the diversity of simulated ENSO periodicity in CMIP5 models results from the biases in mean state and several key parameters that control ENSO dynamics. A larger F factor would result in a shorter ENSO period [e.g., BCC_CSM1.1(m)], whereas a smaller B factor would lead to a longer ENSO period (e.g., HadGEM2-ES). The Wyrtki index serves as a useful tool for a quantitative assessment of the sources for ENSO periodicity in reanalysis data and its biases in CMIP5 model simulations.

Modern instrumental records reveal that El Niño events differ in their spatial patterns and temporal evolutions. Attempts have been made to categorize them roughly into two main types: eastern Pacific (EP; or cold tongue) and central Pacific (CP; or warm pool) El Niño events. In this study, a modified version of the Zebiak–Cane (MZC) coupled model is used to examine the dynamics of these two types of El Niño events. Linear eigenanalysis of the model is conducted to show that there are two leading El Niño–Southern Oscillation (ENSO) modes with their SST patterns resembling those of two types of El Niño. Thus, they are referred to as the EP and CP ENSO modes. These two modes are sensitive to changes in the mean states. The heat budget analyses demonstrate that the EP (CP) mode is dominated by thermocline (zonal advective) feedback. Therefore, the weak (strong) mean wind stress and deep (shallow) mean thermocline prefer the EP (CP) ENSO mode because of the relative dominance of thermocline (zonal advective) feedback under such a mean state. Consistent with the linear stability analysis, the occurrence ratio of CP/EP El Niño events in the nonlinear simulations generally increases toward the regime where the linear CP ENSO mode has relatively higher growth rate. These analyses suggest that the coexistence of two leading ENSO modes is responsible for two types of El Niño simulated in the MZC model. This model result may provide a plausible scenario for the observed ENSO diversity.

This study investigates biases of the climatological mean state of the northern Arabian Sea (NAS) in 31 coupled ocean–atmosphere models. The focus is to understand the cause of the large biases in the depth of the 20°C isotherm [Formula: see text] that occur in many of them. Other prominent biases are the depth [Formula: see text] and temperature [Formula: see text] of Persian Gulf water (PGW) and the wintertime mixed-layer thickness (MLT) along the northern boundary.For models that lack a Persian Gulf (group 1), [Formula: see text] is determined by the wintertime MLT bias [Formula: see text] through the formation of an Arabian Sea high-salinity water mass (ASHSW) that is too deep. For models with a Persian Gulf (group 2), if [Formula: see text] > MLT (group 2B), PGW remains mostly trapped to the western boundary and, again, [Formula: see text] directly controls [Formula: see text]. If [Formula: see text] MLT (group 2A), PGW spreads into the NAS and impacts [Formula: see text] because [Formula: see text] > 20°C; nevertheless [Formula: see text] still influences [Formula: see text] indirectly through its impact on [Formula: see text].The thick wintertime mixed layer is driven primarily by surface cooling [Formula: see text] during the fall. Nevertheless, variations in ΔMLT among the models are more strongly linked to biases in the density stratification (jump) across the bottom of the mixed layer than to [Formula: see text] biases. The jump is in turn determined primarily by sea surface salinity biases (ΔSSS) advected into the NAS by the West India Coastal Current, and the source of ΔSSS is the rainfall deficit associated with the models’ weak summer monsoon. Ultimately, then, ΔD20 is linked to this deficit.

To understand sources of variability in the eastern equatorial Pacific, a region integral to modulating global temperatures, the waters upwelling from the Equatorial Undercurrent (EUC) are characterized. Past work is updated using temperature and salinity measurements from the Argo array and current measurements from Tropical Atmosphere Ocean (TAO) buoys. A larger hemispheric asymmetry is found in the water mass contribution through the ventilated thermocline to the EUC than previously reported, with 80%–90% of waters in the western Pacific originating from the Southern Hemisphere. South Pacific subtropical waters are the dominant source feeding the EUC, although in the central equatorial Pacific upper layers of the EUC experience freshening due to the addition of North Pacific waters. Anomalous volume transport, advection of anomalous waters, and shifts in hemispheric contributions contribute to variability in the EUC. These results suggest that variability in the EUC caused by anomalies in the South Pacific ventilated thermocline can explain variability in the eastern equatorial Pacific.

The coupled atmosphere–ocean variability of the Indo-Pacific domain on seasonal to multidecadal time scales is investigated in CCSM4 and in observations through nonlinear Laplacian spectral analysis (NLSA). It is found that ENSO modes and combination modes of ENSO with the annual cycle exhibit a seasonally synchronized southward shift of equatorial surface zonal winds and thermocline adjustment consistent with terminating El Niño and La Niña events. The surface winds associated with these modes also generate teleconnections between the Pacific and Indian Oceans, leading to SST anomalies characteristic of the Indian Ocean dipole. The family of NLSA ENSO modes is used to study El Niño–La Niña asymmetries, and it is found that a group of secondary ENSO modes with more rapidly decorrelating temporal patterns contributes significantly to positively skewed SST and zonal wind statistics. Besides ENSO, fundamental and combination modes representing the tropospheric biennial oscillation (TBO) are found to be consistent with mechanisms for seasonally synchronized biennial variability of the Asian–Australian monsoon and Walker circulation. On longer time scales, a multidecadal pattern referred to as the west Pacific multidecadal mode (WPMM) is established to significantly modulate ENSO and TBO activity, with periods of negative SST anomalies in the western tropical Pacific favoring stronger ENSO and TBO variability. This behavior is attributed to the fact that cold WPMM phases feature anomalous decadal westerlies in the tropical central Pacific, as well as an anomalously flat zonal thermocline profile in the equatorial Pacific. Moreover, the WPMM is found to correlate significantly with decadal precipitation over Australia.

The interannual fluctuations of the equatorial thermocline are usually associated with El Niño activity, but the linkage between the thermocline modes and El Niño is still under debate. In the present study, a mode function decomposition method is applied to the equatorial Pacific thermocline, and the results show that the first two dominant modes (M1 and M2) identify two distinct characteristics of the equatorial Pacific thermocline. The M1 reflects a basinwide zonally tilted thermocline related to the eastern Pacific (EP) El Niño, with shoaling (deepening) in the western (eastern) equatorial Pacific. The M2 represents the central Pacific (CP) El Niño, characterized by a V-shaped equatorial Pacific thermocline (i.e., deep in the central equatorial Pacific and shallow on both the western and eastern boundaries). Furthermore, both modes are stable and significant on the interannual time scale, and manifest as the major feature of the thermocline fluctuations associated with the two types of El Niño events. As good proxies of EP and CP El Niño events, thermocline-based indices clearly reveal the inherent characteristics of subsurface ocean responses during the evolution of El Niño events, which are characterized by the remarkable zonal eastward propagation of equatorial subsurface ocean temperature anomalies, particularly during the CP El Niño. Further analysis of the mixed layer heat budget suggests that the air–sea interactions determine the establishment and development stages of the CP El Niño, while the thermocline feedback is vital for its further development. These results highlight the key influence of equatorial Pacific thermocline fluctuations in conjunction with the air–sea interactions, on the CP El Niño.

The causes of the change in amplitude of El Niño–Southern Oscillation (ENSO) during the mid-Holocene were investigated by diagnosing the model simulations that participated in the Paleoclimate Modelling Intercomparison Project phases 2 and 3. Consistent with paleoclimate records, 20 out of the 28 models reproduced weaker-than-preindustrial ENSO amplitude during the mid-Holocene. Two representative models were then selected to explore the underlying mechanisms of air–sea feedback processes. A mixed layer heat budget diagnosis indicated that the weakened ENSO amplitude was primarily attributed to the decrease in the Bjerknes thermocline feedback, while the meridional advective feedback also played a role. During the mid-Holocene, the thermocline response to a unit anomalous zonal wind stress forcing in the equatorial Pacific weakened in both models because of the increased ENSO meridional scale. A further investigation revealed that the greater ENSO meridional width was caused by the strengthening of the Pacific subtropical cell, which was attributed to the enhanced mean trade wind that resulted from the intensified Asian and African monsoon rainfall and associated large-scale east–west circulation in response to the mid-Holocene orbital forcing.

Generation and propagation processes of upper-ocean heat content (OHC) in the North Pacific are investigated using oceanic subsurface observations and an eddy-resolving ocean general circulation model hindcast simulation. OHC anomalies are decomposed into physically distinct dynamical components (OHC ρ) due to temperature anomalies that are associated with density anomalies and spiciness components (OHC χ) due to temperature anomalies that are density compensating with salinity. Analysis of the observational and model data consistently shows that both dynamical and spiciness components contribute to interannual–decadal OHC variability, with the former (latter) component dominating in the subtropical (subpolar) North Pacific. OHC ρ variability represents heaving of thermocline, propagates westward, and intensifies along the Kuroshio Extension, consistent with jet-trapped Rossby waves, while OHC χ variability propagates eastward along the subarctic frontal zone, suggesting advection by mean eastward currents. OHC χ variability tightly corresponds in space to horizontal mean spiciness gradients. Meanwhile, area-averaged OHC χ anomalies in the western subarctic frontal zone closely correspond in time to meridional shifts of the subarctic frontal zone. Regression coefficient of the OHC χ time series on the frontal displacement anomalies quantitatively agree with the area-averaged mean spiciness gradient in the region, and suggest that OHC χ is generated via frontal variability in the subarctic frontal zone.

Two state-of-the-art Earth system models (ESMs) were used in an idealized experiment to explore the role of mountains in shaping Earth’s climate system. Similar to previous studies, removing mountains from both ESMs results in the winds becoming more zonal and weaker Indian and Asian monsoon circulations. However, there are also broad changes to the Walker circulation and El Niño–Southern Oscillation (ENSO). Without orography, convection moves across the entire equatorial Indo-Pacific basin on interannual time scales. ENSO has a stronger amplitude, lower frequency, and increased regularity. A wider equatorial wind zone and changes to equatorial wind stress curl result in a colder cold tongue and a steeper equatorial thermocline across the Pacific basin during La Niña years. Anomalies associated with ENSO warm events are larger without mountains and have greater impact on the mean tropical climate than when mountains are present. Without mountains, the centennial-mean Pacific Walker circulation weakens in both models by approximately 45%, but the strength of the mean Hadley circulation changes by less than 2%. Changes in the Walker circulation in these experiments can be explained by the large spatial excursions of atmospheric deep convection on interannual time scales. These results suggest that mountains are an important control on the large-scale tropical circulation, impacting ENSO dynamics and the Walker circulation, but have little impact on the strength of the Hadley circulation.

Coupled dynamics between westerly wind events (WWEs) and the El Niño–Southern Oscillation (ENSO) is examined using an atmosphere–ocean coupled model with intermediate complexity. The model incorporates state-dependent stochastic noise that mimics observed WWEs, which occur at the edge of the Pacific warm pool when the Niño-4 sea surface temperature (SST) anomaly increases positively. The model parameter that controls the efficiency of the thermocline feedback, γ, is perturbed to elaborate the sensitivity of the results to the system’s stability. Without the noise (experiment NO), the model produces an ENSO-like regular oscillation with a 6-yr period, the variance of which increases with γ. When additive noise is introduced over the western Pacific (experiment AD), the oscillations become irregular with a dominant period of 4–6 years and the increase in the variance relative to the NO experiment depends on γ. When state-dependent noise is included (experiment SD), the oscillatory solution is also irregular, and its variance and asymmetry are increased irrespective of the value of γ. Both the additive and state-dependent noise contribute to the occurrence of two types of variability, corresponding to the eastern Pacific (EP) and central Pacific (CP) El Niños. In SD, the state dependence of the stochastic noise guarantees the existence of CP El Niño regardless of γ since the increased likelihood of WWE occurrence with Niño-4 SSTs results in a positive feedback in the central Pacific. The above results suggest that the state dependence of WWEs plays a crucial role in the asymmetry and diversity of ENSO.

For many generations, models simulate an Indian Ocean dipole (IOD) that is overly large in amplitude. The possible impact of this systematic bias on climate projections, including a projected frequency increase in extreme positive IOD (pIOD) using a rainfall-based definition, has attracted attention. In particular, a recent study suggests that the increased frequency is an artifact of the overly large IOD amplitude. In contrast, here the opposite is found. Through intermodel ensemble regressions, the present study shows that models producing a high frequency in the present-day climate generate a small future frequency increase. The frequency is associated with the mean equatorial west-minus-east sea surface temperature (SST) gradient: the greater the gradient, the greater the frequency because it is easier to shift convection to the west, which characterizes an extreme pIOD. A greater present-day gradient is associated with a present-day shallower thermocline, lower SSTs, and lower rainfall in the eastern equatorial Indian Ocean (EEIO). Because there is an inherent limit for a maximum rainfall reduction and for the impact on surface cooling by a shallowing of an already shallow mean EEIO thermocline, there is a smaller increase in frequency in models with a shallower present-day EEIO thermocline. Given that a bias of overly shallow EEIO thermocline and overly low EEIO SSTs and rainfall is common in models, the future frequency increase should be underestimated, opposite to an implied overestimation resulting from the overly large IOD amplitude bias. Therefore, correcting the projected frequency from a single bias, without considering other biases that are present, is not appropriate and should be avoided.

The role of ocean dynamics in the response of the equatorial Pacific Ocean to climate warming is investigated using both an atmosphere–ocean coupled climate system and its ocean component. Results show that the initial response (fast pattern) to an uniform heating imposed on the ocean is a warming centered to the west of the date line owing to the conventional ocean dynamical thermostat (ODT) mechanism in the eastern equatorial Pacific—a cooling effect arising from the up-gradient upwelling. In time, the warming pattern gradually propagates eastward, becoming more El Niño–like (slow pattern). The transition from the fast to the slow pattern likely results from 1) the gradual warming of the equatorial thermocline temperature, which is associated with the arrival of the relatively warmer extratropical waters advected along the subsurface branch of the subtropical cells (STCs), and 2) the reduction of the STC strength itself. A mixed layer heat budget analysis finds that it is the total ocean dynamical effect rather than the conventional ODT that holds the key for understanding the pattern of the SST in the equatorial Pacific and that the surface heat flux works mainly to compensate the ocean dynamics. Further passive tracer experiments with the ocean component of the coupled system verify the role of the ocean dynamical processes in initiating a La Niña–like SST warming and in setting the pace of the transition to an El Niño–like warming and identify an oceanic origin for the slow eastern Pacific warming independent of the weakening trade wind.

Sea surface temperatures (SSTs) have been rising for decades in the Indian Ocean in response to greenhouse gas forcing. However, this study shows that during the recent hiatus in global warming, a striking interhemispheric gradient in Indian Ocean SST trends developed around 2000, with relatively weak or little warming to the north of 10°S and accelerated warming to the south of 10°S. Evidence is presented from a wide variety of data sources showing that this interhemispheric gradient in SST trends is forced primarily by an increase of Indonesian Throughflow (ITF) transport from the Pacific into the Indian Ocean induced by stronger Pacific trade winds. This increased transport led to a depression of the thermocline that facilitated SST warming, presumably through a reduction in the vertical turbulent transport of heat in the southern Indian Ocean. Surface wind changes in the Indian Ocean linked to the enhanced Walker circulation also may have contributed to thermocline depth variations and associated SST changes, with downwelling-favorable wind stress curls between 10° and 20°S and upwelling-favorable wind stress curls between the equator and 10°S. In addition, the anomalous southwesterly wind stresses off the coast of Somalia favored intensified coastal upwelling and offshore advection of upwelled water, which would have led to reduced warming of the northern Indian Ocean. Although highly uncertain, lateral heat advection associated with the ITF and surface heat fluxes may also have played a role in forming the interhemispheric SST gradient change.

Recent studies have highlighted the role of subsurface ocean dynamics in modulating eastern Pacific (EPac) hurricane activity on interannual time scales. In particular, the well-known El Niño–Southern Oscillation (ENSO) recharge–discharge mechanism has been suggested to provide a good understanding of the year-to-year variability of hurricane activity in this region. This paper investigates the influence of equatorial subsurface subannual and intraseasonal oceanic variability on tropical cyclone (TC) activity in the EPac. That is to say, it examines previously unexplored time scales, shorter than interannual, in an attempt to explain the variability not related to ENSO. Using ocean reanalysis products and TC best-track archive, the role of subannual and intraseasonal equatorial Kelvin waves (EKW) in modulating hurricane intensity in the EPac is examined. It is shown first that these planetary waves have a clear control on the subannual and intraseasonal variability of thermocline depth in the EPac cyclone-active region. This is found to affect ocean subsurface temperature, which in turn fuels hurricane intensification with a marked seasonal-phase locking. This mechanism of TC fueling, which explains up to 30% of the variability of TC activity unrelated to ENSO (around 15%–20% of the total variability), is embedded in the large-scale equatorial dynamics and therefore offers some predictability with lead time up to 3–4 months at seasonal and subseasonal time scales.

The double intertropical convergence zone (ITCZ) is a long-standing bias in the climatology of coupled general circulation models (CGCMs). The warm biases in southeastern Pacific (SEP) sea surface temperature (SST) are also evident in many CGCMs. In this study, the role of SEP SST in the double ITCZ is investigated by prescribing the observed SEP SST in the Community Earth System Model, version 1 (CESM1). Both the double ITCZ and dry equator problems are significantly improved with SEP SST prescribed. Both atmospheric and oceanic processes are involved in the improvements. The colder SST over the SEP decreases the precipitation, which enhances the southeasterly winds outside the prescribed SST region, cooling the ocean via increased evaporation. The enhanced descending motion over the SEP strengthens the Walker circulation. The easterly winds over the equatorial Pacific enhance upwelling and shoal the thermocline over the eastern Pacific. The changes of surface wind and wind curl lead to a weaker South Equatorial Countercurrent and stronger South Equatorial Current, preventing the warm water from expanding eastward, thereby improving both the double ITCZ and dry equator. The enhanced Walker circulation also increases the low-level wind convergence and reduces the wind speed in the tropical western Pacific, leading to warmer SST and stronger convection there. The stronger convection in turn leads to more cloud and reduces the incoming solar radiation, cooling the SST. These competing effects between radiative heat flux and latent heat flux make the atmospheric heat flux secondary to the ocean dynamics in the western Pacific warming.

Sea surface temperature (SST) variability associated with El Niño–Southern Oscillation (ENSO) slightly increased in the central Pacific Ocean but weakened significantly in the eastern Pacific at the beginning of twenty-first century relative to 1980–99. This decadal shift led to the greater prominence central Pacific (CP) El Niño events during the 2000s relative to the previous two decades, which were dominated by eastern Pacific (EP) events. To expand upon previous studies that have examined this shift in ENSO variability, temperature and temperature variance budgets are examined in the mixed layer of the Niño-3 (5°S–5°N, 150°–90°W) and Niño-4 (5°S–5°N, 160°E–150°W) regions from seven ocean model products spanning the period 1980–2010. This multimodel-product-based approach provides a robust assessment of dominant mechanisms that account for decadal changes in two key index regions. A temperature variance budget perspective on the role of thermocline feedbacks in the ENSO cycle based on recharge oscillator theory is also presented. As found in previous studies, thermocline and zonal advective feedbacks are the most important positive feedbacks for generating ENSO SST variance, and thermodynamic damping is the largest negative feedback for damping ENSO variance. Consistent with the shift toward more CP El Niños after 2000, thermocline feedbacks experienced a substantial reduction from 1980 to 1999 and into the 2000s, while zonal advective feedbacks were less affected. Negative feedbacks likewise weakened after 2000, particularly thermal damping in the Niño-3 region and the nonlinear sink of variance in both regions.

This study shows the sudden basinwide reversal of anomalous equatorial zonal transport above the thermocline at the peaking phase of ENSO triggers rapid termination of ENSO events. The anomalous equatorial zonal transport is controlled by the concavity of anomalous thermocline meridional structure across the equator. During the developing phase of ENSO, opposite zonal transport anomalies form in the western-central and central-eastern equatorial Pacific, respectively. Both are driven by the equatorial thermocline anomalies in response to zonal wind anomalies over the western-central equatorial ocean. At this stage, the anomalous zonal transport in the east enhances ENSO growth through zonal SST advection. In the mature phase of ENSO, off-equatorial thermocline depth anomalies become more dominant in the eastern Pacific because of the reflection of equatorial signals at the eastern boundary. As a result, the meridional concavity of the thermocline anomalies is reversed in the east. This change reverses zonal transport rapidly in the central-to-eastern equatorial Pacific, joining with the existing reversed zonal transport anomalies farther to the west, and forms a basinwide transport reversal throughout the equatorial Pacific. This basinwide transport reversal weakens the ENSO SST anomalies by reversed advection. More importantly, the reversed zonal transport reduces the existing zonal tilting of the equatorial thermocline and weakens its feedback to wind anomalies effectively. This basinwide reversal is built in at the peak phase of ENSO as an oceanic control on the evolution of both El Niño and La Niña events. The reversed zonal transport anomaly after the mature phase weakens El Niño in the eastern Pacific more efficiently than it weakens La Niña.

Seasonality is a fundamental feature of the coupled ocean–atmosphere system. The hemispherically opposite general pattern at the air–sea interface is well known, but its penetrative behavior below the sea surface is poorly understood. Over 10 years of Argo data reveal for the first time the spiral-like structure of vertical seasonality in the upper extratropical ocean: the amplitude (strength of annual sea temperature variability) decreases rapidly with depth while the phase (peaking time) rotates from August (February) at the sea surface to December (June) at the bottom of the mixed layer in the Northern (Southern) Hemisphere under a clockwise representation. It is found that, analogous to the Ekman current spiral, the oceanic seasonality is almost reversed at approximately 500-m depth with about 5% of the surface intensity left. In contrast, the seasonality of subtropical oceans below the thermocline exhibits a phase-lock pattern around May and October to the north and south of the equator, respectively. Meanwhile, a systematic westward progression of annual phase corresponding to the warmest month is observed in the equatorial regions between 10°S and 10°N. It is suggested that the seasonality spiral of extratropical oceans occurs as a result of the vertical decay of solar penetration in tandem with delayed annual maximum mixing in the context of buoyancy and turbulent induced convections, while the month of full summer appears to be “constant” May (October) in the subtropical oceans of the Northern (Southern) Hemisphere under the stratified subsurface layer due to a 9-month trapping of penetrating solar energy from May (October) to next January (June). The vertically locked westward annual phase progression in the equatorial regions is likely to be a consequence of the first baroclinic mode of β-refracted annual Rossby waves. The spiral and phase-lock behaviors of the upper oceans are of critical significance to the understanding of mixed layer and thermocline dynamics.

The long-term behavior of the wintertime mixed layer depth (MLD) and mixed layer temperature (MLT) are investigated in a region south of the Kuroshio Extension (KE) (30°–37°N, 141°–155°E), an area of the North Pacific subtropical gyre where the deepest MLD occurs, using historical temperature profiles of 1968–2014. Both the MLD and MLT in March have low-frequency variations, which show significant decadal (~10 yr) variations after the late 1980s. Observational data and simulation outputs from a one-dimensional turbulent closure model reveal that surface cooling is the main control on winter MLD in the late 1970s and 1980s, whereas there is a change in the strength of subsurface stratification is the main control after ~1990. In the latter period, a weak (strong) subsurface stratification is caused by a straight path (convoluted path) of the KE and by a deepening (shallowing) of the main thermocline depth due to oceanic Rossby waves formed as a result of positive (negative) anomalies of wind stress curl associated with a southward (northward) movement of the Aleutian low in the central North Pacific. During deeper (shallower) periods of winter MLD, the strong (weak) vertical entrainment process, resulting from a rapid (slow) deepening of the mixed layer (ML) in January and February, forms a negative (positive) anomaly of temperature tendency. Consequently, the decadal variations in wintertime MLT are formed.

The transport of dissolved oxygen (O2) from the surface ocean into the interior is a critical process sustaining aerobic life in mesopelagic ecosystems, but its rates and sensitivity to climate variations are poorly understood. Using a circulation model constrained to historical variability by assimilation of observations, the study shows that the North Pacific thermocline effectively takes up O2 primarily by expanding the area through which O2-rich mixed layer water is detrained into the thermocline. The outcrop area during the critical winter season varies in concert with the Pacific decadal oscillation (PDO). When the central North Pacific Ocean is in a cold phase, the winter outcrop window for the central mode water class (CMW; a neutral density range of γ = 25.6–26.6) expands southward, allowing more O2-rich surface water to enter the ocean’s interior. An increase in volume flux of water to the CMW density class is partly compensated by a reduced supply to the shallower densities of subtropical mode water (γ = 24.0–25.5). The thermocline has become better oxygenated since the 1980s partly because of strong O2 uptake. Positive O2 anomalies appear first near the outcrop and subsequently downstream in the subtropical gyre. In contrast to the O2 variations within the ventilated thermocline, observed O2 in intermediate water (density range of γ = 26.7–27.2) shows a declining trend over the past half century, a trend not explained by the open ocean water mass formation rate.

In the North Atlantic, there are pronounced gyre-scale changes in ocean heat content on interannual-to-decadal time scales, which are associated with changes in both sea surface temperature and thermocline thickness; the subtropics are often warm with a thick thermocline when the subpolar gyre is cool with a thin thermocline, and vice versa. This climate variability is investigated using a semidiagnostic dynamical analysis of historical temperature and salinity data from 1962 to 2011 together with idealized isopycnic model experiments. On time scales of typically 5 yr, the tendencies in upper-ocean heat content are not simply explained by the area-averaged atmospheric forcing for each gyre but instead dominated by heat convergences associated with the meridional overturning circulation. In the subtropics, the most pronounced warming events are associated with an increased influx of tropical heat driven by stronger trade winds. In the subpolar gyre, the warming and cooling events are associated with changes in western boundary density, where increasing Labrador Sea density leads to an enhanced overturning and an influx of subtropical heat. Thus, upper-ocean heat content anomalies are formed in a different manner in the subtropical and subpolar gyres, with different components of the meridional overturning circulation probably excited by the local imprint of atmospheric forcing.

The response of the equatorial Pacific Ocean’s seasonal cycle to orbital forcing is explored using idealized simulations with a coupled atmosphere–ocean GCM in which eccentricity, obliquity, and the longitude of perihelion are altered while other boundary conditions are maintained at preindustrial levels. The importance of ocean dynamics in the climate response is investigated using additional simulations with a slab ocean version of the model. Precession is found to substantially influence the equatorial Pacific seasonal cycle through both thermodynamic and dynamic mechanisms, while changes in obliquity have only a small effect. In the precession experiments, western equatorial Pacific SSTs respond in a direct thermodynamic manner to changes in insolation, while the eastern equatorial Pacific is first affected by the propagation of thermocline temperature anomalies from the west. These thermocline signals result from zonal wind anomalies associated with changes in the strength of subtropical anticyclones and shifts in the regions of convection in the western equatorial Pacific. The redistribution of heat from these thermocline signals, aided by the direct thermodynamic effect of insolation anomalies, results in large changes to the strength and timing of the eastern equatorial Pacific seasonal cycle. A comparison of 10 CMIP5 mid-Holocene experiments, in which the primary forcing is due to precession, shows that this response is relatively robust across models. Because equatorial Pacific SST anomalies have local climate impacts as well as nonlocal impacts through teleconnections, these results may be important to understanding paleoclimate variations both inside and outside of the tropical Pacific.

In this study decadal (≥10 yr) sea level variations in the Indian Ocean (IO) during 1950–2012 are investigated using the Hybrid Coordinate Ocean Model (HYCOM). The solution of the main run agrees well with observations in the western-to-central IO. Results of HYCOM experiments reveal large spatial variations in the mechanisms of decadal sea level variability. Within the tropical IO (north of 20°S), decadal sea level variations achieve maximum amplitude in the south IO thermocline ridge region. They are predominantly forced by decadal fluctuations of surface wind stress associated with climate variability modes, while the impact of other processes is much smaller. The Somali coast and the western Bay of Bengal are two exceptional regions, where ocean internal (unforced) variability has large contribution. Between 28° and 20°S in the subtropical south IO, surface heat flux and ocean internal variability are the major drivers of decadal sea level variability. Heat budget analysis for the upper 300 m of this region suggests that surface heat flux affects regional thermosteric sea level through both local surface heating and heat transport by ocean circulation. In the southwestern IO south of 30°S, where stochastic winds are strong, stochastic wind forcing and its interaction with ocean internal variability generate pronounced decadal variations in sea level. The comprehensive investigation of decadal sea level variability over the IO from an oceanic perspective will contribute to decadal sea level prediction research, which has a high societal demand.

The subsurface ocean response to anthropogenic climate forcing remains poorly characterized. From the Coupled Model Intercomparison Project (CMIP), a robust response of the lower thermocline is identified, where the warming is considerably weaker in the subtropics than in the tropics and high latitudes. The lower thermocline change is inversely proportional to the thermocline depth in the present climatology. Ocean general circulation model (OGCM) experiments show that sea surface warming is the dominant forcing for the subtropical gyre change in contrast to natural variability for which wind dominates, and the ocean response is insensitive to the spatial pattern of surface warming. An analysis based on a ventilated thermocline model shows that the pattern of the lower thermocline change can be interpreted in terms of the dynamic response to the strengthened stratification and downward heat mixing. Consequently, the subtropical gyres become intensified at the surface but weakened in the lower thermcline, consistent with results from CMIP experiments.

An excessive cold tongue error in the equatorial Pacific has prevailed in several generations of climate models. However, the causes of this problem remain a mystery, partly owing to uncertainty and/or a lack of observational datasets. Based on the multimodel ensemble from phase 5 of the Coupled Model Intercomparison Project (CMIP5), this study introduces a novel intermodel approach to identify the bias source by going beyond comparison with observational datasets. Intermodel statistics show that the excessive cold tongue bias could be traced back to a too strong oceanic dynamic cooling linked to a too shallow thermocline along the equatorial Pacific. A heat budget analysis suggests that the excessive oceanic dynamic cooling is balanced by the surface latent heat flux (LHF) adjustment. This is consistent with a variety of oceanic and atmospheric observations but at odds with the popular objectively analyzed air–sea heat fluxes (OAFlux) products. Further analyses suggest an alarming overestimation of OAFlux net surface heat flux (Qnet) into the tropical Pacific, mainly ascribed to observational uncertainly in air specific humidity. Implications for intermodel statistics in assessing model processes, validating observational data, and regulating future climate projections are discussed.

The thermocline shoals in the South China Sea (SCS) relative to the tropical northwest Pacific Ocean (NWP), as required by geostrophic balance with the Kuroshio. The present study examines the effect of this difference in ocean state on the response of sea surface temperature (SST) and chlorophyll concentration to tropical cyclones (TCs), using both satellite-derived measurements and three-dimensional numerical simulations. In both regions, TC-produced SST cooling strongly depends on TC characteristics (including intensity as measured by the maximum surface wind speed, translation speed, and size). When subject to identical TC forcing, the SST cooling in the SCS is more than 1.5 times that in the NWP, which may partially explain weaker TC intensity on average observed in the SCS. Both a shallower mixed layer and stronger subsurface thermal stratification in the SCS contribute to this regional difference in SST cooling. The mixed layer effect dominates when TCs are weak, fast-moving, and/or small; and for strong and slow-moving TCs or strong and large TCs, both factors are equally important. In both regions, TCs tend to elevate surface chlorophyll concentration. For identical TC forcing, the surface chlorophyll increase in the SCS is around 10 times that in the NWP, a difference much stronger than that in SST cooling. This large regional difference in the surface chlorophyll response is at least partially due to a shallower nutricline and stronger vertical nutrient gradient in the SCS. The effect of regional difference in upper-ocean density stratification on the surface nutrient response is negligible. The total annual primary production increase associated with the TC passage estimated using the vertically generalized production model in the SCS is nearly 3 times that in the NWP (i.e., 6.4 ± 0.4 × 1012 versus 2.2 ± 0.2 × 1012 g C), despite the weaker TC activity in the SCS.