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The characteristics of El Niño–Southern Oscillation (ENSO) variability have experienced notable changes since the late 1990s, including a breakdown of the zonal mean upper-ocean heat content as a precursor for ENSO. These changes also initiated a debate on the role of thermocline variations on the development of ENSO events since the beginning of the twenty-first century. In this study, the connection between thermocline variations and El Niño and La Niña events is examined separately for the 1980–98 and 1999–2012 periods. The analysis highlights the important role of thermocline variations in modulating ENSO evolutions in both periods. It is found that thermocline variation averaged in the central tropical Pacific, including both equatorial and off-equatorial regions, is a good precursor for ENSO evolutions before and after 1999, while the traditional basinwide mean of equatorial thermocline variation is a good precursor only before 1999. The new precursor, including both high-frequency variability in equatorial regions and low-frequency variability in off-equatorial regions, is found to be indicative of multiyear persistent warm and cold conditions in the tropical Pacific. Further, it is found that the strength of the subtropical cells (STCs) interior mass transport in both hemispheres increased rapidly around the late 1990s. It is proposed that the strengthened STC interior transports provide a pathway for the enhanced influence of off-equatorial thermocline variations on the development of ENSO events after 1999.

The relationship between the warm water volume (WWV) ENSO precursor and ENSO SST weakened substantially after ∼2000, coinciding with a degradation in dynamical model ENSO prediction skill. It is important to understand the drivers of the equatorial thermocline temperature variations and their linkage to ENSO onsets. In this study, a set of ocean reanalyses is employed to assess factors responsible for the variation of the equatorial Pacific Ocean thermocline during 1982–2019. Off-equatorial thermocline temperature anomalies carried equatorward by the mean meridional currents associated with Pacific tropical cells are shown to play an important role in modulating the central equatorial thermocline variations, which is rarely discussed in the literature. Further, ENSO events are delineated into two groups based on precursor mechanisms: the western equatorial Pacific type (WEP) ENSO, when the central equatorial thermocline is mainly influenced by the zonal propagation of anomalies from the western Pacific, and the off-equatorial central Pacific (OCP) ENSO, when off-equatorial central thermocline anomalies play the primary role. WWV is found to precede all WEP ENSO events by 6–9 months, while the correlation is substantially lower for OCP ENSO events. In contrast, the central tropical Pacific (CTP) precursor, which includes off-equatorial thermocline signals, has a very robust lead correlation with the OCP ENSO. Most OCP ENSO events are found to follow the same ENSO conditions, and the number of OCP ENSO events increases substantially since the start of the twenty-first century. These results highlight the importance of monitoring off-equatorial subsurface preconditions for ENSO prediction and to understand multiyear ENSO.

The seasonal upwelling along the west coast of India (WCI) brings nutrient-rich, oxygen-poor subsurface waters to the continental shelf, favoring very low oxygen concentrations in the surface waters during late boreal summer and fall. This yearly-recurring coastal hypoxia is more severe during some years, leading to coastal anoxia that has strong impacts on the living resources. In the present study, we analyze a 1/4° resolution coupled physical–biogeochemical regional oceanic simulation over the 1960–2012 period to investigate the physical processes influencing the oxycline interannual variability off the WCI, that being a proxy for the variability on the shelf in our model. Our analysis indicates a tight relationship between the oxycline and thermocline variations in this region on both seasonal and interannual timescales, thereby revealing a strong physical control of the oxycline variability. As in observations, our model exhibits a shallow oxycline and thermocline during fall that combines with interannual variations to create a window of opportunity for coastal anoxic events. We further demonstrate that the boreal fall oxycline fluctuations off the WCI are strongly related to the Indian Ocean Dipole (IOD), with an asymmetric influence of its positive and negative phases. Positive IODs are associated with easterly wind anomalies near the southern tip of India. These winds force downwelling coastal Kelvin waves that propagate along the WCI and deepen the thermocline and oxycline there, thus preventing the occurrence of coastal anoxia. On the other hand, negative IODs are associated with WCI thermocline and oxycline anomalies of opposite sign but of smaller amplitude, so that the negative or neutral IOD phases are necessary but not the sufficient condition for coastal anoxia. As the IODs generally start developing in summer, these findings suggest some predictability to the occurrence of coastal anoxia off the WCI a couple of months ahead.

Several previous studies have suggested that a cool western Indian Ocean may induce convection over Indonesia, leading to surface convergence and easterlies over the western equatorial Pacific Ocean. These easterlies then increase the Pacific Warm Water Volume favouring El Niño in the next year. We investigate this mechanism of Indian-Pacific interaction using output from two simulations (at 0 . 1 ∘ and 1 ∘ ocean model resolution) with the Community Earth System Model (CESM). No conclusive evidence for the suggested interaction mechanism is found in CESM. Like many other coupled models, CESM has an overly strong sea surface temperature variability in the eastern Indian Ocean due to an exaggerated sensitivity of the sea surface temperature to thermocline variations. Due to this bias the effect of the western Indian Ocean signals on the Pacific, as found from observational analysis, is hidden. Our analysis shows that this bias can be traced to errors in the time-mean vertical temperature profile in the Indian Ocean. This bias needs to be reduced to allow a better investigation of the subtle interactions between the Indian and Pacific Oceans.

The performance of 23 World Climate Research Programme (WCRP) Coupled Model Intercomparison Project, phase 3 (CMIP3) models in the simulation of the Indian Ocean dipole (IOD) is evaluated, and the results show large diversity in the simulated IOD intensity. A detailed diagnosis is carried out to understand the role of the Bjerknes dynamic air–sea feedback and the thermodynamic air–sea coupling in shaping the different model behaviors. The Bjerknes feedback processes include the equatorial zonal wind response to SST, the thermocline response to the equatorial zonal wind, and the ocean subsurface temperature response to the thermocline variation. The thermodynamic feedback examined includes the wind–evaporation–SST and cloud–radiation–SST feedbacks. A combined Bjerknes and thermodynamic feedback intensity index is introduced. This index well reflects the simulated IOD strength contrast among the strong, moderate, and weak model groups. It gives a quantitative measure of the relative contribution of the dynamic and thermodynamic feedback processes. The distinctive features in the dynamic and thermodynamic coupling strength are closely related to the mean state difference in the coupled models. A shallower (deeper) equatorial mean thermocline, a stronger (weaker) background vertical temperature gradient, and a greater (smaller) mean vertical upwelling velocity are found in the strong (weak) IOD simulation group. Thus, the mean state biases greatly affect the air–sea coupling strength on the interannual time scale. A number of models failed to simulate the observed positive wind–evaporation–SST feedback during the IOD developing phase. Analysis indicates that the bias arises from a greater contribution to the surface latent heat flux anomaly by the sea–air specific humidity difference than by the wind speed anomaly.

The effect of long-term trends and interannual, ENSO-driven variability in the Indian Ocean (IO) on the stability and spatial pattern of ENSO is investigated with an intermediate-complexity two-basin model. The Pacific basin is modeled using a fully coupled (i.e., generating its own background state) Zebiak–Cane model. IO sea surface temperature (SST) is represented by a basinwide warming pattern whose strength is constant or varies at a prescribed lag to ENSO. Both basins are coupled through an atmosphere transferring information between them. For the covarying IO SST, a warm IO during the peak of El Niño (La Niña) dampens (destabilizes) ENSO, and a warm IO during the transition from El Niño to La Niña (La Niña to El Niño) shortens (lengthens) the period. The influence of the IO on the spatial pattern of ENSO is small. For constant IO warming, the ENSO cycle is destabilized because stronger easterlies induce more background upwelling, more thermocline steepening, and a stronger Bjerknes feedback. The SST signal at the east coast weakens or reverses sign with respect to the main ENSO signal [i.e., ENSO resembles central Pacific (CP) El Niños]. This is due to a reduced sensitivity of the SST to thermocline variations in case of a shallow background thermocline, as found near the east coast for a warm IO. With these results, the recent increase in CP El Niño can possibly be explained by the substantial IO (and west Pacific) warming over the last decades.

Variations in eastern Indian Ocean upper-ocean thermal properties are assessed for the period 1970–2004, with a particular focus on asymmetric features related to opposite phases of Indian Ocean dipole events, using high-resolution ocean model hindcasts. Sensitivity experiments, where interannual atmospheric forcing variability is restricted to the Indian or Pacific Ocean only, support the interpretation of forcing mechanisms for large-scale asymmetric behavior in eastern Indian Ocean variability. Years are classified according to eastern Indian Ocean subsurface heat content (HC) as proxy of thermocline variations. Years characterized by an anomalous low HC feature a zonal gradient in upper-ocean properties near the equator, while high events have a meridional gradient from the tropics into the subtropics. The spatial and temporal characteristics of the seasonal evolution of HC anomalies for the two cases is distinct, as is the relative contribution from Indian Ocean atmospheric forcing versus remote influences from Pacific wind forcing: low events develop rapidly during austral winter/spring in response to Indian Ocean wind forcing associated with an enhanced southeasterly monsoon driving coastal upwelling and a shoaling thermocline in the east; in contrast, formation of an anomalous high eastern Indian Ocean HC is more gradual, with anomalies earlier in the year expanding from the Indonesian Throughflow (ITF) region, initiated by remote Pacific wind forcing, and transmitted through the ITF via coastal wave dynamics. Implications for seasonal predictions arise with high HC events offering extended lead times for predicting thermocline variations and upper-ocean properties across the eastern Indian Ocean.

One of the fundamental questions concerning the nature and prediction of the oceanic states in the equatorial eastern Pacific is how the turnabout from a cold water state (La Niña) to a warm water state (El Niño) takes place, and vice versa. Recent studies show that this turnabout is directly linked to the interannual thermocline variations in the tropical Pacific Ocean basin. An index, as an indicator and precursor to describe interannual thermocline variations and the turnabout of oceanic states in our previous paper (Qian and Hu, 2005), is also used in this study. The index, which shows the maximum subsurface temperature anomaly (MSTA), is derived from the monthly 21-year (1980-2000) expendable XBT dataset in the present study. Results show that the MSTA can be used as a precursor for the occurrences of El Niño (or La Niña) events. The subsequent analyses of the MSTA propagations in the tropical Pacific suggest a one-year potential predictability for El Niño and La Niña events by identifying ocean temperature anomalies in the thermocline of the western Pacific Ocean. It also suggests that a closed route cycle with the strongest signal propagation is identified only in the tropical North Pacific Ocean. A positive (or negative) MSTA signal may travel from the western equatorial Pacific to the eastern equatorial Pacific with the strongest signal along the equator. This signal turns northward along the tropical eastern boundary of the basin and then moves westward along the north side of off-equator around 16°N. Finally, the signal returns toward the equator along the western boundary of the basin. The turnabout time from an El Niño event to a La Niña event in the eastern equatorial Pacific depends critically on the speed of the signal traveling along the closed route, and it usually needs about 4 years. This finding may help to predict the occurrence of the El Niño or La Niña event at least one year in advance.[PUBLICATION ABSTRACT]

In case of considering the fact that the tropical ocean thermocline or the lower atmosphere height field has the meridional variation, the topographic Rossby wave will be excited. Also the background geostrophic current emerges. Under such conditions, the topographic Rossby wave can interact with the classical Rossby wave at some parameter band and thus brings a new kind of instability, which may be of potential application in understanding some phenomena of the ENSO cycle.

The thermocline depth in the tropical Indian Ocean has experienced dramatic decadal variations in recent decades. Using analysis and reanalysis datasets, we find that the decadal thermocline depth anomalies show large seasonal differences. The seasonal differences are modulated by two major modes. The first mode shows a zonal dipole pattern, with opposite thermocline depth anomalies in the equatorial eastern Indian Ocean and western Indian Ocean, and is prominent in summer and winter. The second mode is characterized by marked thermocline depth anomalies in the southern Indian Ocean and is significant in spring and fall. The amplitude of the seasonal oscillation in these two modes has increased substantially in the twenty-first century. Their phase change is in good agreement with the observed thermocline depth anomalies in each season. The results also show that the seasonality of the decadal thermocline depth anomalies arises directly from surface wind variations within the Indian Ocean. The first mode is mainly caused by equatorial zonal wind anomalies. The second mode is dominated by local wind stress curl anomalies. These wind anomalies are both significantly correlated with the ENSO-like SST anomalies in the Pacific Ocean. The findings improve our understanding of the decadal thermocline anomalies, and will help to better evaluate their impact on seasonal phase-locked oceanic and atmospheric processes.