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Equatorial subsurface ocean temperature (Tsub) strongly influences tropical Pacific climate variability, yet the Coupled Model Intercomparison Project Phase 5 (CMIP5) and Phase 6 (CMIP6) models exhibit persistent Tsub cold biases and substantial inter‐model uncertainty. Here we diagnose both systematic biases and inter‐model spread in historical Tsub by applying a linear decomposition based on three key parameters: warm‐pool temperature, thermocline depth, and thermocline sharpness. Cooler Tsub tends to occur in models with a cooler warm pool, shallower thermocline, and sharper stratification. These parameters explain most of the systematic biases and inter‐model spread in Tsub among the CMIP historical simulations. Thermocline sharpness consistently dominates the systematic Tsub biases, whereas thermocline depth emerges as the primary source of inter‐model spread in CMIP6. Our method provides an efficient empirical diagnosis of Tsub biases and model uncertainty that can help guide future model development.

The trends over recent decades in tropical Pacific sea surface and upper ocean temperature are examined in observations-based products, an ocean reanalysis and the latest models from the Coupled Model Intercomparison Project phase six and the Multimodel Large Ensembles Archive. Comparison is made using three metrics of sea surface temperature (SST) trend—the east–west and north–south SST gradients and a pattern correlation for the equatorial region—as well as change in thermocline depth. It is shown that the latest generation of models persist in not reproducing the observations-based SST trends as a response to radiative forcing and that the latter are at the far edge or beyond the range of modeled internal variability. The observed combination of thermocline shoaling and lack of warming in the equatorial cold tongue upwelling region is similarly at the extreme limit of modeled behavior. The persistence over the last century and a half of the observed trend toward an enhanced east–west SST gradient and, in four of five observed gridded datasets, to an enhanced equatorial north–south SST gradient, is also at the limit of model behavior. It is concluded that it is extremely unlikely that the observed trends are consistent with modeled internal variability. Instead, the results support the argument that the observed trends are a response to radiative forcing in which an enhanced east–west SST gradient and thermocline shoaling are key and that the latest generation of climate models continue to be unable to simulate this aspect of climate change.

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 Makassar Strait throughflow (MST) is the major component of the Indonesian Throughflow (ITF), transferring Pacific water into the Indian Ocean. In our previous study, we identified a new zonal pathway, a. k.a. the North Equatorial Subsurface Current (NESC), which carried equatorial water into the MST sub‐thermocline (>300 m) in the summer 2016 following the 2015/16 El Niño. We now show continued strong southward MST in the sub‐thermocline during the winter of 2016–2017, with salinity higher than that in the summer 2016, due to direct South Pacific water intrusion into the Sulawesi Sea. The origin of the intrusion is identified from the New Guinea Coastal Undercurrent (NGCUC) and from an anomalous westward flow along 3°N in the western equatorial Pacific. The identified interannual variability of the western Pacific Ocean circulation is particularly strong in the winter following super El Niño events. Plain Language Summary The Indonesian Throughflow (ITF) transfers Pacific waters into the eastern Indian Ocean through the complex passages of the Maritime Continent, affecting the water properties and heat content in both oceans. The vertical structure of the ITF plays an important role in modulating the Indo‐Pacific Ocean heat content and climate. Understanding the Pacific water mass sources of the ITF and their variations is essential to understanding interocean heat and salt transports. The sub‐thermocline (>300 m) throughflow within the Indonesian Seas, has waters drawn from the relatively salty South Pacific thermocline. To date, the pathway of the South Pacific water into the ITF is not understood well. Here we present evidence showing a new pathway for high salinity South Pacific water flowing into the sub‐thermocline Makassar Strait directly after strong El Nino events, which may become more common in the future. This study helps to understand the importance of the South Pacific water in the variations of the Great Ocean Conveyer Belt and in biogeochemical processes with ecological impacts downstream of the ITF. Key Points Strong anomalous southward flow with higher salinity in the winter of 2016–2017 was observed in the sub‐thermocline Makassar Strait The high salinity is due to direct intrusion of South Pacific water from the western boundary current and an anomalous flow along 3 °N The identified direct intrusion of South Pacific water into the Makassar Strait appears strong in the winter following a super El Niño

The El Niño−Southern Oscillation (ENSO), which is tightly coupled to the equatorial thermocline in the Pacific, is the dominant source of interannual climate variability, but its long-term evolution in response to climate change remains highly uncertain. This study uses Mg/Ca in planktonic foraminiferal shells to reconstruct sea surface and thermocline water temperatures (SST and TWT) for the past 142 ky in a western equatorial Pacific (WEP) core MD01-2386. Unlike the dominant 100-ky glacial−interglacial cycle recorded by SST and δ18O, which echoes the pattern seen in other WEP sites, the upper ocean thermal gradient shows a clear half-precessional (9.4 ky or 12.7 ky) cycle as indicated by the reconstructed and simulated temperature (ΔT) and δ18O differences between the surface and thermocline waters. This phenomenon is attributed to the interplay of subtropical-to-tropical thermocline anomalies forced by the antiphased meridional insolation gradients in the two hemispheres at the precessional band. In particular, the TWT shows greater variability than SST, and dominates the ΔT changes which couple with the west−east SST difference in the equatorial Pacific at the half-precessional band, implying a decisive role of the tropical thermocline in orbital-scale climate change.

A large fraction (35%–50%) of observed La Niña events last two years or longer, in contrast to the great majority of El Niño events, which last one year. Here, the authors explore the nonlinear processes responsible for the multiyear persistence of La Niña in the Community Climate System Model, version 4 (CCSM4), a coupled climate model that simulates the asymmetric duration of La Niña and El Niño events realistically. The authors develop a nonlinear delayed-oscillator (NDO) model of the El Niño–Southern Oscillation (ENSO) to explore the mechanisms governing the duration of La Niña. The NDO includes nonlinear and seasonally dependent feedbacks derived from the CCSM4 heat budget, which allow it to simulate key ENSO features in quantitative agreement with CCSM4. Sensitivity experiments with the NDO show that the nonlinearity in the delayed thermocline feedback is the sole process controlling the duration of La Niña events. The authors’ results show that, as La Niña events become stronger, the delayed thermocline response does not increase proportionally. This nonlinearity arises from two processes: 1) the response of winds to sea surface temperature anomalies and 2) the ability of thermocline depth anomalies to influence temperatures at the base of the mixed layer. Thus, strong La Niña events require that the thermocline remains deeper for longer than 1 yr for sea surface temperatures to return to neutral. Ocean reanalysis data show evidence for this thermocline nonlinearity, suggesting that this process could be at work in nature.

The recharge oscillator model of the El Niño Southern Oscillation (ENSO) describes the ENSO dynamics as an interaction between the eastern tropical Pacific sea surface temperatures ( T ) and subsurface heat content (thermocline depth; h ), defining a dynamical cycle with different phases. h is often approximated on the basis of the depth of the 20 °C isotherm ( Z 20 ). In this study we will address how the estimation of h affects the representation of ENSO dynamics. We will compare the ENSO phase space with h estimated based on Z 20 and based on the maximum gradient in the temperature profile ( Z mxg ). The results illustrate that the ENSO phase space is much closer to the idealised recharge oscillator model if based on Z mxg than if based on Z 20 . Using linear and non-linear recharge oscillator models fitted to the observed data illustrates that the Z 20 estimate leads to artificial phase dependent structures in the ENSO phase space, which result from an in-phase correlation between h and T . Based on the Z mxg estimate the ENSO phase space diagram show very clear non-linear aspects in growth rates and phase speeds. Based on this estimate we can describe the ENSO cycle dynamics as a non-linear cycle that grows during the recharge and El Nino state, and decays during the La Nina states. The most extreme ENSO states are during the El Nino and discharge states, while the La Nina and recharge states do not have extreme states. It further has faster phase speeds after the El Nino state and slower phase speeds during and after the La Nina states. The analysis suggests that the ENSO phase speed is significantly positive in all phases, suggesting that ENSO is indeed a cycle. However, the phase speeds are closest to zero during and after the La Nina state, indicating that the ENSO cycle is most likely to stall in these states.

Pine Island Glacier has thinned and accelerated over recent decades, significantly contributing to global sea-level rise. Increased oceanic melting of its ice shelf is thought to have triggered those changes. Observations and numerical modeling reveal large fluctuations in the ocean heat available in the adjacent bay and enhanced sensitivity of ice-shelf melting to water temperatures at intermediate depth, as a seabed ridge blocks the deepest and warmest waters from reaching the thickest ice. Oceanic melting decreased by 50% between January 2010 and 2012, with ocean conditions in 2012 partly attributable to atmospheric forcing associated with a strong La Niña event. Both atmospheric variability and local ice shelf and seabed geometry play fundamental roles in determining the response of the Antarctic Ice Sheet to climate.

During the mature phases of two types of El Niño, the patterns of sea surface temperature anomalies (SSTA) are obviously different, with the centers near eastern Pacific (EP) for the EP El Niño and near the dateline (180° of longitude) for the central Pacific (CP) El Niño. However, contradicting with the observation, in the Community Earth System Model version 2 (CESM2), the SSTA centers of both types of El Niño are close to the CP area, which makes them difficult to be separated, i.e., the CESM2 shows a relatively poor depiction of the realistic El Niño diversity. To explore the possible reasons for this deficiency, a meticulous comparison of the dominant mechanisms for the equatorial Pacific SSTA evolution, i.e., the thermocline feedback (TH) and the zonal advective feedback (ZA), between the model and observation is conducted in this study. The results suggest that comparing with the observation, the weak intensity of TH and the westward shift of the dominant ZA position in the model are the primary causes that induce such proximity of SSTA centers of the two types of El Niño. The deeper thermocline depth, which induce smaller amplitude of thermocline depth variation, cause the deviation of the TH in CESM2. Furthermore, the deviation of the ZA comes from the pronounced westward bias in simulating the background zonal gradient of sea surface temperature, along with the weak zonal current anomalies in the EP area.

The changes in the intensity of the southern tropical Indian Ocean dipole mode (STIOD) are investigated using observations and the Community Earth System Model Large Ensemble (CESM-LE) project in this study. The positive STIOD is characterized as cold SST anomalies over the tropical southeastern Indian Ocean (SEIO) and warm SST anomalies over the southcentral Indian Ocean (SCIO), which peak in boreal summer. It is suggested that the intensity of interannual variability in the STIOD experienced prominent interdecadal changes from 1970 to the present. The STIOD was relatively weak before the late 1980s, while it is enhanced significantly during the late 1980s to early-2000s and displayed some decrease in the intensity after the early-2000s. As an important generation mechanism, changes in the strengths of the Bjerknes feedback between the SCIO and SEIO mainly contribute to the variations of the STIOD intensity. The changes in Bjerknes feedback are associated with the variations in climatological mean states over the tropical Indian Ocean. The warmer climatological SST strengthens the efficiency of the SEIO SST in driving wind. On the other hand, the combined effects of the Indonesian Throughflow and surface climatological zonal winds alter the mean thermocline depth over the SEIO, contributing to the variations in the relationship between the thermocline depth and SST anomalies in situ. Two sets of historical and RCP8.5 simulations from CESM1-LE with 35 ensemble members are analyzed to confirm the roles of mean state changes on the intensity of interannual variability of STIOD.