Mechanisms of asymmetry in sea surface temperature anomalies associated with the Indian Ocean Dipole revealed by closed heat budget

The Indian Ocean Dipole (IOD) is an interannual climate mode of the tropical Indian Ocean. Although it is known that negative sea surface temperature (SST) anomalies in the eastern pole during the positive IOD are stronger than positive SST anomalies during the negative IOD, no consensus has been reached on the relative importance of various mechanisms that contribute to this asymmetry. Based on a closed mixed layer heat budget analysis using a regional ocean model, here we show for the first time that the vertical mixing plays an important role in causing such asymmetry in SST anomalies in addition to the contributions from the nonlinear advection and the thermocline feedback proposed by previous studies. A decomposition of the vertical mixing term indicates that nonlinearity in the anomalous vertical temperature gradient associated with subsurface temperature anomalies and anomalous vertical mixing coefficients is the main driver of such asymmetry. Such variations in subsurface temperature are induced by the anomalous southeasterly trade winds along the Indonesian coast that modulate the thermocline depth through coastal upwelling/downwelling. Thus, the thermocline feedback contributes to the SST asymmetry not through the vertical advection as previously suggested, but via the vertical mixing.

Weakening of the equatorial Atlantic SST variability under global warming

<p>The eastern equatorial Atlantic is the region with the largest seasonal and interannual sea surface temperature (SST) variability in the entire tropical Atlantic Ocean. It is characterized by a rapid cooling during the boreal summer season, between June and September, that has large impacts in the regional climate. In this study we explore climate changes related to global warming in the cold tongue region using the CMIP5 and CMIP6 datasets as benchmarks. The historical simulations of both CMIP generations reproduce fairly well the spatial pattern of the observed warming – although weaker – in the Angola-Benguela region and most of the equatorial Atlantic band. The largest disagreements between model and observations are localized in the eastern equatorial Atlantic. The future business-as-usual scenario shows an intense and zonally homogeneous warming along the equatorial Atlantic band in CMIP5 and CMIP6. We also find a significant reduction of the June-July-August SST variability of 12% (17%) in the ensemble mean of the CMIP5 (CMIP6), in the future scenario (2050-2099) with respect to the historical period (1950-1999). The thermocline feedback, i.e., the local response of the SST anomalies to the thermocline depth anomalies, is weaker in the future scenario and appears to be the main driver of the change in interannual SST variability. The strong warming of the upper equatorial Atlantic Ocean in the future leads to a higher stratification which could explain the weaker thermocline feedback.</p>

Prominent precession-band variance in El Niño–Southern Oscillation Intensity over the last 300,000 years

<p>It remains unclear how El Niño–Southern Oscillation (ENSO)—the prominent interannual anomalous climate mode—varied during the full glacial cycles. We study the evolution of ENSO of the last 300,000 years using continuous fully-coupled climate model simulations. How the slow time‐varying changes in insolation, greenhouse gases concentration, and continental ice sheets could influence the behaviours of El Niño are taken into account. The simulated ENSO variance and the tropical eastern Pacific annual cycle (AC) amplitude change in phase, and both have pronounced precession-band variance (~21,000 years) rather than the obliquity-band (~40,000 years). The precession‐modulated slow (orbital time scales) ENSO evolution is determined linearly by the change of the coupled ocean‐atmosphere instability, notably the Ekman upwelling feedback and thermocline feedback. In contrast, the greenhouse gases and ice sheet forcings (~100,000‐year cycles with sawtooth shapes) are opposed to each other as they influence ENSO variability through changes in AC amplitude via a common nonlinear frequency entrainment mechanism. The relatively long simulations which involve pronounced glacial‐interglacial forcing effects gives us more confidence in understanding ENSO forcing mechanisms, so they may shed light on ENSO dynamics and how ENSO will change in the future.</p>

A sea surface height perspective on El Niño diversity, ocean energetics and energy damping rates

<p>Ocean energetics is a useful framework for understanding El Niño development and diversity; however, its key element, available potential energy (APE), requires accurate ocean subsurface data that are hard to measure. However, sea surface heights (SSH) provide a useful alternative. In this study, we describe an SSH-based index, SSHI, that accurately captures APE variations and can be easily computed from satellite observations. Using SSHI we obtain an observation-based estimate of the APE damping timescale α<sup>-1</sup> of approximately 1.7 years, slightly longer than previous ocean reanalysis-based estimates. We further show that SSHI records the relative strength of the thermocline feedback, serving as an indicator for El Niño “flavors”. SSHI demonstrates a clear decadal shift in El Niño-Southern Oscillation (ENSO) properties that occurred in early 2000s, with a more tilted mean thermocline and weaker thermocline slope variations indicative of the dominance of “Central Pacific” El Niño activity during the past two decades.</p>

Mechanisms Reducing ENSO Amplitude and Asymmetry via an Enhanced Seasonal Cycle in the Mid-Holocene

Paleo proxy records have suggested that El Niño–Southern Oscillation (ENSO) variability during the mid-Holocene [8200 to 4200 years ago (8.2–4.2 ka)] was weaker than during the instrumental periods, but the mechanisms remain unclear. We examined processes of ENSO suppression using a coupled general circulation model (CGCM) that simulates ENSO amplitude and skewness under the present climate reasonably well. Two long simulations were performed: one using the preindustrial condition (CTRL) and the other using the 8-ka insolation having a greater seasonal cycle (MH8K). Consistent with proxy records and previous modeling studies, the ENSO amplitude weakened by 20% in MH8K compared to CTRL, mainly because of reduced thermocline feedback during the mature and decay phases. The weak thermocline feedback, likely a result of the loose equatorial thermocline in the mid-Holocene, suppresses the occurrence of extreme El Niño events and consequently explains the reduction in both ENSO amplitude and asymmetry. In MH8K, strengthened trade winds over the western-central Pacific Ocean act to cool the surface via evaporation while warmer water in the southern subtropical Pacific is transported beneath the equatorial thermocline, both contributing to diffuse the thermocline. Multimodel simulations for the mid-Holocene showed mean state changes and ENSO weakening similar to MH8K, but most models did not show reduced ENSO skewness, probably because of the failure in reproducing extreme El Niño events under the present climate.

ENSO Forecasts near the Spring Predictability Barrier and Possible Reasons for the Recently Reduced Predictability

A cross-validated statistical model has been developed to produce hindcasts for the 1980–2016 November–December–January (NDJ; assumed El Niño peak) mean Niño-3.4 sea surface temperature anomalies (SSTA). A linear combination of two parameters is sufficient to successfully predict the peak SSTA: 1) the 5°N–5°S, 130°E–180°, 5–250-m oceanic potential temperature anomalies in February and 2) the 5°N–5°S, 140°E–160°W cumulative zonal wind anomalies (ZWA), integrated from November (one year before) up to the prediction month. This model is simple but is comparable to, or even outperforms, many NOAA Climate Prediction Center’s statistical models during the boreal spring predictability barrier. In contrast to most statistical models, the predictand Niño-3.4 SSTA is not used as a predictor. The explained variance between observed and predicted NDJ Niño-3.4 SSTA at a lead time of 8 months is 57% using 5 yr for cross validation and 63% in full hindcast mode.Predictive skill is lower after 2000 when the mean climate state is more La Niña–like because of stronger equatorial easterly ZWA. Strengthened Pacific subtropical highs are observed, with weaker westerly ZWA that emerge at a later time during El Niño. The western Pacific is more recharged, with stronger upwelling over the eastern Pacific. The resulting strong zonal subsurface temperature gradient provides a high potential for Kelvin waves being triggered without strong westerly ZWA. However, the persistent easterly ZWA lead to more central Pacific–like El Niños. These are more difficult to predict because the contribution of the thermocline feedback is reduced. Overall, the authors find that the importance of the recharge state for ENSO prediction has increased after 2000, contradicting some previous studies.