Description: El Niño-Southern Oscillation (ENSO) in the Pacific is asymmetric for warm and cold events with respect to amplitude, spatial patterns and temporal evolution. Here the symmetry of the Atlantic Niño mode, which many previous studies have argued is governed by atmosphere–ocean dynamics similar to those of ENSO, is investigated using two different ocean reanalysis products. Calculation of Bjerknes feedback terms for the Pacific reveals a pronounced asymmetry between warm and cold events, though unlike most previous studies, the largest asymmetry is found in the relationship between eastern Pacific thermocline depth and SST anomalies. For the Atlantic, cold events are effectively mirror images of warm events with Bjerknes feedbacks of similar strength. The analysis supports not only the conclusion that Atlantic Niños are more symmetric than ENSO, but the hypothesis itself that the Bjerknes feedback is operative in the Atlantic given the strength of the relationship between the key variables involved.

Description: The ECMWF-T21 atmospheric GCM is forced by observed near-global SST from January 1970 to December 1985. Its response in low level winds and surface wind stress over the Pacific Ocean is compared with various observations. The time dependent SST clearly induces a Southern Oscillation (SO) in the model run which is apparent in the time series of all variables considered. The phase of the GCM SO is as observed, but its low frequency variance is too weak and is mainly confined to the western Pacific. Because of the GCM's use as the atmospheric component in a coupled ocean-atmosphere model, the response of an equatorial oceanic primitive equation model to both the modeled and observed wind stress is examined. The ocean model responds to the full observed wind stress forcing in a manner almost identical to that when it is forced by the first two low frequency EOFs of the observations only. These first two EOFs describe a regular eastward propagation of the SO signal from the western Pacific to the central Pacific within about a year. The ocean model's response to the modeled wind stress is too weak and similar to the response when the observed forcing is truncated to the first EOF only. In other words, the observed SO appears as a sequence of propagating patterns but the simulated SO as a standing oscillation. The nature of the deviation of the simulated wind stress from observations is analyzed by means of Model Output Statistics (MOS). It is shown that a MOS-corrected simulated wind stress, if used to force an ocean GCM, leads to a significant enhancement of low frequency SST variance, which is most pronounced in the western Pacific.

Description: Subtropical western boundary currents are warm, fast-flowing currents that form on the western side of ocean basins. They carry warm tropical water to the mid-latitudes and vent large amounts of heat and moisture to the atmosphere along their paths, affecting atmospheric jet streams and mid-latitude storms, as well as ocean carbon uptake1, 2, 3, 4. The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns5, 6, 7, but detecting such changes is challenging owing to limited observations. Here, using reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products, we find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate. The accelerated warming is associated with a synchronous poleward shift and/or intensification of global subtropical western boundary currents in conjunction with a systematic change in winds over both hemispheres. This enhanced warming may reduce the ability of the oceans to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a need for a long-term monitoring network of the global western boundary currents and their extensions.

Description: Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) is a multinational program initiated in 1997 in the tropical Atlantic to improve our understanding and ability to predict ocean-atmosphere variability. PIRATA consists of a network of moored buoys providing meteorological and oceanographic data transmitted in real time to address fundamental scientific questions as well as societal needs. The network is maintained through dedicated yearly cruises, which allow for extensive complementary shipboard measurements and provide platforms for deployment of other components of the Tropical Atlantic Observing System. This paper describes network enhancements, scientific accomplishments and successes obtained from the last 10 years of observations, and additional results enabled by cooperation with other national and international programs. Capacity building activities and the role of PIRATA in a future Tropical Atlantic Observing System that is presently being optimized are also described.

Description: Naturally occurring tropical Pacific variations at timescales of 7–70 years — tropical Pacific decadal variability (TPDV) — describe basin-scale sea surface temperature (SST), sea-level pressure and heat content anomalies. Several mechanisms are proposed to explain TPDV, which can originate through oceanic processes, atmospheric processes or as an El Niño/Southern Oscillation (ENSO) residual. In this Review, we synthesize knowledge of these mechanisms, their characteristics and contribution to TPDV. Oceanic processes include off-equatorial Rossby waves, which mediate oceanic adjustment and contribute to variations in equatorial thermocline depth and SST; variations in the strength of the shallow upper-ocean overturning circulation, which exhibit a large anti-correlation with equatorial Pacific SST at interannual and decadal timescales; and the propagation of salinity-compensated temperature (spiciness) anomalies from the subtropics to the equatorial thermocline. Atmospheric processes include midlatitude internal variability leading to tropical and subtropical wind anomalies, which result in equatorial SST anomalies and feedbacks that enhance persistence; and atmospheric teleconnections from Atlantic and Indian Ocean SST variability, which induce winds conducive to decadal anomalies of the opposite sign in the Pacific. Although uncertain, the tropical adjustment through Rossby wave activity is likely a dominant mechanism. A deeper understanding of the origin and spectral characteristics of TPDV-related winds is a key priority.

Description: The El Niño-Southern Oscillation (ENSO) is the dominant mode of interannual climate variability on the planet, with far-reaching global impacts. It is therefore key to evaluate ENSO simulations in state-of-the-art numerical models used to study past, present and future climate. Recently, the Pacific Region Panel of the International Climate and Ocean - Variability, Predictability, and Change (CLIVAR) Project, as a part of the World Climate Research Programme (WCRP), led a community-wide effort to evaluate the simulation of ENSO variability, teleconnections and processes in climate models. The new CLIVAR 2020 ENSO metrics package enables model diagnosis, comparison, and evaluation to (1) highlight aspects that need improvement; (2) monitor progress across model generations; (3) help in selecting models that are well suited for particular analyses; (4) reveal links between various model biases, illuminating the impacts of those biases on ENSO and its sensitivity to climate change; and to (5) advance ENSO literacy. By interfacing with existing model evaluation tools, the ENSO metrics package enables rapid analysis of multi-petabyte databases of simulations, such as those generated by the Coupled Model Intercomparison Project phases 5 (CMIP5) and 6 (CMIP6). The CMIP6 models are found to significantly outperform those from CMIP5 for 8 out of 24 ENSO-relevant metrics, with most CMIP6 models showing improved tropical Pacific seasonality and ENSO teleconnections. Only one ENSO metric is significantly degraded in CMIP6, namely the coupling between the ocean surface and subsurface temperature anomalies, while the majority of metrics remain unchanged.

Description: Based on velocity data from a long-term moored observatory located at 0°N, 23°W we present evidence of a vertical asymmetry during the intraseasonal maxima of northward and southward upper-ocean flow in the equatorial Atlantic Ocean. Periods of northward flow are characterized by a meridional velocity maximum close to the surface, while southward phases show a subsurface velocity maximum at about 40 m. We show that the observed asymmetry is caused by the local winds. Southerly wind stress at the equator drives northward flow near the surface and southward flow below that is superimposed on the Tropical Instability Wave (TIW) velocity field. This wind-driven overturning cell, known as the Equatorial Roll, shows a distinct seasonal cycle linked to the seasonality of the meridional component of the south-easterly trade winds. The superposition of vertical shear of the Equatorial Roll and TIWs causes asymmetric mixing during northward and southward TIW phases. Key Points: - Composites of Tropical Instability Waves at 0°N, 23°W show a surface (subsurface) velocity maximum during northward (southward) phases - Meridional wind stress forces a seasonally-varying, shallow cross-equatorial overturning cell-the Equatorial Roll - The superposition of Tropical Instability Waves and Equatorial Roll causes asymmetric mixing during north- and southward phases

Description: Originating in the equatorial Pacific, the El Niño–Southern Oscillation (ENSO) has highly consequential global impacts, motivating the need to understand its responses to anthropogenic warming. In this Review, we synthesize advances in observed and projected changes of multiple aspects of ENSO, including the processes behind such changes. As in previous syntheses, there is an inter-model consensus of an increase in future ENSO rainfall variability. Now, however, it is apparent that models that best capture key ENSO dynamics also tend to project an increase in future ENSO sea surface temperature variability and, thereby, ENSO magnitude under greenhouse warming, as well as an eastward shift and intensification of ENSO-related atmospheric teleconnections — the Pacific–North American and Pacific–South American patterns. Such projected changes are consistent with palaeoclimate evidence of stronger ENSO variability since the 1950s compared with past centuries. The increase in ENSO variability, though underpinned by increased equatorial Pacific upper-ocean stratification, is strongly influenced by internal variability, raising issues about its quantifiability and detectability. Yet, ongoing coordinated community efforts and computational advances are enabling long-simulation, large-ensemble experiments and high-resolution modelling, offering encouraging prospects for alleviating model biases, incorporating fundamental dynamical processes and reducing uncertainties in projections. Key points Under anthropogenic warming, the majority of climate models project faster background warming in the eastern equatorial Pacific compared with the west. The observed equatorial Pacific surface warming pattern since 1980, though opposite to the projected faster warming in the equatorial eastern Pacific, is within the inter-model range in terms of sea surface temperature (SST) gradients and is subject to influence from internal variability. El Niño–Southern Oscillation (ENSO) rainfall responses in the equatorial Pacific are projected to intensify and shift eastward, leading to an eastward intensification of extratropical teleconnections. ENSO SST variability and extreme ENSO events are projected to increase under greenhouse warming, with a stronger inter-model consensus in CMIP6 compared with CMIP5. However, the time of emergence for ENSO SST variability is later than that for ENSO rainfall variability, opposite to that for mean SST versus mean rainfall. Future ENSO change is likely influenced by past variability, such that quantification of future ENSO in the only realization of the real world is challenging. Although there is no definitive relationship of ENSO variability with the mean zonal SST gradient or seasonal cycle, palaeoclimate records suggest a causal connection between vertical temperature stratification and ENSO strength, and a greater ENSO strength since the 1950s than in past centuries, supporting an emerging increase in ENSO variability under greenhouse warming.

Description: Regional climate variability in the tropical Atlantic, from interannual to decadal time scales, is inevitably connected to changes in the strength and position of the individual components of the tropical current system with impacts on societally relevant climate hazards such as anomalous rainfall or droughts over the surrounding continents (Bourlès et al., 2019; Foltz et al., 2019). Furthermore, the lateral supply of dissolved oxygen in the tropical Atlantic upper-ocean is closely linked to the zonal current bands (Brandt et al., 2008; Brandt et al., 2012; Burmeister et al., 2020) and especially to the Equatorial Undercurrent (EUC) and its long-term variations with potential implications for regional marine ecosystems (Brandt et al., 2021). The eastward flowing EUC is located between 70 to 200 m depth and forms one of the strongest tropical currents with maximum velocities of up to 1 m s-1 and maximum variability on seasonal time scales (Brandt et al., 2014; Johns et al., 2014). In the intermediate to deep equatorial Atlantic, variability on longer time scales is mainly governed by alternating, vertically-stacked, zonal currents (equatorial deep jets (EDJs); Johnson and Zhang, 2003). At a fixed location, the phases of these jets are propagating downward with time, implying that parts of their energy must propagate upward towards the surface (Brandt et al., 2011). In fact, a pronounced interannual cycle of about 4.5 years, that is associated with EDJs, is projected onto surface parameters such as sea surface temperature or precipitation (Brandt et al., 2011) further demonstrating the importance of understanding equatorial circulation variability and its role in tropical climate variability. While variability in the zonal velocity component on the equator is focused on seasonal to interannual time scales (Brandt et al., 2016; Claus et al., 2016; Kopte et al., 2018), meridional velocity fluctuations dominate the intraseasonal period range (20 to 50 days) due to the presence and passage of westward propagating Tropical Instability Waves (TIWs; Grodsky et al., 2005; Bunge et al., 2007; Wenegrat and McPhaden, 2015; Tuchen et al., 2018; Specht et al., 2021). In general, intraseasonal variability in the central equatorial Atlantic is mainly attributed to TIWs in the upper ocean (Athie and Marin, 2008), while intraseasonal variability in the deep ocean is associated with the signature of equatorial Yanai waves (Ascani et al., 2015; Tuchen et al., 2018, Körner et al., 2022). The observed and modelled interaction between intraseasonal equatorial waves and the aforementioned EDJs was found to maintain the deep equatorial circulation against dissipation (Greatbatch et al., 2018; Bastin et al., 2020) pointing toward the importance of intraseasonal variability for equatorial ocean dynamics. These findings are largely based on, or underpinned by a unique and steadily expanding data set of current velocity observations in the central equatorial Atlantic Ocean. Since 2001, current velocities have been measured almost continuously as part of a multilateral collaboration, the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA), that regularly services a moored observatory located at 0°N/23°W (Bourlès et al., 2019). The significance of this data set is characterized by the length of the time series and by the full-depth coverage of current velocity observations which allow for a detailed analysis of both upper-ocean and deep-ocean dynamics on a wide range of time scales and frequencies. For instance, it enables the decomposition of the current velocity time series into vertical modes pointing toward the existence of resonant basin modes and identifying different sources of deep intraseasonal variability (Brandt et al., 2016; Claus et al., 2016; Greatbatch et al., 2018; Tuchen et al., 2018, Körner et al. under review). Here, we present 20 years of full-depth current velocity observations at 0°N/23°W. The aim of this study is to provide the scientific community with a publicly available reference data set that could be used in manifold ways, including, for instance, the validation of ocean models or reanalysis products.