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Thirty years (1993–2022) of concurrent satellite and in‐situ observations show a long‐term strengthening of the equatorial Pacific upper‐ocean circulation. Enhanced southeasterly and cross‐equatorial winds have caused an annual mean, basin‐wide acceleration of the equatorial westward near‐surface currents by ∼20% and an acceleration of poleward flow north (south) of the equator by ∼60% (∼20%). Additional moored velocity data reveal a deepening of the Equatorial Undercurrent (EUC) core at 170°W and significant shoaling at 140°W and 110°W, but no significant changes in EUC core velocity. The strongest subsurface zonal velocity trends are observed above the EUC core in the central to eastern Pacific and occur before and after the seasonal maximum of EUC core velocity, causing enhanced upper‐ocean vertical current shear. Consistent with trends of the 20°C isotherm depth along the equatorial Pacific, a significant basin‐wide steepening of the equatorial thermocline is observed. Both the accelerating equatorial current system and the enhanced thermocline slope are consistent with an observed steepening of the zonal sea surface height gradient due to increased wind‐driven westward mass transport at the surface. During February‐March, both surface and subsurface currents along the equator show eastward velocity trends, in contrast to westward near‐surface current trends during the remainder of the year. The trend reversal is attributed to both a long‐term shift in equatorial Kelvin wave activity and to the impact of strong interannual variability due to El Niño Southern Oscillation and other modes of natural variability on decadal to multidecadal time scales. Plain Language Summary: Thirty years (1993–2022) of temporally overlapping satellite and in‐situ observations reveal a long‐term strengthening of the equatorial Pacific circulation in the upper 200 m. Both the west‐east (zonal) and south‐north (meridional) surface atmospheric winds typically observed in the tropical Pacific intensified. As a result, westward near‐surface currents in the central equatorial Pacific accelerated by ∼20%, while poleward currents accelerated by ∼60% (∼20%) north (south) of the equator. The strongest trends in zonal velocity are observed above the core of the Equatorial Undercurrent. While the subsurface current in the central and eastern Pacific shoaled by 12 m, it deepened in the western Pacific by 6 m, which aligns with trends of the 20°C isotherm depth. Both the accelerating equatorial currents and the steeper thermocline slope are consistent with a steepening west‐east slope of sea surface height due to increased westward water mass transport. Our results show that a shift in equatorial wave activity and strong interannual variability cause eastward velocity anomalies in the observed surface and subsurface current trends during February‐March. Between 1993 and 2022, long‐term equatorial circulation trends persist, but these trends are modified by natural events at both interannual timescales such as El Niño and La Niña events, and longer timescales. Key Points: 30 years of observations show a wind‐driven strengthening of equatorial Pacific surface zonal currents by ∼20% and poleward flow by ∼20%−60%The subsurface Equatorial Undercurrent intensified above its core depth, primarily in the central to eastern PacificLong‐term moored observations of subsurface velocity and temperature show a significant basin‐wide steepening of the equatorial thermocline [ABSTRACT FROM AUTHOR]

Indian Ocean Dipole (IOD) prediction is a challenging problem, largely relying on the relationship between IOD and El Niño‐Southern Oscillation (ENSO). This study demonstrates that heat content internal to the Indian Ocean can be an effective predictor providing extra IOD predictability, through constructing statistical prediction models with and without heat content as a predictor. Two recently proposed heat content predictors, equatorial heat content (EQHC) and heat content in the eastern pole of the Dipole (SEHC) are compared in this study. Results show that EQHC is more effective partly because it is relatively independent of ENSO and partly because it does not rely on IOD persistence, as does SEHC. The efficacy of EQHC as an IOD predictor is seasonally dependent, being most effective at 5–8‐month lead times beginning in the preceding late boreal winter and spring. Plain Language Summary: The Indian Ocean Dipole (IOD), a major climate mode in the tropical Indian Ocean, can lead to severe floods and droughts over surrounding continental areas. Prediction of the IOD is very challenging due to the apparent lack of effective predictors besides the El Niño‐Southern Oscillation. This study compares two recently proposed predictors for the IOD based on upper ocean heat content, one along the equator and one in the southeastern tropical Indian Ocean. Equatorial heat content proves to be more effective, especially for predictions starting in the late boreal winter and spring. This study helps to improve our understanding of IOD dynamics and its predictability. Key Points: Heat content internal to the Indian Ocean provides IOD predictability in addition to El Niño‐Southern Oscillation, especially at 5–8‐month lead timesHeat content as a predictor is seasonally dependent, being most effective as an initial condition in the boreal winter and springEquatorial heat content is more useful than heat content in the eastern pole of the Dipole as a predictor when accounting for persistence [ABSTRACT FROM AUTHOR]

The Bay of Bengal (BoB) is characterized by a shallow (∼10–20 m deep) fresh layer associated with 40–60 m deep warm near‐surface layer during the post‐monsoon season (October–November). We use hourly observations from a moored buoy at 15°N, 90°E along with satellite and ocean analysis data sets to understand the evolution of the near‐surface layer during the passage of two category‐3 cyclonic storms: Cyclone Titli (7–11 October 2018) and cyclone Gaja (10–15 November 2018). The mooring was ∼200 km away to the right of the two cyclone tracks. A 15 day (22 September–7 October) break in the Indian summer monsoon resulted in clear skies, calm winds, and sea surface temperature warming (SST) by 2.8°C before Titli. During Titli, near‐surface thermal stratification restricted vertical mixing to 20 m depth and limited SST cooling at the mooring. During passage of cyclone Gaja, fresh Irrawaddy River water associated with a subsurface warm layer was advected to the mooring by mesoscale eddy flow. Cyclone winds could not break through the near‐surface density stratification associated with this river water. Shear associated with near‐inertial currents led to delayed mixed layer deepening after the cyclone's passage but there was no cold wake along the track of Gaja due to mixing of subsurface warm water to the surface. We illustrate the relevant processes at work for the two cyclones based on mixed layer salt and heat balance at 15°N, 90°E. This study demonstrates the importance of ocean preconditioning and thermohaline stratification on SST evolution under the influence of post‐monsoon cyclones in BoB. Plain Language Summary: The Bay of Bengal (BoB) is an active zone for genesis of tropical cyclones. During the post‐monsoon season, the near‐surface ocean in the Bay is stable due to presence of a thin, fresh layer of river water and the subsurface waters are generally warm. In this study, we use in‐situ mooring, satellite, and ocean analysis data sets to understand how the preexisting temperature and the salinity structure of the ocean can influences the ocean's response to two severe tropical cyclones, Titli and Gaja, that occurred in the Bay during October–November 2018. In the case of Titli, we observe that warming of the sea surface prior to arrival of cyclone resulted in strong thermal stratification, which restricted vertical mixing to a depth of 20 m under strong cyclone winds. During the passage of Gaja, river water with a subsurface warm layer advected to the mooring location resulted in stable stratification, which did not allow cyclone‐induced mixing. We observe a delayed deepening of the mixed layer to 40 m depth due to shear induced by inertial currents and near‐surface warming after the passage of cyclone Gaja. This study demonstrates the importance of preexisting upper ocean temperature and salinity stratification for mixed layer evolution under post‐monsoon tropical cyclones in the BoB. Key Points: Strong thermal stratification due to warming during a monsoon break period restricted vertical mixing depth during the passage of cyclone TitliNo cold wake along the track of cyclone Gaja due to cyclone‐induced mixing of subsurface warm water associated with the river waterOcean preconditioning and thermohaline stratification are important for sea surface temperature evolution under post‐monsoon cyclones in the Bay of Bengal [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) variability in the western half of the Niño 4 region (Niño‐4W), bounded by 160°E−175°W and 5°S–5°N, has a significant impact on weather and the marine environment of the Indo‐Pacific. It exhibits significant negative skewness, with the strong penetration of the cold tongue into the western Pacific during strong La Niñas but a more muted response during El Niños. Zonal advection plays a dominant role in driving Niño‐4W SST cooling during the strong La Niña events; vertical processes also contribute, with air‐sea heat fluxes providing weak negative feedback. Advection also plays a key role in driving the Niño‐4W SST warming, which is not always in sync with El Niño events. Among the advection terms, the advection of mean temperature by zonal velocity anomalies, or zonal advective feedback, plays a key role during the development phase of the warm and cold events. Strong negative air‐sea heat flux anomalies during the development of warm events are linked to the negative SST skewness in the Niño‐4W region. Plain Language Summary: Sea surface temperature variability in the western‐central equatorial Pacific (the Niño‐4W region) affects marine heatwaves and rainfall variability in Australia and other areas of the Indo‐Pacific. In contrast with the central‐eastern equatorial Pacific, negative temperature anomalies in the region tend to be stronger than positive temperature anomalies. This study used an upper ocean temperature budget analysis to reveal that zonal temperature advection plays a key role in driving surface temperature variability in the Niño‐4W region. The high surface temperature in the warm pool may damp any further increase in ocean temperature in the region through air‐sea heat flux adjustment. Key Points: Nino‐4W sea surface temperature (SST) variability, with high negative skewness, is a unique climate index in the western Pacific warm poolZonal advective feedback, or advection due to anomalous zonal current, plays a key role in driving SST variability in Nino‐4WStrong negative feedback from air‐sea flux anomalies during the development of the warm events in Nino‐4W contributes to the SST skewness [ABSTRACT FROM AUTHOR]

This study describes biweekly (defined as periods between 10 and 15 days) ocean variability in relation to equatorial wave theory using a comprehensive set of moored ocean current velocities as well as satellite‐retrieved oceanic and atmospheric parameters in the Indian Ocean. Moored biweekly current velocities exhibit the meridional structure of mixed Rossby‐gravity waves: Symmetric for meridional velocity and antisymmetric for zonal velocity around the equator with a decay scale of 228–309 km. The biweekly meridional velocity along the equator has average zonal and vertical wavelengths of 3,430 ± 550 km and 0.76 ± 0.15 km, respectively, with phase propagating westward at speeds of 2.8–3.1 m s−1 and upward at 54–56 m day−1. Conversely, wave energy is inferred to propagate eastward and downward. The range of best fit constants (1.4–2.0 m s−1) that result from the linear separation of horizontal and vertical wave structures suggests the second baroclinic wave mode is dominant over other low baroclinic modes in accounting for the velocity variability. The biweekly waves are forced by surface winds; satellite‐derived wind stress, sea level, and surface current anomalies, regressed against the first principal component of the biweekly moored velocities, illustrate the basin‐scale structure and temporal evolution of the waves. Plain Language Summary: An extensive set of moored ocean current velocities permits evaluation of the three‐dimensional flow field properties, dynamics, and energetics of biweekly waves in the Indian Ocean. We determine, for the first time, the meridional structure of the biweekly flow field from the multiyear in situ measurements and demonstrate that it is dynamically consistent with that of Rossby‐gravity waves, with zonal velocity being antisymmetric and meridional velocity being symmetric about the equator. Our analysis finds that the waves have a decay scale of 228–309 km away from the equator in the meridional direction, a zonal wavelength of 3,430 ± 550 km, and vertical wavelength of 0.76 ± 0.15 km. Phase propagates westward at speeds of 2.8–3.1 m s−1 and upward at 54–56 m day−1, while wave energy flux is eastward and downward. A combination of moored velocities and satellite‐derived data illustrates the basin‐scale structure and temporal evolution of the waves in the Indian Ocean. Key Points: Biweekly variations are prominent in moored buoy velocity and satellite sea level measurements in the equatorial Indian OceanThe properties of these biweekly oscillations are consistent with wind‐forced mixed Rossby‐gravity wavesPhase propagation of the biweekly Rossby‐gravity waves is westward and upward while energy propagation is eastward and downward [ABSTRACT FROM AUTHOR]

Abstract Intraseasonal oceanic variations in the Indian Ocean are prominent mode of variability affecting evolution of the Indian Ocean Dipole, the semiannualWyrtki jets, and the Indonesian throughflow. This study describes the structure and propagation of wind-forced intraseasonal Kelvin waves along the equator in the Indian Ocean and their penetration into the Indonesian seas. For this purpose, we use an unprecedented multiyear (2004–2016) moored velocity time series data set from the equatorial central and eastern Indian Ocean, moored time series from Lombok, Makassar, and Ombai straits; satellite altimetry, and other oceanic and atmospheric data products. The waves are generated by zonal wind fluctuations with periods of approximately 20–90 days. They propagate eastward along the equator at speeds close to that of a second baroclinic mode (with estimates ranging between 1.3 and 1.7 ms−1 depending on method) and have an exponential latitudinal decay scale away from the equator of ∼250 km. Phase also propagates upward, implying downward energy propagation, consistent with wind energy source at the surface. As the waves encounter the coast of Sumatra, they partially reflect into Rossby waves that radiate energy westward back into the ocean interior. Some wave energy also propagates southward along the coasts of Sumatra and Java and can be traced into Lombok, Ombai, and Makassar straits at lags of about 2–3 weeks. The implications of these results for understanding variability in the Indian Ocean and its connectivity with the Indonesian seas are discussed. Plain Language Summary We investigate Indian Ocean equatorial Kelvin waves on weekly to monthly timescales and their propagation into the Indonesian seas. For this purpose, we use ocean current velocity data from an unprecedented number of moorings during 2004–2016, plus satellite data and other data sets. Consistent with theoretical expectations, the wave structures decay exponentially away from the equator to the north and south, with a decay scale of approximately 250 km. The phase of the zonal current variations propagates eastward and upward consistent with eastward and downward energy propagation expected for Kelvin waves forced by surface winds. When the waves encounter the west coast of Sumatra, they partially reflect into Rossby waves that propagate westward back into the interior of the ocean. Some energy also continues poleward as coastal waves along Sumatra and Java. These waves eventually make their way into Lombok, Ombai, and Makassar straits in the Indonesian seas, affecting the velocity there. We discuss the implications of these results for understanding broader regional circulation patterns in the Indian Ocean. [ABSTRACT FROM AUTHOR]

This study investigates basin and regional‐scale eddy (turbulent)‐induced heat and salt transport associated with mesoscale eddies of the Bay of Bengal (BOB). A high level of eddy‐induced transport is found in four subregions: the western boundary of the northern BOB (NB), the western BOB (WB), the southwestern BOB near Sri Lanka (SWB), and the southeastern BOB (SEB). The main seasons for eddy‐induced heat transport in the NB and SWB are identified as the presummer (March to April) and summer monsoon (May to September) seasons, whereas the postsummer monsoon season (October to November) is the main season for the WB and SEB. It is found that not only meridional but also zonal eddy‐induced transport is significant, due to the semienclosed nature of the BOB, which restricts poleward transport in the northern part of the bay. An analysis of upper‐layer eddy energetics reveals that the main contributor to the increased eddy kinetic energy and thus the transport is baroclinic instability in the NB, SWB, and WB, while both barotropic and baroclinic instability are important in the SEB. Using a 2.5‐layer reduced gravity model, baroclinic instability in the NB, SWB, and SEB is further investigated. The results reveal that strong vertical velocity shear and weak stratification are essential for generating baroclinic instability, which enhances the seasonal eddy‐induced transport in the respective subregions. Plain Language Summary: We estimate eddy‐induced heat and salt transport in the Bay of Bengal basin to understand the general characteristics and the key process determining their spatial and seasonal distribution. A linear stability analysis of eddy energetics demonstrates that weak stratification and strong vertical velocity shear generated by the strengthening of mean zonal currents intensify baroclinic instability and enhance the eddy‐induced transport. In addition, it is found that the local wind‐stress curl and remote forcing from the equator partly contribute to the seasonal modulation of eddy‐induced heat transport. Key Points: Baroclinic instability controls the seasonal variation of eddy kinetic energy in the western boundary of the Bay of BengalThe strengthening of mean currents intensifies baroclinic instability and enhances seasonal eddy‐induced transportsEddies associated with local and remote forcing play an important role in the heat and salt transport in the Bay of Bengal [ABSTRACT FROM AUTHOR]

The Indonesian Throughflow (ITF) transport in the upper layer of Makassar Strait was reduced by an unprecedented 25–40% during June to September 2016, the weakest northern summer ITF transport measured through 2004 to mid‐2017. A negative Indian Ocean Dipole (IOD) event occurring through boreal summer and fall 2016 was the main driver for the reduced ITF transport. Elevated sea surface temperature and height off the southern coast of Sumatra and Java islands, attributed to the IOD event, suppressed the Pacific to Indian pressure gradient, resulting in a reduction in the ITF transport. Intensified Wyrtki jets, energetic westerly winds, and downwelling Kelvin waves associated with the strong IOD event contributed to the suppressed interocean pressure gradient. The influence from the 2016 La Niña event on the other hand was secondary. This study showcases the role of coupled ocean‐atmosphere interactions in Indian Ocean in regulating an extreme interannual variation of the ITF in 2016, which is a unique event in the observational record. Key Points: The Indonesian Throughflow in the upper layer of Makassar Strait in June–September 2016 was the weakest in boreal summer in the past decadeThe Pacific to Indian Ocean sea level pressure gradient in 2016 was primarily due to an unprecedented negative Indian Ocean Dipole eventThe unprecedented dipole event was thus responsible for the anomalous Indonesian Throughflow transport [ABSTRACT FROM AUTHOR]

The vertical propagation of middepth (200–1,500 m) zonal velocity in the equatorial Indian Ocean (EIO) and its relationship with surface wind forcing are investigated. We focus on semiannual time scales, using in situ mooring observations and an oceanic reanalysis product that can reasonably well reproduce the structure and magnitude of the observed middepth zonal velocity in the EIO. The pronounced semiannual cycle in observed middepth zonal currents indicates a clear upward phase propagation with particularly robust signatures at depths of 500–1,000 m between 3°S to 3°N, 70 to 85°E. The main characteristics of the middepth currents are consistent with vertically propagating first‐meridional‐mode Rossby wave at semiannual period. Theory suggests that equatorial Kelvin waves, forced by zonal wind variations in the western half of the basin, transfer energy downward and eastward, reflecting into Rossby waves at the eastern boundary. These reflected Rossby waves transfer energy downward and westward, contributing greatly to observed phase propagation at middepths in the EIO. Key Points: The observed middepth (200‐1,500 m) zonal currents in the equatorial Indian Ocean are characterized by a pronounced semiannual cycle exhibiting upward phase propagationThe vertical propagation of middepth zonal currents is controlled by first‐meridional‐mode semiannual period Rossby wave that propagates energy downwardThe middepth Rossby waves are generated by wind‐forced semiannual Kelvin waves that reflect at the eastern boundary [ABSTRACT FROM AUTHOR]

Abstract: Most climate models simulate sea surface temperatures (SSTs) that are consistently 1–4 °C warmer than observed in eastern tropical Atlantic. These biases undermine seasonal prediction efforts and the credibility of climate change projections in this region. To understand what drives the seasonal cycle in upper ocean temperature near the eastern boundary of the tropical Atlantic, we use a 5‐year moored buoy data set from the Prediction and Research Moored Array in the Atlantic at 6°S, 8°E. The buoy is located along the southeastern edge of the Atlantic cold tongue where the seasonal cycle in SST, which is maximum in March and minimum in August, is influenced by the meridional movement of the Intertropical Convergence Zone (ITCZ) and formation of low‐level marine stratocumulus clouds. Associated with these seasonal changes in atmospheric conditions, surface heat fluxes on seasonal timescales are most strongly controlled by shortwave radiation and latent heat flux. The seasonal mixed layer shoals, warms, and freshens in the boreal spring coincident with a southward migration of the ITCZ. The shallow mixed layer amplifies heating from solar radiation on mixed layer temperature at this time. Conversely, during the boreal summer, upwelling leads to entrainment of cold and salty water into the surface layer. From this analysis, we discuss the relative importance of the different components of the seasonal mixed layer heat balance at 6°S, 8°E and how they can be used to better understand the sources of climate model SST biases. [ABSTRACT FROM AUTHOR]

Abstract: Moored measurements between June 2011 and April 2012 at 0°, 90°E in the Indian Ocean are analyzed to assess the surface layer response to the passage of 15 convectively coupled Kelvin waves. During the suppressed phase of the Kelvin waves, intense downward net surface heat flux, weak winds, and a strong diurnal cycle were evident. The net surface heat flux was 3 times larger than turbulent cooling at the base of the surface layer, accounting for an increase of surface layer temperature by +0.2 °C on average. In contrast, strong winds, heavy rainfall, diminished diurnal variations, and air‐sea heat loss marked the active phase, during which surface layer cooling from above by air‐sea fluxes and from below by subsurface turbulent mixing led to a decrease in surface layer temperature by −0.3 °C. Strong salinity stratification formed a thick barrier layer during the active phase, inhibiting the downward diffusion of heat below the surface layer. [ABSTRACT FROM AUTHOR]