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This study provides a quantitative assessment of steric changes associated with sea level rise in the upper (0–500 m) South Indian Ocean (SIO) during 1993–2017, using the latest ocean state estimate of Estimating the Circulation and Climate of the Ocean (ECCO) combined with in‐situ observations from Argo. Both the observations and ECCO estimate show a sea level rise in the low‐latitude (0°–30°S) SIO that is faster than its South Pacific and South Atlantic counterparts by a factor at least two. Much of this fast sea level rise is due to warming and freshening of the upper ocean, with no significant contribution from the deeper layers (>500 m). We emphasize the importance of halosteric effect, whose contribution to sea level rise is comparable with that of thermosteric effect in the eastern basin. On interannual time scale, up to 90% of the region's sea level variability can be explained by steric changes that in turn are dominated by upper ocean convergence. Plain Language Summary Sea level has been rising steadily in the low‐latitude South Indian Ocean (SIO), roughly, between 0° and 30°S, at a rate about two time faster than its South Pacific and South Atlantic counterparts during 1993–2017. This fast sea level rise is largely due to warming and freshening of the upper ocean (0–500 m) that is observed only in the low‐latitude SIO. As is the case over much of the global ocean, thermal expansion has the dominant contribution to sea level rise, except in the eastern basin, where contribution from salinity changes is of comparable strength. The upper SIO has also experienced a large interannual variability in heat and freshwater content that directly contribute to the region's sea level variability. Much of this variability is due to water convergence of the upper ocean, while contribution from surface heat and freshwater fluxes and small‐scale processes is relatively small. Key Points Sea level in the low‐latitude South Indian Ocean has been rising twice as fast as its South Pacific and South Atlantic counterparts Much of this fast sea level rise is due to warming and freshening of the upper ocean, with little contribution from the deeper layers The region's interannual sea level variability is dominated by steric changes associated with water convergence of the upper ocean

This study examines the impact of ocean advection and surface freshwater flux on the non‐seasonal, upper‐ocean salinity variability in two climate model simulations with eddy‐resolving and eddy‐parameterized ocean components (HR and LR, respectively). We assess the realism of each simulation by comparing their sea surface salinity (SSS) variance with satellite and Argo float estimates. In the extratropics, the HR variance is about five times larger than that in LR and agrees with Argo. In turn, the extratropical satellite SSS variance is smaller than that from HR and Argo by about a factor of two, potentially caused by the insufficient resolution of radiometers to capture mesoscale features and their low sensitivity to SSS in cold waters. Using a simplified salinity conservation equation for the upper‐50‐m ocean, we find that the advection‐driven variance in HR is, on average, 10 times larger than the surface flux‐driven variance, reflecting the action of mesoscale processes. Plain Language Summary This study explores the importance of ocean currents, evaporation, and rainfall for driving changes in the salt concentration in the upper ocean (known as salinity) in two climate model simulations with differing ocean resolutions. The high‐resolution model (HR) simulates ocean currents with dimensions of tens of km, while the low‐resolution model (LR) can only simulate currents with hundreds of km in size. When comparing their simulated sea surface salinity variations with those captured by satellites and autonomous floats from the Argo array, the salinity variability in the high‐resolution model is similar to the Argo data at mid to high latitudes and about five times stronger than that in the low‐resolution model. The satellite data show a variability two times smaller than HR and Argo in the same regions, potentially due to their insufficient spatial resolution at higher latitudes and their low sensitivity to the surface salinity in cold waters. Using a simple equation describing the conservation of salinity in the upper ocean, we have shown that small‐scale ocean currents drive most of the salinity variability in HR, while in LR, ocean currents play a much smaller role. Key Points We investigate how advection and surface flux affect upper‐50‐m salinity variance in eddy‐resolving and eddy‐parameterized climate models The extratropical variance in the eddy‐resolving run matches Argo and is much larger than in the eddy‐parameterized run and satellite data The larger upper‐ocean salinity variance in the eddy‐resolving run is predominantly driven by mesoscale ocean processes

Surface westerly winds in the Southern Hemisphere have intensified over the past few decades, primarily in response to the formation of the Antarctic ozone hole, and there is intense debate on the impact of this on the ocean's circulation and uptake and redistribution of atmospheric gases. We used measurements of chlorofluorocarbon-12 (CFC-12) made in the southern oceans in the early 1990s and mid- to late 2000s to examine changes in ocean ventilation. Our analysis of the CFC-12 data reveals a decrease in the age of subtropical subantarctic mode waters and an increase in the age of circumpolar deep waters, suggesting that the formation of the Antarctic ozone hole has caused large-scale coherent changes in the ventilation of the southern oceans.

Ageostrophic baroclinic instabilities develop within the surface mixed layer of the ocean at horizontal fronts and efficiently restratify the upper ocean. In this paper a parameterization for the restratification driven by finite-amplitude baroclinic instabilities of the mixed layer is proposed in terms of an overturning streamfunction that tilts isopycnals from the vertical to the horizontal. The streamfunction is proportional to the product of the horizontal density gradient, the mixed layer depth squared, and the inertial period. Hence restratification proceeds faster at strong fronts in deep mixed layers with a weak latitude dependence. In this paper the parameterization is theoretically motivated, confirmed to perform well for a wide range of mixed layer depths, rotation rates, and vertical and horizontal stratifications. It is shown to be superior to alternative extant parameterizations of baroclinic instability for the problem of mixed layer restratification. Two companion papers discuss the numerical implementation and the climate impacts of this parameterization.

Ocean evaporation (E) and precipitation (P) are the fundamental components of the global water cycle. They are also the freshwater flux forcing (i.e., E‐P) for the open ocean salinity. The apparent connection between ocean salinity and the global water cycle leads to the proposition of using the oceans as a rain gauge. However, the exact relationship between E‐P and salinity is governed by complex upper ocean dynamics, which may complicate the inference of the water cycle from salinity observations. To gain a better understanding of the ocean rain gauge concept, here we address a fundamental issue as to how E‐P and salinity are related on the seasonal timescales. A global map that outlines the dominant process for the mixed‐layer salinity (MLS) in different regions is thus derived, using a lower‐order MLS dynamics that allows key balance terms (i.e., E‐P, the Ekman and geostrophic advection, vertical entrainment, and horizontal diffusion) to be computed from satellite‐derived data sets and a salinity climatology. Major E‐P control on seasonal MLS variability is found in two regions: the tropical convergence zones featuring heavy rainfall and the western North Pacific and Atlantic under the influence of high evaporation. Within this regime, E‐P accounts for 40–70% MLS variance with peak correlations occurring at 2–4 month lead time. Outside of the tropics, the MLS variations are governed predominantly by the Ekman advection, and then vertical entrainment. The study suggests that the E‐P regime could serve as a window of opportunity for testing the ocean rain gauge concept once satellite salinity observations are available. Key Points Ocean salinity and the global water cycle are related through complex dynamics A regime showing salinity sensitivity to change in the water cycle is obtained The finding is a step forward in understanding the oceans as a rain gauge

We use a version of the NOAA Climate Forecast System with enhanced (up to 1‐m) ocean model vertical resolution to investigate the mean diurnal cycles of upper ocean temperature and currents. The model sea surface temperature diurnal cycle agrees well with a global observational analysis. The simulated time‐depth profiles of temperature and current also match closely observations from densely instrumented moorings in the tropical Pacific. Our analyses provide new insights into subsurface ocean diurnal cycles. Significant temperature diurnal range occurs, with seasonal modulation, at depths greater than 10 m across broad areas of the subtropical and midlatitude oceans. Significant current diurnal cycles are evident below 30 m across parts of the tropics, including in areas where deep‐cycle turbulence has been observed. Plain Language Summary We used computer model simulations of Earth's atmosphere and ocean to understand how ocean temperatures and currents vary by time of day. The model has 12 levels in the top 20 m of the ocean—greater than normal for this kind of simulation. This allows realistic simulated diurnal variations of sea surface temperature (compared to global observations), and realistic changes in temperature and current at and below the surface (compared to mooring observations). These results give us confidence in the global simulations of currents, which provide new insights into diurnal variations of surface ocean velocity and turbulence below the surface. Key Points A 1‐m vertical resolution ocean model accurately simulates global patterns of sea surface temperature mean diurnal cycle The model gives new insights into modes of subsurface ocean temperature and current diurnal variation Global maps of current diurnal cycle extend understanding past that from relatively sparse observations

During the past several decades operational forecasts of tropical cyclone (TC) tracks have improved steadily, but intensity forecast skills have experienced rather modest improvements. Here we use 40 years of TC track data to show that storm intensity correlates with translation speed, with hurricanes of category 5 moving on average 1 m s−1 faster than tropical storms. This correlation provides evidence that the translation speed of a storm can exert a significant control on the intensity of storms by modulating the strength of the negative effect of the storm‐induced sea surface temperature (SST) reduction on the storm intensification (i.e., the SST feedback): Faster‐moving storms tend to generate weaker sea surface cooling and have shorter exposure to the cooling, both of which tend to weaken the negative SST feedback. Consistently, there exists a minimum translation speed for intensification and its value grows with TC intensity, resulting in a minimum translation speed for the existence of a TC in each intensity category. Furthermore, a composite analysis of satellite‐based SST measurements reveals that in the tropical region the average strength of the storm‐induced sea surface cooling can be explained by the superposition of an effect due to the storm intensity and an effect associated with the translation speed, and implies that the variability of upper ocean stratification may not be an important factor in this region. Our results suggest that progress in the prediction of TC tracks, particularly in the translation speed of storms, should lead to improved storm intensity prediction. Key Points The intensity of TCs correlates with their translation speed in the tropics Ave cold wake temperature is determined by TC intensity and translation speed TC‐induced ocean cooling feeds back onto the TC intensification process

From February to March 2024, the equatorial Atlantic experienced its highest sea surface temperatures in at least 40 years. This extreme warm event was triggered by a favorable combination of El‐Niño‐induced westerly wind anomalies in the western equatorial Atlantic and Rossby wave reflection at the western boundary, leading to an exceptionally strong downwelling Kelvin wave. The warm event was extinguished abruptly around May by a locally‐forced upwelling Kelvin wave, causing an unprecedented rapid transition to a cold phase which lasted until August. The cold event did not fully develop into an Atlantic Niña due to weak Bjerknes feedback, warming from surface heat fluxes, and thermocline deepening due to a series of equatorial wave reflections. Nevertheless, the cold event is consistent with a northward shift of the intertropical convergence zone, increased rainfall over West Africa, Sahel, and Sahara, reduced rainfall over the Gulf of Guinea, and an earlier onset of the West African monsoon.

Tropical cyclones are among the most destructive natural disasters. However, lack of detailed observations and the simplifications inherent in operational ocean models, lead to incomplete knowledge of underlying ocean processes. Using high‐fidelity large‐eddy simulations and moored observations away from the storm track, we show that mutually interacting shear and convective processes, govern the evolving state of the upper ocean. Our simulation agrees well with observed sea surface temperature and sea surface salinity. Shear driven turbulence due to surface wind stress erodes stratification, deepens the ocean mixed layer and transports freshwater into the mixed layer during rain events. Concurrently, surface buoyancy loss also aids in ocean mixing via convective entrainment. The mixing efficiency and the associated eddy diffusivity shows high spatiotemporal variability throughout the water column during cyclone passage. Thus, a better insight into the upper ocean mixing mechanisms is necessary for developing improved mixing parameterizations for tropical cyclone intensity forecasts. Plain Language Summary Tropical cyclones are among the most destructive natural phenomena on Earth. The powerful winds in a cyclone cause vigorous mixing in the upper ocean, cool the sea surface temperature (SST) and influence cyclone intensification. Using high‐fidelity simulations and observations, we demonstrate the synergy between ocean convection and shear driven processes to change the upper ocean state. Due to temporally varying surface forcings, the upper ocean mixing was also highly variable. Our study provides new insights into the energetics of such an extreme event where both wind stress and surface cooling are present. Key Points Large‐eddy simulations are performed for the upper ocean during the passage of a cyclone Shear and turbulent convection acted in concert to modify the upper ocean properties Irreversible mixing and mixing efficiency show high spatiotemporal variability

An approximate mass (volume) budget in the surface layer of the Southern Ocean is used to investigate the intensity and regional variability of the ventilation process, discussed here in terms of subduction and upwelling. Ventilation resulting from Ekman pumping is estimated from satellite winds, the geostrophic mean component is assessed from a climatology strengthened with Argo data, and the eddy-induced advection is included via the parameterization of Gent and McWilliams, together with eddy mixing estimates. All three components contribute significantly to ventilation. Finally, the seasonal cycle of the upper ocean is resolved using Argo data. The circumpolar-averaged circulation shows an upwelling in the Antarctic Intermediate Water (AAIW) density classes, which is carried north into a zone of dense Subantarctic Mode Water (SAMW) subduction. Although no consistent net production is found in the light SAMW density classes, a large subduction of Subtropical Mode Water (STMW) is observed. The STMW area is fed by convergence of a southward and a northward residual meridional circulation. The eddy-induced contribution is important for the water mass transport in the vicinity of the Antartic Circumpolar Current. It balances the horizontal northward Ekman transport as well as the vertical Ekman pumping. While the circumpolar-averaged upper cell structure is consistent with the average surface fluxes, it hides strong longitudinal regional variations and does not represent any local regime. Subduction shows strong regional variability with bathymetrically constrained hotspots of large subduction. These hotspots are consistent with the interior potential vorticity structure and circulation in the thermocline. Pools of SAMW and AAIW of different densities are found along the circumpolar belt in association with the regional pattern of subduction and interior circulation.