O. T. Sato

The signature of mesoscale eddies on the air‐sea turbulent heat fluxes in the South Atlantic Ocean

Abstract By collocating 10 years (1999–2009) of remotely sensed surface turbulent heat fluxes withsatellite altimetry data, we investigate the impact of oceanic mesoscale eddies on the latent and sensibleheatfluxesintheSouthAtlanticOcean.Instronglyenergeticregions,suchastheBrazil–Malvinasconfluenceand the Agulhas Current Retroflection, eddies explain up to 20% of the total variance in the surfaceturbulent heat fluxes with averaged anomalies of ± (10–20) W/m 2 .Cyclonic(anticyclonic,respectively)eddies are associated with negative (positive) heat flux anomalies that tend to cool (warm) the marineatmospheric boundary layer. A composite analysis of the turbulent heat flux anomalies inside the eddiesrevealsadirectrelationshipbetweeneddyamplitudeandtheintensityofsuchanomalies.Inaddition,theseanomaliesarestrongerneartheeddycenter,decayingradiallytoreachminimumvaluesoutsidetheeddies. 1. Introduction The ocean-atmosphere system is primarily turbulent. In the mesoscale band, oceanic eddies play a crucialrole in transporting heat, salt, momentum, and biogeochemical tr acersalongtheirtrajectories,significantlyimpactingtheoceandynamicsandfluxes[Olson,1991;Wunsch,1999;Morrowetal.,2003].Mesoscaleeddiesare coherent rotating features with typical horizontal scales of 50–300 km and time scales ranging fromweeks to months [Cheltonetal.,2011a].Theyalsoexhibitparticularthermohalineverticalstructuresassoci-ated with strong dynamical perturbations of the density and pressure fields [Olson,1991;Chaigneau et al.,2011; Kurian et al.,2011;Zhang et al.,2013;Roullet et al., 2014]. These perturbations extend up to the seasurface,leadingtoanomaliesonseasurfaceheight,seasurfacetemperature(SST),andwindstress,amongothers[Frengeretal.,2013;Souzaetal.,2014].Asaresult,itisexpectedthateddiesimpacttheair-seafluxesandtheoverlayingatmosphericcirculation[Frengeretal.,2013].The turbulent latent (LHF) and sensible (SHF) heat fluxes are the primary processes by which the oceanreleases heat to the atmosphere [ Cayan, 1992]. Both the LHF and SHF can be derived from bulk formulaelinking oceanic and atmospheric parameters—such as air-specific humidity, SST, and wind speed—to theheat exchanged at the air-sea interface [Liu et al.,1979;Fairall et al.,2003;Large and Yeager, 2009]. Theanomalies caused by mesoscale eddies in some of these parameters have been recently investigated [e.g.,White and Annis, 2003; Hausmann and Czaja, 2012; Frenger et al., 2013; Souza et al., 2014; Chelton and Xie,2010]. Although there is ample evidence that oceanic mesoscale features have a significant impact onair-seaproperties,thereisstillalackofknowledgeabouttheirsignatureonthesurfaceturbulentheatfluxesandtheirpotentialcontributionstothenetsurfaceheatbudget.The recent release of daily satellite-based data from the French Research Institute for Exploitation of theSea (IFREMER) [Bentamy et al., 2013] has sufficient resolution to estimate the LHF and SHF associatedwith mesoscale processes. The present work merges satellite altimetry sea level anomaly (SLA) data withremotely sensed surface heat fluxes to investigate the impact of mesoscale ocean eddies on the LHF andSHFintheSouthAtlanticOcean(SA).The SA encompasses both quiescent areas and two regions of intense energetic variability associated withthe presence of large mesoscale eddies (Figure 1a), namely, the Agulhas Current Retroflection (AGR) [Olsonand Evans,1986;Goni et al.,1997]andtheBrazil-Malvinasconfluence(BMC)[Garzoli and Garraffo,1989;Olson et al., 1988]. More specifically, we aim (i) to assess the mean LHF and SHF anomalies induced bycyclonicandanticycloniceddiesintheSA,(ii)todescribethespatialvariationsoftheheatfluxanomaliesinVILLASBOASETAL.

An algorithm to improve the detection of ocean fronts from whiskbroom scanner images

High-resolution satellite imagery is a valuable data source to analyse ocean submesoscale dynamics (i.e., with spatial scales of the order of 1–10 km) and investigate their impact on turbulent mixing, energetics of mesoscale vortices, instability processes or phytoplankton blooms. However, data acquired by satellite sensors often suffer from instrumental noise that degrades image quality and therefore compromises the detection of ocean fronts as well as the estimation of its physical characteristics. A well-known artefact in data characteristic of whiskbroom scanners is stripe noise. In this article, we propose an algorithm that improves the detection of ocean fronts by removing the impact of striping on the observed gradient field. We use level 2 sea surface temperature and chlorophyll-a products derived from NASA’s Moderate Resolution Imaging Spectroradiometer to illustrate the algorithm performance.

Meridional Overturning Circulation Transport Variability at 34.5°S During 2009–2017: Baroclinic and Barotropic Flows and the Dueling Influence of the Boundaries

Six years of simultaneous moored observations near the western and eastern boundaries of the South Atlantic are combined with satellite winds to produce a daily time series of the basin-wide meridional overturning circulation (MOC) volume transport at 34.5°S. The results demonstrate that barotropic and baroclinic signals at both boundaries cause significant transport variations, and as such must be concurrently observed. The data, spanning ~20 months during 2009–2010 and ~4 years during 2013–2017, reveal a highly energetic MOC record with a temporal standard deviation of 8.3 Sv, and strong variations at time scales ranging from a few days to years (peak-to-peak range = 54.6 Sv). Seasonal transport variations are found to have both semiannual (baroclinic) and annual (Ekman and barotropic) timescales. Interannual MOC variations result from both barotropic and baroclinic changes, with density profile changes at the eastern boundary having the largest impact on the year-to-year variations. Plain Language Summary Changes in the meridional overturning circulation, characterized by north-south flows throughout the Atlantic Ocean basin and vertical exchange between the surface and the deep ocean, are related to changes in important ocean-atmosphere-climate signals like precipitation patterns, sea level, and extreme weather (e.g., drought, heat waves, and hurricane intensification). This study presents, for the first time, a multiyear daily record of the meridional overturning circulation flow based on direct measurements in the South Atlantic Ocean at 34.5°S. The roughly six years of observations presented in this study provided the ability to study seasonal and interannual changes in these important flows with continuous daily data, and they demonstrated a complexity of the ocean circulation as compared to other latitudes where this flow has been studied in the past.

Impacts of Agulhas Leakage on the Tropical Atlantic western boundary systems

AbstractThe influx of warmer and saltier Indian Ocean waters into the Atlantic—the Agulhas leakage—is now recognized to play an important role in the global thermohaline circulation and climate. In this study the results of a ⅞° simulation with the Hybrid Coordinate Ocean Model, which exhibit an augmentation in the Agulhas leakage, is investigated. This increase in the leakage ought to have an impact on the meridional oceanic volume and heat transports in the Atlantic Ocean. Significant linear trends found in the integrated transport at 20°, 15°, and 5°S correlate well with decadal fluctuations of the Agulhas leakage. The augmented transport also seems to be related to an increase in the latent heat flux observed along the northeastern coastline of Brazil since 2003. This study shows that the precipitation on the Brazilian coast has been increasing since 2005, at the same location and with the same regime shift observed for the latent heat flux and the volume transport. This suggests that the increase of ...