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Claude Frankignoul and Elodie Kestenare

Abstract

The dominant air–sea feedbacks that are at play in the tropical Atlantic are revisited, using the 1958–2002 NCEP reanalysis. To separate between different modes of variability and distinguish between cause and effect, a lagged rotated maximum covariance analysis (MCA) of monthly sea surface temperature (SST), wind, and surface heat flux anomalies is performed. The dominant mode is the ENSO-like zonal equatorial SST mode, which has its maximum amplitude in boreal summer and is a strongly coupled ocean–atmosphere mode sustained by a positive feedback between wind and SST. The turbulent heat flux feedback is negative, except west of 25°W where it is positive, but countered by a negative radiative feedback associated with the meridional displacement of the ITCZ. As the maximum covariance patterns change little between lead and lag conditions, the in-phase covariability between SST and the atmosphere can be used to infer the atmospheric response to the SST anomaly. The second climate mode involves an SST anomaly in the tropical North Atlantic, which is primarily generated by the surface heat flux and, in boreal winter, wind changes off the coast of Africa. After it has been generated, the SST anomaly is sustained in the deep Tropics by the positive wind–evaporation–SST feedback linked to the wind response to the SST. However, north of about 10°N where the SST anomaly is largest, the wind response is weak and the heat flux feedback is negative, thus damping the SST anomaly. As the in-phase maximum covariance patterns primarily reflect the atmospheric forcing of the SST, simultaneous correlations cannot be used to describe the atmospheric response to the SST anomaly, except in the deep Tropics. Using instead the maximum covariance patterns when SST leads the atmosphere reconciles the results of recent atmospheric general circulation model experiments with the observations.

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Claude Frankignoul and Elodie Kestenare

Abstract

The Pan-Atlantic sea surface temperature (SST) anomaly pattern that was found in a previous study to have a significant impact on the North Atlantic Oscillation (NAO) in early winter seemed to reflect the nearly uncorrelated influence of a horseshoe SST anomaly in the North Atlantic and an SST anomaly in the eastern equatorial Atlantic. A lagged rotated maximum covariance analysis of a slightly longer dataset shows that the horseshoe SST anomaly influence is robust, but it deemphasizes the center of action southeast of Newfoundland, Canada. On the other hand, it suggests that the link between equatorial SST and the NAO was artificial and due both to ENSO teleconnections and the orthogonality constraint in the maximum covariance analysis.

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Claude Frankignoul, Elodie Kestenare, and Gilles Reverdin

Abstract

Monthly sea surface salinity (SSS) fields are constructed from observations, using objective mapping on a 1°x1° grid in the Atlantic between 30°S and 50°N in the 1970-2016 period in an update of the data set of Reverdin et al. (2007). Data coverage is heterogeneous, with increased density in 2002 when Argo floats become available, high density along Voluntary Observing Ship lines, and low density south of 10°S. Using lag correlation, the seasonal reemergence of SSS anomalies is investigated between 20°N and 50°N in 5°x5° boxes during the 1993-2016 period, both locally and remotely following the displacements of the deep mixed-layer waters estimated from virtual float trajectories derived from the daily AVISO surface geostrophic currents. Although SSS data are noisy, local SSS reemergence is detected in about half of the boxes, notably in the northeast and southeast, while little reemergence is seen in the central and part of the eastern subtropical gyre. In the same period, sea surface temperature (SST) reemergence is found only slightly more frequently, reflecting the short data duration. However, taking geostrophic advection into account degrades the detection of remote SSS and even SST reemergence. When anomalies are averaged over broader areas, robust evidence of a second and third SSS reemergence peak is found in the northeastern and southeastern parts of the domain, indicating long cold-season persistence of large-scale SSS anomalies, while only a first SST reemergence is seen. An oceanic reanalysis is used to confirm that the correlation analysis indeed reflects the reemergence of subsurface salinity anomalies.

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S. Cravatte, Elodie Kestenare, Frédéric Marin, Pierre Dutrieux, and Eric Firing

Abstract

The mean subthermocline and intermediate zonal circulation in the tropical Pacific is investigated using a compilation of shipboard ADCP measurements and absolute geostrophic velocities constructed from a high-resolution 0–2000-m Argo climatology referenced to a 1000-m velocity field derived from Argo float drifts. This reference field is dominated by basinwide alternating zonal jets with a meridional wavelength of about 3°. In regions where the sampling of SADCP data is sufficient, the consistency between the two independent datasets is striking; using the Argo drift reference is crucial to capture the current structures. Two apparently distinct systems of alternating westward and eastward zonal jets are seen in both datasets equatorward of 10°: a series of low-latitude subthermocline currents (LLSCs) below the thermocline, extending from about 200 to 800 m, including the eastward Tsuchiya jets; and a series of low-latitude intermediate currents (LLICs), extending from about 700 to at least 2000 m. These systems seem to merge poleward of 10°. Both series shoal to lighter densities eastward. The subthermocline currents and their associated potential vorticity structures undergo a major shift near 155°W, suggesting some difference in the dynamic regime between the regions west and east of this longitude. Differing behaviors (the LLSCs tend to angle poleward to the east, whereas the LLICs angle slightly equatorward) suggest that these jets may be dynamically distinct, with different forcing mechanisms.

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Frédéric Marin, Elodie Kestenare, Thierry Delcroix, Fabien Durand, Sophie Cravatte, Gérard Eldin, and Romain Bourdallé-Badie

Abstract

A large reversal of zonal transport below the thermocline was observed over a period of 6 months in the western Pacific Ocean between 2°S and the equator [from 26.2 Sv (1 Sv ≡ 106 m3 s−1) eastward in October 1999 to 28.6 Sv westward in April 2000]. To document this reversal and assess its origin, an unprecedented collection of ADCP observations of zonal currents (2004–06), together with a realistic OGCM simulation of the tropical Pacific, was analyzed. The results of this study indicate that this reversal is the signature of intense annual variability in the subsurface zonal circulation at the equator, at the level of the Equatorial Intermediate Current (EIC) and the Lower Equatorial Intermediate Current (L-EIC). In this study, the EIC and the L-EIC are both shown to reverse seasonally to eastward currents in boreal spring (and winter for the L-EIC) over a large depth range extending from 300 m to at least 1200 m. The peak-to-peak amplitude of the annual cycle of subthermocline zonal currents at 165°E in the model is ∼30 cm s−1 at the depth of the EIC, and ∼20 cm s−1 at the depth of the L-EIC, corresponding to a mass transport change as large as ∼100 Sv for the annual cycle of near-equatorial zonal transport integrated between 2°S and 2°N and between 410- and 1340-m depths. Zonal circulations on both sides of the equator (roughly within 2° and 5.5° in latitude) partially compensate for the large transport variability. The main characteristics of the annual variability of middepth modeled currents and subsurface temperature (e.g., zonal and vertical phase velocities, meridional structure) are consistent, in the OGCM simulation, with the presence, beneath the thermocline, of a vertically propagating equatorial Rossby wave forced by the westward-propagating component of the annual equatorial zonal wind stress. Interannual modulation of the annual variability in subthermocline equatorial transport is discussed.

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Lionel Gourdeau, William S. Kessler, Russ E. Davis, Jeff Sherman, Christophe Maes, and Elodie Kestenare

Abstract

The South Equatorial Current (SEC) entering the Coral Sea through the gap between New Caledonia and the Solomon Islands was observed by an autonomous underwater vehicle (Spray glider) and an overlapping oceanographic cruise during July–October 2005. The measurements of temperature, salinity, and absolute velocity included high-horizontal-resolution profiles to 600-m depth by the glider, and sparser, 2000-m-deep profiles from the cruise. These observations confirm the splitting of the SEC into a North Vanuatu Jet (NVJ) and North Caledonian Jet (NCJ), with transport above 600 m of about 20 and 12 Sv, respectively. While the 300-km-wide NVJ is associated with the slope of the main thermocline and is thus found primarily above 300 m, the NCJ is a narrow jet about 100 km wide just at the edge of the New Caledonian reef. It extends to at least a 1500-m depth with very little shear above 600 m and has speeds of more than 20 cm s−1 to at least 1000 m. An Argo float launched east of New Caledonia with a parking depth fixed at 1000 m became embedded in the NCJ and crossed the glider/cruise section at high speed about 3 months before the glider, suggesting that the jet is the continuation of a western boundary current along the east side of the island and extends across the Coral Sea to the coast of Australia. In the lee of New Caledonia, the glider passed through a region of eddies whose characteristics are poorly understood.

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Audrey Delpech, Sophie Cravatte, Frédéric Marin, Yves Morel, Enzo Gronchi, and Elodie Kestenare

Abstract

The middepth ocean circulation in the tropical Pacific is dominated by sets of alternating eastward and westward jets. The origin and transport properties of these flow features remain in many ways an open question, all the more crucial since their usual underestimation in ocean global circulation models has been identified as a potential bias for the misrepresentation of the oxygen minimum zones. In this study, we analyze the water mass properties associated with these systems of jets using velocity and hydrographic sections. Data acquired during a dedicated cruise carried out in the western part of the basin and supplemented by cross-equatorial sections from historical cruises in the central and eastern parts are analyzed. While it is confirmed that the near-equatorial jets carry oxygen anomalies, contributing to the ventilation of the eastern tropical Pacific, the data also revealed unexpected features. Tracer distributions (oxygen, salinity, and potential vorticity) show the presence of fronts extending from 500 to 3000 m and flanked by homogeneous regions. These structures define meridional staircase profiles that coincide with the alternating velocity profiles. Historical data confirm their presence in the off-equatorial deep tropical ocean with a zonal and temporal coherence throughout the basin. These observations support existing theoretical studies involving homogenization by isopycnic turbulent mixing in the formation of staircase profiles and maintenance of zonal jets. The effect of other processes on the equilibration of tracer structures is also discussed.

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