Influence of Atlantic SST anomalies on the atmospheric circulation in the Atlantic-European sector

Recent studies of observational data suggest that Sea Surface Temperature (SST) anomalies in the Atlantic Ocean have a significant influence on the atmospheric circulation in the Atlantic-European sector in early winter and in spring. After reviewing this work and showing that the spring signal is part of a global air-sea interaction, we analyze for comparison an ensemble of simulations with the ECHAM4 atmospheric general circulation model in T42 resolution forced by the observed distribution of SST and sea ice, and a simulation with the ECHAM4/OPA8 coupled model in T30 resolution. In the two cases, a significant influence of the Atlantic on the atmosphere is detected in the Atlantic-European sector. In the forced mode, ECHAM4 responds to SST anomalies from early spring to late summer, and also in early winter. The forcing involves SST anomalies not only in the tropical Atlantic, but also in the whole tropical band, suggesting a strong ENSO influence. The modeled signal resembles that seen in the observations in spring, but not in early winter. In the coupled mode, the Atlantic SST only has a significant influence on the atmosphere in summer. Although the SST anomaly is confined to the Atlantic, the summer signal shows some similarity with that seen in the forced simulations. However, there is no counterpart in the observations

Influence of Atlantic SST anomalies on the atmospheric circulation in the Atlantic-European sector

Recent studies of observational data suggest that Sea Surface Temperature (SST) anomalies in the Atlantic Ocean
 have a significant influence on the atmospheric circulation in the Atlantic-European sector in early winter and in
 spring. After reviewing this work and showing that the spring signal is part of a global air-sea interaction, we
 analyze for comparison an ensemble of simulations with the ECHAM4 atmospheric general circulation model in
 T42 resolution forced by the observed distribution of SST and sea ice, and a simulation with the ECHAM4/OPA8
 coupled model in T30 resolution. In the two cases, a significant influence of the Atlantic on the atmosphere is
 detected in the Atlantic-European sector. In the forced mode, ECHAM4 responds to SST anomalies from early
 spring to late summer, and also in early winter. The forcing involves SST anomalies not only in the tropical
 Atlantic, but also in the whole tropical band, suggesting a strong ENSO influence. The modeled signal resembles
 that seen in the observations in spring, but not in early winter. In the coupled mode, the Atlantic SST only has a
 significant influence on the atmosphere in summer. Although the SST anomaly is confined to the Atlantic, the
 summer signal shows some similarity with that seen in the forced simulations. However, there is no counterpart in
 the observations

Surface salinity in the Atlantic Ocean (30 degrees S-50 degrees N)

Sea surface salinity (SSS) data in the Atlantic Ocean is investigated between 50 degrees N and 30 degrees S based on data collected mostly during the period 1977-2002. Monthly mapping of SSS is done to extract the large-scale variability. This mapped variability indicates fairly long (seasonal) time scales outside the equatorial region. The spatial scales of the seasonal anomalies are regional, but not basin-wide (typically 500-1000 km). These seasonal SSS anomalies are found to respond with a 1-2 month lag to freshwater flux anomalies at the air sea interface or to the horizontal Ekman advection. This relation presents a seasonal cycle in the northern subtropics and north-east Atlantic indicating that the late-boreal spring/summer season is less active than the boreal winter/early- spring season in forcing the seasonal SSS variability. In the north-eastern mid-latitude Atlantic, SSS is positively correlated to SST, with SSS slightly lagging SST. There are noticeable long-lasting larger-scale signals overlaid on this regional variability. Part of it is related to known climate signals, for example ENSO and NAO. A linear trend is present during the first half of the period in some parts of the basin (usually towards increasing salinities, at least between 20 degrees N and 45 degrees N). Based on a linear regression analysis, these signals combined can locally represent up to 20% of SSS variance (in particular near 30 degrees N/60 degrees W or 40 degrees N/10-30 degrees W), but usually represent less than 10% of the variance

Subthermocline and intermediate zonal currents in the tropical Pacific ocean : paths and vertical structure

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 degrees. 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 108: 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 degrees. Both series shoal to lighter densities eastward. The subthermocline currents and their associated potential vorticity structures undergo a major shift near 1558W, 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

Zonal jets entering the Coral Sea

International audienc

Observed tracer fields structuration by middepth zonal jets in the Tropical Pacific

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

Morphodynamic characterisation of the human-impacted Bight of Benin sand barrier coast, West Africa

International audienc