Inspecting the Azores and St. Helena Current Systems from In Situ and Remote Sensing Data Synergy
LŠzaro, Clara1; Nunes, Alexandra L.2; Fernandes, M. Joana3; Juliano, Manuela F.4; Salgado, Miguel3
1Universidade do Porto - Faculdade de CiÍncias, PORTUGAL; 2ISEP; CIIMAR, PORTUGAL; 3Univ. Porto; CIIMAR, PORTUGAL; 4LAMTec; CIIMAR, PORTUGAL
Two decades of satellite altimetry with SST (Sea Surface Temperature) and in situ data synergy are used to study and assess the congeneracy of the Azores and St. Helena currents (AzC and StHC, respectively, the latter also known as Tristan da Cunha current) and their associated fronts. This study aims at achieving a better description of the ocean circulation variability over the North and South Subtropical Atlantic basins on meso and large scales, with focus on the study of the congeneracy between both current systems that has been suggested from previous studies using in situ data. These have shown that both zonal current systems have similar characteristics regarding intensity, depth penetration and volume transport, and their main cores are found at similar latitudes (34°N and 34°S for the AzC and StHC, respectively). Moreover, it is known that both systems have associated subsurface adjacent countercurrent flows.
Satellite altimetry, in combination with a mean dynamic topography (MDT) model computed from in situ data, was previously used to derive a time series of absolute dynamic topography (ADT), which allowed the study of the AzC variability over the 1995-2006 period, namely the existence of inter-annual variability in its axis position. In addition to extending the previously used altimetric data set for the AzC, a similar study for the StHC is performed to assess possible resemblances between both currents in what concerns their inter-annual variability. To identify the most suitable MDT for use in the study region, three state-of-the-art models have been evaluated: Juliano_2007, a regional altimetry-independent MDT only available for the North Atlantic but recently extended to the South Atlantic, with spatial resolution of 1/4°; CNES-CLS09_v1.1, the most recent global MDT from AVISO and the state-of-the-art model using the combined method, provided at 1/4°; DTU10 MDT, derived using the direct method, based on DTU10 MSS and EIGEN-6C geoid and provided at 1-arcmin grid. For this purpose, the three models have been validated with available in situ data, not included in the state-of-the-art models. Special attention has been given to the StHC region, due to its novelty.
Both SLA and eddy kinetic energy (EKE) time series show the existence of interannual variability on both current systems, being significantly larger for the StHC region. Furthermore, the surface thermal signatures of the two currents/fronts, derived from available SST optimally interpolated microwave and infrared products, are also inspected and compared to the corresponding ADT-derived signatures. The results are expected to improve the knowledge of the South Atlantic variability, the Atlantic inter-hemispheric connections and the correlation of long-period variability of the above referred oceanographic fields with the known phenomena of coupled atmosphere-ocean variability that affect the Atlantic, which are expected to occur on inter-annual to decadal time scales.