Satellite and lagrangian observations of mesoscale surface processes in the southwestern Atlantic Ocean

Abstract

This work presents a study of the mesoscale surface processes occurring in the Southwestern Atlantic Ocean. Two regions in this ocean, the Brazil-Malvinas (Falkland) Confluence (BMC) Zone and the South Brazilian Continental Shelf (SBCS) are studied by means of a 14 year long series of low resolution Multi-Channel Sea Surface Temperature (MCSST) images of the Advanced Very High Resolution Radiometer (AVHRR) together with high-resolution data from the same sensor and Lagrangian (buoy) data for the period between March 1993 and July 1994. The AVHRR and buoy data were available from the project COROAS (Oceanic Circulation in the Western Region of the South Atlantic), the Brazilian contribution to the World Ocean Circulation Experiment (WOCE). The variability of the sea surface temperature (SST) fields in the South Atlantic is investigated for the period between January 1982 and December 1995 utilising Principal Component analysis techniques on the MCSST data set. The distribution and oscillation of the SST fields of the South Atlantic are compared to those present in the BMC and SBCS regions, as described by the high-resolution AVHRR and buoy data. The oceanographic surface frontal systems observed in the AVHRR images and buoy trajectories are also studied for the BMC and SBCS regions during 1993 and 1994. Direct measurements of currents taken by the buoys are utilised to describe the characteristics of the Brazil Current (BC), the South Atlantic Current (SAC) and the Brazilian Coastal Current (BCC). These currents are described by their mean surface velocities, kinetic energies, temperature statistics and oscillations. The BCC is a newly described current, very poorly understood in the past and very important for fisheries and, possibly, for the weather of the southern region of Brazil. The surface component of the BCC is described in this thesis as a coastal, northeasterly current flowing in opposition to the BC main flow and with a seasonal behaviour off the South American coast.The eddy field present in the BMC and SBCS regions during 1993 and 1994 is investigated in this work as well. Distinct behaviour and driving mechanisms are reported for the eddies present in these two areas of the Southwestern Atlantic. For the first time in the known literature, small scale and shelfbreak eddies are described for the SBCS region. The nature of these small scale eddies is discussed in relation to that of the mesoscale, geostrophically balanced BMC eddies already known to occur in the study area. The importance of the shear instabilities in the oceanographic front between the BC and the BCC for the eddy generation and mixture processes is emphasised here. Comparing AVHRR and buoy data, empirical relationships are obtained for linking eddy sizes to their rotational periods and tangential velocities. The relationships are useful for monitoring the effects of the eddies in the ocean by remote sensing techniques when in situ data are lacking. The question of whether the high-resolution satellite images utilised in this work are truly representing the SST of the ocean is also addressed here. Moreover, with the support of extra satellite data from the Along-Track Scanning Radiometer (ATSR) and in situ data from ships of opportunity, we investigate the nature of the temperature differences (deltaT) between 'skin' and 'bulk' SSTs in the study area.'Match-ups' between satellite and in situ SSTs demonstrated the presence of a bias in the satellite estimates of SST. DeltaT images also indicated that, owing to the highly dynamic nature of the BMC and BC/BCC fronts, large errors can arise when matching-up buoy with satellite data in these areas.

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