Semi-automatic determination of the Azores Current axis using satellite altimetry: Application to the study of the current variability during 1995–2006

Abstract

Satellite altimetry has been widely used to study the variability of the ocean currents such as the Azores Current (AzC) in the North Atlantic. Most analyses are performed over the region that encloses the current, thus being somehow affected by other oceanographic signals, e.g., eddies. In this study, a new approach for extracting the axis of a zonal current solely based on satellite altimetry is presented. This is a semi-automatic procedure that searches for the maximum values of the gradient of absolute dynamic topography (ADT), using the geostrophic velocity as auxiliary information. The advantage of this approach is to allow the analyses to be performed over a buffer centered on the current axis instead of using a wider region. It is here applied to the AzC for the period June 1995–October 2006.Regular 0.25°×0.25° latitude–longitude grids of multi-mission sea level anomaly (SLA) were generated at a 10-day interval for the Northeast Atlantic using an optimal interpolation methodology with a realistic space-time correlation function. From these, regular grids of ADT and horizontal components of surface geostrophic velocity (U,V) were generated. By applying the methodology to these oceanographic fields, the AzC axis position for each 10-day frame has been derived.The time-mean position of the AzC axis shows a good agreement with the highest root-mean-square values of sea level anomaly in the Azores Current region. Zonal means of axis position, SLA and U and V have been calculated over the longitude range of the current (35°W–15°W) to assess the AzC variability. The decomposition of the time series of the zonal mean of AzC position into trend and seasonal components shows the existence of significant inter-annual variability in the axis position, while seasonal variability is small. The AzC is found at its southernmost position from the beginning of the period to mid-1998, the minimum value occurring in late summer of 1996, and a northward displacement of the jet position is clearly visible up to the end of 2006.