Monitoring the variability of sea level and surface circulation with satellite altimetry

Abstract

Variability in the ocean plays an important role in determining global weather and climate conditions. The advent of satellite altimetry has significantly facilitated the study of the variability of sea level and surface circulation. Satellites provide high-quality regular and nearly global measurements enabling us to study the oceanic variability on the spatial scales from the size of an eddy to global, and on the temporal scales from weeks to interannual and longer. This thesis demonstrates how satellite altimetry measurements can be used to study the mesoscale, seasonal and interannual variability of sea level and surface circulation. Oceanic variability at these time scales is mainly induced by the variations of heat and fresh water fluxes (buoyancy fluxes) at air-sea interface, the variations of heat and salt budget due to the advection of water masses with different properties, eddy generation mechanisms due to the instability of oceanic currents, Rossby waves, etc. It is shown how the sea level in the extratropical North Atlantic Ocean was changing during the investigated time interval from 1993 to 2003. The mesoscale, seasonal and inter-annual modes of the variability are revealed, and the magnitude and relative contribution of each mode to the total variance is assessed. The inter-annual change of the sea surface height in the northern North Atlantic, measured with altimetry, is coupled with in situ observations along the transatlantic section AR7E, repeated almost every year from 1990 to 2003 in the framework of the WOCE (World Ocean Circulation Experiment) and CLIVAR (CLImate VARiability) hydrographic programs. This allowed interpreting the observed inter-annual change of sea level in terms of changes in the sea water properties and the distribution of water masses. A comparative analysis of changes observed in the extratropical North Atlantic and in the extratropical North Pacific is performed. The magnitudes, spatial patterns, and also trends of the observed variations of sea level and associated surface circulation are compared. The variability of surface circulation in the Gulf Stream extension, North Atlantic Current, Azores Current, in the northeastern (Rockall Channel and Iceland Basin) and in the northwestern (Irminger Basin and Labrador Sea) North Atlantic is studied using yearly estimates of eddy kinetic energy as a proxy to define the course of major currents. One of the important issues illustrated by the thesis is the probable dependence of the inter-annual variations of sea level and surface circulation upon the corresponding changes in the atmospheric circulation pattern, related to ocean-atmosphere interactions. The leading modes of these interactions in the North Atlantic and North Pacific ocean basins are known as the North Atlantic Oscillation (NAO) and the Pacific Decadal Oscillations (PDO). The NAO and PDO indices are coupled with the observed inter-annual variations of sea level and surface circulation.