Interannual-to-seasonal variability of the Subpolar North Atlantic Ocean

Abstract

In this thesis we examine the seasonal to interannual variability of the Subpolar North Atlantic Ocean over the last decade using both observations and models.  The Topex/Poseidon altimeter is the primary source for the observational data, which are analysed to examine the seasonal-to-interannual surface variability of the Subpolar North Atlantic Ocean.  These data are also used to infer changes in the positions of the Subpolar Front and the large-scale circulation.  Model output is provided from the Estimating the Circulation and Climate of the Ocean model (ECCO) and had been examined, firstly, to establish the realism of the model in the light of the altimeter data and historical observations.  Then, secondly, to identify any further variability in the ocean system and to document associated forcing mechanisms. The tools used to identify principal modes of variability and any coupling within the climate system are those stemming from Empirical Orthogonal Function analysis.

The analysis of the Topex/Poseidon data reveals that surface variability in the Subpolar Gyre region is primarily driven by the wind field, whereas in the Newfoundland Basin this is not the case.  The study also confirmed previous hydrographic analysis that there had been significant shifts in the position of the Subpolar Front in the Iceland Basin over the last decade.  Analysis of the position of the Subpolar Front in ECCO reveals much the same picture as in the altimeter data. It is demonstrated that the frontal shifts are as a result of a Sverdrup-like response of the ocean to variable wind forcing, of which the North Atlantic Oscillation (NAO) is one of the dominant modes.  It is also thought that this mechanism could be responsible for the generation and subsequent eastward propagation of temperature anomalies along the pathway of the Subpolar Front.

Another common feature present in both the altimeter data and output from ECCO is the weakening of the North Atlantic Current (NAC) and more generally the Subpolar Gyre over the last decade.

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