Northward penetration of Antarctic intermediate water off Northwest Africa

Abstract

In this article, historical and climatological datasets are used to investigate the seasonal northward propagation of Antarctic Intermediate Waters (AAIW) along the eastern margin of the North Atlantic subtropical gyre. A cluster analysis for data north of 26°N shows the presence of a substantial number of hydrographic stations with AAIW characteristics that stretch northeast along the African slope. This water mass extends north during fall, as shown both through the comparison of actual and climatological data, and by applying a mixing analysis to normal-to-shore seasonal sections at both 28.5° and 32°N. The mixing analysis is further used with several fall cruises between 32° and 36°N, and shows that at these latitudes the core of AAIW propagates along the 27.5 isoneutral with contributions that reach as much as 50% at 32.5°N. An idealized Sverdrup-type model is used in combination with climatological hydrographic and wind data to examine what forces this eastern boundary propagation. It is found that column stretching, initiated in the tropical North Atlantic, is the dominant term in the vorticity balance of the AAIW stratum, capable of sustaining a winter–spring–summer northward transport of about 3–4 Sv (1 Sv ≡ 106 m3 s−1) that reaches as far north as the Canary Archipelago (28°N). In fall, this transport may continue beyond 28°N, sustained by a near-slope meridional stretching of this water stratum. AAIW probably fades away in the northeastern region as the result of several processes, specially enhanced double diffusion with surrounding waters and interaction with Mediterranean water lenses.