Variability of the central-North Atlantic subtropical gyre induced by Rossby waves

Abstract:

The Atlantic Ocean meridional overturning circulation (AMOC) contributes to moderate of climate in Europe through the northward transport of 25% of the global heat flux, which is at maximum at around 24.50ºN. Consequently, transatlantic oceanographic sections at this latitude have become a benchmark for monitoring long-term changes in the Atlantic and to study the nature and causes of climate change. This has resulted in the occupation of the North Atlantic Ocean hydrographic transect along 24.5ºN seven times since 1957, more than any other transoceanic section in the world. The east-west slope in the dynamic height at 200dbar referenced to 1800dbar along 24.5ºN has decreased in 12-cm between 1957 and 2011, suggesting a spin-down of the central-North Atlantic subtropical gyre. Over imposed on this long-term trend in the slope there is a decadal variability in the slope with amplitudes up to 5 cm, that persist for several years, as the one during 2001-2004 that preceded large compensated changes in temperature and salinity at 24.5ºN. The hydrographic dataset show that this signal is predominantly due to vertical motion of the isopycnal surfaces, extending to depths of at least 1800 dbar. Using the linearized equations for a stratified ocean separated into vertical modes and for low-frequency, large-scale wind forcing, we explore the role of westward propagating linear Rossby waves in explaining these long-term and decadal changes. The results indicate the long-term change may be explained by the differential trend of wind stress curl between the eastern and western basins. The results are compared with simulations from a two-layer primitive equation model with realistic topography. Given the recent interests in Rossby waves, as the main mechanism explaining the observed seasonal cycle of the AMOC, an analysis of the impact of the seasonal variability induced by Rossby waves at 24.5ºN is also carried out.