Teleconnection between the Atlantic Meridional Overturning Circulation and sea level in the Mediterranean Sea

Abstract

The Mediterranean Sea can be viewed as a “barometer” of the North Atlantic Ocean, because its sea level responds to oceanic-gyre-scale changes in atmospheric pressure and wind forcing, related to the North Atlantic Oscillation (NAO). The climate of the North Atlantic is influenced by the Atlantic Meridional Overturning Circulation (AMOC) as it transports heat from the South Atlantic towards the Subpolar North Atlantic. This study reports on a teleconnection between the AMOC transport measured at 26.5°N and the Mediterranean Sea level during 2004-2017: a reduced/increased AMOC transport is associated with a higher/lower sea level in the Mediterranean. Processes responsible for this teleconnection are analyzed in detail using available satellite and in situ observations, and an atmospheric reanalysis. Firstly, it is shown that on monthly to interannual time scales the AMOC and sea level are both driven by similar NAO-like atmospheric circulation patterns. During a positive/negative NAO state, stronger/weaker trade winds (i) drive northward/southward anomalies of Ekman transport across 26.5°N that directly affect the AMOC, and (ii) are associated with westward/eastward winds over the Strait of Gibraltar that force water to flow out/in the Mediterranean Sea and thus change its average sea level. Secondly, it is demonstrated that interannual changes in the AMOC transport can lead to thermosteric sea level anomalies near the North Atlantic eastern boundary. These anomalies can (i) reach the Strait of Gibraltar and cause sea level changes in the Mediterranean Sea, and (ii) represent a mechanism for negative feedback on the AMOC.

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