Characteristic evolution of the Atlantic Meridional Overturning Circulation from 1990 to 2015: An eddy-resolving ocean model study

Abstract

A 1/12° North Atlantic model is used to investigate the Atlantic Meridional Overturning Circulation (AMOC) variability from 1990 to 2015. The seasonality of the AMOC in depth and density spaces is dominated by the Ekman transport from low to high latitudes. At interannual timescales, the AMOC in depth and density spaces has different characteristics, mostly in high latitudes, which is attributed to strong doming of isopycnals. An Empirical Orthogonal Function analysis of the AMOC in depth space demonstrates that the AMOC can be decomposed into two portions – one associated with Labrador Sea winter convection and one related to Ekman transport. The Ekman transport portion of the depth-space AMOC has a general out-of-phase relationship between high and low latitudes and the AMOC becomes more meridionally coherent after the Ekman transport portion is removed. Our study suggests that the AMOC in depth space appears to have two regimes, a strong one before 2001, and a weak one after 2001, which is seen to be associated with the westward movement of the winter North Atlantic Oscillation centers of action starting from 2001. The two-regime behavior is best reflected in the variability of the western part of the Labrador Current, while the general decline over this period is also seen in the Labrador Sea convection depth, the strength of the eastern part of the Labrador Current, and the downward movement of isopycals in the deep layers of the Labrador Sea The latter suggests that hydrographic changes in Labrador Sea deep layers could be potentially used as a proxy for AMOC variability.