Mechanisms of Low-Frequency Variability in North Atlantic Ocean Heat Transport

Abstract:

Northward ocean heat transport (OHT) plays a key role in climate and its variability. Here, we decompose OHT in the North Atlantic into modes of variability sorted by their dominant timescales, using a low frequency component analysis (LFCA) applied to three global climate models. The first low frequency component (LFC) computed from this method maximizes the ratio of low-frequency variance (occurring at decadal and longer timescales) to total variance. Lead-lag regressions of atmospheric and ocean variables onto the LFC time series illuminate the dominant mechanisms controlling low-frequency OHT variability. Anomalous northwesterly winds from eastern North America over the North Atlantic act to increase upper ocean density in the Labrador Sea region, enhancing convection, which later increases OHT via changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC). Convection peaks two years before OHT and AMOC are at their maximum. The strengthened AMOC carries warm, salty water into the subpolar gyre, which then reduces the convection, weakening AMOC and OHT. This mechanism, where changes in AMOC and OHT are driven primarily by changes in Labrador Sea deep convection, holds not only in models where the climatological (i.e., time-mean) deep convection is concentrated in the Labrador Sea, but also in models where the climatological deep convection is concentrated in the Greenland-Iceland-Norwegian (GIN) Seas. These results suggest that despite recent observations that suggest that the Labrador Sea plays a minor role in driving climatological AMOC, the Labrador Sea may still play an important role in driving low-frequency AMOC and OHT variability.