Varying temperature and heat content signatures in the central Labrador Sea at different layers and timescales

The dominant modes of variability in the temperature and ocean heat content (OHC; 0-1000 m) of the central Labrador Sea were investigated using the Hilbert-Huang Transform (HHT) based on Argo profiles collected during 2003-2012. Warming trends of approximately 0.03°Cyr−1 were observed in the upper 2000 m of the water column. A strong annual temperature cycle exists and dominates at the 500 m depth, while signals at the interannual timescales can explain most of the temperature variability at the 1000 m and 1500 m depths. These interannual signals are closely correlated to the variability of deep convection in the Labrador Sea, which has intermittent enhancement of mixed layer depth (MLD)>1500 m. The Hilbert spectrum from the OHC in the Labrador Sea interior reveals two important components at frequencies of 0.8−1.2cycleyr−1 (T=0.8-1.25 years) and 0.1−0.3cycleyr−1 (T=3.3-10 years), respectively, superimposed on the warming trends. The former corresponds to the dominant seasonal cycle due to surface heating, while the latter is concomitant with the timing of the reoccurrence of convective events. We also found that the cumulative North Atlantic Oscillation (NAO) index significantly correlates to the low-frequency OHC variations. Therefore, the interannual signals in the Labrador Sea at especially the intermediate layers are attributed to changes in the deep convective processes and the atmospheric conditions. By comparing with an extended OHC record (1945-2010), it was further shown that the warming trends obtained from the 10-year Argo record are part of multi-decadal variations that presumably reflect the Atlantic Multi-decadal Oscillation (AMO). In addition, the recent OHC changes in the Labrador Sea (i.e., increased heat in the deeper layers) may be related to the current global warming hiatus, suggesting the potential contributions from the Atlantic Meridional Overturning Circulation (AMOC) to the low-frequency OHC in the Labrador Sea.