On the persistence of decadal SST anomalies in the North Atlantic

Historical wintertime sea surface temperature (SST) data show that a sandwich pattern dominates on the decadal timescales in the North Atlantic, at least after the 1970s. The authors investigated how such decadal SST anomalies can survive against local thermal feedback, which acts to dampen them rapidly. At the surface, winter SST anomalies have a negligible projection with the subsequent summer anomalies while they show a significant projection with the SST anomalies in the next winter. On the other hand, observed summer temperature anomalies below the mixed layer tend to have the same sign as the previous winter SST anomalies, although the magnitude of the formers roughly one-third of the latter. This suggests that a reemergence mechanism of SST anomalies associated with the seasonal cycle of the mixed layer depth (MLD), which has been verified by Alexander and Deser, helps maintain the decadal SST anomalies. In order to examine this scenario, a mixed layer model driven by daily atmospheric data generated by a T42 atmospheric general circulation model was used. The mixed layer model well reproduces the climatology of both SST and MLD in the North Atlantic. An experiment in which a thermal forcing having the observed decadal pattern is added only for the initial winter shows that the SST anomalies disappear until July but reappear in the subsequent winters. This result supports the inference based on the observational evidence, and is explained as follows: (i) SST anomalies are partly detrained to deeper levels in spring when the mixed layer shoals rapidly, (ii) temperature anomalies beneath the shallow mixed layer are preserved during summer, (iii) they are entrained into the surface in the succeeding fall and winter when the mixed layer is again deepened. The recurrence of SST anomalies was found in two centers of the decadal anomaly pattern (30 degrees-45 degrees N, 80 degrees-50 degrees W and 45 degrees-60 degrees N, 50 degrees-20 degrees W), but not in another center in the subtropics (10 degrees-25 degrees N, 40 degrees-10 degrees W) where the MLD reveals only a small seasonality. The magnitude of recurrent SST anomalies is affected by two factors: MLD difference between winter and summer and the persistence of SST anomalies from winter to spring as they determine the amount and the magnitude of detrained temperature anomalies into the mixed layer, respectively. The above results indicate that the effective damping lime for the winter SST anomalies is much longer than the local damping time of several months.