A Mechanism for the Recurrence of Wintertime Midlatitude SST Anomalies

Abstract

In the early 1970s, Namias and Born speculated that ocean temperature anomalies created over the deep mixed layer in winter could be preserved in the summer thermocline and reappear at the surface in the following fall or winter. This hypothesis is examined using upper-ocean temperature observations and simulations with a mixed layer model. The data were collected at six ocean weather stations in the North Atlantic and North Pacific. Concurrent and lead-lag correlations are used to investigate temperature variations associated with the seasonal cycle in both the observations and the model simulations. Concurrent correlations between the surface and subsurface temperature anomalies in both the data and the model indicate that the penetration of temperature anomalies into the ocean is closely tied to the seasonal cycle in mixed layer depth: high correlations extend to relatively deep (shallow) depths in winter (summer). Lead-lag correlations in both the data and the model, at some of the stations, indicate that temperature anomalies beneath the mixed layer in summer are associated with the temperature anomalies in the mixed layer in the previous winter/spring and following fall/winter but are unrelated or weakly opposed to the temperature anomalies in the mixed layer in summer. These results suggest that vertical mixing processes allow ocean temperature anomalies created over a deep mixed layer in winter to be preserved below the surface in summer and reappear at the surface in the following fall, confirming the Namias?Born hypothesis.