Reductions in Strong Upwelling‐Favorable Wind Events in the Pliocene

Abstract

AbstractProxy records of the Pliocene show significant warm anomalies of sea surface temperatures (SST) near midlatitude upwelling sites at the eastern boundaries of the Pacific and Atlantic Oceans. Weaker upwelling‐favorable mean winds or a deeper thermocline have been proposed as explanations, yet the mechanisms involved are still not clear and the dramatic warmings of up to 9°C are only partially accounted for. Here we quantify the response of strong “transient” upwelling‐favorable wind events, defined to have a wind velocity over 5 m s−1 and a duration of at least 3 days, to two reconstructions of Pliocene SST. Such events are responsible for much of the modern upwelling SST signal and may therefore be a more relevant measure than the climatological winds. We find that both the amplitude and duration of upwelling‐favorable events are reduced in the Pliocene scenarios. Using Pliocene Reconstruction Interpretation and Synoptic Mapping SST, we find an annual reduction in upwelling flux between 7.4% and 13.2%, depending on model resolution. With an idealized Pliocene SST, the reduction in upwelling flux is between 46.7% and 50.3%. This reduction is shown to be closely related to changes in anticyclonic atmospheric activity, associated in turn with a weaker large‐scale meridional SST gradient.