The Impact of Ekman Transport on Air-Sea Interaction in the North Pacific during ENSO

The atmospheric response to El Niņo and La Niņa is global, including substantial changes over the North Pacific Ocean. Changes in low-level surface winds, temperature, moisture and clouds associated with sea surface temperature (SST) anomalies in the El Niņo Southern Oscillation (ENSO) region, influence the extratropical oceans, thus forming an "atmospheric bridge" between the tropical and North Pacific Ocean. Most previous studies of the atmospheric bridge have focused on how surface heat fluxes influence SSTs during boreal winter. Here we will expand the atmospheric bridge paradigm by examining how ENSO-induced Ekman transport anomalies influence ocean properties, including SST anomalies, in the North Pacific and how these SST anomalies feed back on the atmosphere. We use coupled atmosphere-mixed layer ocean model experiments with and without Ekman transport, to diagnose its role in the atmospheric bridge.

Preliminary results indicate that Ekman transport has a significant impact on air-sea interaction over the North Pacific during ENSO in winter. The ENSO-induced Ekman transport is of the same order of magnitude as the net heat flux and acts to cool the central North Pacific and warm the Gulf of Alaska during El Niņo events. As a result, the SST anomalies associated with ENSO are enhanced over most of the North Pacific. Relative to model simulations without Ekman transport, these enhanced SST anomalies feedback on the atmosphere enlarging and amplifying the response of the Aleutian Low to ENSO.