An Examination of the Spring 1997 Mid - latitude East Pacific Sea Surface Temperature Anomaly

We seek to document and explain the lifecycle of the warm SST anomaly that intensified and weakened off the west coast of the United States to peak anomalies of 4' K during April-June 1997. We use remotely sensed observational data and model analyses to compute an energy budget for the warm anomaly. The bulk of the anomalous warming was confined to the top 50m of ocean and occurred during May. The immediate cause for the warming was twofold: latent heat fluxes decreased in magnitude as a result of both lower wind speeds and positive moisture anomalies, while the net radiative flux into the ocean increased as a result of lower than normal fractional cloud coverage. During June, the wind speed strengthened and became northerly, resulting in larger than normal latent and sensible heat fluxes that weakened the SST anomaly. Examination of NCEP 1000 mb geopotential height for May shows that the Aleutian low extended far south of its usual position and was responsible for the weaker southerly winds and suppressed latent heat fluxes in the warm anomaly area. Finally,. we note that the near simultaneous appearance of this warm anomaly in conjunction with warm El Niiio waters off Peru makes El Niiio an unlikely trigger for the northeast Pacific warm anomaly. We suggest a possible alternate scenario in which both events are remotely triggered by the 30-60 day intraseasonal oscillation.