On Equatorial Pacific Surface Wind Changes around 1977: NCEP–NCAR Reanalysis versus COADS Observations*

Abstract

This note compares equatorial Pacific surface wind changes around 1977 in the NCEP–NCAR reanalysis and the Comprehensive Ocean–Atmosphere Data Set (COADS) observations. Significant discrepancies are found in wind changes over the equatorial central and eastern Pacific. In the NCEP–NCAR reanalysis, the easterlies weakened over the eastern equatorial Pacific, while the southerlies strengthened over the north equatorial central Pacific. As a result, the low-level convergence and precipitation decreased over the equatorial central Pacific. These wind and precipitation anomalies are opposite to those derived from the COADS observations. Independent observations of ocean heat content are used to validate the changes in equatorial zonal wind, and it is found that the zonal slope of the thermocline in an ocean model forced by the COADS wind is more consistent with ocean observations than forced by the reanalysis wind. The equatorial wind biases are also identified in the NCEP–NCAR reanalysis climatology, reaching a maximum in the cold season from August to October. This seasonality of wind biases calls for improved representation of atmospheric boundary layer processes in climate models.