OCEAN CIRCULATION AND VARIATION
Reconstruction of Sea Surface Wind Fields over the North Pacific Using Sea Level Pressures during the Period of 1899-1995
Kimio Hanawa and Tamaki Yasuda*
Department of Geophysics, Graduate School of Science, Tohoku University
* Present affiliation: Meteorological Research Institute, Japan
Sea surface wind (SSW) fields over the North Pacific are reconstructed using sea level pressure (SLP) data (Trenberth's Corrected Yearly-Monthly Northern Hemisphere SLP, version 010.1) for the period of 97 years from 1899 to 1995.
The reconstruction method is purely empirical. First, since the mean height of the anemometers installed in the merchant vessels during 1980s is approximately 35m, SSW data of the Comprehensive Ocean-Atmosphere Data Set (COADS) are adjusted to that at the standard height of 10m. Then monthly climatologies of SSW from COADS and of SLP averaged for ten years from 1981 through 1990 are computed. Then, monthly anomalies of both SSW and SLP fields during the ten years are calculated. Under the assumption of a geostrophic balance, geostrophic winds are computed using monthly SLP anomalies. Comparing those with monthly COADS SSW anomalies, two adjustment factors of the reduction coefficient and the correction angle are estimated for four seasons. Adopting these adjustment factors for the calculated geostrophic winds, corrected wind anomalies for the rest of the period are calculated. The total wind fields are then obtained as the sum of wind anomalies and COADS SSW climatologies. When we regard the COADS SSWs as the reference fields, the errors of reconstructed SSWs are estimated to be less than 1m/s in magnitude.
Time series of the reconstructed wind speeds in the westerlies region shows decadal to multidecadal scale variation: in 1920s through 1940s, westerlies strengthened compared with the rest of the period. In the same region, wind speeds are also compared to those derived from COADS SSWs. As pointed by theprevious researchers, COADS SSWs show the increasing trend at least from 1950s to the present.
