1899-1995年の間の海面気圧を用いた北太平洋上海上風の再現

Abstract

Sea surface wind (SSW) fields north of 20°N over the North Pacific are reconstructed using sea level pressure (SLP) fields (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 as follows. First, since the mean height of anemometers installed on ships in the 1980s is approximately 35m, values of monthly mean SSW in the 1980s computed using winds of the Comprehensive Ocean-Atmosphere Data Set (COADS) are adjusted to those at the standard height of 10m. Then, monthly climatologies of SSW and of SLP fields averaged for the ten years from 1981 through 1990 are computed. Next, monthly anomalies of both SSW and SLP fields during the ten years are calculated. Under the assumption of a geostrophic wind balance, geostrophic wind (GW) anomalies are computed using monthly SLP anomalies. Based on comparison between GW and COADS SSW anomalies, two adjustment factors of the reduction coefficient and correction angle are simultaneously estimated for each of four seasons using a neutral regression analysis. Adopting these adjustment factors for GW anomaly fields, corrected wind anomalies are obtained. Finally, the reconstructed SSW fields are obtained as the sum of corrected wind anomalies and ten-year monthly climatologies. When we regard height-corrected COADS SSW fields as the reference winds, root-mean-square differences of the reconstructed SSW fields can be estimated to be less than 1m s-1 in magnitude, and less than 15 degrees in wind direction for the winds greater than 2m s-1. Comparison of the reconstructed SSWs with NCEP reanalysis SSWs is also made and shows that NCEP reanalysis SSWs are systematically 8% greater in magnitude than the reconstructed SSWs throughout the comparison period from 1950 to 1995.Time series of magnitudes of the reconstructed SSWs averaged in the wintertime mid-latitude westerlies region shows interannual and decadal/multidecadal time scale variations: from mid-1930s to mid-1940s, westerlies was remarkably strengthened compared with the rest of the period.