To avoid the problem, only valid grid points have been used in this study. Original weekly mean valid-only data at about 18-km resolution have been averaged monthly and over 100km by 100km boxes and smoothed with the 3-month moving average. Then, SST for each month was zonally averaged within 150E-175W. Zonal averaging is appropriate in the central area of midlatitude North Pacific, where, away from western boundary current region, the large-scale structures of OFZ (subpolar and subtropical) have predominantly zonal orientation (Kazmin and Rienecker 1996). As a result, we obtained a 16-years time series of monthly-mean zonally averaged SST for North Pacific (15N-50N) which includes only actually measured data and have no void points to fill in. Then, three-month mean anomalies of SST, magnitude of the meridional gradient of SST and its anomalies for winter (December-February) and summer (June-August) were plotted as a function of latitude and time.
ANALYSIS
Wintertime SST distribution (Fig. 1, upper panel) reveals negative anomaly within 30N-50N in 1982-88 (with the strongest negative values observed along 38N-43N in 1983-87) followed with relatively sharp transition to positive anomaly in 1988-97. The latter was most intensive in 1989-91 within 38N-42N. In subtropical area, south of 25N the . SST pattern was opposite, with the positive anomaly in 1983-90 followed by negative one in 1991-97. In subtropics SST anomalies were weaker and more uniform compare to subpolar area. In general, wintertime SST anomalies derived from the satellite data are consistent with the previous findings based on ship observations, both in spatio-temporal distribution and in magnitude (e.g. Nakamura et al. 1997). They reveal the general decadal trend in SST distribution in North Pacific and show the most pronounced decadal variability to be concentrated around SPFZ location (but not around STFZ as well). In contrast, summertime SST distribution seems to be affected mainly by El Nino - Southern Oscillation (ENSO) time-scale variability (Fig. 1, lower panel). The picture is dominated by the intermittent negative and positive SST anomalies, which exist for 2 to 4 years and stretch out through the entire latitudinal domain as continuous features (except for 1996-97 negative anomaly). Negative SST anomalies' appearance seems to be correlated with the major ENSO events.
Wintertime distribution of SST gradient (Fig. 2, upper panel) reveals two main persistent belts of increased gradients within 38N-46N and 25N-30N, which correspond, respectively, to SPFZ and STFZ. The area of weaker SST gradients separates the two frontal zones all the time except the period of 1986-88 when they virtually merged.
