It happened during transition from the‘cool’to‘warm’period in subtropical area and northward propagation of positive SST anomaly (Fig. 1, upper panel). The SST gradient in STFZ tended to be slightly stronger during the‘cool’phase,which lasted until 1987. Accordingly, SST gradient anomaly was positive at that time and became negative afterward, in‘warm’conditions. In transition period, positive anomaly of SST gradient propagated northward from its subtropical location, providing above mentioned merging of two main frontal zones (Fig. 2, lower panel). Summertime analysis is not applicable to STFZ since its SST manifestation is strongly seasonal and disappears in summer (Kazmin and Rienecker 1996).
The response of SPFZ to decadal changes was apparent in its spatial structure and SST gradient anomalies rather than in SST gradient magnitude. The latter in SPFZ does not show any pronounced temporal variability associated with the decadal climatic shift, except that strongest values (exceeding 1.5degC/100km) have been observed during the strongest stage of positive SST anomaly in 1989-91. The core of SPFZ (defined as an area with the SST gradient magnitude higher than 1deg.C/100km) tended to be wider (up to 6deg. Iatitude) and more diffuse during the‘cool’period of 1982-87, but became narrower and much more concentrated afterwards. By the end of considered period the SPFZ's core shrunk to 3-4deg. latitude with the strongest (higher than 1 .2deg.C/100km) values being concentrated within only 100-150km zone. The SPFZ as a whole appeared to be shifted northward 2-3deg. latitude in 1990s compare to 1980s. This northward shift was more pronounced for the southern boundary of the SPFZ's core. There are indications of the similar pattern of SPFZ (northward shift of southern boundary and core narrowing) during another‘warm’period in 1968-72 obtained from the ship measurements (H. Nakamura, personal communication). Concentration of high gradients in the narrow zone during‘cool’phase may serve as an indirect indication of along-frontal jet intensification in this period.
Decadal time-scale variability in SPFZ appears more obviously in wintertime SST gradient anomalies (Fig. 2, lower panel). In‘cool’period of 1982-88, the main part of SPFZ within 40N-45N revealed relatively strong negative anomaly while its southern edge around 38N-40N was significantly stronger than the long-term average. In‘warm’phase after 1988, significant positive anomaly persisted through the most part of SPFZ within its climatological boundaries. At this time, the southern edge of SPFZ was relatively weaker but negative anomaly was small and concentrated in narrow band. In general, time-space evolution of SST gradient field in SPFZ appeared as a northward propagation of positive anomaly from 38N-40N to 40N-45N during the transition period of 1987-89 from‘cool’to‘warm’conditions. In summer, widening of SPFZ's core due to seasonal enhancement (Kazmin and Rienecker 1996) tend to mask the indications of decadal-scale variability.
