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Indications of decadal variability in the oceanic frontal zones : A preliminary results

 

Alexander S. Kazmin

Institute for Global Change Research, Frontier Research System for Global Change

 

The large-scale oceanic frontal zones (OFZ) are prominent features that originate from and are maintained by the global distribution of surface momentum and heat flux forcing. The variability in the OFZ are important for understanding the ocean's thermodynamic balance and global climate changes. Seasonal and, partly, interannual variability in the main OFZ have been addressed recently using global satellite SST data [e. g., Kazmin and Rienecker, 1996]. However, while ocean-atmosphere system is known to fluctuate also with decadal timescales, the OFZ's response to decadal variability is still poorly investigated. In this study global AVHRR SST for the period of 1982-97 have been used to obtain some evidences of decadal variability in the North Pacific subpolar and subtropical OFZ (as seen in SST gradient field). It is known that SST in mid-latitude North Pacific dropped significantly around 1976 and tended to be lower than the long-term average until 1988 [e. g., Nakamura et al., 1997]. This 'cool' period was followed by 'warm' period till mid-1990's with higher than average SST. Thus, while 16 years-long dataset is insufficient to detect decadal cycle, it still can be used to document changes in OFZ associated with this specific climatic shift. Since the interpolation and smoothing used to fill in missed pixels in satellite SST product may create an artificial trends in SST gradient magnitude in case if there are significant trends in the number of missed pixels (and we found out that this is the case), in this study only valid data was used. Original weekly mean 20km resolution data have been averaged monthly and over 100km by 100km boxes and smoothed with 3-month moving average. Then, SST for each month was zonally averaged within 150E - 175W and meridional gradient of SST was calculated. Finally, wintertime anomalies of SST, SST gradient and anomalies of SST gradient were plotted as a function of latitude and time for N. Pacific mid-latitudes analysis domain (15N-50N). In general, SST distribution reveals negative anomaly between 25N-46N in 1982-88 (with the strongest negative values observed along 38N-43N) followed with relatively sharp transition to positive anomaly in 1988-97, which is consistent with previous studies. South of 25N the SST pattern was opposite, with positive anomaly occurred in 1983-90. The subtropical OFZ tended to be stronger in 1982-88 and weakened substantially afterwards. The subpolar OFZ's response was more complicated. In 'cool' period of 1982-88 it's southern part around 39N-40N revealed strong positive anomaly of SST gradient while area north of 42N was significantly weaker than long-term average. Then, during 1988-90 positive SST gradient anomaly shifted northward to 43N-44N, reached it's maximum in period of sharp transition from 'cool' to 'warm' conditions and stayed in northernmost position during 1991-96. Southern part of subpolar OFZ tended to be weaker in this time. The subpolar OFZ as a whole appeared to be shifted northward. 2-3 degrees latitude in 1990's.

 

 

 

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