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Robust Solar Cycle Signal in the North Pacific Subarctic Frontal Zone
李德琳
广东海洋大学
The Sun is the most fundamental power source of the Earth's climate system. The response of climate system to solar variation is regionally sensitive. In respect of solar impacts on wintertime climate over the Pacific, more attention was paid to the tropical Pacific sector in previous studies. Actually, there are also significant solar cycle signals in the extratropical North Pacific. Based on the observations over the last three decades, we are surprised to find that the positive correlation between the intensity of North Pacific Subarctic Frontal Zone (SAFZ) and sunspot number (SSN) in boreal winter is quite robust. The large-value area of decadal variability in North Pacific mid-latitude sea surface temperature (SST) finely locates in the main body of SAFZ. Moreover, the SAFZ has a significant period of 11-year, which is same as the solar cycle, suggesting their internal association on the common decadal variation. Without regard for the solar effect on the oceanic internal processes, further analyses show that the notable relationship between SSN and SAFZ could be established through the top-down solar influence on the surface wind near the SAFZ region. Specifically, in early winter (November–December), since the increased solar forcing can amplify the stratospheric meridional temperature gradient between subtropics and high latitudes, the stratospheric mid-latitude westerlies accelerate significantly. In the meantime, the planetary waves from the troposphere could be reflected downward and poleward by this strengthened westerly in stratosphere. Then the solar signal can gradually propagate downward into the lower troposphere via the wave mean flow interactions. In December, the solar signal in the low level of troposphere presents as a weakened Aleutian Low, and thereby the North Pacific mid-latitude surface westerlies decelerate markedly in January. This weakened regional surface westerlies avail the rise in local SST over the mid-latitude North Pacific. Therefore, in late winter (February), the significant positive SST anomaly in middle latitudes continually appears and its amplitude increases slightly, accompanying which, the SAFZ strengthens.