Location Variation Of Northern Pacific Calm Belt And Its Response To ENSO In Interannual Variability

Based on the scatterometer wind data from 1999.8 to 2009.8, we find that the Zonal Position of Subtropical Calm Belt (ZPSCB) in North Pacific has not only obvious seasonal change, but also significanl inter-annual variability with periods of 2-7 years and amplitudes of about 2 latitudes. The calm belts in middle and eastern Pacific have moved northward 2 degrees of latitude totally during 18 years, but the belt in west Pacific has nearly no movement of linear trend. On annual scale, the calm belt moves northward in summer and southward in winter; and there are more high pressure centers on this belt in summer than in winter. We also found that the location variation of subtropical clam belt has a positive correlation with SOI and a negative correlation with the tropical westerlies in the interannual scale, which indicates that the calm belt responds well to ENSO. On this basis, a possible mechanism has been proposed as follows:The Hadley cell going up from the tropical western Pacific moves northeastward under the geostrophic effect and eventually sinks in the subtropical areas. During El Nino, the tropical westerlies enhances, leading to a lower sea surface temperature in the tropical western Pacific. The Water evaporation, which is influenced by the sea surface temperature, is reduced. Hence the energy carried by Hadley circulation to make the air flows northward becomes smaller than the normal. Eventually, the subtropical calm belts in North Pacific moves southward abnormally, and during La Nina it changes conversely.