ON THE SUMMER TIME DEVELOPMENT OF THE NORTH PACIFIC SEA-LEVEL PRESSURE ANTICYCLONE

Abstract

With the exception of the North Indian Ocean, subtropical ocean basins are dominated by climatological planetary-scale sea-level pressure (SLP) anticyclones. The seasonal variability of the North Pacific subtropical SLP anticyclone is examined here. The largest ERA-40 and linear diagnostic modeled Northern Hemisphere SLP seasonal variabilities are found in the mid-latitudes with relatively less change in the subtropics; this leads to the poleward boreal summer development of the North Pacific and Atlantic subtropical SLP high. Unlike the Northern Hemisphere, the Southern Hemisphere subtropical SLP highs develop equatorward. The zonal-mean Northern Hemisphere subtropical SLP and  seasonal variabilities are dominated by continental seasonality - a uniform boreal winter descent changing to a zonally asymmetric continental monsoon ascent and heat lows with relatively little change over the oceans. A linear diagnostic model is used to examine the forcing of the SLP seasonal cycle. The modeled North Pacific SLP seasonal variability is forced mainly by winter stormtracks, extra-tropical North Pacific diabatic cooling, and boreal winter ITCZ. Asian monsoon forces a SLP ridge downstream, but the monsoon response is cancelled significantly by East Pacific diabatic heating and transients. North American diabatic heating and transients are also found to have a limited upstream effect. Boreal summer ITCZ forcing has limited North Pacific SLP response, and that is possibly linked to the prescribed tropical zonal-mean easterlies.

ERA-40 and TRMM CSH diabatic heating is inter-compared with other independent measures of diabatic and latent heating. Zonal-mean ERA-40 ITCZ diabatic heating is nearly twice that of NCEP and ERA-15 reanalyses, which indicates a much stronger ERA-40 Hadley Circulation. The ERA-40 Walker Circulation is also stronger than of NCEP Reanalysis, which is consistent with excessive Maritime Continent diabatic heating. Largest differences are also found in the Tropical East Pacific and Atlantic. Vertically integrated TRMM CSH heating is too weak even compare with other TRMM products. However, TRMM CSH mid-tropospheric tropical heating compares well with other datasets. The largest differences appear in the upper and lower troposphere, which implies CSH limitations in handling shallow convection (a known issue) and stratiform precipitation in deep convection.