MECHANISM OF MULTI-DECADAL CLIMATIC VARIABILITY IN A GLOBAL CLIMATIC MODEL

A 500-year run has been made with a global climatic model for current climatic conditions using a simple slab ocean with inferred oceanic he at transfers. The model exhibited multi-decadal warming and cooling ep isodes with changes in globally averaged, annual mean surface temperat ure of up to 0.7 degrees C. The length of the individual episodes vari ed, but 50-60 years was typical for major episodes. Examination of the geographical distribution of climatic variables for warm or cool epis odes revealed distinct differences, particularly of surface temperatur e and low-level zonal wind, with considerable activity concentrated ov er the low-latitude Pacific Ocean. Each multi-decadal warming and cool ing episode experienced pulsations of about 3-5 years duration associa ted with westerly wind bursts over the western Pacific Ocean. These bu rsts were related to the behaviour of the Asian monsoon, and, in turn, a connection between activity over the Pacific Ocean and the Asian mo nsoon was identified via the global distribution of velocity potential . The wind bursts produced warmings of the low-latitude, central Pacif ic Ocean and showed a number of features characteristic of the atmosph eric phase of an ENSO event. The centre of activity producing the mult i-decadal variability was determined to be the low-latitude Pacific Oc ean, and analysis was subsequently concentrated on this region. The ma jor factor controlling the multi-decadal warming and cooling episodes was cloud variability. During a cooling episode low-level cloud amount increased whereas high-level cloud amount decreased, with both variat ions contributing to the overall cooling. The reverse situation applie d during a warming episode. A necessary precursor to a cooling episode was a build up in low-level moisture in the atmosphere sufficient to sustain the subsequent low-level cloud amount as the cooling progresse d. The termination of a cooling episode resulted from a reduction in t he total cloud amount, attributed to the high- and medium-level cloud, despite the increase in low-level cloud amount. This reduction permit ted sufficient solar radiation to reach the surface in low latitudes t o initiate a warming and trigger deep convection and thus recharge the high-level cloud amount, which then enhanced the initial solar-induce d surface warming. (C) 1997 by the Royal Meteorological Society.