A 500-year run has been made with a global climatic model for current climatic conditions using a simple slab ocean with inferred oceanic heat transfers. The model exhibited multi-decadal warming and cooling episodes with changes in globally averaged, annual mean surface temperature of up to 017 C. The length of the individual episodes varied, but 50-60 years was typical for major episodes. Examination of the geographical distribution of climatic variables for warm or cool episodes revealed distinct differences, particularly of surface temperature and low-level zonal wind, with considerable activity concentrated over the low-latitude Pacific Ocean. Each multi-decadal warming and cooling episode experienced pulsations of about 3-5 years duration associated with westerly wind bursts over the western Pacific Ocean. These bursts were related to the behaviour of the Asian monsoon, and, in turn, a connection between activity over the Pacific Ocean and the Asian monsoon was identified via the global distribution of velocity potential. The wind bursts produced warmings of the low-latitude, central Pacific Ocean and showed a number of features characteristic of the atmospheric phase of an ENSO event.
The centre of activity producing the multi-decadal variability was determined to be the low-latitude Pacific Ocean, and analysis was subsequently concentrated on this region.
The major 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 variations contributing to the overall cooling. The reverse situation applied 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 progressed. The termination of a cooling episode resulted from a reduction in the total cloud amount, attributed to the highand medium-level cloud, despite the increase in low-level cloud amount. This reduction permitted sufficient solar radiation to reach the surface in low latitudes to initiate a warming and trigger deep convection and thus recharge the high-level cloud amount, which then enhanced the initial solar-induced surface warming.