Bg. Hunt et Hl. Davies, MECHANISM OF MULTI-DECADAL CLIMATIC VARIABILITY IN A GLOBAL CLIMATIC MODEL, International journal of climatology, 17(6), 1997, pp. 565-580
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.