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Mechanism of Multidecadal Climatic Variability in a Global Climatic Model
Author(s) -
HUNT B. G.,
DAVIES H. L.
Publication year - 1997
Publication title -
international journal of climatology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.58
H-Index - 166
eISSN - 1097-0088
pISSN - 0899-8418
DOI - 10.1002/(sici)1097-0088(199705)17:6<565::aid-joc172>3.0.co;2-6
Subject(s) - climatology , environmental science , sea surface temperature , latitude , monsoon , ocean heat content , climate model , atmospheric sciences , climate change , geology , oceanography , geodesy
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 0·7°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 high‐ and 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. © 1997 by the Royal Meteorological Society.