Analysis of the decrease in the tropical mean outgoing shortwave radiation at the top of atmosphere for the period 1984-2000

[摘要] A decadal-scale trend in the tropical radiative energy budget has beenobserved recently by satellites, which however is not reproduced by climatemodels. In the present study, we have computed the outgoing shortwaveradiation (OSR) at the top of atmosphere (TOA) at 2.5°longitude-latitude resolution and on a mean monthly basis for the 17-yearperiod 1984-2000, by using a deterministic solar radiative transfer modeland cloud climatological data from the International Satellite CloudClimatology Project (ISCCP) D2 database. Anomaly time series for the meanmonthly pixel-level OSR fluxes, as well as for the key physical parameters,were constructed. A significant decreasing trend in OSR anomalies, startingmainly from the late 1980s, was found in tropical and subtropical regions(30° S-30° N), indicating a decadal increase in solar planetaryheating equal to 1.9±0.3Wm^-2/decade, reproducing well thefeatures recorded by satellite observations, in contrast to climate modelresults. This increase in solar planetary heating, however, is accompaniedby a similar increase in planetary cooling, due to increased outgoinglongwave radiation, so that there is no change in net radiation. The modelcomputed OSR trend is in good agreement with the corresponding lineardecadal decrease of 2.5±0.4Wm^-2/decade in tropical mean OSRanomalies derived from ERBE S-10N non-scanner data (edition 2). An attemptwas made to identify the physical processes responsible for the decreasingtrend in tropical mean OSR. A detailed correlation analysis usingpixel-level anomalies of model computed OSR flux and ISCCP cloud cover overthe entire tropical and subtropical region (30° S-30° N), gave acorrelation coefficient of 0.79, indicating that decreasing cloud cover isthe main reason for the tropical OSR trend. According to the ISCCP-D2 dataderived from the combined visible/infrared (VIS/IR) analysis, the tropicalcloud cover has decreased by 6.6±0.2% per decade, in relative terms.A detailed analysis of the inter-annual and long-term variability of thevarious parameters determining the OSR at TOA, has shown that the mostimportant contribution to the observed OSR trend comes from a decrease inlow-level cloud cover over the period 1984-2000, followed by decreases inmiddle and high-level cloud cover. Note, however, that there still remainsome uncertainties associated with the existence and magnitude of trends inISCCP-D2 cloud amounts. Opposite but small trends are introduced byincreases in cloud scattering optical depth of low and middle clouds.

[学科分类] 大气科学