Interannual Variability of Radiative Fluxes and Sea Surface Temperature in the Vicinity of the Indonesian Throughflow

Interannual variability of top-of-atmosphere (TOA) reflected shortwave and outgoing longwave radiation as measured by NASA's Earth Radiation Budget Satellite (ERBS) nonscanner radiometer is described by an empirical orthogonal function (EOF) analysis. ERBE radiative flux data are available for a 15-year time period, from November 1984 through September 1999, in the form of monthly mean values processed onto a 5-degree equal angle grid. Because ERBS is in a 57-degree inclined orbit, data are available between 60N and 60S latitude.

Interannual variability of the sea surface temperature (SST) field is described by an EOF analysis of the NOAA National Centers for Environmental Prediction (NCEP) monthly optimum interpolation (OI) data set over the same time period as the ERBE measurements. The OI data set provides monthly mean SST values for 1-degree grid boxes over the entire globe.

The EOF analyses were performed on the monthly mean anomalies of both the radiative fluxes and the SST, which are computed by subtracting the climatological monthly mean value from the monthly mean values in order to obtain estimates of deseasonalized interannual variability. The analyses were restricted to the tropics, between 35N and 35S latitude. The first EOF, corresponding to the ENSO signal, accounted for approximately 19 percent of the variance in the longwave flux, 9 percent of the variance in the shortwave flux, and 29 percent of the variance in the SST. The time histories of the principal components of first EOF of all three fields are virtually identical, implying that the cloud cover in the tropics is being driven by the SST. Time histories were filtered with a 5-month running mean.

An ENSO signal is also evident in the second EOF of both the longwave and shortwave fluxes and the SST. The time histories of the principal components of the second EOF of the radiative fluxes identify four distinct events over the 15-year period, each with a lifetime of 1 - 2 years. The time history of the principal components of the second EOF of the SST is quite similar to that of the fluxes, which again points to the influence of the ENSO signal in the second EOF.

The third EOF of both fluxes reveals a dipole pattern centered in the vicinity of the Indonesian throughflow, while the SST map shows a minimum of variability in this region. The time histories of the principal components of the third EOF reveal five distinct events over the 15-year period, with events in the SST record lagging those in the radiative flux records by approximately one year. The second EOF accounts for approximately 7 percent of the variance in the longwave flux, 5 percent of the variance in the shortwave flux, and 12 percent of the variance in the SST, while the third EOF accounts for approximately 6 percent of the variance in the longwave flux, 4 percent in the shortwave flux, and 5 percent in the SST. Both of these modes are likely related to variability in the Indonesian throughflow.