EOF Analysis of Zonally Averaged MSU Radiances (1979-1998)

Empirical Orthogonal Function (EOF) analysis was applied to 19 years of zonally averaged MSU Ch 2 and Ch4 radiances in order to investigate zonal patterns and trends in tropospheric and lower stratospheric temperatures. The first EOF is found to be associated with variability of tropical tropospheric temperatures due to El Nino/La Nina. The second EOF describes overall lower stratospheric temperatures and contains both a strong signal from the El Chicon and Pinatubo eruptions and a secular trend of approximately -0.4 degrees per decade. The third EOF is associated with lower stratospheric temperatures over Antarctica and contains considerable interannual variability but no trend. The fourth EOF describes a statistically significant upward trend in tropospheric temperature confined primarily to the northern midlatitudes. At 45N, the maximum magnitude of the trend is +0.3 degree per decade. This finding is contrary to common assertions that the two-decade MSU data record does not exhibit a signficant positive trend in tropospheric temperature. However, the fact that the observed positive trend is confined to a relatively narrow latitude belt is also contrary to the common expectation of widespread tropospheric warming in response to greenhouse gas emissions.