Planetary-Scale Aspects of Outgoing Longwave Radiation and vorticity over the Global Tropics during Winter

Abstract

Fourier analysis was applied to outgoing longwave radiation (OLR) and 200 mb vorticity (VOR) during winter 1974?75, over the global tropics from 20°N to 20°S. Significant OLR and VOR zonal variance (33%) is contained in low wavenumbers (1?4) over the equatorial tropics. Empirical orthogonal function (EOF) analysis was performed on OLR and VOR Fourier coefficients, am, bm (m = 1?4), over the tropical domain. Spectral analyses for the eight largest eigenvectors exhibit marked peaks at periods of about 15?30 days. Only 15?30 day filtered, planetary-scale (1?4) OLR and VOR fields are examined. OLR standard deviations reveal extremely large values over Indonesia and the equatorial Pacific with a maximum at 5°S, 115°E. VOR standard deviations are minimum along the equator with maxima at 5?10° north and south. Correlations between OLR at 5°S, 115°E and global OLR reveal a geographically coherent pattern with OLR over Indonesia out of phase with OLR over the equatorial Indian Ocean and central equatorial Pacific. Correlations between OLR at 5°S, 115°E and global VOR show marked coherence, with VOR over equatorial regions exhibiting a wavenumber 1 distribution symmetric about the equator. Lag correlations (?10 to +10 days) between OLR at 5°S, 115°E and global OLR and VOR reveal systematic eastward movement over equatorial convective regions (70°E?160°W). EOF analysis of 15?30 day filtered a am, bm (m = 1?4), Fourier coefficients reveals major tropical modes of oscillation in OLR of zonal wavenumber 3 and zonal wavenumber 1 in VOR. Comparison of equatorial wavenumber 1 OLR (forcing) and wavenumber 1 VOR (response) shows close resemblance to Matsuno/s (1966) equatorial Kelvin wave model. A composite technique was applied to OLR and VOR to investigate the relationship between long-period changes of OLR at a reference point (5°S, 115°E) and those of OLR and VOR over the tropics. Composite maps constructed by considering only the first five eigenvectors indicate distinct eastward propagation (?4?5° per day) of OLR from 70°E to 160°W and southeast movement over the Bay of Bengal and Malaysia. VOR propagates eastward (?5?9° day?1) around the globe while traversing from 20°N to 20°S.