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.
