The Role of Low-Level Moisture Convergence and Ocean Latent Heat Fluxes in the Madden and Julian Oscillation: An Observational Analysis Using ISCCP Data and ECMWF Analyses

Abstract

This paper examines whether or not low-level moisture convergence and surface latent heat flux act as forcing mechanisms of the Madden and Julian oscillation (MJO), as it is proposed by the theories of wave-CISK (conditional instability of the second kind) and evaporation-wind feedback. The mean brightness temperature of cloudy pixels at 11 ?m, obtained from five years of International Satellite Cloud Climatology Project data, is used as a proxy for tropical convective activity. Five years of European Centre for Medium-Range Weather Forecasts analyses are used to estimate surface latent heat fluxes and moisture divergence integrated in the low levels of the troposphere. Spectral analysis of latent heat fluxes over the Indian and Pacific Oceans shows significant spectral peaks in the frequency band of the MJO. These peaks are due mainly to the oscillation in the surface wind speed rather than in the specific humidity difference. Principal component analysis and tagged correlation patterns of filtered time series 20?70 days are used to investigate the relationships between anomalies in convection, surface latent heat fluxes, and low-level moisture divergence. The correlation patterns show that negative anomalies of latent heat fluxes are systematically observed to the east, whereas positive anomalies are observed to the west of the region of convection. Positive anomalies of surface latent heat flux tag time variations in convection by about 4 days. This result contrasts with the basic requirement of the evaporation-wind feedback theory, which claims that evaporation anomalies are positive on the eastern side of the convective region. In contrast, tag correlation patterns indicate that the region of maximum low-level moisture convergence is located to the east of the region of convection, and low-level moisture convergence leads time variations in convective activity by about 2 days. This observational result supports the frictional wave-CISK theory as a mechanism of the MJO.