J. Atmos. Sci., submitted 2/09.

Diagnosis of the MJO modulation of tropical cyclogenesis using an empirical index

Suzana J. Camargo
IRI, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY

Matthew C. Wheeler
Centre for Australian Weather and Climate Research, Melbourne, Australia.

Adam H. Sobel
Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences, Columbia University, New York, NY.


The modulation of tropical cyclone activity by the Madden-Julian Oscillation (MJO) is explored using an empirical genesis potential (GP) index. Composite anomalies of the genesis index associated with the different MJO phases are consistent with the composite anomalies in TC genesis frequency which occur in the same phases, indicating that the index captures the changes in the environment which are at least in part responsible for the genesis frequency changes. Of the four environmental variables which enter the genesis potential index, the midlevel relative humidity makes the largest contribution to the MJO composite GP anomalies. The second largest contribution comes from the low-level absolute vorticity, and only very minor contributions come from the vertical wind shear and potential intensity.

When basin-integrated MJO composite anomalies of the GP index are regressed against basin-integrated composite anomalies of TC genesis frequency, the results differ quantitatively from those obtained from the analogous calculation performed on the annual climatologies in the two quantities. Thus the index captures the MJO variations in genesis frequency to a lesser degree than the climatological annual cycle of genesis (to which it was originally tuned). This may be in part indicative of the importance of precursor disturbances, not well captured in the GP index computed from weekly data, to the intraseasonal TC genesis frequency fluctuations.