Wednesday, 30 April 2008: 10:15 AM
Palms E (Wyndham Orlando Resort)
The influence of the leading intraseasonal mode of atmospheric and oceanic variability, the Madden-Julian Oscillation (MJO), is known to extend to tropical and extratropical temperature, moisture, sea level pressure, divergent and rotational wind components, and deep convection. The MJO is readily detectable in space-time plots of outgoing longwave radiation (OLR). As a 40- to 50-day oscillation, the MJO is connected to tropical phenomena, including the Southeast Asian, Australian, and North American monsoons and tropical cyclone (TC) activity in various TC basins. In this study, the link between the MJO and TC activity, defined by counts of genesis and landfall of tropical storms, hurricanes, and intense hurricanes, well as by the integrative accumulated cyclone energy (ACE) index, was examined for the North Atlantic, Northeast Pacific, and subbasins. An operational MJO diagnostic derived from the 200 hPa velocity potential was divided into three phases. Strong 200 hPa divergence (convergence) was labeled enhanced (suppressed) and named for its impact on deep convection. Observed and expected levels of TC activity were compared using a binomial distribution hypothesis test. Statistically significant relationships between the MJO and the frequency of TC genesis, intensification, and landfall over nine basins were identified. For example, when strong 200 hPa divergence occurs at 120°W, TC genesis is favored in the Northeast Pacific and not favored in the North Atlantic or Caribbean Sea. Furthermore, when strong 200 hPa convergence is present at 120°W, five times more TCs make landfall in the North Atlantic. Changes in vertical wind shear and column divergence, diagnosed by the upper-tropospheric velocity potential, are proposed as physical explanations for the modulation. A detailed wavelet analysis carried out on of pentad values of accumulated cyclone energy (ACE) confirms the influence of the MJO on TC activity. Peaks in the power spectra of ACE were readily identifiable in this study on the timescale of the MJO.
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