1451 Subseasonal Predictability of Tropical Cyclones over the North Atlantic: Impacts of Rossby Wave Breaking

Wednesday, 25 January 2017
Weiwei Li, University of Illinois, Urbana, IL; and Z. Wang and G. Zhang

Rossby wave breaking (RWB) facilitates the tropical-extratropical interaction and occurs frequently during the warm season. This study investigates how extratropical RWB impacts the atmospheric conditions over the North Atlantic on the subseasonal timescale and the implications for subseasonal predictability, with a special focus on tropical cyclones (TCs). We found that strong vertical shear and midlevel dryness persist more than 5 days over the tropical Atlantic following anticyclonic RWB occurrences over the western North Atlantic. This contributes to a negative correlation between the subseasonal variability of TC activity and RWB occurrence.

The RWB-related subseasonal predictability was examined in the NCEP Global Ensemble Forecasting System (GEFS) Reforecast Version-2. The forecasts initialized before the onsets of RWB exhibit lower prediction skill of TC genesis than those initialized after the onsets of RWB. Ensemble mean error and spread calculations reveal that lower predictability can be explained by larger forecast errors and uncertainties in vertical shear, midlevel humidity and low-level vorticity. We also find that the prediction skill of TC subseasonal variation tends to be lower in years of active RWB. It is suggested that improving RWB prediction may help reduce model forecast errors and enhance TC prediction skill at the subseasonal timescale.

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