Upper-Level Eddy Angular Momentum Fluxes and Tropical Cyclone Intensity Change

Abstract

The eddy flux convergence of relative angular momentum (EFC) at 200 mb was calculated for the named tropical cyclones during the 1989?1991 Atlantic hurricane seasons (371 synoptic times). A period of enhanced EFC within 1500 km of the storm center occurred about every 5 days due to the interaction with upper-level troughs in the midlatitude westerlies or upper-level, cold lows in low latitudes. Twenty-six of the 32 storms had at least one period of enhanced EFC. In about one-third of the cases, the storm intensified just after the period of enhanced EFC. In most of the cases in which the storm did not intensify the vertical shear increased, the storm moved over cold water, or the storm became extratropical just after the period of enhanced EFC. A statistically significant relationship (at the 95% level) was found between the EFC within 600 km of the storm center and the intensity change during the next 48 h. However, this relationship could only be determined using a multiple regression technique that also accounted for the effects of vertical shear and sea surface temperature variations. The EFC was also examined for the ten storms from the 1989?1991 sample that had the largest intensification rates. Six of the ten periods of rapid intensification were associated with enhanced EFC. In the remaining four cases the storms were intensifying rapidly in a low shear environment without any obvious interaction with upper-level troughs.