THE FEBRUARY MINIMUM IN HAWAIIAN RAINFALL AS A MANIFESTATION OF THE PRIMARY INDEX-CYCLE OF THE GENERAL CIRCULATION

Abstract

The climatic minimum in rainfall observed in February at most stations in the Hawaiian Islands is ascribed to the expansion of the circumpolar westerly vortex of the mid-troposphere incident to the annual primary index-cycle. The southward migration of the westerlies in February and early March results in weakening the trade winds, thereby reducing the trade component of Hawaiian rainfall. The migration also makes possible a longer stationary wavelength between the climatologically ?anchored? Asiatic coastal trough and the next down-stream trough, whose position with respect to the islands is critical in that it frequently brings kona storms in January but comparative freedom from such storms in February. The trade-wind regime is restored in strength during March when rainfall again increases. Numerous statistics are cited to establish the reality and time scale of the rainfall minimum and its relation to the index cycle.