On the secular variation of storms in the tropical North Atlantic and Eastern Pacific

Variations in the annual frequency of tropical cyclones in the North Atlantic and of tropical cyclones and temporales in the Eastern North pacific are studied in relation to sea surface temperature (SST), sea level pressure (SLP), and vertical wind shear, the observation period common to most parameters being 1911–72.

Both oceans exhibit a secular SST increase until the 1950’s. Tropical cyclone frequency has a strong negative correlation with SLP in the realm of the subtropical highs, but only a weak negative one with SST in the areas of common storm occurrence. Temporales are positively correlated with SLP but negatively with SST and tropical storms in the Pacific. A minimum threshold of area-averaged SST and a maximum threshold of vertical wind shear can be specified for one or more tropical cyclones to occur.

Principal component analysis was performed on indicative time series from both oceans. As prominent constituents of the first principal component, Pacific SLP, temporales, and Atlantic tropical cyclones have factor loadings of one sign, while Pacific tropical cyclones have the opposite sign. The second principal component is made up primarily of parallel SST variations in both oceans.

Spectral analysis reveals a coupling of SLP variations over the two oceans around 33–34 and 5.5 years. In the Atlantic, SLP minimum precedes the SST maximum at about 12.5 years. Maxima of Pacific SLP and temporales and minima of Ecuador/Peru SST and Atlantic tropical cyclone occurrence broadly coincide at a common frequency of about 8 years. At the 13.6–14.8 year time scale in the Pacific, minima of Ecuador/Peru SST, and minima of tropical cyclone frequency and maxima of SLP in the North Pacific are approximately synchronous. Spectral analysis details the time scale of spatial linkages borne out by linear correlation and principal component analysis.