The Role of Cross-Equatorial Tropical Cyclone Pairs in the Southern Oscillation

The locations and frequencies of cross-equatorial tropical cyclone pairs over the Pacific Ocean are compared between years of contrasting equatorial Pacific sea surface temperature (SST) and atmospheric circulation anomalies, i.e., high versus low Southern Oscillation (SO) indices, during the period 1971–79. Weak southeast trade winds associated with low SO indices allow warm (29°C) surface water in the equatorial (0–5°S) zone to extend eastward across the dateline, encouraging formation of cyclone pairs near the dateline. The cyclone pairs, in turn, provide the linkage between warm water and equatorial convection (as indicated by Canton Island rainfall) during southern summer. In general, the occurrences of cyclone pairs vary in longitude with the eastward excursions of warm equatorial water, furthermore, cyclone pairs appear to provide a positive feedback by advancing the eastward warm water extent. In 1974–75, when no cyclone pairs developed near the dateline in spite of sufficiently high SST's and low SO indices, an apparently developing low SO index anomaly collapsed. The possibility is therefore raised that the occurrence (or absence) of a single synoptic event may influence the outcome of an entire Southern Oscillation anomaly.