Lag Relationships Involving Tropical Sea Surface Temperatures

Abstract

A long historical record (?100 years) of monthly sea surface temperature anomalies from the Comprehensive Ocean?Atmosphere Data Set was used to examined the lag relationships between different locations in the global Tropics. Application of complex principal component (CPC) analysis revealed that the leading mode captures ENSO-related quasi-cyclical warming and cooling in the tropical Pacific Ocean. The dominant features of this mode indicate that SST anomalies in the eastern Pacific lead those of the central Pacific. However, a somewhat weaker aspect of this mode also indicates that SST anomalies in the tropical Indian and western tropical North Atlantic Oceans vary roughly in concert with each other but lag behind those in the central and eastern Pacific. The stability of these lag relationships is indicated by the fact that the leading mode is quite similar in three different 30-year time periods. In order to further examine these relationships some simple indexes were formed as the average over several grid points in each of the four key areas suggested by the CPC analyses. Several different types of analyses including lag correlation, checking the correspondence between extrema, and visual examination of time series plots were used to confirm the relationships implied by the CPC spatial patterns. By aggregating the lag correlations over the three 30-year time periods and performing a Monte Carlo simulation the relationships were found to be statistically significant at the 1% level. Reasonable agreement in the pattern of lag correlations was found using a different SST dataset. Without aggregation of the lag correlations (i.e., considering each 30-year period separately) the area in the Pacific and Indian were consistently well related, but those involving the North Atlantic were more variable. The weaker correlations involving the Atlantic Ocean underscore the more tenuous nature of this remote relationship. While major ENSO-related swings in tropical Pacific SST are often followed by like variations in a portion of the Atlantic, there are times when there is either no obvious association or one of opposite sign. It may be that while ENSO variability tends to have an impact in the Atlantic, more localized factors can override this tendency. This may explain some of the contradictory statements found in the literature regarding such remote associations. In comparing the findings of this project with some studies that utilize very recent data (since about 1982) some discrepancies were noted. In particular, some studies have reported evidence of 1) an inverse relationship between SST anomalies in the tropical Pacific and those in the eastern tropical South Atlantic and 2) the appearance of ENSO-related SST anomalies in the central tropical Pacific prior to those in the eastern tropical Pacific. From a historical perspective both of these characteristics are unusual. Thus, the recent time period may merit special attention. However, it is important to stress that caution should be exercised in generalizing findings based only on this recent time period.