Investigation of the Long-Term Stability of the NAO SST Tripole

Abstract:

The North Atlantic Oscillation (NAO) is a measure of the sea level pressure difference between Iceland and the Azores and impacts the strength and pattern of winds, storms, and wave height from the Eastern US to Western Europe. Many reconstructions of the NAO have been completed using terrestrial proxies such as tree rings, though very few marine records have capitalized on the strong sea surface temperature (SST) correlations to the NAO. Correlations between winter SST and the NAO reveal a tripole pattern of positive correlation in the subtropical Atlantic sandwiched between areas of negative correlation in the subpolar and tropical Atlantic (Visbeck et al, 2001). Coral skeletons serve as an important sub-seasonal climatological archive over time, with coral Sr/Ca ratios inversely correlating to SST.

However, one lingering question is how temporally robust is the correlation between winter SST and the NAO? In this study, we investigate multiple SST records in locations sensitive to the NAO to investigate the stationarity of the SST tripole. Four monthly winter Sr/Ca records located along the correlation gradient from the positive lobe at Bermuda to the negative lobe near Tobago were identified from NOAA’s paleoclimatology database, including Bermuda (Goodkin et al, 2008), Dry Tortugas (DeLong et al, 2014), Little Cayman (von Reumont et al, 2018), and Guadeloupe (Hetzinger et al, 2008). Wavelet coherence analysis using a Morlet base function (Grinsted et al, 2004) was performed on the coral and instrumental NAO record (Hurrell, 1995) to investigate changes to correlation through time. Coral records were then band passed filtered to 3-5 years to investigate high frequency tripole behavior since 1899. Comparing the correlations through time shows that there is shifting of the null zone of the NAO SST tripole across the Caribbean which may imply shifting stationarity through time.