The tropical Atlantic meridional SST gradient index and its relationships with the SOI, NAO and Southern Ocean

Abstract

The tropical Atlantic meridional SST gradient (TAMG) is a mode of climatic variability known to be largely associated with abnormal rainfall regimes in South America and West Africa. A TAMG index is defined by the difference between area indices of sea surface temperature anomaly north (TN index) and south (TS index) of the meteorological equator (∼5°N). We investigate, for the 1964–1998 period, the decadal variability of the TAMG index and the relationship of its TN and TS components with the Southern Oscillation Index (SOI), the North Atlantic Oscillation (NAO), sea level pressure (SLP) signals in South Atlantic islands, the global NCEP-NCAR SLP field, and sea-ice extent data in the Southern Ocean. We explore the relationship between the SOI and the TN and TS signals, after estimating their fluctuating lags with the cross-wavelet transform technique. The SOI always leads TN with an average time lead of 5.4 months. On the contrary, TS leads the SOI by ∼4 months after 1984. When exploring the origin of such a precursory event of the SOI, observed in TS, it is found that the highest correlation between TS, the global SLP field, and the sea-ice extent around Antarctica is located in the Ross sea, where the ice signal leads the SOI. High correlation between TS and the SLP field is also observed in the western tropical Pacific. The continuous wavelet transform is then used to extract the low-frequency components of the TN and TS signals. These components are strong well-defined oscillations with mean periods at 9.6 years for TN and 14 years for TS. Given this asynchronous non-dipole nature of the TN and TS decadal components, we can anticipate that the TAMG undergoes decadal oscillations that are variable on a longer multi-decadal timescale. These components are also very similar to the low-frequency components of the NAO index, for TN (with the NAO leading TN by 14 months) and SLP anomaly signals at St. Helena and Tristan da Cunha islands, for TS. These results indicate that the TN or TS decadal component is each related to the larger climatic milieu in the same hemisphere, but that the climatic milieu of the South Atlantic is distinct from that of the North Atlantic.