The Enhancement of the Impact of the Wintertime North Atlantic Oscillation on the Subsequent Sea Surface Temperature over the Tropical Atlantic since the Middle 1990s

Abstract

The impact of the wintertime North Atlantic Oscillation (NAO) on the subsequent sea surface temperature (SST) anomalies over the tropical Atlantic has experienced obvious interdecadal changes during 1950–2015. During 1995–2015, the negative (positive) phase of the wintertime NAO favors positive (negative) SST anomalies over the tropical Atlantic in the subsequent spring–summer, whereas the NAO–SST connection is insignificant during 1970–94 and is confined to the northern tropical Atlantic (NTA) during 1950–69. Compared to 1970–94, the much stronger influence on the NTA SST during 1995–2015 and 1950–69 is associated with a southward shift of the southern boundary of the NAO. During 1995–2015, the inverted NAO-related warming of the tropical Atlantic consists of three stages: 1) the pronounced increase in SST over the subtropical North Atlantic (SNA) and the tropical South Atlantic (TSA) during December–January, 2) the pronounced increase in SST over the northwestern tropical Atlantic (NWTA) during February–April, and 3) the persistent warming over the tropical Atlantic during May–August. The increases in SST over the SNA and the TSA are attributed to significant positive latent heat flux anomalies via the wind–evaporation effect, which are connected by the suppressed regional Hadley circulation. Afterward, the associated anomalous downward motion over the NWTA persists into February–April, which induces more incoming shortwave radiation and results in a significant increase in the local SST via the cloud–radiation effect. In contrast, during 1950–69, due to the decreased interannual variability of the vertical motion over the NWTA, the anomalous downward branch aloft and the low-level cross-equatorial northwesterly winds associated with the inverted NAO are not evident, and thus the regions with an increase in SST are confined to the Northern Hemisphere.