Summer and winter Atlantic Nino: connections with ENSO and implications

The teleconnection between the Atlantic Nino and the Pacific El Nino Southern Oscillation (ENSO) is revisited using observational and reanalysis data for the 1905-2014 period. Two types of Atlantic Nino are significantly negatively correlated with ENSO, with Atlantic leading ENSO by 6-month to 1-year. The first one is the already well-known connection between the boreal summer Atlantic Nino (ATL3: 3 degrees N-3 degrees S, 20 degrees W-0 degrees) and the subsequent winter ENSO (Nino3: 5 degrees N-5 degrees S, 150 degrees W-90 degrees W). This relationship is strong in the first and last decades of the study period. It is shown that a second Atlantic Nino in boreal fall/early winter (October-December, hereinafter called winter Atlantic Nino) is also significantly correlated with the following year ENSO. This winter Atlantic Nino leads to an early development of ENSO from boreal summer onwards, with a marked multidecadal modulation of the lead time. A nearly 1-year leading connection between winter Atlantic Nino and the following ENSO is generally observed in the mid-twentieth century, mostly when the summer Atlantic Nino teleconnection with the subsequent winter ENSO is weak. The same mechanism of the Atlantic-Pacific Nino connection, which involves the Walker circulation, operates for the two types of Atlantic Nino. Our analysis supports the leading influence of the summer and winter Atlantic equatorial modes on climate variability in South America. These results suggest the relevance of different types of Atlantic Nino for the 6-month to 1-year predictability of ENSO and its climatic impacts.