PMEL Publications Abstract

FY 2012

On the inconsistent relationship between Pacific and Atlantic Niños

Lübbecke, J., and M.J. McPhaden

J. Climate, 25(12), doi: 10.1175/JCLI-D-11-00553.1, 4294–4303 (2012)

The tropical Atlantic wind response to El Niño forcing is robust, with weakened northeast trade winds north of the equator and strengthened southeast trade winds along the equator. However, the relationship between sea surface temperature (SST) anomalies in the eastern equatorial Pacific and Atlantic is inconsistent, with El Niño events followed sometimes by warm and other times by cold boreal summer anomalies in the Atlantic cold tongue region. Using observational data and a hindcast simulation of the global NEMO-ORCA05 model, this inconsistent SST relationship is shown to be at least partly attributable to a delayed negative feedback in the tropical Atlantic which is active in years with a warm or neutral response in the eastern equatorial Atlantic. In these years, the boreal spring warming in the northern Tropical Atlantic that is a typical response to El Niño is pronounced, setting up a strong meridional SST gradient. This leads to a negative wind stress curl anomaly to the north of the equator that generates downwelling Rossby waves. When these waves reach the western boundary they are reflected into downwelling equatorial Kelvin waves that reach the cold tongue region in late boreal summer to counteract the initial cooling that is due to the boreal winter wind stress response to El Niño. In contrast, this initial cooling persists or is amplified in years in which the boreal spring northern Tropical Atlantic warming is weak or absent either due to a positive NAO phase or an early termination of the Pacific El Niño event.

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