On the Inconsistent Relationship between Pacific and Atlantic Niños*

Abstract

The tropical Atlantic wind response to El Niñ o forcing is robust, with weakened northeast trade winds north of the equator and strengthened southeast trade winds along and south of the equator. However, the relationship between sea surface temperature (SST) anomalies in the eastern equatorial Pacific and Atlantic is inconsistent, with El Niñ o 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 Nucleus for European Modeling of the Ocean (NEMO) global model at 0.58 resolution (NEMO-ORCA05), this inconsistent SST relationship is shown to be at least partly attributable to a delayed negative feedback in the tropical Atlantic that 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 Niñ o 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 Niñ o. 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 because of a positive North Atlantic Oscillation (NAO) phase or an early termination of the Pacific El Niñ o event.

A Comparative Stability Analysis of Atlantic and Pacific Niño Modes*

“…As a result, although no SST restoring is used, SST is implicitly damped toward the prescribed surface air temperatures and thus ceases to be a fully prognostic variable. L€ ubbecke and McPhaden (2012) showed that the model simulation successfully captures the interannual variability in the eastern equatorial Atlantic and Pacific. Observed and simulated time series of SST and SSH anomalies are highly correlated and agree in amplitude.…”

On the relationship between Atlantic Niño variability and ocean dynamics

“…Keenlyside and Latif 2007;Burls et al 2011;Lübbecke and McPhaden 2012;Richter et al 2013). Obvious are the differences in timing characteristics: Both positive and negative ENSO events generally peak in boreal winter and last for several months and in some cases even longer than a year.…”

Pacific Influences on Tropical Atlantic Teleconnections to the Southern Hemisphere High Latitudes

“…El Niño events, for example, are known to mediate anomalous warming in the northern tropical Atlantic through a corresponding weakening of the trade winds (Enfield and Mayer 1997;Sutton et al 2000;Alexander et al 2002;Chang et al 2006). The connection with the southern equatorial Atlantic (i.e., the Atlantic Niño domain), however, is far less clear, with El Niño events sometimes followed by warm anomalies, and other times by cold (Huang 2004;Chang et al 2006;Lübbecke and McPhaden 2012). This inconsistency has been interpreted in terms of competing dynamical and thermodynamical processes (Chang et al 2006) and a delayed negative SST feedback (Lübbecke and McPhaden 2012).…”