El Nino-induced suppression of Atlantic ITCZ contributes to the spring warming of the tropical North Atlantic

It is a well-known phenomenon that the Tropical North Atlantic (TNA) warms in boreal spring and early summer (April – June) following El Nino peaks in boreal winter (Enfield and Mayer, 1997). This involves formation of the so-called extratropical atmospheric stationary Rossby wave trains from the tropical Pacific (e.g., Lee et al., 2008). A new paper published online in Climate Dynamics (Jiang and Li, 2019) further explored the atmosphere-ocean processes related to the El Nino-Southern Oscillation (ENSO) – TNA relationship. The highlight of the new findings is that the El Nino-induced suppression of the deep tropical convection in the Atlantic also plays a significant role in the spring warming of the TNA following the El Nino peak. More specifically,  warm sea surface temperature (SST) anomalies in the tropical Pacific induce a global average warming of the tropical troposphere, via a fast tropical teleconnection mechanism (i.e., Kelvin waves), and thus increase the  static stability and suppress deep convection over the tropical North Atlantic (e.g., Chiang and Sobel, 2002). The atmospheric Gill-response to the suppressed Atlantic convection is to reduce the trade winds and thus warms the TNA. Based on the partial regression analysis and idealized numerical model experiments, the study concluded that the ENSO-induced tropical Atlantic forcing may contribute up to 42% of the the ENSO-induced southwesterly anomaly over TNA region, while 58–68% is attributed to the extratropical ENSO teleconnection. It should be noted that the El Nino-induced suppression of Atlantic ITCZ does not depends on the atmospheric basic states and thus should occur in all seasons. However, the correlation between El Nino and TNA SST anomalies is very weak in boreal summer and fall during the development phase of El Nino (Wang et al., 2008). So, it is still unclear why the atmospheric Gill-response to the suppressed Atlantic convection does not  reduce the Atlantic trade winds during the developing phase of El Nino in boreal summer and fall.

Jiang, L. and T. Li, 2019: Relative roles of El Niño‑induced extratropical and tropical forcing in generating Tropical North Atlantic (TNA) SST anomaly. https://doi.org/10.1007/s00382-019-04748-7.  https://link.springer.com/article/10.1007/s00382-019-04748-7

 

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