Origin of the seasonality of the Atlantic Ni&#241;o&#160;
Authors: Nnamchi Hyacinth , Latif Mojib , Keenlyside Noel , Kjellsson Joakim , Richter Ingo 
Published: March 2021

&lt;p&gt;&lt;span&gt;The Atlantic Ni&amp;#241;o is assumed to be largely governed by coupled atmosphere-ocean dynamics described by the Bjerknes-feedback, a positive feedback loop between adjustments in atmospheric and oceanic circulations. &lt;/span&gt;&lt;span&gt;Th&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt; postulat&lt;/span&gt;&lt;span&gt;ion is&lt;/span&gt;&lt;span&gt; that &lt;/span&gt;&lt;span&gt;initial &lt;/span&gt;&lt;span&gt;sea surface temperature&lt;/span&gt;&lt;span&gt; &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;SST&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt; anomal&lt;/span&gt;&lt;span&gt;ies&lt;/span&gt;&lt;span&gt; in the eastern equatorial Atlantic &lt;/span&gt;&lt;span&gt;can &lt;/span&gt;&lt;span&gt;modif&lt;/span&gt;&lt;span&gt;y&lt;/span&gt;&lt;span&gt; &lt;/span&gt;&lt;span&gt;the zonal SST gradient &lt;/span&gt;&lt;span&gt;and &lt;/span&gt;&lt;span&gt;alter&lt;/span&gt;&lt;span&gt; the vertical profile of atmospheric diabatic heating through changes in convection, water vapo&lt;/span&gt;&lt;span&gt;u&lt;/span&gt;&lt;span&gt;r, cloud cover and precipitation&amp;#8288; across the &lt;/span&gt;&lt;span&gt;basin&lt;/span&gt;&lt;span&gt;. The increase&lt;/span&gt;&lt;span&gt;d &lt;/span&gt;&lt;span&gt;diabatic heating &lt;/span&gt;&lt;span&gt;gradient &lt;/span&gt;&lt;span&gt;slows &lt;/span&gt;&lt;span&gt;down&lt;/span&gt;&lt;span&gt; the Walker Circulation &lt;/span&gt;&lt;span&gt;and activates &lt;/span&gt;&lt;span&gt;the oceanic &lt;/span&gt;&lt;span&gt;component of the Bjerknes feedback&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;However, t&lt;/span&gt;&lt;span&gt;he physics underlying the Atlantic Ni&amp;#241;o remain &lt;/span&gt;&lt;span&gt;under debate &lt;/span&gt;&lt;span&gt;but,&lt;/span&gt;&lt;span&gt; &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;role &lt;/span&gt;&lt;span&gt;of &lt;/span&gt;&lt;span&gt;diabatic heating which represents the &lt;/span&gt;&lt;span&gt;atmospheric &lt;/span&gt;&lt;span&gt;component of the Bjerknes feedback loop&lt;/span&gt;&lt;span&gt; &lt;/span&gt;&lt;span&gt;is often overlooked&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;In this study, we use &lt;/span&gt;&lt;span&gt;multiple&lt;/span&gt;&lt;span&gt; &lt;/span&gt;&lt;span&gt;observations &lt;/span&gt;&lt;span&gt;to&lt;/span&gt;&lt;span&gt; show that diabatic heating variability &lt;/span&gt;&lt;span&gt;that is linked to &lt;/span&gt;&lt;span&gt;the seasonal migration of the &lt;/span&gt;&lt;span&gt;i&lt;/span&gt;&lt;span&gt;nter-&lt;/span&gt;&lt;span&gt;t&lt;/span&gt;&lt;span&gt;ropical &lt;/span&gt;&lt;span&gt;c&lt;/span&gt;&lt;span&gt;onvergence &lt;/span&gt;&lt;span&gt;z&lt;/span&gt;&lt;span&gt;one controls the seasonality of the Atlantic Ni&amp;#241;o. &lt;/span&gt;&lt;span&gt;The strongest diabatic heating variability &lt;/span&gt;&lt;span&gt;in spring&lt;/span&gt;&lt;span&gt; leads &lt;/span&gt;&lt;span&gt;that in &lt;/span&gt;&lt;span&gt;the SS&lt;/span&gt;&lt;span&gt;T &lt;/span&gt;&lt;span&gt;in &lt;/span&gt;&lt;span&gt;summer&lt;/span&gt;&lt;span&gt;, whereas the atmospheric response to &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;SST variability is relatively weak. &lt;/span&gt;&lt;span&gt;Th&lt;/span&gt;&lt;span&gt;is &lt;/span&gt;&lt;span&gt;can be linked to n&lt;/span&gt;&lt;span&gt;et surface heat &lt;/span&gt;&lt;span&gt;flux &lt;/span&gt;&lt;span&gt;tendencies which&lt;/span&gt;&lt;span&gt; &lt;/span&gt;&lt;span&gt;drive the mixed-layer temperature anomalies in spring, &lt;/span&gt;&lt;span&gt;but &lt;/span&gt;&lt;span&gt;is the major damping term in June-&lt;/span&gt;&lt;span&gt;July &lt;/span&gt;&lt;span&gt;when the SST &lt;/span&gt;&lt;span&gt;variability &lt;/span&gt;&lt;span&gt;peak&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;although &lt;/span&gt;&lt;span&gt;observational uncertainty &lt;/span&gt;&lt;span&gt;is &lt;/span&gt;&lt;span&gt;quite &lt;/span&gt;&lt;span&gt;large&lt;/span&gt;&lt;span&gt;. Entrainment is the dominant heating term associated with the peak SST &lt;/span&gt;&lt;span&gt;variability&lt;/span&gt;&lt;span&gt; in June. &lt;/span&gt;&lt;span&gt;Our findings point to the existence of a strong meridional &lt;/span&gt;&lt;span&gt;variability &lt;/span&gt;&lt;span&gt;in the atmosphere&lt;/span&gt;&lt;span&gt;, which &lt;/span&gt;&lt;span&gt;by&lt;/span&gt;&lt;span&gt; &lt;/span&gt;&lt;span&gt;terminat&lt;/span&gt;&lt;span&gt;ing&lt;/span&gt;&lt;span&gt; &lt;/span&gt;&lt;span&gt;the Bjerknes feedback,&lt;/span&gt;&lt;span&gt; controls the seasonality of the &lt;/span&gt;&lt;span&gt;Atlantic Ni&amp;#241;o. &lt;/span&gt;&lt;/p&gt;