How do meridional modes structure and growth depend on mean state asymmetry?
We calculate optimal initial structures (named optimals) that experience the greatest transient growth of SSTA over a finite time for different WES parameter and ITCZ mean states. We focused on two opposite cases, ITCZ symmetric and asymmetric with respect to the equator. When the ITCZ was symmetric and thin, a symmetric and zonally moving optimal structure evolves into a Gill-like atmospheric structure with an associated SST response. A positive (negative) geopotential anomaly is phased westward of a positive (negative) SST anomaly implying a westward propagation of the structure due to the WES feedback. When the ITCZ is displaced into one hemisphere (the asymmetric case), the initial structure is more prominent in the hemisphere in which the ITCZ lies. The structure that experiences maximum growth evolves westward and equatorward, again due to the WES feedback sustaining the motion.
We address three observed features of meridional mode variations: (1) the interhemispheric anti-coherence between meridional mode SST variations; (2) preferential forcing of meridional mode variations from one hemisphere; and (3) a maximum in meridional mode variance that occurs in boreal Spring, when the ITCZ is most equatorially symmetric. We find that (1) anticoherence results from stronger coupling in the hemisphere in which the ITCZ lies; (2) initial conditions originating in the Northern Hemisphere produce greater meridional mode growth because the mean ITCZ is generally located in the Northern Hemisphere; and (3) maximum growth is not achieved with a symmetric ITCZ, but rather with an ITCZ that is centered in the northern hemisphere. Results suggest that mean state asymmetry is important for meridional mode structure, but that seasonality and hemispheric preference of stochastic forcing is critical for meridional mode variance and seasonality.