How do meridional modes structure and growth depend on mean state asymmetry?

The focus of this study is to investigate how mean state symmetry or asymmetry affects transient growth of meridional mode-like disturbances in the equatorial region. We conduct a series of analyses using a Gill-Matsuno model coupled with a “slab” mixed layer ocean model. The model includes zonally uniform coupling parameters that are determined by the latitudinal features of the mean state, in particular the central location and width of the mean ITCZ [a latent heating parameter that affects the atmospheric response to sea surface temperature (SST) anomalies] and the latitudinal distribution of the mean zonal wind [the wind-evaporation-SST (WES) feedback parameter]. The total air-sea coupling strength depends on the structure and amplitude of the latent heating and WES feedback parameters. If there is no overlap, no growth is possible and the structures decay at least at the linear ocean damping rate (120d^-1). If the parameters overlap and have large amplitude, linear instability may arise.

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.