The Equatorial Signatures of the Meridional Modes: Dependence on the Mean Climate State

Extratropical climate variability can impact tropical climate in the Pacific sector via the Pacific Meridional Modes (PMMs). The South PMM (SPMM) has a larger equatorial signature than the North PMM (NPMM) for the same amount of extratropical variability. Here we explain this interhemispheric asymmetry using an atmospheric general circulation model coupled to a slab ocean model. By imposing an anomalous interhemispheric heating gradient, we strengthen the northeasterly trades and weaken the southeasterly trades, shifting the Intertropical Convergence Zone (ITCZ) south of the equator. As a result, the SPMM no longer influences the equatorial region while the NPMM shows strengthened linkages to the central-western equatorial Pacific. By demonstrating that background winds determine the propagation of the wind-evaporation-sea surface temperature feedback fundamental for the PMMs, we conclude that the interhemispheric asymmetry between the PMMs is largely attributed to the asymmetric mean trades in the Pacific.

Using an idealized aquaplanet model, the concept of the Meridional Mode as an extratropics-to-tropics linkage is further generalized to the global subtropical ocean-atmosphere coupled system. As in the tropical Pacific, the equatorial signature of the Meridional Mode in the aquaplanet model is constrained by the distribution of tropical trade winds: the Meridional Mode can impact the equator only if the underlying mean trade winds extend onto or across the equator into the other hemisphere. This result implies that, in the modern climate with the ITCZ located in the northern hemisphere in most of the tropical oceans, the southern hemisphere extratropical climate variability may have a larger equatorial impact than its northern counterpart via the Meridional Mode.