Response of Monsoon Rainfall to Changes in the Latitude of the Equatorward Coastline of a Zonally Symmetric Continent

Abstract

Recent studies have shown that the rapid onset of the monsoon can be interpreted as a switch in the tropical circulation, which can occur even in the absence of land-sea contrast, from a dynamical regime controlled by eddy momentum fluxes to a monsoon regime more directly controlled by energetic constraints. Here we investigate how one aspect of continental geometry, that is the position of the equatorward coastal boundary, influences such transitions. Experiments are conducted with an aquaplanet model with a slab ocean, in which different zonally symmetric continents are prescribed in the Northern Hemisphere poleward from southern boundaries at various latitudes, with “land” having a mixed layer depth two orders of magnitude smaller than ocean. For continents extending to tropical latitudes, the simulated monsoon features a rapid migration of the convergence zone over the continent, similar to what is seen in observed monsoons. For continents with more poleward southern boundaries, the main precipitation zone remains over the ocean, moving gradually into the summer hemisphere. We show that the absence of land at tropical latitudes prevents the rapid displacement into the subtropics of the maximum in lower-level moist static energy and, with it, the establishment of an overturning circulation with a subtropical convergence zone that can transition rapidly into an angular momentum conserving monsoon regime.

Monsoon Seasonality Advanced by Opposite‐Hemisphere Continental Drift

“…Interestingly, the critical latitude here is similar to the critical latitude to form a land monsoon (Dirmeyer, P. A. 1998; Hui & Bordoni, 2021). Hui and Bordoni (2021) found that only when the southern coastline of an NH continent extended into the deep tropics (i.e., 20°N), the NH monsoon transformed from an oceanic monsoon into a land monsoon.…”

“…1998; Hui & Bordoni, 2021). Hui and Bordoni (2021) found that only when the southern coastline of an NH continent extended into the deep tropics (i.e., 20°N), the NH monsoon transformed from an oceanic monsoon into a land monsoon.…”

“…Our experiments consist of a series of aquaplanet simulations with varying continental configurations (Table S1 and Figure S1 in Supporting Information S1). Similar to some previous studies (e.g., Hui & Bordoni, 2021;Voigt et al, 2016), here, the land is represented by a thin layer of ocean (0.1 m) with modified parameters. The land albedo is increased by 0.1, and the land evaporation is reduced by half compared to the values of the model's ocean (following Voigt et al, 2016), to represent the increased reflectivity and limitation of moisture over land surface, respectively.…”

“…For example, placing a landmass in the tropics and subtropics leads to regional circulations that strengthen the monsoon intensity, extend its polarward boundary, and accelerate monsoon onset (e.g., Chou et al., 2001; Chou & Neelin, 2001; Hui & Bordoni, 2021; Zhou & Xie, 2018). Recently, a relevant study has noted that an opposite‐hemisphere continent may also help the ITCZ extend further poleward (Hui & Bordoni, 2021). However, to the authors' knowledge, no research has emphasized the effects of an opposite‐hemisphere continent on the monsoon onset timing.…”

Mesoscale Convective Systems Dominate the Energetics of the South Asian Summer Monsoon Onset

“…essential for the onset, removing land in the tropical latitudes prevents the rapid transition into the angular-momentum-conserving monsoon regime (Hui & Bordoni, 2021).…”