Diurnal Forcing and Phase Locking of Gravity Waves in the Maritime Continent

Abstract

An unfiltered zonal Hovmöller depiction of rainfall in the Maritime Continent (MC) reveals remarkable spatiotemporal continuity of zonally propagating disturbances with a diurnal period, which endure over multiple days and propagate faster than the individual convective storms they coupled with. This phenomenon and its sensitivity to the Madden–Julian oscillation (MJO) during the 2011/12 Dynamics of the MJO (DYNAMO) field campaign is examined here through a well-validated, convection-permitting model simulation conducted on a large domain. We find that these disturbances are zonally propagating diurnal gravity waves excited by vigorous nocturnal mesoscale convective systems over Sumatra and Borneo. These gravity waves are diurnally phase locked: their wavelength very closely matches the distance between these two islands (~1500 km), while their particular zonal phase speed (~±17 m s−1) allows them to propagate this distance in one diurnal cycle. We therefore hypothesize that these waves are amplified by resonant interaction due to diurnal phase locking. While these zonal gravity waves decouple from convection once beyond the MC, their divergent flow signature endures well across the Indian Ocean, provoking the notion that they may influence rainfall at far remote locations. The exact controls over this zonal phase speed remain uncertain; we note, however, that it is roughly consistent with diurnal offshore-propagating modes documented previously. Further study is required to tie this down, and more generally, to understand the sensitivity of these modes to background flow strength and the geography of the MC.

Weak cooling of the troposphere by tropical islands in simulations of the radiative‐convective equilibrium

“…The forcing provided by the circulations facilitates frequent initiation of clouds which then convect into a (neutrally stable) atmosphere following a moist adiabatic lapse rate (Betts, 1982; Xu and Emanuel, 1989). Once mature and precipitating, the clouds further enhance rainfall over and next to the islands (Sobel et al ., 2011; Wang and Sobel, 2017; Ruppert and Chen, 2020); and sometimes they organize into Mesoscale Convective Systems (Houze et al ., 1981; Mori et al ., 2004) or excite offshore‐propagating gravity waves (Mapes and Houze, 1993; Love et al ., 2011; Coppin and Bellon, 2019; Ruppert and Zhang, 2019). Factors controlling the spatial distribution and amount of precipitation over islands, in particular interactions between convection and sea breezes, have been studied extensively (e.g., Yang and Slingo, 2001; Qian, 2008; Robinson et al ., 2008; Cronin et al ., 2015; Ulrich and Bellon, 2019).…”

Convective Cold Pool Associated with Offshore Propagation of Convection System over the East Coast of Southern Sumatra, Indonesia

“…Herein, as the CP is a key feature to organize deep convective clouds over midlatitude regions, the existence of CP over the lesser latitude, that is, the Maritime Continent, still had been questionable and not yet understood. However, limited studies have mentioned the CP as a responsible mechanism to develop propagating convective systems related to diurnal rainfall propagation speed over the Maritime Continent [18][19][20][21][22][23][24]. On the other hand, propagating convective systems over coastal regions is the main characteristic that may produce enhanced rainfall related to extreme events [24][25][26].…”

Island Rainfall Enhancement in the Maritime Continent

“…Land‐ and sea‐breeze circulations tied to the many islands of the MC diurnally trigger moist convection like clockwork, which nocturnally grows upscale into vigorous mesoscale convective systems (MCSs; Houze et al, ; Johnson & Priegnitz, ; Carbone et al, ; Mori et al, ; Ichikawa & Yasunari, ; Wu et al, ). While these MCSs endure for many hours, the diurnal gravity waves they excite can endure for days, coupling with and triggering deep convection over large distances (Ruppert & Zhang, , RZ19 henceforth; Ruppert et al, , RCZ20 henceforth).…”