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

An unfiltered zonal Hovmöller depiction of rainfall in the Maritime Continent (MC) reveals remarkable spatiotemporal continuity of zonally propagating disturbances with 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 DYNAMO field campaign (Dynamics of the MJO) is examined here through a well-validated, convection-permitting model simulation conducted on a large domain, and further explored through idealized experiments. 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) 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. Idealized simulations in which individual islands are removed provide support for this hypothesis, and further demonstrate the profound impact of these islands and their diurnal convection in driving mean upward motion over the MC.