Reinstatement of land in tropical meteorology: Dynamical similarity between sea-land breeze and monsoon

A quasi-two-dimensional (zonally uniform but permitted to move) linear convection equation on the equatorial β-plane for a periodically-oscillating equatorially-anti-symmetric heating such as the case of a coastline along the equator (between the northern and southern hemispheres covered totally by land and sea, respectively) is analytically solved. For a periodicity shorter than the local Coriolis period (e.g., diurnal cycle near the equator) the solution becomes a sea-land breeze circulation (purely meridional in this case) consisting of a pair of internal (almost non-inertial) gravity waves, and the motion becomes non-hydrostatic and ageostrophic. For a periodicity longer than the earth's rotational period (e.g., annual cycle in the extratropics) the solution becomes a monsoon circulation consisting of mixed Rossby-gravity and Rossby waves with zero zonal wavenumber, and the motion is quasi-hydrostatic and quasi-geostrophic. In the latter vertical velocity is associated mainly with inertia-gravity waves, as so far shown by Kosaka and Matsuda (2005) for a steady heating.

Above-mentioned theoretical considerations suggest strongly the importance of diurnal cycle in particular along coastlines in the equatorial region. The equatorial rainfall 2,000 mm/year maintaining the earth's climate is concentrated around coastlines, and that the largest regional rainfall and the convection center pumping atmosphere from the surface to the stratosphere appear over the maritime continent with the longest coastlines on the earth. Therefore the temporal and horizontal resolutions of observations and numerical models must be higher than the scales of the diurnally-reversing sea-land breeze circulations. Such high-resolution observations by satellites (e.g., Mori et al., 2004; Sakurai et al., 2005) and a radar-profiler network called HARIMAU (Hydrometeorological Array for Interaseasonal Variation-Monsoon Automonitoring) (e.g., Yamanaka et al., 2008; Wu et al., 2007; Sakurai et al., 2009) have been promoted over the maritime continent.

Similarly the monsoon circulation across the equator between the both hemispheres is concentrated in the Eurasian Continent-Indian Ocean sector including the maritime continent as the eastern boundary, which looks similar to the simplified case considered theoretically here. Because the actual coastline is in the northern hemisphere apart from the equator, there are also intraseasonal variations resulting from the “aqua-planet”-like situation (Hayashi and Sumi, 1986; Miura et al., 2005). Those variations between diurnal and annual periodicities with nonzero zonal wavenumbers may be interacted with the diurnal and annual cycles in the maritime continent and other coastal regions of the Indian Ocean, which can be studied by wave dynamics as mentioned here. An overview of observational results is shown from this viewpoint.