Ventilation revisited: how dry continental air splits the ITCZ and stops the monsoon

Abstract

<p>The Tropical Rain belts with an Annual cycle and Continent Model Intercomparison Project (TRACMIP) ensemble includes slab-ocean aquaplanet controls and experiments with a highly idealized narrow equatorial continent. In the control simulation, the rain band moves between hemispheres over the annual cycle, so that the annual-mean state displays a broad ITCZ straddling the equator. The introduction of the continent causes an equatorial cold tongue to develop off the western coast and, correspondingly, dry anomalies and a split in the oceanic ITCZ.  The oceanic cooling is initiated by the advection of cold, dry air from the winter portion of the continent, but it persists and is amplified by positive feedbacks. In the long wave (LW) feedback, a colder SST leads to drier and colder air, reduced downwelling LW flux, and enhanced net surface LW cooling. On the equator, the wind, evaporation, and SST (WES feedback) also contributes to the establishment and maintenance of the cold tongue.  The annual mean signal over the ocean is dominated by the continental winter cooling and drying because warm, humid anomalies in the summer hemisphere are restricted to the continent by anomalous surface convergence.   Over land itself, aside for the timing of rainfall’s seasonal progression (i.e., the rainy season occurring close to the time of maximum insolation) the continental rain band remains in an ITCZ-like regime akin deep-tropical monsoons, with a smooth latitudinal transition, a poleward reach only slightly farther than the oceanic ITCZ's, and a constant width throughout the year. The confinement of the monsoon to the deep tropics, especially in the western portion of the continent, is the result of advection of dry, low moist static energy air–a process known as ventilation. Contrary to much previous literature, though, we find that ventilation is not achieved by the mean westerly jet aloft bringing colder oceanic air over the continent, but by the anomalous low-level meridional circulation, which brings dry air from the subtropical portion of the continent equatorward. Because the anomalous circulation is in turn a response to the convection anomalies, we conclude that the limiting mechanism for the monsoon is coupled and sensitive to the surface properties of the land. </p>