Séminaire
The deep tropics characterized by moist air and deep convection are separated from the dry, quiescent subtropics often by a sharp horizontal gradient of moisture only loosely tied to SST or other geographical constraints. Mapes et al. (GRL, 2018) showed that this margin of the moist tropics is a true PDF minimum (a regime separatrix), along a column water vapor (CWV) contour around 48 mm in instantaneous data. Quasi-meridional statistical composites of observations across the poleward-most excursion of this sinuous contour retain the sharpness of the margin while increasing signal to noise ratio. Observations primarily from a suite of the A-Train satellites show the meridional structure of thermodynamic state and budget terms across the margin of the moist tropics. Composites are computed around the PDF-minimum CWV value of 48 mm as well as a range of other thresholds from 35 mm to 60 mm for comparison.
Major findings are summarized as follows. (1) CWV increases equatorward from the subtropics for all CWV thresholds but eventually converges to ~48 mm deep into the tropical side. Precipitation abruptly intensifies on the tropical side of the margin but declines equatorward to ~3 mm/d regardless of the CWV thresholds. (2) The diabatic forcing to the atmosphere (radiative heating plus surface heat flux) changes its sign across the CWV=48 mm border, being positive on the tropical side and negative on the subtropics. This contrast is owing to the meridional gradient of radiative heating, principally the longwave effect of high clouds. (3) Vertical mode decomposition applied to vertical moisture advection implies that the second -mode moistening is sharply enhanced on the subtropical side of the margin, suggesting that an efficient “congestus moistening” process may be at work as inflowing lower-tropospheric air masses approach the margin. This second-mode moistening changes abruptly to weaker first-mode advective moistening (with a modest fraction of the drying due to the abrupt jump of precipitation) once the air mass enters the tropics. These observed features are interpreted in terms of a simple theory from the moisture and heat budget perspectives.
caroline.muller@lmd.ens.fr