Person:

Boos, William

A mechanism is presented, based on multiscale interactions via nonlinear wind-induced surface heat exchange (WISHE), that produces eastward-propagating, intraseasonal convective anomalies in the tropical atmosphere. Simulations of convectively coupled disturbances are presented in two intermediate-complexity atmospheric models. One is a shallow water model with a simple WISHE-motivated heating term. The other model is also based on a first baroclinic mode but has an additional prognostic equation for humidity and a simple representation of moist convection based on a quasi-equilibrium approximation. In spite of many differences between the models, they robustly produce a coherent signal in westerly winds and convection that travels eastward at (4–10 m s^{−1}). It is shown here that this slow signal is a forced response to an eastward-propagating Yanai (mixed Rossby–gravity) wave group. The response takes the form of a forced Kelvin wave that is driven nonlinearly, via WISHE, by meridional wind anomalies of the Yanai wave group and that travels considerably more slowly than the free convectively coupled Kelvin waves in these models. The Yanai waves are destabilized in the models used here by WISHE in the presence of mean easterlies, but more generally they could also be excited by stratiform instability in the absence of mean easterlies so that the mechanism described here could also operate without mean easterlies. Similarities to and differences from the Madden–Julian oscillation (MJO) and convectively coupled tropical waves are discussed.

Elevated heating by the Tibetan Plateau was long thought to drive the South Asian summer monsoon, but recent work showed this monsoon was largely unaffected by removal of the plateau in a climate model, provided the narrow orography of adjacent mountain ranges was preserved. There is debate about whether those mountain ranges generate a strong monsoon by insulating the thermal maximum from cold and dry extratropical air or by providing a source of elevated heating. Here we show that the strength of the monsoon in a climate model is more sensitive to changes in surface heat fluxes from non-elevated parts of India than it is to changes in heat fluxes from adjacent elevated terrain. This result is consistent with the hypothesis that orography creates a strong monsoon by serving as a thermal insulator, and suggests that monsoons respond most strongly to heat sources coincident with the thermal maximum.

A high-resolution (40 km horizontal) global model is used to examine controls on the South Asian summer monsoon by orography and surface heat fluxes. In a series of integrations with altered topography and reduced surface heat fluxes, monsoon strength, as indicated by a vertical wind shear index, is highly correlated with the amplitude of the maximum boundary layer equivalent potential temperature (θeb) over South Asia. Removal of the Tibetan Plateau while preserving the Himalayas and adjacent mountain ranges has little effect on monsoon strength, and monsoon strength decreases approximately linearly as the height of the Himalayas is reduced. In terms of surface heat flux changes, monsoon strength is most sensitive to those in the location of the θeb maximum just south of the Himalayas. These results are consistent with the recent idea that topography creates a strong monsoon by insulating the thermal maximum from dry extratropical air. However, monsoon strength is found to be more sensitive to variations in the θeb maximum when topography is altered than when surface heat fluxes are reduced, and it is suggested that free-tropospheric humidity changes lead to deviations from strict convective quasi equilibrium and cause this difference. When topography is reduced, dry extratropical air intrudes into the troposphere over the θeb maximum and is entrained by local deep convection, requiring a higher θeb to achieve convective equilibrium with a given upper-tropospheric temperature and associated balanced monsoon flow. These results illustrate potential complexities that need to be included in simple theories for monsoon strength built on strict convective quasi equilibrium.