Investigating controls on intraseasonal convection and the abrupt seasonal transition in the boundary layer during monsoons

Abstract

As solar insolation increases in the northern hemisphere during boreal summer, it is accompanied by an abrupt change in the circulation associated with the monsoons as well as heightened intraseasonal convective activity in the summer hemisphere tropics. These two aspects shape tropical weather regimes, and understanding their dynamics is crucial for subseasonal forecasting as well as subseasonal to seasonal variability and change in a warming world. In this thesis, we investigate the dynamics of intraseasonal convection, particularly its interaction with circulation and seasonal transition of winds in the boundary layer through observations, theory, and idealized modeling experiments. In the first part of the thesis, we study the intraseasonal variability in the global tropics during boreal summer. An important characteristic of this variability, which climate models do not represent adequately, is the coupling of convection to the large-scale circulation. We investigate this coupling and its systematic latitudinal dependence within the tropics for three dominant timescales of sub-seasonal rainfall variability (synoptic (2-10 days), high frequency (10-25 days), and low-frequency (30-70 days)). By focusing on the evolution and structure of vertical vorticity associated with precipitating convection, we elucidate a stronger spatial and temporal association between the two farther away from the equator. A vertically resolved vorticity budget conditioned on convection across individual events at various timescales further reveals the importance of boundary layer stretching and free tropospheric vertical advection for the maintenance of vertically uniform vorticity during peak convection. Moreover, very close to the equator, where the background absolute vorticity is negligible or negative, the boundary layer stretching is unfavourable for vorticity generation and henceforth, the vorticity associated with convection there is weaker. Although quantitative differences exist due to different background states, this latitude dependence qualitatively persists for all tropical ocean basins. Next, we focus on the intraseasonal convection within the Indian monsoon, specifically focusing on the northward propagating boreal summer intraseasonal oscillation (BSISO) and its evolution at different latitude zones. A detailed event-wise lead-lag analysis of ISO rainfall and its potential precursors reveals that close to the equator (5°N-14°N), barotropic vorticity leads convection by roughly five days, thus suggesting a dynamic control on convection. This relationship reverses away from the equator (15°N-24°N), where a thermodynamic control on ISO convection is observed in which boundary layer moist static energy precedes rainfall while the vorticity follows with a lag of about two days. Large-scale extreme rainfall events, known to be phase-locked with the positive phase of ISO, also depict this thermodynamic control, albeit with a much shorter time-lag (few hours). This latitude- dependent characteristic of BSISO, raises important questions about its northward propagating feature and points to the need to systematically probe this evolving relationship

in simple models of this phenomenon, as well as design benchmarks for general circulation model (GCM) simulations of monsoon intraseasonal variability. In the second part of the thesis, we examine the seasonal transition of lower-level winds during monsoons. Focus is placed on the most intense manifestation of this transition, which is seen as the rapid intensification of the Somali jet at the onset of the Indian monsoon. This highly energetic jet is, in fact, the harbinger of the Indian monsoon. Through the analysis of the kinetic energy (KE) at different sections of the Somali jet, we find distinct balances in the boundary layer at different latitudes. In the Southern Hemisphere, the easterly flow that ultimately feeds the jet exhibits a conventional Ekman balance, with KE generation balanced by frictional dissipation. A unique “advective balance”—balance between KE generation in the northward flow and its advection emerges as the jet begins to form near the equatorial region. The fully formed Somali jet (poleward of 10°N) exhibits a three-way balance between KE generation, its advection, and dissipation. These balances can be characterized by a non- dimensional parameter, namely the local Rossby number (R o ), in the boundary layer. R o , which is closely related to the absolute vorticity, measures the strength of advective acceleration relative to the Coriolis acceleration. For low values of R o (O~(0.01)), the boundary layer is Ekman type, whereas for higher values of R o (O~(1)), it becomes advective. Within this advective boundary layer, the cross-isobaric meridional winds are proportional to meridional geopotential gradients that intensify during boreal summer. This leads to a nonlinear (quadratic) dependence of KE generation on the local meridional pressure gradient, eventually resulting in the rapid intensification of the KE of the Somali jet at the seasonal transition when the cross-equatorial pressure gradient is set up. In this framework, the asymmetrical evolution of the jet during onset and retreat can simply be explained through the evolution of this gradient. Finally, based on the empirical results and momentum balance, simple theoretical arguments are put forth to explain the emergence of the advective boundary layer and the proportionality of meridional winds with geopotential gradient in the high R o regime. The underpinnings of the advective boundary layer are further probed through idealized aquaplanet GCM experiments with zonally symmetric sea surface temperature (SST). These experiments, when SST maximum is located sufficiently away from the equator, reveal boundary layer dynamics qualitatively similar to that of observations. The relationship between meridional windspeed and the meridional geopotential gradient is also present across aquaplanet experiments with different latitudes of SST maxima and moreover matches well with observations, suggesting fundamental constraints on KE generating cross-equatorial flows arising from the emergence of the advective boundary layer. Within the advective boundary layer, the relation between kinetic energy and meridional geopotential gradient is shown to be constrained by the inertial timescale, i.e. inverse of Coriolis parameter. Therefore, we further test the effect of inertial timescale on the advective boundary layer by varying the planetary rotation rate in the aquaplanet GCM. Taken together, the results from this thesis advance our understanding of the seasonal transition during monsoons and the enhanced intraseasonal convection that follows. It points to the importance of boundary layer absolute vorticity and its manifestation as local Rossby number for both these timescales. This thesis also provides frameworks for investigating bidirectional interactions between seasonal and subseasonal timescales.