Observed Convective and Moist Dynamics in Extratropical and Tropical Synoptic-scale Circulation during Boreal Summer

Abstract

This dissertation focuses on the propagation and growth of tropical and extratropical waves during boreal summer by incorporating views based on vorticity and moisture. They are applied to two different synoptic-scale systems that are located in the mid-latitudes and tropics. The first case is the Bermuda High (BH). Its expansion and contraction can be quantified by a Bermuda High Index (BHI). The composites of precipitation based on the phase of BHI show an enhanced (suppressed) rainfall over the southeastern US (SEUS) when BH contracts (expands). Linear regressions of anomalous geopotential height and winds at 200 hPa onto BHI show an eastward-propagating Rossby wave train over the North Hemisphere, with the mid-latitude jet stream being its waveguide. Anomalous troughing over the SEUS associated with this wave train is linked to the contraction of the Bermuda High during July and August. The enhanced precipitation is associated with anomalous ascent to the east and south of this trough where anomalous warm advection is observed. Based on these results, it is hypothesized that this Rossby wave train may partially explain the occurrence of suppressed precipitation tied to midsummer drought in the SEUS. Tropical systems differ from the ones in the mid-latitudes and subtropics because of the high concentration of moisture and weak temperature tendency. The second case focuses on the tropical Indian Ocean. The Indian summer monsoon low pressure systems (MLPSs), as one of the most vigorous systems over the Indian Ocean, have been studied comprehensively over the past decades. However, the dynamics that control the westward movement and growth of the MLPSs remain controversial. According to the analysis of the column-integrated moist static energy (MSE) and perturbation kinetic energy (PKE) budgets, we propose that the MLPSs grow from the moisture-vortex instability (MVI) and barotropic instability. MVI also contribute to the wave propagation. MVI refers to a process in which the strong horizontal MSE advection leads to a moisture tendency. The vortex grows through the enhanced convection brought about by the moisture tendency. The results show a vertically upright structure and, the moisture, precipitation and vorticity have an in-phase component. The meridional mean MSE advected by anomalous winds is the main contributor to anomalous MSE growth and propagation. The tendency and maintenance of PKE are led by the barotropic energy conversion. The moisture in MLPSs plays a central role in their energetics. We used three criteria and a dimensionless number Nmode) to measure the importance of moisture: if (i) the precipitation anomalies and the column water vapor anomalies are highly correlated; (ii) the weak-temperature gradient (WTG) is satisfied; (iii) the latent heat is dominating MSE. The results show that all three criteria were satisfied and Nmode<<1 for MLPSs over the tropical oceans. The “moisture mode” governs the thermodynamics of MLPSs. Using reanalysis products, we examined the moisture mode activity and the meridional eddy moisture flux of MLPSs when they grow from MVI. The available latent energy (ALE) of the Indian monsoon was also calculated. The results show that the meridional eddy moisture flux equals to the tendency of moisture mode activity. The tendency of ALE is coherent with the inverse of meridional eddy moisture flux. The results indicate that the Indian monsoon is weakened through the equatorward eddy moisture flux when MLPSs grow from MVI.