2024 Theses Doctoral

Atmospheric Dynamics Related to Subseasonal Variability in Rainfall over the Tropical Pacific

Bunge, Isabelle Elise

The eastern Pacific is a key region for the study of tropical convection, due to its unique dynamics and year-round rainfall concentrated in a zonal band called the Intertropical convergence Zone (ITCZ). A strong meridional gradient of sea surface temperature (SST) is defined by the equatorial cold tongue and warmer SST to the north, around the typical latitudes of the observed ITCZ. Through sensible heating, the SST imparts its structure onto the overlying atmospheric planetary boundary layer temperature, creating a low-level meridional pressure gradient. Through Ekman balance, this gradient drives low-level convergence towards the highest SST values, where the gradient changes sign. Simultaneously, diabatic heating over the largest SST values also influences upward motion, leading to a kind of chicken-and-egg question: Is it the SST that sets that low-level winds, which then drive the vertical motion and convection where they convergence, or inversely, is it the diabatic heating in moist convection which drives the vertical motion, and then sets the low-level flow through mass conservation.

On the scale of days or weeks, there is very little variation in the underlying SST gradient, but observations demonstrate rapidly evolving convection along the ITCZ. This thesis analyzes the sources of boundary layer geopotential and wind perturbations that contribute to setting low-level convergence and thus convection.

Chapter 1 is an introduction to the theories of tropical convection and its interactions with the large-scale motions in the eastern Pacific. It also provides an overview of Convectively Coupled Equatorial Waves (CCEWs), which will be discussed later as influences on the boundary layer. Chapter 2 applies a Mixed-Layer Model (MLM) to understand the role that the SST gradient and its corresponding low-level pressure gradient play in forcing boundary layer wind and convergence. The MLM results demonstrate that the SST gradient plays a substantial roll in setting the bulk boundary layer winds from OTREC observations.

Chapter 3 focuses on how the planetary boundary layer mass field co-varies with precipitation and examines the relative contributions on the low-level dynamics from a component related to SST and another related to free-tropospheric dynamics. This study finds that on subseasonal timescales, variance in precipitation is strongly associated with variance in the low-level dynamics that is driven by the free troposphere. This result suggests that, under the viewpoint that the boundary layer dynamics forces convection, synoptic-scale variability in rainfall is primarily associated with perturbations to the low-level dynamics driven by free tropospheric transients.

Finally, Chapter 4 covers research performed as a follow-up to Chapter 3, which studies the relative impact of different kinds of CCEWs on boundary layer dynamics along the ITCZ in the eastern Pacific. Through studying the evolution of Kelvin and Easterly waves in the eastern Pacific, the authors find that these waves act on boundary layer dynamics to enhance convection by amplifying vertical velocity modes observed in the region.

The span of the research presented in this thesis can be summarized by a pursuit of understanding the factors that drive rainfall in the eastern Pacific on synoptic scales. On longer timescales, the meridional low-level pressure gradient created by SST is important in driving boundary layer winds; on shorter timescales, variance in low-level dynamics is heavily influenced by free-tropospheric transient (such as CCEWs) in similar manners, such that they that drive a moistening in the lower atmosphere, followed by deep vertical ascents and precipitation.