Orographic Controls on Asian Hydroclimate, and an Examination of Heat Wave Temporal Compounding

Abstract

Global warming is projected to induce changes in phenomena ranging from hurricanes to heat waves to droughts. Preparing for these changes requires both fundamental understanding of the climate system, and applied understanding of the risks posed by such events. This dissertation engages with both these areas of inquiry by analyzing diverse observations and global climate model (GCM) simulations.

Two studies in this dissertation explore a basic climate dynamics question: what role does orography (i.e. mountains and other topographic features) play in shaping patterns of precipitation over Asia? Both studies are conducted by comparing a control GCM simulation incorporating modern day orography to a perturbation simulation where specific orography is flattened.

The first study examines the influence of the Tian Shan, a mountain range branching north from the Tibetan Plateau, on extratropical deserts in Asia. The western and eastern deserts in this region exhibit strikingly different seasonal cycles of precipitation. The Tian Shan's role in this zonal gradient of precipitation is examined, and an important role for the Tian Shan in enhancing the East Asian Monsoon is highlighted.

The second study examines the influence of the Tibetan Plateau and related orography on Asian monsoons and tropical cyclones. Asian orography is found to increase precipitation over the Western North Pacific (WNP) throughout the summer monsoon, but decrease precipitation over the Arabian Sea. The mountains also alter tropical cyclones, enhancing and suppressing their formation in the WNP and Arabian Sea, respectively. The roles of model resolution and atmosphere-ocean coupling in these responses are differentiated using a hierarchy of GCM simulations.

The third study of this dissertation focuses on an applied, policy-motivated question: what is the hazard of heat waves occurring close together in time (i.e. temporally compounding), and how will that change with global warming? Definitions of heat wave compounding are developed and applied to GCM simulations with increased atmospheric carbon dioxide. It is argued that the hazard of compound heat waves will disproportionately increase with global warming. Prior events will then play an increasingly large role in heat wave vulnerability. Policy implications of this conclusion are discussed.