Qualifying Exam: Xinyue Luo
Student: Xinyue Luo
Date: Wednesday, September 15th, 2021
Time: 12:00 p.m.
1) Characterizing the El Nin ̃o-Southern Oscillation and its North American Teleconnections over the Last Millennium
The El Nin ̃o-Southern Oscillation (ENSO) dominates interannual climate variability in the tropical Pacific and affects midlatitude climate through the atmospheric response to shifts in sea surface temperatures (SSTs). These atmospheric responses are often called teleconnections, so named for their far-reaching responses to regional SST changes. ENSO-induced teleconnections were previously assumed to be stationary and predictable. However, observations show that teleconnections differ widely between individual ENSO events. Internal variability of ENSO leads to distinct events and associated impacts: the amplitude of ENSO events, their seasonal timing, SSTA patterns and ocean-atmosphere feedbacks all modulate ENSO teleconnection expression.
Complicating uncertainties in teleconnection rainfall prediction is the fact that our statistical constraints of ENSO events are based on 20th century observations, which span less than 50 ENSO events, too few to sufficiently characterize the complexity of ENSO teleconnections. Highly-resolved paleoclimate archives spanning the Last Millennium (LM) and new techniques in paleoclimate data assimilation (DA) can be used to augment instrumental data and characterize the statistics of ENSO variability and associated teleconnections many centuries into the past. This project proposes to use a comprehensive network of paleo-records and paleoclimate DA products, the Last Millennium Reanalysis (LMR) and the Paleo Hydrodynamics Data Assimilation product (PHYDA). Using an expanded paleoclimate database, we will generate improved statistics with which to constrain ENSO hydroclimate variability over North America (N.A.). This project will investigate changes in ENSO characteristics and its N.A. hydroclimate teleconnection over the LM, and additionally explore shifts in ENSO-driven hydroclimate conditions with background temperature changes in the tropical Pacific.
2) Constraining East Asian Precipitation Variability Associated with Diverse Tropical Pacific Sea Surface Temperature Warming Patterns
Extreme precipitation is of critical socioeconomic importance, affecting freshwater supply and agricultural production in the most populated regions of the world, but also causing devastating floods, particularly in recent decades. Rainfall distribution over East Asia shows large spatiotemporal variability and is jointly impacted by natural and internal climate variability, which makes in situ rainfall prediction difficult. Specifically, precipitation variability over East Asia is closely linked to tropical Pacific Sea Surface Temperature (SST) variability, such as El Nin ̃o-Southern Oscillation (ENSO). However, the relationship between extratropical precipitation and tropical Pacific SST variability has not been thoroughly constrained because of the large influence of internal variability and diverse SST heating patterns. As a result, prediction of East Asian precipitation impacted by tropical SSTs still varies widely in different models; there is no consensus on how regional precipitation will change with anthropogenic warming in future projections.
The atmospheric response to SSTs (e.g. the SST-convection relationship) plays an important role in altering tropical convection and large-scale circulation patterns that affect extratropical rainfall, but is poorly represented in documented ENSO indices. This project proposes to evaluate projections of precipitation in atmospheric general circulation models (AGCMs) and conduct an idealized SST-forcing experiment to identify how precipitation over East Asia responds to distinct SST heating patterns in the tropical Pacific. By incorporating a novel index that explicitly considers SST convection thresholds in defining distinct SST patterns, this project will evaluate the sensitivity of mid-latitude precipitation to shifts in the locus of convection over the tropical Pacific. Using this framework, we will additionally explore how background SST warming patterns, under multiple warming scenarios, alter SST-precipitation relationships over East Asia.