Land-atmosphere interactions and the predictability of the South American low-level jet

Soil moisture plays a critical role in shaping regional atmospheric circulation and hydrological extremes, particularly in regions characterized by strong land-atmosphere coupling. This dissertation investigates how soil moisture anomalies influence the South American low-level jet (SALLJ) and associated moisture transport into southeastern South America (SESA) using a multi-pronged approach that integrates simulations, reanalysis, and forecasts. The overarching goal of this work is to examine how antecedent soil moisture anomalies affect the SALLJ, moisture transport, and precipitation over SESA, and to assess how this knowledge can be leveraged to improve subseasonal forecast skill. Three primary research questions are addressed: 1. How do dry soil moisture anomalies and their spatial distribution affect moisture transport and precipitation over SESA in both simulations and reanalysis at monthly timescales? 2. How do antecedent soil moisture conditions influence the intensity and structure of SALLJ events in reanalysis data at daily timescales? 3. How does the representation of soil moisture in forecast models influence the accuracy of SALLJ predictions at subseasonal timescales? Through three independent yet interconnected studies, this work explores the physical mechanisms, observational evidence, and predictive implications of land surface conditions at subseasonal timescales. In the first study, idealized experiments using the Community Earth System Model (CESM) reveal that the location of large-scale dry soil moisture anomalies significantly affects regional moisture transport and precipitation. The simulations establish a causal mechanism in which dry soils enhance lower tropospheric heating, deepen the thermal low east of the Andes, and strengthen poleward moisture transport. The second study uses the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5) dataset to identify strong Chaco jet events and assess the surface conditions prior to the events. The analysis quantifies the climatological contribution of these jets to regional moisture flux and confirms that dry antecedent soil conditions consistently precede enhanced jet intensity. The third study evaluates ensemble forecasts from the ECMWF Subseasonal to Seasonal (S2S) Prediction Project. It shows that ensemble members with more accurate representations of dry soil moisture are associated with more skillful forecasts of SALLJ-related meridional wind anomalies, with improvements evident up to four weeks in advance. Together, these studies demonstrate that soil moisture is not only a modulator of regional climate dynamics but also a source of predictability at subseasonal timescales. The findings advance our understanding of land-atmosphere interactions in SESA and provide actionable insights into improving subseasonal forecasts of extreme weather events in the region.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Atmospheric Sciences

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Chen, Chu-Chun

Contributors dc:contributor

• Dominguez, Francina
• Wang, Zhuo
• Ford, Trent W.
• Lo, Min-Hui

• Land-atmosphere Interactions
• South American Low-level Jet
• Southeastern South America
• Soil Moisture
• Subseasonal Predictability

Statement dc:rights

• Copyright 2025 Chu-Chun Chen

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/129880

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129880