Understanding the Mechanisms Leading to Early Warning of Meteorological and Hydrological Drought in the U.S. Caribbean

Introduction and Rationale: In groundwater-limited settings, such as the U.S. Caribbean,
societal, ecological, and agricultural water needs are largely supplied by regular rainfall.
Consequently, these islands, Puerto Rico and the U.S. Virgin Islands, are vulnerable to even
short, rapid-onset, dry spells, known as “flash drought,” and drought early warning is immensely
valuable for civil authorities on the islands. In the wake of the 2015 drought, precipitation
deficits were linked to the early arrival of an elevated hot, dry, dust-laden feature, termed the
Saharan air layer (SAL). The SAL increased static stability, largely suppressing convective
precipitation during a typically rainy time of year. The SAL is a precursor of Caribbean drought.

Summary: This project will first examine and diagnose drought through a suite of
hydrometeorological variables, drought indices, and drought definitions, such as the Palmer
Drought Severity Index, Standardized Precipitation Evaporation Index, etc. Episodes of low
drought metrics will be compared to in-situ hydrologic measurements, such as USDA Soil
Climate Analysis Network data, in the U.S. Caribbean to infer their ability to capture flash
drought onset. Next, concurrent meteorological fields from renanalysis products will be
examined during the flash drought periods to identify the local meteorological conditions driving
flash drought and how these differ from conventional drought. Third, drought frequency will be
characterized as a function of SAL activity over the U.S. Caribbean. Self-organizing maps, a
machine learning technique, will mine historical 2D fields of the Galvez-Davison Index, a
recently developed tool well-suited for detecting SAL outbreaks, to determine common historical
SAL behavior during the hydrologically critical early rainfall season in the U.S. Caribbean.
Teleconnection indices and seasonal numerical weather forecasts will be analyzed for their
ability to provide early warning of SAL, and therefore drought, in the U.S. Caribbean.

Broader Impacts: This project provides critical monitoring and drought early warning
improvements in U.S. Caribbean islands with limited water resources for human populations,
unique drought-vulnerable ecosystems, and a recent history of economic hardship and natural
hazards, including a major drought in 2015. The project includes a co-PI located in the region,
and one of the objectives of this project is to actively engage with local stakeholders who have
recently asked for improved drought information. This project will serve a large population that
is an ethnic and linguistic minority in the U.S., and the project will actively recruit and mentor
students who are under-represented in climate science.

Relevance: This project is relevant by engaging Priority Area C by examining the predictability
of U.S. droughts, as well as their multi-scale evolution. Seasonal model forecasts are mined for
the presence of a precursor mechanism that can inform contextualized forecasts of drought
likelihood and severity. The project establishes a new methodology for prediction by identifying
the flash drought indices which correspond most strongly to parameters that can be derived from
existing seasonal forecast models. On a broader, programmatic level, this project advances the
MAPP primary objective #3 to improve methodologies for global to regional scale analysis,
predictions, and projections.

Climate Risk Area: Water Resources