The Influence of Sea Surface Temperature and Variability of the North Atlantic Subtropical High on Multidecadal Drought in Texas

Scatter Plot of PDSI vs. AMO Index vs. PDO Index
Scatter plot of monthly averaged PDSI vs. AMO index vs. PDO index from 1950 – 2010 for the Trans Pecos region. Red circles show severe and extreme dry events (PDSI < -3), and blue circles show severe and extreme wet events (PDSI > 3).

Adam Bowerman

Intense drought is of great concern to the state of Texas, as well as the rest of the American Southwest.  As the population in Texas grows, it becomes increasingly important to understand the climatological mechanisms that cause stress on the supply of fresh water.  Although drought in Texas is linked to La Niña events, the relationship between the strength of the drought and that of La Niña is complex.

This study explores how other factors, in particular sea surface temperature and pressure variability in the Atlantic and Pacific, influence the hydrology of the ten individual climate divisions of Texas.  In confirmation of previous studies, it is shows that the interaction between the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO) exhibits a dipole response.

During regimes with a negative AMO and a positive PDO, which equates to an anomalously cool Atlantic and warm Pacific, Texas precipitation increases and drought occurrence is low.  In the case of a positive AMO and negative PDO, the opposite effect is seen, having a negative hydrological response in Texas with a marked increase in drought occurrence.

Summer SNASH Z-Score and PDO Z-Score
Summer SNASH Z-Score (top) and PDO Z-Score (bottom), each smoothed with a two year running mean.

In an effort to understand the mechanism of Pacific subtropical sea surface temperature forcing on atmospheric circulation in the Atlantic Basin, a detailed look at the relationship between the Pacific Decadal Oscillation and the North Atlantic Subtropical High provides insights into the role of the subtropical high in drought occurrence and severity. It is found that during positive (negative) events of the Pacific Decadal Oscillation, the North Atlantic Subtropical High (NASH) migrates equatorward (poleward) by 1.5° – 3.0° latitude, creating favorable conditions for westward migration of the western boundary of the NASH.  During the summer months, precipitation in Texas is especially susceptible to westward NASH extension, with more precipitation as the NASH moves westward and less precipitation as it moves eastward.  Similar latitudinal motion was found in the 200 hPa Northern Hemisphere subtropical jet, which also shows coherent migrations of 1.5° – 3.0° latitude, and may provide a teleconnection pathway by which the PDO can influence atmospheric circulation across the planet.

Advisor: Rong Fu