Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling
02 April 2019

A New Method for Evaluating Controls on Heat and Drought Extremes

Advanced statistics for understanding when and why heat and drought simultaneously occur.

The effect of the pressure dipole (DPI) on heat+drought extremes. (a) a "histogram" of effects on drought (purple = low DPI, yellow = high DPI). (b) effects on temperature. (c) effects on simultaneous heat+drought, showing that high DPI strongly favors warm, dry conditions in northern California.

An advanced new statistical method is used to show that simultaneous heat and drought extremes in the western U.S. are largely controlled by a semi-permanent ‘dipole’ pattern of high/low pressure straddling the U.S. west coast.


This work shows, for the first time, a method for understanding what controls the joint occurrence of heat and drought: and the joint occurrence of extremes in general. It also shows strong evidence that a semi-permanent ‘dipole’ pattern of high/low pressure straddling the U.S. west coast essentially controls whether winters will be warm and dry or cool and wet for both northern and southern California.


LBL scientists used a fast, robust probability estimation method (the LBL-developed fastKDE) to estimate the effect of various large-scale controls on heat+drought extremes, including global mean temperature, El Niño / Southern Oscillation, the dipole index, and numerous others.  This work introduces a novel methodology for understanding how these large-scale controls control the joint occurrence of extremes in general, and it provides an advanced new set of metrics that can be used to evaluate Earth system models.  The application of this methodology shows that global mean temperature and the ‘dipole index’ cause large variations in the joint statistics of heat and drought extremes for California: with increases in global mean temperature generally favoring warm conditions, and the dipole index favoring either hot/dry conditions or cool/wet conditions depending on the phase.

William D. Collins
Lawrence Berkeley National Laboratory (LBNL)