Hydrometeorological Drivers of Summertime Temperature Variability Over Western US Land Surfaces

Interannual variations in Western United States summertime temperatures are captured by a leading EOF that explains over 50% of the total variance in mean June, July, August (JJA) temperature. This mode of variability, which we term the Western US Summertime Temperature Index (WUS-STI), is well correlated with consequential metrics for drought, wildfire activity, and extreme heat. Whereas interannual variability in near-surface temperature is typically attributed to patterns in sea surface temperature and the atmospheric circulation, we instead find a surprising and significant link between variations in the WUS-STI and hydrological conditions in the preceding spring and winter. To understand how soil moisture coupling with the atmosphere could modulate summertime climate variability, we investigate the behavior of the WUS-STI and its associated spatial pattern in a variety of models and in reanalysis. Using results from the GLACE-CMIP5 experiments and a new soil moisture linear inverse model (LIM), we find evidence that springtime soil moisture anomalies can induce summertime heat anomalies in the Western United States and force an associated circulation response. These findings have important implications for how soil moisture memory and land-induced forcing can impart predictability at seasonal and sub-seasonal time scales.