Land-atmosphere Teleconnections Between Spring Soil Moisture and Summertime Climate

Year-to-year variability in summertime temperature has a large impact on drought, wildfire, and extreme heat across the Western United States. A recent study showed that warmer-than-average summertime temperatures in the Western US are often preceded by drier-than-average springtime soil moisture over the Southwest US. To examine the possibility that land-atmosphere coupling modulates summertime temperature variability over this region, we perform an ensemble of soil moisture depletion experiments within the Community Earth System Model (CESM2) and find that reducing March surface soil moisture over the Southwest US causes positive May-June temperature anomalies throughout the Western US and precipitation anomalies in the Northwest that are consistent with observations. In our experiments, daytime diabatic heating over anomalously dry land surfaces in early spring excites circulation anomalies that evolve into a hemispheric-scale pattern similar to that observed following anomalously dry springtime in the Southwest US. We show that the subsequent late spring and early summer circulation anomalies are associated with large-scale reductions in atmospheric moisture and cloudiness that contribute to the near-surface warming. Our results suggest that spring soil moisture variations are a source of seasonal predictability for summertime climate extremes, through their non-local impact on summertime temperature variability over the Western US.