Variability of Soil Moisture and Sea Surface Temperatures Similarly Important for Warm-Season Land Climate in the Community Earth System Model

Both sea surface temperatures (SSTs) and soil moisture (SM) can influence climate over land. This paper presents a comprehensive comparison of SM versus SST impacts on land climate in the warm season. The authors perform fully coupled ensemble experiments with the Community Earth System Model in which they prescribe SM or SSTs to the long-term median seasonal cycles. It is found that SM variability overall impacts warm-season land climate to a similar extent as SST variability, in the midlatitudes, tropics, and subtropics. Removing SM or SST variability impacts land climate means and reduces land climate variability at different time scales by 10%-50% (temperature) and 0%-10% (precipitation). Both SM- and SST-induced changes are strongest for hot temperatures (up to 50%) and for extreme precipitation (up to 20%). These results are qualitatively similar for the present day and the end of the twenty-first century. Removed SM variability affects surface climate through corresponding variations in surface energy fluxes, and this is controlled to first order by the land-atmosphere coupling strength and the natural SM variability. SST-related changes are partly controlled by the relation of local temperature or precipitation with the El Nino-Southern Oscillation. In addition, in specific regions SST-induced SM changes alter the "direct" SST-induced climate changes; on the other hand, SM variability is found to slightly affect SSTs in some regions. Nevertheless a large level of independence is found between SM-climate and SST-climate coupling. This highlights the fact that SM conditions can influence land climate variables independently of any SST effects and that (initial) soil moisture anomalies can provide valuable information in (sub)seasonal weather forecasts.