Effects of the land-atmosphere coupling on drought predictability

Abstract:

The hypothesis of this research is: Land processes and their coupled interactions with the atmosphere can enhance drought predictability beyond what is attributable to anomalous SST forcing alone. We test this hypothesis using a new two-stage coupled climate modeling experiment design: the GLACE-Hydrology experiment (Kumar et al. 2020). First, as in the GLACE-CMIP5 experiment, twin sets of coupled land-atmosphere climate model (CAM5-CLM4.5) ensembles are performed, with each simulation using the same prescribed observed sea surface temperatures and radiative forcing for the years 1971-2014. In one set, land-atmosphere anomaly coupling is removed by prescribing soil moisture to follow the control model’s seasonally-evolving soil moisture climatology (‘land-atmosphere uncoupled’), enabling a contrast with the original control set (‘land-atmosphere coupled’). Then, the atmospheric output from both sets of simulations is used to force land-only ensemble simulations, allowing investigation of the resulting soil moisture drought under both the ‘coupled’ and ‘uncoupled’ scenarios.

We employ three drought indices: Standardized Precipitation Index (SPI), Standardized Standardised Precipitation-Evapotranspiration Index (SPEI), and Standardized Soil moisture Anomalies (SSA). These three indices represent an increasing level of integration by the coupled land-atmosphere system. The model simulated drought metrics are compared with the observations with the observation-based Evaporative Demand Drought Index (EDDI). We found the temporal correlation between the land-atmosphere coupled experiments and the observed EDDI in the US Great Plains and from 1979 to 2014 analysis period are: 0.79 for SSA, 0.62 for SPEI, and 0.52 for SPI. The corresponding correlations in the land-atmosphere uncoupled experiments are 0.50 for SSA, 0.58 for SPEI, and 0.06 for SPI. Hence, we conclude that land-atmosphere coupling brings significant improvement in the drought prediction skill. We will also discuss the implications of the land-atmosphere coupling for the predictability of the 2012 drought in the US.

Reference

Kumar et al. (2020). The GLACE-Hydrology Experiment: Effects of Land–Atmosphere Coupling on Soil Moisture Variability and Predictability. Journal of Climate, 33(15), pp.6511-6529.