How well do dynamical seasonal forecasts capture soil-moisture atmosphere coupling?

Past observational studies and numerical experimentation suggest that spring soil moisture anomalies in the northern hemisphere can induce significant anomalies in local summer temperature and precipitation as well as in large-scale planetary wave patterns. This is particularly the case in regions where soil-moisture memory is long and there are strong soil-moisture-atmosphere interactions. However, recent studies, where soil-moisture initial conditions of dynamical forecasting systems are scrambled, have found more modest impacts suggesting that current seasonal forecasting systems may not accurately capture this source of potential predictability.

Here we present an evaluation of two important aspects in the dynamical models that contribute to the multi-model seasonal forecasting system of the Copernicus Climate Change Service (C3S). Firstly, assessing the skill of soil-moisture in seasonal hindcasts and secondly analysing the realism of soil-moisture-atmosphere coupling strength. Both of which need to be well represented to achieve accurate predictions. These links between soil-moisture, evapotranspiration and temperature are investigated by way of correlation metrics to identify and compare locations where evaporation is soil-moisture limited or energy limited in seasonal hindcasts and observations.

A key finding of this analysis is that in the summer hindcasts, initialised in May, tend to misrepresent the precise location of the so-called North America coupling “hotspot”, an important region for land-atmosphere coupling. The soil-moisture atmosphere coupling tends to be too weak in the models in the western USA and too strong in the east. Both the reasons for this misplacement and the impact of this on forecasts of temperature, precipitation and large-scale planetary waves will be investigated.