Mechanism behind the spring to summer drought memory and its impact on predictability of the summer drought over US Great Plains

English Abstract:

Observations have shown a significant connection between spring and summer droughts over the US Plains that can improve the predictability of summer droughts. In contrast, such dry memory is largely absent in the dynamic models used for seasonal forecasts. Is the observed dry memory a result of remotely forced large-scale circulation or local/regional soil moisture feedbacks? Why is such dry memory largely missing in the dynamical models and their prediction? We suggest through observational analysis that positive feedbacks between land surface dryness and a cloud and a water vapor radiative effect plays at least as important a role as the positive precipitation feedback in reinforce the initial surface dryness. In particular, the net radiative cooling due to warmer surface and decrease of cloud and water vapor is comparable to or greater than the latent heat reduction in the atmosphere during spring, leading to compensational subsidence in the middle troposphere, which in turn further enhances the anticyclonic circulation anomalies and stabilizes the lower troposphere. Such positive radiative feedbacks amplify the dry anomaly and lead to stronger precipitation and latent heating reduction, which further reduces the diabatic heating of the atmosphere, and so enhances the mid-tropospheric subsidence and anticyclonic circulation in summer. Thus, the radiative feedbacks due to reduction of clouds and water vapor work in concert with precipitation feedbacks in providing sustained drought memory over the Southern Plains. Preliminary evaluation of some NMME models suggest that the models show radiative heating, instead of cooling, during dry springs, leading to anomalous rising motion in the middle troposphere. Such a negative radiative feedback works against positive precipitation feedbacks in the model and damps the dry memory. These results highlight the importance of improving the cloud radiation interactions in models for summer drought prediction over the US Great Plains.