Ocean memory effect on the dynamics of coastal heavy precipitation preceded by a mistral event in the northwestern Mediterranean

Abstract In this study, we examine the possible role of 3D oceanic circulation in 'connecting' a strong mistral windstorm in the northwestern Mediterranean with a heavy precipitating event in southern France in November 1999. These two events are both governed by intense air-sea exchanges and are only separated by a week. From an atmospheric point of view, these two events may be considered as fully uncorrelated and independent. However, using the MORCE regional coupled system with WRF atmospheric model and NEMO-MED12 ocean model, we show that the oceanic cyclonic circulation is a key feature, which can trap strong modifications of the ocean mixed layer over few days. In our specific situation, the mistral (5-9 November 1999) induces a rapid mixed-layer cooling. This cold anomaly is trapped in the cyclonic gyre in the Gulf of Lions and maintained over a week. This directly affects the following heavy precipitating event (12-13 November 1999) with less energy supplied where the cold anomaly is maintained and larger evaporation near the coast. This modulates the rainfall amounts over land, with an increase in the eastern part and a decrease in the western part compared to an uncoupled simulation. These simulations highlight the key role of the general ocean circulation, and especially of the dynamic of the cyclonic gyre in the Gulf of Lions on the ocean memory in the area. The ocean maintains the temperature anomaly over typical periods of a few days, which thus enables some indirect links between two weather systems in which air-sea interactions are significant.