Untangling the Mistral and Seasonal Atmospheric Forcing Driving Deep Convection in the Gulf of Lion: 1993-2013

Abstract

Deep convection in the Gulf of Lion is believed to be primarily driven by the mistral winds. However, our findings show that the seasonal atmospheric change provides roughly two-thirds of the buoyancy loss required for deep convection to occur for the year 2012 to 2013, with the mistral supplying the final third. Two NEMOMED12 ocean simulations of the Mediterranean Sea were run from 1 August 2012 to 31 July 2013, forced with two sets of atmospheric-forcing data from a RegIPSL coupled run within the Med-CORDEX framework. One set of atmospheric-forcing data was left unmodified, while the other was filtered to remove the signal of the mistral. The control simulation featured deep convection, while the seasonal simulation did not. A simple model was derived by relating the anomaly scale forcing (the difference between the control and seasonal runs) and the seasonal scale forcing to the ocean response through the stratification index. This simple model revealed that the mistral's effect on buoyancy loss depends more on its strength rather than its frequency or duration. The simple model also revealed that the seasonal cycle of the stratification index is equal to the net surface heat flux over the course of the year, with the stratification maximum and minimum occurring roughly at the fall and spring equinoxes.

Decomposing the role of dry intrusions for ocean evaporation during mistral

“…These results highlight the importance of DIs to the mistral evaporative features and highlight the role of the mistral as an upper-level-to-surface pathway in which upper-tropospheric momentum is transported downwards into the GOL in the form of DIs, bearing profound influence on mistral air-sea interactions, and further contributing to deep water formation (Keller et al, 2022(Keller et al, , 2023. These results stress the importance of improving our understanding and predictive ability of downward motions in the atmosphere, and their future projections and implications.…”

“…Keller et al (2022) showed the role of the mistral in the build-up process of deep convection for winter 2012-2013 and suggested that long and/or a frequent succession of shorter mistral events is needed during the build-up season (approximately October-January), for the destabilization process of the GOL water column to reach the threshold for the initiation of deep convection. Keller et al (2023), in review) enhanced said analysis to 20 years and concluded that variations in seasonal stratification play a key role in the evolution of deep convection. However, since a high-pass filter is used to separate the mistral forcing from the seasonal, the long-term part of the mistral appears to handle a considerable part of the variability of the 'seasonal' part.…”

Untangling the Mistral and Seasonal Atmospheric Forcing Driving Deep Convection in the Gulf of Lion: 1993–2013

“…The NEMO simulation data used in this article is available at https://doi.org/10.57745/D7ACY9; last accessed 29 March 2024 (Keller, 2024). The Mistral event data is available at https://medcyclones.eu/data/; last accessed 23 August 2022 (Givon, 2021;Givon et al, 2021;Keller et al, 2022).…”