Impact of Diurnal Warm Layers on Atmospheric Convection

Abstract

Diurnal sea surface temperature (SST) anomalies and their interplay with the atmosphere and in particular with the diurnal cycle of convection have been an object of study for many decades. In this study, we investigate this connection for the first time using simulations that can explicitly resolve both the diurnal temperature variations in the ocean and convection in the atmosphere on a global scale.Diurnal variations in SST have already been described in Sverdrup et al. (1942). Since then, there have been numerous studies describing the physics and the conditions of appearance of diurnal SST variations, the seminal work by Price et al. (1986) being the first detailed description of this phenomenon. Under low-wind conditions and with sufficient insolation, a stable near-surface layer forms during the day in the upper layers of the ocean (until the depth of O(10 m)) that leads to a surface warming of up to 5 K (see Wick and Castro (2020)). In the absence of solar radiation during the night, the stratification dissolves as vertical turbulent mixing takes overhand, until a homogeneous mixed layer is restored. The physics of this phenomenon is described in detail in a monograph by Soloviev and Lukas (2013). This stratified, warm layer is known as diurnal warm layer (DWL) and it is ubiquitous in all latitudes, causing SST fluctuations of 0.2 K or more in the entire Northern hemisphere and beyond during boreal summer (see Gentemann et al. (2003)). A comprehensive discussion of its definition and properties can be found in a review by Kawai and Wada (2007). In particular, the authors of the review point out that the presence of DWLs in observations as well as in single column simulations leads to stronger latent and sensible heat fluxes. As surface fluxes connect the surface to the atmospheric boundary layer and since changes in boundary layer properties affect the development of convection, the question of the impacts of DWLs on atmospheric convection arises.Investigating this question in models requires both fine enough vertical resolution in the ocean, to resolve DWLs, and fine enough horizontal grid spacing in the atmosphere, to resolve atmospheric convection. With the development of deca-to kilometer scale simulations in a coupled configuration (Hohenegger et al. ( 2023)) such investigations are becoming possible. Prominent among the newest studies are the papers by Voldoire et al. (2022) and Brilouet et al. (2021). In Voldoire et al. (2022), a single column coupled model has been considered, while