Abstract
Abstract. Local diabatic heating and temperature anomaly fields need to be positively correlated for the diabatic heating to maintain a circulation against dissipation. Here we quantify the thermodynamic contribution of local air–sea heat exchange on the evolution of weather systems using an index of the spatial covariance between heat flux at the air–sea interface and air temperature at 850 hPa upstream of the North Atlantic storm track, corresponding with the Gulf Stream extension region. The index is found to be almost exclusively negative, indicating that the air–sea heat fluxes act locally as a sink on potential energy. It features bursts of high activity alternating with longer periods of lower activity. The characteristics of these high-index bursts are elucidated through composite analysis and the mechanisms are investigated in a phase space spanned by two different index components. It is found that the negative peaks in the index correspond with thermodynamic activity triggered by the passage of a weather system over a spatially variable sea-surface temperature field; our results indicate that most of this thermodynamically active heat exchange is realised within the cold sector of the weather systems.
Highlights
In the Northern Hemisphere, storm tracks have a limited longitudinal extent and are located mainly off the eastern coasts of mid-latitude Asia and North America
Given that the computation of F relies on T at the surface and air temperature is assimilated from observations, their effect on F at the resolved scales would be captured by the reanalysis system and they would still contribute some residual variance, which is included in our analysis
Lorenz (1955) showed that diabatic generation of available potential energy is proportional to the covariance between heating and air temperature
Summary
In the Northern Hemisphere, storm tracks have a limited longitudinal extent and are located mainly off the eastern coasts of mid-latitude Asia and North America. Meridional and vertical heat fluxes act as conversion terms across different types of energy reservoirs, whereas surface heat fluxes are associated with generation and dissipation of available potential energy Global estimates of these terms have been computed (Peixoto and Oort, 1992) and were used to identify the direction of energy flow within the Lorenz energy cycle. This term has been estimated to be positive globally (Oort, 1964; Oort and Peixoto, 1974; Ulbrich and Speth, 1991; Li et al, 2007; Marques et al, 2009), suggesting that diabatic processes are acting as a source of energy in storm development This picture changes when we focus on the contribution of transient eddies, which correspond to synoptic-scale weather systems, to eddy available potential energy. In the final section results are summarised and discussed
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