Abstract
Abstract. Ocean dynamics is predominantly driven by the shear stress between the atmospheric winds and ocean currents. The mechanical power input to the ocean is fluctuating in space and time and the atmospheric wind sometimes decelerates the ocean currents. Building on 24 years of global satellite observations, the input of mechanical power to the ocean is analysed. A fluctuation theorem (FT) holds when the logarithm of the ratio between the occurrence of positive and negative events, of a certain magnitude of the power input, is a linear function of this magnitude and the averaging period. The flux of mechanical power to the ocean shows evidence of a FT for regions within the recirculation area of the subtropical gyre but not over extensions of western boundary currents. A FT puts a strong constraint on the temporal distribution of fluctuations of power input, connects variables obtained with different lengths of temporal averaging, guides the temporal down- and up-scaling and constrains the episodes of improbable events.
Highlights
The exchange of heat, momentum and matter between the atmosphere and the ocean has a strong influence on our climate (Stocker et al, 2013)
In the present paper we focus on the FT put forward in Gallavotti and Cohen (1995a), Gallavotti and Cohen (1995b) and Gallavotti and Lucarini (2014), corresponding to the detailed fluctuation theorem in the limit of large averaging times
We obtain evidence that a FT applies to data within the recirculation area of the subtropical gyre in the Atlantic and Pacific oceans
Summary
The exchange of heat, momentum and matter between the atmosphere and the ocean has a strong influence on our climate (Stocker et al, 2013). In the present work we are not concerned with the details of the exchange in the respective boundary layers (see e.g. Veron, 2015) but suppose that it is well represented through bulk formulas of air–sea interaction (Fairall et al, 1996) In those models the power input is estimated based on the shear stress at the surface and the ocean current near the surface and depends on the sea state and the density stratification in the atmosphere and the ocean. The thermodynamic framework of the quantities considered to be entropy, heat and work is not necessary to establish FTs. Examples of non-thermal fluctuations are the experimental data of the drag force exerted by a turbulent flow (Ciliberto et al, 2004) and the local entropy production in Rayleigh–Bénard convection (Shang et al, 2005). It is important to notice that even in the case of the idealized models the FT was not established by analytical calculation, but it was confirmed numerically that the FT is obtained asymptotically, in the long-time limit, when the averaging time is larger than the timescale of the slow ocean dynamics
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