Latent Heat

Latent Heat in the Ocean Heat Budget

Latent heat in the ocean results from evaporation at the ocean surface. As water changes from liquid in the ocean to vapor in the atmosphere, the ocean loses energy in the form of the latent heat of vaporization, and the atmosphere gains heat when the vapor condenses. Since water vapor pressure depends on temperature, sea surface temperature is an important parameter in determining the latent heat flux. The oceans effectively capture a major portion (about 50%) of the sun's radiated energy and transfer much of it to the atmosphere as latent heat of vaporization and as radiation. The rate of heat flow is expressed in joules per second (J/s) per square meter, or watts per square meter (W/m2) (1 W = 1 J/s).

The analysis of atmospheric modeled data for the 1958–2001 period in the Mediterranean Sea reveals that the time-mean latent heat is negative for the whole domain, with a mean value of −88 W/m2. Its temporal mean ranges from −20 W/m2 in the Alborán Sea (Western Mediterranean) to −125 W/m2 in the Levantine basin, Aegean Sea, and Gulf of Lions. Regarding its seasonal cycle, the minimum value (-125 W/m2) is detected by mid November and the maximum (-51 W/m2) by mid May. The amplitude of the seasonal cycle is 37 W/m2. In terms of total heat [Page 1762]budget, which takes into account all the components (latent heat, sensible heat, long-wave radiation, and shortwave radiation), there is a practical cancellation (-1 W/m2) of the heat flux components for the whole period mentioned above (see Figure 1).

Figure 1 Spatial distribution of the temporal mean of latent heat (in watts per square meter) over the Mediterranean Sea using 44 years (1958–2001) of atmospheric model data. The main regional subbasins are indicated.

Source: Adapted from Ruiz, S., Gomis, D., Sotillo, M. G., & Josey, S. A. (2008). Characterization of surface heat fluxes in the Mediterranean Sea from a 44-year high-resolution atmospheric data set. Global and Planetary Change, 63, 258–274.

In conclusion, latent heat is one of the most variable terms in the global heat budget, and it is crucial at short (e.g., heavy precipitation) and longer timescales (e.g., interannual variability). New international long-term experimental programs aim at better quantification and understanding of the hydrological cycle, and particularly of latent heat. Estimates of latent heat will improve with increasing high-resolution in situ and remote data (ship campaigns, buoy stations, and satellite measurements) and will be useful for both numerical weather prediction models and climate models.

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