The response of the upper ocean to solar heating. I: The mixed layer.

Woods, John D. and Barkmann, Wolfgang (1986) The response of the upper ocean to solar heating. I: The mixed layer. Quarterly Journal of the Royal Meteorological Society, 112 (471). pp. 1-27. DOI 10.1002/qj.49711247102.

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Abstract

The results of two earlier papers on convection in the mixed layer and on the solar heating profile are here introduced into a one-dimensional model in order to investigate the following consequences of the daily cycle of solar heating in the upper ocean:

1. the daytime convection depth becomes less than the turbocline depth;

2. the convective power supply to turbulence in the mixed layer is reduced;

3. the mixed layer below the convection layer becomes stably stratified;

4. the depth of the turbocline is reduced, leaving a diurnal thermocline between it and the top of the seasonal thermocline;

5. the heat content and potential energy of the diurnal and seasonal thermoclines are increased, slowing down the subsequent nocturnal descent of the turbocline.

These diurnal changes are illustrated by integrating a one-dimensional model forced by the astronomical cycle of solar heating and seasonal variation of surface meteorology derived from Bunker's climatology. The model is integrated for 18 months to show the seasonal modulation of the diurnal cycle. Nocturnal convection plays a dominant role. The convection depth closely follows the thermal compensation depth during the day when they are less than the turbocline depth. Integrating the model with a 24-hour time step leads to large errors in the seasonal variation of mixed layer temperature and depth, and in the source term of isopycnic potential vorticity. The errors are reduced by using two time steps per day, one for the daytime when convection is quenched, the other for the night when it is active. A novel parametrization based on tuning the daily equivalent solar elevation to surface temperature further reduces the error. This parametrization is used to investigate the sensitivity of the seasonal cycles of mixed layer depth and temperature to: (1) seasonality in the surface fluxes; (2) systematic changes in the net annual solar heating; (3) random changes in the seasonal cycles of solar heating induced (i) monthly and (ii) daily. The sensitivity to uncertainty in seawater turbidity is investigated in the same way. The profile of isopycnic potential vorticity subducted into the thermocline depends on the vernal correlation of mixed layer depth and density, so gyre circulation is sensitive to solar heating in spring.

Document Type: Article
Keywords: Physical Oceanography; mixed layer; solar heating
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-PO Physical Oceanography
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1002/qj.49711247102
ISSN: 0035-9009
Date Deposited: 18 Jun 2012 12:10
Last Modified: 21 Jan 2019 10:40
URI: http://oceanrep.geomar.de/id/eprint/14692

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