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  • Author or Editor: Roger Proctor x
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Jason T. Holt and Roger Proctor

Abstract

The temperature structure of the Irish Sea is investigated using a 3-yr simulation with a high-resolution (∼1.8 km) three-dimensional baroclinic model (the Proudman Oceanographic Laboratory Coastal-Ocean Modelling System) and CTD and Advanced Very High Resolution Radiometer observations. This paper focuses on the extent to which (horizontal) advection determines the temperature structure. It is found that it has a significant effect on the depth-mean temperatures throughout the region and on the vertical profiles in seasonally stratified areas, such as the Celtic Sea and western Irish Sea. There is depth-mean advective heating during the summer in these stratified regions, whereas in well-mixed regions advection tends to reduce the amplitude of the seasonal cycle. Through an analysis of the terms in the temperature equation, the warming of the “cool pool” waters of the western Irish Sea can be attributed to the advection of partially well-mixed waters into the stratified region from the north. This occurs as an entrainment process with the southward current on the western side of this region folding in this water from the north. This current is seen to originate both as part of the “gyre” circulation and from southward flow through the North Channel of the Irish Sea. The accuracy to which temperatures are modeled (particularly near the seabed in this stratified region), as compared with an experiment without temperature and salinity advection, lends weight to this interpretation of the model results. Overall rms errors against CTD observations are 1.1°C with advection and 1.7°C without. In addition to the direct effects of currents, salinity stratification (which is not present without advection in the western Irish Sea in this model) is seen to play a role in determining the temperature structure, particularly in the spring and early summer. Unlike previous baroclinic simulations in this region, the model run is continued for a further 2 yr, allowing the investigation of the seasonal cycle of temperature far removed from the initial condition. In a number of regions, a systematic overestimation of the winter temperatures is found (the cause of which has yet to be identified), but this bias does not compromise the accuracy of the results between the spring and autumn of subsequent years.

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Patrick J. Luyten, John E. Jones, and Roger Proctor

Abstract

A three-dimensional numerical study is presented of the seasonal, semimonthly, and tidal-inertial cycles of temperature and density-driven circulation within the North Sea. The simulations are conducted using realistic forcing data and are compared with the 1989 data of the North Sea Project. Sensitivity experiments are performed to test the physical and numerical impact of the heat flux parameterizations, turbulence scheme, and advective transport. Parameterizations of the surface fluxes with the Monin–Obukhov similarity theory provide a relaxation mechanism and can partially explain the previously obtained overestimate of the depth mean temperatures in summer. Temperature stratification and thermocline depth are reasonably predicted using a variant of the Mellor–Yamada turbulence closure with limiting conditions for turbulence variables. The results question the common view to adopt a tuned background scheme for internal wave mixing. Two mechanisms are discussed that describe the feedback of the turbulence scheme on the surface forcing and the baroclinic circulation, generated at the tidal mixing fronts. First, an increased vertical mixing increases the depth mean temperature in summer through the surface heat flux, with a restoring mechanism acting during autumn. Second, the magnitude and horizontal shear of the density flow are reduced in response to a higher mixing rate. Thermal and salinity fronts generate a seasonal circulation pattern in the North Sea. Their impact on the horizontal temperature distributions is found to be in good agreement with the observations. It is shown that, in the absence of strong wind forcing, both the vertical temperature distribution and the thermal circulation experience semimonthly variations in response to the spring–neap cycle in tidal mixing. At spring tides, the surface mixed layers are shallower, in agreement with observations at two mooring stations, and the baroclinic circulation intensifies, whereas the opposite occurs at neaps.

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