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  • Author or Editor: Howard P. Hanson x
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Howard P. Hanson

Abstract

Changes in propagation of free linear waves on the equatorial β-plane associated with air-sea heat exchange are in investigated here. By using a mixed-layer model, with the waves considered as perturbations on a specified basic state, the usual separability problems are avoided and the sea surface temperature is carried as a prognostic variable. The heat exchange is limited to that associated with turbulent fluxes, and a simplified air-sea transfer function allows analytic solutions of the various equatorial modes.

The problem is reduced to the classical solutions for a single vertical mode with the air-sea heat flux and mixed layer entrainment feedback effects cast in terms of three adjustments scales: an atmospheric adjustment length scale and two oceanic adjustment time scales, one for the response to surface fluxes and one for the response to entrainment. In order for the feedback to have any effect, both surface fluxes and entrainment must be included.

Propagation speeds of the equatorial waves are affected significantly by the presence of feedback. For an assumed easterly wind, the Kelvin wave speed is decreased by as much as 15% and the Rossby wave speeds are increased by as much as 50%, depending on the magnitude of the feedback parameters. In addition, the feedback increases (decrease) the wave-related SST amplitude for downwind (upwind) propagating waves over that for the no-feedback case. This is not a positive feedback, because the dissipative nature of the feedback causes the solutions to decay.

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Eric B. Kraus
and
Howard P. Hanson

Abstract

The westward propagation of equatorial sea surface temperature anomalies exceeds the surface drift velocity and is probably associated with propagating changes in the depth of the surface mixed layer and upper thermocline. These can be caused by equatorial Rossby waves and/or by air-sea interactions. In the present paper, it is shown that changes in the stress and heat flux associated with the passage of air over an ocean surface of variable temperature can produce a westward propagation of the temperature pattern regardless of Coriolis effects.

The phenomenon is investigated in the framework of a two-layer channel model. A physical description of the mechanism is followed by the discussion of an approximate solution to the steady state and by a linear wave analysis which deals with the propagation and modification of an initially stipulated departure from the equilibrium.

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Rainer Bleck
,
Dingming Hu
,
Howard P. Hanson
, and
Eric B. Kraust

Abstract

The annual buildup and obliteration of the seasonal thermocline and the associated ventilation of the permanent thermocline in a wind- and thermally driven ocean basin are simulated numerically. The model developed for this purpose is a combination of a single-layer model of the oceanic mixed layer, based on a simple closure of the turbulence kinetic energy equation, and a three-dimensional isopycnic coordinate model of the stratified oceanic interior. The joint model, set in a rectangular ocean basin, is forced by annually varying wind stress and radiative plus turbulent heal fluxes approximating zonnaly averaged conditions over the North Atlantic. Special emphasis is placed on the description of the mixed-layer detrainment process, which requires distributing mixed-layer water of continuously variable density among constant-density interior layers. The truncation errors associated with this process are found to be numerically tolerable.

The quasi-Lagrangian character of the model's vertical coordinate permits easy tracking of water masses left behind during the annual retreat of the mixed layer to form the seasonal thermocline. Likewise, the subduction of ventilated water into the permanent thermocline by the horizontal gyre motion is explicitly simulated.

While a comparison of simulated mixed-layer characteristics with actual observations is problematic due to the idealized basin configuration, the model appears to be reasonably successful in duplicating the seasonal cycle of the zonally averaged conditions over the North Atlantic.

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