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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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Howard P. Hanson
,
Claire S. Hanson
, and
Brenda H. Yoo

Analysis of ice observations made by cooperative observers from shoreline stations reveals significant changes in the ice season on the North American Great Lakes over the past 35 years. Although the dataset is highly inhomogeneous and year-to-year variability is also quite large, there is a statistically significant indication that the end of the ice season (as defined by the time at which ice departs from the observer stations in spring) has come increasingly early at a number of locations. The earlier ice departure is reflected in a somewhat earlier spring runoff through the St. Lawrence River over the same time period and correlates with increases in springtime temperatures at stations in the region. This example of a trend toward warmer, earlier springs in the upper Midwest is consistent with results from a number of other regional datasets. Because the ice observations began in the mid-1950s, other analyses, including comparisons with modern satellite datasets, could provide a useful tool for monitoring future climate change.

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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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Power from the Florida Current

A New Perspective on an Old Vision

Howard P. Hanson
,
Susan H. Skemp
,
Gabriel M. Alsenas
, and
Camille E. Coley
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