Elise Ralph

Wind-Driven Currents in the Tropical Pacific

Abstract An analysis is made of the ensemble mean ageostrophic near-surface circulation measured by 1503 Lagrangian drifters drogued to 15-m depth in the tropical Pacific between 1987 and 1994. It was found that the physical model of the wind-driven currents in a weakly stratified upper layer, in which the Richardson number remains near unity, accounts for 40% of the vector variance of the observations. In such a model, the amplitude of the current is proportional to u∗(N/f)1/2 and the depth scale of the wind-driven layer is proportional to u∗(N/f)−1/2. When the ageostrophic currents at 15-m depth are binned by the scaled Ekman depth, a net rotation of 0.87 radians was observed through the layer depth. These measurements suggest that the vertical austausch coefficient of the upper ocean is proportional to u2∗/N. Ekman proposed such a model based on two reports of wind-driven currents in 1905 at widely separated latitudes.

Prognostic Modeling Studies of the Keweenaw Current in Lake Superior. Part I: Formation and Evolution

Abstract The formation and evolution of the Keweenaw Current in Lake Superior were examined using a nonorthogonal-coordinate primitive equation numerical model. The model was initialized by the monthly averaged temperaturefield observed in June and September 1973 and run prognostically under different forcing conditions with and without winds. As a Rossby adjustment problem, the model predicted the formation of a well-defined coastal current jet within an inertial period of 16.4 h after the current field adjusted to the initial temperature field. The magnitude and direction of this current jet varied with the cross-shelf temperature gradient and wind velocity. It tended to intensify during northeastward (downwelling favorable) winds, and to lessen, or even reverse, during southwestward to northwestward (upwelling favorable) or southeastward (downwelling favorable) winds. In a case with strong stratification and without external atmospheric forcings, a well-defined clockwise warm-core eddy formed near the ...

A Lagrangian description of the western equatorial Pacific response to the wind burst of December 1992 : Heat advection in the warm pool

Abstract During the Tropical Oceans Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE) intensive observing period (IOP), sustained westerly winds were observed between 20 December 1992 and 10 January 1993 in the area between 155°E and 180°. The oceanic response to this event was monitored by 33 Lagrangian mixed layer drifters, six of which were equipped with SEACAT salinity sensors. The drifters were distributed over several hundred kilometers meridionally and over a zonal extent of 2400 km. During the wind event, the drifters accelerated eastward and formed a strong equatorial jet that was relatively independent of longitude. Following the drifters, the water parcels cooled and became more saline; Sea surface temperature (SST) maps suggest that evaporative cooling occurred. In order to consider the dynamics and thermodynamics of this jet in more detail, wind stress and buoyancy forcing along the track of each individual drifter were constructed from the TOGA COARE European Cent...

A General Theory for Equivalent Barotropic Thin Jets

Abstract The so-called thin-jet approximation, in which variations along the jet axis are assumed gradual in comparison with variations normal to the axis, allows the calculations of along- and cross-axis structures to be decoupled. The result is a nonlinear equation, with one lesser spatial dimension, governing the meandering of the jet. Here a new such “path equation” is constructed in the context of a one-layer, reduced-gravity model. The formalism retains two distinct physical processes: a vortex-induction mechanism, originating from the jet curvature, that causes meanders to travel downstream (i.e., usually eastward), and the planetary (beta) effect, induced by meridional displacements, that gives the meanders the allure of Rossby waves and generates a westward (i.e., usually upstream) propagation. After a brief comparison with previous path equations, analytical solutions of the new equation are explored, including solitons and other exact nonlinear wave forms. The presentation concludes with numeri...

Prognostic Modeling Studies of the Keweenaw Current in Lake Superior. Part II: Simulation

The Keweenaw Current, observed along the coast of the Keweenaw Peninsula in Lake Superior during July 1973, was simulated using a 3D, nonorthogonal coordinate transformation, primitive equation coastal ocean model. The model domain covered the entire lake with a high resolution of 250‐600 m in the cross-shelf direction and 4‐6 km in the alongshelf direction along the peninsula. The model was initialized using the monthly averaged temperature field observed in June 1973 and was run prognostically with synoptic wind forcing plus monthly averaged heat flux. Good agreement was found between model-predicted and observed currents at buoy stations near Eagle Harbor. Comparison of the model results with and without inclusion of heat flux suggested that combined wind and heat fluxes played a key role in the intensification of the Keweenaw Current during summer months. The model-predicted relatively strong near-inertial oscillations occurred episodically under conditions of a clockwise-rotating wind. These oscillations intensified at the surface, were weak near the coast, and increased significantly offshore.

Bottom stress generation and sediment transport over the shelf and slope off of Lake Superior's Keweenaw peninsula

[1] Data from near-bottom instruments reveal that the mechanisms responsible for generating bottom stresses and resuspending sediment over the shelf and slope off of Lake Superior’s Keweenaw peninsula exhibit distinct seasonal variations. Notably, near-bottom flows over the slope are persistently weak (<10 cm s � 1 ) during summer but frequently attain high speeds, in excess of 20 cm s � 1 , in autumn and winter. During the intense storms of autumn and winter the generation of bottom stress is enhanced by the action of near-bottom orbital velocities due to surface waves. Even at 90-m depth, orbital velocities can increase bottom stress by a factor of up to 20% during storms. Where the seasonal thermocline intersects the lake floor, bottom stress is also considerably enhanced, often by more than a factor of 2, by high-frequency motions in the internal wave band. Over the Keweenaw slope, sediment resuspension is largely confined to autumn and winter episodes of high bottom stress. Our analysis indicates that this resuspended material tends to be carried offshore, a phenomenon that is partly due to the coincidence of the direction of the buoyancy-driven component of the Keweenaw Current with downwelling favorable alongshore winds. As a result of this coincidence, currents and bottom stresses tend to be greater during periods of downwelling, as opposed to upwelling, circulation. A potential challenge to modeling storm-driven resuspension in the study region is indicated by observations that the minimum stress required for resuspension may vary significantly with time over the autumn and winter. INDEX TERMS: 4558 Oceanography: Physical: Sediment transport; 4211 Oceanography: General: Benthic boundary layers; 4239 Oceanography: General: Limnology; KEYWORDS: sediment transport, sediment dynamics, bottom boundary layer dynamics

A Non-orthogonal Primitive Equation Coastal Ocean Circulation Model: Application to Lake Superior

A non-orthogonal coordinate primitive equation model has been developed for the study of the Keweenaw Current in Lake Superior. This model provides a more accurate fitting of the coastline. A comparison with a curvilinear orthogonal model shows that the non-orthogonal transformation model provided a better simulation of the current jet in the near-shore region. Accurate fitting of both bathymetry and irregular coastlines plays an essential role in capturing the magnitude of the Keweenaw Current and cross-shelf structure of the thermal bar near the coast. The formation of the Keweenaw Current and thermal front was directly driven by a westerly or southwesterly wind and seasonal development of stratification over steep bottom topography. Under a condition with accurate fitting of steep bathymetry, failure to resolve the irregular geometry of the coastline can result in an underestimation of the magnitude of the Keweenaw Current by about 20 cm/s.

Cross-frontal transport along the Keweenaw coast in Lake Superior: a Lagrangian model study

Offshore transport across the thermal front along the Keweenaw coast in Lake Superior was examined by tracking the trajectories of water particles in the model-simulated three-dimensional (3D) flow field of July 1973. Particles were released at different depths and horizontal locations within the Keweenaw Current during various wind events and were tracked until the end of the month. The trajectories of water particles showed a remarkable offshore cross-frontal water transport at the topographic-splitting point on the eastern side of the Keweenaw Waterway and near Eagle Harbor. This transport was driven dominantly by wind-induced Ekman flow near the surface but was controlled by local bottom topography in the deep region. A northeastward wind prevailed over the lake during July 1973. This wind tended to produce onshore water transports near the surface and hence caused downwelling against the coast. An offshore current was expected in the deep region based on the conservation of water mass. The vortex shedding off coastal bathymetry abutments plus baroclinic instability of the thermal front also led to offshore meandering of the temperature field in the deep region over local varying bottom topography. This meandering tended to produce a cyclonic vorticity and drove particles offshore across the thermal front along the northern coast of the Keweenaw Peninsula.