Projects
Circulation in the Chukchi Sea and Ventilation of the Arctic Halocline
Circulation and Water Mass Transformation in a model of the Chukchi Sea
Ventilation of the Arctic Halocline by Shelfbreak Eddies
The circulation and water mass transformation in a regional ocean-ice model of the Chukchi Sea are discussed. The model has horizontal resolution of O( 4 km), is forced by fluxes derived from daily NCEP reanalysis fields, and has seasonally varying transport, temperature, and salinity imposed at Bering Strait. Many of the observed characteristics of the mean circulation and seasonal cycle in the Chukchi Sea are reproduced. The discussion focuses on: the branching of the inflow transport into pathways following Herald Canyon, Central Channel, and the Alaskan coast; the pattern of ice melt; and the water mass transformation and formation of winter water and hypersaline water. The ice melt pattern and timing is strongly influenced by advection through Bering Strait. High frequency forcing results in a larger region of ice melt, particularly over the shoals and in the northern Chukchi Sea, compared to monthly mean forcing.
Air-Sea Coupling in the Vicinity of Ocean Fronts
Observations in the vicinity of ocean fronts indicate that sea surface temperature (SST) is positively correlated with near-surface wind speed such that stronger winds are found over warmer SST. This indicates that the ocean is forcing the atmosphere, and is opposite to what is commonly observed on large scales, where a negative correlation is indicative of the atmosphere driving the ocean. I have been exploring several aspects of the coupling near ocean fronts, including the behavior in strong cross-front, mid-latitude regimes and the feedback of such coupling on the ocean circulation. Traditional mechanisms for the acceleration of near-surface winds over warm water are an adjustment to local pressure gradients and the downward mixing of high momentum from aloft due to increased instability of the planetary boundary layer over warm water.
Circulation around Islands and Ridges
I am studying the influences of planetary scale topographic features such as islands and abyssal ridges on the mean and low frequency variability of the large scale circulation using both numerical models and simple theories. The circulation integral around an island places a strong constraint on the properties of the flow, both in the vicinity of the island and in the adjacent basins. Single islands or island chains allow for the passage of some forms of low frequency variability quite freely, while other forms are entirely blocked by the topography. Interesting circulation features arise in the mean flow, such as recirculation gyres, jets, and eastern boundary currents as a result of circulation constraints around the topography.
Watermass Transformation in Marginal Seas
Waters formed at high latitudes have distinct water mass characteristics that can be traced throughout the worlds oceans. This water mass transformation and transport is a fundamental component of the oceanic thermohaline circulation and plays an important role in the global climate system. I am interested in several aspects of how these waters are formed, where they sink, and how they are transported away from their formation regions. I am also interested in how the thermohaline circulation interacts with the wind-driven circulation, both at mid-latitudes and at high latitudes.
Mixing near Boundaries
The presence of lateral boundaries fundamentally changes the circulation driven by turbulent mixing processes. In the open ocean, weak diapycnal mixing can drive strong horizontal recirculation gyres, such as is found in the classical Stommel / Arons abyssal circulation model. However, as this mixing is confined more closely to a lateral boundary, the potential vorticity balance changes such that the strong horizontal recirculations, which cross planetary vorticity contours, are replaced by weak unidirectional flow along potential vorticity contours into and out of the mixing region. If the horizontal scale of the mixing region drops below the relevant viscous length scale, the horizontal recirculation is essentially eliminated.
Instability and Nonlinear Equilibrations of the Large Scale Wind-driven Flow
While it has been known for several decades that the variability of the ocean circulation in the gyre interiors is much stronger than the mean flow, the sources of these strong eddies are still not well understood. Baroclinic instability of the large scale circulation is a likely source, but it has been shown that zonal flows with weak shear typical of the gyre interiors can not generate very strong variability. I have recently addressed the question of whether the meridional component of the wind-driven gyres might generate strong mesoscale eddies more effectively than can zonal flows. Scale analaysis and and nonlinear numerical modelling studies show that the potential energy stored in the meridional component of the large-scale wind-driven gyres may be effectively converted to eddy kinetic energy even for very weakly sheared mean flows. For parameters typical of the interior of the mid-latitude subtropical gyres the eddy kinetic energy at equilibration is two orders of magnitude greater than the mean kinetic energy.
Equatorial Rossby Wave Reflection and Transmission
(Collaboration with J. Pedlosky)
The interaction of equatorial Rossby waves with a western boundary perforated with one or more narrow gaps is investigated using a shallow water numerical model and supporting theory. It is found that very little of the incident energy flux is reflected into eastward propagating equatorial Kelvin waves provided that at least one gap is located within approximately a deformation radius of the equator. Due to the circulation theorem around an island, the existence of a second gap off the equator reduces the reflection of short Rossby waves and enhances the transmission of the incident energy into the western basin. The westward energy transmitted past the easternmost island is further reduced upon encountering islands to the west, even if these islands are located entirely within the "shadow" of the easternmost island.