Roland W. Garwood

The Labrador Sea Deep Convection Experiment

In the autumn of 1996 the field component of an experiment designed to observe water mass transformation began in the Labrador Sea. Intense observations of ocean convection were taken in the following two winters. The purpose of the experiment was, by a combination of meteorological and oceanographic field observations, laboratory studies, theory, and modeling, to improve understanding of the convective process in the ocean and its representation in models. The dataset that has been gathered far exceeds previous efforts to observe the convective process anywhere in the ocean, both in its scope and range of techniques deployed. Combined with a comprehensive set of meteorological and air-sea flux measurements, it is giving unprecedented insights into the dynamics and thermodynamics of a closely coupled, semienclosed system known to have direct influence on the processes that control global climate.

Sea-Surface Temperature Anomaly Generation in Relation to Atmospheric Storms

Abstract An extended period of reduced surface heat and momentum fluxes due to the absence of atmospheric storms may result in upper-ocean temperature anomalies that persist for months. The predominance of either anomalously high or low temperatures is related to the ocean thermal structure that is established on the transition date between the winter and summer regimes.

A numerical study of three‐dimensional dense bottom plumes on a Southern Ocean continental slope

Three-dimensional features of dense bottom plumes flowing over continental slopes, with or without along-slope topographic variations, are investigated by simulating the evolution of a density front at a Southern Ocean continental shelf break, using a three-dimensional, primitive equation numerical model with a second-order turbulence closure scheme embedded. The focus of our investigation is the role of topography in determining mixing and the offshore transport of dense shelf water during the transient adjustment process of a density front over a continental slope. We compare and discuss the numerical simulations for two cases: a uniform shelf and slope case and a case with a canyon that leads from the coast to the deep ocean crossing the shelf and the slope. The numerical simulations indicate that baroclinic instability, planetary rotation, bottom friction, and topography play major roles in determining the surface and bottom plume formation, the growth and penetration depth of the bottom plumes, and the characteristics of water mass on the slope. Mesoscale eddies play a fundamental role in transporting mass, heat, and salt from shelf to deep ocean. The effects of the depth and width of the canyon are examined with two more experiments. The presence of a wide and deep canyon in the continental shelf and slope enhances considerably the drainage of coastal shelf water into the deep ocean.

Fully Lagrangian Floats in Labrador Sea Deep Convection: Comparison of Numerical and Experimental Results

Measurements of deep convection from fully Lagrangian floats deployed in the Labrador Sea during February and March 1997 are compared with results from model drifters embedded in a large eddy simulation (LES) of the rapidly deepening mixed layer. The deep Lagrangian floats (DLFs) have a large vertical drag, and are designed to nearly match the density and compressibility of seawater. The high-resolution numerical simulation of deep convective turbulence uses initial conditions and surface forcing obtained from in situ oceanic and atmospheric observations made by the R/V Knorr. The response of model floats to the resolved large eddy fields of buoyancy and velocity is simulated for floats that are 5 g too buoyant, as well as for floats that are correctly ballasted. Mean profiles of potential temperature, Lagrangian rates of heating and acceleration, vertical turbulent kinetic energy (TKE), vertical heat flux, potential temperature variance, and float probability distribution functions (PDFs) are compared for actual and model floats. Horizontally homogeneous convection, as represented by the LES model, accounts for most of the first and second order statistics from float observations, except that observed temperature variance is several times larger than model variance. There are no correspondingly large differences in vertical TKE, heat flux, or mixed layer depth. The augmented temperature variance may be due to mixing across large-scale temperature and salinity gradients that are largely compensated in buoyancy. The rest of the DLF statistics agree well with the response of correctly ballasted model floats in the lowest 75% of the mixed layer, and are less consistent with results from buoyantly ballasted model floats. Other differences between observation and simulation in the mean profiles of heat flux, vertical TKE, and Lagrangian heating and vertical acceleration rates are confined to the upper quarter of the mixed layer. These differences are small contributions to layer-averaged quantities, but represent statistically significant profile features. Larger observed values of heat flux and vertical TKE in the upper quarter of the mixed layer are more consistent with model floats ballasted light. Float buoyancy, however, cannot fully account for the observed PDFs, temperature profiles, and Lagrangian rates of heating and acceleration. A test of Lagrangian self-consistency comparing vertical TKE and Lagrangian acceleration also shows that DLF measurements are not significantly affected by excess float buoyancy. These upper mixed layer features may instead be due to the interaction of wind-driven currents and baroclinicity.

Effects of topographic steering and ambient stratification on overflows on continental slopes : A model study

The three-dimensional flow structures of cold dense overflow water on continental slopes with and without along-slope topographic variations and ambient slope water stratification are investigated, using a three-dimensional numerical ocean model. We discuss the effects of topographic steering and ambient stratification on the downslope transport of dense overflow on continental slopes. A constant upstream inflow of dense water is specified at the upper edge of the slope that represents an overflow from a marginal sea. We present the numerical simulations for overflow plumes in the presence of three topographic features: a cross-slope canyon that leads from the coast to the deep ocean, a cross-slope ridge, and a seamount. We compared the numerical results with the previously published results of overflow plumes on a uniform slope without ambient water stratification. In the presence of a canyon, a portion of the dense water descends into the canyon, forming a bottom-trapped plume that flows offshore along the right side (facing the ocean) of the canyon. The numerical result indicates that intensive mixing and entrainment occur in the canyon plume. The remainder of the overflow flows across the canyon and keeps descending on the slope while being deflected to the right-hand side of the inflow. In the presence of a cross-slope ridge, part of the water is blocked by the ridge and the flow is confined along the left side of the ridge. The remaining portion of the plume water flows over the ridge toward the right-hand side of the inflow. The presence of a canyon or a ridge significantly enhances the downslope transport of dense water compared with the uniformly sloping bottom case. A seamount does not affect the cross-isobath transport of dense water as much as does a canyon or a ridge. A seamount does influence the mixing, entrainment, and the plume trajectories. Ambient slope water stratification has significant influence on the mixing and cross-slope penetration of the overflow plumes. It has hindered the downslope penetration of the plumes as a result of enhanced mixing and entrainment when the plumes encounter deep denser water.

Surface Heating and Patchiness in the Coastal Ocean off Central California During a Wind Relaxation Event

The field work was funded by the U.S. Office of Naval Research (ONR) Coasta,I Sciences Program, Code I 122CS, as part of the Coastal Transition Zone project. S.R.R. re- ceived additional support from Direct Research Funding at the U.S. Naval Postgraduate School (NPS). The portion of this work carried out at the Jet Propulsion Laboratory, California Institute of Technology, was sponsored by ONR and the National Aeronautics and Space Administration. Model simulatiotis were conducted by the Oceanic Planetary Boundary Layer Laboratory, sponsored by ONR and funded by NPS. The boom probe was designed and built by the Oceanography Department support staff at the Naval Postgraduate School. Paul Jessen assisted with the data collection and processing. The satellite image was provided by Toby Garfield and the NPS Interactive Digital Environmental Analysis (IDEA) Laboratory.

Identification of modeled ocean plumes in Greenland Gyre ERS‐1 SAR data

Oceanic convective plumes modeled with a thermobaric large-eddy simulation and driven by conditions similar to those of the Greenland Sea are compared to observations from ERS-1 Synthetic Aperture Radar (SAR) data from the Greenland Sea for the winter of 1992. In both form and size the two representations are seen to compare favorably. The plume-filled area of the SAR image occupies a region about 20 km by 90 km at the ice edge of the open water in “Nordbukta”, the large seasonal ice retreat, in the “Odden” ice protuberance in the southern Greenland gyre. In the SAR data the plumes appear to be ice covered while the convective-return areas are open.

On the two-phase thermodynamics of the coupled cloud-ocean mixed layer

The rudiments of a self-consistent two-phase thermodynamical theory of intraseasonal and interannual variability for the tropical cloud-ocean mixed layer system are presented. In this paper we study some basic properties of low-frequency phenomena in the tropical coupled cloud-ocean mixed layer system under low mean wind speed conditions and attempt to seek a unified thermodynamical framework in which the thermodynamical oscillation can be studied and understood. In order to do so, all waves in the atmosphere and in the ocean are initially filtered out, and the coupled system is purely thermodynamic. An air-ocean coupled model designed especially for the low wind speed condition is employed to test the basic thermodynamic feedback mechanism between clouds and the ocean mixed layer. The model has four parts: (1) a shallow-water system for the large-scale atmosphere motion; (2) a cloud model, (3) a marine atmospheric boundary layer, in which the physical processes are parameterized into the bulk formulae through a geostrophic drag coefficient and corresponding heat and moisture exchange coefficients; and (4) an oceanic mixed layer model. The coupled model is solved analytically as an eigenvalue problem. Three nondimensional model parameters are found to be very important in separating growing or decaying, oscillatory or nonoscillatory modes: (1) the ocean surface stability index, ϵ; (2) the surface water budget index, γ; and (3) the mean diapycnal gradient of the spiciness (χ) in the entrainment zone, δ. The sign of ϵ divides the ocean mixed layer into shallowing (ϵ > 0) and entrainment (ϵ 20ϵ + 11 for γ = −0.1, δ > 20ϵ + 11.8 for γ = −0.2, and δ > 20ϵ + 14.6 for γ = −0.5. The model results also demonstrate that the exchanges of heat and water across the sea surface lead to both growing and decaying modes of oscillation on two different time scales, owing to the stability of the atmosphere. For an unstable atmosphere the time scale is about 20–30 days. However, the time scale is approximately 1–3 years for a stable atmosphere. This work introduces the new concept of two-phase thermodynamics for the coupled air-ocean system. Both atmosphere and ocean have two important thermodynamical variables: temperature and moisture (or fractional cloudiness) for the atmosphere, and temperature and salinity for the ocean. If salinity is neglected in the ocean model, no positive feedback mechanism will be possible in the coupled air-ocean system.

Numerical Modeling of Wave-Enhanced Turbulence in the Oceanic Upper Layer

A coupled model of air-wave-sea interaction is modified based on a new roughness formulation and the latest data. The model parameters for aerodynamic roughness from below (ARB) and wave-dependent roughness from above (ARA, z0a) are assumed equal. The combined roughness is assumed to be a function of friction velocity, gravity, air and seawater densities, and wave age (cw). The model is used in a study of wave-enhanced turbulence under breaking waves to predict turbulent dissipation (ε), ARA, and drag coefficient (Cd). Both waves and shear production are considered as sources of ocean turbulent energy. The atmospheric part of the model is used only to specify a correct condition at the interface. Numerical experiments are performed to study the ε-distribution, z0a and Cd, and to compare with data. The major achievement is model verification using all available data. The first full application of this model is in conjunction with an ocean circulation model in a coupled circulation-wave system. Simulations show that the ε-distribution is strongly dependent on local wind-forced wave heights. For each wind and wave state there is a particular wave-dependent depth that is verified by data. The comparison shows that the model predicted ε agrees well with the observed ε of the z−4 law distribution of Gargett (1989). Simulations also show that waves have an important role in causing ε to differ from the classical wall-layer theory and z0a, with a value of 0.30 for the empirical constant aa. The model-predicted ε, z0a, Cd and Cgd agree well with data.

UNSTABLE AND DAMPED MODES IN COUPLED OCEAN MIXED LAYER AND CLOUD MODELS

The ocean mixed layer and clouds are coupled by the fluxes of momentum, heat, and water mass at the interface. The importance of the fluxes of momentum and heat is well recognized by both meteorologists and oceanographers. However, the water mass flux has been given considerable attention only in atmospheric models since the latent heat release is an important source of energy for the atmospheric general circulation. The water mass flux is given less attention in ocean models although it is realized that evaporation and precipitation contribute to the surface buoyancy flux which influences the depth of mixing and the thermohaline circulation. Clouds and the ocean mixed layer are coupled by both the heat and moisture fluxes across the air-ocean interface. Two time scales are demonstrated in this paper: a sea surface temperature (SST) evolution time scale, τT, that is virtually controlled by the oceanic planetary boundary layer (OPBL) and a cloud-SST coupling time scale, τn.T. These two time scales depend on the stability of the marine atmospheric boundary layer (MABL). The more unstable the atmosphere, the shorter the time scales will be. For a stable atmosphere, τT ∼ 1–3 years, and τn.T ≈ 0.3–1 years. However for an unstable atmosphere, τT ≈ 20–30 days, and τn.T — 3–6 days. An air-ocean coupled model is presented in this paper for two different regimes: (1) the non-entraining ocean mixed layer case and (2) the entraining mixed layer case. The model results demonstrate that the exchanges of heat and water across the sea surface lead to both growing and decaying modes of oscillation on the two time scales due to the stability of the atmosphere. These oscillatory solutions are entirely thermodynamic and do not require wave dynamics for their existence.