Robert L. Haney

Surface Thermal Boundary Condition for Ocean Circulation Models

Abstract By employing a heat budget analysis appropriate to zonally and time averaged conditions within the atmosphere, it is shown that the net downward heat flux Q at the oceans surface can be expressed as Q = Q2 (TA*–Ts), where TA* is an apparent atmospheric equilibrium temperature, Ts the sea surface temperature, and Q2 a coefficient determined from the zonally and time averaged data. The latter coefficient, which is of the order of 70 ly day−1 (°C)−1, varies with latitude by as much as 20%. It is suggested that the use of the above relation as a flux-type thermal boundary condition would allow for large-scale thermal coupling of ocean and atmosphere. The more common use of specified Ts as a boundary condition clearly does not allow for such coupling.

On the pressure gradient force over steep topography in sigma coordinate ocean models

Abstract The error in computing the pressure gradient force near steep topography using terms following (σ) coordinates is investigated in an ocean model using the family of vertical differencing schemes proposed by Arakawa and Suarez. The truncation error is estimated by substituting known buoyancy profiles into the finite difference hydrostatic and pressure gradient terms. The error due to “hydrostatic inconsistency,” which is not simply a space truncation error, is also documented. The results show that the pressure gradient error is spread throughout the water column, and it is sensitive to the vertical resolution and to the placement of the grid points relative to the vertical structure of the buoyancy field being modeled. Removing a reference state, as suggested for the atmosphere by Gary, reduces the truncation error associated with the two lowest vertical modes by a factor of 2 to 3. As an example, the error in computing the pressure gradient using a standard 10-level primitive equation model appl...

Circulation in the Alboran Sea as determined by quasi-synoptic hydrographic observations. Part I: Three-dimensional structure of the two anticyclonic gyres

Abstract The circulation and dynamics of the Modified Atlantic Water have been studied using data from an intensive field experiment carried out between 22 September and 7 October 1992. Data included 134 CTD casts, ADCP, and satellite imagery. A well-defined wavelike front was observed with two significant anticyclonic gyres in the western and eastern Alboran Sea. Smaller-scale cyclonic eddies were also observed. The front separates the more saline, older modified Atlantic water (S>38) in the northern region from the fresher, more recent modified Atlantic water (S<36.8) in the south. The associated baroclinic jet had a mean transport of 1 Sv and maximum geostrophic velocities of 1.0 ms−1. The three-dimensional structure and spatial scales of both gyres were similar, that is, 90 km long and 220 m deep. In the eastern Alboran, northeast of Oran, the origin of the Algerian Current was also detected with an eastward transport of 1.8 Sv. The general picture can be presented as a structure formed by a wavelike ...

On the upper layer circulation in the Alboran Sea

We present new experimental results and assemble them with previous results in order to develop an improved picture of the upper layer circulation in the Alboran Sea. It is suggested that the key idea to understanding this upper layer circulation is the tendency of the Atlantic jet (AJ) to have negative curvature. Local interactions with the western Alboran gyre (WAG) or the African coast can, however, counterbalance this tendency and modify the anticyclonic path of the AJ. It is also proposed that the density gradients in the WAG can be maintained in part by means of an intermittent surface cross-gyre current which results in an input, mixing, and renewal of Atlantic water. The static stability at the bottom of the gyre increases because of the mixing of Atlantic water with Western Mediterranean Deep Water which is uplifted close to the African coast. This mixing process thereby acts as a local source of potential vorticity. We also report the existence in the Alboran Sea of subsurface anticyclonic eddies (located between 100 and 400 m) of relatively cold water that appear to be detached from the Iberian shelf. Regarding the large-scale variability of the AJ-WAG system, we present evidence of an eastward migration of the WAG and the subsequent formation of a new anticyclonic gyre in the western Alboran basin on a timescale of 1 month. This eastward gyre migration process temporarily allows the simultaneous presence of three anticyclonic gyres in the Alboran Sea.

Midlatitude Sea Surface Temperature Anomalies: A Numerical Hindcast

Abstract A multilevel primitive equation, ocean circulation model with surface layer physics is used to study the interannual variability of sea surface temperatures (SST) in the central midlatitude North Pacific Ocean. Results from a 10-year model simulation (hindcast) driven by observed winds are analyzed and compared with observations. The hindcast SSTs exhibit a significant amount of nonseasonal variability despite being damped toward a regular annual cycle by the thermal boundary condition at the surface. This variability is due to the horizontal and vertical redistribution of heat by currants and by parameterized turbulence processes caused by the winds. The resulting hindcast SST anomalies are correlated with observed SST anomalies at a statistically significant level over a large part of the central midlatitude North Pacific Ocean. This suggests that wind forcing by itself, through the mechanisms noted before, makes an important contribution to the development of SST anomalies in this area. The hi...

An embedded mixed-layer-ocean circulation model

Abstract The rationale and numerical technique of embedding an oceanic bulk mixed-layer model with a multi-level primitive equation model is presented. In addition to the usual prognostic variables that exist in a multi-level primitive equation model, the embedded model predicts the depth of the well-mixed layer as well as the jumps in temperature and velocity that occur at the base of that layer. The depth of the mixed layer need not coincide with any of the fixed-model levels used in the primitive equations calculations. In addition to advective changes, the mixed layer can deepen by entrainment and it can reform at a shallower depth in the absence of entrainment. When the mixed layer reforms at a shallower depth, the vertical profile of temperature below the new, shallower mixed layer is adjusted to fit the fixed-level structure used in the primitive equations calculations using a method which conserves heat, momentum and potential energy. Finally, a dynamic stability condition, which includes a consideration of both the vertical current shear and the vertical temperature gradient, is introduced in place of the traditional ‘convective adjustment’. A two-dimensional version of the model is used to test the embedded model formulations and to study the response of the ocean to a stationary axisymmetric hurricane. The model results indicate a strong interdependence between vertical turbulent mixing and advection of heat.

Offshore propagation of eddy kinetic energy in the California Current

Low-pass-filtered velocities obtained from surface drifters and surface geostrophic velocities estimated from TOPEX/Poseidon altimeter data have recently revealed a clear and robust seasonal cycle in the surface eddy kinetic energy (EKE) in the California Current (CC) [Kelly et al., 1998; Strub and James, 2000]. The seasonal cycle begins in spring when a surface-intensified baroclinic equatorward jet develops next to the coast in response to strong upwelling favorable winds. This jet, and a developing eddy field, then moves offshore during summer and fall. The EKE maximum associated with the jet progresses only as far as 127°W, beyond which it decreases rapidly. This is a robust characteristic of the seasonal cycle that has been previously attributed only to an unspecified dissipation process. To investigate this aspect of the surface EKE, a multiyear simulation of the CC is carried out using the Dietrich/Center for Air-Sea Technology primitive equation regional ocean model [Dietrich, 1997]. The simulation accurately reproduces many aspects of the observed annual cycle, including the offshore propagation of the EKE at the surface. The model results indicate that the decrease of surface EKE west of 127°W in the simulation is not due to dissipation but rather is caused by the vertical redistribution of EKE to the deep ocean. This redistribution occurs through the transformation of kinetic energy from the vertical shear flow to the vertical mean flow. The transformation is a nonlinear process inherently associated with the life cycle of baroclinically unstable waves, and in the CC, it effectively energizes the deeper ocean at the expense of the upper ocean. The process is also known to be important in the atmosphere [Wiin-Nielsen, 1962]. Taken together, the recent California Current observations and the new model results strongly suggest that the CC regularly supplies EKE to the deep waters of the eastern North Pacific.

Circulation in the Alboran See as Determined by Quasi-Synoptic Hydrographic Observations. Part II: Mesoscale Ageostrophic Motion Diagnosed through Density Dynamical Assimilation

Abstract The 3D velocity field in the Alboran Sea (Western-Mediterranean) is diagnosed through density dynamical assimilation in a primitive equation (PE) model with mesoscale resolution. The ageostrophic motion is computed from fields produced by short-term backward and forward integrations of the PE model initialized with quasi-synoptic CTD data. A weight function based on a low-pass digital filter (Digital Filter Initialization method) is applied to the resulting time series of model variables to obtain the final, dynamically balanced, density and 3D velocity fields. The diagnosed ageostrophic motion is interpreted by comparing the vertical velocity field with that obtained from the quasigeostrophic (QG) ω equation. The two methods produce very similar results with maximum vertical motions in the range of 10–20 md−1 associated with the differential advection of relative vorticity in small-scale jet meanders [upward (downward) motion upstream (downstream) of the ridges]. Small local differences between ...

A Numerical Study of the Response of an Idealized Ocean to Large-Scale Surface Heat and Momentum Flux

Abstract A numerical model of a 6-level, baroclinic ocean with a flat bottom and a regular coast line extending from 51.25S to 48.75N is integrated over 125 years of simulated time using a finite-difference analog of the primitive equations. The surface atmospheric conditions which drive the circulation, both mechanically and thermally, are prescribed and depend on latitude only. The numerical integration is done in two phases. In the first phase (100 years), the temperature is predicted from the complete thermal energy equation, while the equations of horizontal motion are linear and the vertical mean current is constant in time. In the second phase (25 years), the complete primitive equations are used, and the coefficients of eddy viscosity and eddy conductivity are reduced. Integration of the first phase produces western boundary currents in both hemispheres, a surface counter-current at 7N, an eastward undercurrent at the equator, and a narrow band of cold surface water along the equator which is main...

Diagnosis of the Three-Dimensional Circulation in Mesoscale Features with Large Rossby Number

Several diagnoses of three-dimensional circulation, using density and velocity data from a high-resolution, upper-ocean SeaSoar and acoustic Doppler current profiler (ADCP) survey of a cyclonic jet meander and adjacent cyclonic eddy containing high Rossby number flow, are compared. The Q-vector form of the quasigeostrophic omega equation, two omega equations derived from iterated geostrophic intermediate models, an omega equation derived from the balance equations, and a vertical velocity diagnostic using a primitive equation model in conjunction with digital filtering are used to diagnose vertical and horizontal velocity fields. The results demonstrate the importance of the gradient wind balance in flow with strong curvature (high Rossby number). Horizontal velocities diagnosed from the intermediate models (the iterated geostrophic models and the balance equations), which include dynamics between those of quasigeostrophy and the primitive equations, are significantly reduced (enhanced) in comparison with the geostrophic velocities in regions of strong cyclonic (anticyclonic) curvature, consistent with gradient wind balance. The intermediate model relative vorticity fields are functionally related to the geostrophic relative vorticity field; anticyclonic vorticity is enhanced and cyclonic vorticity is reduced. The iterated geostrophic, balance equation and quasigeostrophic vertical velocity fields are similar in spatial pattern and scale, but the iterated geostrophic (and, to a lesser degree, the balance equation) vertical velocity is reduced in amplitude compared with the quasigeostrophic vertical velocity. This reduction is consistent with gradient wind balance, and is due to a reduction in the forcing of the omega equation through the geostrophic advection of ageostrophic relative vorticity. The vertical velocity diagnosed using a primitive equation model and a digital filtering technique also exhibits reduced magnitude in comparison with the quasigeostrophic field. A method to diagnose the gradient wind from a given dynamic height field has been developed. This technique is useful for the analysis of horizontal velocity in the presence of strong flow curvature. Observations of the nondivergent ageostrophic velocity field measured by the ADCP compare closely with the diagnosed gradient wind ageostrophic velocity.