Search Results

You are looking at 1 - 10 of 16 items for :

  • Author or Editor: Frank Bryan x
  • Journal of Physical Oceanography x
  • Refine by Access: All Content x
Clear All Modify Search
Frank Bryan

Abstract

Experiments with a low resolution, primitive equation ocean general circulation model with idealized basin geometry and surface forcing have been carried out in order to identify the processes controlling the climatically important aspects of the circulation. Emphasis was placed on the sensitivity of the model solutions to the magnitude of the vertical diffusivity. Scaling arguments suggest, and the numerical experiments confirm, that the solutions are most sensitive to the magnitudes of the wind stress curl and the vertical diffusivity. For small vertical diffusivity, wind forcing dominates the solution. The vertical scale of the thermocline is set by the strength of the Ekman pumping, and there is a multiple gyre circulation in the upper layers. For large vertical diffusivity, diabatic surface forcing dominates the solution. Vertical diffusion controls the vertical scale of the thermocline, and there is a single large anticyclonic gyre in the upper layers. Both the meridionally and zonally integrated overturning circulations are sensitive to the vertical diffusivity, though not to the same degree. The poleward heat transport is dominated by the zonally integrated meridional overturning circulation and, hence, also shows a sensitivity to the vertical diffusivity. The maximum poleward heat transport for the model used in this study varies by an order of magnitude as the vertical diffusivity is varied over a range comparable to that of estimates based on observations. The sensitivity to the prescription of the surface diabatic forcing and to other closure parameters is also described.

Full access
Frank Bryan and Scott Bachman

Abstract

In this study, the salinity budget of the North Atlantic subtropical salinity maximum region for control volumes bounded by isohaline surfaces is analyzed. The authors provide closed budgets based on output from a high-resolution numerical simulation and partial budgets based on analyses of observational climatologies of hydrography and surface fluxes. With this choice of control volume, advection is eliminated from the instantaneous volume-integrated salt budget, and time-mean advection is eliminated from the budget evaluated from time-averaged data. In this way, the role of irreversible mixing processes in the maintenance and variability of the salinity maximum are more readily revealed. By carrying out the analysis with both near-instantaneous and time-averaged model output, the role of mesoscale eddies in stirring and mixing for this water mass is determined. This study finds that the small-scale mixing acting on enhanced gradients generated by the mesoscale eddies is approximately equal to that acting on the large-scale gradients estimated from climatological-mean conditions. The isohaline salinity budget can be related to water mass transformation rates associated with surface forcing and mixing processes in a straightforward manner. The authors find that the surface net evaporation in the North Atlantic salinity maximum region accounts for a transformation of 7 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) of water across the 37-psu isohaline outcrop into the salinity maximum in the simulation, whereas the estimate based on climatological observations is 9 to 10 Sv.

Full access
Stuart P. Bishop and Frank O. Bryan

Abstract

For the first time estimates of divergent eddy heat flux (DEHF) from a high-resolution (0.1°) simulation of the Parallel Ocean Program (POP) are compared with estimates made during the Kuroshio Extension System Study (KESS). The results from POP are in good agreement with KESS observations. POP captures the lateral and vertical structure of mean-to-eddy energy conversion rates, which range from 2 to 10 cm2 s−3. The dynamical mechanism of vertical coupling between the deep and upper ocean is the process responsible for DEHFs in POP and is in accordance with baroclinic instability observed in the Gulf Stream and Kuroshio Extension. Meridional eddy heat transport values are ~14% larger in POP at its maximum value. This is likely due to the more zonal path configuration in POP. The results from this study suggest that HR POP is a useful tool for estimating eddy statistics in the Kuroshio Extension region, and thereby provide guidance in the formulation and testing of eddy mixing parameterization schemes.

Full access
Frank O. Bryan, Claus W. Böning, and William R. Holland

Abstract

This paper describes, and establishes the dynamical mechanisms responsible for, the large-scale, time-mean, midlatitude circulation in a high-resolution model of the North Atlantic basin. The model solution is compared with recently proposed transport schemes and interpretations of the dynamical balances operating in the sub-tropical gyre. In particular, the question of the degree to which Sverdrup balance holds for the subtropical gyre is addressed. At 25°N, thermohaline-driven bottom flows cause strong local departures from the Sverdrup solution for the vertically integrated meridional mass transport, but these nearly integrate to zero across the interior of the basin. In the northwestern region of the subtropical gyre, in the vicinity of the Gulf Stream, higher-order dynamics become important, and linear vorticity dynamics is unable to explain the model's vertically integrated transport. In the subpolar gyre, the model transport bears little resemblance to the Sverdrup prediction, and higher-order dynamics are important across the entire longitudinal extent of the basin.

The sensitivity of the model transport amplitudes, patterns, and dynamical balances are estimated by examining the solutions under a range of parameter choices and for four different wind stress forcing specifications. Taking into account a deficit of 7–10 Sv (Sv ≡ 106 m3 s−1) in the contribution of the model thermohaline circulation to the meridional transports at 25°N, the wind stress climatology of Isemer and Hasse appears to yield too strong of a circulation, while that derived from the NCAR Community Climate Model yields too weak of a circulation. The Hellerman and Rosenstein and ECMWF climatologies result in wind-driven transports close to observational estimates at 25°N. The range between cases for the annual mean southward transport in the interior above 1000 m is 14 Sv, which is 40%–70% of the mean transport itself. There is little sensitivity to the model closure parameters at this latitude. At 55°N, in the subpolar gyre, there is little sensitivity of the model solution to the choice of either closure parameters or wind climatology, despite large differences in the Sverdrup transports implied by the different wind stress datasets. Large year to year variability of the meridional transport east of the Bahamas makes it difficult to provide robust estimates of the sensitivity of the Antilles and deep western boundary current systems to forcing and parameter changes.

Full access
Stuart P. Bishop, Frank O. Bryan, and R. Justin Small

Abstract

Observational and model evidence has been mounting that mesoscale eddies play an important role in air–sea interaction in the vicinity of western boundary currents and can affect the jet stream storm track. What is less clear is the interplay between oceanic and atmospheric meridional heat transport in the vicinity of western boundary currents. It is first shown that variability in the North Pacific, particularly in the Kuroshio Extension region, simulated by a high-resolution fully coupled version of the Community Earth System Model matches observations with similar mechanisms and phase relationships involved in the variability. The Pacific decadal oscillation (PDO) is correlated with sea surface height anomalies generated in the central Pacific that propagate west preceding Kuroshio Extension variability with a ~3–4-yr lag. It is then shown that there is a near compensation of O(0.1) PW (PW ≡ 1015 W) between wintertime atmospheric and oceanic meridional heat transport on decadal time scales in the North Pacific. This compensation has characteristics of Bjerknes compensation and is tied to the mesoscale eddy activity in the Kuroshio Extension region.

Full access
R. Justin Small, Frank O. Bryan, and Stuart P. Bishop

Abstract

The water mass transformation (WMT) framework describes how water of one class, such as a discrete interval of density, is converted into another class via air–sea fluxes or interior mixing processes. This paper investigates how this process is modified at the surface when mesoscale ocean eddies are present, using a state-of-the-art high-resolution climate model with reasonable fidelity in the Southern Ocean. The method employed is to coarse-grain the high-resolution model fields to remove eddy signatures, and compare the results with those from the full model fields. This method shows that eddies reduced the WMT by 2–4 Sv (10%–20%; 1 Sv ≡ 106 m3 s−1) over a wide range of densities, from typical values of 20 Sv in the smoothed case. The corresponding water mass formation was reduced by 40% at one particular density increment, namely, between 1026.4 and 1026.5 kg m−3, which corresponds to the lighter end of the range of Indian Ocean Mode Water in the model. The effect of eddies on surface WMT is decomposed into three terms: direct modulation of the density outcrops, then indirectly, by modifying the air–sea density flux, and the combined effect of the two, akin to a covariance. It is found that the first and third terms dominate, i.e., smoothing the outcrops alone has a significant effect, as does the combination of smoothing both outcrops and density flux distributions, but smoothing density flux fields alone has little effect. Results from the coarse-graining method are compared to an alternative approach of temporally averaging the data. Implications for climate model resolution are also discussed.

Restricted access
Yiming Guo, Stuart Bishop, Frank Bryan, and Scott Bachman

Abstract

We use an interannually forced version of the Parallel Ocean Program, configured to resolve mesoscale eddies, to close the global eddy potential energy (EPE) budget associated with temperature variability. By closing the EPE budget, we are able to properly investigate the role of diabatic processes in modulating mesoscale energetics in the context of other processes driving eddy–mean flow interactions. A Helmholtz decomposition of the eddy heat flux field into divergent and rotational components is applied to estimate the baroclinic conversion from mean to eddy potential energy. In doing so, an approximate two-way balance between the “divergent” baroclinic conversion and upgradient vertical eddy heat fluxes in the ocean interior is revealed, in accordance with baroclinic instability and the relaxation of isopycnal slopes. However, in the mixed layer, the EPE budget is greatly modulated by diabatic mixing, with air–sea interactions and interior diffusion playing comparable roles. Globally, this accounts for ∼60% of EPE converted to EKE (eddy kinetic energy), with the remainder being dissipated by air–sea interactions and interior mixing. A seasonal composite of baroclinic energy conversions shows that the strongest EPE to EKE conversion occurs during the summer in both hemispheres. The seasonally varying diabatic processes in the upper ocean are further shown to be closely linked to this EPE–EKE conversion seasonality, but with a lead. The peak energy dissipation through vertical mixing occurs ahead of the minimum EKE generation by 1–2 months.

Restricted access
Elizabeth M. Douglass, Steven R. Jayne, Synte Peacock, Frank O. Bryan, and Mathew E. Maltrud

Abstract

A climatologically forced high-resolution model is used to examine variability of subtropical mode water (STMW) in the northwestern Pacific Ocean. Despite the use of annually repeating atmospheric forcing, significant interannual to decadal variability is evident in the volume, temperature, and age of STMW formed in the region. This long time-scale variability is intrinsic to the ocean. The formation and characteristics of STMW are comparable to those observed in nature. STMW is found to be cooler, denser, and shallower in the east than in the west, but time variations in these properties are generally correlated across the full water mass. Formation is found to occur south of the Kuroshio Extension, and after formation STMW is advected westward, as shown by the transport streamfunction. The ideal age and chlorofluorocarbon tracers are used to analyze the life cycle of STMW. Over the full model run, the average age of STMW is found to be 4.1 yr, but there is strong geographical variation in this, from an average age of 3.0 yr in the east to 4.9 yr in the west. This is further evidence that STMW is formed in the east and travels to the west. This is qualitatively confirmed through simulated dye experiments known as transit-time distributions. Changes in STMW formation are correlated with a large meander in the path of the Kuroshio south of Japan. In the model, the large meander inhibits STMW formation just south of Japan, but the export of water with low potential vorticity leads to formation of STMW in the east and an overall increase in volume. This is correlated with an increase in the outcrop area of STMW. Mixed layer depth, on the other hand, is found to be uncorrelated with the volume of STMW.

Full access
Eric P. Chassignet, Linda T. Smith, Rainer Bleck, and Frank O. Bryan

Abstract

A series of medium-resolution (∼1°) numerical simulations for the equatorial and North Atlantic basin have been performed with two primitive equation models, one employing depth and the other density as the vertical coordinate. The models have been configured for this exercise in as similar a fashion as their basic formulations allow, and with fundamentally identical initialization, boundary conditions, and forcing functions for each of the experiments. The purpose of comparing the models’ results is twofold: 1) to understand the degree to which model-generated circulation fields depend on the particular model architecture by examining the rate of divergence of the solutions of an isopycnic and a depth coordinate model given the same initial conditions and 2) to uncover and remedy possible defects in either model design. The comparison is focused on the importance in each simulation of the choice of mixing parameterization, which has a crucial impact on the meridional overturning circulation, on the associated northward heat transport, and on the evolution of water masses. Although the model-generated horizontal fields viewed at specific times during the integrations do not appear to be strongly dependent on the design of each model and are in good agreement with one another, the integrated properties of the depth coordinate model and the isopycnic coordinate model diverge significantly over time, with the depth coordinate model being unable to retain its most dense water masses after long integration periods.

Full access
Benjamin K. Johnson, Frank O. Bryan, Semyon A. Grodsky, and James A. Carton

Abstract

Six subtropical salinity maxima (S max) exist: two each in the Pacific, Atlantic, and Indian Ocean basins. The north Indian (NI) S max lies in the Arabian Sea while the remaining five lie in the open ocean. The annual cycle of evaporation minus precipitation (EP) flux over the S max is asymmetric about the equator. Over the Northern Hemisphere S max, the semiannual harmonic is dominant (peaking in local summer and winter), while over the Southern Hemisphere S max, the annual harmonic is dominant (peaking in local winter). Regardless, the surface layer salinity for all six S max reaches a maximum in local fall and minimum in local spring. This study uses a multidecade integration of an eddy-resolving ocean circulation model to compute salinity budgets for each of the six S max. The NI S max budget is dominated by eddy advection related to the evolution of the seasonal monsoon. The five open-ocean S max budgets reveal a common annual cycle of vertical diffusive fluxes that peak in winter. These S max have regions on their eastward and poleward edges in which the vertical salinity gradient is destabilizing. These destabilizing gradients, in conjunction with wintertime surface cooling, generate a gradually deepening wintertime mixed layer. The vertical salinity gradient sharpens at the base of the mixed layer, making the water column susceptible to salt finger convection and enhancing vertical diffusive salinity fluxes out of the S max into the ocean interior. This process is also observed in Argo float profiles and is related to the formation regions of subtropical mode waters.

Full access