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William G. Large, Gokhan Danabasoglu, James C. McWilliams, Peter R. Gent, and Frank O. Bryan

Abstract

Horizontal momentum flux in a global ocean climate model is formulated as an anisotropic viscosity with two spatially varying coefficients. This friction can be made purely dissipative, does not produce unphysical torques, and satisfies the symmetry conditions required of the Reynolds stress tensor. The two primary design criteria are to have viscosity at values appropriate for the parameterization of missing mesoscale eddies wherever possible and to use other values only where required by the numerics. These other viscosities control numerical noise from advection and generate western boundary currents that are wide enough to be resolved by the coarse grid of the model. Noise on the model gridscale is tolerated provided its amplitude is less than about 0.05 cm s−1. Parameter tuning is minimized by applying physical and numerical principles. The potential value of this line of model development is demonstrated by comparison with equatorial ocean observations.

In particular, the goal of producing model equatorial ocean currents comparable to observations was achieved in the Pacific Ocean. The Equatorial Undercurrent reaches a maximum magnitude of nearly 100 cm s−1 in the annual mean. Also, the spatial distribution of near-surface currents compares favorably with observations from the Global Drifter Program. The exceptions are off the equator; in the model the North Equatorial Countercurrent is improved, but still too weak, and the northward flow along the coast of South America may be too shallow. Equatorial Pacific upwelling has a realistic pattern and its magnitude is of the same order as diagnostic model estimates. The necessary ingredients to achieve these results are wind forcing based on satellite scatterometry, a background vertical viscosity no greater than about 1 cm2 s−1, and a mesoscale eddy viscosity of order 1000 m2 s−1 acting on meridional shear of zonal momentum. Model resolution is not critical, provided these three elements remain unaltered. Thus, if the scatterometer winds are accurate, the model results are consistent with observational estimates of these two coefficients. These winds have larger westward stress than NCEP reanalysis winds, produce a 14% stronger EUC, more upwelling, but a weaker westward surface flow.

In the Indian Ocean the seasonal cycle of equatorial currents does not appear to be overly attenuated by the horizontal viscosity, with differences from observations attributable to interannual variability. However, in the Atlantic, the numerics still require too large a meridional viscosity over too much of the basin, and a zonal resolution approaching 1° may be necessary to match observations. Because of this viscosity, increasing the background vertical viscosity slowed the westward surface current; opposite to the response in the Pacific.

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Anna-Lena Deppenmeier, Frank O. Bryan, William S. Kessler, and LuAnne Thompson

Abstract

The tropical Pacific Ocean cold tongue (CT) plays a major role in the global climate system. The strength of the CT sets the zonal temperature gradient in the Pacific that couples with the atmospheric Walker circulation. This coupling is an essential component of the El Niño–Southern Oscillation (ENSO). The CT is supplied with cold water by the Equatorial Undercurrent that follows the thermocline as it shoals toward the east, adiabatically transporting cold water toward the surface. As the thermocline shoals, its water is transformed through diabatic processes, producing water mass transformation (WMT) that allows water to cross mean isotherms. Here, we examine WMT in the cold-tongue region from a global high-resolution ocean simulation with saved budget terms that close its heat budget exactly. Using the terms of the heat budget, we quantify each individual component of WMT (vertical mixing, horizontal mixing, eddy fluxes, and solar penetration) and find that vertical mixing is the single most important contribution in the thermocline and solar heating dominates close to the surface. Horizontal diffusion is much smaller. During El Niño events, vertical mixing, and hence cross-isothermal flow as a whole, are much reduced, whereas, during La Niña periods, strong vertical mixing leads to strong WMT, thereby cooling the surface. This analysis demonstrates the enhancement of diabatic processes during cold events, which in turn enhances cooling of the CT from below the surface.

Open access
Richard D. Smith, Mathew E. Maltrud, Frank O. Bryan, and Matthew W. Hecht

Abstract

In this paper an initial analysis of an 0.1° simulation of the North Atlantic Ocean using a level-coordinate ocean general circulation model forced with realistic winds covering the period 1985–96 is presented. Results are compared to the North Atlantic sector of a global 0.28° simulation with similar surface forcing and to a variety of satellite and in situ observations. The simulation shows substantial improvements in both the eddy variability and the time-mean circulation compared to previous eddy-permitting simulations with resolutions in the range of 1/2°–1/6°. The resolution is finer than the zonal-mean first baroclinic mode Rossby radius at all latitudes, and the model appears to be capturing the bulk of the spectrum of mesoscale energy. The eddy kinetic energy constitutes 70% of the total basin-averaged kinetic energy. Model results agree well with observations of the magnitude and geographical distribution of eddy kinetic energy and sea-surface height variability, with the wavenumber–frequency spectrum of surface height anomalies in the Gulf Stream, with estimates of the eddy length scale as a function of latitude, and with measurements of eddy kinetic energy as a function of depth in the eastern basin. The mean circulation also shows significant improvements compared to previous models, although there are notable remaining discrepancies with observations in some areas. The Gulf Stream separates at Cape Hatteras, and its speed and cross-stream structure are in good agreement with current meter data; however, its path is somewhat too far south and its meander envelope too broad to the west of the New England Seamounts. The North Atlantic Current is remarkably well simulated in the model: it exhibits meanders and troughs in its time-mean path that agree with similar structures seen in float data, although the separation of this current in the region of the “Northwest Corner” is displaced somewhat too far to the northwest. The Azores Current appears in the simulation, perhaps for the first time in a basin-scale model, and its position, total transport, and eddy variability are consistent with observational estimates.

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R. Justin Small, Frank O. Bryan, Stuart P. Bishop, and Robert A. Tomas

Abstract

A traditional view is that the ocean outside of the tropics responds passively to atmosphere forcing, which implies that air–sea heat fluxes are mainly driven by atmosphere variability. This paper tests this viewpoint using state-of-the-art air–sea turbulent heat flux observational analyses and a climate model run at different resolutions. It is found that in midlatitude ocean frontal zones the variability of air–sea heat fluxes is not predominantly driven by the atmosphere variations but instead is forced by sea surface temperature (SST) variations arising from intrinsic oceanic variability. Meanwhile in most of the tropics and subtropics wind is the dominant driver of heat flux variability, and atmosphere humidity is mainly important in higher latitudes. The predominance of ocean forcing of heat fluxes found in frontal regions occurs on scales of around 700 km or less. Spatially smoothing the data to larger scales results in the traditional atmosphere-driving case, while filtering to retain only small scales of 5° or less leads to ocean forcing of heat fluxes over most of the globe. All observational analyses examined (1° OAFlux; 0.25° J-OFURO3; 0.25° SeaFlux) show this general behavior. A standard resolution (1°) climate model fails to reproduce the midlatitude, small-scale ocean forcing of heat flux: refining the ocean grid to resolve eddies (0.1°) gives a more realistic representation of ocean forcing but the variability of both SST and of heat flux is too high compared to observational analyses.

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Claus W. Böning, William R. Holland, Frank O. Bryan, Gokhan Danabasoglu, and James C. Mcwilliams

Abstract

Many models of the large-scale thermohaline circulation in the ocean exhibit strong zonally integrated upwelling in the midlatitude North Atlantic that significantly decreases the amount of deep water that is carried from the formation regions in the subpolar North Atlantic toward low latitudes and across the equator. In an analysis of results from the Community Modeling Effort using a suite of models with different horizontal resolution, wind and thermohaline forcing, and mixing parameters, it is shown that the upwelling is always concentrated in the western boundary layer between roughly 30° and 40°N. The vertical transport across 1000 m appears to be controlled by local dynamics and strongly depends on the horizontal resolution and mixing parameters of the model. It is suggested that in models with a realistic deep-water formation rate in the subpolar North Atlantic, the excessive upwelling can be considered as the prime reason for the typically too low meridional overturning rates and northward heat transports in the subtropical North Atlantic. A new isopycnal advection and mixing parameterization of tracer transports by mesoscale eddies yield substantial improvements in these integral measures of the circulation.

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Xiaomin Chen, George H. Bryan, Andrew Hazelton, Frank D. Marks, and Pat Fitzpatrick

Abstract

Accurately representing boundary layer turbulent processes in numerical models is critical to improve tropical cyclone forecasts. A new turbulence kinetic energy (TKE)-based moist eddy-diffusivity mass-flux (EDMF-TKE) planetary boundary layer scheme has been implemented in NOAA’s Hurricane Analysis and Forecast System (HAFS). This study evaluates EDMF-TKE in hurricane conditions based on a recently developed framework using large-eddy simulation (LES). Single-column modeling tests indicate that EDMF-TKE produces much greater TKE values below 500-m height than LES benchmark runs in different high-wind conditions. To improve these results, two parameters in the TKE scheme were modified to ensure a match between the PBL and surface-layer parameterizations. Additional improvements were made by reducing the maximum allowable mixing length to 40 m based on LES and observations, by adopting a different definition of boundary layer height, and by reducing nonlocal mass fluxes in high-wind conditions. With these modifications, the profiles of TKE, eddy viscosity, and winds compare much better with LES results. Three-dimensional idealized simulations and an ensemble of HAFS forecasts of Hurricane Michael (2018) consistently show that the modified EDMF-TKE tends to produce a stronger vortex with a smaller radius of maximum wind than the original EDMF-TKE, while the radius of gale-force wind is unaffected. The modified EDMF-TKE code produces smaller eddy viscosity within the boundary layer compared to the original code, which contributes to stronger inflow, especially within the annulus of 1–3 times the radius of maximum wind. The modified EDMF-TKE shows promise to improve forecast skill of rapid intensification in sheared environments.

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Anna-Lena Deppenmeier, Frank O. Bryan, LuAnne Thompson, and William S. Kessler

Abstract

The equatorial Pacific zonal circulation is composed of westward surface currents, the eastward equatorial undercurrent (EUC) along the thermocline, and upwelling in the eastern cold tongue. Part of this upwelling arises from water flowing along isotherms sloping up to the east, but it also includes water mass transformation and consequent diabatic (cross-isothermal) flow (w ci) that is a key element of surface-to-thermocline communication. In this study we investigate the mean seasonal cycle and subseasonal variability of cross-isothermal flow in the cold tongue using heat budget output from a high resolution forced ocean model. Diabatic upwelling is present throughout the year with surface-layer solar penetration driven diabatic upwelling strongest in boreal spring, and vertical mixing in the thermocline dominating during the rest of the year. The former constitutes warming of the surface layer by solar radiation rather than exchange of thermal energy between water parcels. The mixing driven regime allows heat to be transferred to the core of the EUC by warming parcels at depth. On subseasonal timescales the passage of tropical instability waves (TIWs) enhances diabatic upwelling on and north of the equator. On the equator the TIWs enhance vertical shear and induce vertical mixing driven diabatic upwelling, while off the equator TIWs enhance the sub-5-daily eddy heat flux which enhances diabatic upwelling. Comparing the magnitudes of TIW, seasonal, and interannual w ci variability, we conclude that each timescale is associated with sizeable variance. Variability across all of these timescales needs to be taken into account when modeling or diagnosing the effects of mixing on equatorial upwelling.

Open access
Frank O. Bryan, Gokhan Danabasoglu, Norikazu Nakashiki, Yoshikatsu Yoshida, Dong-Hoon Kim, Junichi Tsutsui, and Scott C. Doney

Abstract

The response of the North Atlantic thermohaline circulation to idealized climate forcing of 1% per year compound increase in CO2 is examined in three configurations of the Community Climate System Model version 3 that differ in their component model resolutions. The strength of the Atlantic overturning circulation declines at a rate of 22%–26% of the corresponding control experiment maximum overturning per century in response to the increase in CO2. The mean meridional overturning and its variability on decadal time scales in the control experiments, the rate of decrease in the transient forcing experiments, and the rate of recovery in periods of CO2 stabilization all increase with increasing component model resolution. By examining the changes in ocean surface forcing with increasing CO2 in the framework of the water-mass transformation function, we show that the decline in the overturning is driven by decreasing density of the subpolar North Atlantic due to increasing surface heat fluxes. While there is an intensification of the hydrologic cycle in response to increasing CO2, the net effect of changes in surface freshwater fluxes on those density classes that are involved in deep-water formation is to increase their density; that is, changes in surface freshwater fluxes act to maintain a stronger overturning circulation. The differences in the control experiment overturning strength and the response to increasing CO2 are well predicted by the corresponding differences in the water-mass transformation rate. Reduction of meridional heat transport and enhancement of meridional salt transport from mid- to high latitudes with increasing CO2 also act to strengthen the overturning circulation. Analysis of the trends in an ideal age tracer provides a direct measure of changes in ocean ventilation time scale in response to increasing CO2. In the subpolar North Atlantic south of the Greenland–Scotland ridge system, there is a significant increase in subsurface ages as open-ocean deep convection is diminished and ventilation switches to a predominance of overflow waters. In middle and low latitudes there is a decrease in age within and just below the thermocline in response to a decrease in the upwelling of old deep waters. However, when considering ventilation within isopycnal layers, age increases for layers in and below the thermocline due to the deepening of isopycnals in response to global warming.

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Peter R. Gent, Frank O. Bryan, Gokhan Danabasoglu, Keith Lindsay, Daisuke Tsumune, Matthew W. Hecht, and Scott C. Doney

Abstract

An ensemble of nine simulations for the climate of the twentieth century has been run using the Community Climate System Model version 3 (CCSM3). Three of these runs also simulate the uptake of chlorofluorocarbon-11 (CFC-11) into the ocean using the protocol from the Ocean Carbon Model Intercomparison Project (OCMIP). Comparison with ocean observations taken between 1980 and 2000 shows that the global CFC-11 uptake is simulated very well. However, there are regional biases, and these are used to identify where too much deep-water formation is occurring in the CCSM3. The differences between the three runs simulating CFC-11 uptake are also briefly documented.

The variability in ocean heat content in the 1870 control runs is shown to be only a little smaller than estimates using ocean observations. The ocean heat uptake between 1957 and 1996 in the ensemble is compared to the recent observational estimates of the secular trend. The trend in ocean heat uptake is considerably larger than the natural variability in the 1870 control runs. The heat uptake down to 300 m between 1957 and 1996 varies by a factor of 2 across the ensemble. Some possible reasons for this large spread are discussed. There is much less spread in the heat uptake down to 3 km. On average, the CCSM3 twentieth-century ensemble runs take up 25% more heat than the recent estimate from ocean observations. Possible explanations for this are that the model heat uptake is calculated over the whole ocean, and not just in the regions where there are many observations and that there is no parameterization of the indirect effects of aerosols in CCSM3.

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Frank O. Bryan, Robert Tomas, John M. Dennis, Dudley B. Chelton, Norman G. Loeb, and Julie L. McClean

Abstract

The emerging picture of frontal scale air–sea interaction derived from high-resolution satellite observations of surface winds and sea surface temperature (SST) provides a unique opportunity to test the fidelity of high-resolution coupled climate simulations. Initial analysis of the output of a suite of Community Climate System Model (CCSM) experiments indicates that characteristics of frontal scale ocean–atmosphere interaction, such as the positive correlation between SST and surface wind stress, are realistically captured only when the ocean component is eddy resolving. The strength of the coupling between SST and surface stress is weaker than observed, however, as has been found previously for numerical weather prediction models and other coupled climate models. The results are similar when the atmospheric component model grid resolution is doubled from 0.5° to 0.25°, an indication that shortcomings in the representation of subgrid scale atmospheric planetary boundary layer processes, rather than resolved scale processes, are responsible for the weakness of the coupling. In the coupled model solutions the response to mesoscale SST features is strongest in the atmospheric boundary layer, but there is a deeper reaching response of the atmospheric circulation apparent in free tropospheric clouds. This simulated response is shown to be consistent with satellite estimates of the relationship between mesoscale SST and all-sky albedo.

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