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W. G. Large

Abstract

The climatological heat and salt budgets at OWS Papa are examined over the years 1960 to 1981. The average surface fluxes of heat and freshwater are estimated to be 25±15 W m−2 and 4.4±4.3 mg m−2 s−1, respectively. Year to year changes in these fluxes are found to be uncorrelated with changes in ocean heat and salt content above 200-m depth. Different hypotheses of how the surface fluxes are balanced are tested using a one-dimensional model of vertical mixing with prescribed horizontal and vertical heat and salt transport. The surface freshwater flux must be at least 3 mg m−2 s−1, on average, in order to balance model-computed vertical diffusion. Additional surface input is balanced by the vertical advection of salt from below. Other processes are required to balance the heat budget. Assuming that the surface flux estimates are in error and correcting them to give zero net fluxes of both heat and freshwater leads to a systematic erosion of the main pycnocline and deepening of winter mixed layers, such that the surface waters become too cold and too salty. These trends are reduced but not eliminated by including a steady vertical advection due to Ekman pumping and accounting for vertical diffusion in the salt and heat budgets. Balancing the remaining heat by horizontal advection throughout the water column results in a local minimum of too cold water at about 165-m depth. However, acceptable long-term simulations are achieved if the required cold water is transported into the seasonal thermocline and isothermal layer only during the fall and winter months. Observations supporting this scenario are reviewed. Model sensitivity experiments with this balance show which combinations of surface heat and freshwater fluxes produce the observed average amplitude of the annual cycle of sea surface temperature while achieving a realistic salt balance. Coupling the model by using its upper-layer temperature in the surface flux calculations greatly improves the simulators compared to observations and only changes the average heat flux by 1 W m−2. It is concluded that using constant surface flux corrections does not properly account for oceanic transport deficiencies that are not both concentrated in the mixed layer and steady throughout the year.

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W. G. Large and G. Danabasoglu

Abstract

The largest and potentially most important ocean near-surface biases are examined in the Community Climate System Model coupled simulation of present-day conditions. They are attributed to problems in the component models of the ocean or atmosphere, or both. Tropical biases in sea surface salinity (SSS) are associated with precipitation errors, with the most striking being a band of excess rainfall across the South Pacific at about 8°S. Cooler-than-observed equatorial Pacific sea surface temperature (SST) is necessary to control a potentially catastrophic positive feedback, involving precipitation along the equator. The strength of the wind-driven gyres and interbasin exchange is in reasonable agreement with observations, despite the generally too strong near-surface winds. However, the winds drive far too much transport through Drake Passage [>190 Sv (1 Sv ≡ 106 m3 s−1)], but with little effect on SST and SSS. Problems with the width, separation, and location of western boundary currents and their extensions create large correlated SST and SSS biases in midlatitudes. Ocean model deficiencies are suspected because similar signals are seen in uncoupled ocean solutions, but there is no evidence of serious remote impacts. The seasonal cycles of SST and winds in the equatorial Pacific are not well represented, and numerical experiments suggest that these problems are initiated by the coupling of either or both wind components. The largest mean SST biases develop along the eastern boundaries of subtropical gyres, and the overall coupled model response is found to be linear. In the South Atlantic, surface currents advect these biases across much of the tropical basin. Significant precipitation responses are found both in the northwest Indian Ocean, and locally where the net result is the loss of an identifiable Atlantic intertropical convergence zone, which can be regained by controlling the coastal temperatures and salinities. Biases off South America and Baja California are shown to significantly degrade precipitation across the Pacific, subsurface ocean properties on both sides of the equator, and the seasonal cycle of equatorial SST in the eastern Pacific. These signals extend beyond the reach of surface currents, so connections via the atmosphere and subsurface ocean are implicated. Other experimental results indicate that the local atmospheric forcing is only part of the problem along eastern boundaries, with the representation of ocean upwelling another likely contributor.

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W. G. Large and S. Pond

Abstract

Measurements of the momentum flux were made by the Reynolds flux and dissipation methods on a deep water stable tower operated by the Bedford Institute of Oceanography, A modified Gill propeller-vane anemometer was used to measure the velocity. Drag coefficients from 196 Reynolds flux measurements agree well with those reported in Smith (1980) based on independent observations at the same site. Based on 192 runs, a comparison of the dissipation and Reynolds flux results shows excellent agreement on average, for wind speeds from 4 to 20 m s−1. The much more extensive dissipation data set (1086 h from the tower and 505 h from the weathership PAPA, CCGS Quadra) was used to investigate the dependence of the drag coefficient on wind speed, fetch and stability. The drag coefficient reduced to 10 m height and neutral conditions (CDN), is independent of stability and fetch (for fetch/height ≳800) but increases with wind speed above 10 m s−1. Some time series of the momentum flux and drag coefficient are shown to demonstrate additional sources of variation in the drag coefficient. CDN is observed to be smaller, on average. during rising winds than during failing winds or after a change in wind direction. Based on our results and many deep water results of others, we obtainwhere U10 is the wind speed at a height of 10 m. A method for calculating the stress from this CDN and observations of wind speed and surface minus air temperature at heights other than 10 m is also given.

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W. G. Large and S. Pond

Abstract

This papar presents an extensive act of sensible heat (Reynolds flux and dissipation methods) and latent heat (dissipation method) flux measurements from a stable deep water tower and from ships on the deep sea. Operational difficulties associated with ship spray and flow distortion and with sensor calibration, response and contamination are discussed. The influence of atmospheric stability on the dissipation measurements and the bulk transfer coefficients is considered and a parameterization of Z/ L in terms of wind speed and the sea-air potential temperature difference is found to be adequate. Temperature variances, Stanton numbers and wt cospectra from the Roynolds flux measurements are compared to previous results.

The dissipation method is shown to be a viable means of measuring the heal fluxes over the deep sea by comparison with simultaneous Reynolds flux measurements, using our data for the sensible heat and the data of others for the latent heat. The neutral drag coefficient at 10 m height, CDN, because it is relatively well established, is used to check the performance of the shipboard measurements The dissipation sensible and latent heat fluxes are well described, on average, by the neutral transfer coefficients at 10 m height, CTN and CEN, respectively:Previously published results are considered, indicating that 103 CTN = 0.75 may be preferable in stable conditions Some data suggest a slight wind-speed dependency above 10 m s−1, which is mostly accounted for with CTN and CEN proportional to CDN½, as implied by constant roughness lengths

A bulk aerodynamic method of estimating the heat fluxes from CDN, CTN and CEN, wind speed, sea temperature, and air temperature and humidity is described and compared to time series of the dissipation method boat fluxes. Potential problem with the data are discussed using the time series.

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G. B. Crawford and W. G. Large

Abstract

A one-dimensional model of upper-ocean vertical mixing is used to investigate the ocean's response to idealized atmospheric storms over short (1–2 day) timescales. Initial ocean conditions are based on observations from the northeast Pacific. When the wind rotation is resonant at the inertial frequency, the surface input of kinetic energy to the currents, KE0, is maximized, as are the net changes in inertial kinetic energy, potential energy, and sea surface temperature. The KE0 is a key air–sea interaction parameter because of its strong dependence on the time histories of the wind forcing and surface current, and because some of this kinetic energy input can go to increasing potential energy when dissipated in regions of large buoyancy gradients below the mixed layer. Energy input and the ocean response are rapidly reduced for less inertial winds, indicating that the upper ocean has highly tuned inertial resonant responses. The degree of tuning is highest for the inertial kinetic energy response, followed by KE0 input, the potential energy, and temperature responses.

For storms of varying strength, duration, shape, and wind rotation, about 20% of the final inertial current energy is found beneath the mixed layer, regardless of the stratification. The magnitude of inertial current response depends on KE0 and wind rotation, but not stratification, and is approximately 0.532 KE0[1–e −2.81], where Γ is a function of wind rotation that varies from 1 for purely inertial winds to 0 for winds with no energy at the inertial frequency. Changes in potential energy and surface temperature depend mainly on KE0 and stratification, but not systematically on wind rotation other than as accounted for in KE0. Initial currents can modulate KE0 and the responses significantly; the modulation varies roughly linearly with initial current speed, consistent with a simple scale analysis. Modulation of each measure of ocean response is similar, so that there is little effect on general relationships formed by normalizing the responses with KE0, except for certain special phase relationships between the initial current direction and wind direction. Parameterizations of KE0 and of the mechanical production of turbulent kinetic energy should include both wind speed (or friction velocity) and rotation of the wind.

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W. G. Large and G. B. Crawford

Abstract

The Ocean Storms dataset is used to compile observations of the oceanic response to midlatitude storms. Of particular interest are episodic mixed layer temperature cooling events whose characteristics are reviewed. The data include subsurface temperatures from drifting thermistor chains, mixed layer temperature and velocity from mixed layer drifters, conductivity-temperature-depth profiles, and radiation measurements from ships, and the surface meteorological parameters produced by the European Centre for Medium-Range Weather Forecasts. A method for processing irregular drifting buoy position fixes to yield estimates of the geostrophic, ageostrophic, and inertial mixed layer currents is developed and shown to yield residuals that can mostly be attributed to errors in the positioning. From these currents the ocean's dynamic responses, namely, the change in mixed layer inertial kinetic energy and the ageostrophic particle displacement, are computed. The process of removing horizontal and vertical advection and surface heating from potential energy and mixed layer temperature responses is described. Temperature change responses are shown to be related to inertial current generation. Large responses. including episodic cooling, are found to be forced not necessarily by large storms, but by storms whose wind stress vector rotates inertially. The observations suggest that the phase of preexisting inertial currents may modulate the responses. The spatial scale of response to one particular storm is found to be about 150 km.

The compiled dataset is also used to provide the initial conditions and the surface forcing required to run three one-dimensional numerical models of ocean vertical mixing. All three models are shown to qualitatively exhibit the observed behavior, including episodic cooling. Quantitatively, all the models predict the dynamic responses well, considering the uncertainty in the wind stress forcing. However, one model, a nonlocal K-profile parameterization of the oceanic boundary layer, is found to be somewhat better in reproducing the observed vertical profile of temperature change. This model's success is due to its more realistic exchange of mixed layer water with water from much deeper in the thermocline. In particular, the deepest extent of this exchange is accurately observed and well simulated.

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W. G. Large and S. G. Yeager

Abstract

Global satellite observations show the sea surface temperature (SST) increasing since the 1970s in all ocean basins, while the net air–sea heat flux Q decreases. Over the period 1984–2006 the global changes are 0.28°C in SST and −9.1 W m−2 in Q, giving an effective air–sea coupling coefficient of −32 W m−2 °C−1. The global response in Q expected from SST alone is determined to be −12.9 W m−2, and the global distribution of the associated coupling coefficient is shown. Typically, about one-half (6.8 W m−2) of this SST effect on heat flux is compensated by changes in the overlying near-surface atmosphere. Slab Ocean Models (SOMs) assume that ocean heating processes do not change from year to year so that a constant annual heat flux would maintain a linear trend in annual SST. However, the necessary 6.1 W m−2 increase is not found in the downwelling longwave and shortwave fluxes, which combined show a −3 W m−2 decrease. The SOM assumptions are revisited to determine the most likely source of the inconsistency with observations of (−12.9 + 6.8 − 3) = −9.1 W m−2. The indirect inference is that diminished ocean cooling due to vertical ocean processes played an important role in sustaining the observed positive trend in global SST from 1984 through 2006, despite the decrease in global surface heat flux. A similar situation is found in the individual basins, though magnitudes differ. A conclusion is that natural variability, rather than long-term climate change, dominates the SST and heat flux changes over this 23-yr period. On shorter time scales the relationship between SST and heat flux exhibits a variety of behaviors.

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W. Timothy Liu and W. G. Large

Abstract

The values of sea surface stress determined with the dissipation method and those determined with a surface-layer model from observations on F.S. Meteor during the Joint Air-Sea Interaction (JASIN) Experiment are compared with the backscatter coefficients measured by the scatterometer SASS on the satellite Seasat. This study demonstrates that SASS can be used to determine surface stress directly as well as wind speed. The quality of the surface observations used in the calibration of the retrieval algorithms, however, is important. This sample of measurements disagrees with the predictions by the existing wind retrieval algorithm under non-neutral conditions and the discrepancies depend on atmospheric stability.

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W. G. Large and H. Van Loon

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The surface response of the Southern Hemisphere's oceans to the large spatial scale, interseasonal changes in wind forcing during the FGGE year of 1979 is investigated. The primary data are the analyzed daily wind fields, and the trajectories of the FGGE drifting buoy array. The zonal wind forcing is characterized by large spatial patterns of low frequency (annual and semiannual) variability. particular attention is paid to the second harmonic, which has amplitude peaks at 35°–40° S with solstitial maxima, and amplitude peaks at 60°S with equinoctial maxima. The distinct phase change occurs at 50°S.

The analysis of the drifting buoy data is guided by the wind patterns, but first the question of the current-following characteristics of the FGGE buoys is addressed. Compared to the wind, the buoy drift has even larger spatial scales, and more low frequency contributions to its intra-annual variance. Like the wind, amplitude peaks in the second harmonic of monthly mean zonal drift are found in each ocean basin sector at 40 ± 5° S and at 55° −60° S, with a phase change at about 50° S. These wind and drift patterns extend from 30°S to antarctica, and so encompass the entire antarctic Circumpolar Current (ACC) and the poleward halves of the subtropical gyres.

The results are discussed in relation to Southern Ocean dynamics and previous studies. A simple barotropic calculation shows that interseasonal changes in buoy drifts are small enough relative to the wind forcing that neither baroclinic surface enhancement nor slip error need be invoked to explain them. Latitudinal shear in zonal drift is shown to have a great deal of temporal variability implying momentum transports across the ACC, to the center or from the center of the ACC, depending on the time of year. The observed buoy drift is consistent with the view of the ACC consisting of multiple narrow cores. Furthermore, it suggests that as the latitude of the peak in zonal wind shifts with the half-year waves, different underlying cores of the ACC are accelerated to be the one with the greatest velocity. The Seasat satellite altimetric results are interpreted as capturing a half-cycle of the second harmonic, and as showing a phase change in zonal geostrophic flow at about 50°S. A second harmonic with equinoctial maxima is found in the 500 m depth pressure difference across the Drake Passage, although we find that this area is not very representative of the ACC as a whole.

We propose that the semiannual signals in the winds and surface currents should be important diagnostics in coupled ocean-atmosphere models of the Southern Ocean. This wave is, however, faithfully represented only in products from daily analyzed pressure fields and in their climatological analyses, but not in atmospheric general circulation models nor in wind climatologies based on ship observatons.

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W. G. Large and J. A. Businger

Abstract

The DISSTRESS system for remote measurements of the surface wind stress over the ocean from ships and buoys is described. It is fully digital, utilizing the inertial dissipation technique. Parallel processing allows anemometer data to be filtered in natural frequency space; that is, the fitter cutoffs shift linearly with the mean wind speed of the data to be filtered. The construction of the digital Butterworth bandpass filters is presented in detail.

The performance of the system is evaluated by analyzing the results from 28 days of operation during FASINEX. The mean wind speed is checked, the anemometer response function is established, and drag coefficients are compared to previous studies. The capability of the system is demonstrated by continuous time series of the friction velocity computed every 20 min. The conclusion is that the surface wind stress can be measured more reliably and accurately (20%) with this system than from anemometer wind speeds and a bulk formula.

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