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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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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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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. 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

Abstract

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, J. Morzel, and G. B. Crawford

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Marine wind measurements at three heights (3.0,4.5, and 5.0 m) from both moored and drifting buoys during the Ocean Storms Experiment are described. These winds are compared with each other, with winds from ships, from subsurface ambient acoustic noise, and from the analyses of three numerical weather prediction centers. In the mean, wind directions generally differ by only a small constant offset of a few degrees. No wave influence on the wind direction is evident, because the differences are not systematic and, with few exceptions, they are less than the expected error. After correcting for some apparent calibration and instrument bias, the Ocean Storms wind speeds display similar behavior when compared to the analyzed wind products. There is excellent agreement up to a transition wind speed between 7 and 10 m s−1, above which all the measured winds tend to be relatively low. The transition speed is found to increase with anemometer height, so this behavior is interpreted as being due to the distortion of the wind profile by surface waves. The wave effects are shown to be profound. By increasing the stress by 40% or more in high winds, the corrections are shown to be essential for numerical models to simulate the oceanic response to storm events. The Ocean Storms corrections are used to construct functions describing wave influence on both the vertical wind shear and the mean wind speed profile. These functions can only be regarded as crude approximations because the Ocean Storms data are far from ideal for determining them. However, they can be used to assess potential influences of surface waves on any low-level wind measurement.

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W. G. Large, W. R. Holland, and J. C. Evans

Abstract

A one-degree, flat bottom, eight-layer quasi-geostrophic model of the North Pacific Ocean is forced by six different wind stress curl datasets, all derived from seven years of twice daily analyses at the European Centre for Medium Range Weather Forecasts, 1980–86. The six datasets, with nominal averaging times of 1, 2, 3, 7, 14, and 30 days, are obtained by carefully filtering in the frequency domain. This filtering greatly reduces the variance, typically by 90% for 30-day averaging, because the wind stress curl spectrum is nearly white in frequency. It also smoothes spatially, reducing high wavenumber variance to a greater degree than variance near wavenumber zero. The climatology of the ECMWF wind stress curl does not show any unexpected differences from climatologies based on historical marine wind observations. The wind stress curl is neither temporally nor spatially stationary with the high frequency variance being much larger during the winter season and over the northern half of the North Pacific. Its spectrum does not appear to be isotropic in wavenumber.

From a common initial state, the baroclinic fields in the model ocean runs evolve nearly identically regardless of the forcing bemuse their frequencies are all lower than the Nyquist frequency of even 30-day sampling. The higher frequency forcing generates Rossby waves that dominate the instantaneous barotropic stream function throughout the basin. These barotropic waves are not found at frequencies above the Nyquist frequency of the forcing. There is negligible rectification into basin scale, six year mean flows. There are only small scale differences in mean monthly barotropic streamfunction fields. Thus, the barotropic ocean response diminishes as the nominal averaging time increases. Furthermore, these Rossby waves appear to be natural modes of the model basin, and they could be artificially forced unless the wind data processing carefully avoids aliasing unresolved frequencies. Overall the spectrum of ocean response is found to be more red in frequency than the nearly white wind curl spectrum and even more red in wavenumber.

High frequency forcing produces higher kinetic energies at all depths with an annual cycle that is related to the annual cycle of the high frequency variance in the wind stress curl. The deep kinetic energy and the intra-annual streamfunction variance are used to quantify the relative importance of the high frequency barotropic Rossby waves. There is considerable advantage in using 3-day average (over 7- and 14-day average) wind forcing, however, there is little more to be gained with 2-day averaging and nothing further added by 1-day averaging. When forced with 30-day averaged wind curls, the intra-annual stream function variance is typically only 30% of its value when forced with 3-day or shorter averages.

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W. G. Large, J. C. McWilliams, and P. P. Niiler

Abstract

CASID free-drifting thermistor chain buoys that utilized Service ARGOS positioning and data collection were deployed in the northeast Pacific Ocean in the vicinity of OWS-P in late autumn in both 1980 and 1981 as part of the Storm Transfer and Response Experiment (STREX). It is argued that because of the large drag on their 120–125 m lines, CASID buoy drift is tightly coupled to currents. The response function of buoy motion and line shape to a two-dimensional current profile is determined, and an inversion technique is developed to infer relative flow past the buoy. In the mixed layer 6 cm s−1 errors in the inferred horizontal flow are acceptable, because advective temperature changes in the drifting CASID frame of reference are small. They are not acceptable in the thermocline where advection is large. These advective effects are removed from observed subinertial thermal evolution and the result compared to the effects of vertical heat redistribution processes and of surface heat flux, estimated from STREX synoptic analyses of air-sea interaction parameters.

A number of processes are responsible for the late autumn mixed-layer temperature change over both 50 days and a 1–2 day storm period. A 50-day SST change of −13.2°C following the mixed-layer flow and averaged over a three-buoy array is due to surface cooling (−0.04°C), entrainment (−1.1°C, of which −0.8°C is due to mixed layer deepening), and vertical mixing or diffusion (−1.5°C). Of the latter, −1.4°C occurs episodically in response to some, but not all storms, and the resulting thermocline heating appears clearly in a composite of twenty cooling/mixing events. The SST cooling and the distinctive heating pattern in the seasonal thermocline imply vertical diffusivities greater than 10 × 10−4 m2 s−1 at the base of the mixed layer and about 4 × 10−4 m2 s−1 in the lower two-thirds of the thermocline. When such enhanced diffusion is accounted for, the imbalance in the mixed layer heat budget (−0.2 ± 0.8°C) is well within measurement uncertainty. Enhanced diffusion is even more important in the 1–2 day episodic cooling response to a storm. Averaged over nine such events, it accounts for 63% of the −0.41°C of SST cooling at a CASID buoy. Only about 10% and 13% of the cooling is due to surface cooling and entrainment, respectively. Thus the mixed-layer heat budget imbalance on this time scale is only −0.06 (±0.10)°C. Over two to ten days there can be substantial horizontal advective heating or cooling, but these periods average to a small net effect on SST and do not appear to be necessarily associated with episodic cooling.

The sub-mixed layer estimates of subinertial flow and of temperature gradients indicate that most of the heat mixed vertically into the thermocline during episodic cooling is advected to the south and east. In both 1980 and 1981 the rate of change in heat content of the upper 120 m was about −140 W m−2, of which about −50 W m−2 was due to this flow. The processes of surface cooling, vertical advection and diffusion, and mixed layer advection each tend to cool too, but only at rates less than about 20 W m−2. Over 50 days of late autumn in both 1980 and 1981, the heat budget of 120 m has an imbalance of about −20 ± 36 W m−2.

A striking 50–200 km horizontal variability appears in the upper ocean response to a single storm. For two to four inertial periods following the onset of the storm, the magnitude of the inertial shear and low-frequency currents show a marked correspondence with the inferred strength of episodic SST cooling due to enhanced vertical diffusion. This correlation suggests a link between cooling and ocean dynamics, with strong vertical mixing between the thermocline and mixed layer receiving its energy locally from supercritical storm-driven currents. These currents depend largely on the preexisting current field whose horizontal scale determines the scale of episodic cooling. No such correspondence is found with either mixed layer depth, mixed layer deepening, the surface heat flux, or the surface wind forcing.

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D. A. Cherian, D. B. Whitt, R. M. Holmes, R.-C. Lien, S. D. Bachman, and W. G. Large

Abstract

The equatorial Pacific cold tongue is a site of large heat absorption by the ocean. This heat uptake is enhanced by a daily cycle of shear turbulence beneath the mixed layer—“deep-cycle turbulence”—that removes heat from the sea surface and deposits it in the upper flank of the Equatorial Undercurrent. Deep-cycle turbulence results when turbulence is triggered daily in sheared and stratified flow that is marginally stable (gradient Richardson number Ri ≈ 0.25). Deep-cycle turbulence has been observed on numerous occasions in the cold tongue at 0°, 140°W, and may be modulated by tropical instability waves (TIWs). Here we use a primitive equation regional simulation of the cold tongue to show that deep-cycle turbulence may also occur off the equator within TIW cold cusps where the flow is marginally stable. In the cold cusp, preexisting equatorial zonal shear u z is enhanced by horizontal vortex stretching near the equator, and subsequently modified by horizontal vortex tilting terms to generate meridional shear υ z off of the equator. Parameterized turbulence in the sheared flow of the cold cusp is triggered daily by the descent of the surface mixing layer associated with the weakening of the stabilizing surface buoyancy flux in the afternoon. Observational evidence for off-equatorial deep-cycle turbulence is restricted to a few CTD casts, which, when combined with shear from shipboard ADCP data, suggest the presence of marginally stable flow in TIW cold cusps. This study motivates further observational campaigns to characterize the modulation of deep-cycle turbulence by TIWs both on and off the equator.

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