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James J. Simpson and Clayton A. Paulson

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James J. Simpson and Clayton A. Paulson

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Observations of sea surface temperature and wave height were made from a large, manned spar buoy (R/P FLIP) ∼100 km off the coast of Baja California. Surface temperature was measured with a radiation thermometer which viewed a disc on the surface 12 cm in diameter. The instrument responded to frequencies up to 3 Hz. Wave height was measured with a resistance gage located close to the field of view of the radiometer.

Log-log plots of spectra of sea surface temperature exhibit a plateau between 0.05 and 0.5 Hz, followed by a rapid decrease in energy at frequencies >1 Hz. A coherence of 0.5 between waves and surface temperature occurs at the same frequency as the peak in the wave spectrum. Phase spectra show that warm temperatures associated with the thinning of the surface viscous layer occur systematically upwind of the crests of the dominant gravity waves and downwind of the crests of steeply sloping, shorter period gravity waves. The warm temperatures are hypothesized to be caused by enhanced wind stress upwind from the crests and by surface instability and surface convergence downwind from the crests.

The magnitude of the mean temperature difference between the surface and the warmer, well-mixed water below is estimated from the surface temperature record. It is assumed that the warmest surface temperatures observed are associated with thinning of the viscous layer and are representative of the well-mixed water below. The dimensionless constant in a formula due to Saunders (1967), which relates the temperature difference to wind stress and heat flux, is found to be seven.

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James J. Simpson and Clayton A. Paulson

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Mid-ocean observations (35°N, 155°W) of temperature and salinity were made from R/P Flip during the period 28 January-14 February 1974 as part of the NORPAX POLE Experiment.

Autocorrelations for the time series of depth of several σt surfaces confirm the presence of a semidiurnal internal tide whose amplitude is about 10 m. The period of 12.7 h determined from the autocorrelation analysis is not statistically significantly different from the period of the M2 semidiurnal tide (12.4 h). The coherence between pairs of time series of the depth of the σt surfaces is high, ranging from 0.97 to 0.91 at the frequency of the peak in the spectrum corresponding to the semi-diurnal tide. The coherence between a given σt surface and deeper lying surfaces decreases slowly with the mean separation between surfaces. The vertical coherence scale suggests that most of the energy of the semi-diurnal internal tide is in the low-order modes. The data show that the phase difference between surfaces increases with the mean separation between surfaces at the approximate rate of +35° (100 m). Estimates of the vertical and horizontal wavelengths of the observed semi-diurnal internal tide are 1 km and 35 km, respectively.

One-dimensional mixed-layer deepening models fail to predict the mixed-layer depths and temperatures observed during POLE. Horizontal advection, as evidenced from the salinity maximum frequently occurring at the bottom of the mixed layer and other near-surface changes in salinity and temperature not associated with local surface forcing, are responsible for the failure. During the one period in which the one-dimensional models may be applicable a value of the mixing energy flux coefficient m = 0.0017 was obtained.

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Clayton A. Paulson and James J. Simpson

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Observations were made of downward solar radiation as a function of depth during an experiment in the North Pacific (35°N, 155°W). The irradiance meter employed was sensitive to solar radiation of wavelength 400–1000 nm arriving from above at a horizontal surface. Because of selective absorption of the short and long wavelengths, the irradiance decreases much faster than exponential in the upper few meters, falling to one-third of the incident value between 2 and 3 m depth. Below 10 m the decrease was exponential at a rate characteristic of moderately clear water of Type IA. Neglecting one case having low sun altitude, the observations are well represented by the expression I/I 0=Re z/ζ1+(1−R)e 2, where I is the irradiance at depth −z, I 0 is the irradiance at the surface less reflected solar radiation, R=0.62, ζ1 and ζ2 are attenuation lengths equal to 1.5 and 20 m, respectively, and z is the vertical space coordinate, positive upward with the origin at mean sea level. The depth at which the irradiance falls to 10% of its surface value is nearly the same as observations of Secchi depth when cases with high wind speed or low solar altitude are neglected. Parameters R, ζ1, and ζ2 are computed for the entire range of oceanic water types.

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Murray D. Levine, Clayton A. Paulson, and James H. Morison

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A thermistor chain was moored below the pack ice from 50–150 m in the Arctic Ocean for five days in 1981. Oscillations in temperature are attributed to the vertical dispalcement of internal waves. The spectral shape of isotherm dispalcement is consistent with the Garrett-Munk model and other internal wave observations, but the spectral level is significantly lower. Other observations from the Arctic Ocean also exhibit lower internal-wave energy when compared with historical data from lower latitudes. The lower energy may be related to the unique generation and dissipation mechanisms present in the ice-covered Arctic Ocean. Significant peaks in vertical coherence occur at 0.81 and 2.6 cph. The peak at 2.6 cph coincides approximately with the high-frequency spectral cutoff near the local buoyancy frequency; this feature has been observed in many other internal wave experiments. The coherent oscillations at 0.81 cph exhibit a node in vertical dispalcement at 75–100 m. This is consistent with either the second, third or fourth vertical mode calculated from the mean buoyancy frequency profile. Evidence is presented which suggests that, contrary to the Garrett-Munk model, the frequency spectrum does not scale with the Coriolis parameter.

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Hongbo Qi, Roland A. De Szoeke, Clayton A. Paulson, and Charles C. Eriksen

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Current meter data from two sites were analyzed for near-inertial motions generated by storm during the ten-month period of the Ocean Storms Experiment in the northeast Pacific Ocean. The most striking feature of the inertial wave response to storms was the almost instantaneous generation of waves in the mixed layer, followed by a gradual propagation into the thermocline that often lasted many days after the initiation of the storm. The propagation of near-inertial waves generated by three storms in October, January, and March was studied by using group propagation theory based on the WKB approximation. It was found that wave frequencies were slightly superinertial, with inertial shifts 1%–3% in October and March and around 1% in January. The phase of near-inertial currents propagated upward below the mixed layer, confirming the downward radiation of energy by these waves. The average downward energy flux during the storm periods was between 0.5 and 2.8 mW m−2. The vertical wavelengths indicated by the vertical phase differences ranged from 150 to 1500 m. The vertical group velocity was estimated from the arrival times of the groups at successive depths. Using this in the dispersion relation, horizontal wavelengths ranging from 140 to 410 km were obtained. A relation between density and velocity that gives the horizontal directionality of internal waves was derived. During the storm periods examined, the propagation directions of near-inertial waves mainly lay between northeast and south, indicating sources west of moorings. The directions tended to rotate clockwise with increasing depth, consistent with the expected effect of the earth's curvature. The estimated horizontal wavelength and propagation direction were consistent with the horizontal phase difference between inertial currents at the two sites.

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Eric A. D'Asaro, Charles C. Eriksen, Murray D. Levine, Clayton A. paulson, Peter Niiler, and Pim Van Meurs

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A strong, isolated October storm generated 0.35–0.7 m s−1 inertia] frequency currents in the 40-m deep mixed layer of a 300 km×300 km region of the northeast Pacific Ocean. The authors describe the evolution of these currents and the background flow in which they evolve for nearly a month following the storm. Instruments included CTD profilers, 36 surface drifters, an array of 7 moorings, and air-deployed velocity profilers. The authors then test whether the theory of linear internal waves propagating in a homogeneous ocean can explain the observed evolution of the inertial frequency currents.

The subinertial frequency flow is weak, with typical currents of 5 cm s−1, and steady over the period of interest. The storm generates inertial frequency currents in and somewhat below the mixed layer with a horizontal scale much larger than the Rossby radius of deformation, reflecting the large-scale and rapid translation speed of the storm. This scale is too large for significant linear propagation of the inertial currents to occur. It steadily decreases owing to the latitudinal variation in f, that is, β, until after about 10 days it becomes sufficiently small for wave propagation to occur. Inertial energy then spreads downward from the mixed layer, decreasing the mixed layer inertial energy and increasing the inertial energy below the mixed layer. A strong maximum in inertial energy is formed at 100 m ("the Beam"). By 21 days after the storm. both mixed layer inertial energy and inertial frequency shear maximum just below the mixed layer have been reduced to background levels. The total depth-average inertial energy decreases by about 40% during this period.

Linear internal wave theory can only partially explain the observed evolution of the inertial frequency currents. The decrease in horizontal wavelength is accurately predicted as due to the β effect. The decrease in depth-average inertial energy is explained by southward propagation of the lowest few modes. The superinertial frequency and clockwise rotation of phase with depth are qualitatively consistent with linear theory. However, linear theory underpredicts the initial rate at which inertial energy is lost from the mixed layer by 20%–50% and cannot explain the decrease of mixed layer energy and shear to background levels in 21 days.

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David J. Raymond, Steven K. Esbensen, Clayton Paulson, Michael Gregg, Christopher S. Bretherton, Walter A. Petersen, Robert Cifelli, Lynn K. Shay, Carter Ohlmann, and Paquita Zuidema

Coupled global ocean–atmosphere models currently do a poor job of predicting conditions in the tropical east Pacific, and have a particularly hard time reproducing the annual cycle in this region. This poor performance is probably due to the sensitivity of the east Pacific to the inadequate representation of certain physical processes in the modeled ocean and atmosphere. The representations of deep cumulus convection, ocean mixing, and stratus region energetics are known to be problematic in such models. The U.S. Climate Variability and Predictability (CLIVAR) program sponsored the field experiment East Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System 2001 (EPIC2001), which has the goal of providing the observational basis needed to improve the representation of certain key physical processes in models.

In addition to physical processes, EPIC2001 research is directed toward a better understanding and simulation of the effects of short-term variability in the east Pacific on climate. This variability is particularly important in the region because conditions in the intertropical convergence zone are highly variable on daily to intraseasonal time scales. The effects of such variability rectify strongly onto climate time scales in this region.

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Taneil Uttal, Judith A. Curry, Miles G. McPhee, Donald K. Perovich, Richard E. Moritz, James A. Maslanik, Peter S. Guest, Harry L. Stern, James A. Moore, Rene Turenne, Andreas Heiberg, Mark. C. Serreze, Donald P. Wylie, Ola G. Persson, Clayton A. Paulson, Christopher Halle, James H. Morison, Patricia A. Wheeler, Alexander Makshtas, Harold Welch, Matthew D. Shupe, Janet M. Intrieri, Knut Stamnes, Ronald W. Lindsey, Robert Pinkel, W. Scott Pegau, Timothy P. Stanton, and Thomas C. Grenfeld

A summary is presented of the Surface Heat Budget of the Arctic Ocean (SHEBA) project, with a focus on the field experiment that was conducted from October 1997 to October 1998. The primary objective of the field work was to collect ocean, ice, and atmospheric datasets over a full annual cycle that could be used to understand the processes controlling surface heat exchanges—in particular, the ice–albedo feedback and cloud–radiation feedback. This information is being used to improve formulations of arctic ice–ocean–atmosphere processes in climate models and thereby improve simulations of present and future arctic climate. The experiment was deployed from an ice breaker that was frozen into the ice pack and allowed to drift for the duration of the experiment. This research platform allowed the use of an extensive suite of instruments that directly measured ocean, atmosphere, and ice properties from both the ship and the ice pack in the immediate vicinity of the ship. This summary describes the project goals, experimental design, instrumentation, and the resulting datasets. Examples of various data products available from the SHEBA project are presented.

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