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William E. Johns

Abstract

During November 1986, a 6-day record was collected from a 150 kHz Acoustic Doppler Current Profiler (ADCP) mounted in the upward-looking mode on a subsurface mooring in the Gulf Stream near Cape Hatteras. The flotation unit used for the ADCP was a newly developed streamlined float, designed to minimize the effects of drag-induced tilt and high-frequency buoy motion on the range and precision of the Doppler measurements. The overall performance of the float was found to be excellent, with a mean tilt of less than 2° in up to 2 kt of current and a high apparent stability to vortex-induced oscillations. As a result, good velocity data were obtained to within 30 m of the surface from a mean depth of 375 m. A comparison of the near-field ADCP velocity data with a conventional Aanderra current meter moored 20 m below the ADCP yielded mean and root-mean-square speed and direction differences of 1.0 ± 3.7 cm s−1 and 0.5 ± 2.9°, respectively. Also, a comparison with Pegasus velocity profiles taken within 1 n mi of the mooring site showed qualitatively good agreement, with the ADCP reproducing well the small-scale vertical structure. Significant fluctuations in the vertical component were also observed, related to diurnal migration of biological scatterers, with vertical “speeds” often in excess of 3–4 cm s−1.

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William Bluman and John E. Hart

Abstract

Airbone Doppler lidar wind measurements were obtained in the lee of Mount Shasta in northern California on 28 August 1984. These data consist of line of sight wind vectors at flight level (3000 m) and along planes tilted at 1, 2 and 3 degree below the 3000 m level. The observed field is confined to a rectangular box, encompassing the mountain, that extends about 40 km downwind and about 20 km crosswind. The spatial resolution of the measured wind field is approximately 330 m.

The upstream southwesterly flow tended to circumvent the mountain although some air did rise over the peak (at 4317 m) to initiate three-dimensional internal gravity waves in the lee. These waves are delineated in the two-dimensional divergence field D, determined from the downwind velocity components on each of the tilted planes with line of sight wind vector measurements. The observed field of D exhibits a peak in its power spectrum, determined along the downstream direction, at a wavelength of about 8 km with a secondary peaks at about 17 km. Data from upper air soundings at Medford, Oregon and from onboard sensors establish that the 8 km wavelength represents the free wave response, which is determined by the airstream characteristics. Comparison with the power spectrum of the mountain slope indicates that the longer wavelength is a forced response.

Qualitative aspect of the lee-wave pattern are reproduce in a linear model with uniform airstream characteristics. However, the amplitude of the free wave response is underestimated by a factor of two, and the forced wave amplitude is about three time that of the free wave. In addition, the wave disturbance produced by the linear model decays more rapidly in the downstream direction than the observed wave. These discrepancies are interpreted in relation to physical features that are contained in the linear model.

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Hartmut Peters and William E. Johns

Equation (5) in Peters and Johns (2006) is incorrect. The correct equation is where u j is the velocity of beam j, w′ is the velocity from the fifth, near-vertical beam, and θ = 30° is the ADCP beam angle. Equation (5) as shown above was used in processing the ADCP velocity data, and thus all results presented in Peters and Johns (2006) are valid.

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Hartmut Peters and William E. Johns

Abstract

South of the Strait of Bab el Mandeb, saline Red Sea Water flows downslope into the Gulf of Aden mainly along the narrow 130-km-long “Northern Channel” (NC) and the shorter and wider “Southern Channel” (SC). In the NC, the Red Sea plume simultaneously exhibited weak entrainment into a 35–120-m-thick, weakly stratified bottom layer while a 35–285-m-thick interfacial layer above showed signs of vigorous mixing, overturns up to 30 m thick, and extensive zones of gradient Richardson numbers below 1/4. Turbulent overturning scales, or Thorpe scales, are extracted from regular CTD profiles and equated to Ozmidov scales. On this basis, interfacial mixing is quantified in terms of estimated turbulent dissipation rates, vertical turbulent salt flux, and interfacial stress. Even though these estimates are subject to significant uncertainty, they demonstrate the intensity of mixing during strong winter outflow in terms of eddy diffusivities Kρ on the order of 10−2 m2 s−1. The large Kρ occur in strong stratification such that vertical turbulent salt fluxes are also large. Along the NC, relative maxima of Kρ correspond to maxima in the bulk Froude number. Direct short-term measurements of the Reynolds stress just above the seafloor at two locations, one in the NC and one in the SC, allow comparisons of the bottom stress τb with the interfacial turbulent stress τi. The ratio τi/τb shows large scatter in a small sample, with maximum values on the order of 1. An outlines procedures of making and reducing lowered acoustic Doppler current profiler measurements optimized for observing descending plumes.

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Hartmut Peters and William E. Johns

Abstract

Turbulence in the Red Sea outflow plume in the western Gulf of Aden was observed with an upward-looking, five-beam, 600-kHz acoustic Doppler current profiler (ADCP). The “Bottom Lander” ADCP was deployed on the seafloor in two narrow, topographically confined outflow channels south of Bab el Mandeb for periods of 18–40 h at three locations at 376-, 496-, and 772-m depths. Two deployments were taken during the winter season of maximum outflow from the Red Sea and two in the summer season of minimum outflow. These short-term observations exhibit red velocity spectra with high-frequency fluctuations of typically a few centimeters per second RMS velocity during strong plume flow as well as strong subtidal variations. In one winter season event, the plume flow was reduced by a factor of 4 over an 18-h time span. In variance-preserving form, velocity spectra show a separation at frequencies of 0.3–3 cycles per hour between low-frequency and high-frequency signals. The latter show significant coherence between horizontal and vertical velocity components; hence they carried turbulent stress. Based on a comparison with velocity spectra from atmospheric mixed-layer observations, the authors argue that large variance at frequencies of the order of 1 cph was possibly associated with bottom-generated, upward-propagating internal waves. One coherent feature that matched such waves was observed directly. Higher frequencies correspond to turbulent motions of energy-carrying scales. The turbulent Reynolds stress at heights above the bottom between 4 and 30–40 m was computed for most of the ADCP observations. Near the bottom, the streamwise turbulent stress and the streamwise velocity followed a quadratic drag law with drag coefficients ranging from 0.002 to 0.008. There was also significant spanwise stress, hinting at the three-dimensional nature of the boundary layer flow. The time–height variations of the stress and its spectrum proved to be complex, one of its most striking features being angles of up to ∼40° between the direction of the stress and that of the low-frequency flow. The turbulent shear production and eddy viscosity were also examined. On the technical side, the paper discusses the role of the fifth, center-beam velocity measurements in correcting for instrument tilt along with the effect of beam spreading in the 30° Janus configuration of the “regular” four ADCP beams. Instrumental noise and detection limits for the stress are also established.

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Silvia Matt and William E. Johns

Abstract

The Red Sea outflow exhibits strong seasonal variability in outflow transport due to effects of monsoon winds and seasonal fluctuations in buoyancy forcing. As it descends the continental slope in the western Gulf of Aden, it entrains significantly less-dense near-surface water, which itself varies on seasonal time scales. High-resolution hydrographic and direct velocity data collected during the 2001 Red Sea Outflow Experiment (REDSOX) are used herein to characterize and quantify the pathways of the Red Sea Outflow Water (RSOW) and the associated entrainment of Gulf of Aden Water. The outflow transport exhibits a maximum in winter of about 0.29 Sv (Sv ≡ 106 m3 s−1) at the exit of the Bab-el-Mandeb and approximately doubles to 0.56 Sv as it descends into the Gulf of Aden and entrains ambient water. In summer, the outflow is much weaker, reaching about 0.06 Sv at the strait and about 0.18 Sv downstream. The outflow plume divides into three distinct branches in winter, consisting of descending branches along two bathymetrically confined channels (the “Northern” and “Southern” channels, respectively), and an adjusted intrusion layer at shallower depths in the water column. Estimates of transport of “pure” Red Sea Outflow Water through salt flux conservation show the general partitioning of the outflow between the individual plumes, where the Northern Channel (NC) accounts for 52% of Red Sea Outflow Water, the Southern Channel (SC) carries 31%, and the intrusion layer (IL) the remaining 17%. The results also indicate that the transport of Red Sea Outflow Water is subject to considerable synoptic temporal variability that is unresolved by the present study.

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William E. Johns and Friedrich Schott

Abstract

Current meter observations were collected from a three-dimensional array moored in the Florida Straits between December 1983 and June 1984 as part of the Subtropical Atlantic Climate Studies (STACS) program. Approximately one-fourth of the total subinertial velocity and temperature variance contained in these records is associated with meandering of the Florida Current on time scales ranging from several days to a few weeks. There approach to be no strong correlation between the occurrence of meanders and variations in Florida Current volume transport or local wind forcing.

Utilizing frequency-domain empirical mode analysis we find the most coherent, energetic meandering signals within two limited frequency bands centered near periods of 12 days and 5 days. These meanders propagate downstream (northward) with phase speeds and wavelengths of approximately (28 km d−1, 340 km) and (36 km d−1, 170 km) respectively. Periodic waveforms composed from these modes indicate an asymmetric meander pattern with wave crests and troughs leading on the eastern side of the Florida Straits. These meanders appear to be giving up significant energy to the mean flow through up-gradient eddy momentum and buoyancy fluxes in the cyclonic shear zone of the Florida Current, with the net energy transfer being generally dominated by barotropic (eddy momentum flux) processes.

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John R. Gerhardt and William E. Gordon

Abstract

Selected portions of microtemperature data obtained continuously and with near simultaneity at several levels up to six feet over a desert surface are plotted on expanded height-time coordinates. The resulting isotherm patterns are shown to be strikingly consistent at all levels and are qualitatively analyzed in relation to the turbulence field present. Correlation coefficients between temperature fluctuations simultaneously at two levels and at a point for various time intervals are evaluated and their variation with separation, time, wind speed, and thermal stability is discussed. Tentative intensity and scale measures of turbulence derived from iemperature data are presented.

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William L. Chapman and John E. Walsh

Abstract

Monthly surface air temperatures from land surface stations, automatic weather stations, and ship/buoy observations from the high-latitude Southern Hemisphere are synthesized into gridded analyses at a resolution appropriate for applications ranging from spatial trend analyses to climate change impact assessments. Correlation length scales are used to enhance information content while limiting the spatial extent of influence of the sparse data in the Antarctic region. The correlation length scales are generally largest in summer and over the Antarctic continent, while they are shortest over the winter sea ice. Gridded analyses of temperature anomalies, limited to regions within a correlation length scale of at least one observation, are constructed and validated against observed temperature anomalies in single-station-out experiments. Trends calculated for the 1958–2002 period suggest modest warming over much of the 60°–90°S domain. All seasons show warming, with winter trends being the largest at +0.172°C decade−1 while summer warming rates are only +0.045°C decade−1. The 45-yr temperature trend for the annual means is +0.082°C decade−1 corresponding to a +0.371°C temperature change over the 1958–2002 period of record. Trends computed using these analyses show considerable sensitivity to start and end dates, with trends calculated using start dates prior to 1965 showing overall warming, while those using start dates from 1966 to 1982 show net cooling over the region. Because of the large interannual variability of temperatures over the continental Antarctic, most of the continental trends are not statistically significant. However, the statistically significant warming over the Antarctic Peninsula is the strongest and most seasonally robust in the spatial patterns of temperature change.

Composite (11-model) global climate model (GCM) simulations for 1958–2002 with forcing from historic aerosol and greenhouse gas concentrations show warming patterns and magnitudes similar to the corresponding observed trends for the 45-yr period. GCM projections for the rest of the twenty-first century, however, discontinue the pattern of strongest warming over the Antarctic Peninsula, but instead show the strongest warming over the Antarctic continent.

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William L. Chapman and John E. Walsh

Abstract

Gridded fields of sea ice concentration are used to evaluate weekly and monthly anomalies of sea ice coverage in 22 Arctic subregions. The primary period of study is 1972–1988, although statistical comparisons are made with data of lesser quality from 1953–1971. The various time series of regional ice coverage permit the evaluation of ice anomaly persistence as a function of region, season, and lag (forecast range). The fractions of variance explained by anomaly persistence in most regions are considerably larger than corresponding atmospheric values. The fractions typically decrease from 50% to 10% as the forecast range increases from several weeks to several months. Anomaly persistence from the winter months is generally largest, although the regions of greatest persistence-derived forecast skill tend to migrate seasonally with the marginal ice zone. Biases in the regional analyses of the 1950s and 1960s inflate the apparent persistences in the North Atlantic during 1953–1971, but the persistences in most other regions are generally similar in the pre-1972 and post-1972 data. The inclusion of lagged regional cross-correlations provides little increment of forecast skill over persistence at the 1-month range, but this strategy appears to have the potential to enhance the usefulness of ice forecasts at ranges of several months. Analog-based forecasts show statistically significant skill but are generally unable to outperform persistence at the 1-month range.

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