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David Halpern

Abstract

Moored buoy wind measurements were made at 15 min intervals at 3.5 m height at 6, 7 and 8°N along 150°W in the central equatorial Pacific from November 1977 to March 1978. During November, December and January the Intertropical Convergence Zone occurred near 8, 7 and 6°N, respectively, indicating a southward movement of ∼3 km day−1. The east and north spectra of each record did not contain any significant (at the 95% confidence level) peaks and the spectral estimates decreased with increasing frequency with a slope of about −1.5. Wind fluctuations at frequencies <0.01 and 0.005 cycles per hour (cph) were coherent (at the 95% confidence level) with zero phase difference for horizontal distances of 110 and 22O km, respectively. The noise level of the coherent wind fluctuations was less than 50%. The mean value of a first-order approximation of the divergence between 6 and 7°N was (−1.0 ± 6.5) × 10−5 s−1. On short time-scales, such as 28 h, the divergence between 6 and 7°N changed from zero to −1.3 × 10−4 s−1. Spectral estimates of the divergence contained a small peak at ∼6 days which was not expected because of the absence of a corresponding peak in the north spectrum.

Satellite-derived low-level cloud motion vectors determined on a routine basis by NOAA's National Environmental Satellite Service at four locations within ∼275 km of the buoys were compared with coincident 3 h vector-averaged buoy wind measurements. The average rms difference between all the satellite and buoy wind speeds was 4.4 m s−1. Approximately 3 and 44% of all the buoy and satellite wind speeds, respectively, were greater than 10 m s−1. For wind speeds the average orthogonal regression line was satellite wind speed (m s−1)= −7.1+2.5 buoy wind speed (m s−1). For wind directions the average regression line was satellite wind direction (deg) = 069+0.23 buoy wind direction (deg), indicating that the winds veered (clockwise turning) with height for northeast trade winds and backed (counterclockwise turning) with height for southeast trade winds. Because of the turning of the wind vector with height, the directions of the satellite winds were more easterly than the surface winds. Although the results of the comparison between buoy and satellite wind vectors were presumably a priori expected because of the vertical separation between the measurements, our results provide an estimate of the difference between the two types of measurements.

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David Halpern

Abstract

Moored buoy wind measurements made at 3 m height at three sites (8°27′N, 150°45′W; 0°, 150°W; 0°, 125°W) in the central equatorial Pacific during 1976 and 1977 were compared with satellite-derived low-level (∼1 km) cloud motion vectors within a 5° square centered on each buoy site. The satellite winds were determined on a routine basis by NOAA's National Environmental Satellite Service and the buoy wind measurements were made by NOAA's Pacific Marine Environmental Laboratory. To compare satellite and buoy wind measurements, 3 h vector averages of the buoy data were computed at times of satellite wind observations. The number of pairs of buoy winds and winds derived from infrared satellite imagery was 265. Directions of the low-level cloud motion vectors were similar to the buoy wind directions. The average satellite wind speed was 8.0 m s−1, which was 3.3 m s−1 or 70% greater than the average buoy wind speed. At the 0°, 125°W site where 190 satellite-buoy wind pairs were obtained, the average wind speed difference was 3.8 m s−1 and 30% of the deviations were greater than 5 m s−1. The standard deviation of the satellite winds was nearly 80% larger than the wind recorder data. At the 0°, 125°W site the correlation between the satellite and buoy wind speeds was 0.25. Our results indicated that the mean and fluctuations of low-level cloud motion vectors were not in satisfactory enough agreement with the buoy wind data to be considered representative of the near-surface wind field. Although our results were presumably a priori expected because of the vertical separation between the measurements, this note provides an estimate of the difference between the two types of measurements.

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David Halpern

Abstract

Measurements from a Savonius rotor and vane vector-averaging current meter (VACM) and dual orthogonal propeller vector-measuring current meter (VMCM) placed 1 m apart beneath a surface-following buoy moored on the Pacific equator for 6-month intervals are described. Experiments at 13, 98, 120 and 160 m depths were completed in light winds (≈5 m s−1), small surface wave heights (1–2 m), strong current speeds (monthly averaged speeds ≈65 cm s−1) and large current shears (0.0 1 s−1). Excellent coherence was found between each VACM-VMCM doublet's data. As expected, the near-surface VACM recorded slightly larger speeds (≈10% for monthly mean 15-min vector-averaged values) than the VMCM. The only kinetic energy density mismatch at the 95% confidence level was between the 13-14 m dyad and occurred above 1.3 cph. The most striking feature of them intercomparison tests was that the VACM and VMCM observations were virtually identical below the new-surface layer. Below 98 m the average rms amplitude difference within a decadal frequency interval was <1 cm s−1. The average maximum usable frequency and mean frequency of the 50% noise level were 1.8 and 1.5 cph, respectively.

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David Halpern

Abstract

Yearlong in situ surface wind measurements at three sites along the Pacific equator (95°, 110°, 152°W) are used to estimate the required number of random observations per month for monthly mean wind speed components accurate to 1.0 and 0.5 m s. The three-site average amplitude of wind speed fluctuations with time scales less than a month was 2.8 m s, which was about 64% of the annual vector-mean speed. For the zonal (meridional) wind component, the average numbers of random observations at the three sites were about 10 (8) and 39 (30), respectively, for accuracies of 1.0 and 0.5 m s. The number of random observations increased westward and was highly correlated (R = 0.97) with monthly mean standard deviations. Neglect of wind variations yields an approximate 50% underestimate of monthly mean wind stress computed from the square law and monthly mean wind components.

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David Halpern

The origin of the Tropical Oceans–Global Atmosphere (TOGA) Program was closely related to the response of global atmospheric circulation to sea surface temperature variations in the tropical Pacific Ocean, which is evident by the El Niño phenomenon. During the two decades before the 1985 start of TOGA, advancements in scientific understanding of the tropical ocean and global atmosphere and advancements in technology provided strong foundations for TOGA. By the early 1980s, research had demonstrated a strong linkage between tropical SST variations and global atmospheric circulation, and discussions of an international ocean–atmosphere program had begun. Probably the single most important event leading to the creation of TOGA was the unannounced arrival in 1982 of the largest El Niño in a century.

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David Halpern

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In 1976, a pilot experiment, called first Equatorial Mooring (EQUA-1), tested an innovative technique for anchoring a taut-line surface mooring at 0°, 150°W where the water depth is 4.5 km. The 36-day deployment contained a wind recorder and fixed-level current meters at 50 and 100 m in the Equatorial Undercurrent (EUC). The following year, in a second pilot experiment, named EQUA-2, a similar mooring was deployed at 0°, 125°W for 99 days. EQUA-2, with current meters at 10, 50, 100, 150, and 200 m, recorded a surge in EUC transport during April 1977 when 3-day-averaged eastward current speeds at 50-m depth reached 2 m s−1. The associated eastward transport per unit meridional width over the 50–200-m layer was 190 m2 s−1. Based on observations recorded in April 1980, the EQUA-2 pulse would correspond to a total EUC transport surge of about 38 Sv (1 Sv ≡ 106 m3 s−1) and would represent an equatorially trapped first-mode baroclinic Kelvin wave. This paper describes EQUA Project observations and why and how I created the high-risk-of-failure opportunity to record pioneering time series measurements at the equator. The enduring legacy of the EQUA Project is the sustained maintenance of in situ surface wind and upper-ocean current and temperature measurements at numerous sites in the equatorial oceans, starting in the Pacific to improve forecasts of the El Niño and La Niña phenomenon. For example, the 40-yr records of surface wind and upper-ocean current and temperature measurements at 0°, 110°W and 0°, 140°W are some of oceanography’s longest time series recorded far from land.

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David Halpern

Abstract

Mooted surface wind measurements were recorded along the Pacific equator at 140°, 124°, 110°, and 95°W during portions of 1980–85. Minimum record length is one year. The annual mean and monthly mean westward speeds at 110°W were about 1.5 m s−1 higher during the year preceding the 1982–83 El Niño than in the year following this event. The annual cycle, which moved westward at ≈0.8 m s−1, consisted of weak westward and northward speeds in February–April and vice versa in September–October. The spectral slope between 5-day and 0.05-day periods was −1.5. The rms amplitude of the 95% statistically significant diurnal period oscillation was 0.3 m s−1, and the meridional component was nearly twice as large as the zonal component. The diurnal period wave was coherent (at the 95% confidence level) between 95° and 124°W with westward phase propagation of about 138 m s−1. No statistically significant (at the 95% confidence level) spectral peak was found in the 40- to 50-day intraseasonal period band. The surface zonal ocean current component, which reached approximately 0.5 and −0.5 m s−1 in April and October, respectively at 110°W, influenced the surface wind stress computed from the quadratic bulk aerodynamic formulation by 10%–20%.

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David Halpern

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Measurements of wind at 5 m height, currents at 7.6, 15.4. 21.4 and 27.5 m depths, and temperature at 19 depths between 0.4 and 43.3 m depths were made from 19 August to 12 September 1974 at 8°41′N, 23°10′W (Site E3 of the GATE C-scale Oceanographic Experiment). In the uppermost 30 m where the temperature was isothermal to within 0.1°C m−1, the 24-day mean wind-driven trans-port represented ∼20% of the total transport and the geostrophic transport was 80%. Time variations of the current records were dominated by near-inertial period and semidiurnal tidal period motions with rms amplitudes of 0.05-0.08 m s−1 and 0.05 m s−1, respectively. Throughout the frequency range the currents were coherent with zero phase difference between 7.6 and 27.5 M.

Several squalls occurred in which the wind stress was greater than 0.1 N m−2 and the temperature records did not indicate entrainment of cold water into the mixed layer from the upper thermo-cline. The upper pycnocline was very strongly stratified. The vigorous wind-generated turbulence in the mixed layer, as represented by a vertical eddy viscosity coefficient of about 6 × 10−3 m2 s−1, and the measured current shears of ∼10−2 s−1 were not large enough to induce entrainment.

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David Halpern

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This paper describes the temporal and spatial variations of the wind stress (computed from the square of the wind velocity vector) and wind-stress curl recorded during July and August 1973 at two moored buoy stations, one 13 km and the other 120 km from the Oregon coast, along 45°15′N. Some facets of the relationship between wind stress and the physical oceanography over the continental shelf off 0regon are discussed.

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David Halpern

Abstract

Measurements of winds and of near-surface temperatures and currents made during March and April 1974 on the continental shelf off northwest Africa were extremely time-dependent. Alternating land and sea breezes were well-developed and produced temperature and current fluctuations in the uppermost 15 m. Time-averaged speed of the surface current (28 an s−1) was much larger than the geostrophic current computed from the density field over the shelf. Approximately 60% of the variance of the current measurements occurred at frequencies less than the inertial period. Inertial and tidal period currents were large. Water stratification was very weak and tidal internal gravity wave motions were not detected. During a coastal upwelling event the Ekman transport, the offshore transport and the onshore transport were nearly equivalent, and the vertical eddy viscosity coefficient over the upper 10 m was about 125 cm2 s−1.

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