Wayne V. Burt

Use of the Southern Oscillation in Weather Prediction

Abstract Information and data recently received from Chilean meteorological authorities provide an improved atmospheric pressure record for Easter Island for 1912–13 and 1942–67. This study indicates that variations in the difference of mean monthly sea level pressure between Easter Island and Darwin, Australia, provide a much more effective aid to forecasting extended periods of abnormally heavy rainfall over the central and western equatorial Pacific than our earlier approach which used just the Darwin pressure trend. The principal advantages of using this forecasting aid over the earlier method are that. 1) it provides a physically significant index with regard to atmospheric and hydrospheric circulation over the Indo-Pacific region, and more clearly reflects fluctuations associated with the Southern Oscillation; 2) it provides a 1–2 month increase in lead time for forecasts of abnormally heavy equatorial Pacific rainfall; and 3) it is much more effective over the 1943–61 period (when the Darwin pressu...

Surface Currents off Oregon as Determined from Drift Bottle Returns

Abstract It is generally known that surface currents off Oregon flow equatorward in summer when winds are from the northwest and poleward in the winter when winds are from the south. A detailed pattern of surface currents has been derived from drift bottle returns and releases observed during a 10-year study from 1961–71. A total of 21,285 drift bottles were released within 165 n mi of the Oregon coast, of which 2937 bottles were eventually recovered. The annual cycle of surface currents indicated from these returns was as follows: Northward–October through February Southward–May through August Variable–March, April and September The Davidson current or (northward) winter current was considered to be any northward prevalence of surface flow. The Davidson current commonly reached 48–50N. Every year of the study, some surface blow off Oregon reached as far north as southeast Alaska to about 55N. Although drift bottle observations often indicate northward surface flow all the way to at least the edge of the ...

Prediction of Abnormally Heavy Precipitation over the Equatorial Pacific Dry Zone

Abstract A simple objective method for using the barometric pressure changes associated with the Southern Oscillation to predict abnormally heavy rainfall over the Pacific equatorial dry zone was applied to 79 years of Darwin sea-level pressure data and precipitation data for a large number of small islands in the equatorial Pacific. It appears that this method, based on a consideration of pressure changes from one year to the next, could provide a prediction one to eight months in advance of heavy precipitation over most of the central and western part of the zone with about 76% effectiveness. There also appears to be some degree of correlation between the amount of pressure departure and the extensiveness (in time and space) of this heavy rainfall. A check into those cases where an associated El Nino occurred indicated that the El Nino usually sets in a few to several months earlier than the heavy precipitation over the central and western equatorial Pacific dry zone and prior to the obvious Darwin pres...

Selective transmission of light in tropical Pacific waters

Abstract Transmission of light was measured at thirteen wave lengths on forty-six water samples drawn from the ocean in the eastern tropical part of the Pacific Ocean. Samples were taken at fifteen stations at depths ranging from the surface down to 1170 m. Transmission increased with wave length in a characteristic manner. Application of the Mie Theory of light scattering to the data indicated a decrease in the size of suspended particles with depth and with the clarity of the samples. A comparison with other oceanographic light-transmission data is made.

Buoy and satellite observations of mesoscale cellular convection during AMTEX 75

Buoy and satellite observations of mesoscale cellular convection (MCC) over the East China Sea in the vicinity of the Kuroshio current have been made during 14–18 February 1975, as a part of the Air Mass Transformation Experiment (AMTEX). Surface observations of solar radiation from spar buoys indicate the distinct passage of open and closed MCC that formed and continued for three consecutive days during an outbreak of cold polar air over the much warmer Kuroshio. A critical air-sea temperature of −5 °C for the occurrence of MCC has been substantiated. The time required for the passage of solar radiation peaks coupled with the buoy wind speed gave a computed closed cell diameter of 28 km, comparable to estimates from satellite photographs.The horizontal component of wind beneath the cloudy portion of a closed cell, due to convection, has been estimated as 0.6 m s−1. This represents the speed at which air near the sea surface moves from the edge toward the center of a closed cell. Also, the temperature difference obtained near the sea surface between the relatively cold descending branch and the warm ascending branch is 0.2°C. Similarly, the specific humidity difference of the less moist descending air near the edge and the moist ascending air near cell center is 9% (∼0.4 g/kg). Some indications were also found in the variation of horizontal wind direction with the passage of closed cells, since wind variations at the edge of passing cells exceeded the mean sequential variability (10.6 ° compared to 9.4 °).Sensible heat flux calculations associated with closed MCC suggest that strong surface heating can be associated with closed cells, previously reported by Hubert (1966) to be a characteristic of only open cells.Finally, the results of this study should remove any disclaimers that MCC appear in satellite photography simply because of a resolution bias and that the consideration of all visible clouds actually present would remove any periodicity one might expect to see in surface observations.

Diurnal variations of winds over an upwelling region off Oregon

The diurnal variation of surface winds off the coast of Oregon is described and compared with a recent analysis of winds off the coast of Peru. The Oregon wind speeds have a distinct 24-h periodicity, while the Peru wind speeds were reported to have an irregular 12-h variation. The long-and trans-shore components of both winds exhibit 24-h periodicities; the ratio of the long-shore to trans-shore diurnal amplitudes off Oregon is 2.8, twice the ratio found off Peru. Although meteorological conditions off Oregon were quasi-stationary during the period investigated, there were considerable day-to-day variations in diurnal amplitudes and phases. Diurnal amplitudes were found to be correlated with the daily mean long-shore winds.

The surface wind over an upwelling area near Pisco, Peru

Harmonic analysis of longshore and onshore components of surface winds over an upwelling area off the coast of Peru shows regular, nearly sinusoidal, diurnal variations in both velocity components. Spectral analysis of wind speed also indicates strong diurnal variations in kinetic energy. The data are compatible with a recent model of coastal winds proposed by Lettau and Lettau.

A comparison of methods for forecasting wave generation

Abstract Wave heights and periods in the generating areas of an unusually severe storm and its forerunner were hindcast by three different methods and compared with observations. Wave height agreement was good for all methods. Period agreement was poor; however, all reported periods fell within the spectral range determined by the Pierson-Neuman-James method. The description of a sea in terms of its spectrum is indicated.

Computation of Incoming Solar Radiation over the Equatorial Pacific

Abstract This paper discusses findings in regard to cloud distributions over the equatorial Pacific and the development of a method, compatible with actual cloud distributions, for using certain existing formulas to compute incoming solar radiation from meteorological data. U. S. Air Force Uniform Summaries of Surface Weather Observations for 10 central and western equatorial Pacific sites indicate that the very large cirriform cloud contribution to total sky cover, noted in the July 1957–June 1958 Canton Island weather data, is a general characteristic of the cloud distribution over a large part of the western equatorial Pacific. Since most formulas for computing incoming solar radiation depend on a mean total cloud cover entry, a large bias toward thin cirriform cloud could lead to sizeable underestimations of incoming solar radiation and cause significant errors in heat budget calculations for this region. In order to determine a suitable cloud cover term, monthly mean total sky cover, low cloud cover ...

Wintertime current meter measurements from the East China Sea

Abstract An array of three current meters were anchored on the continental shelf of the East China Sea during the last half of February 1975 as part of the Japanese Air Mass Transformation Experiment, AMTEX-75. The results indicate that the currents are dominated by the rotational semidiurnal M2 component superimposed on a slow mean drift to the northeast. Differences in direction of several days duration between two of the current meters suggest the presence of transient mesoscale eddies or meanders in the flow regime. Rotary spectra indicated that the tidal currents are characterized by a clockwise ellipse of eccentricity ≈0.6 and that there was some inertial energy present. The current rose for bandpassed velocity fluctuations (0.2–2 eph) was elongated in a direction 25–30° to the right of the wind direction indicative of Ekman veering.