Eugene M. Rasmusson

Variations in Tropical Sea Surface Temperature and Surface Wind Fields Associated with the Southern Oscillation/El Niño

Abstract Surface marine observations, satellite data, and station observations of surface pressure and precipitation are used to describe the evolution of sea surface temperature (SST) anomalies, surface wind fields, and precipitation anomaly patterns during major warm episodes in the eastern and central tropical Pacific. The sequence of events is described in terms of composite SST and wind fields (30°N–30°S) for six warm episodes since 1949, and time series and cross-spectral analyses of mean monthly data along six shipping lanes which cross the equator between the South American coast and 170°W. During the months preceding a warm episode, the equatorial easterlies are stronger than normal west of the dateline. This and other coherent and strongly developed anomaly patterns over the western equatorial Pacific and South Pacific are associated with a South Pacific Convergence Zone (SPCZ) located southwest of its normal position. During October-November prior to El Nino, the equatorial easterly anomalies i...

The Relationship Between Eastern Equatorial Pacific Sea Surface Temperatures and Rainfall over India and Sri Lanka

Abstract Monsoon season (June–September) precipitation data from 31 Indian subdivisions and mean monthly precipitation data from 35 Indian and Sri Lanka stations, spanning the period 1875–79, were analyzed to determine the relationship between equatorial Pacific warm episodes (El Nino events) and interannual fluctuations in precipitation over India and Sri Lanka. The data reveal a strong tendency for a below normal summer monsoon during the 25 moderate/strong warm episodes which occurred during the period. For example, area-averaged June–September rainfall over India was below the median value in 21 of the 25 warm episode years. These years include the five largest, and nine of the eleven largest, negative monsoon season anomalies. Considerably smaller average departures occurred during the monsoon seasons of the year preceding and the year following the warm episode, and during the remaining months of the warm episode year. The anomaly pattern over Sri Lanka and extreme southern India is quite different,...

Measurements of the Atmospheric Mass, Energy, and Momentum Budgets Over a 500-Kilometer Square of Tropical Ocean

Abstract The atmospheric budgets of mass, water vapor, heat, momentum, and mechanical energy have been analyzed for a 5-day undisturbed period (June 22–26, 1969) during the Barbados Oceanographic and Meteorological Experiment (BOMEX). Data were obtained from rawinsondes launched every 1½ hr from the four corner ships of the BOMEX array. Computations were limited to the lowest 500 mb within a 500-km square centered at 15°N, 56°30′W. The mass budget is characterized by mean, low-level divergence and downward motion. Maximum divergence (5 × 10−6 s−1) is found approximately 80 mb above the surface and maximum subsidence near the base of the trade-wind inversion. The computed average evaporation is 6.0 mm/day. The computations show an upward flux of moisture through the trade inversion by subgrid-scale eddies, but the mean downward flux dominates, leading to a net downward transfer of moisture through the trade inversion. A surface stress of 0.065 Newtons (N)·m−2 is computed from the momentum balance with the ...

ATMOSPHERIC WATER VAPOR TRANSPORT AND THE WATER BALANCE OF NORTH AMERICA: PART I. CHARACTERISTICS OF THE WATER VAPOR FLUX FIELD

Abstract The atmospheric water vapor flux over North America and the Central American Sea (Gulf of Mexico and Caribbean Sea) has been investigated for the period May 1, 1961–April 30, 1963, as part of a more general study of the water balance of these areas. Mean monthly values of the total vapor flux components are analyzed and the more important aspects of the regional vapor flux climatology are discussed and illustrated by maps and cross-sections. Additional insight into the seasonal march is obtained from the computation of the total monthly vapor flux across selected regional boundaries. Major features of the North American total vapor flux field previously described by Benton and Estoque are confirmed, but the more extensive data used in this study bring out additional significant detail, particularly over the southern United States. Important diurnal variations are found in the flux field, particularly during the summer south of 50° N. These result primarily from diurnal variations in the average m...

ATMOSPHERIC WATER VAPOR TRANSPORT AND THE WATER BALANCE OF NORTH AMERICA

Abstract The atmospheric water vapor flux divergence and certain aspects of the water balance of North America are investigated, using data from the period May 1, 1958–Apr. 30, 1963. The mean vertical distribution of flux divergence is computed for the United States for the months of January (1962, 1963) and July (1961, 1962). Strong flux convergence in the lowest kilometer and divergence in the remainder of the troposphere were found in July. Flux convergence was found throughout the troposphere over the eastern half of the area in January, with a maximum between 900 and 950 mb.; while in the west, convergence (with no particularly pronounced maximum) was found above 800 mb., with weak divergence below. Corresponding features of the profiles were found at higher elevations over the west, where the flux divergence above 500 mb. is quite significant. Particular emphasis is placed on computation of the vertically integrated flux divergence, and its use in estimating P-E, the mean difference between precipit...

ON THE ANNUAL VARIATION OF THE MONTHLY MEAN MERIDIONAL CIRCULATION

Abstract Data for a 5-yr period from a dense network of upper air stations have been used to determine the annual cycle in the mean meridional circulation north of 15°S. Only during the transition months April, May and October, November is there some degree of symmetry with respect to the Equator. During the other months of the year, the Hadley cell of the winter hemisphere with a maximum strength of about 23 × 1013 gm sec–1 appears always to dominate the circulation. The Hadley cell of the summer hemisphere practically disappears, except possibly near the surface. Maximum meridional velocities connected with the winter Hadley cell are about 2.5 m sec–1 near 1000 mb and over 3 m sec–1 near 200 mb. Mean vertical velocities attain values of about 5 and 8 mm sec–1 in the downward and upward branches of the winter Hadley cells. A rather weak Ferrel circulation (about 4 × 1013 gm sec–1) and a very weak polar circulation (about 1 × 1013 gm sec–1) are computed in middle and high latitudes throughout the year. Wi...

Mississippi Moisture Budgets on Regional Scales

Abstract Two years of regional analyses based on the Eta Data Assimilation System (EDAS) are used to examine the mesoscale features of the moisture budgets of the Mississippi River basin and its subbasins. Despite the short period, basic aspects of the regional-scale seasonal means, annual cycle, and even diurnal cycle of the atmospheric water cycle are represented. The ability of the Eta Model to resolve mesoscale features of the low-level circulation is an important factor in improving the estimates of moisture flux convergence at regional scales. It appears that the internal consistency of moisture budgets estimated from EDAS analyses for basins of nearly 5 × 105 km2 is comparable to that computed from radiosondes for basins of about 2 × 106 km2 or larger. In other terms, the spatial scale of basins where consistent moisture budgets can be estimated appears to be reduced by almost one order of magnitude. Area-averaged evaporation estimates (computed as residuals of the moisture budget equation) for bas...

Biennial Variations in Surface Temperature over the United States as Revealed by Singular Decomposition

Abstract Surface temperature variations over the contiguous United States during the period 1931–75 are examined using mean monthly averages for the 344 climate divisions. This data matrix is decomposed into orthogonal components using the method of singular decomposition. The third empirical orthogonal function, which accounts for nine percent of the nonseasonal variance, exhibits a significant quasi-biennial oscillation (QBO). The phase and amplitude of the QBO implied by this analysis were further studied using an extension of the singular decomposition method which we call Hilbert Singular Decomposition (USD). HSD uses the Hilbert Transformer to augment the data matrix and transform the real elements into complex elements so that coherent “wavelike” variations can be represented in terms of a complex singular decomposition. Additional cross-spectral analyses were performed for selected climate division aggregates. Two areas of maximum QBO amplitude are indicated; one over the northeastern United State...

A STUDY OF THE HYDROLOGY OF EASTERN NORTH AMERICA USING ATMOSPHERIC VAPOR FLUX DATA

Abstract The atmospheric water vapor flux divergence and certain aspects of the water balance of Eastern North America are investigated, using data from the period May 1, 1958, to Apr. 30, 1963. Mean monthly values of evapotranspiration and storage change are computed as residuals, using measured values of vapor flux divergence, precipitation, and streamflow. Computations are performed for regions varying in size from 42 × 105 km2 to approximately 5 × 105 km2. The results for the smaller areas, which are the least reliable, are critically examined. Computed values of evapotranspiration and storage change are compared with the climatological estimates Thornthwaite Associates and Budyko. The Thomthwaite climatic water balance data appear to overestimate , the difference between precipitation and evapotranspiration, during winter and underestimate it during summer. Budykos values of evapotranspiration generally show a slightly smaller seasonal variation and appear to lead the values obtained from the atmosp...

Studies of North American continental‐scale hydrology using Eta model forecast products

In this study, the forecast products of the National Meteorological Centers Eta model during the period August 1993 to March 1994 are critically examined to gain insight into the quality of the first guess fields used in the models four-dimensional data assimilation and the usefulness of the forecast products in diagnostic studies involving unmeasured hydrological variables. We find that the Eta model 12–36 hour forecasts produce patterns of monthly precipitation that are a reasonably good approximation of the monthly observed precipitation fields for the period under consideration. The diurnal cycle of the water vapor fluxes and their seasonal changes obtained from the model forecasts are also in good agreement with observations. During the warm months the stationary component of the forecast fluxes includes a good representation of the low-level jet (LLJ) that is an important means for the transport of moisture from the Gulf of Mexico into the Great Plains. The LLJ decays during autumn, when it is replaced by the transient component of the fluxes as the means of transport of water vapor from the Gulf of Mexico region into the United States. Finally, evaporation estimates were derived from the convergence of the fluxes of the forecast fields and model and observed values of precipitation. All estimates agree within 0.5 mm d−1 and showed a reasonable seasonal cycle, with maximum evaporation during the warm season and a minimum evaporation during February.