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John D. Horel

Abstract

The annual cycle in sea surface temperature (SST), surface wind and other atmospheric variables in the tropical Pacific are described. The primary data sets of SST and surface wind are derived from ship observations in the Pacific between 29°N and 29°S during the period 1946–76.

The annual cycle in SST away from the equator can be attributed to the annual cycle in solar heating. However, in the eastern equatorial Pacific, the annual cycle in SST undergoes systematic longitudinal changes in phase and amplitude. Near the coast of Peru, the warmest temperatures occur during March, while further west along the equator, the warmest temperatures occur progressively later and with diminished amplitude. The annual cycle in surface wind convergence along the equator displays similar changes with longitude.

The annual cycle in surface wind is dominated by the meridional migrations of the trade wind belts. Near the equator, the amplitude of the annual cycle in meridional wind is larger than that in zonal wind with the zonal flow (relative to the annual mean) directed into the summer hemisphere. The annual cycles in wind speed and pressure gradient are shown to be kinematically consistent. The annual cycles in rainfall, surface wind convergence, and satellite-derived outgoing infrared radiation and albedo exhibit many similarities in the regions dominated by tropical convection.

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John D. Horel

Abstract

The persistence of the planetary-scale circulation during the Northern Hemisphere winters from 1965/66 to 1981/82 is documented. National Meteorological Center analyses of 500 mb geopotential height for the Northern Hemisphere poleward of 20°N are used. Instead of constructing auto-correlation maps based on lime series at grid points, time series are constructed that show the pattern correlations between daily hemispheric maps. These time series provide information on the day-to-day changes in persistence of the winter circulation and allow comparisons of the intraseasonal and interannual variability of persistence.

The hemispheric circulation is usually quite persistent from day-to-day; correlations between successive maps usually fall within the range 0.7 to 0.9. As the time between the maps is increased the correlations between these maps decreases at a rate faster than that expected of an autoregressive (red-noise) process. The hemispheric circulation rarely resembles closely the circulation a few days before unless synoptic scale waves are removed.

Subjectively defined criteria are used to identify quasi-stationary regimes i.e. periods during which the planetary-scale circulation is more persistant than usual. A total of 58 regimes encompassing 25% of the 2040 days am identified. Multiple regimes are evident during several winter. The quasi-stationary regimes exhibit considerable diversity in their spatial configurations. Comparison of the regimes using principal component analysis suggests that the most frequently reoccurring regime consists of a superposition of a wavenumber 3 pattern at roughly 50°N upon zonally symmetric components at middle and polar latitudes of opposing signs. However, this principal component explains only 17% 6f the variance contained in the 58 regimes.

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John D. Horel

Abstract

The persistence of daily 500 mb geopotential height anomalies during the 17 winters from 1966 to 1982 is documented for a region over the central Pacific that is noted for its frequent blocking activity. Persistence is expressed here in terms of the degree to which a height value remains constant during the next several days. For a 9 grid-point region centered on 50°N, 165°W, blocks (large positive geopotential height anomalies) tend to be less persistent than negative height anomalies between −100 and −200 m. The greater persistence of negative height anomalies in this region is related to periods of stronger than normal zonal flow centered near 40°N (high zonal index). These periods of stronger than normal zonal flow are more persistent than expected of a first-order autoregressive process.

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John D. Horel

Abstract

The principal components derived by Wallace and Gutzler (1981) from a 500 mb height data set are linearly transformed using the varimax method. Their data set consists of 45 winter months of National Meteorological Center analyses of Northern Hemisphere 500 mb height. The linear transformation (or rotation) of the principal components emphasizes the strongest relationships within the 500 mb height data set; hence, spatial patterns associated with the rotated principal components are simpler to interpret than the spatial patterns associated with the unrotated components. The teleconnection patterns identified by Wallace and Gutzler (1981) on the basis of the negative extrema approach closely resemble several of the spatial patterns of the rotated principal components.

In order to show the seasonal dependence of the rotated principal components, an expanded data set consisting of 30 years of 500 mb height data is used. Most of the teleconnection patterns derived from the 90 winter month data set are “seesaws” with the southernmost center of high correlation located in the subtropics. In some cases, additional centers of high correlation are located downstream of the two primary centers. The spatial patterns associated with the rotated principal components based on 90 summertime months are analogous to those for the wintertime months but are displaced northward along with the displacement of the time mean jet streams and storm-track regions.

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John D. Horel
and
John M. Wallace

Abstract

No abstract available.

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John D. Horel
and
John M. Wallace

Abstract

Atmospheric phenomena associated with the Southern Oscillation are examined, with emphasis on vertical structure and teleconnections to middle latitudes. This paper is specifically concerned with the interannual variability of seasonal means for the Northern Hemisphere winter during the period 1951–78. Among the variables considered are sea surface temperature in the equatorial Pacific, precipitation at selected equatorial Pacific stations, a “Southern Oscillation Index” of sea level pressure, 200 mb height and tropospheric mean temperature at stations throughout the tropics, and Northern Hemisphere geopotential height fields. Selected statistics derived from surface data also are examined for the period 1910–45. Results are presented in the form of time series and correlation statistics for the variables listed above.

Results concerning the relationships between sea surface temperature, sea level pressure and rainfall are consistent with the major conclusions of previous studies by J. Bjerknes and others. Fluctuations in mean tropospheric temperature and 200 mb height are shown to vary simultaneously with equatorial Pacific sea surface temperature fluctuations, not only in the Pacific sector, but at stations throughout the tropics. The zonally symmetric component of these 200 mb height fluctuations is considerably larger than the Southern Oscillation in 1000 mb height, and the corresponding fluctuations in the mean temperature of the tropical troposphere are on the order of nearly 1 K.

The correlations between the tropical time series and Northern Hemisphere geopotential height fields exhibit well-defined teleconnection patterns. Warm episodes in equatorial Pacific sea surface temperature tend to be accompanied by below-normal heights in the North Pacific and the south–eastern United States and above-normal heights over western Canada.

Recent theoretical work by Opsteegh and Van den Dool (1980), Hoskins and Karoly (1981) and Webster (1981) on Rossby wave propagation on a sphere provides a basis for understanding the teleconnection in terms of the distribution of sea surface temperature and rainfall in the equatorial Pacific. The theory successfully explains several characteristics of the observed teleconnection patterns, including their horizontal scale and shape, their vertical structure and their seasonal dependence.

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Timothy W. Barker
and
John D. Horel

Abstract

The average skill of 61 thirty-day forecasts of 500 mb geopotential height from the Dynamic Extended Range Forecast (DERF) experiment are investigated. These forecasts were made using the National Meteorological Center's Medium Range Forecast model and starting from initial analyses during the winter of 1986/87. The impact upon extended range forecast skill of the removal or retention of systematic errors and low-frequency climate variability is studied. If the systematic error is removed a posteriori at each forecast lead time, then the skill of forecasts of time averages may be improved considerably. The magnitude of this improvement is difficult to quantify with forecasts from a single season. Nearly all of the skill at extended range in the DERF experiments arises from the successful forecast of low-frequency fluctuations in the large-scale circulation.

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Neil P. Lareau
and
John D. Horel

Abstract

The position and variability of storm tracks across western North America are examined during the October–April cool seasons spanning 1989–2010. The location and intensity of storms are represented by strong synoptic-scale ascent, which is diagnosed by the alternative balance omega equation applied to ECMWF Re-Analysis Interim data. This dynamically filtered method removes poorly resolved updrafts arising from subsynoptic-scale phenomena such as convection and mountain waves. The resulting vertical motions are illustrated for the case of a strong storm traversing the western United States.

Summary statistics of synoptic-scale ascent are compiled over months, seasons, and the entire 21-yr period. Locations exhibiting high mean values of ascent are deemed to represent storm tracks. The climatological-mean storm track exhibits a sinusoidal shape across the eastern Pacific and western North America. The composite evolution of strong storms moving along specific segments of the storm track show regional differences (e.g., storms poleward of 50°N tend to result from progressive low-amplitude troughs progressing through the mean planetary ridge, while storms over the western United States are initiated by digging troughs that temporarily suppress the mean ridge).

Seasonal shifts in the storm track are pronounced and exhibit coherent regional patterns. Interannual variations in synoptic-scale ascent indicate meridional shifts in position as well as changes in the degree of amplification within the dominant sinusoidal storm track. These changes in structure are related to the phase of ENSO: El Niño (La Niña) winters favor zonal (amplified) and southern (northern) storm tracks.

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John D. Horel
and
Angel G. Cornejo-Garrido

Abstract

Streamflow and historical records indicate that flooding in northern Peru was more severe during 1983 than during any year since 1891. A case study of the meteorological conditions along the northwest coast of South America from 10°S to 10°N during 1982–83 is presented. Station rainfall and satellite-derived outgoing infrared observations are used to deduce the structure and time evolution of convection in this region.

Substantial rainfall amounts were first observed along the western slopes of the Andes Mountains and coastal plain of southern Ecuador during November and December 1982, and it continued to rain in this region through June 1983. In northern Peru, the onset of the rains along the coastal plain was delayed until January 1983 and ended abruptly during mid-June 1983. Convective activity was much greater along the coastal strip than over the eastern equatorial Pacific to the cast of 100°W until May-June 1983. In addition, cloudiness was strongly modulated on the diurnal time scale, with more clouds during the night and early morning than during the afternoon.

The distribution of rainfall along the Peruvian littoral is compared to local changes in sea surface temperature, surface equivalent potential temperature, and surface wind. These comparisons suggest that abnormally high coastal ocean temperatures during the first half of 1983 aided the outbreak of convection during this period. However, the latitudinal extent and timing of the rainfall was quite different from that of sea surface temperature or surface equivalent potential temperature.

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Kim M. Waldron
,
Jan Paegle
, and
John D. Horel

Abstract

Numerical filters required to control spatial computational modes in a limited-area model (LAM) that uses the unstaggered. A grid are developed and tested over the complex topography of the Great Basin of the western United States. The filters are founded upon Fourier expansions of forecast deviation fields and function equally effectively for both periodic and aperiodic local structures. Unlike other spatial filters, the approach used here avoids any direct contamination of larger scales. Provided that the shortest resolved wavelength of two grid intervals is removed, the results do not depend strongly on the range of filtered short waves or on the type and order of horizontal space difference approximations.

This approach leads naturally to methods in which the large scales predicted by an ambient outer model can be directly incorporated within the complete domain of the inner LAM, rather than just through conditions applied at the lateral boundaries of the LAM. This technique has some similarities to methods used both in operational regional models in Japan and in recent regional research models at the National Centers for Environmental Prediction (formerly National Meteorological Center) of the United States. Several methods to incorporate the large scales into the LAM are evaluated in a winter storm case study and in an ensemble of seven forecasts.

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