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Henry F. Diaz

Abstract

Eigenvector analysis was performed on spring, summer, fall and annual temperature and precipitation over the contiguous United States and on the frequency of occurrence of cyclonic and anticyclonic conditions over an area extending roughly from 60–130°W and 20–50°N.

The first eigenvector in all cases represents anomalies of one sign over nearly all the contiguous United States. Correspondence of the other eigenvector forms across seasons was also good. These patterns appear to be associated with latitudinal shifts in the westerlies and to the amplitude and position of the long waves.

An opposition in precipitation anomaly was found between the northwestern and southwestern United States which was mirrored in a similar pattern in variations of cyclone frequency over these two areas.

Significant changes occurred in the frequency of occurrence of individual eigenvector forms, suggesting that particular circulation regimes may become established, persist and disappear in time. The length of time that a particular regime lasts is thus connected with regional changes in climate.

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Henry F. Diaz

Abstract

The separate contribution of December, January and February temperature to the net seasonal anomaly for the 1975–76 through 1981–82 winter seasons is analyzed. It is found that the January departures contributed by far the most toward making these seven winter seasons well below average in much of the contiguous United States, particularly in the eastern half of the country.

Each of these Januaries averaged from below to much-below the long-term average. By contrast, the Decembers and Februaries ranged from much-below to much-above the long-term mean.

Intraseasonal relationships were explored to see what role monthly persistence may have played during these seven winters. It was found that although there was a general tendency for anomalies to persist in sign from one month to the next, particularly in the eastern third and the far West of the United States, the coldness of the Januaries appears to be the result of an intensified meridional circulation occurring for the most part, during the calendar month of January.

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Henry F. Diaz

Abstract

The month of March 1843 has been characterized as exhibiting the greatest temperature anomaly of any month during the period of instrumentally measured meteorological data in the United States. The March 1843 departure patterns from recent temperature normals are compared to those of February 1936 which were of the same magnitude (−30°F/−16.7°C), though perhaps of lesser areal extent. The frigidity of the air during February 1936 set some all time low temperature records that still stand today.

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Henry F. Diaz

Abstract

Areally weighted time series of temperature and precipitation have been compiled for Alaska for the period 1931–1977. Correlations of the temperature values with those of the contiguous United States indicate that, at both the monthly and seasonal time scales, the temperatures over the eastern two-thirds of the contiguous United States and Alaska are basically out of phase. However, with regard to long-term trends, the; temperatures in both Alaska and the lower 48 states exhibit a similar pattern.

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Henry F. Diaz

Abstract

Using state monthly values of the Palmer Drought Index from January 1895 through April 1981, thespatial and temporal features of dry and wet episodes over the contiguous United States were analyzed. Thevariance spectrum of the area under both drought and wet spells in the western United States (17 westernmoststates) was also investigated.

The main results are as follows. Consistent with the findings of previous investigators, the interior andwestern portions of the United States are found to be more drought-prone than other parts of the country.By contrast, the likelihood of drought occurrence in states near coastal areas is considerably less. Prolongedmoisture abnormalities also tend to occur over the more drought-prone states indicating a tendency towardbimodality (either too dry or too wet).

The variance spectrum of the area under drought in the western United States exhibits a red-type spectrum,whereas the wet spell area exhibits relatively greater variance at the highest and intermediate frequencies(2 and 3-9 years). In a few regions of the United States, the initiation and termination of drought episodestend to occur more often at certain times of the year. For most regions, however, this preference is onlymarginal or non-existent.

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Henry F. Diaz

Abstract

The spatial patterns and temporal trends of temperature and precipitation for northern North America (Alaska Canada and western Greenland) have been analyzed. Over approximately the past hundred years, three temperature regimes are identified that correspond roughly to similar climatic regimes identified in separate studies for the contiguous United States. Through 1980, warming is evident only from around the mid-1920s to about the early 1960s. No recent trends are present in winter or fall. Some cooling is evident during summer while spring shows cooling from 1963 to 1976 and warming thereafter.

Spatially, the largest changes occur in areas where variations in the amplitude of the long waves result in large advective differences; these areas are also sensitive to fluctuations in the mean position of the arctic front. Changes from one temperature regime to another occur quite abruptly and last for several years to a few decades.

There are two areas where well-defined precipitation changes coincide with temperature changes: the southern Canadian Plains near the 100°W meridian; and from the Great Lakes to James Bay northeastward toward Labrador. The location of these areas within a principal storm corridor suggests that the changes are associated with southward and northward shifts in the storm track that runs from the upper Midwest/Great Lakes region along the St. Lawrence River Valley toward the North Atlantic.

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Henry F. Diaz and Jerome Namias

Abstract

The relationship between anomalous seasonal surface temperature and precipitation in the contiguous United States and concomitant fluctuations of western Northern Hemisphere 700 mb height as a function of latitude are examined. Findings are consistent with other studies dealing with the spatial patterns of midtropospheric teleconnections and their surface manifestations. A strong inverse relationship between temperature in the eastern United States and heights in the western Northern Hemisphere polar regions is present during winter and spring, especially in winter. In summer the association is positive and is most evident in the southern tier of states. Fewer significant correlations are found between seasonal precipitation and midtropospheric height, with the strongest ones occurring in winter and summer.

It is shown that in winter, during the period 1948–81. the frequency of below-average 700 mb heights has increased in the temperate latitudes of the western Northern Hemisphere. At the same time, a general increase in above normal polar latitude heights has taken place. This in turn has been associated with lower winter temperatures in the eastern United States.

Some suggestions are offered as to the physical-synoptic mechanisms underlying these relationships, although this report is mainly of a fact finding nature. Several data tables and charts are presented for purposes of application to climate studies and long range forecasting.

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Oliver Timm and Henry F. Diaz

Abstract

A linear statistical downscaling technique is applied to the projection of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) climate change scenarios onto Hawaiian rainfall for the late twenty-first century. Hawaii’s regional rainfall is largely controlled by the strength of the trade winds. During the winter months, disturbances in the westerlies can produce heavy rainfall throughout the islands. A diagnostic analysis of sea level pressure (SLP), near-surface winds, and rainfall measurements at 134 weather observing stations around the islands characterize the correlations between the circulation and rainfall during the nominal wet season (November–April) and dry season (May–October). A comparison of the base climate twentieth-century AR4 model simulations with reanalysis data for the period 1970–2000 is used to define objective selection criterion for the AR4 models. Six out of 21 available models were chosen for the statistical downscaling. These were chosen on the basis of their ability to more realistically simulate the modern large-scale circulation fields in the Hawaiian Islands region.

For the AR4 A1B emission scenario, the six analyzed models show important changes in the wind fields around Hawaii by the late twenty-first century. Two models clearly indicate opposite signs in the anomalies. One model projects 20%–30% rainfall increase over the islands; the other model suggests a rainfall decrease of about 10%–20% during the wet season. It is concluded from the six-model ensemble that the most likely scenario for Hawaii is a 5%–10% reduction of the wet-season precipitation and a 5% increase during the dry season, as a result of changes in the wind field. The authors discuss the sources of uncertainties in the projected rainfall changes and consider future improvements of the statistical downscaling work and implications for dynamical downscaling methods.

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Henry F. Diaz and Robert G. Quayle

Abstract

No abstract available.

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Henry F. Diaz and Robert G. Quayle

Abstract

Analysis of monthly-mean temperature and precipitation data for each of the 48 contiguous United States for the 1976–77 through 1978–79 winter seasons shows that the temperature and precipitation departures from the long-term means were extreme. The consecutive occurrence of such severely cold winters is unprecedented in the available 85-year record.

Variability of temperature and precipitation has increased in the past 5-year period, compared to previous pentads, mainly as a result of much greater frequency of extreme anomalies. An “extreme anomaly”is defined as a mean monthly or seasonal value exceeding two standard deviations from the long-term mean.

Statistical estimates of average return periods of winter mean temperatures equal to or lower than the actual values recorded for the past three seasons are close to the empirical values. However, the implausibly low probabilities for the occurrence of consecutive severe winters suggest that the development of large-scale anomalies in atmospheric circulation, which these low temperatures represent, may have a common dynamical forcing and that these forcing mechanisms possess time scales on the order of several years.

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