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- Author or Editor: Henry F. Diaz x
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Abstract
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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.
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.
Abstract
Principal component (PC) analysis performed on irregularly spaced data can produce distorted loading patterns. We provide an example to demonstrate some distorted patterns which can result from the direct application of PC analysis (or eigenvector analysis, factor analysis, or asymptotic singular decomposition) on irregularly spaced data. The PCs overestimate loadings in areas of dense data. The problem can be avoided by interpolating the irregularly spaced data to a grid which closely approximates equal-area.
Abstract
Principal component (PC) analysis performed on irregularly spaced data can produce distorted loading patterns. We provide an example to demonstrate some distorted patterns which can result from the direct application of PC analysis (or eigenvector analysis, factor analysis, or asymptotic singular decomposition) on irregularly spaced data. The PCs overestimate loadings in areas of dense data. The problem can be avoided by interpolating the irregularly spaced data to a grid which closely approximates equal-area.
Abstract
The possible effects of climatic fluctuations on renewable water supplies in the western United States was examined, especially as it is impacted by the growth of population and water consumption in recent decades.
Precipitation fluctuations in the Colorado River Basin states have differed depending upon their location, but have tended to fluctuate with a time scale of one to two decades. Longer-term regimes may also be operative. For example, the Upper Basin states (Colorado and Utah) experienced a prolonged wet interval from about the turn of this century to around 1930; from 1930 to around 1978, drier than normal years tended to outnumber wet ones; and since 1978 the Upper Basin has been exceedingly wet. Lower Basin states also experienced the early wet period and drier conditions after the mid-1940s, but they undergo somewhat different alternations of wetness and dryness. However, from the point of view of water supply, precipitation variability in the Upper Basin, particularly in Colorado, is more critical.
Reservoir capacity in the and western states is expected to gain little in additional storage capacity during the next couple of decades; in addition, withdrawal of water from the Colorado River is approaching the legal limits. The effect of a future prolonged drought on the order of those which have occurred in recent decades, or in a worse case, those which have occurred in past centuries from tree ring studies, could have far more serious consequences than any in previous experience due to the large population increases in the region. These population trends show all signs of continuing, at least in the near future. The impact of a drought, however, would depend on the level of reservoir capacity that is present at the time of drought onset as well as its intensity and longevity; reservoirs in the West are presently at or near capacity.
Abstract
The possible effects of climatic fluctuations on renewable water supplies in the western United States was examined, especially as it is impacted by the growth of population and water consumption in recent decades.
Precipitation fluctuations in the Colorado River Basin states have differed depending upon their location, but have tended to fluctuate with a time scale of one to two decades. Longer-term regimes may also be operative. For example, the Upper Basin states (Colorado and Utah) experienced a prolonged wet interval from about the turn of this century to around 1930; from 1930 to around 1978, drier than normal years tended to outnumber wet ones; and since 1978 the Upper Basin has been exceedingly wet. Lower Basin states also experienced the early wet period and drier conditions after the mid-1940s, but they undergo somewhat different alternations of wetness and dryness. However, from the point of view of water supply, precipitation variability in the Upper Basin, particularly in Colorado, is more critical.
Reservoir capacity in the and western states is expected to gain little in additional storage capacity during the next couple of decades; in addition, withdrawal of water from the Colorado River is approaching the legal limits. The effect of a future prolonged drought on the order of those which have occurred in recent decades, or in a worse case, those which have occurred in past centuries from tree ring studies, could have far more serious consequences than any in previous experience due to the large population increases in the region. These population trends show all signs of continuing, at least in the near future. The impact of a drought, however, would depend on the level of reservoir capacity that is present at the time of drought onset as well as its intensity and longevity; reservoirs in the West are presently at or near capacity.
Abstract
Several equations were developed that related the effect of urban growth, measured by increasing population, to the mean seasonal and annual temperature: the diurnal maximum, minimum, average, and range. These equations were derived from a network of 1219 stations across the United States, which were analyzed for the years 1901–84. The results indicate that urban effects on temperature are detectable even for small towns with populations under 10 000. Stations with populations near 10 000 are shown to average 0.1°C warmer for the mean annual temperature than nearby stations located in rural areas with populations less than 2000. Urbanization decreases the daily maxima in all seasons except winter and the temperature range in all seasons. It increases the diurnal minima and the daily means in all seasons.
The equations indicate that, for the annual mean temperature, urbanization during the twentieth century accounts for a warm bias of about 0.06°C in the U.S. Historical Climatology Network (HCN). Due to the large number of stations located in sparsely populated arms [(over 85% (70%) of all stations had a 1980 population of less than 25 000 (10 000)], the impact of urbanization is not large in relation to decadal changes of temperature in the United States. The average heat island impact during the period 1901–84 for the HCN is largest for the daily minima (0.13°C) and the temperature range (−0.14°C), while the impact on the daily maxima (−0.01°C) is an order of magnitude smaller.
Abstract
Several equations were developed that related the effect of urban growth, measured by increasing population, to the mean seasonal and annual temperature: the diurnal maximum, minimum, average, and range. These equations were derived from a network of 1219 stations across the United States, which were analyzed for the years 1901–84. The results indicate that urban effects on temperature are detectable even for small towns with populations under 10 000. Stations with populations near 10 000 are shown to average 0.1°C warmer for the mean annual temperature than nearby stations located in rural areas with populations less than 2000. Urbanization decreases the daily maxima in all seasons except winter and the temperature range in all seasons. It increases the diurnal minima and the daily means in all seasons.
The equations indicate that, for the annual mean temperature, urbanization during the twentieth century accounts for a warm bias of about 0.06°C in the U.S. Historical Climatology Network (HCN). Due to the large number of stations located in sparsely populated arms [(over 85% (70%) of all stations had a 1980 population of less than 25 000 (10 000)], the impact of urbanization is not large in relation to decadal changes of temperature in the United States. The average heat island impact during the period 1901–84 for the HCN is largest for the daily minima (0.13°C) and the temperature range (−0.14°C), while the impact on the daily maxima (−0.01°C) is an order of magnitude smaller.
Abstract
Consistent increases in the strength and frequency of occurrence of the trade wind inversion (TWI) are identified across a ~40-yr period (1973–2013) in Hawaii. Changepoint analysis indicates that a marked shift occurred in the early 1990s resulting in a 20% increase in the mean TWI frequency between the periods 1973–90 and 1991–2013, based on the average of changes at two sounding stations and two 6-month (dry and wet) seasons. Regional increases in the atmospheric subsidence are identified in four reanalysis datasets over the same ~40-yr time period. The post-1990 period mean for the NCEP–NCAR reanalysis shows increases in subsidence of 33% and 41% for the dry and wet seasons, respectively. Good agreement was found between the time series of TWI frequency of occurrence and omega, suggesting that previously reported increases in the intensity of Hadley cell subsidence are driving the observed increases in TWI frequency. Correlations between omega and large-scale modes of internal climate variability such as El Niño–Southern Oscillation (ENSO) and the Pacific decadal oscillation (PDO) do not explain the abrupt shift in TWI frequency in the early 1990s in both seasons. Reported increases in TWI frequency of occurrence may provide some explanation for climate change–related precipitation change at high elevations in Hawaii. On average, post-1990 rainfall was 6% lower in the dry season and 31% lower in the wet season at nine high-elevation sites. Rainfall was significantly correlated with TWI frequency at all of the stations analyzed.
Abstract
Consistent increases in the strength and frequency of occurrence of the trade wind inversion (TWI) are identified across a ~40-yr period (1973–2013) in Hawaii. Changepoint analysis indicates that a marked shift occurred in the early 1990s resulting in a 20% increase in the mean TWI frequency between the periods 1973–90 and 1991–2013, based on the average of changes at two sounding stations and two 6-month (dry and wet) seasons. Regional increases in the atmospheric subsidence are identified in four reanalysis datasets over the same ~40-yr time period. The post-1990 period mean for the NCEP–NCAR reanalysis shows increases in subsidence of 33% and 41% for the dry and wet seasons, respectively. Good agreement was found between the time series of TWI frequency of occurrence and omega, suggesting that previously reported increases in the intensity of Hadley cell subsidence are driving the observed increases in TWI frequency. Correlations between omega and large-scale modes of internal climate variability such as El Niño–Southern Oscillation (ENSO) and the Pacific decadal oscillation (PDO) do not explain the abrupt shift in TWI frequency in the early 1990s in both seasons. Reported increases in TWI frequency of occurrence may provide some explanation for climate change–related precipitation change at high elevations in Hawaii. On average, post-1990 rainfall was 6% lower in the dry season and 31% lower in the wet season at nine high-elevation sites. Rainfall was significantly correlated with TWI frequency at all of the stations analyzed.
Abstract
The development of serially complete (no missing values) daily maximum–minimum temperatures and total precipitation time series over the western United States is documented. Several estimation techniques based on spatial objective analysis schemes are used to estimate daily values, with the &ldquost” estimate chosen as a missing value replacement. The development of a continuous and complete daily dataset will be useful in a variety of meteorological and hydrological research applications.
The spatial interpolation schemes are evaluated separately by interpolation method and calendar month. Cross validation of the results indicates a distinct seasonality to the efficiency (error) of the estimates, although no systematic bias in the estimation procedures was found. The resulting number of serially complete daily time series for the western United States (all states west of the Mississippi River) includes 2034 maximum–minimum temperature stations and 2962 total daily precipitation locations.
Abstract
The development of serially complete (no missing values) daily maximum–minimum temperatures and total precipitation time series over the western United States is documented. Several estimation techniques based on spatial objective analysis schemes are used to estimate daily values, with the &ldquost” estimate chosen as a missing value replacement. The development of a continuous and complete daily dataset will be useful in a variety of meteorological and hydrological research applications.
The spatial interpolation schemes are evaluated separately by interpolation method and calendar month. Cross validation of the results indicates a distinct seasonality to the efficiency (error) of the estimates, although no systematic bias in the estimation procedures was found. The resulting number of serially complete daily time series for the western United States (all states west of the Mississippi River) includes 2034 maximum–minimum temperature stations and 2962 total daily precipitation locations.
Abstract
The overall amount of precipitation deposited along the West Coast and western cordillera of North America from 25° to 55°N varies from year to year, and superimposed on this domain-average variability are varying north–south contrasts on timescales from at least interannual to interdecadal. In order to better understand the north–south precipitation contrasts, their interannual and decadal variations are studied in terms of how much they affect overall precipitation amounts and how they are related to large-scale climatic patterns. Spatial empirical orthogonal functions (EOFs) and spatial moments (domain average, central latitude, and latitudinal spread) of zonally averaged precipitation anomalies along the westernmost parts of North America are analyzed, and each is correlated with global sea level pressure (SLP) and sea surface temperature series, on interannual (defined here as 3–7 yr) and decadal (>7 yr) timescales. The interannual band considered here corresponds to timescales that are particularly strong in tropical climate variations and thus is expected to contain much precipitation variability that is related to El Niño–Southern Oscillation; the decadal scale is defined so as to capture the whole range of long-term climatic variations affecting western North America.
Zonal EOFs of the interannual and decadal filtered versions of the zonal-precipitation series are remarkably similar. At both timescales, two leading EOFs describe 1) a north–south seesaw of precipitation pivoting near 40°N and 2) variations in precipitation near 40°N, respectively. The amount of overall precipitation variability is only about 10% of the mean and is largely determined by precipitation variations around 40°–45°N and most consistently influenced by nearby circulation patterns; in this sense, domain-average precipitation is closely related to the second EOF. The central latitude and latitudinal spread of precipitation distributions are strongly influenced by precipitation variations in the southern parts of western North America and are closely related to the first EOF. Central latitude of precipitation moves south (north) with tropical warming (cooling) in association with midlatitude western Pacific SLP variations, on both interannual and decadal timescales. Regional patterns and zonal averages of precipitation-sensitive tree-ring series are used to corroborate these patterns and to extend them into the past and appear to share much long- and short-term information with the instrumentally based zonal precipitation EOFs and moments.
Abstract
The overall amount of precipitation deposited along the West Coast and western cordillera of North America from 25° to 55°N varies from year to year, and superimposed on this domain-average variability are varying north–south contrasts on timescales from at least interannual to interdecadal. In order to better understand the north–south precipitation contrasts, their interannual and decadal variations are studied in terms of how much they affect overall precipitation amounts and how they are related to large-scale climatic patterns. Spatial empirical orthogonal functions (EOFs) and spatial moments (domain average, central latitude, and latitudinal spread) of zonally averaged precipitation anomalies along the westernmost parts of North America are analyzed, and each is correlated with global sea level pressure (SLP) and sea surface temperature series, on interannual (defined here as 3–7 yr) and decadal (>7 yr) timescales. The interannual band considered here corresponds to timescales that are particularly strong in tropical climate variations and thus is expected to contain much precipitation variability that is related to El Niño–Southern Oscillation; the decadal scale is defined so as to capture the whole range of long-term climatic variations affecting western North America.
Zonal EOFs of the interannual and decadal filtered versions of the zonal-precipitation series are remarkably similar. At both timescales, two leading EOFs describe 1) a north–south seesaw of precipitation pivoting near 40°N and 2) variations in precipitation near 40°N, respectively. The amount of overall precipitation variability is only about 10% of the mean and is largely determined by precipitation variations around 40°–45°N and most consistently influenced by nearby circulation patterns; in this sense, domain-average precipitation is closely related to the second EOF. The central latitude and latitudinal spread of precipitation distributions are strongly influenced by precipitation variations in the southern parts of western North America and are closely related to the first EOF. Central latitude of precipitation moves south (north) with tropical warming (cooling) in association with midlatitude western Pacific SLP variations, on both interannual and decadal timescales. Regional patterns and zonal averages of precipitation-sensitive tree-ring series are used to corroborate these patterns and to extend them into the past and appear to share much long- and short-term information with the instrumentally based zonal precipitation EOFs and moments.
Abstract
Decadal (>7- yr period) variations of precipitation over western North America account for 20%–50% of the variance of annual precipitation. Spatially, the decadal variability is broken into several regional [O(1000 km)] components. These decadal variations are contributed by fluctuations in precipitation from seasons of the year that vary from region to region and that are not necessarily concentrated in the wettest season(s) alone. The precipitation variations are linked to various decadal atmospheric circulation and SST anomaly patterns where scales range from regional to global scales and that emphasize tropical or extratropical connections, depending upon which precipitation region is considered. Further, wet or dry decades are associated with changes in frequency of at least a few short-period circulation “modes” such as the Pacific–North American pattern. Precipitation fluctuations over the southwestern United States and the Saskatchewan region of western Canada are associated with extensive shifts of sea level pressure and SST anomalies, suggesting that they are components of low-frequency precipitation variability from global-scale climate processes. Consistent with the global scale of its pressure and SST connection, the Southwest decadal precipitation is aligned with opposing precipitation fluctuations in northern Africa.
Abstract
Decadal (>7- yr period) variations of precipitation over western North America account for 20%–50% of the variance of annual precipitation. Spatially, the decadal variability is broken into several regional [O(1000 km)] components. These decadal variations are contributed by fluctuations in precipitation from seasons of the year that vary from region to region and that are not necessarily concentrated in the wettest season(s) alone. The precipitation variations are linked to various decadal atmospheric circulation and SST anomaly patterns where scales range from regional to global scales and that emphasize tropical or extratropical connections, depending upon which precipitation region is considered. Further, wet or dry decades are associated with changes in frequency of at least a few short-period circulation “modes” such as the Pacific–North American pattern. Precipitation fluctuations over the southwestern United States and the Saskatchewan region of western Canada are associated with extensive shifts of sea level pressure and SST anomalies, suggesting that they are components of low-frequency precipitation variability from global-scale climate processes. Consistent with the global scale of its pressure and SST connection, the Southwest decadal precipitation is aligned with opposing precipitation fluctuations in northern Africa.
Developing accurate reconstructions of past climate regimes and enhancing our understanding of the causal factors that may have contributed to their occurrence is important for a number of reasons; these include improvements in the attribution of climate change to natural and anthropogenic forcing, gaining a better appreciation for the range and magnitude of low-frequency variability and previous climatic regimes in comparison with the modern instrumental period, and developing greater insights into the relationship between human society and climatic changes. This paper examine upto- date evidence regarding the characteristics of the climate in medieval times (A.D. ~950–1400). Long and high-resolution climate proxy records reported in the scientific literature, which form the basis for the climate reconstructions, have greatly expanded in the last few decades, with greater numbers of sites that now cover more areas of the globe. Some comparisons with the modern climate record and discussion of potential mechanisms associated with the patterns of medieval climate are presented here, but our main goal is to provide the reader with some appreciation of the richness of past natural climate variability in terms of its spatial and temporal characteristics.
Developing accurate reconstructions of past climate regimes and enhancing our understanding of the causal factors that may have contributed to their occurrence is important for a number of reasons; these include improvements in the attribution of climate change to natural and anthropogenic forcing, gaining a better appreciation for the range and magnitude of low-frequency variability and previous climatic regimes in comparison with the modern instrumental period, and developing greater insights into the relationship between human society and climatic changes. This paper examine upto- date evidence regarding the characteristics of the climate in medieval times (A.D. ~950–1400). Long and high-resolution climate proxy records reported in the scientific literature, which form the basis for the climate reconstructions, have greatly expanded in the last few decades, with greater numbers of sites that now cover more areas of the globe. Some comparisons with the modern climate record and discussion of potential mechanisms associated with the patterns of medieval climate are presented here, but our main goal is to provide the reader with some appreciation of the richness of past natural climate variability in terms of its spatial and temporal characteristics.
