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David S. Gutzler and Roland A. Madden

Abstract

Seasonal and geographical variations in tropical intraseasonal wind variance are described using bandpass filtered 850 and 150 mb wind time series derived from rawinsonde observations. Three bandpass filters, with central response periods of 31, 47, and 99 days, are applied to the daily time series. The intermediate filter is designed to isolate variance associated with the “40–50 day oscillation.” The spatial coherence of the bandpass filtered wind fluctuations is examined using complex eigenvector analysis.

Comparisons are made of u and v variance and large-scale structure of filtered wind anomalies for each season and frequency band, with emphasis on the u component. At stations across the western Pacific the 47-day filtered u 150 variance is nearly constant with season. The largest seasonal variability in 47-day filtered zonal wind variance is at 150 mb at stations along and to the north of the equator between Africa and Southeast Asia, and in the central Pacific. Compared to the u 150 variance over the western Pacific, the variance at these stations is much larger in the boreal winter and much smaller in the boreal summer. Large variance at 850 mb is found in each frequency band from the central Indian Ocean eastward to the dateline, with u 850 and u 150 fluctuating out-of-phase and the largest u 850 variance in the summer hemisphere. Eastward propagation of u 150 anomalies is found in each season and frequency band. A longitudinally varying wavenumber structure fits the eigenvectors reasonably well. Across the western Pacific, the u 150 anomalies have a wavenumber 2 structure, consistent with the leading pattern of large-scale convection anomalies. From the dateline eastward across Africa the scale of the u 150 anomalies is broader, closer to a wavenumber 1 scale.

The results suggest that the 40–50 day oscillation in the global tropics has a “two-regime” character. Across the eastern Indian and western Pacific Oceans (the “convective regime”) the 40–50 day oscillation occurs year-round and its spatial structure indicates that it is closely coupled to convection. Elsewhere (the “dry regime”) the oscillation is clearly evident only in the upper troposphere and is subject to strong seasonal modulation.

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David S. Gutzler and Tamara M. Wood

Abstract

Geographical variations in the variance and cross-correlation of monthly mean sea surface temperature (SST), outgoing longwave radiation (OLR, a proxy for deep convection and vertical motion), and convergence of winds at the surface and at 850 mb across the tropical Indian and Pacific oceans are examined. Within about 10° of the equator at most longitudes the variance of these quantities associated with the seasonal cycle is less than the variance associated with anomalies from the seasonal cycle. Largest variances in the SST and surface convergence data occur across the eastern near-equatorial Pacific, whereas OLR and 850 mb convergence variances are largest across the western Pacific. OLR anomalies are significantly correlated with collocated SST and surface convergence anomalies from the date line eastward to the South American coast, but are uncorrelated west of the. date line. The OLR and 850 mb convergence anomalies are significantly correlated from about 120°W westward but are uncorrelated east of that longitude. The near-surface convergence field thus contains a complicated vertical structure that may not be adequately represented in models with a single lower layer.

These calculations suggest that the relative effectiveness of different mechanisms for large ocean ocean-atmosphere coupling varies considerably across the near-equatorial oceans. Direct thermodynamic linkage between SST and convection anomalies is consistent with the results only across the eastern near-equatorial Pacific. Surface gradients of SST are most effective at forcing low-level atmospheric circulation anomalies over the eastern Pacific, where SST and surface convergence have large variances and OLR and 850 mb convergence anomalies are small and not well correlated. West of the date line, forcing by midtropospheric latent heating seems most consistent with the observed relationships.

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David S. Gutzler and Richard D. Rosen

Abstract

Digitized maps of Northern Hemisphere snow cover derived from visible satellite imagery are examined to assess the interannual variability of snow cover in winter months for years 1972–90. The secular trend of winter snow cover over the landmasses of Eurasia and North America during this period is extremely small in December and January. A decreasing trend of somewhat larger magnitude is observed in Eurasian snow cover in February. Fluctuations of detrended interannual snow-cover anomalies averaged over the Eurasian and North American continents are positively correlated. By subdividing the continents into longitudinal sectors it is determined that this intercontinental relationship is due to high correlations between European and North American sectors. The relationship of snow-cover fluctuations to large-scale circulation anomalies is described using lime series of teleconnection pattern indices derived from monthly mean geopotential height fields. A pattern of height anomalies resembling the North Atlantic Oscillation is correlated with snow-cover anomalies in North America and Europe. The Pacific-North American teleconnection pattern is highly correlated with snow-cover anomalies in western North America but has limited influence on intercontinental snow-cover fluctuations.

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John M. Wallace and David S. Gutzler

Abstract

Contemporaneous correlations between geopotential heights on a given pressure surface at widely separated points on earth, referred to as teleconnections in this paper, are studied in an attempt to identify and document recurrent spatial patterns which might be indicative of standing oscillations in the planetary waves during the Northern Hemisphere winter, with time scales on the order of a month or longer. A review of existing literature on the subject reveals the existence of at least four such patterns: the North Atlantic and North Pacific Oscillations identified by Walker and Bliss (1932). a zonally symmetric seesaw between sea level pressures in polar and temperature latitudes, first noted by Lorenz (1951), and what we will refer to as the Pacific/North American pattern, which has been known to operational long-range forecasters in this country since the 1950's.

A data set consisting of NMC monthly mean sea level pressure and 500 mb height analyses for a 15-year period is used as a basis for calculating the temporal correlation coefficients between all possible pairs of grid points. An objective method is used to identify and describe the strongest teleconnection patterns in this correlation matrix. The five leading patterns are compared with, and found to bear some similarity to, the leading eigenvectors of the correlation matrix. Certain of the above calculations are repeated on an independent data set in order to test the reproducibility of the patterns.

The North Atlantic Oscillation and the Pacific/North American patterns are strongly evident in both data acts. The former is associated with fluctuations in the strength of the climatological mean jet stream over the western Atlantic. The Pacific/North American pattern includes a north–south seesaw in the central Pacific somewhat reminiscent of the North Pacific Oscillation mentioned by Walker and Bliss (1932) and Bjerknes (1969), together with centers of action over western Canada and the southeastern United States. Several other teleconnection patterns are revealed by the analysis of the primary data set, but are not found to be as reproducible in the independent data set.

The sea level pressure statistics are dominated by negative correlations between the polar region and temperature latitudes, whereas the 500 mb statistics are dominated by patterns of a more regional scale, which display a nearly equivalent barotropic structure with amplitudes increasing with height. Most of the regional patterns have only one or two well-defined centers of action at the earth's surface, but at mid-tropospheric levels they are more wavelike in appearance and characterized by multiple centers of action; at these levels their structure resembles that of forced stationary wakes on a sphere.

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Shannon M. Jones and David S. Gutzler

Abstract

Southwestern North America (SWNA) is projected to become drier in the twenty-first century as both precipitation (P) and evaporation (E) rates change with increasing greenhouse gas concentration. The authors diagnose the relative contributions of changes in P and E to the local surface moisture balance (PE) in cold and warm halves of the year across SWNA. Trends in PE vary spatially between the arid southern subregion (mostly northern Mexico) and the more temperate northern subregion (southwest United States), although both subregions exhibit a negative trend in PE (trending toward more arid conditions) in CMIP5 projections for the twenty-first century. The PE trend is biggest in the cold season, when much of the base flow to rivers in the southwest United States is generated. The downward trend in cold season PE across SWNA is caused primarily by increasing E in the north and decreasing P in the south. Decreasing P is the primary contributor to modest warm season drying trends in both northern and southern subregions. Also, P accounts for most of the interannual variability in SWNA PE and is strongly correlated with modes of oceanic natural variability during the cold season. SWNA aridification is therefore most readily distinguished from the region’s large natural climate variability in the cold season in the northern subregion, where the projected temperature-driven increase in E is greater than the projected decrease in P.

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William J. Gutowski Jr., David S. Gutzler, and Wei-Chyung Wang

Abstract

We examine surface energy balances simulated by three general circulation models for current climatic boundary conditions and for an atmosphere with twice current levels of CO2. Differences between model simulations provide a measure of uncertainty in the prediction of surface temperature in a double-CO2 climate, and diagnosis of the energy balance suggests the radiative and thermodynamic processes responsible for these differences. The scale dependence of the surface energy balance is examined by averaging over a hierarchy of spatial domains ranging from the entire globe to regions encompassing just a few model grid points.

Upward and downward longwave fluxes are the dominant terms in the global-average balance for each model and climate. The models product nearly the same global-average surface temperature in their current climate simulations, so their upward longwave fluxes are nearly the same, but in the global-average balance their downward longwave fluxes, absorbed solar radiation, and sensible and latent heat fluxes have intermodel discrepancies that are larger than respective flux changes associated with doubling CO2. Despite the flux discrepancies, the globally averaged surface flux changes associated with CO2 doubling are qualitatively consistent among the models, suggesting that the basic large-scale mechanisms of greenhouse warming are not very sensitive to the precise surface balance of heat occurring in a model's current climate simulation.

The net longwave flux at the surface has small spatial variability, so global-average discrepancies in surface longwave fluxes are also manifested in the regional-scale balances. For this reason, increasing horizontal resolution will not improve the consistency of regional-scale climate simulations in these models unless discrepancies in global-average longwave radiation are resolved. Differences between models in simulating effects of moisture in the atmosphere and in the ground appear to be an important cause of differences in surface energy budgets on all scales.

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David S. Gutzler, Deirdre M. Kann, and Casey Thornbrugh

Abstract

Seasonal predictability of winter precipitation anomalies across the U.S. Southwest derived from knowledge of antecedent, late-summer Pacific Ocean surface temperatures is examined empirically. Previous studies have shown that equatorial Pacific SST anomalies associated with the El Niño–Southern Oscillation (ENSO) cycle, which are persistent from late summer through winter, exhibit a strong relationship with winter precipitation in Arizona and New Mexico. Here the degree to which seasonal predictability in this region is modulated by longer-term oceanic fluctuations associated with the Pacific decadal oscillation (PDO) is assessed. When all years from 1950 through 1997 are considered as a single dataset, inclusion of the PDO signal adds only slightly to the ENSO-based statistical predictability of Southwest winter precipitation anomalies. However, when the dataset is split into two subperiods delineated by a major shift in the PDO (before and after 1977), the ENSO-based predictability and, to a lesser extent, PDO-based predictability are substantially modified. Before 1977, negative winter precipitation anomalies are strongly tied to ENSO cold years but warm years do not systematically lead to positive precipitation anomalies. After 1977, this asymmetry is reversed and positive precipitation anomalies predictably follow warm ENSO years but cold years yield no precipitation predictability. Within each subperiod, interannual PDO fluctuations yield less predictability than ENSO fluctuations. Thus ENSO-based predictability seems to undergo a profound decadal modification that might be associated statistically with the PDO, but the physical link to North Pacific Ocean temperatures is problematic.

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David S. Gutzler, Sharon M. Sullivan, and Deirdre M. Kann

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The wettest year, by a huge margin, in the instrumental history for the state of New Mexico was 1941. The authors describe the extraordinary magnitude and persistence of above-average precipitation across the seasonal cycle during this year and consider possible climatic causes of this exceptional annual anomaly through examination of a wide variety of historical records and modern analysis tools. Indices of the Pacific decadal oscillation and the El Niño–Southern Oscillation were both extremely positive in 1941, consistent with the historical tendency for above-average precipitation across the southern United States under such conditions. However, the largest precipitation anomalies occurred in transition season months that do not fit the typical seasonality associated with strong ENSO- or PDO-related continental climate anomalies in the more recent historical record. The difficulty in attributing this extreme annual anomaly to any specific climatic cause is a reminder that the radiosonde era provides only a limited sample of natural climatic variability. The number and quality of data sources available for preradiosonde years allows for surprisingly in-depth observational analysis of early twentieth-century climatic anomalies.

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David S. Gutzler, Richard D. Rosen, David A. Salstein, and JoséP. Peixoto

Abstract

Interannual fluctuations of observed winter seasonal mean 850 mb temperatures over the Northern Hemisphere during 1958–73 are documented and compared with midtropospheric height variations. Interannual temperature variance maxima are found over the Eurasian and North American continents, in striking contrast to the height field which exhibits variance maxima over the midlatitude oceans. Patterns of interannual variability are defined objectively using eigenvector analysis. The first two spatial eigenvectors of temperature variability describe hemisphere-scale patterns. The gravest eigenvector contains elements of the Eurasian and Pacific–North American (PNA) height patterns defined in earlier studies. One-point correlation maps confirm the strong positive correlation between temperature fluctuations over Siberia and western Canada found in the first eigenvector but indicate that other elements of intercontinentality are not so strong as the eigenvectors suggest.

To isolate more regionalized patterns of variability, therefore, the leading temperature eigenvectors are subjected to varimax rotation. The leading rotated pattern contains North American centers coincident with the PNA height pattern and an additional Caribbean center, and its temporal fluctuations are highly correlated with a PNA index derived from 500 mb height anomalies. Over the Asian continent, two temperature patterns are found that incorporate north–south anomaly dipoles not clearly depicted in height patterns. One of these patterns also describes the tendency for positive correlation between temperatures over the Gulf of Alaska and Siberia. Another pattern contains two centers over Europe and a broad center across western and central Asia. A fifth rotated pattern describes temperature fluctuations associated with the North Atlantic Oscillation in the sea level pressure field.

Time series associated with patterns containing centers over Siberia and northwest North America are also correlated with interannual fluctuations of hemisphere-averaged temperature. In particular, temporal fluctuations of the PNA height pattern or the leading rotated temperature pattern are significantly correlated with hemisphere-mean seasonal temperature anomalies at 850 mb and at the surface.

The roles of anomalous temperature advection and transient eddy heat flux divergence in the heat balance of seasonal temperature anomalies are examined using correlation statistics. Over much of the hemisphere, seasonal anomalies of temperature advection appear to maintain seasonal temperature anomalies, while eddy heat flux divergence anomalies tend to dissipate them. However, advection anomalies are poorly correlated with temperature anomalies over the Asian continent west of about 90°E, so another term in the heat balance must play a dominant role. We speculate that radiative forcing due to snow cover anomalies may be important in this region.

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Elizabeth A. Ritchie, Kimberly M. Wood, David S. Gutzler, and Sarah R. White

Abstract

Forty-three eastern North Pacific tropical cyclone remnants with varying impact on the southwestern United States during the period 1992–2005 are investigated. Of these, 35 remnants (81%) brought precipitation to some part of the southwestern United States and the remaining 8 remnants (19%) had precipitation that was almost entirely restricted to Mexico, although cloud cover did advect over the southwestern United States in some of these cases. Although the tropical cyclone–strength winds rapidly diminish upon making landfall, these systems still carry a large quantity of tropical moisture and, upon interaction with mountainous topography, are found to drop up to 30% of the local annual precipitation.

Based on common rainfall patterns and large-scale circulation features, the tropical cyclones are grouped into five categories. These include a northern recurving pattern that is more likely to bring rainfall to the southwestern United States; a southern recurving pattern that brings rainfall across northern Mexico and the Gulf Coast region; a largely north and/or northwestward movement pattern that brings rainfall to the west coast of the United States; a group that is blocked from the southwest by a ridge, which limits rainfall to Mexico; and a small group of cases that are not clearly any of the previous four types. Composites of the first four groups are shown and forecasting strategies for each are described.

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