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David S. Gutzler

Abstract

Seasonal precipitation anomalies associated with the continental North American monsoon system are characterized using a land-based dataset derived from in situ observations across the southwestern United States and northwestern Mexico. Coherent regions of interannual continental precipitation variability are derived from principal component analysis, after defining separate “early” and “late” summer monsoon seasons. The gravest mode of late-season interannual variability captures precipitation anomalies in the core of the continental monsoon domain. A simple spatial average is developed as an index of this core variability. The seasonal separation allows examination of persistence of precipitation anomalies as an indicator of practical late-season predictability. Possible influences of large-scale oceanic interannual fluctuations [ENSO and Pacific decadal oscillation (PDO)] on core index precipitation anomalies are also considered. The core precipitation index exhibits considerably more early-to-late-season persistence than an ocean-centered precipitation index. Implications of these results for monthly/seasonal predictability of warm-season precipitation are discussed.

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David S. Gutzler

Abstract

Geographical and seasonal variations of the variance and vertical structure of interannual anomalies of seasonally averaged zonal winds are calculated from rawinsonde records at tropical stations between 80°E and 140°W, the longitudes spanned by the “Walker Circulation.” Wind anomalies are negatively correlated in the upper and lower troposphere only in a latitude band within about 10° of the equator, defining the latitudinal extent of the Walker Circulation; about 70% of the interannual variance of tropospheric zonal winds is accounted for by a single vertical mode within this band. The band shifts seasonally north and south slightly, in conjunction with seasonal shifts in large-scale convection. The level of zonal overturning occurs between 300 and 500 mb, and is highest near the seasonal convective maximum. Other vertical structures, suggestive of interannual variability not associated with thermally forced circulations, are also found near the equator at some longitudes in the boreal winter season.

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David S. Gutzler

Abstract

Interannual fluctuations of observed summer rainfall across the monsoon region of the southwestern United States are analyzed to ascertain their spatial coherence and to test the hypothesis that antecedent spring snowpack anomalies may modulate the monsoon and exhibit an inverse correlation with summer rainfall anomalies. To characterize the spatial coherence of seasonal rainfall anomalies, an objective linear analysis of interannual variability is applied to climate divisional data across the Southwest. Three coherent subregions are identified, broadly representing rainfall anomalies across Arizona, eastern New Mexico/western Texas (the Southwest Plains), and most of New Mexico. Interannual fluctuations of summer rainfall in the New Mexico region exhibit a very significant negative correlation with a large-scale index of the antecedent 1 April snowpack over the southern U.S. Rocky Mountains during the 1961–90 climatic averaging period. This strong relationship seems to break down in the years before and after this period, possibly indicating a shift in climate associated with other forcing factors.

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David A. Portman and David S. Gutzler

Abstract

A study was conducted to identify and separate possible signals of volcanic eruptions and of the El Niño–Southern Oscillation (ENSO) in U.S. surface climate records. Anomalies of monthly mean surface air temperature and total precipitation taken from the U.S. Historical Climatology Network were composited (averaged) over years of major explosive volcanic eruptions. ENSO warm events, and ENSO cold events since the year 1900. It was assumed that volcanic eruptions and ENSO events occur independently of each other. All composite anomalies were assessed for significance with regard to several statistical and physical criteria. The composite ENSO-related anomalies were then subtracted from anomalies of temperature and precipitation associated with the volcanic eruptions.

Removal of large magnitude and highly significant anomalies associated with the ENSO warm and cold events is found to facilitate detection of volcanic signals in monthly records of U.S. temperature and precipitation. Volcanic signals are strongly suggested cast of the Continental Divide, for example, where positive monthly temperature anomalies exceeding 1°C occur during the first fall and winter after eruptions. Negative temperature anomalies occur west of the Continental Divide during the first winter and spring after eruptions and in the southern United States during the summer of the first post-eruption calendar year. Positive monthly precipitation anomalies exceeding 15 mm in magnitude are found in the southeastern United States during the first winter and spring after eruptions. Precipitation anomalies that are smaller in magnitude and yet significant, such as positive anomalies in the northwestern United States and negative anomalies in the central and eastern United States, are found during the summer of the first post-eruption calendar year.

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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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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, 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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