David S. Gutzler

Teleconnections in the Geopotential Height Field during the Northern Hemisphere Winter

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 1950s. 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 fo...

Toward a Unified View of the American Monsoon Systems

An important goal of the Climate Variability and Predictability (CLIVAR) research on the American monsoon systems is to determine the sources and limits of predictability of warm season precipitation, with emphasis on weekly to interannual time scales. This paper reviews recent progress in the understanding of the American monsoon systems and identifies some of the future challenges that remain to improve warm season climate prediction. Much of the recent progress is derived from complementary international programs in North and South America, namely, the North American Monsoon Experiment (NAME) and the Monsoon Experiment South America (MESA), with the following common objectives: 1) to understand the key components of the American monsoon systems and their variability, 2) to determine the role of these systems in the global water cycle, 3) to improve observational datasets, and 4) to improve simulation and monthly-to-seasonal prediction of the monsoons and regional water resources. Among the recent observational advances highlighted in this paper are new insights into moisture transport processes, description of the structure and variability of the South American low-level jet, and resolution of the diurnal cycle of precipitation in the core monsoon regions. NAME and MESA are also driving major efforts in model development and hydrologic applications. Incorporated into the postfield phases of these projects are assessments of atmosphere–land surface interactions and model-based climate predictability experiments. As CLIVAR research on American monsoon systems evolves, a unified view of the climatic processes modulating continental warm season precipitation is beginning to emerge.

A U.S. Clivar project to assess and compare the responses of global climate models to drought-related SST forcing patterns: Overview and results

Abstract The U.S. Climate Variability and Predictability (CLIVAR) working group on drought recently initiated a series of global climate model simulations forced with idealized SST anomaly patterns, designed to address a number of uncertainties regarding the impact of SST forcing and the role of land–atmosphere feedbacks on regional drought. The runs were carried out with five different atmospheric general circulation models (AGCMs) and one coupled atmosphere–ocean model in which the model was continuously nudged to the imposed SST forcing. This paper provides an overview of the experiments and some initial results focusing on the responses to the leading patterns of annual mean SST variability consisting of a Pacific El Nino–Southern Oscillation (ENSO)-like pattern, a pattern that resembles the Atlantic multidecadal oscillation (AMO), and a global trend pattern. One of the key findings is that all of the AGCMs produce broadly similar (though different in detail) precipitation responses to the Pacific for...

Estimating the Effect of Stochastic Wind Stress Forcing on ENSO Irregularity

Abstract One open question in El Nino–Southern Oscillation (ENSO) simulation and predictability is the role of random forcing by atmospheric variability with short correlation times, on coupled variability with interannual timescales. The discussion of this question requires a quantitative assessment of the stochastic component of the wind stress forcing. Self-consistent estimates of this noise (the stochastic forcing) can be made quite naturally in an empirical atmospheric model that uses a statistical estimate of the relationship between sea surface temperature (SST) and wind stress anomaly patterns as the deterministic feedback between the ocean and the atmosphere. The authors use such an empirical model as the atmospheric component of a hybrid coupled model, coupled to the GFDL ocean general circulation model. The authors define as residual the fraction of the Florida State University wind stress not explained by the empirical atmosphere run from observed SST, and a noise product is constructed by ran...

Interannual Variability of Wintertime Snow Cover across the Northern Hemisphere

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 N...

Evidence for a relationship between spring snow cover in North America and summer rainfall in New Mexico

Two possible mechanisms for interannual modulation of precipitation anomalies in the Southwest U.S. are examined. The association between El Nino/Southern Oscillation (ENSO) fluctuations and New Mexico precipitation anomalies is found to vary with season: positive Pacific Ocean surface temperature anomalies are associated with positive winter-spring precipitation anomalies in New Mexico but negative summer anomalies. We propose that the spring snow extent across the west-central U.S. acts to enhance or suppress the subsequent summer monsoon circulation affecting the American Southwest, such that excessive snow leads to deficient summer rain, and deficient snow leads to abundant rain. This mechanism is consistent with the observed seasonal variation in the ENSO—precipitation relationship.

The NAME 2004 Field Campaign and Modeling Strategy

The North American Monsoon Experiment (NAME) is an internationally coordinated process study aimed at determining the sources and limits of predictability of warm-season precipitation over North America. The scientific objectives of NAME are to promote a better understanding and more realistic simulation of warm-season convective processes in complex terrain, intraseasonal variability of the monsoon, and the response of the warm-season atmospheric circulation and precipitation patterns to slowly varying, potentially predictable surface boundary conditions. During the summer of 2004, the NAME community implemented an international (United States, Mexico, Central America), multiagency (NOAA, NASA, NSF, USDA) field experiment called NAME 2004. This article presents early results from the NAME 2004 campaign and describes how the NAME modeling community will leverage the NAME 2004 data to accelerate improvements in warm-season precipitation forecasts for North America.

Covariability of Spring Snowpack and Summer Rainfall across the Southwest United States

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 be...

Seasonal and Interannual Variability in a Hybrid Coupled GCM

Abstract A hybrid coupled model for the tropical Pacific ocean-atmosphere system is used to simulate El Nino Southern Oscillation (ENSO) interannual variability and to investigate the role of coupling in the seasonal cycle. An ocean GCM (OGCM) is coupled to an empirical atmospheric model that specifies a wind stress field from a given sea surface temperature (SST) field. The stress is estimated by singular value decomposition of the covariance between observed surface wind stress and SST fluctuations. Two versions of the atmospheric model are employed: one includes only spatial patterns of the atmospheric feedbacks associated with interannual variability, whereas the other also includes spatial patterns associated with the annual cycle. In the latter version, wind stress coupling in the seasonal cycle is modeled on the same basis as in the interannual variability. The seasonal cycle enters through prescribed beat flux and is modified by momentum-flux feedbacks. In the OGCM, two vertical mixing schemes-Phi...

Geographical Variations in the Vertical Structure of Geopotential Height Fluctuations1

Abstract Temporal (but nonseasonal) fluctuations in the geopotential height field exhibit large regional contrasts in vertical structure, as manifested in the geographical distributions of the correlation between 1000 and 500 mb height, and the ratio of the amplitudes of the fluctuations at those levels. This geographical variability is investigated in order to ascertain its seasonal, frequency and zonal wavenumber dependence and its relation to other indicators of vertical structure: statistics involving the 1000–500 mb thickness, and the structure of the dominant mode in an eigenvector analysis expansion of geopotential height in the vertical. Results are based on operational analyses by the United States National Meteorological Center over a 15-year period. Particularly striking is the contrast between transient fluctuations over the eastern oceans, which exhibit a highly barotropic structure with strong vertical coherence in the geopotential height field and small temperature variability, and those ov...