Gerald D. Bell

Statistical Relationships between Topography and Precipitation Patterns

Abstract Statistical relationships between topography and the spatial distribution of mean annual precipitation are developed for ten distinct mountainous regions. These relationships are derived through linear bivariate and multivariate analyses, using six topographic variables as predictors of precipitation. These predictors are elevation, slope, orientation, exposure, the product (or interaction) of slope and orientation, and the product of elevation and exposure. The two interactive terms are the best overall bivariate predictors of mean annual precipitation, whereas orientation and exposure are the strongest noninteractive bivariate predictors. The regression equations in many of the climatically similar regions tend to have similar slope coefficients and similar y-intercept values, indicating that local climatic conditions strongly influence the relationship between topography and the spatial distribution of precipitation. In contrast, the regression equations for the tropical and extratropical regi...

Leading Tropical Modes Associated with Interannual and Multidecadal Fluctuations in North Atlantic Hurricane Activity

Abstract Interannual and multidecadal extremes in Atlantic hurricane activity are shown to result from a coherent and interrelated set of atmospheric and oceanic conditions associated with three leading modes of climate variability in the Tropics. All three modes are related to fluctuations in tropical convection, with two representing the leading multidecadal modes of convective rainfall variability, and one representing the leading interannual mode (ENSO). The tropical multidecadal modes are shown to link known fluctuations in Atlantic hurricane activity, West African monsoon rainfall, and Atlantic sea surface temperatures, to the Tropics-wide climate variability. These modes also capture an east–west seesaw in anomalous convection between the West African monsoon region and the Amazon basin, which helps to account for the interhemispheric symmetry of the 200-hPa streamfunction anomalies across the Atlantic Ocean and Africa, the 200-hPa divergent wind anomalies, and both the structure and spatial scale ...

Atmospheric Circulation Associated With the Midwest Floods of 1993

Abstract This paper presents an observational analysis of the large-scale atmospheric circulation prior to and during the Midwest floods of June–July 1993. The floods developed and persisted in association with three major circulation features, none of which alone would likely have produced such intense and prolonged flooding. First, a persistent, positive phase of the North Pacific teleconnection pattern was observed throughout the Pacific sector for four months prior to the onset of the floods. This anomalous circulation was associated with much above-normal cyclone activity over the middle latitudes of the North Pacific and with below-normal cyclone activity over the western and central United States. Second, a major change in this pattern occurred over the western United States in late May, which established very strong zonal flow from the western Pacific to the eastern United States. This flow provided a “duct” for the intense cyclones to propagate directly into the Midwest throughout the month of Ju...

Tropical Multidecadal and Interannual Climate Variability in the NCEP–NCAR Reanalysis

Abstract The leading tropical multidecadal mode (TMM) and tropical interannual (ENSO) mode in the 52-yr (1949– 2000) NCEP–NCAR reanalysis are examined for the December–February (DJF) and June–August (JJA) seasons based on seasonal tropical convective rainfall variability and tropical surface (land + ocean) temperature variability. These combined modes are shown to capture 70%–80% of the unfiltered variance in seasonal 200-hPa velocity potential anomalies in the analysis region of 30°N–30°S. The TMM is the dominant mode overall, accounting for 50%–60% of the total unfiltered variance in both seasons, compared to the 22%–24% for ENSO. The robustness of the tropical multidecadal mode is addressed, and the results are shown to compare favorably with observed station data and published results of decadal climate variability in the key loading regions. The temporal and spatial characteristics of this mode are found to be distinct from ENSO. The TMM captures the global climate regimes observed during the 1950s–6...

A Statistical Forecast Model for Atlantic Seasonal Hurricane Activity Based on the NCEP Dynamical Seasonal Forecast

Abstract A hybrid dynamical–statistical model is developed for predicting Atlantic seasonal hurricane activity. The model is built upon the empirical relationship between the observed interannual variability of hurricanes and the variability of sea surface temperatures (SSTs) and vertical wind shear in 26-yr (1981–2006) hindcasts from the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS). The number of Atlantic hurricanes exhibits large year-to-year fluctuations and an upward trend over the 26 yr. The latter is characterized by an inactive period prior to 1995 and an active period afterward. The interannual variability of the Atlantic hurricanes significantly correlates with the CFS hindcasts for August–October (ASO) SSTs and vertical wind shear in the tropical Pacific and tropical North Atlantic where CFS also displays skillful forecasts for the two variables. In contrast, the hurricane trend shows less of a correlation to the CFS-predicted SSTs and vertical wind shear in...

The Global Climate of June–August 1991: Tropical Pacific Warm Episode Conditions Evolve; New Base Period Means Established

Tropical Pacific atmospheric and oceanic indices continued to indicate early stages of an ENSO episode. The increase in convection during August 1991 confirmed a continued development of the 1991-92 warm episode in the equatorial Pacific. Circulation anomalies, persistent in the Northern Hemisphere, were associated with well above normal surface temperatures and below normal precipitations. In the southern Hemisphere, the primary circulation anomaly, a blocking anticyclone, dissipated during July. 9 refs., 31 figs., 1 tab.

The Global Climate of March–May 1991: Anomalous Low-Frequency Fluctuations Dominate the Midlatitudes; ENSO Becomes Established in the Tropics

Positive sea surface temperatures (SST) anomalies increased and spread throughout the central and eastern equatorial Pacific during April and May 1991, the largest since the 1986-87 ENSO event and accompanied by sharp decreases in the ENSO index during March-May 1991. All atmospheric and oceanic indices indicated the initial stages of ENSO by the end of the season. Persistent circulation anomalies were observed in both the Northern and Southern hemispheres extratropics. Mean seasonal temperatures were above normal over many land regions, continuing a trend of warmer than normal March-May globally that began in the early 1980s. 6 refs., 26 figs., 1 tab.

State of the climate in 2012 - eScholarship

Special supplement to the Bulletin of the American Meteorological Society vol.94, No. 8, August 2013