Ph.D. Princeton University, 1982
Senior Research Scientist
NOAA/ESRL/Physical Sciences Division
Diagnosis, modeling, and predictability of large-scale weather and climate variations on time scales of days to millennia.
Current Research: Is global warming significantly affecting daily weather extremes?
The answer to this trillion-dollar question depends not just on the mean shift of the probability density function (PDF) of daily weather anomalies, but also on changes in the width and shape of the PDF. The PDFs of daily weather are generally not Gaussian and are, therefore, not characterized by their mean and variance. One also has to account for their generally asymmetric and heavy-tailed character when assessing changes in tail probabilities. We are addressing this important issue using the longest global-atmospheric-circulation data set currently available, an ensemble of 56 equally likely estimates of the global atmospheric state within observational error bounds generated for every six hours from 1871 to the present in the Twentieth Century Reanalysis (20CR) project, a major international effort led by CIRES and NOAA (Compo et al., QJRMS, 2011). Specifically, we are using the mean, variance, skewness, and kurtosis of the daily data to fit so-called SGS (stochastically generated skewed) PDFs (Sardeshmukh and Sura, J. Clim., 2009) to the histograms of the daily values, and then using the fitted PDFs to draw inferences about tail probabilities. We have initially focused on the PDFs of daily indices of four prominent modes of sea-level pressure variability: the North Atlantic Oscillation (NAO), the North Pacific Oscillation (NPO), the tropical Pacific Walker Circulation (PWC), and the Annular Antarctic Oscillation (AAO). We have fitted SGS distributions to the histograms of these indices separately in the first and second halves of our 136-year record (1874 to 1942 and 1943 to 2010) and assessed the statistical significance of changes in the PDFs through extensive Monte Carlo integrations with a “weather generator” model whose parameters are consistent with those of the fitted distributions. Applying this rigorous significance-testing procedure, we find no significant change in the mean of the NAO and NPO, and a small but significant positive shift in the mean of PWC and AAO from the first to the
second half of the 136- year period. For the PDF as a whole, we find no significant changes in the PDFs of the NAO and NPO. The small positive
mean shifts of the PWC and AAO PDFs are associated with increased probabilities of large positive values and reduced probabilities of large negative values, but these changes are much smaller and statistically insignificant for extreme positive values, beyond about 2.5 standard deviations. These are important results and also underscore the danger of drawing inferences about changes in extreme-value statistics merely from shifts of the mean.