Arthur V. Douglas

Changes in the Probability of Heavy Precipitation: Important Indicators of Climatic Change

A simple statistical model of daily precipitation based on the gamma distribution is applied to summer (JJA in Northern Hemisphere, DJF in Southern Hemisphere) data from eight countries: Canada, the United States, Mexico, the former Soviet Union, China, Australia, Norway, and Poland. These constitute more than 40% of the global land mass, and more than 80% of the extratropical land area. It is shown that the shape parameter of this distribution remains relatively stable, while the scale parameter is most variable spatially and temporally. This implies that the changes in mean monthly precipitation totals tend to have the most influence on the heavy precipitation rates in these countries. Observations show that in each country under consideration (except China), mean summer precipitation has increased by at least 5% in the past century. In the USA, Norway, and Australia the frequency of summer precipitation events has also increased, but there is little evidence of such increases in any of the countries considered during the past fifty years. A scenario is considered, whereby mean summer precipitation increases by 5% with no change in the number of days with precipitation or the shape parameter. When applied in the statistical model, the probability of daily precipitation exceeding 25.4 mm (1 inch) in northern countries (Canada, Norway, Russia, and Poland) or 50.8 mm (2 inches) in mid-latitude countries (the USA, Mexico, China, and Australia) increases by about 20% (nearly four times the increase in mean). The contribution of heavy rains (above these thresholds) to the total 5% increase of precipitation is disproportionally high (up to 50%), while heavy rain usually constitutes a significantly smaller fraction of the precipitation events and totals in extratropical regions (but up to 40% in the tropics, e.g., in southern Mexico). Scenarios with moderate changes in the number of days with precipitation coupled with changes in the scale parameter were also investigated and found to produce smaller increases in heavy rainfall but still support the above conclusions. These scenarios give changes in heavy rainfall which are comparable to those observed and are consistent with the greenhouse-gas-induced increases in heavy precipitation simulated by some climate models for the next century. In regions with adequate data coverage such as the eastern two-thirds of contiguous United States, Norway, eastern Australia, and the European part of the former USSR, the statistical model helps to explain the disproportionate high changes in heavy precipitation which have been observed.

A climatological perspective of transient synoptic features during NAME 2004

Abstract This note provides a first look at a recently developed long-term climatology of transient synoptic features in northern Mexico. Key features investigated include inverted troughs, cutoff lows, cold fronts, and open troughs (westerly short waves). This 35-yr analysis of transient systems crossing northern Mexico (1967–2001) was developed to help place the summer climatology of the 2004 North American Monsoon Experiment (NAME) into a broader perspective. Inverted troughs are found to be the most commonly occurring transient synoptic feature during the monsoon with a mean frequency of occurrence of 55 days per summer season (June–September). Inverted troughs are found to contribute from 20% to 25% of the average summer rainfall observed in northern Mexico. Rainfall doubles during inverted trough days compared to days without transient systems being present. In 2004 the monsoon season was greatly shortened due to a poorly developed subtropical high. Compared to long-term means, inverted troughs cont...

Long-Term Observations for Monitoring Extremes in the Americas

In this paper we discuss data available from Argentina, Mexico, the United States and Canada that are suitable for the analysis of extreme temperature and precipitation events. We also discuss some of the problems of homogeneity and quality control that can potentially affect the observation of extreme values. It is clear that even in countries like the United States and Canada that have a potentially rich source of climate data there are still problems in obtaining homogeneous data necessary to perform thorough studies of time varying changes in extreme events. These types of problems may be compounded if data are needed from countries that do not have a tradition of maintaining large climate archives or observing networks.We also provide two examples of analyses that can be performed with these types of data: (1) the development of climate extremes indices for Canada, and (2) analyses of freezing events for Florida and the effect on the citrus industry. The Canadian example provides a good basis for countries to take indices developed for one country or region and through minor modifications make the index relevant to their own needs. The analysis of freezing events in Florida is a timely example of how extreme events have both ecological and societal impact.

Defining Intraseasonal Rainfall Variability within the North American Monsoon

Abstract This study provides an empirical description of intraseasonal rainfall variability within the North American monsoon (NAM) region. Applying particular definitions to historical daily rainfall observations, it demonstrates that distinct intraseasonal rainfall modes exist and that these modes differ considerably from the monsoon core region in northwest Sonora (SON), California, to its northward extension in southeast Arizona (AZ). To characterize intraseasonal rainfall variability (ISV), separate P-mode principal component (PC) analyses were performed for SON and AZ. The results indicate that in each area, much of the ISV in rainfall can be described by three orthogonal modes. The correlations between ISV modes and total seasonal rainfall reinforce the notion of differing behaviors between the monsoon’s core and extension. For SON all three ISV modes exhibit significant correlation with seasonal rainfall, with the strongest relationship in evidence for the ISV mode, which is related to rainfall in...

Assessing Warm Season Drought Episodes in the Central United States

Abstract Recurrent drought episodes are a signature feature of the climate of the central United States. Using a particular set of definitions, this study examines some of the basic statistical features of previous drought episodes. It shows that there are distinct intraseasonal preferences for the onset of long periods of dry weather and that these preferences vary regionally in ways that are consistent with seasonal climate controls. Evaluation of past drought occurrences indicates that there is no simple or strong teleconnection between drought and the El Nino–Southern Oscillation (ENSO) phenomenon. Individual drought episodes certainly evolve with unique character. However, based on analysis of large-scale variations in sea level pressure patterns, evidence is presented for systematic links between drought and the interdecadal climate mode often referred to as the Pacific decadal oscillation (PDO).

Dissecting the Macro-scale Variations in Mexican Maize Yields (1961-1997)

This paper offers a macro-scale view of maize production in Mexico. It provides the quantitative association between interannual variability in maize yield and fluctuations in seasonal rainfall. It also highlights trends in production variables, and shows that significant trends are present in many of Mexicos state-level rainfall series. The study presents a three-variable model relating yield to rainfall variations. Analysis of model residuals indicates a tendency for nonrandom serial behavior, particularly in the 1990s where the model consistently underestimates yield. This result is consistent with the major geographic shift in maize cultivation from the traditional, rain-fed production core to states in the Northwest. Here, maize production is largely irrigated so that yields presumably are less sensitive to climate fluctuations.

Interannual variability of tropical cyclone activity along the Pacific coast of North America

The interannual variability of near-coastal eastern North Pacific tropical cyclones is described using a data set of cyclone tracks constructed from U.S. and Mexican oceanic and atmospheric reports for the period 1951-2006. Near-coastal cyclone counts are enumerated monthly, allowing us to distinguish interannual variability during different phases of the May-November tropical cyclone season. In these data more tropical cyclones affect the Pacific coast in May-July, the early months of the tropical cyclone season, during La Nina years, when equatorial Pacific sea surface temperatures are anomalously cool, than during El Nino years. The difference in early season cyclone counts between La Nina and El Nino years was particularlypronounced during the mid-twentieth century epoch when cool equatorial temperatures were enhanced as described by an index of the Pacific Decadal Oscillation. Composite maps from years with high and low near-coastal cyclone counts show that the atmospheric circulation anomalies associated with cool sea surface temperatures in the eastern equatorial Pacific are consistent with preferential steering of tropical cyclones northeastward toward the west coast of Mexico.