Changes in the frequency of U.S. landfalling hurricanes with respect to the El Nino-Southern Oscillation (ENSO) cycle are assessed. Ninety-eight years (1900-97) of U.S. landfalling hurricanes are classified, using sea surface temperature anomaly data from the equatorial Pacific Ocean, as occurring during an El Nino (anomalously warm tropical Pacific waters), La Nina (anomalously cold tropical Pacific waters), or neither (neutral). The mean and variance of U.S. landfalling hurricanes are determined for each ENSO phase. Each grouping is then tested for Poisson distribution using a chi-squared test. Resampling using a "bootstrap" technique is then used to determine the 5% and 95% confidence limits of the results. Last, the frequency of major U.S. landfalling hurricanes (sus-tained winds of 96 kt or more) with respect to ENSO phase is assessed empirically. The results indicated that El Nino events show a reduction in the probability of a U.S. landfalling hurricane, while La Nina shows an increase in the chance of a U.S. hurricane strike. Quantitatively, the probability of two or more landfalling U.S. hurricanes during an El Nino is 28%, of two or more landfalls during neutral conditions is 48%, and of two or more landfalls during La Nina is 66%. The frequencies of landfalling major hurricanes show similar results. The probability of one or more major hurricane landfall during El Nino is 23% but is 58% during neutral conditions and 63% during La Nina.

Abstract.The authors document and explain changes in the rates of North Atlantic major hurricanes over the 20th century. A change-point analyses identifies two contrasting regimes of activity. The regimes have significantly different occurrence rates that coincide with changes in the climate over the extratropical North Atlantic. In conjunction with the recent Arctic warming and a relaxation of the North Atlantic oscillation, it is speculated that we are beginning a new period of greater major hurricane activity.

The authors develop and apply a model that uses hurricane-experience data in counties along the U.S. hurricane coast to give annual exceedence probabilities to maximum tropical cyclone wind events. The model uses a maximum likelihood estimator to determine a linear regression for the scale and shape parameters of the Weibull distribution for maximum wind speed. Model simulations provide quantiles for the probabilities at prescribed hurricane intensities. When the model is run in the raw climatological mode, median probabilities compare favorably with probabilities from the National Hurricane Center's risk analysis program ''HURISK'' model. When the model is run in the conditional climatological mode, covariate information in the form of regression equations for the distributional parameters allows probabilities to be estimated that are conditioned on climate factors. Changes to annual hurricane probabilities with respect to a combined effect of a La Niña event and a negative phase of the North Atlantic oscillation mapped from Texas to North Carolina indicate an increased likelihood of hurricanes along much of the coastline. Largest increases are noted along the central Gulf coast.

A long‐term (9.5 yr) study addressed the relationship of the trophic organization of a river‐dominated Gulf of Mexico estuary with interannual trends of freshwater input and biological controlling features. Alluvial river flow characteristics were evaluated with respect to seasonal and interannual changes in physical, chemical, and biological trends in the receiving estuary. Infaunal and epifaunal macroinvertebrates and fishes taken over the period of sampling in the Apalachicola Bay system were transformed into their trophic equivalents. The long‐term trophic organization of the bay was then related to observed changes in the physical and chemical conditions in the receiving estuary with particular attention to long‐term response to a 2‐yr drought. Within limited natural bounds of freshwater flow from the Apalachicola River, there was little change in the trophic organization of the receiving estuary over prolonged periods. The physical instability of the estuary was actually a major component in the continuation of a biologically stable estuarine system. However, when a specific threshold of freshwater reduction was reached during a prolonged natural drought, we suggest that the clarification of the normally turbid and highly colored river–estuarine system led to rapid changes in the pattern of primary production, which, in turn, were associated with major changes in the trophic structure of the system. Increased light penetration due to the cessation of river flow was an important factor in the temporal response of bay productivity and herbivore/omnivore abundance. There was a dichotomous response of the estuarine trophic organization, with herbivores and omnivores responsive to river‐dominated physicochemical factors whereas the carnivores responded to biological factors. Trophic response time could be measured in months to years from the point of the initiation of low‐flow conditions. The reduction of nutrient loading during the drought period was postulated as a major cause of the loss of productivity of the river‐dominated estuary during and after the drought period. Recovery of such productivity with resumption of increased river flows was likewise a long‐term event. Based on the observed trends in the bay, postulated permanent reductions of freshwater flows due to anthropogenous activities could lead to major reductions of biological productivity in the Apalachicola Bay system. The long‐term data indicated that, with reduction of freshwater flow below a level specific for the receiving system, the physically controlled, highly productive river–estuarine system would become a species‐rich, biologically controlled bay with substantially reduced productivity.

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