Increased Hurricane Activity Research Articles

ALTERATION OF ISLAND FOOD-WEB DYNAMICS FOLLOWING MAJOR DISTURBANCE BY HURRICANES

Major abiotic disturbance can be an important factor influencing food-web dynamics, particularly in areas impacted by the recent increase in hurricane activity. We present a unique set of data on key food-web processes occurring on 10 small islands for three relatively calm years and then four subsequent years during which two hurricanes passed directly over the study site. Herbivory, as measured by leaf damage, was 3.2 times higher in the year after the first hurricane (2000) than in the previous year and was 1.7 times higher in the year after the second hurricane (2002) than in 2001. The effect of a top predator (the lizard, Anolis sagrei) on herbivory strengthened continuously after the first hurricane and overall was 2.4 times stronger during the disturbance period than before. Overall abundance of lizards was 30% lower during the disturbance period than before, and abundances of web spiders and hymenopteran parasitoids were 66% and 59% lower, respectively. We suggest that increased herbivory observed on all islands was caused, at least in part, by the overall reduction in predation by both lizards and arthropods, whereas magnification of the lizard effect on herbivory was caused by reduced compensatory predation by arthropods.

Hurricane impacts on coastal ecosystems

Hurricanes, typhoons, and other cyclonic disturbances often produce sudden, massive disturbances in estuaries and other coastal ecosystems around the world. Compared with other ecological disturbances, such as prairie and forest fires, tree falls, and coral bleaching events, the effects of hurricanes are usually studied in isolation of each other and rarely as repeated or even ecologically predictable events across an area of sustained hurricane activity. Studies of their effects have rarely contributed to the growing body of theory on ecological disturbance and ecosystem resiliency and they are conspicuously absent from ecological textbooks. Since 1996, there has been an increase in the frequency of hurricane landfalls in the southeastern United States that is predicted to continue for some years into the future (Goldenberg et al. 2001; Emanuel 2005; Webster et al. 2005). Coupled with the availability of results from many water quality and biological monitoring programs now in place in these coastal systems, the increase in hurricane activity now provides an opportunity to begin the systematic investigation of how a series of hurricanes might produce cumulative and perhaps even irreversible effects on coastal ecosystems. One particular example that is highlighted in this special issue of Estuaries and Coasts is the occurrence of four major hurricanes to make landfall in Florida in a two-month period of 2004. In some localities, sequential hurricanes made landfall in close proximity to each other, while in other regions of coastal Florida, the repeated heavy rains associated with the multiple hurricanes had effects that were distinct from the wind and storm surge effects often associated with hurricanes.

Increased hurricane activity expected in 2006

Increased hurricane activity expected in 2006

The 2005 hurricane season: An echo of the past or a harbinger of the future?

Recent Atlantic hurricane activity raises several questions. For example, why was the 2005 season so early and active and are there similarities with the past? We show that parallels exist between 2005 and previously active years. Tropical Atlantic sea surface temperature (SST) anomalies beginning in winter aided the formation of early storms. On longer timescales SST, sea level pressure and zonal winds exhibit multidecadal variability; weaker easterlies during the positive phase of the Atlantic Multidecadal Oscillation (AMO) result (via ocean‐atmosphere interactions) in warmer water and increased hurricane activity. However, individual active years appear independent of the AMO phase.

Seasonal variation in the attachment strength of blue mussels: Causes and consequences

The midintertidal zone of temperate rocky coasts is often dominated by mussels that form dense beds and exclude other primary space holders. The rate at which storms remove mussels from their substrate consequently has a controlling influence on intertidal community structure. The rate of disturbance by waves can be predicted mechanistically by comparing the attachment strength of an organism to the force it encounters. One simplifying assumption often implemented with this approach is that organismal strength is temporally constant. This study demonstrates that such an assumption is not valid for Mytilus edulis in Rhode Island, where byssal attachment strength (tenacity) increases twofold in winter compared to summer. Time series analysis of monthly samples (1998–2000) indicates tenacity generally tracks seasonal fluctuations in wave height, which suggests that mussels sense and respond to changes in their flow environment. However, the ability of mussels to respond to wave climate is not precise; during September and October (hurricane season), mussels remain weakly attached and are prone to major mortalities. Mussel reproductive condition also displays seasonal cycles, peaking 3–4 months after attachment strength. This suggests a possible energetic constraint on the ability of mussels to respond to their flow environment, as mussels trade byssal thread production for gonad development. This study demonstrates that accurate predictions of disturbance events must consider not only a storm’s magnitude, but its timing relative to the cycle in attachment strength. Given the recent shift to increased hurricane activity in the North Atlantic, it is likely that mussels will begin to suffer more frequent and more severe disturbances compared to those that occurred during 1971–1994.

Are Gulf Landfalling Hurricanes Getting Stronger?

Recent predictions of increased hurricane activity in the Atlantic basin, as well as explosive coastal population growth, have prompted a study of the trends in quantity and intensity of U.S. landfalling hurricanes in the Gulf of Mexico. Gulf of Mexico hurricane landfalls from Cape Sable, Florida, to Brownsville, Texas, are binned by decade from 1886 to 1995 to determine whether gulf hurricane landfalls are becoming more or less frequent. From these bins, subsets of intense hurricanes (sustained winds of 96 kt or more) per decade are also made. The results show that there is no sign of an increase of hurricane frequency or intensity in the Gulf of Mexico at this time.

The Extremely Active 1995 Atlantic Hurricane Season: Environmental Conditions and Verification of Seasonal Forecasts

The 1995 Atlantic hurricane season was a year of near-record hurricane activity with a total of 19 named storms (average is 9.3 for the base period 1950–90) and 11 hurricanes (average is 5.8), which persisted for a total of 121 named storm days (average is 46.6) and 60 hurricane days (average is 23.9), respectively. There were five intense (or major) Saffir–Simpson category 3, 4, or 5 hurricanes (average is 2.3 intense hurricanes) with 11.75 intense hurricane days (average is 4.7). The net tropical cyclone activity, based upon the combined values of named storms, hurricanes, intense hurricanes, and their days present, was 229% of the average. Additionally, 1995 saw the return of hurricane activity to the deep tropical latitudes: seven hurricanes developed south of 25°N (excluding all of the Gulf of Mexico) compared with just one during all of 1991–94. Interestingly, all seven storms that formed south of 20°N in August and September recurved to the northeast without making landfall in the United States. The sharply increased hurricane activity during 1995 is attributed to the juxtaposition of virtually all of the large-scale features over the tropical North Atlantic that favor tropical cyclogenesis and development. These include extremely low vertical wind shear, below-normal sea level pressure, abnormally warm ocean waters, higher than average amounts of total precipitable water, and a strong west phase of the stratospheric quasi-biennial oscillation. These various environmental factors were in strong contrast to those of the very unfavorable conditions that accompanied the extremely quiet 1994 hurricane season. The favorable conditions for the 1995 hurricane season began to develop as far back as late in the previous winter. Their onset well ahead of the start of the hurricane season indicates that they are a cause of the increased hurricane activity, and not an effect. The extreme duration of the atmospheric circulation anomalies over the tropical North Atlantic is partly attributed to a transition in the equatorial Pacific from warm episode conditions (El Niño) to cold episode conditions (La Niña) prior to the onset of the hurricane season. Though the season as a whole was extremely active, 1995’s Atlantic tropical cyclogenesis showed a strong intraseasonal variability with above-normal storm frequency during August and October and below normal for September. This variability is likely attributed to changes in the upper-tropospheric circulation across the tropical North Atlantic, which resulted in a return to near-normal vertical shear during September. Another contributing factor to the reduction in tropical cyclogenesis during September may have been a temporary return to near-normal SSTs across the tropical and subtropical North Atlantic, caused by the enhanced tropical cyclone activity during August. Seasonal hurricane forecasts for 1995 issued at Colorado State University on 30 November 1994, 5 June 1995, and 4 August 1995 correctly anticipated an above-average season, but underforecast the extent of the extreme hurricane activity.