Xufeng Niu

Effect of El Niño on U.S. Landfalling Hurricanes, Revisited

Abstract 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 (sustained 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 t...

FRESHWATER INPUT TO A GULF ESTUARY: LONG-TERM CONTROL OF TROPHIC ORGANIZATION

A long-term (9.5 yr) study addressed the relationship of the trophic orga- nization 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 fresh- water 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 ac- tivities 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-estu- arine system would become a species-rich, biologically controlled bay with substantially reduced productivity.

Changes in the rates of North Atlantic major hurricane activity during the 20th century

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.

Detecting Shifts in Hurricane Rates Using a Markov Chain Monte Carlo Approach

Time series of annual hurricane counts are examined using a changepoint analysis. The approach simulates posterior distributions of the Poisson-rate parameter using Gibbs sampling. A posterior distribution is a distribution of a parameter conditional on the data. The analysis is first performed on the annual series of major North Atlantic hurricane counts from the twentieth century. Results show significant shifts in hurricane rates during the middle 1940s, the middle 1960s, and at 1995, consistent with earlier published results. The analysis is then applied to U.S. hurricane activity. Results show no abrupt changes in overall coastal hurricane rates during the twentieth century. In contrast, the record of Florida hurricanes indicates downward shifts during the early 1950s and the late 1960s. The shifts result from fewer hurricanes passing through the Bahamas and the western Caribbean Sea. No significant rate shifts are noted along either the Gulf or East Coasts. Climate influences on coastal hurricane activity are then examined. Results show a significant reduction in U.S. hurricane activity

Secular changes to the ENSO‐U.S. hurricane relationship

Analysis of the statistical relationship between annual U.S. hurricane activity and the El Nino-Southern oscillation (ENSO) is performed. The legitimacy of considering annual U.S. hurricane counts as a Poisson process is checked. Then, Poisson regression is used to model the ENSO-U.S. hurricane connection. A bivariate regression model verifies a significant negative correspondence between tropical Pacific sea-surface temperature (SST) and U.S. hurricane activity. When equatorial SSTs are cold, U.S. hurricanes are more likely. Secular changes to the ENSO-U.S. hurricane relationship are examined using moving regressions. A nonlinear downward trend in the relationships strength is evident. Variations in sea-level pressures over the extra-tropical North Atlantic Ocean during months immediately prior to the hurricane season provide an explanation for a portion of this secular variability. Atmospheric synoptic conditions associated with the North Atlantic oscillation (NAO) result in hurricanes tracking parallel to southern latitudes en route to the United States.

Comparison of Methodologies for Probabilistic Quantitative Precipitation Forecasting

Abstract Twenty-four-hour probabilistic quantitative precipitation forecasts (PQPFs) for accumulations exceeding thresholds of 0.01, 0.05, and 0.10 in. are produced for 154 meteorological stations over the eastern and central regions of the United States. Comparisons of skill are made among forecasts generated using five different linear and nonlinear statistical methodologies, namely, linear regression, discriminant analysis, logistic regression, neural networks, and a classifier system. The predictors for the different statistical models were selected from a large pool of analyzed and predicted variables generated by the Nested Grid Model (NGM) during the four cool seasons (December–March) from 1992/93 to 1995/96. Because linear regression is the current method used by the National Weather Service, it is chosen as the benchmark by which the other methodologies are compared. The results indicate that logistic regression performs best among all methodologies. Most notable is that it performs significantly...

Factors controlling seagrass growth in a gulf coastal system: Water and sediment quality and light

Abstract A combined field descriptive/experimental and laboratory experimental study was carried out to determine the relationships of water quality, qualitative and quantitative light factors and sediment characteristics in the definition of the distribution of submerged aquatic vegetation (SAV) offshore of two streams, one polluted and one natural, that drain into the northeastern Gulf of Mexico. Release of pulp mill effluents into a small drainage system were associated with increased loading of dissolved organic carbon (DOC), water color and nutrients to offshore areas relative to an unpolluted reference system. This loading resulted in changes of water quality factors and light transmission characteristics in the receiving Gulf area. Sediments in affected offshore areas were characterized by increased silt/clay fractions and altered particle size relative to reference sites. Based on the field data, the best (statistically significant) predictors of SAV distribution were photic depths, qualitative aspects of wave length distributions, water quality factors (color, DOC and chlorophyll a ) and sediment characteristics. Photic depths were good predictors of SAV distribution, with depth as an important modifying factor. Mesocosm experiments showed that pulp mill effluent in direct contact with Thalassia testudinum and Syringodium filiforme had significant adverse impacts on growth at relatively low concentrations (1–2%) of effluent. Light mesocosm experiments indicated that light levels in inshore Fenholloway areas were associated with reduced growth of Halodule wrightii , S. filiforme and T. testudinum . Field transfer experiments showed that altered sediment and water quality in inshore polluted areas induced significant adverse effects on growth indices of all three species. By comparing the field and experimental results, a hierarchy of habitat requirements for the subject seagrass species was determined. Salinity, temperature and depth restraints are important habitat variables that control seagrass growth; when such variables are not limiting, light, sediment and nutrient characteristics become important in the determination of the distribution of seagrasses in coastal areas.

Seasonal and altitudinal variation in decomposition of soil organic matter inferred from radiocarbon measurements of soil CO2 flux

The rate of carbon (C) cycling in soils is controlled by an array of processes and conditions. It has been widely accepted that an increase in temperature would accelerate microbial decomposition of soil organic matter (SOM) and provide a positive feedback to global warming, other factors being equal. However, soil moisture has received little attention in C cycle studies. In this project, we developed a technique for sampling soil-respired CO2 for isotopic measurements and a model that relates the radiocarbon (14C) content of soil respired CO2 to the rate of C cycling in soils. We measured soil CO2 flux, carbon isotopic content (both 13C and 14C) of soil-respired CO2, soil temperature, and soil moisture on a monthly basis along an elevation transect in the Sierra Nevada in an attempt to determine the relationship between the rate of soil C cycling and soil environmental conditions. Both soil CO2 flux and its 14C content displayed significant variations (spatially and temporally), which reflect natural variations in the rate of SOM decomposition and in the relative amount of SOM-derived CO2 versus root-respired CO2 caused by seasonal changes in soil temperature, moisture, and plant activity. The relative contribution of SOM decomposition to total soil CO2 production changed throughout the year from ∼20–50% at the peak of the growing season to close to 100% in the nongrowing season. The apparent decay rate of SOM determined from the 14C content of soil-respired CO2 varied from ∼0.2 yr−1 in the spring to ∼0.01 yr−1 in the fall at the lowest-elevation site and from 0.1 yr−1 in the summer to ∼0.01 yr−1 in the late fall at the highest-elevation site. It appears that the apparent decay rate of SOM increased with increasing temperature when soil moisture was adequate but decreased with increasing temperature when soil moisture became limited. The apparent decay rate of SOM also varied with soil moisture. Higher soil moisture content accelerated decomposition of SOM until it reached an optimal level of ∼14–25 wt % water content and then inhibited decomposition when more pores in soils became saturated with water and perhaps oxygen availability (for microbes) became limited. Although the rate of SOM decomposition varied throughout the year in response to fluctuations in soil temperature and moisture, the maximum apparent decay rate was higher at the low-elevation site (i.e., maximum apparent decay rate = 0.22 yr−1) than at the high-elevation sites (i.e., maximum apparent decay rate = 0.10 yr−1). Litter decomposition simulated by measuring changes in mass of litter in litter bags placed in the field also showed a similar decomposition pattern with decreasing decomposition rate with elevation. Multivariable regression analyses including various terms of soil temperature, moisture, and site variability suggest that soil moisture was a major factor, but not the only factor, controlling the rate of SOM decomposition and soil CO2 flux in the Sierra Nevada soils. Both decay rate and total soil CO2 flux are related significantly to soil moisture, temperature, and site effects.

Trends in column ozone based on TOMS data: Dependence on month, latitude, and longitude

On the basis of the TOMS satellite column ozone data in latitudes 70°S–70°N from November 1978 to May 1990, we use a statistical model to estimate the trends in ozone as a function of latitude, longitude, and month. The trends in the TOMS ozone data are highly seasonal and dependent on location. Near the equator, the estimated monthly trends are not significantly different from zero. For high latitudes, most of the estimated monthly trends are negative. In January, February, and March, there are some positive trend estimates in the western hemisphere around latitude 60°N. The most negative trends for these 3 months also appear in the high latitudes of the northern hemisphere. Starting in June, more negative trends appear in the latitudes 50°S–70°S than the trends in the rest of the world considered. A large depletion develops during the spring time (September to November) in the southern high-latitude region, and the area of peak ozone decline is moving eastward during the period. The largest negative trends (about −29% per decade) for the area considered in this study appear in October around the latitude 70°S and longitudes 20°W–100°W region. Since the magnitudes of the estimated trends in the southern hemisphere increase toward the pole, more negative trends occur beyond the latitude 70°S. For the northern hemisphere, the year-round trend estimates for latitudes 30°N–70°N range from −0.96% to −7.43% per decade. In the latitudes 30°N–50°N, the winter trend estimates are more negative than those for the summer and the fall. However, this pattern did not hold for latitudes 50°N–70°N.

A Dynamic Probability Model of Hurricane Winds in Coastal Counties of the United States

Abstract 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 Nina event and a negative phase of the North Atla...