Passage Of Storm Research Articles

Near-Inertial Wave Wake of Hurricanes Katrina and Rita over Mesoscale Oceanic Eddies

Abstract Tropical cyclones (TCs) Katrina and Rita moved as major hurricanes over energetic geostrophic ocean features in the Gulf of Mexico. Increased and reduced oceanic mixed layer (OML) cooling was measured following the passage of both storms over cyclonic and anticyclonic geostrophic relative vorticity ζg, respectively. This contrasting thermal response is investigated here in terms of the evolution of the storms’ near-inertial wave wake in geostrophic eddies. Observational data and ray-tracing techniques in realistic geostrophic flow indicate that TC-forced OML near-inertial waves are trapped in regions of negative ζg, where they rapidly propagate into the thermocline. These anticyclonic-rotating regimes coincided with the distribution of reduced OML cooling because rapid downward dispersion of near-inertial energy reduced the amount of kinetic energy available to increase vertical shears at the OML base. By contrast, TC-forced OML near-inertial waves were stalled in upper layers of cyclonic circulations, which strengthened vertical shears and entrainment cooling. Upgoing near-inertial energy propagation dominated inside a geostrophic cyclone that interacted with Katrina; the salient characteristics of these upward-propagating waves were the following: (i) they were radiated from the ocean interior because of geostrophic adjustment following upwelling–downwelling processes; (ii) rather than with the buoyancy frequency, they amplified horizontally as they encountered increasing values of f + ζg/2 during upward propagation; and (iii) they produced episodic vertical mixing through shear instability at a critical layer underneath the OML. To improve the prediction of TC-induced OML cooling, models must capture geostrophic features and turbulence closures must represent near-inertial wave processes such as dispersion and breaking between the OML base and the thermocline.

Windborne debris risk analysis - Part I. Introduction and methodology

Windborne debris is a major cause of structural damage during severe windstorms and hurricanes owing to its direct impact on building envelopes as well as to the \'chain reaction\' failure mechanism it induces by interacting with wind pressure damage. Estimation of debris risk is an important component in evaluating wind damage risk to residential developments. A debris risk model developed by the authors enables one to analytically aggregate damage threats to a building from different types of debris originating from neighboring buildings. This model is extended herein to a general debris risk analysis methodology that is then incorporated into a vulnerability model accounting for the temporal evolution of the interaction between pressure damage and debris damage during storm passage. The current paper (Part I) introduces the debris risk analysis methodology, establishing the mathematical modeling framework. Stochastic models are proposed to estimate the probability distributions of debris trajectory parameters used in the method. It is shown that model statistics can be estimated from available information from wind-tunnel experiments and post-damage surveys. The incorporation of the methodology into vulnerability modeling is described in Part II.

Intense Rainfalls on August 17, 1968 over the Kiso-Hida and Nagara River Basin in Japan Associated with Intrusion of Middle Tropospheric Dry Airs over the Low-level Moist Belt

The synoptic-scale condition related to the intense rainfalls on August 17, 1968, over the Kiso?Hida and Nagara River Basin in the central part of Japan is studied by using European Center for Medium-Range Weather Forecasts 40-year reanalysis data and upper, synoptic-surface, local rain-gauge observation data and satellite cloud images.The intense rainfalls occurred within a long cloud belt formed with a low-level moist belt (LMB), which had formed along the northwestern rim of the North Pacific subtropical anticyclone (NPSA). The LMB was sustained by large-scale moisture transport along the northern rim of the NPSA and mesoscale northward moisture transport along a small anticyclone embedded in the NPSA. In the lower and middle troposphere, dry air spread over the Japan Sea after the passage of a severe tropical storm (STS) over the Japan Sea owing to the prevailing westerlies to the southwest of the STS. The northern edge of the LMB was bounded by the dry air. The northern boundary of the LMB was signified as the dry front because of the strong moisture gradient and very weak thermal gradient.Intense rainfalls occurred in the cloud belt, at a distance of ∼1200 km south of the STS, over the river basin where the large-scale and the mesoscale moist flows converged in association with the orographic convergence. Intense convective rainfalls were accompanied by the increase in the convective instability due to the southeastward intrusion of the middle tropospheric dry air over the LMB. Significant low-level jet stream and mesoscale depression were not found around the intense rainfall area.

Effect of wintertime low‐pressure systems on the Atlantic inflow to the Nordic seas

The response of the Atlantic inflow to the Nordic seas to a variable wind field is investigated using storm track data and observed current meter data from the three inflowing Iceland, Faroe, and Shetland branches and from the Svinøy section off the west coast of Norway. The Atlantic inflow is found to be anomalously large in the Shetland and Svinøy sections for 4–5 days after a storm passage with maximum response after 2–3 days. No similar inflow response was found in the Iceland and Faroe sections. On the contrary, the Faroe current becomes weaker 2 days after the inflow maxima in the Shetland and Svinøy sections. We speculate that this is a lagged response to increased sea level height north of the Greenland‐Scotland Ridge. For longer timescales, most of the winter anomalies in the volume transports through the Shetland and Svinøy sections are related to cyclone variability, with the number of storms, the residence time, and the intensity of the storms being critical factors.

Detection and classification of typhoons using underwater acoustic sensors in the western Pacific Ocean.

About two years of underwater ambient noise were collected using Passive Aquatic Listeners (PALs) in the western Pacific Ocean. PALs were deployed using the bottom-mount or surface-mount buoys on the possible typhoon paths. Several episodes of intensive typhoon noise were recorded during the passage of these powerful tropical storms. These acoustic data are compared with other ancillary data for the classification of typhoons. Using the underwater ambient noise to classify the intensity of typhoons is a relatively reliable method. The instrument is placed away from the air-sea interface in a less destructible environment and provides relative in situ measurement of the wind speed. The ambient noise recordings from the deployment also include the biology, shipping, and other surface noises, but the noises from the typhoons are the most distinguishable against all other sources of noise. The noise budgets from different deployment sites are also presented.

Flow division at a channel crossing with subcritical or supercritical flow

AbstractThe passage of an extreme storm over an urban area can lead to the flooding of the streets if the rainfall intensity exceeds the design value and/or the drainage system is not functional. The study of flow distribution in street networks thus is important for the design of flood protection measures. The flow distribution is affected by the junction flow characteristics, inflow discharges and downstream water depths. To reduce the degree of empiricism, a 3D Reynolds‐averaged Navier–Stokes equations model has been implemented in this study to investigate the flow phenomena in a cross junction. The Spalart–Allmaras model is used for turbulence closure. The numerical model utilizes the split‐operator approach, in which the advection, diffusion and pressure propagations are solved separately. The numerical model predicts accurately the flow distribution in a channel crossing under different subcritical flow conditions, for which experimental data are available. Recirculation zones exist at both the downstream channels and the associated contraction coefficient varies linearly with the ratio of the discharges at the two inlets. Secondary currents are apparent for the flow with strong asymmetric outlet conditions. Under supercritical inflow conditions, the model reproduces the hydraulic jump and hydraulic drop phenomena and predicts accurately the relationship between the input power ratio and the outflow discharge ratio of the street crossing. Copyright © 2009 John Wiley & Sons, Ltd.

Biogeochemical responses to late-winter storms in the Sargasso Sea, III—Estimates of export production using 234Th: 238U disequilibria and sediment traps

Direct measurements of new production and carbon export in the subtropical North Atlantic Ocean appear to be too low when compared to geochemical-based estimates. It has been hypothesized that episodic inputs of new nutrients into surface water via the passage of mesoscale eddies or winter storms may resolve at least some of this discrepancy. Here, we investigated particulate organic carbon (POC), particulate organic nitrogen (PON), and biogenic silica (BSiO 2) export using a combination of water column 234Th: 238U disequilibria and free-floating sediment traps during and immediately following two weather systems encountered in February and March 2004. While these storms resulted in a 2–4-fold increase in mixed layer NO 3 inventories, total chlorophyll a and an increase in diatom biomass, the systems were dominated by generally low 234Th: 238U disequilibria, suggesting limited particle export. Several 234Th models were tested, with only those including non-steady state and vertical upwelling processes able to describe the observed 234Th activities. Although upwelling velocities were not measured directly in this study, the 234Th model suggests reasonable rates of 2.2–3.7 m d −1. Given the uncertainties associated with 234Th derived particle export rates and sediment traps, both were used to provide a range in sinking particle fluxes from the upper ocean during the study. 234Th particle fluxes were determined applying the more commonly used steady state, one-dimensional model with element/ 234Th ratios measured in sediment traps. Export fluxes at 200 m ranged from 1.91±0.20 to 4.92±1.22 mmol C m −2 d −1, 0.25±0.08 to 0.54±0.09 mmol N m −2 d −1, and 0.22±0.04 to 0.50±0.06 mmol Si m −2 d −1. POC export efficiencies (Primary Production/Export) were not significantly different from the annual average or from time periods without storms, although absolute POC fluxes were elevated by 1–11%. This increase was not sufficient, however, to resolve the discrepancy between our observations and geochemical-based estimates of particle export. Comparison of PON export rates with simultaneous measurements of NO 3 − uptake derived new production rates suggest that only a fraction, <35%, of new production was exported as particles to deep waters during these events. Measured bSiO 2 export rates were more than a factor of two higher ( p<0.01) than the annual average, with storm events contributing as much as 50% of annual bSiO 2 export in the Sargasso Sea. Furthermore it appears that 65–95% (average 86±14%) of the total POC export measured in this study was due to diatoms. Combined these results suggest that winter storms do not significantly increase POC and PON export to depth. Rather, these storms may play a role in the export of bSiO 2 to deep waters. Given the slower remineralization rates of bSiO 2 relative to POC and PON, this transport may, over time, slowly decrease water column silicate inventories, and further drive the Sargasso Sea towards increasing silica limitation. These storm events may further affect the quality of the POC and PON exported, given the large association of this material with diatoms during these periods.

Wind profiler radar investigation on typhoon-orography interaction

[1] Two typhoons Kaemi (200605) and Bopha (200609) crossing over Taiwan were continuously monitored by a wind profiler radar located on the lee side of the Central Mountain Range (CMR) of the island. Wind fields rotating systematically associated with the storm passage is pronounced for both typhoons. Nevertheless, significant wind shear takes place above and below the altitude of about 3.5 km owing to the mountain blocking before typhoon landing is reported in this paper for the first time. Winds deflected due to the orography behave diversely with respect to different typhoon tracks and strengths that may cause a secondary circulation to modify the original one. In this study, a moderate typhoon, Kaemi re-organizes the winds from the CMR blocking after landfall. On the other hand, weak typhoon Bopha shows a very complicated wind pattern structure from this typhoon-orography interaction. As the winds are disturbed dramatically, it dissipates over the sea soon after the typhoon center leaves Taiwan.

Enhancement of oceanic parameters associated with dust storms using satellite data

Atmospheric aerosols play a vital role on the dynamics of climate processes through direct and indirect effects. Dust storms originating over the world's arid regions contribute a large fraction of aerosols in the atmosphere. Using remote sensing data, an anomalous enhancement in the biological productivity of sea was observed in the Gulf of Oman which was attributed only to cold sea surface temperature (SST) eddies (during November to early December months of 1996–1999), whereas recent study has shown that during dust storms (June–July–August and October–November–December months of 1997–2004), major nutrient supply is from atmospheric dust deposition. We have carried out a study of individual cases of major dust storms over the Arabian Sea during the entire year (December 2003–December 2006) to quantify role of dust storms and changes in ocean surface due to chlorophyll bloom. Using Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua, we have found that the deposition of dust along the passage of major dust storms (aerosol optical depth (AOD) ∼0.25–0.41) occuring over the Arabian Sea causes chlorophyll blooming (usually 10–22.43 mg/m3) within a period of 1–2 to up to 3–4 days. However, we have also found significant anomalous cooling of the ocean surface (SST) and relatively higher ocean wind speeds (QuikSCAT) during dust storms that may lead to favorable conditions for blooming. Exact nature and cause of chlorophyll bloom in the semienclosed northern Arabian Sea, surrounded by one of the world's major sources of dust storms (Africa, Middle East, Iran, and Afghanistan), are very important in understanding the productivity and the biogeochemical cycles of the marine ecosystem. The results have been validated using the Indian Remote Sensing Polar‐4 Ocean Color Monitor (IRS P4 OCM) data.

Biogeochemical responses to late-winter storms in the Sargasso Sea, I—Pulses of primary and new production

We have hypothesized that the weekly/biweekly passage of winter storms in the subtropical open ocean destabilizes the water column leading to pulsed NO 3 − inputs, resulting in new production that is not accounted for in most annual estimates. This paper presents data on nitrogen and carbon cycling in the Sargasso Sea at approximately daily resolution, during the period prior to seasonal stratification in 2004 and 2005; these data permit us to assess the importance of winter storms for introducing NO 3 − and the contribution of these inputs to annual new and export production. The two sampling years were in stark contrast to each other with 2004 characterized by periods of relative calm between winter storms, and 2005 characterized by nearly continuous storm activity. As a result, temporal variability in mixed layer depth (MLD) and euphotic zone [NO 3 −] were very different between years. MLDs in 2004 increased to >150 m in response to the passage of storms and then rapidly shoaled to <100 m leading to the pulsed injection of NO 3 − (∼100 nmol l −1) into the lower half of the euphotic zone, while in 2005 MLDs were consistently >300 m and euphotic zone [NO 3 −]>100 nmol l −1. Despite the very different [NO 3 −], rates of daily NO 3 − uptake were similar from year to year because of significant nocturnal uptake in 2004. Similar rates of new production did not translate into similar rates of particulate nitrogen and carbon export however, as observed export from the upper 200 m was 2–5-fold greater in 2004 than in 2005. Furthermore, the decrease of particulate nitrogen and carbon flux with depth between 200 and 400 m in 2004 was substantially lower than in 2005; this is consistent with the observed biological response in which diatoms and coccolithophores exhibited rapid growth following pulsed NO 3 − inputs in 2004. A combination of data from the Bermuda Testbed Mooring, which provides a longer temporal record than the cruise, and the observations presented in this study show that in the winter of 2004, there were 8–10 storm events that likely resulted in pulsed NO 3 − inputs. Summed over all the events, new production prior to seasonal stratification was estimated to be ∼0.12–0.18 mol N m −2 or ∼14–21% of current annual estimates.

Isla Hispaniola: A trans-boundary flood risk mitigation plan

It is sadly known that over the past decades Isla Hispaniola (Haiti and the Dominican Republic) has been exposed to the devastating passage of several hurricanes and tropical storms. Territories that are economically weak and extremely poor in terms of natural resources have been shaken by severe flood events that caused the loss of thousands of human lives, displacement of people and damage to the environment. On May 24th 2004, the flooding of the trans-boundary river Soliette killed over 1000 Haitian and Dominican people, wiping out villages and leaving behind desolation and poverty. After this catastrophic flood event, the General Direction for Development and Cooperation of the Italian Department of Foreign Affairs funded through the Istituto Italo-Latino Americano (IILA, www.iila.org) an international cooperation initiative (ICI), coordinated and directed by the University of Bologna. The ICI involved Haitian and Dominican institutions and was twofold: (a) institutional capacity building on flood risk management and mitigation measures and policies; (b) hydrological and hydraulic analysis of the May 2004 flood event aimed at formulating a suitable and affordable flood risk mitigation plan, consisting of structural and non-structural measures.

Whitecapping and wave field evolution in a coastal bay

Evolution of the wave field in a coastal bay is investigated, by comparison between field observations and numerical simulations using a spectral wave model (Simulating WAves Nearshore (SWAN)). The simulations were conducted for the passage of an extratropical storm, during which surface elevation spectra were bimodal owing to local wind‐sea generation and swell propagation into the bay. SWAN was run in stationary and nonstationary mode for two whitecapping source term formulations. The first was developed by Komen et al. (1984) and is dependent on spectrally averaged wave steepness, and thus includes swell in the calculation of whitecapping dissipation and typically overestimates wind sea in the presence of swell. The second, proposed by van der Westhuysen et al. (2007), estimates whitecapping of wind sea locally in the wave spectrum and is not coupled to swell energy. This formulation reproduced the magnitude and shape of the observed wind‐sea spectral peak much better than the previous formulation. Whitecapping dissipation rates have been estimated from observations, using the equilibrium range theory developed by Phillips (1985), and are well correlated with both wind speed and acoustic backscatter observations. These rates agree with SWAN estimates using the spectrally local expression, and provide additional physical validation for the whitecapping source term.

Estuarine Phytoplankton Responses to Hurricanes and Tropical Storms with Different Characteristics (Trajectory, Rainfall, Winds)

We examined the short-term (<1 month post-storm) impact of storms [Tropical Storm (TS) Helene in 2000, Hurricane (H) Isabel in 2003, H Alex, Tropical Depression (TD) Bonnie and TS Charley in 2004] varying in their trajectory, wind and rainfall characteristics, on water column structure, nutrients, and phytoplankton biomass in North Carolina’s Neuse R. Estuary (NRE). Data are presented from two sampling programs, ModMon (biweekly) and FerryMon (measurements made every 3 min daily). Helene’s winds mixed the previously stratified water column, delivering sediment-bound nutrients to the euphotic zone, and localized freshwater input from Helene was also evident. Mean chlorophyll a concentrations in the mesohaline portion of the NRE, where N was strongly limiting before the storm (molar DIN:DIP < 1), more than doubled after the storm. Unlike with Helene, the water column was well mixed before passage of Isabel, and nutrient concentrations were high. As a result, minimal impact on phytoplankton biomass was detected despite Isabel’s high winds and significant freshwater input. In fact, conditions became less favorable for phytoplankton growth after the storm. Alex was fast moving and relatively small, but its winds were sufficient to mix the water column. Although data from ModMon suggest that chlorophyll a was only slightly higher after passage of Alex, FerryMon detected an ephemeral bloom that was missed by ModMon. Overall, these results suggest that relatively small tropical storms and hurricanes can lead to significant increases in phytoplankton biomass. However, the phytoplankton response depends on both the characteristics of a particular storm and the physical–chemical conditions of the water column before storm passage. Finally, the ephemeral bloom that developed as a result of Alex, the strong response of phytoplankton in the mesohaline portion of the estuary to nutrient inputs, and their patchiness on several other occasions suggests that storms may create “hot spots” for trophic transfer and biogeochemical dynamics in estuaries. Adaptive sampling is necessary to capture these features and to fully understand the impact of perturbations such as storms on estuarine ecosystem functioning.

Seasonal evolution of hydrographic fields in the central Middle Atlantic Bight from glider observations

The first sustained glider observations in the Middle Atlantic Bight are used to describe the seasonal evolution of hydrographic fields off New Jersey. Near‐surface temperatures respond to the seasonal cycle of surface heating, while waters at depth are primarily influenced by advection of cold waters from the north in the cold‐pool during spring/summer, and warming due to mixing during fall. The thermocline thickness increases in the offshore direction. Salinity presents seasonal variability due to river discharge and wind variations, with low‐salinity waters spanning ∼100 km across the shelf from May to September in a ∼10 m thick surface layer. Stratification intensifies from April/May to late summer, especially within 80 km from the coast. The pycnocline deepens in the water column during late summer, while the passage of storms during fall rapidly reduces the stratification. The glider high‐resolution observations allowed for unprecedented detailed characterization of the spatial scales of variability.

Of Natmats, Terrorists, and Toxics: Regulatory Adaptation in a Changing World

I. INTRODUCTION II. THE PERFECT STORM A. Natural Accidents B. Climate Change C. Industrial Terrorism III. DUELING PARADIGMS: MANAGEMENT VS. PREVENTION A. Implementing the Dominant Paradigm B. Inherently Safer Design IV. ADAPTATION: INTEGRATION OF ISD V. CONCLUSION I. INTRODUCTION A Category four hurricane is bearing down upon the Gulf Coast. Managers at the Trident Oil refinery in Pascagoula, Mississippi, invoke shutdown procedures three days prior to expected landfall. All crude oil, sulfuric acid, hydrofluoric acid, and other hazardous materials are drained from the process units and the nearby storage tanks and pumped to a more distant emergency storage area at the facility. Most equipment is shut down, although some steam boilers, power generating units, and cooling systems continue to run in order to maintain temperature and pressure in tanks and to support safety devices. A skeleton crew remains at the facility to deal with emergencies and to restart the facility after the storm passes. The storm arrives, lashing the plant with thick sheets of rain and howling 125 mile per hour winds. Heavy rains have overwhelmed the storm water drains throughout the plant, flooding various buildings. The automatic monitoring system associated with Tank 213, which contains a highly toxic liquid material, is located in one of the flooded buildings. High winds rip Tank 213 from its foundations, causing a massive spill of the contents. The facility operator is unaware of the release due to the failure of the submerged monitoring system, and takes no action until after the released material has reached a nearby channel to the gulf. Just south of downtown Philadelphia, the Atlantic Refining Company's facility is nestled among several working class neighborhoods. A sprawling complex on 25 acres, the Atlantic refinery produces 350,000 barrels per day of gasoline, diesel, and jet fuel each year. In the alkylation process unit, light hydrocarbons react with a catalyst to create alkylate, an important additive used to produce high octane gasoline. At the Atlantic refinery hydrofluoric acid is the catalyst of choice. The hydrofluoric acid is stored in a large green cylindrical tank located adjacent to the alkylation process unit. One July morning the neighborhoods surrounding the Atlantic refinery are slowly rousing. Dogs sniff grass, weeds, and flowers on morning walks or in backyards. Children rush to depart for daycare, or play in the street or in small green patches behind their homes. Distracted parents finish coffee on the run, packing up for a day of work. Unnoticed, a lone figure carrying a large duffle bag emerges from the stairs leading to the roof of a weathered apartment building. He sets it down by the roof's edge facing the refinery, and methodically removes dark machined items from the bag. Once assembled, he lifts the tubular contraption to his shoulder, sighting the dull green cylinder at the refinery some 500 yards away. A projectile erupts from the tube, streaking towards the refinery. A large puff of smoke and flame appear on the face of the cylinder, and an orange-tinted haze slowly oozes out, drifting in silence towards the neighborhood. As the 21st century unfolds, regulators charged with overseeing chemical production and use in the United States face a perfect storm of sorts. Three important phenomena with critical implications for chemical management policy are converging at once. First, production and use of toxic chemicals continue to proliferate. Eighty thousand chemicals are in commerce in the United States, with 1,000 added each year. (1) Although current government and industrial codes are in place to minimize known risks, there are troubling gaps in our understanding of the use and impacts of these chemicals. (2) Despite best efforts, even under normal circumstances chemical releases regularly occur. Normal accident theory posits that unanticipated human and system failures are inevitable at complex facilities like oil refineries and chemical plants. …

Hybridation of generation propagation models and SPH model to study severe sea states in Galician Coast

The Atlantic and Cantabrian coasts of the Iberian Peninsula suffer the periodic passage of storms which give rise to severe sea states. Standard generation- propagation models (WAM, WAVEWATCH, SWAN) are commonly used for operational oceanography purposes with satisfactory results both in the open ocean and in near shore areas but they do not provide information about the possibility of flooding in coastal areas. A 2D HV version of SPH (Smoothed Particle Hydrodynamics) model is considered to propagate the signal provided by generation- propagation models to the coast and to analyze the nature of coastal structures overtopping as a tool to create maps of risk. SPH is shown to provide valuable information about overtopping (maximum water height and water velocity) under the stormy conditions suffered by the Galician coast on February 17, 2006.

Analysis of the 13–14 July Gulf Surge Event during the 2004 North American Monsoon Experiment

Abstract Gulf surges are disturbances that move northward along the Gulf of California (GOC), frequently advecting cool, moist air from the GOC or eastern tropical Pacific Ocean into the deserts of the southwest United States and northwest Mexico during the North American Monsoon (NAM). Little attention has been given to the dynamics of these disturbances because of the lack of reliable high-resolution data across the NAM region. High temporal and spatial observations collected during the 2004 North American Monsoon Experiment are used to investigate the structure and dynamical mechanisms of a significant gulf surge on 13–14 July 2004. Integrated Sounding Systems deployed along the east coast of the GOC and an enhanced network of rawinsonde sites across the NAM region are used in this study. Observations show that the 13–14 July gulf surge occurred in two primary stages. The first stage was preceded by anomalous low-level warming along the northern GOC on 13 July. Sharp cooling, moistening, and increased low-level south-southeasterly flow followed over a 12–18-h period. Over the northern gulf, the wind reached ∼20 m s−1 at 750 m AGL. Then there was a brief respite followed by the second stage—a similar, but deeper acceleration of the southerly flow associated with the passage of Tropical Storm (TS) Blas on 14 July. The initial surge disturbance traversed the GOC at a speed of ∼17–25 m s−1 and resulted in a deepening of the mixed layer along the northern gulf. Dramatic surface pressure rises also accompanied the surge. The weight of the evidence suggests that the first stage of the overall surge itself consisted of two parts. The initial part resembled borelike disturbances initiated by convective downdrafts impinging on the low-level stable layer over the region. The secondary part was characteristic of a Kelvin wave–type disturbance, as evident in the deeper layer of sharp cooling and strong wind that ensued. Another possible explanation for the first part is that the leading edge of this Kelvin wave steepened nonlinearly into a borelike disturbance. The second stage of the surge was associated with the increased circulation around TS Blas.

Observational evidence for an ocean heat pump induced by tropical cyclones

Ocean mixing affects global climate and the marine biosphere because it is linked to the ocean's ability to store and transport heat and nutrients. Observations have constrained the magnitude of upper ocean mixing associated with certain processes, but mixing rates measured directly are significantly lower than those inferred from budget analyses, suggesting that other processes may play an important role. The winds associated with tropical cyclones are known to lead to localized mixing of the upper ocean, but the hypothesis that tropical cyclones are important mixing agents at the global scale has not been tested. Here we calculate the effect of tropical cyclones on surface ocean temperatures by comparing surface temperatures before and after storm passage, and use these results to calculate the vertical mixing induced by tropical cyclone activity. Our results indicate that tropical cyclones are responsible for significant cooling and vertical mixing of the surface ocean in tropical regions. Assuming that all the heat that is mixed downwards is balanced by heat transport towards the poles, we calculate that approximately 15 per cent of peak ocean heat transport may be associated with the vertical mixing induced by tropical cyclones. Furthermore, our analyses show that the magnitude of this mixing is strongly related to sea surface temperature, indicating that future changes in tropical sea surface temperatures may have significant effects on ocean circulation and ocean heat transport that are not currently accounted for in climate models.

Air–Sea Exchange in Hurricanes: Synthesis of Observations from the Coupled Boundary Layer Air–Sea Transfer Experiment

The Coupled Boundary Layer Air–Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air–sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (CD) values derived from CBLAST momentum flux measurements show CD becoming invariant with wind speed near a 23 m s−1 threshold rather than a hurricane-force threshold near 33 m s−1 . Values above 23 m s−1 are lower than previous open-ocean measurements. The Dalton number estimates (CE) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s −1 which is in approximate agreement with previous measurements at lower winds. These observations imply a CE/CD ratio of approximately 0.7, suggesting that additional energy sources are necessary for hurricanes to achieve their maximum potential intensity. One such additional mechanism for augmented moisture flux in the boundary layer might be “roll vortex” or linear coherent features, observed by CBLAST 2002 measurements to have wavelengths of 0.9–1.2 km. Linear features of the same wavelength range were observed in nearly concurrent RADARSAT Synthetic Aperture Radar (SAR) imagery. As a complement to the aircraft measurement program, arrays of drifting buoys and subsurface floats were successfully deployed ahead of Hurricanes Fabian (2003) and Frances (2004) [16 (6) and 38 (14) drifters (floats), respectively, in the two storms]. An unprecedented set of observations was obtained, providing a four-dimensional view of the ocean response to a hurricane for the first time ever. Two types of surface drifters and three types of floats provided observations of surface and subsurface oceanic currents, temperature, salinity, gas exchange, bubble concentrations, and surface wave spectra to a depth of 200 m on a continuous basis before, during, and after storm passage, as well as surface atmospheric observations of wind speed (via acoustic hydrophone) and direction, rain rate, and pressure. Float observations in Frances (2004) indicated a deepening of the mixed layer from 40 to 120 m in approximately 8 h, with a corresponding decrease in SST in the right-rear quadrant of 3.2°C in 11 h, roughly one-third of an inertial period. Strong inertial currents with a peak amplitude of 1.5 m s−1 were observed. Vertical structure showed that the critical Richardson number was reached sporadically during the mixed-layer deepening event, suggesting shear-induced mixing as a prominent mechanism during storm passage. Peak significant waves of 11 m were observed from the floats to complement the aircraft-measured directional wave spectra.

Possible biogeochemical response to the passage of Hurricane Fabian observed by satellites

Physical and biogeochemical changes induced by the Hurricane Fabian in the Northwest Atlantic in early September 2003 were observed using composite satellite images. After the passage of the hurricane, the mean sea surface temperature (SST) along the track decreased on average by about 1.3°C with maximum decrease of 10°C. At the same time, the mean Chl a concentration increased by about 42%. Entrainment of cold, nutrient-rich waters by vertical mixing induced by the hurricane seems to have enhanced the phytoplankton production. Asymmetric distribution of changes in SST and Chl a (strong intensity on the right side of the storm track) was observed from the satellite data. The storm-induced nitrate increase estimated from the satellite SST, using a local relationship between nitrate and temperature measurements was about 40% on average along the track of the storm. A numerical model study and climatological nutrient profile showed an increase in mixed-layer depth of 26 m and nitrate increase of about 0.2 μmol L -1 after the storm passage. In addition to altering the physicochemical conditions of the water column, physical forcing by the hurricane also changed the taxonomic composition of phytoplankton. It is inferred that the dominance of diatoms after the storm is a result of the increase in nutrient concentration within the mixed layer due to the wind forcing of the storm.