Papers by Michael Alexander
Case Reports, 2010
Spontaneous intracranial hypotension (SIH) is an uncommon condition characterised by postural hea... more Spontaneous intracranial hypotension (SIH) is an uncommon condition characterised by postural headache secondary to low cerebrospinal fluid pressure. Here we present a case of recurrence of SIH in early pregnancy in a 26-year-old woman. She first presented at the age of 21 years at 15 weeks’ gestation with a history of headache, nausea, vomiting, neck stiffness and photophobia. Findings from
1] A basin-wide interdecadal change in both the physical state and the ecology of the North Pacif... more 1] A basin-wide interdecadal change in both the physical state and the ecology of the North Pacific occurred near the end of 1976. Here we use a physical-ecosystem model to examine whether changes in the physical environment associated with the 1976-1977 transition influenced the lower trophic levels of the food web and if so by what means. The physical component is an ocean general circulation model, while the biological component contains 10 compartments: two phytoplankton, two zooplankton, two detritus pools, nitrate, ammonium, silicate, and carbon dioxide. The model is forced with observed atmospheric fields during 1960-1999. During spring, there is a $40% reduction in plankton biomass in all four plankton groups during 1977-1988 relative to 1970-1976 in the central Gulf of Alaska (GOA). The epoch difference in plankton appears to be controlled by the mixed layer depth. Enhanced Ekman pumping after 1976 caused the halocline to shoal, and thus the mixed layer depth, which extends to the top of the halocline in late winter, did not penetrate as deep in the central GOA. As a result, more phytoplankton remained in the euphotic zone, and phytoplankton biomass began to increase earlier in the year after the 1976 transition. Zooplankton biomass also increased, but then grazing pressure led to a strong decrease in phytoplankton by April followed by a drop in zooplankton by May: Essentially, the mean seasonal cycle of plankton biomass was shifted earlier in the year. As the seasonal cycle progressed, the difference in plankton concentrations between epochs reversed sign again, leading to slightly greater zooplankton biomass during summer in the later epoch.
Declines of Steller sea lion (Eumetopias jubatus) populations in the Aleutian Islands and Gulf of... more Declines of Steller sea lion (Eumetopias jubatus) populations in the Aleutian Islands and Gulf of Alaska could be a consequence of physical oceanographic changes associated with the 1976--77 climate regime shift. Changes in ocean climate are hypothesized to have affected the quantity, quality, and accessibility of prey, which in turn may have affected the rates of birth and death of sea lions. Recent studies of the spatial and temporal variations in the ocean climate system of the North Pacific support this hypothesis. Ocean climate changes appear to have created adaptive opportunities for various species that are preyed upon by Steller sea lions at mid-trophic levels. The east--west asymmetry of the oceanic response to climate forcing after 1976-- 77 is consistent with both the temporal aspect (populations decreased after the late 1970s) and the spatial aspect of the decline (western, but not eastern, sea lion populations decreased). These broad-scale climate variations appear to b...
The Influence of Thermocline Topography on the Oceanic Response to Fluctuating Winds: A Case Study in the Tropical North Pacific
Fluid Mechanics and Its Applications, 2001
Thermocline processes likely play an important role in climate variability at decadal timescales.... more Thermocline processes likely play an important role in climate variability at decadal timescales. Surface anomalies subducting at midlatitudes propagate in the main thermocline toward the tropics with a timescale close to decadal. Thermocline variability is also associated with baroclinic Rossby waves, a fundamental agent in the adjustment of the ocean circulation, whose propagation time can introduce a delay of several
This paper describes the development and prelimi- nary testing of a functional upper-limb orthosi... more This paper describes the development and prelimi- nary testing of a functional upper-limb orthosis for people that have limited strength in their arms. This is symptomatic of con- ditions such as muscular dystrophy (MD), spinal muscular atrophy (SMA), and partial spinal cord injury . The exoskeletal orthosis is wheelchair mounted, has two links and four degrees of freedom . It
We explored the relationship between amphibian declines and climate variations in Colorado (U.S.A... more We explored the relationship between amphibian declines and climate variations in Colorado (U.S.A.), Puerto Rico, Costa Rica-Panama, and Queensland (Australia) through two sources of data: output from the National Center for Environmental Prediction "reanalysis system" and area-averaged station data. The reanalysis system merges observations from airplanes, land stations, satellites, ships, and weather balloons with output from a weather-forecast model to create global fields of atmospheric variables. Station data consisted of temperature and precipitation measured with thermometers and rain gauges at fixed locations. Temperatures were near normal in Colorado when the amphibian declines occurred in the 1970s, whereas in Central America temperatures were warmer than normal, especially during the dry season. The station data from Puerto Rico and Australia indicated that temperatures were above normal during the period of amphibian declines, but reanalysis did not show such a clear temperature signal. Although declines occurred while the temperature and precipitation anomalies in some of the regions were large and of extended duration, the anomalies were not beyond the range of normal variability. Thus, unusual climate, as measured by regional estimates of temperature and precipitation, is unlikely to be the direct cause of amphibian declines, but it may have indirectly contributed to them. Previous researchers have noted that the declines appear to have propagated from northwest to southeast from Costa Rica to Panama and from southeast to northwest in Queensland, Australia. Wind has the potential to transport pathogens that cause amphibian mortality. The mean direction of the near-surface winds tended to parallel the path of amphibian declines from July-October in Central America and from May-July in Australia. The wind direction was highly variable, however, and the propagation rate of amphibian declines was much slower than the mean wind speed. In addition, the most likely pathogen is a chytrid fungus that does not produce desiccation-resistant spores. Thus, if wind is involved in the propagation of amphibian declines, it is through a complex set of processes.
An extended reanalysis, a combination of observations and model output, is used to examine the sp... more An extended reanalysis, a combination of observations and model output, is used to examine the spatial patterns of physical variables associated with the Atlantic Multidecadal Oscillation (AMO) from 1871 to 2008. The results are presented as anomalies during positive and negative phases of the AMO. As in previous studies, during positive (negative) AMO phases the sea surface temperature (SST) is anomalously warm (cold) over most of the North Atlantic, with the exception of the east coast of the United States. The atmospheric patterns, associated with the positive phase of the AMO, include anomalous low pressure over the Atlantic between 20°S and 50°N, cyclonic surface winds around the low, reduced wind speeds over the tropical Atlantic and enhanced precipitation in the eastern tropical Atlantic, with roughly opposite conditions during negative AMO phases. There are, however, substantial differences in the SST and the atmospheric anomalies between periods of the same phase, especially in the extratropics. Correlations between the AMO and air temperature anomalies are positive over much of the globe between 40°S and 50°N, with correlations exceeding 0.6 (~95% significance level) over the Maritime Continent and northern rim of the Pacific Ocean. Most of the sea level pressure (SLP) anomalies beyond the Atlantic are not statistically significant.
Interactions between the north Pacific Ocean and the Northern Hemisphere atmosphere during El Nino(Ph. D. Thesis)
Abstract Model simulations are used to examine sea surface temperature (SST) anomalies in the Nor... more Abstract Model simulations are used to examine sea surface temperature (SST) anomalies in the North Pacific during the fall and winter of El Nino episodes and the extent to which these anomalies influence the atmospheric circulation. A mixed layer ocean model of the ...
The seasonal footprinting mechanism in CFSv2: simulation and impact on ENSO prediction
Two atmospheric general circulation model experiments are conducted with specified terrestrial sn... more Two atmospheric general circulation model experiments are conducted with specified terrestrial snow conditions representative of 1980-99 and 2080-99. The snow states are obtained from twentieth-century and twenty-first-century coupled climate model integrations under increasing greenhouse gas concentrations. Sea surface temperatures, sea ice, and greenhouse gas concentrations are set to 1980-99 values in both atmospheric model experiments to isolate the effect of the snow changes. The reduction in snow cover in the twenty-first century relative to the twentieth century increases the solar radiation absorbed by the surface, and it enhances the upward longwave radiation and latent and sensible fluxes that warm the overlying atmosphere. The maximum twenty-first-century minus twentieth-century surface air temperature (SAT) differences are relatively small (,38C) compared with those due to Arctic sea ice changes (;108C). However, they are continental in scale and are largest in fall and spring, when they make a significant contribution to the overall warming over Eurasia and North America in the twenty-first century. The circulation response to the snow changes, while of modest amplitude, involves multiple components, including a local low-level trough, remote Rossby wave trains, an annular pattern that is strongest in the stratosphere, and a hemispheric increase in geopotential height.
Atlas of climatology and variability in the GFDL R30S14 GCM
We examine processes that influence North Pacific sea surface temperature (SST) anomalies includi... more We examine processes that influence North Pacific sea surface temperature (SST) anomalies including surface heat fluxes, upper ocean mixing, thermocline variability, ocean currents, and tropical-extratropical interactions via the atmosphere and ocean. The ocean integrates rapidly varying atmospheric heat flux and wind forcing, and thus a stochastic model of the climate system, where white noise forcing produces a red spectrum, appears to provide a baseline for SST variability even on decadal time scales. However, additional processes influence Pacific climate variability including the "reemergence mechanism," where seasonal variability in mixed layer depth allows surface temperature anomalies to be stored at depth during summer and return to the surface in the following winter. Wind stress curl anomalies in the central/east Pacific drive thermocline variability that propagates to the west Pacific via baroclinic Rossby waves and influences SST by vertical mixing and the change in strength and position of the ocean gyres. Atmospheric changes associated with the El Niño-Southern Oscillation (ENSO) also influence North Pacific SST anomalies via the "atmospheric bridge." The dominant pattern of North Pacific SST anomalies, the Pacific Decadal Oscillation (PDO), exhibits variability on interannual as well as decadal time scales. Unlike ENSO, the PDO does not appear to be a mode of the climate system, but rather it results from several different mechanisms including (1) stochastic heat flux forcing associated with random fluctuations in the Aleutian Low, (2) the atmospheric bridge augmented by the reemergence mechanism, and (3) wind-driven changes in the North Pacific gyres.
Observations indicate the existence of two bands of maximum thermocline depth variability centere... more Observations indicate the existence of two bands of maximum thermocline depth variability centered at ϳ10ЊS and 13ЊN in the tropical Pacific Ocean. The analysis of a numerical integration performed with the National Center for Atmospheric Research ocean general circulation model (OGCM) forced with observed fluxes of momentum, heat, and freshwater over the period from 1958 to 1997 reveals that the tropical centers of thermocline variability at 10ЊS and 13ЊN are associated with first-mode baroclinic Rossby waves forced by anomalous Ekman pumping. In this study the factors that may be responsible for the Rossby wave maxima at 10ЊS and 13ЊN, including the amplitude and spatial coherency of the forcing at those latitudes, are systematically investigated. A simple Rossby wave model is used to interpret the OGCM variability and to help to discriminate between the different factors that may produce the tropical maxima. These results indicate that the dominant factor in producing the maximum variability at 10ЊS and 13ЊN is the zonal coherency of the Ekman pumping, a characteristic of the forcing that becomes increasingly more pronounced at low frequencies, maximizing at timescales in the decadal range. Local maxima in the amplitude of the forcing, while not explaining the origin of the centers of variability at 10ЊS and 13ЊN, appear to affect the sharpness of the variability maxima at low frequencies.
A band of enhanced thermocline variability at 10Њ-15ЊN in the Pacific found in nature also occurs... more A band of enhanced thermocline variability at 10Њ-15ЊN in the Pacific found in nature also occurs in an ocean general circulation model forced with observed fluxes of momentum, heat, and freshwater over the period 1958-97. The variability in the model is primarily associated with long baroclinic Rossby waves characterized by periods in the decadal range (7-10 yr). The waves are forced by westward propagating Ekman pumping anomalies east of the date line and propagate at a speed of ϳ13 cm s Ϫ1 , which is slower than the phase speed of the first mode unforced baroclinic waves (15-16 cm s Ϫ1 ). West of the date line, the correlations between thermocline displacements and local Ekman pumping are relatively small, and the ocean signals have a phase speed of ϳ20 cm s Ϫ1 , very similar to the phase speed of the first baroclinic mode in the western half of the basin (18-20 cm s Ϫ1 ). The phase speeds of the ocean model signals have been estimated using cospectral analysis, while the WKB approximation has been used to evaluate the phase speed of the baroclinic Rossby wave modes for the given model stratification. The thermocline displacements are coherent all the way across the basin in the 10Њ-15ЊN latitude band. After reaching the western boundary the signal appears to propagate along the boundary, both to the north and the south. Along the southern branch, the signal reaches the equator and propagates along the equator, contributing to low-frequency equatorial thermocline variability.
Increased concern over climate change is demonstrated by the many efforts to assess climate effec... more Increased concern over climate change is demonstrated by the many efforts to assess climate effects and develop adaptation strategies. Scientists, resource managers, and decision makers are increasingly expected to use climate information, but they struggle with its uncertainty. With the current proliferation of climate simulations and downscaling methods, scientifically credible strategies for selecting a subset for analysis and decision making are needed. Drawing on a rich literature in climate science and impact assessment and on experience working with natural resource scientists and decision makers, we devised guidelines for choosing climate-change scenarios for ecological impact assessment that recognize irreducible uncertainty in climate projections and address common misconceptions about this uncertainty. This approach involves identifying primary local climate drivers by climate sensitivity of the biological system of interest; determining appropriate sources of information for future changes in those drivers; considering how well processes controlling local climate are spatially resolved; and selecting scenarios based on considering observed emission trends, relative importance of natural climate variability, and risk tolerance and time horizon of the associated decision. The most appropriate scenarios for a particular analysis will not necessarily be the most appropriate for another due to differences in local climate drivers, biophysical linkages to climate, decision characteristics, and how well a model simulates the climate parameters and processes of interest. Given these complexities, we recommend interaction among climate scientists, natural and physical scientists, and decision makers throughout the process of choosing and using climate-change scenarios for ecological impact assessment.
1] Changes in upper ocean stratification during the second half of the 21st century, relative to ... more 1] Changes in upper ocean stratification during the second half of the 21st century, relative to the second half of the 20th century, are examined in ten of the CMIP3 climate models according to the SRES-A2 scenario. The upper ocean stratification, defined here as the density difference between 200 m and the surface, is larger everywhere during the second half of the 21st century, indicative of an increasing degree of decoupling between the surface and the deeper oceans, with important consequences for many biogeochemical processes. The areas characterized by the largest stratification changes include the Arctic, the tropics, the North Atlantic, and the northeast Pacific. The increase in stratification is primarily due to the increase in surface temperature, whose influence upon density is largest in the tropical regions, and decreases with increasing latitude. The influence of salinity upon the stratification changes, while not as spatially extensive as that of temperature, is very large in the Arctic, North Atlantic and Northeast Pacific. Salinity also significantly contributes to the density decrease near the surface in the western tropical Pacific, but counteracts the negative influence of temperature upon density in the tropical Atlantic.
The meridional shifts of the Oyashio Extension (OE) and of the Kuroshio Extension (KE), as derive... more The meridional shifts of the Oyashio Extension (OE) and of the Kuroshio Extension (KE), as derived from high-resolution monthly sea surface temperature (SST) anomalies in 1982-2008 and historical temperature profiles in 1979-2007, respectively, are shown based on regression analysis to significantly influence the large-scale atmospheric circulation. The signals are independent from the ENSO teleconnections, which were removed by seasonally varying, asymmetric regression onto the first three principal components of the tropical Pacific SST anomalies. The response to the meridional shifts of the OE front is equivalent barotropic and broadly resembles the North Pacific Oscillation / Western Pacific pattern in a positive phase for a northward frontal displacement. The response reaches 35 m at 250 hPa for a typical OE shift, a strong sensitivity since the associated SST anomaly is 0.5 K. However, the amplitude depends on an assumed 2-month atmospheric response time. The response is stronger during winter and when the front is displaced southward. The response to the northward KE shifts primarily consists of a high centered in the northwestern North Pacific and hemispheric teleconnections. It is also equivalent barotropic, except near Kamchatka where it tilts slightly westward with height. The typical amplitude is half as large as that associated with OE shifts.
This study presents an overview of the El Niñ o-Southern Oscillation (ENSO) phenomenon and Pacifi... more This study presents an overview of the El Niñ o-Southern Oscillation (ENSO) phenomenon and Pacific decadal variability (PDV) simulated in a multicentury preindustrial control integration of the NCAR Community Climate System Model version 4 (CCSM4) at nominal 18 latitude-longitude resolution. Several aspects of ENSO are improved in CCSM4 compared to its predecessor CCSM3, including the lengthened period (3-6 yr), the larger range of amplitude and frequency of events, and the longer duration of La Niñ a compared to El Niñ o. However, the overall magnitude of ENSO in CCSM4 is overestimated by ;30%. The simulated ENSO exhibits characteristics consistent with the delayed/recharge oscillator paradigm, including correspondence between the lengthened period and increased latitudinal width of the anomalous equatorial zonal wind stress. Global seasonal atmospheric teleconnections with accompanying impacts on precipitation and temperature are generally well simulated, although the wintertime deepening of the Aleutian low erroneously persists into spring. The vertical structure of the upper-ocean temperature response to ENSO in the north and south Pacific displays a realistic seasonal evolution, with notable asymmetries between warm and cold events. The model shows evidence of atmospheric circulation precursors over the North Pacific associated with the ''seasonal footprinting mechanism,'' similar to observations. Simulated PDV exhibits a significant spectral peak around 15 yr, with generally realistic spatial pattern and magnitude. However, PDV linkages between the tropics and extratropics are weaker than observed.
Detailed basement mapping in the deepwater Gulf of Mexico from integrated modeling
SEG Technical Program Expanded Abstracts 2001, 2001
Summary Detailed mapping of Moho relief, crustal thickness, detailed basement structure, and a te... more Summary Detailed mapping of Moho relief, crustal thickness, detailed basement structure, and a tectonic framework for the deepwater Gulf of Mexico has recently been completed. Input data for this work includes a 100 deepwater well velocity database, 95,000 line miles ...
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Papers by Michael Alexander