Browsing by Subject "Texas-Louisiana Shelf"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item Characterizing the Weather Band Variability of the Texas Coastal Current(2014-07-17) Zimmerle, HeatherCurrent velocities from 21 years (1992-2012) of near-continuous observations are used to investigate the Texas Coastal Current on the western Texas-Louisiana continental shelf in the northwestern Gulf of Mexico. Observations were made using the moored current meters deployed as part of the Texas Automated Buoy System (TABS) and historical current meter data. The general coastal circulation is known to be deterministic, with downcoast flow (westward) in the non-summer months (September-May) and a reversal to upcoast (eastward) flow in the summertime (June-August). This study focuses on characterizing features of the Texas Coastal Current that include the onset, frequency, magnitude, and persistence of current reversals along with the upcoast transport that occurs during reversals. The determined interannual variability of the Texas Coastal Current is imperative for understanding the surface transport of water and mitigating associated coastal hazards, including oil, harmful algal blooms, and hypoxia. Results show the onset of the upcoast reversal during the summer with a mostly downcoast flow during the non-summer at upper Texas coastal locations. More persistent currents are observed during the non-summer in the downcoast direction within the weather band frequencies (2-15 days). Currents with longer persistence are found to be relatively slow, generally below 10 cm s^(-1). Fast currents (> 50 cm s^(-1)) tend to be short-lived, typically lasting less than 72 hours. Maximum upcoast transport is observed along the upper coast during the summer, reaching a minimum in the winter and fall. A relationship between the along-shore wind stress and along-shore current velocity is indicated, signaling that the Texas Coastal Current is mostly wind-driven. Spatial variability is present along the southern Texas coast. Current flow is directed downcoast during the summer and slightly downcoast during the non-summer at buoy J, the southernmost location. Currents near the coastal bend tend to be upcoast during the non-summer and slightly downcoast during the summer. Longer persistence is observed at the southern location in the downcoast direction during the summer, with several currents lasting longer than 15 days. Maximum upcoast transport is present during the winter along the southern Texas Coast, reaching a minimum during the summer. Some evidence of a relationship between the along-shore wind stress and along-shore current flow are present, indicating some wind-driven forcing on the current flow. Less seasonal variability is present at offshore locations. Locations on the outer shelf display a general upcoast flow regardless of season. Longer persistence is observed in the upcoast direction on the outer shelf during the summer and non-summer. Maximum upcoast transport is present during the non-summer at all offshore locations. Little correlation is found between seasonal winds and along-shore current flow, meaning mesoscale features, such as Loop Current eddies, provide offshore current forcing.Item Coastal Trapped Waves Generated By Hurricane Andrew on the Texas-Louisiana Shelf(2012-02-14) Pearce, StuartThe Texas-Louisiana Shelf Circulation and Transport Study featured moorings that covered the shelf during 1992 to 1994, and captured the oceanic response on the shelf to category 4 Hurricane Andrew in August of 1992. Eighty-one current meters distributed over 31 moorings along several contours of isobaths provided excellent spatial and temporal coverage over the shelf. The low-frequency variability (2 days and longer) of current observations and tide gauges to the West of the storm are analyzed after the passage of Andrew, focusing on the region outside of direct hurricane forcing. Wavelet analyses are utilized to investigate the dominant periods excited by the storm over the shelf and their temporal evolution after forcing has subsided. Subsequent to the storm's passage, the observations and wavelet transforms show a two-to-four day period coastal trapped wave that propagate westward at speeds near 6 m/s and then around the Texas bend along the bathymetry. The signal remains detectable in observations as far south as Port Isabel, Texas. The prominent frequencies determined from wavelet analysis are compared with predicted coastal trapped wave dispersion modes and show good agreement in the predicted group speed and cross-shelf structure of the first mode. The energies calculated from the data indicate a largely barotropic shelf wave response which is corroborated in the observed currents and by theory.Item Physical Mechanisms Driving Harmful Algal Blooms Along the Texas Coast(2012-12-12) Ogle, Marcus 1982-Commonly referred to as ?red tide?, harmful algal blooms (HABs) formed by Karenia brevis occur frequently in the Gulf of Mexico (GOM). A bloom is defined as cell abundances >105 cells L-1. This thesis will focus primarily on Karenia brevis, formerly known as Gymnodinium breve, in the Gulf of Mexico. K. brevis is harmful because it produces brevetoxin, a ladder-frame polyether that acts as a potent neurotoxin in vertebrates. K. brevis commonly causes fish kills, respiratory irritation in humans, and Neurotoxic Shellfish Poisoning (NSP) if ingested. Blooms of K. brevis occur almost annually along the West Florida Shelf (WFS) in the late summer and early fall, when the coastal current is favorable for bloom initiation. Along the Texas-Louisiana shelf (TLS) however, blooms of K. brevis are infrequent and sporadic. While much is known of the blooms along the WFS due to their frequent presence, little is known of the mechanisms driving the blooms along the TLS due to their inconsistent presence. To understand the stochastic nature of HABs along the TLS, historical data of bloom occurrences from 1996 to present were compared with NOAA station PTAT2 wind, sea-level pressure, air and water temperature data and NCEP NARR-A sea-level pressure data. The difference in the monthly-mean along-shore component of the wind was statistically significant between bloom and non-bloom years in September (p<<0.001) and April (p=0.0015), with bloom years having a strong downcoast current. Monthly mean water temperature values yielded similar results between bloom and non-bloom years. Both March and September monthly-mean water temperature values were lower during non-bloom years with p-values of 0.01 and 0.048, respectively. These results suggest the possibly of forecasting for HABs along the TLS with currently measured, publicly available data.Item Relationships between nutrients and dissolved oxygen concentrations on the Texas-Louisiana shelf during summer of 2004(2009-06-02) Lahiry, SudeshnaHypoxia (dissolved oxygen concentrations less than 1.4 ml/l) is a recurrent seasonal phenomenon on the Louisiana Shelf, caused by the combined effects of nutrient loading by the Mississippi and Atchafalaya River System (MARS), and density stratification. In 2004, three shelf wide cruises (in April, June and August) were conducted on the Louisiana Shelf to understand the mechanisms controlling hypoxia on the shelf, and examine the relationship between dissolved oxygen and nutrient concentrations during the hypoxic periods. The shelf was divided into three geographically separate zones: A (off the mouth of the Mississippi River), B (off the Terrebonne Bay) and C (off the mouth of the Atchafalaya). Each zone was different in terms of the physical and biochemical processes occurring there. In April, no hypoxia was observed on the shelf because of water column mixing by winds, even though high discharge occurred from the MARS. Nutrients were abundant in the surface waters but present only in little amounts at the bottom. In June, the water column was highly stratified. Because of the presence of upwelling favorable winds no vertical mixing occurred and caused extensive hypoxia on the shelf. Dissolved oxygen concentrations were negatively correlated with nutrients at the bottom of the water column. Nutrients were considerably higher at the bottom than at the surface (except for zone A, where high nutrients were seen even at the bottom), indicating remineralization below the pycnocline. Resuspension of organic material and remineralized nitrogen were sustaining hypoxia far from the river sources. In August, hypoxia was patchy on the Louisiana Shelf. Correlations between dissolved oxygen and nutrient concentration varied seasonally with highest correlations occurring during hypoxic conditions in June and August. The spatial distribution of nutrients and other oceanographic parameters, such as light transmission, fluorescence, and dissolved oxygen concentrations, indicate seasonal variability of biochemical processes that are related to physical processes that affect stratification.