Worth D. Nowlin

Near‐inertial oscillations over the Texas‐Louisiana shelf

Wind-induced, near-inertial oscillations over the Texas-Louisiana shelf in spring and summer 1992 are described using the current and wind observations taken during the first year of the Texas-Louisiana Shelf Circulation and Transport Processes Study (LATEX A). Rotary spectral analysis shows clockwise-rotating energy peaks at near-inertial frequencies for records from all current meter moorings after the suppression of principal tidal signals. The vertical structure of near-inertial oscillations is characterized by a first baroclinic mode with a near 180° phase difference between the upper mixed layer and the lower stratified layer. The oscillations are intermittent with a modulation timescale of about 5–10 days. They are surface-intensified and have maximum values near the shelf break, decaying gradually toward the coast but rapidly offshore. Near-inertial oscillations appear to accompany a sudden change of the wind stress during frontal passages. Diagnostic analysis suggests that the large near-inertial oscillations over the LATEX shelf are mainly generated by high-frequency (near-inertial) variation of the wind stress accompanying the passage of atmospheric fronts. When the downward transfer of the near-inertial energy to the deep stratified layer is small, a simple mixed layer model forced by the observed wind stress provides a reasonable prediction of the near-inertial currents in the mixed layer.

Low‐Frequency Circulation Over the Texas‐Louisiana Continental Shelf

Low-frequency circulation over the Texas-Louisiana continental shelf is examined. Currents over the inner shelf are upcoast (Rio Grande to Mississippi River) in summer and downcoast in nonsummer and are driven by an annual cycle of winds. This results in an annual signal for salinity, with lowest salinity waters occurring (a) in late spring along the inner portion of the western shelf when downcoast flows carry the high discharges from the Mississippi-Atchafalya and other rivers to the Mexican border, and (b) in summer over the inner and outer eastern shelf when the upcoast flow causes a pooling of the discharges from the Mississippi-Atchafalya rivers over that shelf. Upcoast winds during summer also result in high salinities over the western shelf due to advection from off Mexico and upwelling. Currents over the outer shelf are variable, but predominantly upcoast throughout the year, probably a result of the integrated effects of anticy-clonic eddies impinging on the shelf edge. Comparison of currents in the weather band (2-10 d) with the mesoscale band (10-100 d) suggests the shelf is divided at approximately the 50-m isobath. The weather band predominates over the inner shelf, reflecting frequent passage of fronts over the region. The mesoscale band predominates over the outer shelf, indicating the presence of offshelf eddies that frequent this region.

Mean hydrographic fields and their interannual variability over the Texas-Louisiana continental shelf in spring, summer, and fall

New hydrographic data from the Texas-Louisiana continental shelf were combined with data from older cruises covering significant portions of this shelf to produce spatial distributions of surface and bottom temperature and salinity as well as of surface geopotential anomaly relative to 70 dbar. These were used to calculate mean fields with their standard deviations for spring (May), summer (July–August), and fall (November). For each season, histograms were prepared of differences between properties in the individual fields and our seasonal mean values at each grid point in the individual fields. These histograms have highly tuned Gaussian distributions centered on zero differences, proving that a distribution selected randomly will likely be quite similar to the mean for the season in which the sample was made. The individual fields of salinity for summer and geopotential anomaly for spring are included for comparison with the mean fields. The mean fields, produced by adding a large data set to that used by Cochrane and Kelly [1986], substantiate the bimodal annual patterns of circulation and property distributions over the inner shelf region described by them. Essentially, there is downcoast (directed from the Mississippi toward Brownsville) nearshore flow except during the summer months. That flow is driven by downcoast along-shelf wind and enhanced by Mississippi-Atchafalaya River discharge. In July and August the average wind has an upcoast component and the nearshore flow is reversed. Patterns and values of the standard deviations are used to infer causes and magnitudes of interannual variability, respectively. Three examples of anomalous property distributions are presented to illustrate the effects of the principal external forcing mechanisms affecting interannual variability on the Texas-Louisiana shelf. These mechanisms are wind stress, Mississippi-Atchafalaya River discharge, and mesoscale eddies in the offshore circulation near the shelf-slope break. For each cruise examined, residuals of geopotential anomaly and surface salinity relative to the seasonal mean are examined in relation to departures of river discharge from the long-term (64 year) average and an index of along-shelf wind component appropriate to the times of the cruises. The residuals of geopotential anomaly were found to be significantly negatively correlated with those of surface salinity, with an intercept of approximately zero indicating that salinity plays the dominant role relative to temperature in year-to-year variability of the geopotential anomaly. Positive river discharge residuals were correlated with negative surface salinity residuals; enhanced downcoast wind resulted in negative surface salinity residuals; and enhanced upcoast wind resulted in positive surface salinity residuals. Most correlations were significant (different from zero) at the 95% confidence level.

Satellite observations of upwelling on the continental shelf south of Madagascar

We report on upwelling seen in satellite AVHRR sea surface temperature imagery over the continental slope and shelf of southern Madagascar during February and March 2000. The upwelling is concurrent with anomalously high pseudo wind-stress over the region during this period. However, the western boundary East Madagascar Current, which is seen over the continental slope region, may contribute to the upwelling effect. The upwelling covers an area of 2° longitude by 1° latitude and at its peak is about 3–5°C cooler than the local ambient sea surface temperature. The paucity of in situ wind and current data in the region, however, prohibit a quantitative assessment of the relative forcing.

A Statistical Description of the Velocity Fields from Upper Ocean Drifters in the Gulf of Mexico

We analyzed 1397 drifter records collected in the Gulf of Mexico and northwestern Cayman Basin between the years 1989 and 1999 to describe the general features of the upper ocean circulation. These drifters were generally drogued at 50 m below the surface and exclude those of the Surface CUrrent Lagrangian Program (SCULP). In addition to the dominant flows through the Yucatan Channel and Straits of Florida, robust circulation features clearly seen include: a weak cyclone south of 21°N in the Bay of Campeche, westward zonal flow across the Gulf between 21°N and 24°N, a northward western boundary current between 95°W and 97°W and 24°N and 26°N, mean downcoast (westward) flow on the Texas-Louisiana shelf, highly variable mean flow on the shelves and slope of the northeastern Gulf, mean upcoast (southward) flow on the lower West Florida Shelf, and a large region of high variability in the deep regions of the central Gulf of Mexico. Although much of the driving of the Gulf of Mexico is attributed to currents associated with the Loop Current and its associated Loop Current Eddies (particularly in the deep waters), other features can be directly correlated with seasonal wind driving, particularly on the shelves of the northern Gulf, near the western boundary, and in the Bay of Campeche.

Spatial‐scale analysis of hydrographic data over the Texas‐Louisiana continental shelf

On the basis of hydrographic data collected by the Texas-Louisiana Shelf Circulation and Transport Processes Study (LATEX A) and on earlier cruises, we examined the energetic scales of spatial variability over the Texas-Louisiana continental shelf. Shelf-scale spatial reference fields were sought to represent the general distributions of circulation and water properties over the shelf at the time of the observations. Various methods were explored for determining such reference fields of potential temperature, salinity, and geopotential anomaly at the sea surface relative to 70 dB. Spatial reference fields obtained from mean May fields and from polynomials fitted to individual May cruise data were compared. On the basis of those comparisons, quadratics were selected to fit property distributions from individual cruises and so to yield reference fields. Smaller-scale anomaly fields were obtained by removing the reference fields from the observed distributions. Calculation of correlation versus separation distance based on these anomaly fields then allowed estimation of spatial scales of anomaly fields for cross-shelf and along-shelf transects. The zero-crossing scale and the Gaussian decay scale are shown to be essentially the same, and the zero-crossing scale is used. The principal results for the anomaly scales are (1) cross-shelf scales over the western shelf are shorter (order 15 km) than those in the eastern and central regions (order 20 km), (2) along-shelf spatial scales are of the order of 35 km, (3) there is no significant difference in cross-shelf scales at the surface, middepth, and bottom, and (4) along-shelf scales are essentially the same over the western and eastern regions of the shelf, over the midshelf (50-m isobath) and along the shelf break (200-m isobath), and at different depths along the 200-m isobath. The same spatial scales are found when using data with spatial resolution of 1–10 km cross shelf and 10–20 along shelf to obtain the anomaly fields, so the data resolution used is adequate to represent the scales. The variances of the observed (shelf-wide) salinity, temperature, and geopotential anomaly are greater cross shelf than along shelf. The variance of the cross-shelf anomaly fields is around 10% of the shelf-wide fields; that of the along-shelf anomaly fields is about 35% of that in the shelf-wide fields. The analysis of scales when grouped by season did not show persuasive evidence of seasonal variation.

Observations of kinetic energy levels in the antarctic circumpolar current at Drake Passage

Based on 31 nearly year-long records of current in Drake Passage the kinetic energy levels in the deep water (below 2000 m) across the passage and through the water column in the central passage are examined. The energy spectra show no significant temporal variability; by contrast, the spatial differences are pronounced, with more fluctuation kinetic energy (KF) in the northern than the southern passage and more above 1000 m than below. Partitioning by frequency bands shows that approx. 28% of the KF results from fluctuations with periods between 2 h (the Nyquist period for the sampling rate) and 2 days and that the energy level for this band is rather uniform across the passage. Large values of KF at northern passage locations result primarily from more activity in the period band between 2 and 50 days. Although that band accounts for almost half of the total KF, a large fraction (23%) of observed kinetic energy is associated with longer periods. The long-term records allowed examination of the representativeness of results—cumulative plots of KF and kinetic energy of mean motion (KM) vs time indicate that the kinetic energy densities reach equilibrium values (for specific long-term records) only after intervals of the order of 4 months. As in the case for the Gulf Stream system, abyssal KF values in the Antarctic Circumpolar Current system at Drake Passage are one or two orders of magnitude greater than KF values in the interior of the North Atlantic subtropical gyre. Moreover, deep KF values south of Cape Horn equal deep water values beneath the Gulf Stream. Values of KM from records deeper than 2500 m increase southward from northern to central locations in Drake Passage. In the central passage KM increases upward from values near 10 cm2 s−2 at 2700 m to over 300 cm2 s−2 at 300 m. KF increases from deep water values near 25 cm2 s−2 to just over 200 cm2 s−2 near 300 m. Relative to mid depths, ratios of KF to KM increase near the surface and near bottom in the central passage.

Research and GOOS

A Global Ocean Observing System (GOOS) is needed for: detecting and predicting climate variability and consequences, protecting and restoring healthy marine ecosystems, reducing human health risks, managing resources, facilitating safe and efficient marine operations, and predicting and mitigating against coastal hazards. To these ends, an operational system of integrated observations, data communications and management, and data analysis is being developed to serve the data and information requirements of a broad spectrum of user groups from industry, government, nongovernmental organizations, and academia. Successful development of a system that guarantees the continuous and sustained provision of useful information requires effective collaboration between the research and operational communities and between data providers and users to ensure that system development is user-driven and sustained in perpetuity. Development of GOOS requires a managed process that selectively incorporates, enhances, and supplements existing elements consistent with user needs. Successful transition of new elements from research to operational oceanography is a big step and is inherently difficult. It will require ongoing guidance from both data providers and users as well as cooperation and good will on the part of the research and operational groups involved. In some cases it may be necessary to transition new elements into the operational system prior to obtaining user acceptance because that acceptance may be obtained only after the usefulness of new products is demonstrated. A process that selectively migrates new knowledge and technology from research into an operational mode and promotes synergy between research and operational oceanography is described herein.

Development of a Coastal Ocean Observing System for the Gulf of Mexico

A regional coastal ocean observing system being developed under the U.S. Integrated Ocean Observing System, a Global Ocean Observing System contribution, is the Gulf of Mexico Coastal Ocean Observing System (GCOOS). A wide range of users will be provided with estuarine and marine system products, information, and data by GCOOS, which will be a sustained ocean observing system. GCOOS is being developed through the GCOOS Regional Association. GCOOS building activities include real-time physical data connection via the Internet from extant observing systems to the National Data Buoy Center, extant observing systems inventory development, and mechanism establishment and implementation for ongoing identification of regional need priority pilot projects, and stakeholder priorities and requirements. Identification of products and measurements emergency responders and managers need for better southeastern United States and Gulf of Mexico inundation and storm surge prediction and mitigation is being provided through an inundation and storm surge workshop. GCOOS enhancement funding from private and governmental sources is being sought. The authors argue that new federal resources for regional coastal ocean observing systems must be committed for GCOOS to evolve to its full potential.

Developing the U.S. sustained and integrated ocean observing system

Serious planning for a U.S. component of the Global Ocean Observing System (GOOS) began in 1998 with the establishment of a U.S. GOOS Steering Committee. With the establishment in October 2000 of the Ocean.US Office by the National Oceanographic Partnership Program (NOPP) that planning has greatly accelerated. That interagency office is charged to integrate existing and planned activities to achieve a U.S. integrated and sustained ocean observing system (IOOS) to meet long-term needs of operational and research users. In March 2002, the Ocean.US Office held a national workshop to provide the basis for an initial plan for the IOOS. Attendees represented all sectors of potential system users and scientific and technical disciplines involved in its implementation. General consensus was reached on the vision for the observing system, the core elements of the system that should be federally supported, and an approach to planning the data management and communications subsystem. A report form the workshop and the initial plan for the system have been prepared by the Ocean.US Office; they have or will be reviewed by the cognizant agencies and submitted to Congress. The U.S. IOOS will obtain the observations and provide the data management and communications subsystem required to produce needed products and services. The U.S. IOOS will consist of two major components. (1) A global, open ocean component will be of primary interest to users in the climate, defense, research, and maritime commerce sectors. (2) A coastal component, consisting of a federation of regional observing systems, will focus on products and services from the estuaries to the edge of the nations EEZ and will be of interest to the aforementioned sectors plus others, including resource management, public health, recreation, and energy. The user base for such a system is large. To complete the planning and implement the observing system needed to meet the vision many activities will be needed. Key activities are described and briefly discussed in the paper.