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Ocean Surface Current Climatology in the Northern Gulf of Mexico

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Ocean Surface Current Climatology in the Northern Gulf of Mexico Ocean Surface Current Climatology in the Northern Gulf of Mexico by Donald R. Johnson Center for Fisheries Research and Development Gulf Coast Research Laboratory University of Southern Mississippi Project funded by the Marine Fisheries Initiative (MARFIN) program of NMFS/NOAA Published by Gulf Coast Research Laboratory Ocean Springs, Mississippi 39564 May 2008 Table of Contents List of Figures ………………………………………1 Abstract ………………………………………2 I. Introduction ………………………………………3 II. Data and Statistics ………………………………………4 A. Scalar ………………………………………6 B. Vector ……………………………………..10 III. Wind Stress Climatology ……………………………………14 IV. Surface Current Patterns ..….....……………………………20 A. Methodology ………...…………………………...20 B. Monthly Averaged Currents .......................................21 Acknowledgements ……………………………………..36 References ……………………………………..37 ii List of Figures: Figure Page 1. Sea Surface Temperature in the Gulf of Mexico 3 2. Location of daily current vectors in data set 4 3. Grid distribution 5 4. Average Speed (Savg) 6 5. Standard Deviation of Speed (Sstd) 7 6. Maximum Speed (Smax) 8 7a. Scatter plot U/V(Dog Key Pass) 9 7b. Scatter plot U/V (South of Petit Bois Island) 9 8. Vector Resultant Speed (Sr) 11 9. Vector Resultant Direction 12 10. Correlation U/V (r) 13 11. Monthly climatological wind stress for the northern Gulf of Mexico. Taken from Harrison(1989). 15-18 12. Climatological wind stress averages for non-summer (upper) And summer (lower) months. 19 13. Example of observed daily currents between year-days 172-192 and within 50 km of a single grid point (red square). The inner red circle is 20 km from the grid point and the outer red circle is 50 km away. 20 14. Weight of current observation vs distance from each grid point. 21 15. Monthly vector averages of observed currents gridded To 12th degree longitude/latitude and decimated by 5 for better visual resolution. 22-33 16. Non-summer climatology of surface currents (Sep-May). 34 17. Summer climatology of surface currents (June-August) 35 1 Abstract The goal of this technical report is to present a synthesis of 23 years of current observations on the continental shelves of the northern Gulf of Mexico (NGOM). As a result of several large Minerals Management Service programs, a rich set of satellite tracked drifter and moored current meter observations are available. These observations have been previously described in the literature (e.g., Ohlmann and Niiler, 2005 ). In this report, we add observations from other programs and synthesize the data set into statistical and circulation climatology for the continental shelves of the NGOM. This climatology is targeted especially for fishery researchers and managers who need to understand the effects of ocean currents on fishery problems. Both the scalar and vector (resultant) statistical synthesis of the entire set of near- surface currents are presented. Since wind stress plays a major role in establishing seasonal circulation patterns, monthly wind stress climatology has been taken from the literature (Harrison, 1989) and included here. The circulation climatology was formed from the observational data set by calculating daily current vectors and putting them into a year-day time frame (days 1-365) regardless of year. To elucidate patterns of currents, the data set was smoothed temporally (21-year-day moving window) and spatially (gridded by optimal interpolation). The result is a 365 day data set of surface current vectors, gridded at 1/12th degree longitude and latitude resolution. For presentation, this data set is averaged monthly and the results presented as monthly climatology. Since it has been noted that the circulation on the NGOM shelves can be separated into summer (June-August) and non-summer (rest of year) patterns, this climatology is included. 2 I. Introduction. The Northern Gulf of Mexico (NGOM) has been the site of several large observational programs involving measurements of ocean currents. A major goal for much of this work has been to develop a risk assessment for the NGOM involving oil spill spread by currents (Ohlmann and Niiler, 2005). As a result of these programs, a rich set of surface and near-surface current observations are available for other projects. Fishery programs involving larval transport, migratory patterns, aquaculture, marine protected areas, and the impact of climate change also need an understanding of fundamental circulation patterns and statistics of currents over broad regional areas. For example, the synthesized observational data described herein have previously been applied to the transport of red snapper (Lutjanus campechanus) larvae across the entire NGOM (Johnson et al., 2008). The goals of this technical report are to present the geographical distribution of observed current statistics and to describe and present the synthesis of observations into a current-climatology for the continental shelves of the NGOM. Over the continental shelf the principal driving forces are wind stress, buoyancy driven outflows, tides, and interactions with deep-basin events. Wind stress is important in driving the shallow waters over the shelf on both seasonal and storm scales (2-6 days in mid-latitudes). The Ekman Depth, or depth of wind stress momentum mixing, can represent a significant portion of the water column over the shelf. In addition, the near presence of the coastline and other boundaries contributes to rapid wind set-up and set- down. Buoyancy driven river and estuarine outflows also contain both seasonal and storm scales although storm scales are somewhat mitigated by interior drainage time. In the Gulf of Mexico, tidal currents are relatively small and require special handling for adequate resolution. They will not be further discussed in this report; however, they should be noted when assessing current forcing on structures and for short time scale (sub-diurnal) advection. Deep basin events and their effects on the continental shelf are difficult to predict, and can have substantial but transient impacts, particularly on the outer shelf and upper slope. Two major deep-basin circulation features created by basin geography and ocean dynamics are the Loop Current and spin-off rings (Figure 1). The Gulf of Mexico (GOM) is a semi-enclosed sea with inflow of North Atlantic/Caribbean Water through the Yucatan Channel. The Loop Current is formed when this inflow penetrates northward in the GOM before turning in an anti-cyclonic (clockwise) motion and exiting through the Straits of Florida. The loop at times turns back on itself and forms a ring, which eventually breaks off from the main flow. Under the influence of the earth’s rotation the separated ring migrates to the western basin. The rings are large and relatively deep; they spread warm, salty water through the central and western GOM, significantly affecting the oceanic and atmospheric climatology of the entire Gulf. Northward penetration of the loop and break-off of a ring, however, is a dynamically unstable process (Hurlbert and Thompson, 1980) with an unpredictable time scale of roughly 9-13 months (Vukovich, 1995). Since they do not contribute to seasonal patterns, these features affect current statistics but cannot be adequately captured in our 3 climatology. Most of the action from the Loop Current and its rings are contained in the deep basin. However, it should be noted that they can interact with continental shelf waters, particularly over the outer shelf and upper slope region (Oey, 1995). Spin-off ring Loop Current Figure 1: Sea surface temperature in Gulf of Mexico In this report, we 1.) Introduce the data set and derive current statistics. 2.) Present wind stress climatology. 3.) Present the methodology for determining circulation climatology. 4.) Present monthly and seasonal circulation climatology. II. Data and Statistics. The data set is derived from a variety of sources covering the period 1980-2002. Both satellite tracked drifters and moored current meters contributed to the set. Daily averaged current vectors were calculated from the drifters after interpolation along the drifter track to daily positions. Vector components (north and east positive) were formed from the daily positions and tagged with date and location. Moored current meters were both fixed level and Acoustic Doppler Current Profilers (ADCPs). Daily averages of near-surface currents were calculated for the moored instruments and each daily current vector component tagged with date and location. In the case of fixed level current meter 4 strings, the uppermost current meter was used; for acoustic profilers, the bin nearest to a depth of 15% of water depth was used (reduces the error from surface gravity waves). Each daily value in the data set contains [datei, longitudei, latitudei, Ui,Vi], where the subscript i represents an individual data value, U represents the positive eastward component of the current vector and V represents its positive northward component. There are 77,667 daily data values in the set. Figure 2 shows the location of all daily vectors and demonstrates the relatively high density of currents available to the data set, particularly in the northeastern Gulf. The black plus signs represent locations of daily current values for the drifters and the red Figure 2 squares represent locations of moored current meters where daily values were extracted. Since observational data was sparse south of 26o N latitude, the study area was confined to the GOM north of that latitude. Statistics of the observed currents were derived on a homogeneous
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