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Observation of the Deepwater Manifestation of the Loop Current and Loop Current Rings in the Eastern Gulf of Mexico

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Observation of the Deepwater Manifestation of the Loop Current and Loop Current Rings in the Eastern Gulf of Mexico OCS Study MMS 2009-050 Coastal Marine Institute Observation of the Deepwater Manifestation of the Loop Current and Loop Current Rings in the Eastern Gulf of Mexico U.S. Department of the Interior Cooperative Agreement Minerals Management Service Coastal Marine Institute Gulf of Mexico OCS Region Louisiana State University OCS Study MMS 2009-050 Coastal Marine Institute Observation of the Deepwater Manifestation of the Loop Current and Loop Current Rings in the Eastern Gulf of Mexico Authors Susan E. Welsh Masamichi Inoue Lawrence J. Rouse, Jr. Eddie Weeks October 2009 Prepared under MMS Contract 1435-01-04-CA-32806-36189 (M05AZ10669) by Louisiana State University Coastal Marine Institute Baton Rouge, Louisiana 70803 Published by U.S. Department of the Interior Cooperative Agreement Minerals Management Service Coastal Marine Institute Gulf of Mexico OCS Region Louisiana State University DISCLAIMER This report was prepared under contract between the Minerals Management Service (MMS) and Louisiana State University (LSU). This report has been technically reviewed by the MMS, and it has been approved for publication. Approval does not signify that the contents necessarily reflect the view and policies of the MMS, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. It is, however, exempt from review and compliance with the MMS editorial standards. REPORT AVAILABILITY This report is available only in compact disc format from the Minerals Management Service, Gulf of Mexico OCS Region, at a charge of $15.00, by referencing OCS Study MMS 2009-050. The report may be downloaded from the MMS website through the Environmental Studies Program Information System (ESPIS). You will be able to obtain this report also from the National Technical Information Service in the near future. Here are the addresses. You may also inspect copies at selected Federal Depository Libraries. Minerals Management Service U.S. Department of Commerce Gulf of Mexico OCS Region National Technical Information Service Public Information Office (MS 5034) 5285 Port Royal Road 1201 Elmwood Park Boulevard Springfield, Virginia 22161 New Orleans, Louisiana 70123-2394 Phone: (703) 605-6040 Telephone requests may be placed at Fax: (703) 605-6900 (504) 736-2519, 1-800-200-GULF, or Email: [email protected] Rush Orders: 1-800-553-6847 Fax: (504) 736-2620 CITATION S.E. Welsh, M. Inoue, L.J. Rouse, Jr., and E. Weeks. 2009. Observation of the deepwater manifestation of the Loop Current and Loop Current rings in the Eastern Gulf of Mexico. U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 2009-050. 110 pp. iii ABSTRACT Following the previous successful observations involving three deployments covering the periods June 2000 – June 2002 and April 2003 – June 2004, two additional years of deployment of a deep water mooring at the center of the eastern Gulf of Mexico produced additional observations of deep water manifestations of the Loop Current and Loop Current rings. These deployments placed an emphasis on observing the interface between the upper- and lower-layers and variability of water mass properties in deep water. The interface between the upper and lower layers appears to be located close to the sill depth (~800 m) at Strait of Florida. In the modeling component of this project, very high horizontal (.075º) and vertical (100 levels) resolution allowed more realistic representation of the bottom topography including the gentle rise and the steep escarpments. Consequently, it resulted in more realistic simulation of deep water currents in the eastern Gulf of Mexico, i.e., more energetic deep water and more chaotic eddy field in deep water, consistent with the observations at the mooring site. It appears that the Loop Current and Loop Current rings drive deep water currents in the eastern Gulf of Mexico. A modon pair forms underneath the Loop Current when the Loop Current extends northward prior to the formation of a Loop Current ring. However, deep water eddy-eddy and eddy-topography interaction due to the topographic constriction in the central gulf and the limited size of the eastern basin make clear identification of “a modon pair” problematic. The observations of currents and water mass characteristics at the mooring site appear to be consistent with the Cushman-Roisin et al. (1990) mechanism for the generation of the deep anticyclone-cyclone pair beneath the Loop Current. Another mechanism to transmit significant energy to deep water in the Gulf of Mexico is the barotropic oceanic response to the elevated sea surface near the center of tropical and extra-tropical storms often observed in the Gulf of Mexico. Due to frequent occurrences of these storms within the Gulf of Mexico, this could represent another important forcing mechanism to transmit significant energy to deep water in the Gulf of Mexico, thus contributing to deep water energetics and its well-mixed deep water conditions below the Yucatan sill depths. v TABLE OF CONTENTS Page LIST OF FIGURES ....................................................................................................................... ix LIST OF TABLES.........................................................................................................................xv ACKNOWLEDGMENTS .......................................................................................................... xvii 1 INTRODUCTION ......................................................................................................................1 1.1 Water Mass Characteristics at the Mooring Site..................................................................2 2 MOORING STATISTICS..........................................................................................................7 2.1 Deployment..........................................................................................................................7 2.2 Basic Statistics...................................................................................................................10 2.3 Spectra ..............................................................................................................................19 3 LC, LCE, AND THE DEEP CIRCULATION .........................................................................25 3.1 Model Simulations ............................................................................................................25 3.2 Evidence of Deep Modon Formation in the Mooring Data ...............................................36 3.3 Locating the Interface between the Upper and Lower Layers...........................................41 3.4 Temperature Variability in Deep Water ............................................................................44 3.5 The Fate of Cold Deep Caribbean Water...........................................................................49 4 IMPACTS OF 2005 HURRICANES........................................................................................55 4.1 Hurricane Katrina...............................................................................................................55 4.1.1 Currents Observed at the Mooring.........................................................................59 4.1.2 Temperature and Salinity Observed at the Mooring..............................................71 4.2 Hurricane Rita....................................................................................................................79 4.3 Summary............................................................................................................................85 5 DISCUSSION AND SUMMARY............................................................................................87 4.1 Summary of Observations and Modeling ..........................................................................87 4.2 Conclusions........................................................................................................................88 6 REFERENCES..........................................................................................................................91 APPENDIX A. HIGH-RESOLUTION MODEL DESCRIPTION ..............................................95 APPENDIX B. MONTHLY SSH MAPS OF THE GULF OF MEXICO FOR DEPLOYMENTS 4 AND 5.......................................................................99 APPENDIX C. DEPLOYMENT 5 HIGH-SPEED CURRENT EVENT...................................105 vii LIST OF FIGURES Page Figure 1-1. Bottom bathymetric contours (in meters) in the Gulf of Mexico. ..............................3 Figure 1-2. Vertical temperature profile at the mooring site estimated for the mean CTD profile based on the first three cruises ...............................................................4 Figure 1-3. Vertical salinity profile at the mooring site estimated for the mean CTD profile based on the first three cruises ........................................................................5 Figure 1-4. Temperature-salinity relationships at the mooring site...............................................6 Figure 2-1. 40-HRLP current vectors during Deployment 4 for the indicated depths................12 Figure 2-2. 40-HRLP current vectors during Deployment 5 for the indicated depths................13 Figure 2-3. Record-length statistics (mean, maximum, 1 standard deviation around mean) of current speed during Deployment 4...........................................................14
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