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Doug Wilson C/O NOAA Chesapeake Bay Office, 410 Severn Avenue 2.4 THE NOPP YEAR OF THE OCEAN DRIFTER PROGRAM Doug Wilson NOAA/AOML, Miami, FL organize themselves until they approach the 1. INTRODUCTION Yucatan Channel, where they form into a coherent northward flow known as the Yucatan Current. Beginning in March of 1998, over 150 WOCE Once through the Yucatan Channel, the flow type drifting buoys, drogued at 15 meters depth, were launched in the Gulf of Mexico and the becomes the Loop Current in the Gulf of Mexico Caribbean Sea and their approaches, providing in and then the Florida Current east of the excess of 30,000 drifter days of data to date. southeastern U.S. coast. The manner in which Buoys were provided by the U.S. National Ocean these warm western Atlantic and Caribbean currents organize into the powerful Florida Current / Partnership Program; launch co-ordination was provided by NOAA and academic research Gulf Stream system is not well understood. scientists interested in regional ocean circulation studies; and logistical and data processing support The Caribbean Sea is also the location of some was provided by the NOAA/AOML Global Drifter of the richest coastal and reef habitats in the tropical oceans. As the Caribbean Current flows and Data Assembly Centers. Buoys were launched with the cooperation of commercial ships, the westward on its way to the Yucatan Peninsula, it Colombian Navy, the U.S. Coast Guard, and passes through very productive zones of coastal research vessels working in the region. Drifter track upwelling (produced by the easterly winds) north of figures and data have been made available in real Venezuela and Colombia, fertile coastal "nurseries" and estuaries such as the Cienaga Grande de time via the WWW at www.IASlinks.org and www.drifters.doe.gov. In addition to the web-based Santa Marta in Colombia, the continental shelves educational resources, articles appeared in east of Nicaragua and south of western Cuba, and Scientific American and NMEA’s Current magazine. a large number of reef regions along the coasts of The result was a program that generated an Panama, Costa Rica, Jamaica, Cuba and other sites (reference: The Sea of Streams, Nature important oceanographic research data set and also numerous opportunities for interactive Conservancy, May/June 1999). The Caribbean Current is then clearly also a possible "highway" educational involvement. along which drifting objects such as fish or 2. SCIENCE: TRANSPORT PATHWAYS crustacean (e.g. lobster) THROUGH THE CARIBBEAN larvae can be carried downstream. It is strongly The Gulf Stream is one of the most well known, suspected by marine closely studied, and significant features of the biologists that larvae ocean. For centuries those traveling the seas have traveling along this used or avoided the Gulf Stream and other strong current could replenish currents. Palm trees in the British Isles at the populations farther latitude of Moscow stand as a tribute to the Gulf downstream, such as on Stream's ability to transport its warm water over the Campeche Bank long distances and impact coastal climates. This north of the Yucatan powerful North Atlantic ocean current has a Peninsula, or even more relatively humble beginning in the Caribbean Sea. importantly for the U.S., The large-scale wind field over the North Atlantic in the shallows of Florida Ocean, with westerlies at mid-latitudes and Bay and the Florida easterlies at lower latitudes, drives a warm surface Keys. Of course, if larvae westward ocean flow in the tropics that finally can be transported over reaches the islands of the Lesser Antilles. Most of these long distances these currents flow through the passes between the through the Caribbean islands of the Caribbean, where they eventually and ultimately to the U.S. converge in the western Caribbean as the mainland, then it is Caribbean Current. However, these "fragments" of equally true that a highly variable current with many eddies do not pollutants introduced by man farther "upstream" in the Caribbean could also Corresponding author address: Doug Wilson be transported to the c/o NOAA Chesapeake Bay Office, 410 Severn U.S. coast by the Figure 1. WOCE SVP Avenue, Suite 107, Annapolis, MD 21403; e-mail identical mechanism. drifter deployed [email protected] Severn Ave One way to study the motion and trajectories of Circulation Experiment) drifter design, and all are surface water parcels, larvae, or pollutants, is to drogued at a depth of 15 meters (information is introduce surface drifters into the flow. We hope to available from the NOAA/AOML GDC web site use these buoys to study the how the Caribbean www.aoml.noaa.gov/phod/dac/gdc.html). Since the Current forms, to better understand current beginning of the YOTO experiment over 150 patterns, and to determine how and where nutrients surface drifters have been launched. Due to the or larvae are distributed. These drifters can make high expense of launching drifters from measurements of quantities such as surface water oceanographic research vessels, most of the temperature as they move along with the flow, and drifters shown in Fig. 2 were launched from so- can automatically report their positions to satellites called "ships of opportunity", such as commercial at predetermined intervals. Drifters are commonly freighters, U.S. Coast Guard vessels in the eastern "drogued" with a parachute or similar high-drag Caribbean, or vessels of the Colombian Navy in the element so that the influence of surface winds on southwestern Caribbean. Typically, groups of the drifter is reduced and so that the drifter follows drifters were introduced into the flow every few approximately the flow at the depth of the drogue. months, and lifetimes of individual drifters were Clearly, drifters can only be rough representation of usually at least six months. the manner in which larvae are carried along by the currents. For example, larvae do not all drift at the Several features stand out in Fig. 2. First, there is same depth, nor do they even stay at the same an immense range of distances over which drifters depth over periods as short as a day. can travel. While there is no preferred path for drifters through the eastern and central Caribbean, Until recently, the number of surface drifters an organized westward-flowing Caribbean Current deployed in the Caribbean Sea could almost have begins to form at around 80o W. Most drifters been counted on one hand. Therefore, many of the entering this region enter the Gulf of Mexico, where basic features of the Caribbean flow field remained the "looped" pattern of the Loop Current is clearly only poorly defined. However, beginning in 1998 a seen, most of those are advected far north into the major deployment of drifters took place as part of Atlantic by the Gulf Stream. Second, some drifters the so-called "Year of the Ocean" ("YOTO") can be "trapped" within the Caribbean Sea for experiment (Fig. 2). The drifter tracks shown in this extended periods. In fact, one region of the figure are all based on the "WOCE" (World Ocean Caribbean that appears to be effective in trapping Figure 2. Tracklines of all NOPP Year of the Ocean drifting buoys, March 1998 – October 1991. drifters was only suspected before these drifters necessity to repeat drifter launches into the same were deployed; this region is formed by a typically region so that a large number of "realizations" of closed pattern of cyclonic (counterclockwise in the the flows can be observed; only then can we form Northern Hemisphere) flow north of Panama called some statistically accurate picture of the flow field the "Panama-Colombia Gyre". Another region as a whole. This becomes even more important where drifters seem often to become trapped is when biology is introduced into the problem, since south of the western part of Cuba. Finally, although many marine organisms have seasonal some overall patterns of flow can be discerned in reproductive cycles that may or may not coincide Fig. 2, there is also clearly a very high level of with favorable physical conditions. Information variability in the flow. from the NOPP YOTO drifters, in conjunction with numerical model outputs, is already being used to To try to make some order from the apparent study regional larval drift patterns and determine chaos shown in Fig. 2, we can create an average appropriate locations for Marine Protected Areas. picture of the currents using all the drifter data (Fig. 3). Here, we have gathered together all the velocities inferred from any drifter paths that passed 3. EDUCATION: REAL-LIFE, REAL-TIME through geographic boxes of approximately 50 km SCIENCE, MATH, AND GEOGRAPTHY by 50 km on a side and averaged them together to obtain a field of the mean velocities. Velocities Drifting buoys make an excellent educational tool along each drifter track were simply determined by because of their dynamic nature and direct links to differencing sequential drifter positions and dividing mathematic, physics, geography, and technology the resulting differences by the time intervals as well as oceanography and meteorology. The between each sequential pair of positions. From selection of the Caribbean Sea and Gulf of Mexico Fig. 3 we can more clearly see the broad, westward as a location was also a good choice, due to its drift of the Caribbean Current in the central proximity and impact on the United States as well Caribbean Sea, the "organizing" of the currents in as its scientific significance. During the experiment, the approaches to the Yucatan Channel, and the the AOML Global Drifter Center, in conjunction with cyclonic Panama-Colombia Gyre north of Panama. oceanographers, balanced the needs of continuous drifter distribution with the limited resources One conclusion from all this is that the variability available for deployment.
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