oceannews float found | measured

Oceanographic Instrument Returns After Two Years By David Hebert and Tom Rossby

In early January 2003, 92 sion Experiment (LIDEX), funded by the surfaces, respectively). In the last few subsurface isopycnal RAFOS floats were National Science Foundation, is to deter- years, significant effort has been made loaded on a truck for the start of a long mine the processes that mix the waters to determine the processes responsible voyage. The floats were delivered to Fort in the horizontally (along constant for diapycnal mixing in the ocean’s inte- Pierce, Florida and loaded on the R/V density surfaces) and at what rate. When rior, and now these processes are known. Seward Johnson II at the Harbor Branch these scientists departed from the Canary However, the stirring and mixing of Oceanographic Institution. In March, Islands to deploy the floats, little did they water along isopycnals is poorly un- Drs. Hebert and Rossby started on a know that one of their floats would even- derstood, and even the use of the terms month-long expedition from the Canary tually be returned by a fisherman. “stirring” and “mixing” depends on the Islands to deploy these floats south of Mixing in the ocean is separated horizontal scales that are being resolved. the Cape Verde Islands. The goal of this into two components: diapycnal and The effective isopycnal diffusivity is project, the Lagrangian Isopycnal Disper- isopycnal (i.e., across and along density generally believed to be scale-dependent (i.e., process-dependent). Models cur- rently used for climate studies cannot resolve the mesoscale eddies and subme- soscale mixing processes because the grid size they use in their models is larger than the size of a mesoscale eddy (tens to hundreds of kilometers across). Thus, the fluxes by these eddies must be repre- sented in terms of quantities resolved by the models (i.e., parameterized). To date,

Figure 1. Clusters of isopycnal RAFOS floats were deployed at five locations (stars) and at two depths in March and April 2003. The trajectory of one isopycnal RAFOS float (#329) is shown while under- water (red line) and on the surface (black line), and at deployment location off of northwestern Africa (red star). The blue contours show the mean dis- solved oxygen concentration at 500 m depth. This float was recovered (black diamond) by a local fish- erman, taken the U.S. Embassy in Guinea (red circle), and returned to the University of Rhode Island.

This article has been published inOceanography , Volume 18, Number 2, a quarterly journal of The Society. Copyright 2005 by The Oceanography Society. All rights reserved. Reproduction of any portion of this article by photo- 8 Oceanography Vol.18, No.2, June 2005 copy machine, reposting, or other means without prior authorization of The Oceanography Society is strictly prohibited. Send all correspondence to: [email protected] or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. there is a lack of good field observations to test any eddy flux parameterizations. In LIDEX, the processes and dynamics of Figure 2. Picture of the RAFOS float in on isopycnal surfaces on scales Guinea and the fisherman, Mr. Kamara from several kilometers to hundreds of (center), who found it. The float was in kilometers will be examined. These data excellent condition. will significantly improve ocean models. Clusters of isopycnal RAFOS floats were deployed at five locations and on two density surfaces in the low-oxygen tongue off of northwest Africa (Figure 1). The nominal depths of the density remain in the region for their two-year not known. The mean movement of the surfaces were 500 m and 800 m. The mission. Second, the mean dissolved floats can answer this question. floats were to remain underwater for oxygen concentration isopleths (blue One of the floats that was deployed nearly two years. All of the floats mea- contours in Figure 1) are not parallel to at 7°N, 28°W on April 6, 2003 (red star sure pressure and temperature every six the mean isotherms, which are mainly in Figure 1) surfaced prematurely. This hours. The 50 floats on the shallower zonal. The low-oxygen tongue off north- float remained underwater for 420 days density surface measured dissolved oxy- western Africa is due to both isolation of before surfacing. While underwater, its gen using the new Aanderaa Optode sen- the water from the surface and enhanced position was determined from acoustic sor. The location of each float is deter- productivity and organic regeneration beacons (red track) every six hours. The mined every six hours by triangulation in the coastal zone off North float moved slowly westward while mak- using sound sources. The data are sent Africa. It is believed that the low-oxy- ing large looping motions due to eddies back to the laboratory through the Argos gen water is advected westward from that stir the ocean. While at the surface, satellite system (for more information go the coastal upwelling sites. With this as- the location of the float was determined to http://noaasis.noaa.gov/ARGOS/ and sumption, and the fact that the dissolved from the Argos satellites (black line in http://www.argosinc.com/) after the float oxygen concentration is increasing to the Figure 1). During the 185 days on the surfaces at the end of its mission. Thus, west, only isopycnal and diapycnal mix- ocean surface, surface currents carried estimates of the isopycnal fluxes of tem- ing of higher-oxygenated waters can be the float eastward to Africa, down along perature and oxygen can be made after responsible for the observed dissolved the coast, and then back northwards all the data are received. oxygen concentration horizontal struc- along the coast off of the Republic of The study area off of northwestern ture. This description assumes a mean Guinea (black diamond). At that lo- Africa (Figure 1) was chosen for two westward zonal current, which has not cation, a local fisherman, Mr. Osman reasons. First, the mean currents in the been conclusively proven. The direction Kamara, found the float (Figure 2). He region are weak. Thus, the floats should of the weak circulation in this region is brought it to Conakry, Guinea (red circle

Oceanography Vol.18, No.2, June 2005 9 ocean news

Figure 3. Picture of the RAFOS float after its return to University of Rhode Island to Professors Hebert and Rossby.

in Figure 1), the country’s capital. two years since it left the state. The float Although the actual recovery of was in excellent condition (Figure 3). the floats is not critical or expected, Drs. Hebert and Rossby appreciate the it is often useful to see how well the assistance of Mr. Kamara and the Em- floats survive after being underwater bassy in returning this instrument. for long periods. Thus, contact infor- This float was one of those equipped mation is included within each float. with the novel oxygen sensor from Aan- Mr. Kamara contacted Dr. Hebert in deraa. The return of the instrument al- order to return the float to Rhode lowed them and us to assess how well the Island. After several telephone calls sensor fared from prolonged exposure during the Thanksgiving holiday pe- in the ocean and to see how much bio- riod, it was not clear how to send Mr. logical fouling occurred. Visual inspec- Kamara a box for the float and pay tion of the float and the sensor suggests for the shipping charges. Dr. Hebert only modest fouling took place during convinced Mr. Kamara to deliver the its half-year drift on the surface in near- float to the U.S. Embassy in Conakry. equatorial waters. A box for the float could be shipped to the Embassy. Although wary of David Hebert ([email protected]) is trying to deliver such a strange item Professor, Graduate School of Oceanog- to an embassy in Africa, Mr. Kamara raphy, University of Rhode Island, Narra- did just that. With the assistance of gansett, RI, USA. Tom Rossby is Professor, Special Agent Daniel Wilhelm and Graduate School of Oceanography, Univer- the Embassy staff in Conakry, the sity of Rhode Island, Narragansett, RI, USA. float was return to Rhode Island at the end of January, a little more than

10 Oceanography Vol.18, No.2, June 2005