Environmental Forecasting Program 



The Environmental Forecasting Program focuses on proj- 

 ects designed to explain the large-scale, long-term behavior of 

 the ocean and the ocean's influence on weather and climate. 

 Experiments and studies include: the Joint U.S.-U.S.S.R. 

 Mid-Ocean Dynamics Experiment (POLYMODE); the North 

 Pacific Experiment (NORPAX); the International Southern 

 Ocean Studies (ISOS); and the Climate: Long-Range Investi- 

 gation, Mapping, and Prediction (CLIMAP) Study. 





MODE 



Joint U.S.-U.S.S.R. Mid-Ocean Dynamics Experiment 

 (POLYMODE) 



The purpose of POLYMODE is to establish the dy- 

 namics and statistics of mesoscale motions in the ocean, their 

 energy source, and their role in the general circulation of the 

 ocean. POLYMODE is based on: 1) the U.S.S.R. Polygon 

 project — a continuing series of experiments investigating meso- 

 scale phenomena in the Atlantic and Pacific Oceans and in the 

 Arabian Sea, and 2) the Mid-Ocean Dynamics Experiment 

 (MODE project) of the United States and the United King- 

 dom. The POLYMODE experiment is under the direction of 

 a Joint U.S.-U.S.S.R. POLYMODE Organizing Committee, 

 established under the Agreement between the Governments of 

 the United States and the U.S.S.R. on Cooperation in Studies 

 of the World Ocean. Other countries have been invited to par- 

 ticipate in POLYMODE by the UNESCO/International 

 Oceanographic Commission's Scientific Committee on Oceano- 

 graphic Research (SCOR) Working Group 34. 



The overall description of the U.S. POLYMODE efl^ort 

 is described in WOE Progress Report Volume 5. After a series 

 of feasibility experiments in fall and winter 1976, the U.S.S.R. 

 recommended that the location of the Northern Synoptic Ex- 

 periment be moved to 29 °N, 70° W to coincide geographically 

 with the Southern Synoptic Experiment. There is now only 

 one POLYMODE Local Dynamics Experiment. This report 

 describes U.S. POLYMODE deep-current measurements, joint 

 U.S.-U.S.S.R. activities, and the Local Dynamics Experiment. 

 U.S. POLYMODE statistical-geographical experiments (Ar- 

 ray 3, XBT sections) and theoretical and numerical studies 

 are not described in this report. 



U.S. participation in POLYMODE is sponsored jointly 

 by the IDOE Office and the Office of Naval Research. Projects 

 in POLYMODE are listed in table 6. 



Deep-Current Measurements 



The purpose of U.S. POLYMODE Arrays 1 and 2 was 

 to extend the study of low-frequency fluctuations from the 

 MODE-I site to the east along 28°N and to the north along 

 55°W to other regions of the subtropical gyre. The location 

 of these arrays is shown in figure 5. Array 1 measurements 

 took place from July 1974 to April 1975. Array 2 measure- 

 ments span the 27-month period from March 1975 to June 

 1977. 



Also shown in figure 5 are volume transport streamhnes 

 for the deep (potential temperature less than 4°C) general 

 circulation of the North Atlantic as deduced by Worthington 

 from hydrographic measurements. The shape of this sub- 

 tropical gyre is concentrated more north and north-westward 

 and is smaller than previously suggested. A medium-scale gyre 

 west of the Mid-Atlantic Ridge transports about 30 Sverdrups 

 (Sv) through the Straits of Florida. A smaller gyre, which 

 transports about 120 Sv, is embedded in the northern and 

 western corner of the larger gyre. The smaller scale gyre con- 

 tains all of the so-called downstream increase in transport of 

 the Gulf Stream and all of the deep, general circulation of the 

 entire subtropical gyre. A controversial element of this circula- 

 tion pattern as drawn by Worthington is that approximate geo- 

 stropic balance is violated in the westward return flow east of 

 the Gulf Stream. 



Current meter measurements have found the deep, easterly 

 flow associated with the Gulf Stream to be comparable in 

 magnitude (5 to 10 cm s"') to that suggested by Worthington, 

 although displaced 100 to 200 km to the south in moored data 

 (fig. 6A). A deep, westward mean flow north of and under the 

 Gulf Stream, stronger and broader than implied by Worthing- 

 ton, is characteristic of the moored current meter data. In the 

 offshore return flow near 4,000 m depth, westerly velocity 

 components are about three times as large as assigned by 

 Worthington, and the width of the return flow is three times 

 smaller. 



Figure 6B presents estimates of low-frequency ("eddy") 

 kinetic energy for deep currents (4,000 m) for two sections 

 across the subtropical gyre at 55°W and 70°W. The eddy 

 kinetic energy distribution at each longitude has nearly the 

 same characteristic shape. Peak values of the eddy kinetic 

 energy occur about 200 km south of the Gulf Stream axis and 

 decrease to the north and south. The properties of the eddy 

 field vary with position by about two orders of magnitude and 

 appear to be associated with properties of the general circu- 

 lation as well as the bottom topography. The intensity and 

 spatial variability of the eddy field is in sharp contrast to that 

 of higher frequency (one cycle per hour to one cycle per day). 

 For example, along 55°W near 4,000 m depth between 28° 

 and 41.5°N, the kinetic energy of the higher frequency band 

 varies between 2 and 6.5 cm-s -, whereas the low-frequency 

 kinetic energy varies from less than 1 to 162 cm^s^^. 



13 



