WINDWARD ISLANDS PASSAGES MONITORING PROGRAM


Principal Investigators: Doug Wilson
William E. Johns of (RSMAS, U. Miami)

Objective: The objective of this project, sponsored by the Atlantic Climate Change Program, is to determine the quantity and sources of transport between the Atlantic Ocean and the Caribbean Sea. This transport includes the upper ocean portion of the Meridional Overturning Cell (MOC) of the North Atlantic Ocean. In the MOC, cold deep water exits the North Atlantic southward across the equator, balanced by northward-flowing, warmer upper ocean waters from the South Atlantic. This program seeks to better understand the mechanisms and pathways of upper ocean warm water interhemispheric transport, since the resultant heat flux into the northern hemisphere is thought to be a key determinant affecting climate in the North Atlantic and adjacent regions.

Rationale: Leetmaa et al. (1977) showed that the Florida Current transport at 25°N was in close agreement with the integrated annual mean Sverdrup transport at that latitude, which led to a general perception that the Florida Current was simply a return conduit for the southward transport of the subtropical gyre. However, it is now generally well established that a vigorous thermohaline circulation exists in the Atlantic that transports approximately 15 Sv of cold, deep water southward across the equator, balanced by a warm return flow in the upper layers (Gordon, 1986; Schmitz and McCartney, 1993). This warm water return, made up of South Atlantic thermocline and intermediate waters, is believed to proceed into the North Atlantic along the western boundary, although the precise pathways of this return flow are unknown. Lee et al. (1990) suggested that the Florida Current may carry up to 5 Sv of this upper level thermohaline flow, while Schmitz and Richardson (1991) (hereafter SR91) suggested that as much as 13 Sv, or nearly half of the Florida Current transport, may be derived from South Atlantic source waters. The amount of southern hemisphere throughflow, and how and when this flow into the Caribbean occurs, is an important element in understanding the Atlantic thermohaline circulation.

Constraints imposed by the gyre circulation in the North Atlantic suggest that the southern passages are the most likely location for South Atlantic inflow into the Caribbean (Figure 1). Click to expand the map and interactively access CTDO2 and Lowered ADCP Velocity data. The most complete current and hydrographic survey of the southern passages was conducted in March and April of 1970 and reported in Stalcup and Metcalf (1972, SM72 hereafter ), Metcalf et al. (1971), and Stalcup et al. (1971). SM72 found a total transport through the southern passages of approximately 26 x 106 m3/s (106 m3/s = 1 Sverdrup, hereafter abbreviated Sv), with transports of 9 Sv, 10 Sv, 6 Sv, and 1 Sv through the Grenada, St. Vincent, St. Lucia, and Dominica Passages, respectively. ) SR91 reanalyzed the SM72 data and concluded that 22 of the approximately 30 Sv flowing through the Florida Current is derived from the southern passages, with most of the remainder (7 Sv) coming through Windward Passage. The South Atlantic waters carry a distinctive temperature-salinity-dissolved oxygen signature, being relatively fresher and higher in dissolved oxygen compared to waters from the North Atlantic on the same density surfaces ( Metcalf, 1968; Cochrane, 1969; SR91). Wust's (1964) analysis of historical hydrographic data suggested that flow of these waters into the Caribbean is strongly concentrated in the southernmost passages off the northern coast of South America. However, several surveys in these passages have found waters that were clearly of North Atlantic origin (Metcalf et al., 1971; Mazeika et al., 1980; Parr, 1937), suggesting at least a strong time dependence of the South Atlantic throughflow.


Method: The WIPP observational program was designed to investigate the above issues by obtaining repeated sampling in the southern Caribbean passages on seasonal time scales over a period of years. Cruises have taken place on the Barbados Coast Guard vessel HMBS TRIDENT, the NOAA Ship MALCOM BALDRIGE , and the University of Puerto Rico's RV ISLA MAGUEYES. To facilitate the use of regional vessels not necessarily dedicated to oceanographic research, we have developed an instrument package that can be deployed from any vessel of suitable size and stability. The unit includes a self-contained Seabird SBE19 CTD, six 1.2 liter sample bottles tripped automatically at preset depths (the rosette interface was developed at AOML, Bitterman and Wilson, 1993), and a self-contained RDI 300 kHz Broadband Acoustic Doppler Current Profiler. Profiles of pressure, temperature, salinity, dissolved oxygen, and current velocity are collected along with water samples for CTD calibration or other analysis at depths of up to 2000 meters. Data reports containing all individual station data have been published for cruises W1 through W7 (Wilson et al., 1993, 1994, 1995, 1995) and are in preparation for the remaining cruises; the most recent cruise was W11 in September 1995.

During a cruise stations are occupied in each passage; see Figure 2 passage close-ups inset for the locations of repeated stations in the southern passages. Station velocity profiles are compiled into sections across a passage, and integrated to estimate transport entering or leaving the Caribbean. Profiles of water properties are stored in a database for analysis of property fluxes and water mass origins.


Accomplishment: Physical oceanographic measurements from 11 cruises (through September 1995) in the eastern Caribbean have been analyzed to study water exchange between the Atlantic Ocean and Caribbean Sea. This region is generally thought to be an important site for the entrainment of upper ocean waters of South Atlantic origin into the North Atlantic circulation system, a process necessary to balance southward North Atlantic outflow in deeper layers. The magnitude of this exchange is generally thought to be around 15 Sv (Schmitz and McCartney, 1993). The following results are summarized from Wilson and Johns (1995).

In this program, we have made significant advances in the area of collecting high-quality physical oceanographic measurements using regional ship resources. The standard measurement package used in this program, consisting of a CTD, six-bottle rosette, and 300 kHz Lowered ADCP (Fischer and Visbeck, 1993), can be lowered to depths of 2000 meters using a self-contained electric-hydraulic winch and non-conducting cable. By utilizing smaller, locally based ships, repeat sampling programs in dynamically important regions such as this are more economically sustainable.

Mean velocity sections in the three southernmost passages are shown in Figure 3. Mean velocities in the Dominica Passage were small. Mean total transport through the four southern passages was 9.0 Sv, with an estimated standard error of less than 2.5 Sv, partitioned as follows: Grenada, 5.4 Sv ; St. Vincent, 3.0 Sv; St. Lucia, 1.1 Sv, and Dominica, -0.5 Sv. The range for the total of the three southern passages was 3 Sv to 17 Sv, only slightly larger than the annual variablity predicted by Sverdrup theory and purely wind-driven models; these models, however, predict an approximately zero mean transport. The mean transport through these passages therefore appears to be attributable mainly to the large scale Atlantic thermohaline circulation.

Our mean transports for these passages are significantly smaller than those estimated by Schmitz and Richardson (1991) based on the velocity measurements of Stalcup and Metcalf (1972) in March and April, 1970. We feel that the SM72 values are biased towards high transports and not representative of present mean conditions. Based on our mean transport values and the SR91 percentages of North and South Atlantic waters in each temperature class, we estimate a maximum transport of 7 to 9 Sv of South Atlantic water through the southern passages, compared to SR91's estimate of 13 Sv. Continuing field measurements and analysis of temperature, salinity, and dissolved oxygen data will further refine this estimate.

Based on our smaller total transport values, we estimate that roughly 20 Sv of the Florida Current's 30 Sv transport must enter the Caribbean north of Dominica. This is twice as much as required by SR91, but not unreasonable. Approximately 8 Sv could be entering through the Windward Passage (Roemmich, 1981) and 2 Sv through the Bahamas (Leaman et al., 1995). This leaves 10 Sv to enter through the northern passages between Hispaniola and Dominica, just about the amount suggested by the mean Sverdrup transport streamlines. Unfortunately, insufficient observations exist in the northern passages to verify these estimates. Assuming a maximum of 7 to 9 Sv of South Atlantic inflow through the southern passages and 14 Sv of North Atlantic Deep Water exported, a significant amount of South Atlantic water appears to take a different route northward. Some may enter through the northern passages, probably water from the North Brazil Current (NBC) retroflection carried northward in the surface layers and returning eastward in the North Equatorial Current (NEC); this would likely have a seasonal signal dependent on interaction between the NBC annual cycle and NEC axis location. Again, transport and water mass measurements in the northern passages are necessary for verification. There may also be South Atlantic water transport that does not go through the Straits of Florida and need not be considered in this balance. Finally, our lower South Atlantic water transport values may accurately represent the entire total at this time; little is known about variability in the strength of the Atlantic meridional overturning cell on interannual and decadal time scales.


References:
Bitterman D., and W. D. Wilson (1993). A compact CTD system with programmable controller for obtaining in situ water samples. OCEANS '93 Proceedings, IEEE/OES, Victoria, B.C. , I, 144- 148.

Cochrane J. D. (1969). Low sea-surface salinity off northeastern South America in summer 1964. Journal of Marine Research, 27, 327 - 334.

Gordon A. L. (1986). Interocean exchange of thermocline water. Journal of Geophysical Research, 91, 5037 - 5046.

Lee, T. N., W.E. Johns, F.A. Schott, and R. Zantopp (1990). Western Boundary Current Structure and Variability off Abaco, Bahamas, at 26.5° N. Journal of Physical Oceanography, 20, 446 - 466.

Leetmaa A., P.P. Niiler and H. Stommel (1977). Does the Sverdrup relation account for the mid-Atlantic circulation? Journal of Marine Research, 35, 1-10.

Mazeika P. A., D. A. Burns and T.H. Kinder (1980). Mesoscale circulation east of the southern Lesser Antilles. Journal of Geophysical Research, 85, 2743 - 2758.

Metcalf W. (1968). Shallow currents along the northeastern coast of South America. Journal of Marine Research, 26, 232 - 243.

Metcalf W. G., M.C. Stalcup, and M.E. Zemanovic (1971). Hydrographic station data from Atlantis II Cruise 56 to the southeastern approached to the Caribbean Sea, February-April 1970. Wood Hole Oceanographic Institution Technical Report, WHOI-71-13, 103 pp.

Parr A. E. (1937). A contribution to the hydrography of the Caribbean and Cayman Seas, based upon the observations made by the research ship "Atlantis", 1933-34. Bulletin of the Bingham Oceanographic Collection, 5(4), 110 p.

Roemmich, D. (1981). Circulation of the Caribbean Sea: a well-reolved inverse problem. Journal of Geophysical Research, 86, 7993 - 8005.

Schmitz W. J., and P.L. Richardson (1991). On the Sources of the Florida Current. Deep -Sea Research , 38(S1), S379 - S409.

Schmitz W. J., and M. S. McCartney (1993). On the North Atlantic Circulation. Reviews of Geophysics, 31(1), 29 - 50.

Stalcup M.C, W. G. Metcalf, and M.E. Zemanovic (1971). Current Measurements in the Lesser Antilles. Wood Hole Oceanographic Institution Technical Report, WHOI-71-51, 78 pp.

Stalcup M. C., and W. G. Metcalf (1972). Current Measurements in the Passages of the Lesser Antilles. Journal of Geophysical Research, 77, 1032 - 1049.

Wilson W. D., W. E. Johns, M. D. Hendry, and J. A. Routt (1993). Windward Island Passages Monitoring Program: Physical oceanographic data collected on cruise WI-91-01, HMBS Trident, 15-21 December 1991. NOAA Technical Memorandum ERL-AOML-76, 55 pp.

Wilson W. D., W. E. Johns, J. A. Routt, and M. D. Hendry (1994). Windward Island Passages Monitoring Program: Physical oceanographic data collected on cruises WI-02, HMBS Trident, 6-10 May 1992 and WI-03, HMBS Trident, 19-23 September 1992. NOAA Technical Memorandum ERL-AOML-79, 99 pp.

Wilson W. D., W. E. Johns, J. A. Routt, and M. D. Hendry (1995). Windward Island Passages Monitoring Program: Physical oceanographic data collected on cruises WI-04, HMBS Trident, 7-13 December 1992, WI-05, HMBS Trident, 7-12 February 1993, and WI-06, HMBS Trident, 24-28 March 1993. NOAA Technical Memorandum ERL-AOML-86, 106 pp.

Wilson, W.D. and W.E. Johns (1995) Velocity Structure and Transport in the Windward Islands Passages. Submitted to Deep-Sea Research.

Wust G. (1964). Stratification and Circulation in the Antillean-Caribbean Basins, Columbia University Press, New York. .



Caribbean Nations - WIPP



Oceanographic data for the Windward Islands Passages Monitoring project are collected in the territorial waters of fourteen Caribbean nations. Click here for more information on these countries.



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