Western Boundary Currents. Global Distribution of Western Boundary Currents and their importance

Transcription

1 Western Boundary Currents In previous chapters you have learned about the processes that cause the intensification of currents along the western boundaries of the oceans. In this chapter we will examine the descriptive physical oceanography of the major western boundary currents. Of all the currents in the ocean the western boundary currents are the strongest and often easiest to observe. This is because of the strong contrast between the western boundary current waters that are often warm flowing colder, higher latitude waters. Global Distribution of Western Boundary Currents and their importance The western boundary currents are poleward flowing currents along the western boundary of the ocean basins. They are the Kuroshio off Japan, the East Australian Current off Australia, the Gulf Stream off North America, and the Brazil Current off South America. The Mozambique Current and its continuation as the Agulas Current off southeast Africa while shortened by the termination of the African continent technically is also a western boundary current. Figure of Global SST with Western Boundary Currents noted. The importance of these currents is many fold. Here are a few: " major pathway for heat, salt and nutrients transport to high latitudes " frontal processes important to ecosystems. Students should have an appreciation for the physical processes in the WBCs and the physical oceanographic features relevant to biogeochemical processes. To illustrate the processes we will rely on examples from the Gulf Stream system as it is the best studied. We will also discuss briefly the Kuroshio and Brazil Current. While some of the processes are analogous from one WBC to the other there are significant differences that must be considered. The Gulf Stream System FIGURE OF GENERAL FLOW AND GEOGRAPHY The Gulf Stream system originates in the tropics off northeast South America, flows through the Windward Islands, into the Caribbean Sea, through the Straits of Yucatan, forms a loop in the Gulf of Mexico (the Loop Current), passes through the Straits of Florida (where is often called the Florida Current), flows northward pressed against the confining walls of the SE U.S. continental slope, leaves the slope at Cape Hatteras and forms the North Atlantic Current. It receives additional water along the way, especially as it passes the Old Bahama Channel where it gains up to 4 Sv and in the transit from the Straits of Florida to Cape Hatteras where it gains an additional 5 to 10 Sv. 1

2 The presence of the Gulf Stream was no doubt well known to ancient mariners. The charts shown in the following figure show the path of the Gulf Stream as deduced from sailing ship observations. Clearly knowing the location and characteristics of the Gulf Stream was important then as it is now. FIGURE FROM DE BRAHM Maury organized the countries of the world to submit ship observations and from that he developed sailing charts such as the one shown below. Note that in addition to stream lines he included temperature cross sections. FIGURE FROM MAURY Typical Section across the Gulf Stream The classic sections across the Gulf Stream are shown in the following figure. FIGURE OF HYDRO SECTION FROM CAPE LOOKOUT TO BERMUDA FIGURE OF TEMPERATURE AND SALINITY FIGURE OF SILICATE FIGURE OF SILICATE VS POT. TEMP. AND SALINITY VS. POT. TEMP FIGURE OF GEOSTROPHIC VELOCITY Waters in the Gulf Stream have a variety of sources. Considering that the Gulf Stream extends to several thousand meters depth one would expect nearly all the waters of the North Atlantic represented save Antarctic Bottom Water. The following figure shows a cross section of the Gulf Stream. One of the classic studies of Gulf Stream waters was done by Wüst. Much of the following is based on his studies. Subtropical Mode Water FIGURE: The maps from Wüst. Antarctic Intermediate Water AAIW is formed even farther from the Gulf Stream than STMW. From its formation in the sub-antarctic Convergence it travels northward to merge into the Gulf Stream in the sub- Tropical Atlantic. Its low salinity is characteristic of waters in the 5 to 12C range. Continental Shelf Waters and other modifications 2

3 As Gulf Stream waters flow through the Caribbean, Gulf of Mexico and along the southeast U.S. the waters are slightly modified. Considering the volume of the Gulf Stream and the relatively minute river flows we expect little dilution of the waters. FIVE FIGURES FROM WENNEKIN Long Time Transport Measurements in the Straits Question: Assume the combined flow of all rivers entering the ocean from the Windward Islands to the Straits of 3-1 Florida equaled 50,000 m s. How much would the salinity of the Gulf Stream be reduced? Should that be detectable? What waters in the Gulf Stream might be most effected? There are several ways to measure the transport in the Gulf Stream. Since the Gulf Stream is in near geostrophic balance a measure of sea level slope across the Stream would be a measure of flow. A second technique that has been used extensively makes use of the flow of the Gulf Streams waters through the Earth's magnetic field. This flow produces an electrical current that is measurable. The following figure shows the results of such measurements. FIGURE: Gulf Stream transport measurement from Larsen. The results of this and other studies have confirmed that the Gulf Stream has a seasonal component to its transport. Eddy Processes in the Gulf Stream Eddies in the Gulf Stream front have important effects on the Gulf Stream itself and probably even more effect on the adjacent shelf and slope waters. In this section we discuss frontal eddies that occur where the Gulf Stream flows along a shelf break, permanent meanders such at 32N, and ring structures that occur downstream from Cape Hatteras. Frontal Eddies Frontal eddies occur where the Gulf Stream interacts with the slope and shelf creating areas of high shear and instability. This process is found between Key West and Cape Hatteras and along the west Florida shelf when the loop current flows along the shelf break. It no doubt 3

4 occurs anywhere a high speed current flows along a shelf break. The structure and SST properties of a frontal eddy are shown in the following figures. FIGURE: Sequence of slides The importance of frontal eddies in boundary currents is that they tend to cause enhanced interaction with the adjacent shelf waters. These effects were summarized in Lee Yoder and Atkinson (1991). FIGURE: Schematic FIGURE: Propagation FIGURES: Cross shelf interaction. Permanent Meanders A nearly permanent meander is found in the Gulf Stream at about 32N off Charleston, S.C. This meander is caused by a 150 m rise in the bottom at a depth of 450 to 500m. The change in depth causes the Gulf Stream accelerate and then decelerate. This causes the Stream to swing offshore then back onshore and resume its bath towards Cape Hatteras. The structure of the gyre is shown in the following figure: FIGURE: CZCS from McClain and Atkinson. The core of the Gulf Stream is to the right of the light blue to dark blue transisition. The Charleston Bump causes a seaward displacement in the front and an upwelling area forms in the gyre. The upwelling is seen as the dark green area south of Cape Fear. From McClain and Atkinson (1985). See WWW page for color image. 4

5 FIGURE: Bathymetry of the Charleston Bump or Gyre. Isobaths from 400 to 700 m are deflected offshore between 31 to 32N creating a rise up to 300 m in the path of the Gulf Stream. From Brooks and Bane (1978). FIGURE: Temperature section through Charleston Gyre. The shaded area marks the zone of high chlorophyll seen in the CZCS figure. From McClain and Atkinson (1985). Rings-Formation Gulf Stream rings were one of the first well studied mesoscale features in the ocean. First observed by detailed ship observations and later by satellite they dominate the oceanography of the western North Atlantic second only to the Gulf Stream itself. The formation process is shown schematically in the following figure: 5

6 FIGURE: figure of ring formation Rings-Cold Cold core rings spin off into the North Atlantic between Cape Hatteras and about xx W. Many studies of cold core rings have shown their path to be to the SW where they are eventually absorbed back into the Gulf Stream. This figure shows the formation of a cold core ring as a pinch off from a meander in the Gulf Stream. Sections through cold core rings look like the following: This figure shows temperature sections through the Gulf Stream and through many cold core rings. The path of cold core rings is shown in the following figure. 6

7 Rings-Warm Warm core rings spin off from the anti-cyclonic side of the Gulf Stream and drift into the Slope Sea wedged between the Gulf Stream and the continental slope of North America. Their path is to the southwest leading to eventual absorption in to the Gulf Stream near Cape Hatteras. FIGURE: figure of paths of the eddies. Warm core rings are anti-cyclonic storms in the Slope Sea causing strong currents and episodic exchange between the Slope Sea and the adjacent shelf waters. Key References: Blanton, J. O., L. P. Atkinson, L. J. Pietrafesa, and T. N. Lee", 1981, The intrusion of Gulf Stream water across the continental shelf due to topographically-induced upwelling. Deep-Sea Research, 28: McClain, C. R. and L. P. Atkinson,1985,A note on the Charleston Gyre,J. Geophys. Res.,90: C6, 11,857-11,861. T. N. Lee, J. A. Yoder and L. Atkinson, 1991, Gulf Stream frontal eddy influence on productivity of the southeast U. S. continental shelf, J. Geophys. Res.,96: Lee, T. N. and L. P. Atkinson, 1983, Low-frequency current and temperature variability from Gulf Stream frontal eddies and atmospheric forcing along the southeast U.S. outer continental shelf, J. Geophys. Res., 88: Larsen, J. c Transport and heat flux of the Florida Current at 27N derived from crossstream voltages and profiling data: theory and observations. Phil. Trans. R. Soc., London A: 338: Robinson, A. J Eddies in Marine Science. Springer-Verlag. New York. 609pp. Singer, J.J., L.P. Atkinson, J.O. Blanton and J.A. Yoder, 1983, Cape Romain and the Charleston Bump: Historical and Recent Hydrographic Observations, J. Geophys. Res.,88: C8, Stommel, Henry. "The Gulf Stream". University of California Press Wennekens, M. P Water Mass Properties of the Straits of Florida and related waters", Bull. Mar. Sci. Gulf and Caribbean. 9(1):

8 Wüst, Georg, Circulation and Stratification in the Caribbean Sea 8