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by: Dr. Estrella Hessel


Dr. Estrella Hessel
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Class Notes
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This 5 page Class Notes was uploaded by Dr. Estrella Hessel on Friday September 18, 2015. The Class Notes belongs to OCP 6050 at University of Florida taught by Valle-Levinson in Fall. Since its upload, it has received 17 views. For similar materials see /class/206650/ocp-6050-university-of-florida in Physical Oceanography at University of Florida.

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Date Created: 09/18/15
Geomorphology and Sea Level slowt ml 1 lull The shape of the ocean basins has much to do with the my the ocean moves the way the wind blows 4L 439 39L39 at r A quot39amongorlrer39 quot39 This chapter discusses the geomorphology of the ocean basins in the context of physical oceanography This chapter also discussesthe recent trends in sealevel The Ocean Basins u JULIA I I HZHHK m c quot 39 39 I I global ocean Features that are important to the physics ofthe ocean include The placement ofthe continents is asymmetrical larger aream northern hemisphere I The Southern Ocean is continuous zonally along colatitude lines large marginal seas some ofwhich are evaporative basins are found mostly in the orthem Hemis here The South Pole is land and the North Pole is ocean River input to the Nonhem Hemisphere is higher than that to the Southern Hemisphere The paci c Atlantic and Indian are connected only at Very high southern latitudes be considered amargmal sea The global system of rises and rifts stands several thousand meters above the abyssal ocean oor Such relief is sufficient to affect the ow of deep and bottom waters This is most noticeable in the Atlantic basin where the MidAtlantic Ridge isolates the northward ow of Antarctic Bottom Water Some of the gaps in the ridge system such as the Vema Channel and the IcelandFaroe gap exemplify the effect of topography on water mass distribution A secondary effect of the distribution of land and water is relatedto the advection of cold air masses from continents to the ocean Clearly because the major continental land masses lie in the Northern Hemisphere the major air sea exchanges will occur in the northem Hemisphere The exception is the cold air owing off the Antarctic continent that modifies the Southern Ocean The typical ocean basin has several characteristic that are shown schematically in Figure 2 Note the following The broad abyssal plain interrupted by the mid ocean ridge system The presence of sea mounts Deep trenches adjacent to island arc systems The continental rise sloping up from the abyssal plain to the continental slope The shallow shelf break forming the boundary between the relatively steep slope and the gently sloping continental shelf The slope of the shelf break is like approaching the Blue Ridge or Rockies while the shelf itself would resemble a coastal plain The hypsomem39c curve Fig 3 shows the frequency distribution of depth in the ocean basins The following should be noted about the depths of the ocean Average depth of ocean is 4000 m in contrast to mean land height of 1000 In Most ofthe ocean is deep Shelves and slopes occupy a small percentage of the ocean Continental Margin Continental margins are composed of the Continental Rise Continental Slope Continental Shelf and Shelf Break Fig 4 Some regions have large shelf areas others have steep shelves Example of California continental margin Fig 5 Shelf Break As you will leam the shelf break depth is very important This is because winddriven process obviously originate at the surface and penetrate downward into the ocean Over most of the ocean the downward penetration never reaches the bottom However as we approach the coast the surface processes on the ocean eventually reach the bottom Thus the depth of the shelf break is important in de ning how the waters over the continental shelf interact with the deeper waters of the adjacent ocean Figure 6 shows a plot of the shelf break depth along the east coast of the Us Note how shallow the shelf break depth is off Florida compared to farther north Off Virginia it is about 100 m and deepens to 300 m off Newfoundland Figure 7 shows the width of various bathymetric features Note that the widest feature is the abyssal plain Continental shelf and rise are also typically wide Seamounts trenches and canyons are typically very narrow These dimensions have important implications for the productivity of the region as they may determine the transport and exchange of nutrients between different regions of the ocean Continental shelves are among the most productive regions of the ocean On the continental shelves as we approach the coast we usually see one of two types of coasts steep or shallow Fig 8 The shallow coast slowly shoals to the beach Often barrier islands lie offshore creating shallow lagoons These lagoons because they are shallow respond quicklyto atmospheric variability The steep coast as we see off the west coast features steep shore narrow shelf and often rocky headland Here the currents of the open ocean often reach to within several kilometers of the coast Estuaries such as the Chesapeake Bay Fig 9 can get extremely complicated just in their bathyrnetry let alone the dynamics and biology Here we see the complex system of channels deeps and shoals that make up the Chesapeake Note how the deep channels are not connected Submarine Canyons Submarine canyons Fig 10 are important because they cut through the slope and often the shelf providing a route for shelf and deeper slope waters to interact A more obvious indicator of their importance is the way fisherman congregate around them because of the concentrations of fish Canyons incise much of the U S coastline Fig 11 but some areas are noticeably lacking in them Gulf of Mexico SoutheastU 8 part of Oregon When studying shelf areas you must be aware of the proximity of canyons as they will affect the ow field significantly Many studies seek to avoid them The canyon system draining Baffin Bay Fig 12 is important in the ow of North Atlantic Deep Water NADW that is sinking in this area A multitude of canyons concentrate in the Nova Scotia Shelf Fig 13 Also note the large channels Northeast and Laurentian connecting the deeper ocean to basins Gulf of Maine and Gulf of St Lawrence respectively Also note the basin system on this shelf partially isolated from the deep ocean by shelf edge banks The Hudson canyon Fig 14 reaches the coast at the entrance to the New YorkNew Jersey Harbor estuary Represents an important source of nutrients to the shelf but also may become anoxic in the stratified summer The Monterey and Indus canyonsFig 15 are other examples of canyons that extend across the shelfclose to the coast or river mouth The Congo canyon Fig 16 is the extreme It extends into the river mouth The Magdalena in Colombia is similar The Mississippi River because of human control has extended its delta completely across the shelf and spills over the slope Fig 17 Thus Mississippi River waters do not enterthe shelf area directly and may be returned onshore by the slopetoshore ow Mississippi River waters can interact directly with currents in the Gulf of Mexico Deep Ocean Basin Submarine Trenches In addition to the extensive abyssal plains a relevant feature in the deep basins are the trenches and the seamounts Fig 18 Trenches Fig 19 are the deepest parts of the ocean and represent areas of crust subduction The Mariana Trench 11 km is the deepest point on Earth Ocean currents in trenches are very weak but it would be interesting to see what kind of vertical ows develop around these features are very 39 39 J quot quot 39 the world s oceans Fig 20 They tend to disturb the typical flow pattem in a given region Fig 21 Such disturbances may enhance the productivity of the area by retaining nutrients or early life stages of different organisms thus providing a strong base for a food chain Importance of bathymetric features The importance of bathymetric features in coastal waters is illustrated in three figures The rst US eastcoast Fig 22shows surface colorthatiSuearly j 39 to39 39 39 39 39 r J 39 Note the ow of low chlorophyll water into the Gulf of Maine through Northeast Channel Note the high values over shoals and headlands The second Fig 23 illustrates pigment concentration in the Mediterranean and the Arabian Sea Note the highest productivity around coastal areas The third is a sea surface temperature SST image for the Us west coast and the coast of Chile Fig 24 Note the activity off some of the major headlands Sills analogouslyto seamounts but over shallower water also produce important disturbances to the ow Sills are rises in the bottom that isolate one deep water area from another The sills at the Strait of Gibraltar are classic examples Figure 25 shows how dense waters formed in the Mediterranean are isolated and ow over the sill as the basin fills What other large ocean basins have sills Look at Figure 1 One last look at sills Fjords are by definition formed by glaciers and often the entrance to the fjord is blocked by the terminal moraine of the glacier Here is an example of such a sill and the effect First figure Fig 26a shows the inlet with bathymetry The bottom rises from 80 to 10 meters over the sill The water accelerates as it ows overthe sill and causes disturbances at the pycnocline This process enhances mixing between bottom and surface waters and likely provides food to upper trophic levels Sea Level The final part of this lecture deals with sea level variations Obviously sea level changes will not have an immediate effect on the open ocean However you must be aware that sea level has changed in the past and is now This is because interpretation of historical data from coastal areas may require this knowledge The fact that sea level is now rising may be of little signi cance to people who live on shorelines that have relief in regions close to the coast But for those who live in relatively at regions like the western Paci c there are reasons to be very concemed Within several generations their territories may disappear Approximately 15000 years ago the sea level was over 100 m lower Fig 27 The coastal line during these low stands of sea level did not have a continental shelf Fig 28 But what is sea level doing now Where is it going Sea level is rising at a rate of about 2 mm per year or about 20 cm per century Douglas 1991 What is the significance of this There is probably little significance to ocean dynamics at less than century time scales The noticeable short term effect is on coastlines and thus indirectly on ocean circulation For a shallow slope coastline having a slope of 1500 such a rise in sea level means a horizontal transgression speed of 1 meter per year Such transgression will have significant effects on the coast by causing morphological changes that will no doubt affect coastal ocean ow The sea level changes produced by glaciation deglaciati on can be understood quantitatively through the principle of Isostacy This principle can also be used to explain ocean currents It assumes that the pressure at a certain depth underneath the earth s crust is the same everywhere Thus if we add mass to a continent in the form of ice the continent will sink Quantitatively this is explored with the relationship also know as the hydrostatic equation that relates pressure to the density of the material the acceleration due to gravity and the thickness of the material This can be illustrated with a couple of examples in class Summary We have discussed the shape ofthe ocean and changes in sea level While this will not be discussed any further you must keep these ideas in mind as you think about the deep or shallow ocean Size and shape are important they often are the most significant factors in ocean dynamics Sea level has changed and is presently rising The effects on the coastal ocean may be significant especially on shallow slope coasts


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