Topic 3: surface ocean circulation
F. Upwelling and Downwelling in Gyres
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- G. Gulf Stream
- VIII. KEY POINTS
- IX. QUESTIONS/ PROBLEMS
F. Upwelling and Downwelling in Gyres1. In regions where the wind, via Ekman transport, has piled up surface water the increased pressure in theses regions causes surface water to move outward and downward (downwell). 2. In regions where the wind, via Ekman transport, has removed surface water the decreased pressure in theses regions causes subsurface water to move inward and upward (upwell). 3. The direction Ekman transport caused by the Trades Winds and Westerlies cause surface waters to converge (pile up) in the subtropical gyres at around 30ºN&S and to diverge (be removed) from the subpolar regions at around 60ºN&S and at the equator (see Fig. 8) 4. Thus the subtropical gyres are regions of downwelling. 5. In contrast, the equator and subpolar regions are areas of upwelling, with the strongest upwelling rates occurring at the equator. 6. Upwelling and downwelling have a major impact on the concentration of nutrients in the surface layer. - upwelling increases the supply of nutrients to the surface layer because nutrient concentrations increase with depth - downwelling reduces the supply of nutrients to the surface layer because the surface layer has low nutrient concentrations
-similarly in certain regions of the coastal ocean (e.g. in particular off Peru), there is strong coastal upwelling that supplies nutrients to the surface ocean 8. Since photosynthesis depends, in part, on the availability of nutrients, the rates of photosynthesis are generally higher in upwelling regions where nutrients are available in the photic layer (upper ~100m). 9. Distribution of chlorophyll in the surface ocean generally indicate that chlorophyll abundances are higher in upwelling regions where there are high nutrient concentrations in surface waters (e.g, equatorial ocean, subpolar regions and certain coastal regions) than in downwelling regions with low nutrients concentrations in surface waters (Fig. 19) -chlorophyll is the compound plankton use to capture sunlight (energy) required for photosynthesis and its concentration in surface waters can be estimated by satellites -generally, there is a positive correlation between higher levels of chlorophyll and higher rates of photosynthesis by plankton -however in certain regions, like the Southern Ocean, other factors (possibly the supply rate of iron from dust in the atmosphere and light during winter) limits the photosynthesis rate despite high concentrations of nutrients in the surface layer 10. The subtropical gyres, regions of downwelling and low surface nutrient concentrations (Fig. 18), have the lowest chlorophyll levels observed in the surface ocean (Fig. 19) G. Gulf Stream1.The Gulf Stream (GS) is probably the best-studied geostrophic current in the ocean. - originally identified during period of sea going exploration in late 1500s - mapped in late 1700s by Ben Franklin (Fig. 20) 2. It lies at the western edge of the subtropical gyre in the N Atlantic Ocean. (Fig. 20) -it is an example of a Western Boundary Current -it is relatively narrow (50-100 km wide) and deep (1500m) -it is one of the fastest currents in the ocean (up to 3 m/s or 300 cm/s) 3. The GS flows northeastward along the eastern coast of the US. It turns eastward (off the coast of around Newfoundland) and heads across the N. Atlantic where it is called the N. Atlantic Current. -some of the N Atlantic Current flows northward into the far N. Atlantic where a portion of its transport cools, sinks and forms North Atlantic Deep Water (off the coasts of Greenland, Norway and Labrador) (Fig. 20) -much of the N Atlantic Current flows eastward across the Atlantic towards Europe where it becomes part of the southward flowing Canary Current
5. There is a Western Boundary Current analogous to the Gulf Stream for the subtropical gyre in each of the ocean basins (see Fig. 1).
6. The Gulf Stream (and all the Western Boundary Currents) is a geostrophic current that lies in a region of strong sea surface height gradients at the western boundary of the subtropical gyres (see Figs. 9 and 14) 7. The Gulf Stream is located at a front between the warm/salty water flowing northward and the cold/less saline water near the coast (Fig. 21). -this front is seen in the rapid southeastward increase in depths of the isotherms and isohalines 8. Remember that the trend in the depth of the isopycnals is the mirror image (opposite) of the sea surface height trend under baroclinic conditions - thus the strongest increase in sea surface height occurs at the location where there is the sharpest increase in depth of the isotherms Class Exercise: Draw the trend in sea surface height across the temperature cross section in Fig 21 assuming the isotherms approximate the trend in isopycnals. 9. The Gulf Stream lies at the location of the steepest gradient in sea surface height (Fig. 14) - this means the Gulf Stream is a fast geostrophic current (~2m/s) -Remember: the velocity of a geostrophic current depends on the gradient in sea surface height - thus Velocity = g/f*(Δz/Δx), where Δz/Δx is the sea surface height gradient - Western Boundary Currents are some of the fastest currents in the ocean
-there is evidence for southward flow beneath the Gulf Stream (>~1500m) at this location 11. Class Problem: Using the east-west trend in geostrophic current speeds shown in Fig 22, sketch the east-west trend in sea surface height. Notice that the geostrophic current switches direction (from northward to southward towards the east in this velocity cross section. What does this imply about the direction of the horizontal pressure gradient at 1000-1500m compared to the surface? 12. In contrast to Western Boundary Currents, the equatorward flowing currents at the other side of the subtropical gyres have much slower speeds (e.g. Canary Current in the N. Atlantic, California Current in the N. Pacific, Peru Current in the S. Pacific, see Fig. 1) 13. Western Boundary Currents (WBC) effectively transport heat poleward (Fig. 23) -generally, the fast WBCs transport more heat poleward than their slower counterparts in the eastern edge of the gyres transport heat equatorward - WBCs are fed by tropical currents that supply warm and salty water (e.g., the North Equatorial Current feeds the Gulf Stream) (see Fig. 20) -in contrast, the slower and broader equatorward flowing currents at the other side of the subtropical gyres (like the Canary Current in the N Atlantic) carry colder water towards the equator -thus the subtropical gyre circulation scheme in each ocean basin are major pathways of poleward heat transport on earth
-both warm core and cold core rings are produced by the Gulf Stream 15. Warm core rings are eddies of warm water surrounded by cold water and Cold core rings are patches of cold water surrounded by warm water (Fig. 25) -these rings are typically 100-300km in diameter and can last for 1-3 years before they completely mix with surrounding water -these rings are a very efficient mechanism for transporting heat between the cold subpolar waters and warm subtropical waters
- these rings often have anomalous biological properties than the surrounding water - cold core rings typically have higher nutrients and chlorophyll levels than surrounding subtropical water and warm core rings typically have lower nutrients and chlorophyll levels than surrounding continental slope water
1. The patterns of surface winds have a strong influence on both surface current speeds and directions. 2. Viscosity is the water property that allows the transfer of energy and momentum between winds and ocean and within the ocean 3. Surface currents, under steady winds, move at an angle to the wind direction (to the right in the northern hemisphere and to the left in the southern hemisphere). 4. Surface Ekman currents exposed to steady winds are not accelerating or changing direction (over the long term), which implies that a force balance exists (acceleration = 0) where wind stress force is balanced by the combination of Frictional and Coriolis forces. 5. The depth-integrated average water transport in the Ekman layer is perpendicular to direction of wind stress (to right in northern hemisphere and to left in southern hemisphere) 6. Ekman transport, given the global pattern of surface winds, piles up surface water is some regions (generally at ~30°) and removes surface water from other regions (generally at 0° and 60°) 7. Irregularities in sea surface height caused by Ekman transport, result in horizontal pressure gradients (HPGs), which in turn cause water to move from regions of high to low pressure. 8. The irregularity of the sea surface elevation is measured in dynamic meters (which represents potential energy). 9. Geostrophic currents result when a force balance exists between the Coriolis and HPG forces (frictional force for most of the ocean (away from boundaries) is negligible compared to the Coriolis and HGP forces). The geostrophic current moves at right angles (perpendicular) to the direction of the HPG force. 10. The geostrophic current velocity (v) depends on the magnitude of the HPG and latitude and is expressed as v = (1/ρ) * (1/f) * ΔP/Δx or, when expressed in terms of sea surface height gradients, then v = g/f * ΔZ/Δx. 11. Geostrophic surface currents move along contours of equal dynamic height. Thus the general pattern of surface current circulation can be approximated by a contour map of sea surface height. 12. Under baroclinic conditions the isopycnals are inclined to the sea surface, which cause the HPG, and thus geostrophic current speed, to decrease with depth. This is the result of a sub-surface adjustment of the density field that has dense water under regions of low sea surface height and less dense water under regions of high sea surface height. 13. In subtropical gyres, where sea surface height is high, there is downwelling of surface water, whereas at the equator and in the subpolar gyres where sea surface height is low there is upwelling of subsurface water. 14. Surface nutrient concentrations and chlorophyll levels are generally higher in regions of upwelling and lower in regions of downwelling. 15. The Gulf Stream in the N Atlantic is the most notable example of a geostrophic current and an example of a strong Western Boundary Current in a subtropical gyre. 16. The asymmetry between fast poleward flowing Western Boundary Currents and slower equatorward flowing currents along the eastern boundary of the subtropical gyres provides an efficient mechanism for the poleward transport of heat.
1. If the wind direction is from east to west, what would be the compass direction of the average Ekman transport in the northern hemisphere? What is the compass direction of the current at the surface of the Ekman Layer (z = 0m)? Is this an easterly or westerly wind? What is the typical depth of the Ekman Layer? 2. At 30°N, there is a 2m increase in sea surface height in the eastward direction that occurs over 10° of longitude. What is the direction and speed of the resulting geostrophic surface current? Assume uniform density. 3. Assume at locations A and B the average seawater densities above the 990 dbar isobaric surface are 1024 and 1026 kg/m3, respectively (1 dbar= 1.01m). Calculate the difference in column height of water above the 990 dbar surface (~1000m) between these two sites. Does this situation represent baroclinic or barotropic conditions? 4. In which region of the ocean are the lowest sea surface dynamic height anomalies located? Why? (see Fig. 14). 5. Plot the north-south (meridional) trend in sea surface dynamic height from 60ºS to 60ºN along 180°W in the Pacific (use Fig. 14).
6. Calculate the geostrophic current speed at the surface of the Gulf Stream if the sea surface height increases by 1.4m over a distance 115 km at a latitude of 35°N? -How does this compare to the geostrophic velocity calculations presented for the Gulf Stream in Fig. 22?
8. How does the sea surface height of a Warm Core Ring compare to the sea surface height of its surrounding water (see Fig. 25)? What would be the likely rotational direction (clockwise or counterclockwise) of a cold core ring? How would the photosynthesis rate inside a cold core ring likely compare to the photosynthesis rate outside? Directory: oc210 -> OC210 -> Lecture%20Notes Lecture%20Notes -> Oc 210 topic 2: circulation in the deep sea Lecture%20Notes -> Oc 210 topic 2: circulation in the deep sea (Fall 2009) Download 106.11 Kb. Share with your friends:
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