Monday, October 2, Watch for new assessment (Week 4/5 review) TA s have your tests, please see key (at course website)

Transcription

1 Monday, October 2, 2017 Watch for new assessment (Week 4/5 review) TA s have your tests, please see key (at course website) Homework 3 Due date Wednesday, Oct 11 (8 pm) Be ready to watch another important video in entirety Crude, the Incredible Journey of Oil. This will also be accompanied by questions on Mid-term 2. I used to show in my T/Th class, but it is longer than 45 minutes, and it is important to watch it in a single session!

2 Fall Week 6 Chapter 5 Oceans (Continued)

3 Our goals to examine the role of atmosphere/ocean exchange (starting with physics) in Earth s climate Drill down on phenomena that determine local weather/climate Examine climate variability in context of atmospheric and oceanic circulation (both the averages and the extremes)

4 Today Wind-driven surface circulation Gyres major ocean basins Ekman transport (winds, friction, Coriolis Effect, geostrophy) Next time composition of the oceans and deep-water formation

5 Typically, the trade winds in the northern hemisphere blow from the northeast. This is because the heating in the tropics is relatively uniform, so the rising branch of the Hadley circulation is also a relatively uniform feature. But at high latitudes, seasonal temperature contrasts lead to very different wind patterns. Winds are strongest in the winter when the equator-to-pole temperature gradient is strongest, and there are seasonal patterns that reverse (a high pressure in the summer over the north Pacific ocean due to continental heating and relatively cool ocean waters (cool air that is next to warm air is more dense, so the column density is higher near the cooler water so surface pressure is higher). In winter, the land is colder than the ocean, so there is a relative high at the surface over the ocean. This reversal of pressures causes a shift in location and pattern of mid-latitude winds.

6 Note that in January, there are relatively weak northeasterly trade winds in the equatorial western Pacific (weak pressure gradient means weak winds), whereas there are strong westerlies at o N latitudes. These high latitude storm tracks take wet weather in the direction of the Northwest US (Oregon, Washington), and pull the high latitude waters from west to east. In July, the trade winds strenghen, become more easterly, pulling equatorial waters from east to west, and the winds at higher latitudes slacken and the westerlies becomes more southerly. This drags water at high latitudes still from west to east

7 This surface circulation affects the top meters of ocean Continents then deflect the currents, leading to gyres Fig 5.2

8 Even before satellites could measure the oceans surface, we knew a lot about circulation in Alaska from cargo that spilled out from ships here s one story; Nike shoes!

9 Strangely, a similar accident happened a few years later, and even more bizarre this time it was rubber duckies!

10 Fig 5.5 Geostrophy flow that is determined by the balance of the pressure force (from the center of the gyre) and the Coriolis effect, so that the flow of the top 100 m of the ocean is to the right of the direction of the pressure force (clockwise ) in the northern hemisphere.

11 Ekman Spiral refers to the pattern of flow of water at different depths due to the balance of wind-driven surface flow, the coriolis effect, and friction.

12 Ekman Spiral looking from above, we see that water at the very surface flows to the right of the wind direction, but at an angle that is less than 90 o. The net movement of water over the entire ~few hundred meters is about 90 o to the right of the wind direction.

13 Ekman Spiral The coriolis effect turns the surface flow in a direction that is about o from the wind direction, with a net (average) movement of water that is 90 o from the direction of the wind (to the right in the northern hemisphere). Fig 5.3

14 Coupled layers: Friction drag=> Energy Dissipated=> slower motion below; Coriolis effect: deflection=> Ekman spiral Ekman spiral Figure 5-3a

15 Ekman as a theory, vs. observation Reality: True Ekman flow of surface waters is hard to observe. This is due to turbulence in the upper layers (i.e., waves ) that mix the waters, smoothing out the flow shear that is formed by the wind. A true Ekman pattern (called the spiral ) is what would happens in the absence of turbulence what we call laminar (or smooth) flow. However, it is often the case that the very surface of the ocean flows in a direction that is between 20 and 45 degrees of the wind.

16 Even so, this Ekman transport (pulling of the upper ~100 meters of water to the right of the surface winds) results in the convergence of water at the center of the wind-drive gyre, so that a bulge in sea level occurs. Fig 5.3

17 Surface layer thickening (or bulge) leads to vertical (downward) motion because of differences in weight of water columns. (The opposite occurs in regions where the surface layer thins and this is responsible for upwelling. (note that the arrows for geostrophic current are labeled incorrectly in Ed 2 of the textbook) Pressure gradient force Fig 5.4

18 Why do Humpback whales migrate to Southeast Alaska in summer? (a) It s where the best clubs are (b) It s when sea-ice melts there (c) because of prevailing winds and the Coriolis effect (d) They have been doing it forever, why stop now?

19 Why do Humpback whales migrate to Southeast Alaska in summer? (a) It s where the best clubs are (b) It s when sea-ice melts there (c) because of prevailing winds and the Coriolis effect (d) They have been doing it forever, why stop now?

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21 Chlorophyll grows where there are nutrients either coming from river runoff into the ocean or where there is upwelling of nutrientrich deep water. River runoff is warm, whereas upwelling waters are cold. So combined with ocean surface temperatures, scientists can prove that the chlorophyll is forming in regions of upwelling. upwelling

22 Another consequence of the coriolis effect (Ekman Transport) is for upwelling and downwelling to be influenced by the direction of surface winds next to coastlines:

23 Ekman transport Ekman transport - the net movement of water in the entire layer - is at the right angle (90 o ) to the wind direction. Northern Hemisphere to the right and southern Hemisphere to the left. When add up all layers in the spiral, the net direction of transport within the water column is at right angle to the wind direction. Northern Hemisphere

24 Sea Surface Temperatures show regions of upwelling at California, Oregon coastline

25 Sea Surface Temperatures show regions of upwelling at California, Oregon coastline Northerly Wind 30 o C 10 o C

26 Sea Surface Temperatures show regions of upwelling at California, Oregon coastline Northerly Wind Ekman Transport 30 o C 10 o C

27 However, note that surface waters are generally warmer and less dense than deep water. So there is no easy way for surface water to sink very far. thermo = heat halo = salt pycno = dense Fig 5.13

28 2. Sea surface temperature (SST) cold warm cold cold cold Figure 3

29 So, how does water get from the surface layer of the oceans to the deepest parts of the ocean? Not from heating note that sunlight can t penetrate very deeply into the ocean (maybe a few hundred feet at best). So there must be some other way for water at the surface to exchange with deeper water or, maybe the surface water never sinks? We will look at tracers from the upper ocean to learn that deep ocean water actually forms at the surface so something must be happening that causes this to occur.

30 Next time - The Thermohaline Circulation Fig 5.19