Wednesday, September 27, 2017 Test Monday, about half-way through grading. No D2L Assessment this week, watch for one next week

Transcription

1 Wednesday, September 27, 2017 Test Monday, about half-way through grading No D2L Assessment this week, watch for one next week Homework 3 Climate Variability (due Monday, October 9) Quick comment on Coriolis Effect (p 63-65) General circulation on a rotating Earth (p 65-68) Geostrophy force balance (p 66) Global Distributions and Continental effects (p 70-75) Water and phase changes (p 75-82)

2 The coriolis effect works in three dimensions like a vector, or a force. We have to deal with the horizontal component of that force. The vertical can t do much working against the much larger vertical force of gravity.

3 The Geostrophic Wind At mid and high latitudes, the coriolis effect is larger than it is in the tropics (in fact, in the tropics it is very small). Air will move if subjected to a force. Therefore, it will respond to both differences in pressure and to the coriolis force. Because air experiences very little friction, it will eventually travel in a direction where the two forces cancel. This direction ends up being perpendicular to the pressure force. It s easy to know where way the wind blows simply by noting that in the northern hemisphere, high pressure will be to the right of the wind direction. We call the balanced wind the geostrophic wind.

4 When winds obey the condition where the pressure force balances the coriolis force, we call it geostrophy. At high latitudes, winds are usually nearly geostrophic, in that the air travels in a direction perpendicular to the pressure force. However, friction causes the true winds to be a little slower than expected, which leads to waves that create moving weather systems (what we often hear called steering currents on news reports). Fig. 4.13

5 For this class, we will use that the winds are nearly geostrophic, which will allow us to use maps of pressure to estimate the direction of the winds. For predicting weather, this won t be perfect (in fact, the deviations from geostrophic balance are responsible for changes in the weather). However, for climate and trends, assuming winds are geostrophic will be quite useful. We will look for droughts in places where surface pressure tends to be high. We ll look for rain or snow where surface pressures are low, and we ll look for fronts to move in the direction of geostrophic winds winds that, in the northern hemisphere, maintain high pressure to the right of the wind direction. This approach will tell us a lot about climate zones places on earth where certain types of biomes occur. It will also tell us the directions of the major ocean gyres!

6 What air flow at the surface would look like, based on observed pressures alone air would move from high to low pressure. Areas where air is rising due to solar heating (e.g., tropics) have low pressure at the surface, areas where air is sinking due to cooling (e.g., north and south poles) have high pressures. Areas in between can have either high or low pressures, depending on what is happening to air at other latitudes. We can now begin to see climate bands! Places where it is often warm and wet, like the tropics, places that are cool and dry, like the polar regions. Places in between (like the US) will have more variable climates! Air over the poles is sinking because it is cooling. Fig. 4.8

7 As a consequence of the Coriolis Effect (due to Earth s rotation), air does not travel in a N/S direction, but is turned in an E/W direction giving rise to gradients that separate warm and cold air masses Fig. 4.11

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10 Using geostrophy to deduce climates Note that, based on geostrophic flow and mean surface pressure fields, we can deduce general patterns of climate in different places. For example, the west coast of the US experiences a mean wind direction from the north-northwest during July, keeping weather cool near of the coasts of states like Oregon, Washington, and many parts of California. In January, the mean flow towards Washington state is from the southwest, bringing warm, wet air from near Hawaii (the pineapple express. Note that weather in the UK comes from the west-southwest both months, even though the pattern switches from high to low. This is because the position of the highs and lows shifts. The UK typically has wet weather in both July and January.

11 Water and it s phases Fig. 4.23

12 Plus, we can t ignore the seasons! To build a more complicated picture of climate at Earth s surfaces, let s start with some basic information. Maps of surface temperatures (summer and winter).

13 July temperatures Fig. 4.18

14 January temperatures Fig. 4.18

15 Summer/winter temperature differences Fig. 4.18

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18 Biomes (regions with distinct types of plants) can tell us something important about climate! Fig. 4.27

19 tundra deciduous forest alpine taiga grasslands rainforest savanna desert chaparral

20 chaparral

21 alpine

22 deciduous forest

23 savanna

24 desert

25 taiga

26 Chapter 5 - Oceans

27 Average sea-level pressure patterns - January L H L H H Fig. 4.19

28 Northern hemispheric average winds, January L H L H H Air flows in a direction that keeps high pressure the right of the wind direction in the NH Fig. 4.19

29 Average sea-level pressure patterns - July H H L Fig. 4.19

30 Mean wind directions in NH, July H H L Fig. 4.19

31 End of Chapter 4, Start of Chapter 5 Climate Zones (p 75-82) see especially Figures 4-26 and 4-27 Wind-drift currents trade winds and mid-latitude westerlies (p 84-85) Gyres and convergence (p 85) Explaining convergence and divergences the Ekman spiral (p 86) Upwelling and downwelling (p 87)

32 Next let s look at how water gets from the surface layer of the oceans to the deepest parts of the ocean. It won t come directly 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.

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

34 Wind-drift currents friction (wind stress) drags ocean surface in general direction of winds Fig 5.1

35 Using ocean surface currents, and some knowledge of atmospheric mean winds and phases of water, we can explain more details the Earth s climate zones Fig 5.2

36 There was a recent story in the news about a pile of trash that was discovered in the center of the Pacific Ocean gyre. How is it that a circular circulation can carry trash in to the center?

37 Let s start with the mean circulation and wind-drift currents friction (wind stress) will drag ocean surface in general direction of the winds General mean direction of the midlatitude surface winds General direction of the trade winds Fig 5.1

38 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.

39 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

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

41 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.

42 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.

43 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.

44 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

45 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

46 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

47 The vertical structure of the ocean thermo = heat halo = salt pycno = dense Note that surface waters are warm and less dense than deep water. There is no easy way for surface water to sink. Fig 5.6

48 Next time we ll look at how water gets from the surface layer of the oceans to the deepest parts of the ocean. It won t come directly 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.

49 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:

50 Upwelling at California, Oregon coastline

51 Upwelling at California, Oregon coastline Northerly Wind