ERTH 535:
Planetary Climate Change
(Spring 2018)
Reading Questions
(Responses due in class Monday, May 14)
Dr. Dave Dempsey
Dept. of Earth & Climate Sci.,
SFSU

"Evidence linking Arctic amplification
to extreme weather in mid-latitudes
"
Jennifer Francis and Stephen Vavrus,
Geophysical Research Letters, March 2012


"Future loss of Arctic sea-ice cover could drive
a substantial decrease in California's rainfall
"
Ivana Cvijanovic et al., Nature Communications, Nov. 2017

Recommended video background lecture (5.5 minutes):
Jennifer Francis: Understanding the Jetstream

Respond briefly to each of the "Reading Questions" below about the two required articles above, in the manner outlined by the assignment "Reading, Discussing, and Writing about Articles from the Literature". We will have a round-table discussion of the topic of this reading in class Monday, May 14. Completing this assignment in advance should help prepare you to participate actively. (Your response to the reading questions is worth 10 pts. Your participation in the discussion is worth 5 pts.)
Some terminology and concepts, in roughly the order in which they appear in the first article (with a couple of others thrown in for good measure):
  1. hectoPascal (hPa): A Pascal is a MKS unit of pressure, and "hecto" means 100, so 1 hPa is 100 Pascals.
  2. thickness: The vertical distance between the altitudes at which two specified pressures in the atmosphere are found. A common pair of pressure values specified for this purpose is 1000 hPa and 500 hPa (the "1000–500 hPa thickness"). For any pair of specified pressures, the thickness is proportional to the mean temperature in the layer between them: a warmer average temperature between the two pressure levels implies a greater thickness. Since 1000 hPa is typically near sea level and 500 hPa is roughly half way up in the troposphere, the 1000–500 hPa thickness at any particular place and time is an indirect measure of the average temperature of the lower half of the troposphere at that place and time.

  3. (geopotential) height: The altitude above sea level at which a specified atmospheric pressure is found. For example, the 500 hPa (geopotential) height at midlatitudes is typically between 5 and 6 km above sea level (roughly half way up in the troposphere).

  4. Rossby wave: In the atmosphere, Rossby waves are the largest-scale meanders ("wobbles" or waves) in the pattern of pressure in the upper troposphere and lower stratosphere at midlatitudes, and hence in the wind pattern (in particular, the jet stream) at those altitudes and latitudes. Rossby waves tend to propagate (shift, progress, migrate) eastward, but they can occasionally stall in place or even progress slowly westward. Features of midlatitude weather patterns, notably precipitation-producing midlatitude cyclones, are closely tied to Rossby waves.

  5. gradient: For a quantity (say, temperature or pressure or thickness) that varies from place to place (that is, spatially), the gradient of that quantity is the difference in the quantity across a (that is, per) unit of distance. The larger the gradient in some quantity, the bigger the difference you'd see in that quantity across a unit of distance. For example, suppose that the temperature gradient at any particular place is 3°C/100 km. In that case you'd see a difference of 0.03°C between two places 1 km apart in that area. It is a measure of how rapidly a quantity varies with respect to spatial position—that is, from place to place at any moment in time.

  6. zonal: In the west-east direction (parallel to lines of latitude). (Horizontal wind velocities can be broken into two parts or "components" in various ways. One such way is to break them into zonal (east/west) and meridional (north/south) components.)

  7. ridge: A region of relatively higher pressures aloft (compared to areas east or west of it), corresponding to a "tongue" of relatively warm air "protruding" poleward beneath the ridge. Winds aloft tend to blow around the edge of a ridge, outlining it and creating a poleward bend in the east-west jet stream pattern (in the Northern Hemisphere, a hump- or ridge shaped feature), so "ridge" is often used to describe this feature in the jet stream pattern, too.

  8. trough: A region of relatively lower pressure aloft (compared to areas east or west of it), corresponding to a "tongue" of relatively cool air "protruding equatorward beneath the trough. The area along the leading (eastern) edge of a trough tend to be favored locations for midlatitude cyclones (storms). Winds aloft tend to blow around the edge of a trough, outlining it and creating an equatorward bend in the east-west jet stream pattern (in the Northern Hemisphere, a dip or trough-shaped feature), so "trough" is often used to describe this feature in the jet stream pattern, too.

  9. blocking: An atmospheric phenomenon in which a long-wave (that is, large) ridge becomes quasi-stationary for an extended period of time. As a result, midlatitude cyclones, which follow the long-wave jet stream pattern, deflect around the edge of the ridge and miss the area beneath the ridge for a sustained period (that is, the ridge "blocks" storms from passing through the area beneath the ridge, which can lead to drought). Since large-scale troughs and ridges typically alternate, blocking means that the long-wave trough that follows a ridge can become relatively stationary, too.

  10. anomaly: The difference between a quantity and the average value of that quantity over some period of time. It tells you how far the quantity deviates at any particular time from the norm or mean

  11. JFM, AMJ, JAS, and OND: January, February, and March; April, May, and June; July, August, and September; and October, November, and December, respectively.

  12. thermal wind relationship: A quantitative relationship between (a) the horizontal gradient of thickness between to pressure levels; and (b) the vertical difference between winds the two pressure levels. The relationship gets its name ("thermal") from the fact that the "thickness" of an atmospheric layer between two specified pressure levels is proportional to the mean temperature of the layer. Hence, the thermal wind relationship relates (a) the horizontal gradient in average temperature of a layer, and (b) the difference in winds between the top and bottom of the layer. A larger horizontal temperature gradient means a bigger difference in the winds. Since winds tend to be smaller near the Earth's surface than aloft, a bigger difference in winds between the surface and aloft means stronger winds aloft. The (zonal) jet stream at midlatitudes aloft, in particular, is associated with a large north-to-south gradient across the midlatitudes (between the tropics and polar regions) in the thickness (mean temperature) of the layer below the jet stream.

  13. meridional: in the north-south direction (along a line of longitude, or a meridian).

  14. polar jet stream: A part of the wind pattern in the middle and upper troposphere (and lower stratosphere) at midlatitudes (sometimes edging into the high latitudes) in which wind speeds exceed a certain threshold (typically around 60 knots). This part of the wind pattern forms a relatively narrow, wavy band around the globe in midlatitudes.

"Evidence linking Arctic amplification to extreme weather in mid-latitudes" (Jennifer Francis and Stephen Vavrus, 2012)

  1. What is meant by "Arctic amplification" (AA)? What linkage to Arctic amplification does this paper explore?

  2. What two effects that this linkage has on Rossby wave characteristics do the authors say each contributes to slowing the eastward progression of midlatitude Rossby waves?

  3. When are these effects particularly evident? To what other phenomena do the authors suggest might these effects be tied?

  4. What impact would slower eastward progression of Rossby waves have on midlatitude weather? [Why?]
  5. 1. Introduction (p. 1)

  6. In the last few decades, how much has the Arctic warmed relative to the Northern Hemisphere as a whole? To what do the authors attribute AA?

  7. How much sea ice has been lost since the 1980s? What consequence does the loss of sea ice have on the budget of heat in the lower atmosphere in the Arctic?

  8. What is one effect that climate scientists have expected greenhouse-gas-induced tropospheric warming to have on storms and flooding? What is a different factor (the subject of this article) that can affect the severity of extreme weather events?
  9. 2. Analysis and Results (p. 1)

  10. What question does this article address? How do address it?

  11. Figure 1 (p. 2) shows seasonal anomalies in 2000-2010 (10-year) average 1000-500 hPa thicknesses north of 40°N, relative to 1970-1999 (a 30-year period). What does a positive anomaly indicate? When and where are there widespread positive anomalies?

  12. Based on Figure 1, in what area(s) and in which "season" do the authors assert that positive anomalies are associated with sea ice loss? In what areas and season do they associate them with earlier snow melt?

  13. When are positive anomalies absent? To what do the authors attribute this absence?

  14. The authors say they expect to find two effects of Arctic amplification on patterns of pressure (and hence winds) aloft (that is, in the upper troposphere and lower stratosphere).
    1. What effect do they expect to find on poleward thickness gradients? What impact would this have on zonal wind speeds aloft?
    2. What effect do they expect to find on the latitude and amplitude of Rossby waves? (See Figure 2(b).)

  15. In theory, what effect should both of the effects in question 12.a and 12.b have on the rate (speed) at which Rossby waves progress eastward? (See Figure 2(b).)

  16. What does Figure 3 (left and right graphs) show? Do these graphs show the effect that the authors were looking for in question 12.a above? If so, do they see it at all times of year?

  17. What impact should slower-progressing Rossby waves have on certain types of extreme weather? Which types?

  18. What does the first row of graphs in Figure 4 show? Do these graphs show the effect that the authors were looking for in question 12.b above? If so, do they see it at all times of the year? What is the statistical correlation between (a) maximum latitude of Rossby wave ridge peaks in the OND "season", and (b) September sea ice extent? What is the statistical correlation between (a) maximum latitude of Rossby wave ridge peaks in the JJA "season", and (b) northern hemisphere snow cover?

  19. 3. Conclusions (p. 5)

  20. Do the authors attribute particular events of extreme weather in 2010 and 2011 in parts of the U.S. and Europe to the effects of Arctic amplification on the jet stream? Why or why not?

Some terminology and concepts, in roughly the order in which they appear in the first article (with a couple of others thrown in for good measure):
  1. (geopotential) height: The altitude above sea level at which a specified atmospheric pressure is found. For example, the 500 hPa (geopotential) height at midlatitudes is typically between 5 and 6 km above sea level (roughly half way up in the troposphere).

  2. ridge: A region of relatively higher pressures aloft (compared to areas east or west of it), corresponding to a "tongue" of relatively warm air "protruding" poleward beneath the ridge. Winds aloft tend to blow around the edge of a ridge, outlining it and creating a poleward bend in the east-west jet stream pattern (in the Northern Hemisphere, a hump- or ridge shaped feature), so "ridge" is often used to describe this feature in the jet stream pattern, too.

  3. anomaly: The difference between a quantity and the average value of that quantity over some period of time. It tells you how far the quantity deviates at any particular time from the norm or mean

  4. teleconnections: a causal connection or correlation between meteorological or other environmental phenomena that occur a long distance apart.

  5. CMIP-5: a climate model.

  6. ITCZ: The Intertropical Convergence Zone, a zone in the tropics near the equator where the northeast and southeast trade winds in the northern and southern hemispheres, respectively, converge near the earth's surface. The converging air rises, expands under lower pressure as it rises, and cools, leading to condensation to form clouds, typically thunderstorms with high, cold tops. This makes the ITCZ very visible on infrared satellite images as a band of cold cloud tops near the equator, stretching across the Pacific Ocean and sometimes the Atlantic Ocean.

"Future loss of Arctic sea-ice cover could drive a substantial decrease in California's rainfall" (Cvijanovic et al., 2017)

    Introduction (p. 2)

  1. Why is there so much uncertainty in climate model projections of changes in precipitation in California in the next century?

  2. How do high temperatures exacerbate the impact of drought?

  3. What feature of atmospheric pressure pattern (in the middle and upper troposphere) accompanied the exceptionally dry conditions in California during the winters of 2012-2015? What was it's connection to the drought?

  4. What phenomena might have helped sustain the high-pressure ridge in the North Pacific?

  5. Aside from anomalous sea-surface temperatures (SSTs) in the tropics and convection (thunderstorms driven by sensible heat fluxes [conduction] from the warm surface of the earth), what other candidate cause for the creation and maintenance of the North Pacific high-pressure ridge has been suggested?

  6. What do the authors of this article aim to accomplish in this article? What methodology do they employ that they consider novel? What do they use for their climate model "control simulations" (which they compare with their "low Arctic ice" simulations)?

  7. Based on their atmospheric general circulation model (AGCM) simulations, what impact do the authors expect a decline in Arctic sea ice to have on precipitation in California?
  8. Results: Impacts of Arctic sea-ice loss on California’s precipitation (p. 3)

  9. How is a "nearly ice-free Arctic" defined? Based on climate model simulations and extrapolations from the recent past, when (that is, by what years) is the Arctic expected to become virtually ice-free in August and September of each year?

  10. How do Figures 2(b) and (c) illustrate the author's assertion that under low Arctic ice conditions, (i) precipitation in the tropics will shift northward, and (ii) California will receive less precipitation (and Alaska and Canada more)? To what do they attribute result (ii)? How does Figure 2(e) support this attribution?

  11. The authors attribute the projected change in precipitation in California not just to changes in the jet stream caused by Arctic warming, as others have proposed, but to what alternative mechanism?
  12. Results: Equatorward propagation (p. 4)

  13. The authors argue that loss of sea ice results in more absorption of solar radiation and hence warming of the surface at high latitudes. Sensible and latent heat fluxes then increase to balance the increased solar absorption by the surface. This warms the atmosphere, but emission of LWIR radiation to space by the atmosphere doesn't increase enough to balance the increased sensible and latent heat fluxes into the atmosphere. To compensate and enable a balanced atmospheric energy budget at high latitudes, what happens to transport of heat by the atmosphere from mid-latitudes into the high latitudes?

  14. To achieve a balanced energy budget, the earth must ultimately compensate for the increased solar absorption at high latitudes as a result of loss of sea ice, by radiating more LWIR radiation to space. According to the authors' climate model simulations, how and where does this increased LWIR radiation occur?

  15. By what mechanism to the authors propose that disturbances (perturbations—in this case, increases) in lower-atmospheric and sea-surface temperatures at high latitudes propagate into the tropics and change the pattern of convection (thunderstorms) there? (Also see Figure 4.)
  16. Results: Poleward propagation (p. 4)

  17. By what mechanism do that authors propose that changes in tropical convection in the Pacific might lead to the development of a high-pressure ridge in the North Pacific, diverting storms along the jet stream northward, away from California and into Alaska and Canada? (Also see Figure 4.)

  18. The authors identify two hypotheses about the primary influence on precipitation variability in California. What are they? Why do the authors say that these two hypotheses turn out not to be easily separable?
  19. Results: Atmospheric impacts of Antarctic sea ice loss (p. 5)

    Discussion (p. 6)

  20. Do the authors claim that their climate model simulations predict that California will be drier every year in response to loss of Arctic sea ice? How large a decrease in precipitation in California do their model simulations suggest we should expect, in a 20-year average?

  21. Do the authors claim that loss of sea ice in the Arctic caused the 2012-2015 drought in California? If not, what do they say instead? What other possible causes of the drought do they mention (and what is their opinion about each)?

  22. The climate model simulations that the authors performed to study the impacts of the loss of Arctic sea ice, differ in what the authors consider two important ways. What were they?

  23. Do coupled (atmosphere/ocean) climate model simulations of 21st century climate reproduce the recent California drought and the atmospheric circulation associated with it? Do such simulations even agree on on whether precipitation will increase or decrease this century? What does the mean of all current climate model simulations predict for the future of precipitation in California? What do the authors suggest might be a common problem with these simulations?
  24. Methods (p. 8)


Recommended video lecture (5.5 minutes): Jennifer Francis: Understanding the Jetstream


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