1. Lecture 18: Map discussion + Atmospheric circulation & pressure distrib’ns (Ch 8) map discussion; focus on series of lows forming in trough over Ab this week (powerpoint “animation”) Rossby waves Chinook and sea breeze – examples of a synoptic-scale and of a meso- scale wind system El Nino

2. We’ve had a week I would characterize as cool, moist, cloudy, calm. We’ve had drizzle, a few flakes of snow, a frost one morning. Let’s focus on Thursday 19 Oct… 19-Oct-06TRHTd 13:00Light Rain3902SE 9 12:00Light Rain3911SE 15 11:00Cloudy2901SE 13 10:00Cloudy2921SE 9 09:00Light Rain2931ESE 11 08:00Cloudy2941calm 07:00Cloudy1940SSW 5 06:00Cloudy1940calm 05:00Cloudy1971calm 04:00Mostly Cloudy1971calm 03:00Cloudy1970NNE 5 02:00Cloudy2971N 15 01:00Cloudy1951NW 9 00:00Cloudy2941W 9

3. at or near saturation absolutely stable layers NW wind all levels

4. widespread low cloud (not very different Friday)

5. Let’s watch the progression of a trough across BC into Alberta… … and formation of surface lows… 00Z Wed 18 Oct. Why? Because science can be said to be all about recognizing, recording and (if possible) explaining patterns … no patterns? … then not possible to “do” science. This is one of the Ab weather “patterns” Path of “this” low

6. 06Z Wed 18 Oct. Let’s watch the progression of a trough across BC into Alberta… … and formation of surface lows…

7. Let’s watch the progression of a trough across BC into Alberta… … and formation of surface lows… 12Z Wed 18 Oct.

8. Let’s watch the progression of a trough across BC into Alberta… … and formation of surface lows… 18Z Wed 18 Oct.

9. Let’s watch the progression of a trough across BC into Alberta… … and formation of surface lows… 00Z Thurs 19 Oct.

10. Let’s watch the progression of a trough across BC into Alberta… … and formation of surface lows… 06Z Thurs 19 Oct.

11. Let’s watch the progression of a trough across BC into Alberta… … and formation of surface lows… Edmonton: (T=) 0 (Td=) 0 ie. sfc chart gives Td directly 12Z Thurs 19 Oct.

12. Let’s watch the progression of a trough across BC into Alberta… … and formation of surface lows… 18Z Thurs 19 Oct.

13. Let’s watch the progression of a trough across BC into Alberta… … and formation of surface lows… 00Z Fri 20 Oct.

14. Let’s watch the progression of a trough across BC into Alberta… … and formation of surface lows… 06Z Fri 20 Oct.

15. Let’s watch the progression of a trough across BC into Alberta… … and formation of surface lows… 12Z Fri 20 Oct.

16. 12Z Thurs 19 Oct. During these events, consistently low pressure at all levels in eastern Canada

17. Edmonton: (T=) -7 (T-Td=) 0 ie. upper charts give T and T-Td T and T-Td 12Z Thurs 19 Oct. 3x5+2.5 m/s Heavy stippling on 700 mb analysis: T-T d <= 2 o C T-T d <= 2 o C This ridge moves eastward by Fri

18. 12Z Fri 20 Oct. Edmonton: (T=) -9 (T-Td=) 1 ie. upper charts give T and T-Td T and T-Td 2x5+2.5 m/s Heavy stippling on 700 mb analysis: T-T d <= 2 o C T-T d <= 2 o C

19. 12Z Thurs 19 Oct. note the weak ridge note the weak ridge and NW flow onto AB and NW flow onto AB

20. Long (Planetary/Rossby) Waves — wavelength  1000’s kilometers typically 3 - 7 around globe (fewer, longer, stronger in winter) not always unambiguously identifiable - but can be shown mathematically to exist in ideal atmos. due to N-S variation of Coriolis force (usually) move slowly eastward Waves in mid-latitude mid/upper troposphere causing convergence and divergence aloft... 500 mb con div con divL

21. Fig. 8-13 Example of high amplitude Rossby wave over N. America Sept. 22, 1995 Edmonton low +4 o C, high +20 o C Winnipeg low -2 o C, high +11 o C

22. Fig. 8-12 Rossby wave moving eastward

23. Notice the “short wave” here; covered later – are associated with storms Rossby wave moving eastward

26. Foehn/Chinook/etc. – synoptic scale wind down mountain slope “when winds warmed by compression descend the eastern slopes of the Rocky Mountains in North America, they are called Chinooks” (native-American term meaning “snow-eater”). Condensation of vapour on the windward slopes also contributes to the warmth of the dry, descending lee-side current “Low-pressure systems east of the mountains cause these strong winds to descend the eastern slopes…” Warm lee current may ride above a layer of cold dense air on the lee side… frontal boundary may move back and forth – rapid temperature changes

27. Chinook conditions in SW Alberta Strong SW 700 mb flow strikes mountains perpendicularly Warm air at 850 mb in lee of Rockies (adiabatic compression)… “trowal” Surface lee trough of low pressure; tight contours, windy.

28. Sea breeze mesoscale caused by differential warming (“thermally driven”) shallow circulation masked (ie. effect over-ridden) by strong synoptic-scale winds Fig. 8-24

29. El Nino/ La Nina ENSO an internally-generated disruption or instability of the ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe and giving some basis for long range weather forecasting (though presently with low skill) Is connected with the Southern-Oscillation, a pattern of reversing sea-level east- west pressure gradient across the equatorial Pacific… El Niño and La Niña are opposite phases of the El Niño-Southern Oscillation (ENSO) So potentially can be forecast by coupled oc/atmos prediction model Fig. 8-27

30. easterly Pacific tradewinds pile up warm surface water in the west Pacific - so that sea surface is about 1/2 meter higher at Indonesia than at Ecuador. A weak surface ocean counter-current then develops The sea surface temperature is about 8 o C higher in the west, with cool temperatures off South America, due to an upwelling of cold (nutrient-rich) water from deeper levels Normal conditions JDW, EAS, U. Alberta EAS270 22 Nov/04 strong convection/rain over the warmest water, and the east Pacific is relatively dry strong equatorward-flowing coastal current (“Humboldt” or “Peruvian”) sustains the upwelling cold deep-water Equivalent to Fig. 8-17, for S. hemisphere

31. Reversal in sea-level east-west pressure gradient across equatorial Pacific. Abnormal winds (weakened easterly trades, or even westerlies) The warm surface water normally found in the western equatorial Pacific “sloshes eastward”, and the warm sea surface temperature anomaly in the eastern Pacific suppresses upwelling of cold deep-water El Nino phase Fig. 8-29… sea sfc temperature anomaly averaged over eight El-Nino’s (red = + 2 o C)

32. La Nina phase La Niña phase is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific Fig. 8-31… sea sfc temperature anomaly averaged over eight La-Nina’s (blues, about = minus 1 o C)