1. Southwest Climate
Bimodal and Unimodal rainfall
Winter precipitation
Pacific frontal storms
El Niño
Summer monsoons
Arid Foresummer
What SW climate (precipitation) means
Vegetation
Humans
Temperature less interesting for now

3. MAT (°F)
MAP (in.)
JFMAMJJASOND

4. Air Masses
Large (tens of thousands of mi.2)
Homogeneous temperature and moisture

5. Air Mass Origins
Cold (polar) or warm (tropical)
Dry (continental) or moist (maritime)

6. Summer: cT, mT Pacific, mT Gulf mP, mT, cP Air Masses Impacting SW
Winter:
mP, mT, cP
Summer:
cT, mT Pacific, mT Gulf

7. Summer: cT, mT Pacific, mT Gulf mP, mT, cP Air Masses Impacting SW
Winter:
mP, mT, cP
Summer:
cT, mT Pacific, mT Gulf

8. Ocean – Atmosphere Interaction
Evaporation always occurs to some degree
Affected by temperature of water and air, moisture in air
Once over land, air must cool to rain:
Uplift.

9. Northern Hemisphere Winter
Typical NH Winter Circulation (strong)
Oceanic Lows
N. Am. High pressure
Westerly air flows around these features

10. Northern Hemisphere Winter
Typical NH Winter Circulation (strong)
Oceanic Lows
N. Am. High pressure
Westerly air flows around these features

11. Average
Winter Flow
Slightly north of the SW

12. Winter Pacific front Seattle wet Tucson wet Pacific Storms
Where will they cross land?
North:
Seattle wet
South
Tucson wet

13. Maybe to SW Satellite View Incoming Pacific moisture Counter-clockwise
Reaches N. Am. at about CA-OR state line
Long tail may bring rain to SW

14. Cold water stays deep current: weakens, convective uplift: moves east
Normal – La Niña
Cold water wells up, current: east-west, convective uplift: west Pacific
El Niño
Cold water stays deep current: weakens, convective uplift: moves east

15. Cold water stays deep current: weakens, convective uplift: moves east
La Niña = normal
Upwelling of cold water to surface of Peruvian coast
Westward flow of warm surface water
Hot air rises over western Pacific
Clouds miss American SW
Normal – La Niña
Cold water wells up, current: east-west, convective uplift: west Pacific
El Niño
Cold water stays deep current: weakens, convective uplift: moves east

16. Cold water stays deep current: weakens, convective uplift: moves east
El Niño = abnormal
Upwelling of cold water slackens, fails
Warm surface water stalls over central Pacific
Hot air rises over central Pacific
Clouds hit the American SW
Normal – La Niña
Cold water wells up, current: east-west, convective uplift: west Pacific
El Niño
Cold water stays deep current: weakens, convective uplift: moves east

17. La Niña
Pacific moisture goes north, misses SW

19. El Niño-Southern Oscillation (ENSO)
Pacific moisture stays south, hits SW

20. U.S. Temperature and Precipitation Departures During the Last 30 and 90 Days
Last 30 Days
30-day (ending 16 Jan 2011) % of average precipitation
30-day (ending 15 Jan 2011) temperature departures (degree C)
Last 90 Days
90-day (ending 16 Jan 2011) % of average precipitation
90-day (ending 15 Jan 2011) temperature departures (degree C)

24. Watch these videos

25. Oaks, manzanitas now dieing
ENSO and Wildflowers
An even better show took place 1998
Vegetation prospers with rain, vice versa
Oaks, manzanitas now dieing

26. El Niño  Wet Winters in SW
Peak runoff SW rivers during El Niño years

29. Should we stick a toe in the water?
Expensive, but worth it

32. Niño
Niña

33. North Hemisphere Summer
Summer NH Circulation (weak)
Pacific and Bermuda highs over oceans
Continental low pressure
North Hemisphere Summer

34. North Hemisphere Summer
Summer NH Circulation (weak)
Pacific and Bermuda highs over oceans
Continental low pressure
North Hemisphere Summer

35. Gulf of CA Gulf of Mexico Monsoon Circulation
Surface low pressure at hot SW deserts
Surface high pressure over cool tropical oceans
Moisture source:
Gulf of CA
Gulf of Mexico

36. Gulf of CA Gulf of Mexico Monsoon Circulation
Surface low pressure at hot SW deserts
Surface high pressure over cool tropical oceans
Moisture source:
Gulf of CA
Gulf of Mexico

37. Monsoon Circulation
Hot air rises over land (forms clouds)
Cold air sinks over oceans (erases clouds)

38. Thermal Uplift
Hot air rises, cold air sinks
Very effective during summer
Looks like Hadley cell also

39. 6. Cooling air condenses 1. Incoming solar 7. Rain 5. Rising air cools
Thermal Uplift
Hot air rises, cold air sinks
Very effective during summer
Looks like Hadley cell also
5. Rising air cools
4. Hot air rises
2. Surface absoprtion
3. Surface emission

40. Tucson, Water Year-to-Date (2007)
We ended up ca. 1” below normal—few winter rains

41. Tucson rainfall by month through time
Winter is quite variable
Summer relatively consistent
Foresummer drought extremely faithful

42. Tucson rainfall by month through time
Winter is quite variable
Summer relatively consistent
Foresummer drought extremely faithful

43. Monsoon Rainfall & SW
Some modern societal segments “immune” to rainfall variation
Stored, imported, or ground water
Critical to non-irrigated agriculture
Upland “dry” farming
Ranching
Flooding.

44. Key Points
SW has bimodal (W) and Unimodal (E) rainfall
Arid Foresummer—always (?)
Winter
Frontal westerlies
El Niño
Decadal variability
Summer
Monsoonal southerlies
Fairly consistent