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

MAT (°F) MAP (in.) JFMAMJJASOND

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

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

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

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

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.

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

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

Average Winter Flow Slightly north of the SW

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

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

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

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

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

La Niña Pacific moisture goes north, misses SW

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

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)

Watch these videos http://www.youtube.com/watch?v=fN_NmCpry38&feature=related http://www.youtube.com/watch?v=1oKIJIfpR6U&feature=related http://www.youtube.com/watch?v=OsYneLonQF4&feature=related

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

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

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

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml

Niño Niña

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

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

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

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

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

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

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

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

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

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

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.

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