NATS 101 Lecture 19 Monsoons Review LandSea Breeze

NATS 101 Lecture 19 Monsoons

Review Land-Sea Breeze (Courtesy of Mohan Ramamurthy, WW 2010) 6 Due to differential 5 heating between land-sea Gives diurnal reversal in temperature contrast Onshore winds PM - Offshore winds AM Sea Breeze PM - Land Breeze AM DIV CON Rising Sinking CON DIV Heat

Review: Mountain-Valley Breeze DIV Sun warms slopes Density decreases Air rises CON Rising Sinking CON DIV Heat IR cools slopes Density increases Air drains Ahrens, Older Ed. Differential heating along mountain slopes Gives diurnal reversal in temperature contrast Upslope winds PM - Downslope winds AM Valley Breeze PM - Mountain Breeze AM

Thermally Direct Circulation DIV CON Heat Warm Rising Sinking Cold CON DIV Heat

Monsoon • Seasonal Reversal of Prevailing Wind shift often accompanied by Major Change in Weather Summer Rains - Often Abrupt Onset Winter Dryness Major Monsoon occurs over Asia Weaker Monsoon occurs in North America

Monsoon • Land mass is colder than ocean in winter • Land-sea temp contrast reverses in summer • Wind forced by seasonal changes in PGF Higher SLP over land in winter Offshore flow at Surface Lower SLP over land in summer Onshore flow at Surface

Monsoon • Onshore flow leads to surface convergence Rising motion over land during summer • Offshore flow leads to surface divergence Sinking motion over land during winter • Monsoon is Thermally Direct Circulation Warm Air Rises - Cold Air Sinks

JANUARY Ahrens Fig 7. 17 a

JULY Ahrens Fig 7. 17 b

Asian Winter H Cherrapunji offshore flow Aguado & Burt Fig 8 -17

Asian Summer L Cherrapunji onshore flow Aguado & Burt Fig 8 -17

Monthly Average Rain Cherrapunji Aguado & Burt Fig 8 -18

January COLD offshore

July HOT onshore

Geography of Region Douglas et al (1993)

Terrain o. C 20 • Terrain (300 m) • Steep slopes of Sierra Madre Occidental • Warm Waters wa 30 o ter High mountains C r Stensrud et al (1995) te wa Mountain Barrier Steep western slope Sea of Cortez warm Pacific cool

Douglas et al (1993) Monthly Rainfall Mexican Monsoon Similar onset Similar behavior but… Much less intense A Wimp Compared to Asian Monsoon

Percentage of Annual Rainfall Accounts for up to 70% total rain in monsoon core Tucson ~50% Phoenix ~40% Douglas et al (1993)

July minus June Rainfall Douglas et al (1993)

Monsoon Evolution from Satellite June July August CCT < -38 o. C Frequency Centered over W. Mexico June start over Mexico AZ at northern fringes of heart of monsoon Rains reach SE Arizona by July Douglas et al (1993)

June-July 500 mb Flow Dry Moist Douglas et al (1993)

May 500 mb Flow Dry Douglas et al (1993) Moist

July 500 mb Flow Dry Moist Douglas et al (1993)

Monthly Winds and RH (Guaymas, Sonora) Dry Moist Douglas et al (1993)

July 900 mb Flow Mountains block flow of low-level moisture from Gulf of Mexico tropical moisture Douglas et al (1993)

Diurnal Winds 450 m AGL 5 PM LST e or sh /on pe slo up Stensrud et al (1995) 5 AM LST AZ moisture source

Summary • Monsoons Differential Heating Between Land Oceans Seasonal Reversal of Wind Summer Rain - Winter Aridity Thermally-Direct Circulation • Regions Major Monsoon Occurs over SE Asia Weaker Monsoon Occurs over North America Africa, Australia, South America in SH

Assignment • Topic- Global Scale Winds General Circulation • Reading - Ahrens 181 -189, 343 -349 • Problems - 7. 11, 7. 12, 7. 13