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NATS 101 Lecture 19 Monsoons

Learn about monsoons, the differential heating between land and sea, and the thermally-direct circulation that causes major weather changes. Explore how monsoons occur globally and affect regions like Asia and North America.

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NATS 101 Lecture 19 Monsoons

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  1. NATS 101Lecture 19Monsoons

  2. DIV CON Rising Sinking CON DIV Heat Heat Review Land-Sea Breeze(Courtesy of Mohan Ramamurthy, WW2010) 5 6 Due to differential heating between land-sea Gives diurnal reversal in temperature contrast Onshore winds PM - Offshore winds AM Sea Breeze PM - Land Breeze AM

  3. DIV CON Rising Sinking CON DIV Heat Heat Review: Mountain-Valley Breeze Sun warms slopes Density decreases Air rises 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

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

  5. Monsoon • Seasonal Reversal of Prevailing Wind 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

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

  7. 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

  8. JANUARY Ahrens Fig 7.17a

  9. JULY Ahrens Fig 7.17b

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

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

  12. Monthly Average RainCherrapunji Aguado & Burt Fig 8-18

  13. January COLD offshore

  14. July HOT onshore

  15. Geography of Region Douglas et al (1993)

  16. Terrain • Terrain (300 m) • Steep slopes of Sierra Madre Occidental • Warm Waters 20oC water High mountains Steep western slope Sea of Cortez warm Pacific cool 30oC water Mountain Barrier Stensrud et al (1995)

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

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

  19. July minus June Rainfall Douglas et al (1993)

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

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

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

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

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

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

  26. Diurnal Winds 450 m AGL 5 PM LST 5 AM LST AZ moisture source upslope/onshore Stensrud et al (1995)

  27. Summary • Monsoons Differential Heating Between Land and 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

  28. Assignment • Topic- Global Scale Winds General Circulation • Reading -Ahrens 181-189, 343-349 • Problems -7.11, 7.12, 7.13

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