NATS 101 Lecture 19 Monsoons

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NATS 101Lecture 19Monsoons

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Supplemental References for Todays Lecture

• Aguado, E. and J. E. Burt, 2001 Understanding
  Weather Climate, 2nd Ed. 505 pp. Prentice Hall.
  (ISBN 0-13-027394-5)

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Review Land-Sea Breeze(Courtesy of Mohan
Ramamurthy, WW2010)
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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
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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
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Thermally Direct Circulation
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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

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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

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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

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JANUARY
Ahrens Fig 7.17a
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JULY
Ahrens Fig 7.17b
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Asian Winter
H
Cherrapunji
offshore flow
Aguado Burt Fig 8-17
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Asian Summer
L
Cherrapunji
onshore flow
Aguado Burt Fig 8-17
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Monthly Average RainCherrapunji
Aguado Burt Fig 8-18
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January
COLD
offshore
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July
HOT
onshore
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Geography of Region
Douglas et al (1993)
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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)
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Monthly Rainfall
Douglas et al (1993)

• Mexican Monsoon
• Similar onset
• Similar behavior but
• Much less intense
• A Wimp Compared to Asian Monsoon

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Percentage of Annual Rainfall

• Accounts for up to 70 total rain in monsoon core
• Tucson 50
• Phoenix 40

Douglas et al (1993)
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July Average Rainfall

• Peak Values
• West Coast of Mexico
• West Slopes of Sierra Madre Occidental

July Rain (mm)
Douglas et al (1993)
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July minus June Rainfall
Douglas et al (1993)
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Monsoon Evolution from Satellite
June

• CCT lt -38oC Frequency
• Centered over W. Mexico
• June start over Mexico
• AZ at northern fringes of heart of monsoon
• Rains reach SE Arizona by July

July
August
Douglas et al (1993)
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June-July 500 mb Flow
Dry
Dry
Moist
Moist
Douglas et al (1993)
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May 500 mb Flow
Dry
Moist
Douglas et al (1993)
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June 500 mb Flow
Dry
Moist
Douglas et al (1993)
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July 500 mb Flow
Dry
Moist
Douglas et al (1993)
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Monthly Winds and RH (Guaymas, Sonora)
Dry
Dry
Moist
Douglas et al (1993)
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July 700 mb Flow
Douglas et al (1993)
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July 900 mb Flow
Mountains block flow of low-level moisture from
Gulf of Mexico
tropical moisture
Douglas et al (1993)
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Diurnal Winds 450 m AGL
5 PM LST
5 AM LST
AZ moisture source
upslope/onshore
Stensrud et al (1995)
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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

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Assignment

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