NATS 101 Lecture 19 Monsoons

Supplemental References for Today’s Lecture Aguado, E. and J. E. Burt, 2001: Understanding Weather & Climate, 2 nd Ed. 505 pp. Prentice Hall. (ISBN )

Review Land-Sea Breeze (Courtesy of Mohan Ramamurthy, WW2010) 56 RisingSinking DIV CON Heat 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

Review: Mountain-Valley Breeze Ahrens, Older Ed. Sun warms slopes Density decreases Air rises IR cools slopes Density increases Air drains RisingSinking DIVCON Heat CON DIV 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 Heat WarmColdRisingSinking DIV CON Heat Less Dense More Dense

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

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

Ahrens Fig 7.17a JANUARY

Ahrens Fig 7.17b JULY

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

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

Monthly Average Rain Cherrapunji Aguado & Burt Fig 8-18

January offshore COLD

July onshore HOT

Geography of Region Douglas et al (1993)

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

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

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

July Average Rainfall Peak Values West Coast of Mexico West Slopes of Sierra Madre Occidental July Rain (mm) Douglas et al (1993)

July minus June Rainfall Douglas et al (1993)

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

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

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

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

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

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

July 700 mb Flow Douglas et al (1993)

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

Diurnal Winds 450 m AGL upslope/onshore AZ moisture source Stensrud et al (1995) 5 PM LST5 AM LST

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

Assignment Topic- Global Scale Winds General Circulation Reading - Ahrens , Problems , 7.12, 7.13