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NATS 101 Lecture 19 Monsoons

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Page 1: 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

NATS 101

Lecture 19Monsoons

Page 2: 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

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

5 6

RisingRising SinkingSinking

DIV

DIV

CON

CONHeatHeat

Due to differential heating between land-seaGives diurnal reversal in temperature contrastOnshore winds PM - Offshore winds AM Sea Breeze PM - Land Breeze AM

Page 3: 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

Review: Mountain-Valley Breeze

Ahrens, Older Ed.

Sun warms slopes Density decreases Air rises

IR cools slopes Density increases

Air drains

RisingRising SinkingSinking

DIV CON

HeatHeat

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

Page 4: 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

Thermally Direct Circulation

Heat Heat

WarmWarm ColdColdRisingRising SinkingSinking

DIV

DIV

CON

CON

HeatHeat

Less Less DenseDense

More More DenseDense

Page 5: 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

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

Page 6: 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

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

Page 7: 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

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

Page 8: 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

Ahrens Fig 7.17aJANUARY

Page 9: 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

Ahrens Fig 7.17bJULY

Page 10: 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

Asian Winter

H

Aguado & Burt Fig 8-17

offshore flow

Cherrapunji

Page 11: 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

Asian Summer

onshore flow

Aguado & Burt Fig 8-17

L

Cherrapunji

Page 12: 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

Monthly Average RainCherrapunji

Aguado & Burt Fig 8-18

Page 13: 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

January

offshoreoffshore

COLDCOLD

Page 14: 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

July

onshoreonshore

HOTHOT

Page 15: 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

Geography of Region

Douglas et al (1993)

Page 16: 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

Terrain

• Terrain (300 m)• Steep slopes of Sierra

Madre Occidental • Warm Waters30 oC water

Stensrud et al (1995)

High mountains

Steep western slope

Sea of Cortez warm Pacific cool

20 oC w

ater

Mountain Barrier

Page 17: 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

Monthly Rainfall

Mexican Monsoon

Similar onset

Similar behavior but…

Much less intense

A Wimp Compared to Asian Monsoon

Douglas et al (1993)

Page 18: 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

Percentage of Annual Rainfall

Accounts for up to Accounts for up to 70% total rain in 70% total rain in monsoon coremonsoon core

Tucson Tucson ~50%~50%

Phoenix Phoenix ~40%~40%Douglas et al (1993)

CORE

Page 19: 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

Monsoon Evolution from Satellite

CCT < -38oC 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

AugustDouglas et al (1993)

Page 20: 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

June-July 500 mb Flow

Moist

Dry

Douglas et al (1993)

Moist

Dry

Page 21: 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

July 900 mb Flow

tropical moistureDouglas et al (1993)

Mountains block flow of low-level moisture from Gulf of Mexico

Page 22: 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

Diurnal Winds 450 m AGL

upslope/onshore

AZ moisture source

Stensrud et al (1995)

5 PM LST 5 AM LST

Page 23: 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

Summary

• MonsoonsDifferential Heating Between Land and OceansSeasonal Reversal of WindSummer Rain - Winter AridityThermally-Direct Circulation

• Regions

Major Monsoon Occurs over SE Asia

Weaker Monsoon Occurs over North America

Africa, Australia, South America in SH

Page 24: 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

Assignment

• Topic- Global Scale Winds

General Circulation• Reading - Ahrens 181-189, 343-349• Problems - 7.11, 7.12, 7.13