NATS 101

Lecture 19Monsoons

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

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

Thermally Direct Circulation

Heat Heat

WarmWarm ColdColdRisingRising SinkingSinking

DIV

DIV

CON

CON

HeatHeat

Less Less DenseDense

More More DenseDense

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 7.17aJANUARY

Ahrens Fig 7.17bJULY

Asian Winter

H

Aguado & Burt Fig 8-17

offshore flow

Cherrapunji

Asian Summer

onshore flow

Aguado & Burt Fig 8-17

L

Cherrapunji

Monthly Average RainCherrapunji

Aguado & Burt Fig 8-18

January

offshoreoffshore

COLDCOLD

July

onshoreonshore

HOTHOT

Geography of Region

Douglas et al (1993)

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

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 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

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)

June-July 500 mb Flow

Moist

Dry

Douglas et al (1993)

Moist

Dry

July 900 mb Flow

tropical moistureDouglas 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 LST 5 AM LST

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

Assignment

• Topic- Global Scale Winds

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