ES 520 - Monsoons

monsoons arise from the coupled interactions between the ___________ and _______

atmosphere, ocean

there is a net ______ in outgoing radiation when you have conducting storms (i.e. low clouds which you get with monsoons)

loss

monsoons can be defined either _______ or _________

regionally, globally

seasonal variations in winds and precipitation are due to...

seasonal variations in heating and T, p gradients

role of the sierra madres in the NAM (barron et al)

sierra madre oriental - acts as a barrier to low-level moisture transport from the GoM into the western part of the monsoon region sierra madre occidental - channels and confines low-level moisture from the tropical pacific up the axis of the GoC toward Arizona/western New Mexico

somali jet

the winds resulting from the pressure gradient

onset of SAM (arias et al)

- NAM retreats southward from the southwestern US and w/ mexico to central american during sept-oct then into N south america by nov-dec - wet season spreads from equatorial w/ amazon to east and southeast - by oct, monsoon activity starts over amazon and brazilian highlights - mature SAM form late nov to late feb, where large amounts of precip fall across amazon basin - march → may, regions of high precip reduce in size and migrate slow toward the equator, in association with a southerly regime of the low-level cross equatorial flow, indicating the retreat of SAM and transition toward a NAM regime

why are monsoons so important?

- affect ~3 million people - critical for agricultural planning, flood, and drought mitigation

south american monsoon (SAM)

- affects brazil, argentina, bolivia, and paraguay - not as strong as indian monsoon but result of the same mechanism - still not well defined

african monsoon

- african low level jet that brings moisture into N. Africa and the opposite in northern hemisphere winter - moisture sources from the atlantic

western north pacific monsoon

- an oceanic component of the asian monsoon system - important for tropical cyclone tracks in the pacific

monsoon circulation

- arise from seasonal reversals in heating and temperature gradient between land and ocean - think land and sea breezes which is the same idea on a very small scale

asia-australian monsoon

- associated with the seasonal migration of heating between the northern and southern hemispheres

onset of NAM (arias et al)

- between july and september - upper level anticyclone develops in june/july and low level heat low is formed which disrupts the normal zonal wind flow - NAM precip is driven by northward migration of the ITCZ along w/ convectional uplift generated by the thermal low (primarily over the mexican plateau) which entrains atmospheric moisture derived from nearby ocean sources inlcuding the Gulf of CA and Gulf of MX

asian monsoon

- can be divided into subsections: 1. indian summer monsoon/south asian monsoon 2. east asian monsoon

east asian monsoon

- carries moist air from south asia to east asia - affects approximately one third of global populations (china, korea, japan) - driven by temperature differences between asia and the pacific ocean

monsoons

- derived from the arabic word for season - seasonal reversal in the direction of the wind - monsoonal regions defined by significant change in the wind direction between winter and summer - extends over large part of the tropics

why is the north american monsoon weaker than than its indian/asian counterparts?

- due to the mexican plateau not being as high/large as the tibetan plateau

progression of the asian monsoon

- earliest onset: southern Bay of Bengal in late April - over the indo-chinese peninsula and south india in early may - progresses north and northwestward into the continent reaching japan by late june to july - by the end of the peak season over japan, the monsoon is retreating over india

what do holocene ssts indicate about the north american monsoon ? (barron et al)

- indicate that SSTs from 11000-6200 BP were too cold to support modern climatology in the southwestern US

the global monsoon

- monsoon climates characterized by seasonal reversal of winds, this reversal is global not just regional - regional monsoons are coordinated primarily by the annual cycle of solar heating

what is hypothesized to have triggered the change in the NAM climatology between the holocene and present day? (barron et al)

- paleo proxies indicate that the NAM use to impact a greater portion of north america but has since narrows - hypothesize that the change may have been triggered by new upwelling in baja california

what is the mechanism behind a shortening of the wet seasons over the american monsoon systems since the late 1970s? (arias et al)

- positive AMO phase - northward shift of subtropical jets over N. America - westward shift of the NASH

North american monsoon (NAM)

- provides ~60% of annual precip to central/northern mexico and ~35-45% of mean annual precip to arizona and new mexico - not really "discovered" until 1970s - not as strong as indian monsoon but result of the same mechanism

what is driving the increase on the length of the transition period of the NAM/SAM (arias et al)

- shortening of the wet season

South asian/ indian summer monsoon

- strongest and best known of the regional monsoons - carries moist air from the indian ocean to south asian - accounts for approximately 80% of the rainfall in india - driven by strong heating over the tibetan plateau, which reverses the low-level pressure gradient

ocean atmosphere feedbacks

- the ocean moves energy from summer hemisphere to winter hemisphere while the atmosphere moves energy from the winter hemisphere to summer hemisphere - monsoon seasonal cycle modulated by negative atmosphere-ocean feedback associated with ekman transport

simulating the asian monsoon

- when models are averaged you get closer to the truth than the models alone - cmip 3 and 5 both simulate lower precipitation than reality - wind simulated better than rainfall which makes sense because we understand this better than precip

american monsoons

NAM: multiple moistire sources -> pacific probably contributes more SAM: sameish mechanism - there is debate over whether these are true monsoons