Tropical Atmosphere Test 1

Edmund Halley

1686, had a theory for the NE and SE trade winds that the winds were the result of cool dense air flowing to a region of hot air, then rises

George Hadley

1735, proposed a single axisymmetric cell in each hemisphere that transports heat from the tropics to the poles

Lord Kelvin

1857 & 1892, presented a more complex near surface complication but retained the single "Hadley cell" circulation aloft

Where is the Hadley circulation confirmed?

Between the equator and subtropics, plus/minus 30deg

William Ferrel

1856-1861, proposed midlatitude circulation cells with westerly winds were caused by air deflected by the Coriolis force

What causes the subtropical jet at 30deg lat?

air flowing upward and poleward in the Hadley cell

Trade winds

lowest branch of the Hadley cells that play a significant role in shaping the weather and ocean interactions in the tropics

Tropical Easterly Jet

What controls the subtropics and stj?

angular momentum conservation

Main reasons for existing global circulation pattern

earth's rotation, atmospheric gases change density in response to temperature changes, topography of earth, unequal heating of earth

what jet stream has substantial influence on south asian monsoon weather

tropical easterly

barotropic

tropospheric atmosphere! A situation that results in upper-level weather maps having height contours and isotherms parallel to each other.

baroclinic

midlatitude atmosphere! A situation that results in upper-level weather maps having height contours and isotherms that intersect each other. This situation may enhance or reduce upper-level divergence or convergence.

mechanisms fundamental to global monsoons

processes of moist convection that modulate cloud distribution and radiative effects, seasonal oscillation of net heating in the summer hemisphere and associated migration of the equatorial trough and rainfall, rotation of the earth which causes curving of large-scale cross-equatorial flow, ability of land to gain and lose heat faster than water

source of moisture for south american monsoon

tropical atlantic and amazon basin

ITCZ

Intertropical Convergence zone. Belt of low pressure around the equator. formed by the vertical ascent of warm, moist air from the latitudes north and south of the equator. Lots of precipitation.

ITCZ qualities

has its mean latitude in the Northern Hemisphere and in a region where convective lifting is common

Classical criteria for monsoons

-prevailing wind shifts 120deg btwn Jan-July
-average frequency of prevailing wind >40%
-speed of mean wind exceeds 3m/s
-pressure patterns satisfy steadiness criterion

When is the asian monsoon

early onset: southern bay of bengal in late april
by early may it's over Indo-chinese peninsula and south india
reaches japan late june

South Asian monsoon is what branch

northern branch of the seasonal migration of the ITCZ

South Asian monsoon precipitation belt

migrates from the southern to the northern hemisphere in the boreal summer and vice versa in the boreal winter

Most northern part of the South Asian tropical rain belt

20degN and retreats 5degS during winter

South Asian monsoon favorable rain band

over the heated subcontinent and the warm eastern equatorial indian ocean

what regulates annual monsoon season

heat transported across the equator by the ocean and atmosphere

East Asian monsoon timing

clouds and evaporation increases mid-june, longitudinal extent of the southwesterly winds shifts east into pacific
pacific high shifts E and N after the mid-may onset, expands, and strengthens

Mei-Yu/Baiu front location + time

begins mid-may through mid summer shift north, semi-permanent, quasi-stationary, weak frontal zone that extends from eastern china E-NEward into pacific

Mei-yu front

focal point for persistent heavy precipitation produced by mesoscale convective systems that form and track eastward along the front

When does Australian-Maritime Continent monsoon happen

onset over Malaysia in late august, reaches most south in february over north Australia

Maritime monsoon heat source

near the equator, thus not strongly affected by earth's rotation

where does most precipitation over the Maritime continent come from

mesoscale convective systems that are forced by sea/land breezes or large-scale disturbances

Hovmöller diagram

plots meteorological data to highlight the role of waves. axes are lat/long and time. invented by Ernest H in 1949

how to use Hovmöller diagram

We set the boundaries of the area to focus on. In this case, it was most of Africa, from 20°W to 55°E longitude and 30°N to 30°S (avoiding locations in South Africa and dry northern Africa where rains are not dominated by the movement of the ITCZ).
Starting with January 1, we averaged the rainfall amounts in all the grid boxes (pixels) across each latitude strip between 30°N and 30°S. For example, we averaged all the rainfall values along the equator from 20°W to 55°E.
Then we repeated Step 2 for every day in the year (365 days). Each day's data is one column in the Hovmöller Diagram. Imagine that each day is like a book tucked vertically into a bookshelf, spine facing out. Each day is a different book on the shelf, a different story of the rainfall patterns from north to south on each day of the year.

Walker circulation

zonal distribution fo tropical surface heating (both continental and oceanic) produces and E-W circulation.
cycles between neutral (normal walker circ), el nino, and la nina

How does ENSO mess with Walker Circulation

triggers major shifts in rainfall and convection, disrupting atmospheric circulations and climate across the globe

how long is the walker circulation

2-7 years

El Nino

first noticed in ocean when water was way too warm in Decemer 1980
trade winds weaken and SST in equatorial pacific rise. high moves from asia to low makes it hot water in the middle of equatorial basin and no upwelling bc everything is so hot, we get nothing in the Atlantic bc of this bc the water is so choppy

how long does el nino last

9-15 months

La Nina

stronger than normal trade winds and cold SSTs, less extreme than el nino. high moves from north America to asia and makes a warm water thing by asia lots of upwelling from south america

how long is la nina

1-3 years

Southern Oscillation Index

talks about a common metric to identify ENSO, proxy for strength of trade winds

SOI Equation

a. SOI= 10 x ([Pdiff-Pdiffav]/ SD(Pdiff)); Pdiff= Tahiti MSLP- Darwin MSLP monthly averaged, Pdiffav= long term average of Pdiff for the particular month, SD(Pdiff)= long term standard of deviation of Pdiff, the multiplication by 10 is a convention.
i.

SOI Negative

pressure gradient is weaker than usual; corresponds to El nino (or ENSO warm event).

SOI positive

bigger difference means stronger Pacific trade winds and warmer sea temperatures to the north of Australia; corresponds to la nina

Evidence of ENSO signature

SST, mixed layer depth, upper ocean currents, surface wind stress, shift in Walker Circulation

Anamolous westerly winds

form when trades weaken or westerly winds develop with large scale phenom (MJO)

Conditions for westerly wind bursts

5m/s wind speed, cover at least 10deg lat, last for at least 2 days, center longitudes of 2 continuous days are within 7.5deg of each other

What phase of MJO increased frequency in tropical cyclones?

Active MJO