Chapter2 part 2 Flashcards
ENSO
(coupled ocean-atmospheric phenomenon)
El Nino - Southern Oscillation
ENSO components:
- Trade Winds
- Pressure Distribution (zonal distribution)
- Walker Circulation
- Precipitation
compare between Normal and Elnino
Normal:
- walker circulation
- rise is west and sinks in east (clockwise)
- Pressure system and SST
- Cold upwelling in South america (HIGH) and warm above Australia (LOW)
- trades
- Southeast
- thermocline
- high towards SA and low towards australia
- precipitation
- on the western side
Elnino:
- Walker Circulation
- rise in east and sinking is west
- Walker circulation either weekns or reverse
- Pressure and SST
- cold above australia (HIGH) and warm SST in central and east pacific equator (LOW)
- trades
- become weeker and may reverse (depending on SST)
- Thermocline
- sinks in east and rises in west
- Precipitation:
- on the eastern side
what is thermocline?
Zone of transition (similar to front)
occur in cold water (places of fish)
difference between normal circulation and la nina
la nina is stronger than normal circulation (area of thermocline rises near to the surface)
Southern Oscillation:
The seesaw variation between the surface atmospheric pressure of the eastern and western pacific
what is SOI?
the southern oscillation index
pressure of tahiti - pressure of darwin
compare different SOI values
In normal Circulation SOI>0
In El Nino SOI<0
to asses the strength of el nino you should
devide the pacific basin into three nino regions (eastern, center, western) the eastern and central witness more significant changes
the trade wind charactaristics:
- steady but have inter annual variations (IAV)
- mean velocity 3.6-7.2 m/s
- stronger in winter because the meridional temperature gradient is strong
- weaker in summer
trades layer structure:
- Sub cloud layer (bellow the cloud base)
- cloud layer
- Inversion layer (stable layer with NEGATIVE lapse rate)(Temp increasing w/ht)
- this is called trade wind inversion
trade wind inversion location
- Decending branch of the hadley cell and subsidence in the subtropical anticyclone
trade wind inversion structure:
two dimentional variation in space (zonal and meridional variation)
- the inversion height and strength vary spacially (2D variation) –> zonally and meridionally
compare between zonally and merionally variability
Zonally:
- Height: Inversion increase towards west
- strenth decrease towards west
meridionally:
- Height: Inversion increases towards the equator
- strength: decrease towards equator
subsidence - upwelling cold currents —> more intense
formation and maintenece of TWT depends on
the factos that lead to dO-/dz >0 (static stability)
factors that contribute to static stability:
- strong divergence (anticyclone) because of anticyclone it decreases E to W & 30 to the equator
- sensible heat flux (cold in surface making the air cold above it) its more in the east (air is warmer than the cold ocean currents or cold upwelling water)
Factors to distroy (destabilize) TWI
- NORMAL CONDITIONS: warm SST’s to the west (east upwelling and west warmest) lead to:
- More unstable
- Weakening of TWI
- The presence of Clouds
- through sensible heating
- turbulent mixing
- turbulent mixing occure only in thunderstorm clouds (trap long wave rad and reflect more radiation from the top)
- give rise to temperature decrease with height (unstable)
- Descending branch brings warm O- downwards (mixing). the ascending branch brings colder O-‘s upwards. this will cause weekening as the warmer heat flux is directed downwards weakening TWI.
- In clouds in general O- increases with height while temperature decrease with height
The ITCZ:
- Region of convergence of the trades from each hemisphere
- thermally generated trough of low pressure
structure of ITCZ:
- confluence zone. zone of:
- lowest pressure
- highest temp
- max wind
- convergence zone. zone of:
- maximum cloudiness and
- rainfall
distance between confluence and conv.
300 - 1000 km
the seperation of the regions of …………. is necessary for the ……….
maximum temperature and pressure (confluence) and maximum cloudiness (divergence) is necessary for the maintenance of the thermal trough
speration between regions of maximum temperature and pressure (confluence) and maximum cloudiness (divergence) is necessary for the maintenance of the thermal trough because
maximum cloudiness decreases solar energy (-ve feedback) for mainaining ITCZ
explation the seperation of the two zones
- corriolis force turns the cross equatorial flow anti cyclonically
- in the regions of maximum anticyclonic curvatire the flow is faster
- the deceleration of the flow downstream (the same direction the wind is moving) results in convergence before reaching the equatorial trough)
the position and movement of ITCZ:
the mean position is 15N and 5 s in northern hemisphere and southern hemispherre in summer
What would happen to the ITCZ if the earth surface was uniform:
- the ITCZ would be parallel to the latitude
- its annual migration would be symmetric with a reference to the equator
the asymetry in the ……………….. is due to …………
position and migration is due to land - ocean contrast
circulation associated with ITCZ:
- trade wind trough (closer to the equator) - trade wind confluence with no directional shift across the trough
- monsoon trough: trough with directional shift along the equator
……………………. is proportional to …………………
the variation of zonal wind with height is proportional to the meridional temperature gradient
do ug/do z is proportional to -do T/do y
(thermal wind relationship)
if temperature decreases poleward
westerlies increase with ht (ug>0)
easterlies decrease with ht (ug<0)
