MJO, ENSO, and QBO

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MJO, ENSO, QBO, and Other
Patterns
Madden Julian Oscillation (MJO)
• Wavenumber-1 (possibly, wavenumber 2 at
times) equatorially trapped Kelvin wave which is
convectively coupled
• 1st baroclinic mode atmospheric structure
• 30-60 day period
• Propagation ~5-10 ms-1 to east
• Most prominent signal over Indian Ocean as well
as Western and Central Pacific
• Sometimes weak or virtually absent; generally
strongest in boreal winter (austral summer)
Madden
Julian
Oscillation
(MJO)
• Structure and
evolution of wave
Madden Julian Oscillation (MJO)
• UL Divergence and Convergence strongest and most
consistent signal global
• In addition to UL Div and Con, may be seen through lowlevel wind strength and div, RH, OLR, PW, T, P, precip
• Remember, large LR, so Div/Con and convection can force
equatorial waves, not necessarily the other way around
• Kelvin wave may induce surface wind stress anomalies,
affecting the upper ocean and sometimes affecting ENSO
• Often affects areas by enhancing or suppressing
convection for ~2-3 weeks in an ocean basin, which also
impacts tropical cyclogenesis and monsoons
Madden Julian Oscillation (MJO)
• Hovmoller of OLR
anomalies and horizontal
plots of OLR anomalies- not
always obvious in latter
Madden Julian Oscillation (MJO)
• Easterly
propagating
Kelvin wave
(negative slope,
propagate to the
right)
• Higher frequency
waves- here,
WIG- embedded
within (positive
slope, propagate
to the left)
Madden Julian Oscillation (MJO)
• Velocity potential at 200 mb and upper ocean heat
anomalies- MJO can affect surface wind stress (proportional
to wind speed squared, with west anomalies warming here)
Walker Circulation
• Walker Circulation best developed in South
Pacific tropics during their summer and early
fall.
• Zonal circulation between dry eastern Pacific,
with low SSTs, UL con and LL div, and moist
western Pacific, with high SSTs, UL div and LL
con.
• Often weakens temporarily in midwinter as
part of the annual cycle.
Walker Circulation
El Nino-Southern Oscillation (ENSO)
• Irregular periods of warming and cooling of
upper ocean in eastern and central equatorial
Pacific; period about 2-7 yr
• Warm phase (El Nino); Cool (La Nina)
• Maximum amplitude of El Nino exceeds La
Nina, probably due to average background
state
• El Ninos typically have shorter durations (1 yr
v. 1-3 yr on av)
El Nino-Southern Oscillation (ENSO)
• ENSO warm phase onset: SE Pacific surface high
generally weakens
• Pressure gradient decrease: trades weaker and
westerly wind anomalies
• SE Pac SST increase due to less upwelling
• Increased SST in eastern and central Pacific helps
lead to increased rainfall
• Equatorial Kelvin and n=1 Rossby waves also appear
to do similarly to trade wind and ocean effects as
well as deepen eastern and central Pacific
thermocline
El Nino-Southern Oscillation (ENSO)
• Negative feedback: as central and eastern equatorial
Pacific SSTs increase, pressure near equator falls
• Pressure gradient between subtropical high and ITCZ
increases
• Trade wind strength increases, which would increase
upwelling of cooler water from below
• Also remember (complicating factors): ocean
response and adjustment is longer than atmosphere
• Other wave forcing is likewise important
• LH release in mid to upper troposphere, as well
(ENSO)
La Nina is
simply an
amplification
of “normal”
conditions
El Nino-Southern Oscillation (ENSO)
• ENSO warm phase: ITCZ in eastern and central
Pacific shifts equatorward from Northern
Hemisphere
• Enhanced UL Div in Pacific: intensify UL High
and Subtropical Jet (STJ) and displace STJ
further south
• Walker Circulation strength decreases while
Hadley Cell strength increases
El Nino-Southern Oscillation (ENSO)
• Various hypotheses to try to explain ENSO formation;
equatorial Kelvin and n=1 Rossby waves important to
many conjectures and are supported by observations
• Various ways to measure ENSO via indices; a robust
measure is the Multivariate ENSO Index (MEI): 6
variables- SLP, surface wind zonal component,
surface wind meridional component, SST, surface air
T, and total cloud fraction; bimonthly measure and
takes first principal component of all 6 combined
fields
(ENSO)
• Delayed
Oscillator
Model
El Nino-Southern Oscillation (ENSO)
El Nino-Southern Oscillation (ENSO)
• It’s all about MEI!
El Nino-Southern Oscillation (ENSO)
• Teleconnection patterns can be modified by
ENSO (and vice versa)
• Some areas are strongly correlated with ENSO
according to surface T and P, precipitation
• Shear and SST patterns altered in ocean
basins, often affecting regional convection,
monsoons, and TCs
El Nino-Southern Oscillation (ENSO)
FYI… not be tested on this:
-Warm ENSO and TC correlations:
• N.H. number overall decrease
• More in Eastern North Pacific
• Less in North Atlantic
• Tracks shift eastward in Western North Pacific
• S.H. numbers increase, still not as many as N.H. in
total, though
• Numbers increase particularly e of Australia
El Nino-Southern Oscillation (ENSO)
Quasi-Biennial Oscillation (QBO)
• UL winds in lower equatorial stratosphere (20-50
hPa) oscillate between east and west
• 28 month mean period; varies from ~20-36 months
• Easterly winds stronger
• Winds propagate downwards ~ 1 km/mo
• W momentum from vert propagating equatorial
trapped Kelvin waves, while mixed Rossby gravity
waves provide e momentum
• May effect vertical wind shear and deep convection
in various ocean basins
Quasi-Biennial Oscillation (QBO)
• Earlier link of QBO to North Atlantic TC activity is
now in doubt. Appears not to alter tropospheric
VWS as much as thought earlier.
Some Sources Exhibiting Decadal
Variability
• Pacific Decadal Oscillation (PDO)
20-30 year period
strongest in boreal winter
positive phase: warm W Coast of N America,
central and eastern Pacific warm, too;
Kuroshio and Oyashio relatively cold
opposite for negative phase
PDO
Some Sources Exhibiting Decadal
Variability
• Atlantic Multidecadal Oscillation (AMO):
Period 20-40 yr in SST and VWS
Shift in subtropical ridges and jets
Warm phase, such as 1930’s: more hurricanes,
less US rain and ne S America; weaker links
also include more S Atlantic rain as well as in n
Eur, w Afr, and se US; Midwest drought
AMO
Some Sources Exhibiting Decadal
Variability
• North Atlantic Oscillation (NAO)
Pressure difference between Bermuda-Azores
high and Icelandic low
Positive when stronger pressure difference,
which also increases strength of trade winds in
the tropical Atlantic, generally increasing
upwelling and VWS
NAO
NAO
• End of Material for Test 2
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