MJO, ENSO, QBO, and Other Patterns

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 low-

level 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)


• 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


• 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


• 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


• 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


• It’s all about MEI!



• 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


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


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 periodstrongest 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


• 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


• North Atlantic Oscillation (NAO)Pressure difference between Bermuda-Azores

high and Icelandic lowPositive 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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MJO, ENSO, QBO, and Other Patterns