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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 1 st baroclinic mode atmospheric structure 30-60 day period Propagation ~5-10 ms -1 to east
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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.
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) • 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
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
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
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