African Easterly Waves - PowerPoint Presentation

African Easterly Waves (Barotropic Instability, Structure) SO 442 – Tropical Meteorology

The Wave Moves Over WaterOnce the wave moves over the Atlantic, it loses the horizontal temperature gradient from which it was drawing its strength…what happens to the wave now? 1000 hPa 700 hPa L L   WAA   CAA  West East  Atlantic Ocean Africa Over the tropical ocean, density tends to be more horizontally uniform (since is more horizontally uniform) and varies mostly only with respect to   Such an atmosphere is referred to as a barotropic atmosphere So the question now is, does the wave continue to grow, and if so, by what process? Though the baroclinic mechanism (horizontal temperature/density gradient) is gone, the zonal flow in the vicinity of the AEJ remains horizontally sheared (i.e., faster easterly flow in the jet axis near ~12°N and slower flow to the south) jet So we’ll now consider the atmospheric reaction to a disturbance in the presence of such a sheared profile…

  North 8°N 4°N     AEJ max Consider the easterly flow as pictured Max vertical vorticity is found where the wind speed is increasing fastest with latitude (at the bottom)   Individual parcels translating westward in the flow have a rotational tendency proportional to the average of the flow at their latitude   Suppose a parcel at 4°N is displaced to the north, conserving its as it goes   The it introduces at 8°N induces the advection of parcels from lower latitudes (which wind up to its right) and parcels from higher latitudes (which wind up to its left)   Since the advected parcel to the right has higher than the advected parcel to the left, a northerly flow is induced over the original displaced parcel which sends it back toward 4°N   This flow profile is thus stable to parcel displacements

  North   North 8°N 8°N 4°N 4°N           AEJ max Now consider this easterly flow Max vertical vorticity is found where the wind speed is increasing fastest with latitude (at the inflection point in the profile)   Suppose a parcel at 4°N is displaced to the north, conserving its as it goes   The it introduces at 8°N is small compared to the ambient at 8°N and so the parcels to its left and right dominate the rotational tendency at the displaced parcel’s location   Since the advected parcel to the left has higher than the advected parcel to the right, a southerly flow is induced over the original displaced parcel which accelerates it farther away from its original position   This flow profile is thus unstable to parcel displacements AEJ max That rotational tendency induces the advection of parcels from lower latitudes (which wind up to its left) and parcels from higher latitudes (which wind up to its right)

  North   North 8°N 8°N 4°N 4°N           Jet max Jet max Thus for a steady barotropic flow with a velocity gradient, a criterion for the flow to be unstable to cross-flow displacements is that the flow have an inflection point in velocity profile (i.e., for a zonal flow changes sign somewhere in the flow)   Another way of stating this would be to say that a steady barotropic flow with a velocity gradient is unstable to cross-flow displacements if there is a change in the vorticity gradient somewhere in the flow This is a form of barotropic instability (the wave is drawing energy to grow from the momentum and vorticity of the ambient flow) In reality, this effect is also present over the African continent so that AEWs are said to grow through a combination baroclinic-barotropic instability … realistic for horizontal flow away from rigid boundaries which would be true for the AEJ (the zonal flow is not up against a “wall” to the south )

Africa Barotropic Instability Consider first the case of an AEJ that is purely zonal with no disturbances (i.e., no troughs/ridges) 20 kts 16 kts 12 kts 10 kts maximum   The cyclonic shear vorticity maximum straddles the inflection point in the velocity profile…thus a criteria for barotropic instability exists   North   The AEJ is only purely zonal in the average data…we’ll consider how barotropic instability feeds back on periodic disturbances within the jet (that would have grown by baroclinic mechanisms over the continent and then emerged over the Atlantic)

Africa A A B B Barotropic Instability We’ll now consider a wavy maximum and track what occurs at the points A and B   A and B translate westward over time due to the DCVA (and DAVA) mechanisms previously described The high vorticity parcels east of A have been displaced northward into faster flow closer to the AEJ core…induced vorticity advection there is thus strongly westward The high vorticity parcels east of B have been displaced southward into slower flow farther from the AEJ core…induced vorticity advection there is comparatively slow westward

Africa A A B B The high vorticity parcels east of A have been displaced northward into faster flow closer to the AEJ core…induced vorticity movement there is thus strongly westward The high vorticity parcels east of B have been displaced southward into slower flow farther from the AEJ core…induced vorticity movement there is comparatively slow westward The effect of this is that vorticity begins to become concentrated at A and depleted at B over time

Africa A A B B The high vorticity parcels east of A have been displaced northward into faster flow closer to the AEJ core…induced vorticity movement there is thus strongly westward The high vorticity parcels east of B have been displaced southward into slower flow farther from the AEJ core…induced vorticity movement there is comparatively slow westward The effect of this is that vorticity begins to become concentrated at A and depleted at B over time

Africa A A B B The high vorticity parcels east of A have been displaced northward into faster flow closer to the AEJ core…induced vorticity movement there is thus strongly westward The high vorticity parcels east of B have been displaced southward into slower flow farther from the AEJ core…induced vorticity movement there is comparatively slow westward The effect of this is that vorticity begins to become concentrated at A and depleted at B over time

Structure of AEWsWe’ve seen how AEWs may grow due to baroclinic processes over Africa and then due to barotropic processes over the Atlantic The extent to which these processes operate varies from case to case and, in the absence of a strong AEJ over the Atlantic, the barotropic mechanism may be weak In the absence of strong barotropic instability over the ocean, the wave would slowly weaken as it progresses westward Though weaker, it would still possess a structure where the 700 hPa trough lags the surface trough and the system leans eastward with height This structure is commonly observed with AEWs throughout their journey across the Atlantic and even into the Caribbean, Central America, and the eastern Pacific Indeed, the surface trough is maintained by the increasing advection of cyclonic vorticity with height overhead (DCVA)

divergence convergence Structure of AEWs 1002 mb 1004 mb 1006 mb 1008 mb 1010 mb The surface trough presents as an inverted V shape in the surface isobars The surface winds cross the isobars at a shallow angle due to friction The kink in the isobars associated with the trough results in surface wind convergence behind the trough axis and divergence ahead of the trough Surface convergence forces ascent which supports convection just behind the trough axis As previously discussed, the 700 hPa trough lags the surface trough The 700 hPa trough (near AEJ level) is also typically stronger than the surface trough such that the AEW has somewhat of a cold core structure—typical of baroclinic disturbances Since the convection lags the surface trough, it may become nearly collocated with the 700 hPa trough What is the significance of this? This also implies that CVA increases with height (i.e., DCVA exists) which provides forcing for ascent via 700 hPa divergence just downwind of the trough axis…the 700 hPa divergence maintains the surface trough

Africa A A B B Structure of AEWs Convection initially organizes in the convergent region behind the surface trough (nearly collocated with the 700 hPa trough) Sfc 700 Remember, the surface trough is forced by the differential (increasing with height) advection of cyclonic vorticity The 700 hPa trough axis is a lobe of maximum cyclonic vorticity (curvature and shear)

Africa A A B B As cyclonic vorticity accumulates in the vicinity of point A over time, the surface trough migrates just ahead of point A (since DCVA is forcing the surface trough) If adequate barotropic instability exists (i.e., a strong AEJ), the 700 hPa troughs will amplify over time causing induced vorticity movement along the shear zone (as discussed previously) Sfc Surface convergence, and resulting convection, remains maximized just behind the surface trough (now close to point A)

Africa A A B B As time moves on, cyclonic vorticity continues to increase at point A and convection continues behind the surface trough—which is just ahead of the cyclonic vorticity max at point A Sfc What is happening here??

Africa A A B B The (approximate) collocation of convection with the 700 hPa cyclonic vorticity maximum results in the convection acquiring mid-level (~700 hPa) cyclonic spin If that spin is able to subsequently be transferred to the surface by convective downdrafts and precipitation drag, a surface circulation may develop (an incipient disturbance) and begin to activate sea surface fluxes of sensible and latent heat in accordance with WISHE The organization and concentration of convection and latent heating by the developing spin begins to generate a zone where latent heating is maximized in convective updrafts and a warm core disturbance develops (tropical depression) The cyclonic vorticity filaments extending east and west from point A may play a role in developing the outer rain bands (or feeder bands ) of the developing disturbance

Structure of AEWsThe inverted V structure of AEWs (tropical waves) is clearly evident in microwave data of deep tropospheric moisture content t ropical waves The breakdown of waves into discrete patches of high cyclonic vorticity (tropical cyclones) is sometimes evident

21 Jul 2300Z22 Jul 1100Z22 Jul 2300Z 23 Jul 1100Z 23 Jul 2300Z 24 Jul 1100Z 25 Jul 0000Z 25 Jul 1200Z 25 Jul 2300Z 26 Jul 1100Z Structure of AEWs Tropical waves (AEWs) tend to move westward at about 10 to 15 kts on average though they may speed up, slow down, stall, or even retrograde depending on external forcing and their individual wavelength Disturbances tend to have a wavelength of ~3000-4000 km and a given location experiences the passage of a tropical wave about once every 3 to 4 days on average (marked by a wind shift and an increase in precipitation) That general propagation is evident by tracking individual disturbances via satellite imagery