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African Easterly Wave trough

Peak rainfall ahead of trough

2-3000km Wavelength

11-12oN

Relative Vorticity 700hPa (10-5s-1)

Average precipitation rate (mm/day)

Note distinctive horizontal tilts

Note distinctive vertical tilts

Consensus has developed in the community that AEWs arise in association with a linear instability mechanism

Recall the Charney-Stern necessary conditions for instability:

In the fluid interior and

at the surface have opposite signs

or

has opposite signs in the fluid interior

925hPa q

315K PV

- PV ‘strip’ present on the cyclonic shear side of AEJ.

- Strong baroclinic zone 10o-20oN

925hPa qe

- High qe strip exists near 15oN

See Dickinson and Molinari (2000) for more discussion of PV sign-reversals)

Note definition of meridional gradient in qgpv

Discussion of Interacting Rossby Waves

Different types of modeling are possible:

Idealized modeling

Climate modeling using a global model

Regional modeling (e.g. for case studies)

Here modeling of the AEWs and AEJ are both considered.

Nonlinear life-cycles

Following is an animation of a predicted AEW sequence in WRF (Berry)

Basic State:

AEJ is maintained by deep moist convection to the south and dry convection to the north (tephigram construction)

From a PV-thinking perspective in addition to the surface temperature gradient the AEJ is associated with two distinctive PV anomalies – a negative one linked to the dry convection and a +ve one linked to the moist convection (and to a lesser extent, descent in the heat low circulation)

AEWs:

Idealised modeling confirms that AEWs can grow via a mixed barotropic-baroclinic instability mechanisms. Normal modes tend to be dominated by barotropic growth. As these modes grow and become more nonlinear they become more dominated by baroclinic growth.

Climate modeling studies tend to be more about the models than the processes (biased view).

Regional Modeling can be used for process studies.

AEWs and AEJ

So the observed WAM is characterized by convective processes that maintain the AEJ and processes linked to the growth of AEWs that weaken the AEJ. The observed climate represents the long-term balance of these two aspects of the WAM system.

Question: Do we expect an active AEW year to be associated with a strong AEJ or weak AEJ?

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