551 Tropical Advanced Topics

Trade Wind Inversion

One starts with subsidence in the subtropics

But the subsidence is not zonally uniform: greater in the eastern side of the oceans

The resulting greater subsidence on the eastern sides depressing heights on the eastern sides of the ocean relative to the central portions

But why more subsidence on the eastern side? Asymmetry of meridional motions Northerly on western side Southerly on eastern side Thus, air parcels are experiencing progressively smaller f on the eastern side. If relative vorticity is maintained, need differential subsidence to balance.

Trade Wind or Subtropical Inversion The height of the base of this inversion varies from about 500 m at the eastern extremities of the subtropical highs to about 2000 m at the western and equatorial extremities. In the equatorial trough zone and over the western portions of the trade-wind belt, the inversion does not exist as a mean condition, although it appears in certain weather patterns. The inversion is generally strongest when the height of its base is lowest, and vice versa. The thickness of the inversion layer varies from tens of m to more than 1000 m. On the average its thickness is about 400 m.

Tropical Analyses See both midlatitude style, isobaric analysis, and streamline/isotach analyses. Why the latter is most popular?

Why streamline/isotachs better than isobars/heights in the tropics? Flow not geostrophic within roughly 5 degrees of the equator (so connection between pressure and winds are not necessarily straightforward) Lot of pibals (just winds) in the tropics compared to radiosondes. Pressure variations normally weak in the equator (large noise to signal)

Thus, why not analyze winds instead of pressure?

Streamline/isotach analysis is used but there are alternatives (isogon, isotach) Not intuitive

Tips on streamline/isotach analysis (you will do this) Do streamlines first if possible. You will see a lot of familiar patterns (circulation, deformation, convergence, etc.) Winds should go to zero at singular points: such as centers of rotation, convergence/divergence, and deformation.

Easterly Waves: The Major Synoptic Disturbances in the Tropics Westward moving synoptic waves characterize the whole tropics They are tropospheric waves that modulate clouds/rainfall and move at about 5-10 m/s and have wavelengths of 2000-4000 km. Assignment: METED African Easterly Wave module

Composite African Wave Structures (Reed et al., 1977)

African Easterly Waves Are Closely Associated with and Propagate in a Midtropospheric Easterly Jet Centered around 650-700 hPa near 15N Associated with large temperature difference between the hot Sahara and cool Gulf of Guinea

The Mean State over West Africa: The African Easterly Jet (AEJ) Burpee, R.W. 1972 The origin and structure of easterly waves in the lower troposphere of North Africa, J. Atmos. Sci. 29, 77-90 Why is the AEJ there?

Strong baroclinic zone 10o-20oN Hint: Surface temps Strong baroclinic zone 10o-20oN

Reed, R. J. , Norquist, D. C. and Recker, E. E Reed, R.J., Norquist, D.C. and Recker, E.E., The structure and properties of African wave disturbances as observed during Phase III of GATE, Mon. Wea. Rev. 105, 317-333 (1977).

Zonal Variations in the Mean State Mean 700hPa U wind, 16th July – 15th August 2000 Berry and Thorncroft 2005

Observations of African Easterly Waves Carlson 1969ab Carried out case studies of several AEWs Peak amplitudes at 600-700mb and at surface Eastward tilt with height from the surface to the level of the AEJ Synoptic variations in cloud cover Peak of cloudiness close to AEW trough

Observations of African Easterly Waves Burpee (1970) Eastward tilt beneath the AEJ – Westward tilt above the AEJ Northerlies dry and warm Southerlies wet and cold

Observations of African Easterly Waves Reed et al, 1977 Composite AEW structures from phase III of GATE (after Reed et al, 1977). (a) and (b) are relative vorticity at the surface and 700hPa respectively with a contour interval of 10-5s-1. (c) and (d) show percentage cover by convective cloud and average precipitation rate (mm day-1) respectively. Category 4 is location of 700hPa trough and the “0” latitude is 11oN over land and 12oN over ocean.

Origin of African Easterly Waves A joint baratropic/baroclinic instability on the African easterly jet since there are large vertical and horizontal shears. Release of latent heat in convection can help initiate and support these waves.

Theories for the Genesis of AEWs AEWs are generated via a linear mixed barotropic-baroclinic instability mechanism AEJ satisfies the necessary conditions for barotropic and baroclinic instability: Burpee (1972), Albignat and Reed, 1980; Charney and Stern 1962). Therefore we expect AEWs to arise from small random perturbations 315K PV 925hPa q

Barotropic Instability Associated with reversal of horizontal gradient of absolute or potential vorticity

Barotropic Instability Can Exist in the Midlatitudes When Horizontal Shear is Very Large Hakim examples: http://www.atmos.washington.edu/~hakim/542/baro_wv.html

(1) Two Theories for the Genesis of AEWs I: AEWs are generated via a linear mixed barotropic-baroclinic instability mechanism (evidence against!) The AEJ is too short! The jet is typically 40-50o long. It can only support two waves at one time. It is therefore not possible for AEWs to develop via a linear instability mechanism. The AEJ is only marginally unstable! Hall et al (2006) showed that in the presence of realistic boundary-layer damping the AEW growth rates are very small or zero. It is therefore not possible for AEWs to develop sufficiently fast to be important.

Some have suggested that barotropic/baroclinic instability is not a complete explanation The AEJ is short The jet is typically 40-50o long. It can only support two waves at one time.. The AEJ is only marginally unstable! Hall et al (2006) showed that in the presence of realistic boundary-layer damping the AEW growth rates are small. It is may not be possible for AEWs to develop sufficiently fast to be important. So what can account for the existence of AEWs, their genesis and intermittancy?

Perhaps AEWs are generated by finite amplitude forcing upstream of the region of observed AEW growth, with barotropic/baroclinic instability taking over, assisted by latent heat release in the waves. Carlson (1969) suggested the importance of convection and upstream topography for the initiation of AEWs.

Thorncroft and Hodges 2001

The End