1. Past and future changes in Sahel rainfall: Possible mechanisms
Kerry H. Cook
Department of Earth and Atmospheric Sciences
Cornell University
Ithaca NY

2. Present some of the dynamical processes that are
responsible for variability in the Sahel on all time
scales
paleoclimate – the African Humid Period
decadal (Samson Hagos)
interannual
intraseasonal
not organized by time scale because they may be relevant at various time scales – for example, depending on the time scale of the forcing; it is important to think about the basic dynamics of how this system works because the GCM simulations are often poor in this region, we can’t rely on them without closer scrutiny, we need to understand how the system works in order to evaluate the quality of model simulations

3. The African Humid Period
Cold air surges have been observed to have an impact on climate for other regions of the world.
Most notable regions where outbreaks are observed include east of the Rocky Mountains over the US and Gulf of Mexico, over South America, east of the Andes, and Asia.
These outbreaks are not limited to just the cold season….For example Garreaud and Wallace (1998) and Garreaud (2001) note the importance of these events during the warm season over South America for convection.
In this observational study we will investigate the relevance of cold air surges over northern Africa during the boreal summer.
with Christina Patricola

4. African Humid Period AHP Present Day
Vegetation for (a) present day (b) and African Humid Period according to Hoelzmann et al. (1998) with grassland - 7, shrubland - 8, savanna - 10, evergreen broadleaf forest - 13, and
desert -19.

6. Enhancement of the westerly low-level jet is a primary
moisture source.
Note that the southerly low-level southerly flow
is unchanged.

7. The African easterly jet is not a part of the AHP climate

8. The Monsoon Jump
with Samson Hagos

9. Coastal

10. “Sahel”

11. Smoothed rainfall in mm/day from TRMM (top) and FEWS (bottom)
2004
Coastal
Sahel

12. Daily rainfall in mm/day from TRMM
2002, 2003, 2005, 2006
trmm

13. Precipitation difference:
“Sahel” – “Coast”

14. Precipitation difference:
“Sahel” – “Coast”
monsoon onset
2002: July 14
2003: June 24
2004: June 16
2005: July 8
2006: July 10

15. The regional model captures the monsoon jump
Fig. 1. Precipitation from the (a) RCM simulation, (b) GPCP 6-year mean, (c) TRMM 6-year mean, and (d) CMAP 6-year mean. All fields are averaged between and and are in mm/day. Dashed lines indicate approximate date of the monsoon jump and the bold lines represent the approximate latitude of the coastline.

17. X the change in sign of the meridional acceleration is related to a
change in the balance between the Coriolis and pressure gradient forces, while friction delays
the process by about three days. Thus, the condition for northward acceleration and the
associated shift in meridional convergence is a change in sign of -fu-dphi/dy
But the significant meridional acceleration over both the ocean and the continent
Makes the assumption of purely zonal flow during the pre-monsoon period questionable.

18. Pre-monsoon onset
A permanent sensible heating maximum exists from about 10N-12N:
relatively low albedo => shortwave radiation maximum and net total radiative heating maximum
This sensible heating drives a shallow meridional circulation (Zhang et al. 2006)
low-level moisture convergence
moisture transport into the middle layer ( hPa), divergence
The radiative forcing increases through the spring and, near the middle of May, the gradually increasing moisture supply from the boundary layer begins condensing in the middle layer
=> condensation and precipitation increases in the continental interior

19. Monsoon Onset
The condensational heating in the hPa layer introduces
a meridional pressure gradient in this layer which results in an
inertial instability
=> coastal region becomes unfavorable for convergence
=> maximum precipitation abruptly shifts from the coast into the Sahel

20. Eastern Sahel: Another "Monsoon Jump"
Two-Stage Monsoon Onset over Ethiopia
with Emily Riddle

23. Low-level 910 mb winds Pre-onset Mar 1 – Mar 31 Transitional
Apr 20 – May 15
Post-onset
Jun 1 – Jun 30

24. The precipitation dipole response to SSTAs in the Gulf of Guinea
with Edward Vizy

25. of interannual variability:
Surface Temperature Anomalies
A prominent mode
of interannual variability:
~ 25% of the years 1950 – 2000 are identified as dipole years (12 years)
Extremely high correlation
with warm SSTAs in the
Gulf of Guinea during dipole years
1984 Precipitation Anomalies

26. Streamlines (v, wx10-2) and meridional velocity (m/s)
A north/south cross-section along the Greenwich meridian
Need to look a little deeper, if we are to trust the simulations of the future - for example, the circulation
Streamlines (v, wx10-2) and meridional velocity (m/s)

27. Streamlines (v, wx10-2) and meridional velocity (m/s)
A north/south cross-section along the Greenwich meridian
Vertically-confined
monsoon inflow
Need to look a little deeper, if we are to trust the simulations of the future - for example, the circulation
A north/south cross-section along the Greenwich meridian
Streamlines (v, wx10-2) and meridional velocity (m/s)

28. Streamlines (v, wx10-2) and meridional velocity (m/s)
2nd selection criterion: Reasonable monsoon circulation
Subsidence over
the Gulf of
Guinea
Need to look a little deeper, if we are to trust the simulations of the future - for example, the circulation
Streamlines (v, wx10-2) and meridional velocity (m/s)

29. Streamlines (v, wx10-2) and meridional velocity (m/s)
Southward mid-tropospheric
flow (African easterly jet)
Saharan high
Need to look a little deeper, if we are to trust the simulations of the future - for example, the circulation
thermal low
Streamlines (v, wx10-2) and meridional velocity (m/s)

30. Top: Climatological circulation
From a regional climate model.
Bottom: Circulation anomalies
associated with warming in the Gulf of Guinea and the dipole
precipitation mode.
Anomalously high rainfall along the Guinean coast occurs in association with an increase in the moisture content of the monsoon inflow. Subsidence over the Gulf of Guinea suppresses the precipitation anomaly over the ocean.

31. With warm SSTAs in the
Gulf of Guinea, the
southward outflow from the
Saharan high has a larger
meridional extent, and is
located closer to the surface.
These differences in the outflow
generate subsidence
and drying over the Sahel
due to shrinking of both
planetary and relative
vorticity.

32. Cold Air Surges and Monsoon Breaks
Cold air surges have been observed to have an impact on climate for other regions of the world.
Most notable regions where outbreaks are observed include east of the Rocky Mountains over the US and Gulf of Mexico, over South America, east of the Andes, and Asia.
These outbreaks are not limited to just the cold season….For example Garreaud and Wallace (1998) and Garreaud (2001) note the importance of these events during the warm season over South America for convection.
In this observational study we will investigate the relevance of cold air surges over northern Africa during the boreal summer.
with Edward (Ned) Vizy

33. What is a cold surge? Mid-tropospheric ridge/trough pattern
Shallow dome of cold air with a sharp temperature gradient along it’s leading edge
Typically moves along topography, e.g., east of the Rockies and Andes
into the sub-tropics. The leading edge of the surge is indicated by the surface cold front. The thin curves represent surface isobars. The dashed lines indicate the position and phase of the mid-level wave.
So what are the general characteristics of cold air surges? This schematic from Garreaud (2001) illustrates the main features.
First, cold air surges are generally associated with the passage of an mid-level extratropical disturbance, building up a cold air pool and a poleward large-scale pressure gradient to the east of the mountain range.
The subsequent equatorward advance of the relatively colder air occurs in the form of a shallow dome of cold air with a sharp temperature gradient along it’s leading edge.
The cold air dome typically moves parallel to contours of surface elevation and is accompanied by a hydrostatically induced ridge of surface pressure.
As the surge moves into low-latitudes, strong surface heating fluxes weaken the cold air anomalies, and the surge may begin to lose it’s “cold” character. However, strong meridional winds and low dew points remain as clear signatures of the surge.
These surges tend to enhance subtropical and tropical deep convection because of the intense low-level convergence along their leading edge.
In the wake of the passage of the leading edge, convergence, hence precipitation can be reduced.
Thus, cold surges have been observed to influence the regional climates of other regions of the world, and we will show evidence that they are also operating over Africa and do influence the regional climate in a similar fashion.
Fig 2. from Garreaud (2001): Conceptual model of a cold surge moving from mid-latitudes

34. The climatology summer mid-tropospheric geopotential height field does have the ridge/trough pattern
While Africa does not have the solid mountain barrier that has been associated with other cold air surge regions (e.g., the Rockies, Andes, Himalayas) the climatological summer mid-tropospheric height field does indicate a ridge/trough pattern often associated with cold air surges anchored in the west by the Saharan High, and with troughing over the eastern Mediterranean Sea.
Topography (m) and June-August climatological 500 hPa geopotential heights (m) and winds (m/s) from the NCEP2 reanalysis

35. The climatological summer mid-tropospheric height field has the ridge/trough pattern
eastern
Mediterranean
Saharan
high
While Africa does not have the solid mountain barrier that has been associated with other cold air surge regions (e.g., the Rockies, Andes, Himalayas) the climatological summer mid-tropospheric height field does indicate a ridge/trough pattern often associated with cold air surges anchored in the west by the Saharan High, and with troughing over the eastern Mediterranean Sea.
Topography (m) and June-August climatological 500 hPa geopotential heights (m) and winds (m/s) from the NCEP2 reanalysis

36. A B C D E F Local rate of change of temperature (negligible)
Mean diabatic heating and cooling term (calculated as a residual from the NCEP2)
Mean vertical advection of potential temperature term
Mean horizontal advection of temperature term (Zonal + Meridional components)
Vertical transient term
Horizontal transient term

37. 850 hPa JJA Thermodynamical Budget AnalysisF
850 hPa JJA Thermodynamical Budget Analysis B C
E+F D
Warm land surface associated with low-level diabatic heating while the cooler water bodies are associated with low-level diabatic cooling.
Over northeastern Africa the diabatic heating is balanced by the equatorward transport of cooler air from the Mediterranean region. There is some upward vertical advection of colder potential temperatures, but not enough to offset the strong diabatic heating found over northeastern Africa.
Over the Mediterranean Sea the low-level diabatic cooling is balanced by the downward advection of higher potential temperatures
Response to low-level heating over northwestern Africa weaker, but the upward advection of colder potential temperatures balances the diabatic heating term plus the meridional temperature advection term. D D

38. June-August Climatological Vertical-p velocity along 35N
Strong mid-tropospheric subsidence over the eastern Mediterranean Sea
June-August Climatological Vertical-p velocity along 35N
Upward vertical motions are generally associated with the upward advection of lower potential temperature air from below, and vice versa. At 35N, the topography over NW Africa is associated with rising motions (denoted by the blue colors), while over the eastern Mediterranean there is strong sinking motions.
This region of sinking air is associated with a region of strong upper tropospheric converge at the subtropical westerly jet entrance region.
NW Africa
E. Med Sea

39. Daily TRMM rainfall rates (mm/day) and 850 hPa wind convergence (contoured) for a JULY 2005 cold air surge event
Here is an example of a cold air surge event over Africa
Peak is approximately on July 15th. Strong event that pushes southward of 15N
See a strong enhancement of convection over eastern Sahel on July 16-17th, then a marked decrease in activity on the 18th-19th as the wind convergence weakens.
Strong enhancement of low-level divergence in the wake of the frontal boundary shifting equatorward.

40. Precipitation climatology in the current generation of climate models
1949 – 2000
JJAS
Here are the summertime precipitation climataologies from the 18 models (1) eliminate if precipitation isn’t o the continent, (2) look for maxima

44. Coastal

45. “Sahel”

46. Daily rainfall in mm/day from TRMM (top) and FEWS (bottom)
2004
Coastal
Sahel

47. Smoothed rainfall in mm/day from TRMM (top) and FEWS (bottom)
2004
Coastal
Sahel

48. Daily rainfall in mm/day from TRMM
2002, 2003, 2005, 2006
trmm

49. Daily rainfall in mm/day from FEWS
2002, 2003, 2005, 2006
fews

50. Precipitation difference:
“Sahel” – “Coast”
monsoon onset
2002: July 14
2003: June 24
2004: June 16
2005: July 8
2006: July 10

51. The regional model captures the monsoon jump
Fig. 1. Precipitation from the (a) RCM simulation, (b) GPCP 6-year mean, (c) TRMM 6-year mean, and (d) CMAP 6-year mean. All fields are averaged between and and are in mm/day. Dashed lines indicate approximate date of the monsoon jump and the bold lines represent the approximate latitude of the coastline.

53. Cold Surges
A type of monsoon break

55. Long term goal: Predicting monsoon onset (monsoon jump)
Why does the jump occur?
What controls the timing of the monsoon onset?
Does the timing of the onset correlate with seasonal precipitation totals?
Is there a relationship with interannual variability?
…. etc

56. Long term goal: Predicting monsoon onset (monsoon jump)
Why does the jump occur?
What controls the timing of the monsoon onset?
Does the timing of the onset correlate with seasonal precipitation totals?
Is there a relationship with interannual variability?
…. etc

57. The West Frican monsoon jump is a consequence of inertial
instability that develops in the coastal region
above the boundary layer ( hPa layer)
Hagos and Cook 2007: Dynamics of the West African
Monsoon Jump. J .Climate)

58. A reminder about inertial instability …
The West Frican monsoon jump is a consequence of inertial
instability that develops in the coastal region
above the boundary layer ( hPa layer)
Hagos and Cook 2007: Dynamics of the West African
Monsoon Jump. J .Climate)
A reminder about inertial instability …

59. Consider a geostrophic, zonal basic state flow in the
Northern Hemisphere.
stability test – perturb the parcel to the north and see if it stay, returns (stable) or goes farther north (unstanble case)

60. Perturb the parcel to the north …
stability test – perturb the parcel to the north and see if it stay, returns (stable) or goes farther north (unstanble case)

61. the parcel will return southward (stable).
stability test – perturb the parcel to the north and see if it stay, returns (stable) or goes farther north (unstanble case); so inertial instability is caused by an umbalance between pressure gradient forces and inertial forces If
the parcel will return southward (stable). If
the parcel will continue northward (unstable).

62. So inertial instability is caused by an imbalance between
pressure gradient forces and inertial forces:

64. X
For example, in line with the idea of inertial instability, consider a parcel of air located at
point X on the zero contour of acceleration (Fig. 10a). Initially its acceleration is zero. Any
northward displacement would move the parcel into a region of positive net force and cause it to
accelerate further into the continent. Likewise, a parcel displaced southward is also accelerated
further southward. Therefore, because of inertial instability the coastal region (the region
surrounded by the contour of zero acceleration) becomes unfavorable for meridional
convergence in the end of May and the meridional wind convergence jumps into the continental
interior where convergence is sustainable.
Comparing Fig. 10b, which shows the sum of the first two right hand side terms of Eq.
(5), with Fig. 10a indicates that the change in sign of the meridional acceleration is related to a
change in the balance between the Coriolis and pressure gradient forces, while friction delays
the process by about three days. Thus, the condition for northward acceleration and the
associated shift in meridional convergence is a change in sign of -fu-dphi/dy For a geostrophic,
zonally uniform flow, this condition can be simplified to the change in sign of absolute vorticity
as discussed above. The significant meridional acceleration over both the ocean and the continent
throughout the period of simulation, however, makes assumption of purely
zonal flow during the pre-monsoon period questionable.

65. So inertial instability is caused by an imbalance between
pressure gradient forces and inertial forces:

66. So inertial instability is caused by an imbalance between
pressure gradient forces and inertial forces:

67. Inertial instability is related to angular momentum and
vorticity by considering the stability of a parcel that is
displaced meridionally to
in the geostrophic, zonal background flow.
Apply the v-momentum equation at the new location for the displaced parcel

68. But
since the parcel’s velocity at y0 is
the geostrophic background velocity
and
using a 1st order expansion
about y0 So
absolute vorticity or
for
and

69. For the application to the WAM jump, we are looking for the conditions
under which a northward displacement in the Northern Hemisphere is
unstable:
Unstable solution
for
and
This is the condition for
inertial instability
over West Africa relevant
to the monsoon onset

70. This is the theory, assuming
purely zonal, geostrophic basic flow
no friction
neglected terms in Coriolis force/curvature, vertical velocity
But is this really what happens over northern
Africa to reposition the precipitation maximum in a relatively
short time?
Can’t tell (so far!) from the observations – not fine enough,
going to try using AMMA observations.
But we have a modeling study completed that I want to tell
you about, and how you the inertial instability at work.

71. the advantage of high resolution

74. The regional model captures the monsoon jump
Fig. 1. Precipitation from the (a) RCM simulation, (b) GPCP 6-year mean, (c) TRMM 6-year mean, and (d) CMAP 6-year mean. All fields are averaged between and and are in mm/day. Dashed lines indicate approximate date of the monsoon jump and the bold lines represent the approximate latitude of the coastline.

76. X
For example, in line with the idea of inertial instability, consider a parcel of air located at
point X on the zero contour of acceleration (Fig. 10a). Initially its acceleration is zero. Any
northward displacement would move the parcel into a region of positive net force and cause it to
accelerate further into the continent. Likewise, a parcel displaced southward is also accelerated
further southward. Therefore, because of inertial instability the coastal region (the region
surrounded by the contour of zero acceleration) becomes unfavorable for meridional
convergence in the end of May and the meridional wind convergence jumps into the continental
interior where convergence is sustainable.
Comparing Fig. 10b, which shows the sum of the first two right hand side terms of Eq.
(5), with Fig. 10a indicates that the change in sign of the meridional acceleration is related to a
change in the balance between the Coriolis and pressure gradient forces, while friction delays
the process by about three days. Thus, the condition for northward acceleration and the
associated shift in meridional convergence is a change in sign of -fu-dphi/dy For a geostrophic,
zonally uniform flow, this condition can be simplified to the change in sign of absolute vorticity
as discussed above. The significant meridional acceleration over both the ocean and the continent
throughout the period of simulation, however, makes assumption of purely
zonal flow during the pre-monsoon period questionable.

77. condensation and precipitation along the coast gradually disappear.
Because of the distribution of albedo and surface moisture availability, a
permanent sensible heating maximum exists around 10N. This sensible heating drives a
shallow meridional circulation (Zhang et al. 2006) and moisture convergence at that latitude.
During the second half of May, an imbalance between the moisture flux
from the boundary layer and divergence in the middle layer results in a net supply of
moisture and condensation (Figs. 5b and 7b). This condensation warms up the
continental middle layer, while the evaporation of rain and radiation cool the middle layer
along the coast (Fig. 11).
The resulting pressure gradient results in an inertial instability, which abruptly
shifts the meridional wind convergence maximum from the coast into the continental
interior on around May 29. This introduces a net total moisture convergence, net upward
moisture flux and condensation in the upper layer, and the enhancement of precipitation
in the continental interior (Figs. 10, 8, and 5a).
During the month of June, because of the shift of the meridional convergence into
the continent and downward flux of moisture into the boundary layer, upper layer
condensation and precipitation along the coast gradually disappear.

78. North/south circulation in coupled GCMs with reasonable precipitation
climatologies
NCEP/NCAR Reanalysis
Get new figures, maybe just shown some of them, or show them on 2 pages

85. Governing equations, neglecting friction and assuming that the
basic state is
i.e., v = 0 and
Then the approximate momentum equations are
and

86. Governing equations, neglecting friction and assuming that the
basic state is
i.e., v = 0 and
Then the approximate momentum equations are
and

87. Consider the stability of a parcel that is displaced meridionally fromto
in this geostrophic, zonal background flow. When it is displaced
northward (poleward) over West Africa, will it
return southward? = stable solution,
continue northward? = unstable solution,
stay in the new location? = neutral solution

88. Evaluate the v-momentum equation at the new location for the displaced parcel
Again, Holton’s derivation doesn’t distinguish between f at the displaced location and the initial location:
The above equation provides a good physical interpretation of inertial instability. If the displaced parcel’s zonal velocity is different from the
geostrophic zonal velocity at the new location, there will be a net meridional acceleration because the velocity-dependent Coriolis force will not balance the pressure gradient for in the new location.

89. Evaluate the v-momentum equation at the new location for the displaced parcel
Again, Holton’s derivation doesn’t distinguish between f at the displaced location and the initial location:
The above equation provides a good physical interpretation of inertial instability. If the displaced parcel’s zonal velocity is different from the
geostrophic zonal velocity at the new location, there will be a net meridional acceleration because the velocity-dependent Coriolis force will not balance the pressure gradient in the new location.

90. If the parcel velocity at the new location is greater than the
geostrophic velocity at the new location, then the parcel is
“super-rotating” and will be directed back toward the equator
by Coriolis accelerations. This is the stable case. If the parcel
velocity at the new location is less than the geostrophic velocity
at the new location, then the parcel is “sub-rotating” and will be
directed away from the equator by Coriolis accelerations.
This is the unstable case.
stable
unstable

91. Holton goes on to rewrite the above equation.
from the u-momentum equation,
since the parcel’s velocity at y0 is
the geostrophic background velocity
and
using a 1st order expansion
about y0 So or

92. Why does this happen over West Afric and not over other places?
For example, does the South America monsoon onset this way? Is
this common in mid-latitude flows?

93. JJAS GPCP (1979 – 1999)
JJAS CRU (1961 – 1990)
identify “Sahel”

94. JJAS GPCP (1979 – 1999)
JJAS CRU (1961 – 1990)

95. JJAS GPCP (1979 – 1999)
JJAS CRU (1961 – 1990)

96. JJAS GPCP (1979 – 1999)
JJAS CRU (1961 – 1990)

97. Regional Model Summer Precipitation Climatology (mm/day)
A tropical, climate
version of MM5
grid spacing 90 km
23 vertical levels
time step 90 s