Past%20and%20future

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Past and future changes in Sahel rainfall
Possible mechanisms
Kerry H. Cook Department of Earth and Atmospheric
Sciences Cornell University Ithaca NY
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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
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The African Humid Period

• with Christina Patricola

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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.
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Enhancement of the westerly low-level jet is a
primary moisture source. Note that the southerly
low-level southerly flow is unchanged.
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The African easterly jet is not a part of the AHP
climate
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The Monsoon Jump
with Samson Hagos
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Coastal
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Sahel
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Smoothed rainfall in mm/day from TRMM (top) and
FEWS (bottom) 2004
Coastal
Sahel
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Daily rainfall in mm/day from TRMM 2002, 2003,
2005, 2006
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Precipitation difference Sahel Coast
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Precipitation difference Sahel Coast
monsoon onset
2002 July 14 2003 June 24 2004 June
16 2005 July 8 2006 July 10
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The regional model captures the monsoon jump
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X
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Pre-monsoon onset
A permanent sensible heating maximum exists from
about 10N-12N relatively low albedo gt
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 moistur
e transport into the middle layer (825 -525 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 gt condensation and precipitation
increases in the continental interior
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Monsoon Onset
The condensational heating in the 825 - 525 hPa
layer introduces a meridional pressure gradient
in this layer which results in an inertial
instability gt coastal region becomes
unfavorable for convergence gt maximum
precipitation abruptly shifts from the coast
into the Sahel
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Eastern Sahel Another "Monsoon Jump"
Two-Stage Monsoon Onset over Ethiopia
with Emily Riddle
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Low-level 910 mb winds
Pre-onset Mar 1 Mar 31
Transitional Apr 20 May 15
Post-onset Jun 1 Jun 30
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The precipitation dipole response to SSTAs in the
Gulf of Guinea
with Edward Vizy
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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
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A north/south cross-section along the Greenwich
meridian

Streamlines (v, wx10-2) and meridional velocity
(m/s)
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A north/south cross-section along the Greenwich
meridian
Vertically-confined monsoon inflow

A north/south cross-section along the Greenwich
meridian
Streamlines (v, wx10-2) and meridional velocity
(m/s)
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2nd selection criterion Reasonable monsoon
circulation
Subsidence over the Gulf of Guinea

Streamlines (v, wx10-2) and meridional velocity
(m/s)
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Southward mid-tropospheric flow (African easterly
jet)
Saharan high
thermal low
Streamlines (v, wx10-2) and meridional velocity
(m/s)
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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.
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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.
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Cold Air Surges andMonsoon Breaks

• with Edward (Ned) Vizy

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What is a cold surge?

• Mid-tropospheric ridge/trough pattern
• Shallow dome of cold air with a sharp temperature
  gradient along its leading edge
• Typically moves along topography, e.g., east of
  the Rockies and Andes

Fig 2. from Garreaud (2001) Conceptual model of
a cold surge moving from mid-latitudes
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The climatology summer mid-tropospheric
geopotential height field does have the
ridge/trough pattern
Topography (m) and June-August climatological 500
hPa geopotential heights (m) and winds (m/s) from
the NCEP2 reanalysis
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The climatological summer mid-tropospheric
heightfield has the ridge/trough pattern
eastern Mediterranean
Saharan high
Topography (m) and June-August climatological 500
hPa geopotential heights (m) and winds (m/s) from
the NCEP2 reanalysis
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E
B
C
D
A
F

1. Local rate of change of temperature (negligible)
2. Mean diabatic heating and cooling term
   (calculated as a residual from the NCEP2)
3. Mean vertical advection of potential temperature
   term
4. Mean horizontal advection of temperature term
   (Zonal Meridional components)
5. Vertical transient term
6. Horizontal transient term

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A
B
C
D
E
F
850 hPa JJA Thermodynamical Budget Analysis
B
C
EF
D
D
D
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• Strong mid-tropospheric subsidence over the
  eastern Mediterranean Sea

June-August Climatological Vertical-p velocity
along 35N
NW Africa
E. Med Sea
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Daily TRMM rainfall rates (mm/day) and 850 hPa
wind convergence (contoured) for a JULY 2005 cold
air surge event
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Precipitation climatology in the
current generation of climate models 1949
2000 JJAS
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Coastal
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Sahel
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Daily rainfall in mm/day from TRMM (top) and FEWS
(bottom) 2004
Coastal
Sahel
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Smoothed rainfall in mm/day from TRMM (top) and
FEWS (bottom) 2004
Coastal
Sahel
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Daily rainfall in mm/day from TRMM 2002, 2003,
2005, 2006
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Daily rainfall in mm/day from FEWS 2002, 2003,
2005, 2006
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Precipitation difference Sahel Coast
monsoon onset
2002 July 14 2003 June 24 2004 June
16 2005 July 8 2006 July 10
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The regional model captures the monsoon jump
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Cold Surges A type of monsoon break
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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
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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
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The West Frican monsoon jump is a consequence of
inertial instability that develops in the coastal
region above the boundary layer (825 -525 hPa
layer) Hagos and Cook 2007 Dynamics of the West
African Monsoon Jump. J .Climate)
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The West Frican monsoon jump is a consequence of
inertial instability that develops in the coastal
region above the boundary layer (825 -525 hPa
layer) Hagos and Cook 2007 Dynamics of the West
African Monsoon Jump. J .Climate)
A reminder about inertial instability
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Consider a geostrophic, zonal basic state flow in
the Northern Hemisphere.
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Perturb the parcel to the north
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If
the parcel will return southward (stable).
If
the parcel will continue northward (unstable).
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So inertial instability is caused by an imbalance
between pressure gradient forces and inertial
forces
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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.
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So inertial instability is caused by an imbalance
between pressure gradient forces and inertial
forces
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So inertial instability is caused by an imbalance
between pressure gradient forces and inertial
forces
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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
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But
since the parcels velocity at y0 is the
geostrophic background velocity
and
using a 1st order expansion about y0
So
absolute vorticity
or
for
and
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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
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• 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?
• Cant 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.

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The regional model captures the monsoon jump
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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
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.
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North/south circulation in coupled GCMs
with reasonable precipitation climatologies
NCEP/NCAR Reanalysis
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Governing equations, neglecting friction and
assuming that the basic state is
i.e., v 0 and
Then the approximate momentum equations are
and
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Governing equations, neglecting friction and
assuming that the basic state is
i.e., v 0 and
Then the approximate momentum equations are
and
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Consider the stability of a parcel that is
displaced meridionally from
to
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, Holtons derivation doesnt 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 parcels 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, Holtons derivation doesnt 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 parcels 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
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Holton goes on to rewrite the above equation.
from the u-momentum equation, since the parcels
velocity at y0 is the geostrophic background
velocity
and
using a 1st order expansion about y0
So
or
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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?
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JJAS GPCP (1979 1999)
JJAS CRU (1961 1990)
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JJAS GPCP (1979 1999)
JJAS CRU (1961 1990)
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JJAS GPCP (1979 1999)
JJAS CRU (1961 1990)
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JJAS GPCP (1979 1999)
JJAS CRU (1961 1990)
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Summer Precipitation Climatology (mm/day)
Regional Model A tropical, climate version of
MM5 grid spacing 90 km 23 vertical levels time
step 90 s