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Past and future changes in Sahel rainfall: Possible mechanisms

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### The African Humid Period

### Cold Air Surges andMonsoon Breaks

changes in Sahel

rainfall: Possible

mechanisms

Kerry H. Cook

Department of Earth and Atmospheric Sciences

Cornell University

Ithaca NY

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

with Christina Patricola

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.

Enhancement of the westerly low-level jet is a primary

moisture source.

Note that the southerly low-level southerly flow

is unchanged.

with Samson Hagos

Daily rainfall in mm/day from TRMM

2002, 2003, 2005, 2006

“Sahel” – “Coast”

“Sahel” – “Coast”

monsoon onset

2002: July 14

2003: June 24

2004: June 16

2005: July 8

2006: July 10

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 (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

=> condensation and precipitation increases in the continental interior

The condensational heating in the 825 - 525 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

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

A north/south cross-section along the Greenwich meridian

Streamlines (v, wx10-2) and meridional velocity (m/s)

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)

2nd selection criterion: Reasonable monsoon circulation

Subsidence over

the Gulf of

Guinea

Streamlines (v, wx10-2) and meridional velocity (m/s)

flow (African easterly jet)

Saharan high

thermal low

Streamlines (v, wx10-2) and meridional velocity (m/s)

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.

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.

with Edward (Ned) Vizy

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

Fig 2. from Garreaud (2001): Conceptual model of a cold surge moving from mid-latitudes

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

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

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

Strong mid-tropospheric subsidence over the eastern Mediterranean Sea

June-August Climatological Vertical-p velocity along 35N

NW Africa

E. Med Sea

Daily TRMM rainfall rates (mm/day) and 850 hPa wind convergence (contoured) for a JULY 2005 cold air surge event

Daily rainfall in mm/day from TRMM

2002, 2003, 2005, 2006

Daily rainfall in mm/day from FEWS

2002, 2003, 2005, 2006

“Sahel” – “Coast”

monsoon onset

2002: July 14

2003: June 24

2004: June 16

2005: July 8

2006: July 10

A type of monsoon break

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

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

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)

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 …

Consider a geostrophic, zonal basic state flow in the

Northern Hemisphere.

So inertial instability is caused by an imbalance between

pressure gradient forces and inertial forces:

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.

So inertial instability is caused by an imbalance between

pressure gradient forces and inertial forces:

So inertial instability is caused by an imbalance between

pressure gradient forces and inertial forces:

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

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

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

- 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.

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.

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.

in coupled GCMs with

reasonable precipitation

climatologies

NCEP/NCAR Reanalysis

Governing equations, neglecting friction and assuming that the

basic state is

i.e., v = 0 and

Then the approximate momentum equations are

and

Governing equations, neglecting friction and assuming that the

basic state is

i.e., v = 0 and

Then the approximate momentum equations are

and

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

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.

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.

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

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

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?

JJAS CRU (1961 – 1990)

JJAS CRU (1961 – 1990)

JJAS CRU (1961 – 1990)

JJAS CRU (1961 – 1990)

Summer Precipitation Climatology (mm/day)

Regional Model

A tropical, climate

version of MM5

grid spacing 90 km

23 vertical levels

time step 90 s

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