Past and future changes in Sahel rainfall: Possible mechanisms

Past and futurechanges in Sahelrainfall: Possiblemechanisms

Kerry H. CookDepartment of Earth and Atmospheric SciencesCornell UniversityIthaca NY

Present some of the dynamical processes that areresponsible for variability in the Sahel on all timescales

paleoclimate – the African Humid Perioddecadal (Samson Hagos)interannualintraseasonal

The African Humid Period

with Christina Patricola

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, anddesert -19.

African Humid Period

Present Day AHP

Enhancement of the westerly low-level jet is a primarymoisture source.

Note that the southerly low-level southerly flowis unchanged.

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

The Monsoon Jump

with Samson Hagos

Coastal

“Sahel”

Smoothed rainfall in mm/day from TRMM (top) and FEWS (bottom)2004

Coastal

Sahel

Daily rainfall in mm/day from TRMM2002, 2003, 2005, 2006

Precipitation difference:“Sahel” – “Coast”

monsoon onset

2002: July 142003: June 242004: June 162005: July 82006: July 10


The regional modelcaptures the monsoonjump

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 convergencemoisture 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

Pre-monsoon onset

The condensational heating in the 825 - 525 hPa layer introducesa meridional pressure gradient in this layer which results in aninertial instability

=> coastal region becomes unfavorable for convergence=> maximum precipitation abruptly shifts from the coast into the Sahel

Monsoon Onset

Eastern Sahel: Another "Monsoon Jump"

Two-Stage Monsoon Onset over Ethiopia

with Emily Riddle

Pre-onsetMar 1 – Mar 31

TransitionalApr 20 – May 15

Post-onsetJun 1 – Jun 30

Low-level 910 mb winds

The precipitation dipole responseto SSTAs in the Gulf of Guinea

with Edward Vizy

A prominent modeof interannual variability:

~ 25% of the years 1950 – 2000 are identified as dipole years (12 years)

Extremely high correlationwith warm SSTAs in theGulf of Guinea during dipole years

Surface Temperature Anomalies

1984 Precipitation Anomalies

A north/south cross-section along the Greenwich meridian

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


Vertically-confinedmonsoon inflow



2nd selection criterion: Reasonable monsoon circulation

Subsidence overthe Gulf ofGuinea



thermal low

Saharan high

Southward mid-troposphericflow (African easterly jet)

Top: Climatological circulationFrom a regional climate model.

Bottom: Circulation anomaliesassociated with warming in the Gulf of Guinea and the dipoleprecipitation 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.

With warm SSTAs in theGulf of Guinea, the southward outflow from the Saharan high has a largermeridional extent, and is located closer to the surface.

These differences in the outflowgenerate subsidenceand drying over the Saheldue to shrinking of bothplanetary and relativevorticity.

Cold Air Surges andMonsoon Breaks

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

Topography (m) and June-August climatological 500 hPa geopotential heights (m) and winds (m/s) from the NCEP2 reanalysis

Saharanhigh

easternMediterranean

Tvpp

pTv

pp

p

c

Q

t

Tp

op

op

EB C DA F

A.Local rate of change of temperature (negligible)

B.Mean diabatic heating and cooling term (calculated as a residual from the NCEP2)

C.Mean vertical advection of potential temperature term

D.Mean horizontal advection of temperature term (Zonal + Meridional components)

E. Vertical transient term

F. Horizontal transient term

850 hPa JJA Thermodynamical Budget Analysis

B C

E+F D

Tvpp

pTv

pp

p

c

Q

t

Tp

op

op

A B C D E F

D D

• 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

Precipitation climatology inthe currentgeneration ofclimate models

1949 – 2000JJAS

Coastal

“Sahel”

Coastal

Sahel

Daily rainfall in mm/day from TRMM (top) and FEWS (bottom)2004

Smoothed rainfall in mm/day from TRMM (top) and FEWS (bottom)2004

Coastal

Sahel

Daily rainfall in mm/day from TRMM2002, 2003, 2005, 2006

Daily rainfall in mm/day from FEWS2002, 2003, 2005, 2006

monsoon onset

2002: July 142003: June 242004: June 162005: July 82006: July 10


Cold Surges

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

The West Frican monsoon jump is a consequence of inertialinstability that develops in the coastal region above the boundary layer (825 -525 hPa layer)

Hagos and Cook 2007: Dynamics of the West AfricanMonsoon Jump. J .Climate)

The West Frican monsoon jump is a consequence of inertialinstability that develops in the coastal region above the boundary layer (825 -525 hPa layer)

Hagos and Cook 2007: Dynamics of the West AfricanMonsoon Jump. J .Climate)

A reminder about inertial instability …

Consider a geostrophic, zonal basic state flow in theNorthern Hemisphere.

11 ,1p gF f u

y

1 ,1cor gF f u

,1gu

,2gu

Perturb the parcel to the north …

2,1p gF f u

y

2 ,1cor gF f u

,1gu

,2gu

22 ,2 2 ,1p g gF f u f u

y

2 ,1cor gF f u

,1gu

,2gu

If 22 ,1p gF f u

y

the parcel will return southward (stable).

If 22 ,1p gF f u

y

the parcel will continue northward (unstable).

So inertial instability is caused by an imbalance between

pressure gradient forces and inertial forces:

dvfu

dt y

For example, in line with the idea of inertial instability, consider a parcel of air located atpoint X on the zero contour of acceleration (Fig. 10a). Initially its acceleration is zero. Anynorthward displacement would move the parcel into a region of positive net force and cause it toaccelerate further into the continent. Likewise, a parcel displaced southward is also acceleratedfurther southward. Therefore, because of inertial instability the coastal region (the regionsurrounded by the contour of zero acceleration) becomes unfavorable for meridionalconvergence in the end of May and the meridional wind convergence jumps into the continentalinterior 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 achange in the balance between the Coriolis and pressure gradient forces, while friction delaysthe process by about three days. Thus, the condition for northward acceleration and theassociated 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 vorticityas discussed above. The significant meridional acceleration over both the ocean and the continent throughout the period of simulation, however, makes assumption of purelyzonal flow during the pre-monsoon period questionable.

X



dvfu

dt y

gfu

gdv

f u udt



dvfu

dt y

gfu

gdv

f u udt

afu

in the geostrophic, zonal background flow.

Inertial instability is related to angular momentum andvorticity by considering the stability of a parcel that isdisplaced meridionally

0y to 0y y

Apply the v-momentum equation at the new location for the displaced parcel

0 0 0

0

gy y

dvf y y u y y u y y

dt

since the parcel’s velocity at y0 isthe geostrophic background velocity 0 0gu y y u y f y

So

But

and

0 0g

g g

uu y y u y y

y

using a 1st order expansionabout y0

0 0

0

gg g

y y

udvf u y y u y f y

dt y

0

g

y y

udvf f y

dt y

or absolute vorticityf

for 0v and gu u

0

0y y

dv

dt Unstable solution

For the application to the WAM jump, we are looking for the conditionsunder which a northward displacement in the Northern Hemisphere isunstable:

for 0f

0

0 0g g

y y

u udvf f y if f

dt y y

and 0y

This is the condition forinertial instability

over West Africa relevantto the monsoon onset

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 northernAfrica to reposition the precipitation maximum in a relativelyshort 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 tellyou about, and how you the inertial instability at work.

xdu

fv Fdt x

ydv

fu Fdt y

For example, in line with the idea of inertial instability, consider a parcel of air located atpoint X on the zero contour of acceleration (Fig. 10a). Initially its acceleration is zero. Anynorthward displacement would move the parcel into a region of positive net force and cause it toaccelerate further into the continent. Likewise, a parcel displaced southward is also acceleratedfurther southward. Therefore, because of inertial instability the coastal region (the regionsurrounded by the contour of zero acceleration) becomes unfavorable for meridionalconvergence in the end of May and the meridional wind convergence jumps into the continentalinterior 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 achange in the balance between the Coriolis and pressure gradient forces, while friction delaysthe process by about three days. Thus, the condition for northward acceleration and theassociated 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 vorticityas discussed above. The significant meridional acceleration over both the ocean and the continent throughout the period of simulation, however, makes assumption of purelyzonal flow during the pre-monsoon period questionable.

X

Because of the distribution of albedo and surface moisture availability, apermanent sensible heating maximum exists around 10N. This sensible heating drives ashallow meridional circulation (Zhang et al. 2006) and moisture convergence at that latitude.

During the second half of May, an imbalance between the moisture fluxfrom the boundary layer and divergence in the middle layer results in a net supply ofmoisture and condensation (Figs. 5b and 7b). This condensation warms up thecontinental middle layer, while the evaporation of rain and radiation cool the middle layeralong the coast (Fig. 11).

The resulting pressure gradient results in an inertial instability, which abruptlyshifts the meridional wind convergence maximum from the coast into the continentalinterior on around May 29. This introduces a net total moisture convergence, net upwardmoisture flux and condensation in the upper layer, and the enhancement of precipitationin the continental interior (Figs. 10, 8, and 5a).

During the month of June, because of the shift of the meridional convergence intothe continent and downward flux of moisture into the boundary layer, upper layercondensation and precipitation along the coast gradually disappear.

North/south circulationin coupled GCMs withreasonable precipitationclimatologies

NCEP/NCAR Reanalysis

du dyfv f u f y

dt dt

gdv

fu f u udt y

Governing equations, neglecting friction and assuming that thebasic state is ˆ

gv u i

i.e., v = 0 and 0x

Then the approximate momentum equations are

and

du dyfv f u f y

dt dt

gdv

fu f u udt y

Governing equations, neglecting friction and assuming that thebasic state is ˆ

gv u i

i.e., v = 0 and 0x

Then the approximate momentum equations are

and gfugfu

in this geostrophic, zonal background flow. When it is displacednorthward (poleward) over West Africa, will it

return southward? = stable solution,

continue northward? = unstable solution,

stay in the new location? = neutral solution

Consider the stability of a parcel that is displaced meridionally from

0y to 0y y

0dv

dt

0dv

dt

0dv

dt

Evaluate the v-momentum equation at the new location for the displaced parcel

0 0 0

0

gy y

dvf y y u y y u y y

dt

Again, Holton’s derivation doesn’t distinguish between f at the displaced location and the initial location:

0f y y f 0 0

0

gy y

dvf u y y u y y

dt

The above equation provides a good physical interpretation of inertial instability. If the displaced parcel’s zonal velocity is different from thegeostrophic 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

0 0 0

0

gy y

dvf y y u y y u y y

dt

Again, Holton’s derivation doesn’t distinguish between f at the displaced location and the initial location:

0f y y f 0 0

0

gy y

dvf u y y u y y

dt

The above equation provides a good physical interpretation of inertial instability. If the displaced parcel’s zonal velocity is different from thegeostrophic 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.

ageostrophicu

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

0ageostrophicu

0ageostrophicu

stable

unstable

from the u-momentum equation,since the parcel’s velocity at y0 isthe geostrophic background velocity

0 0gu y y u y f y

So

Holton goes on to rewrite the above equation.

and

0 0g

g g

uu y y u y y

y

using a 1st order expansionabout y0

0 0

0

gg g

y y

udvf u y y u y f y

dt y

0

g

y y

udvf f y

dt y

or

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

JJAS GPCP (1979 – 1999)

JJAS CRU (1961 – 1990)

Regional Model

A tropical, climateversion of MM5

grid spacing 90 km23 vertical levels time step 90 s

Summer Precipitation Climatology (mm/day)

Past and future changes in Sahel rainfall: Possible mechanisms

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