Sahel Precipitation - Examining regional
teleconnections across Asia and the Indian Ocean
Ellen Dyer, Dylan Jones, Lawrence Mudryk
University of Toronto
Jesse Nusbaumer, David Noone
University of Colorado
African Climate Conference 2013
Chapter 11
Regional Climate Projections
Multi Model Dataset-A1B scenario simulations ((2080 to 2099) minus (1980 to 1999))
• Most extreme drying in June-July-August
• Change in outer rain belt regions vs equator
[From IPCC AR4]
CESM
JJAS 1950-2000 average
Objective: examine the climatological
impact of changes in the spatial gradient in
Indian Ocean sea surface temperature
(SST) on precipitation in the Sahel region
• The Community Earth System Model (CESM 1.0.4) with Community Atmosphere Model (CAM5),
Community Land Model (CLM4) and a data ocean (DOCN7)
• 1.9x2.5 degree
• Hadley-NOAA/OI Sea surface temperature climatology for an AMIP style uncoupled run
[Hurrell et al 2008]
• Modifications:
• Sea surface temperature (SST) perturbations
• Sensible heat flux perturbation between land and atmosphere coupling
Mean Circulation Patterns
168hpa
JAS Average (1940 - 2004)
Winds in CESM
(m/s) wind vectors over zonal
wind contours
• The Tropical Easterly Jet (TEJ) occurs at about 150 hPa and
extends across the Indian Ocean and terminates over
Western Africa
690hpa
• The African Easterly Jet (AEJ) occurs at about 650 hPa and
extends across the African continent
• The Westerly African Jet (WAJ) occurs about 850 hPa
• Near surface flow - inflow from Atlantic Ocean
912hpa
distinct modes in diabatic heating profiles. Such twomode heating profiles have been produced by an idealized model for the SMC (Nolan et al. 2007), and they
may indeed exist in reality (Shige et al. 2007). This
seems to be contradictory to the idealized simulation of
Schneider and Lindzen
(1977) in
which
only Winds
a deep over
JJAS Average
(1950
- 2000)
diabatic heating profile was prescribed but both a deep
Vertical Velocity at 690 hpa CESM-CAM5
and a shallow meridional overturning circulation were
generated. However, in their model, the circulation
does not feed back to diabatic heating. The boundary
layer meridional wind forced by the prescribed gradient
in SST was essentially a dry flow and resulted in a SMC
exactly because of the sea-breeze mechanism of Nolan
et al. (2007). It remains an open question, however,
whether the two distinct vertical modes in the meridional overturning circulation can be explained solely in
terms of large-scale fluid dynamics in the tropics.
The monsoon SMC may play an active role in the
seasonal evolution of monsoon rainfall. Over West Africa, for example, the shallow return flow of the SMC is
likely to advect dry, warm, and perhaps dusty, air from
the heat low into the monsoon rainband (Parker et al.
2005). It is known that dry air in the lower to midtroposphere is detrimental to deep convection (e.g.,
Local circulation patterns
FIG. 24. Schematic diagrams of the shallow meridional circulations (solid arrows) associated with (a) the ITCZ and (b) the
monsoon rainband (represented by cloud symbols). The proximate locations of the equator and heat low are marked. The
boundary layer is shaded. The deep meridional circulations are
indicated by open arrows. The arrows depict only the direction of
the circulations, not their strength. Surface convergence is expected to occur below vertical motions.
[Zhang et al 2008]
•Shallow meridional circulation
from the hot dry area north of
the Sahel which can damp
convection over the Sahel reduce
precipitation
•The hot dry region to the north
is the Saharan heat low,
characterised by a low level
anticyclonic circulation
Indian Ocean Sea Surface Temperature Experiments
• Perturb SSTs to amplify or diminish the JJA latitudinal and longitudinal SST gradients
• Changes are constant over the course of the experiment (1940-2005)
• SST changes range from 0 to 3 K
• CESM using an AMIP configuration (active land and atmosphere with a data ocean)
• These experiments are an attempt to explore a number of issues including the effect of a
warming Indian Ocean, altering the Indian Ocean dipole and changing the heating gradient
of the larger Indian Ocean/Asian Monsoon heating region.
Seasonal variability of Indian Ocean
SSTs
CESM-CAM5 JJA - DJF (1950-2000
average)
K
Sea Surface Temperature Experiments
Cold West (CW)
Cool South (CS)
Warm West (WW)
Warm South (WS)
SST Experiments: Precipitation response
Cold West (CW) JJAS 1950-2000 average
( Experiment - Baseline difference)
Cold South (CS) JJAS 1950-2000 average
( Experiment - Baseline difference)
• A cooling (increase in the SST
gradient) generally induces
reduced precipitation across the
Sahel, with increases in Central
Africa
Warm West (WW)
Warm South (WS)
• Warming (decrease in the SST
gradient) induces increased
precipitation across the Sahel and
in Eastern Africa, with a decrease in
the eastern Guinean coast
(Cameroon)
• Changing the longitudinal SST
gradient strongly impacts the
western and eastern Sahel,
whereas changes in the latitudinal
gradient has the largest impact in
the eastern Sahel
Cold West (CW) JJAS 1950-2000 average
( Experiment - Baseline difference)
Warm West (WW) JJAS 1950-2000 average
( Experiment - Baseline difference)
• Increasing the longitudinal SST gradient induces increased Asian Monsoon inflow, strengthening
the circulation
• In the lower troposphere over North Africa the circulation is also altered: cooling in the west
shifts easterlies in the Sahel further south, whereas a warming causes an acceleration in the Sahel
region
Cold West (CW) JJAS 1950-2000 average
( Experiment - Baseline difference)
Warm West (WW) JJAS 1950-2000 average
( Experiment - Baseline difference)
• Increasing the longitudinal SST gradient induces increased Asian Monsoon inflow, strengthening
the circulation
• In the lower troposphere over North Africa the circulation is also altered: cooling in the west
shifts easterlies in the Sahel further south, whereas a warming causes an acceleration in the Sahel
region
Cold West (CW) JJAS 1950-2000 average
( Experiment - Baseline difference)
• An increase in the longitudinal
SST gradient is accompanied by
increased descent over the Sahel
and decreased moisture in the
lower troposphere
Warm West (WW) JJAS 1950-2000 average
( Experiment - Baseline difference )
• A decrease in the longitudinal
SST gradient is accompanied by
the opposite effect - an increase
in ascent and low level moisture
in the Sahel
Sensible Heat Flux Experiment
Perturb the longitudinal gradient in heating without altering SSTs
W/m2
W/m2
• Reduce the heat flux in the region of the Arabian heat low by a factor of 2
(the range of the seasonal cycle of heat flux in the region of the Arabian heat low is
about 30-70 W/m2)
• Use the same configuration as previous experiments
Arabian Low - Reduced heat flux
Cold West (CW) JJAS 1980-2000 average
( Experiment - Baseline difference)
• The change in precipitation is similar to that obtained
when the western Indian Ocean was cooled
• There is increased descent over the Sahel region and a
decrease in specific humidity
• Monsoon inflow is increased and the zonal wind in the
African Easterly Jet region is enhanced as with the
other decreased SST experiments
Summary
• Changes in SSTs in the Indian Ocean can influence precipitation in the Sahel through
modulation of the Asian Monsoon.
• Enhanced ascent over South Asia is linked with enhanced descent and reduced specific
humidity over North Africa.
• An increase in the longitudinal SST gradient decreases precipitation in the western and
eastern Sahel, whereas an increase in the latitudinal SST gradient produces the largest
decrease in precipitation in the eastern Sahel.
• A decrease in surface heating to the West of the Asian Monsoon region results in a decrease
in Sahel precipitation, highlighting the importance the longitudinal heating gradient to the
strength of ascent and descent in Asia and North Africa, respectively.
➡Interpreting observed SST trends in the Indian Ocean in the context of their impact on the
strength of the monsoon flow is important for understanding the response of Sahel
precipitation to observed Indian Ocean warming.