The ANNUAL CYCLE OF TEMPERATURE in the SAHEL and its

GC33A
1004
The ANNUAL CYCLE OF TEMPERATURE in the SAHEL and its CLIMATIC SENSITIVITY
Françoise GUICHARD 1*, Laurent KERGOAT 2 , Eric MOUGIN 2 and Frédéric HOURDIN 3
(*) [email protected]
1: CNRM-GAME (CNRS & Météo-France) 2: GET (CNRS, IRD, UPS, CNES) 3: LMD, IPSL (CNRS & UPMC)
1. CLIMATOLOGY and QUESTIONS
3. WARMING TREND and SEASONS
▶ In the subtropical semi-arid Sahel, a major climatic feature is the bimodal
structure of the annual cycle of temperature at the surface, with two maxima
surrounding the rainy season (e.g. Guichard et al. 2009). The first one, in Spring,
is particularly large ( monthly mean temperature = 35°C at 15°N). [Fig 1]
▶
▶
▶
▶
Observations: a strong warming in 60 years, > 1 K in Spring [Fig 2] - larger increase at night.
In Spring: weaker short-term inter-annual variability (> 10 years), stronger multidecennal trend.
Spring warming: not the same processes at play than during the rainy season (rain-T2m couplings)
A strong warming in Spring (already very hot) is likely to have strong societal implications.
▶ We analyse this specific structure, explore how it has been affected by the
multi-decadal warming observed in this region, and evaluate whether reanalyses
and climate models (CMIP5) can reproduce observations.
EACH DOT
=
1 MONTH
FROM
1950 to 2009
TREND
annual
maximum
Figure 1 : an example of time
series of temperature and
specific humidity at 2m in the
Sahel (function of day of year).
Temperature
Observations from a few years
are overplotted, using automatic weather station data from
Agoufou (Mali, 1.5°W, 15.3°N)
– 10-day mean values.
Specific humidity
rainfall
dominated by short
interannual variability
2. OBSERVATIONS, DATASETS, MODELS
▶ local meteorological data: SYNOP, Hombori historical data, also GISS & BEST [> 60 years]
▶ CRU [> 60 years] and MSU [~ 30 years] gridded datasets
▶ high-frequency (15 min) AMMA-Catch ground station data [~ 10 years]
▶ reanalyses : ERA40 [ ~ 50 years ] and ERA Interim, NCEP CFSR, MERRA [~ 60 years]
▶ CMIP5 climate simulations: amip [~ 30 years] , (nat) historical [> 60 years] …
4. WARMING TREND in REANALYSES
▶ CRU gridded dataset consistent with local SYNOP data
▶ ERA40 & ERA-Interim consistent with CRU [Fig 3]
▶ MERRA & NCEP-CFSR: data assimilation pb? [Fig 3b]
(b)
TEMPERATURE TREND 1950-2010, MONTH per MONTH
1K
0K
J
F M A M J J A S O N D
coupling with rainfall
(70's 80's severe droughts)
multi-decacal
trend dominates
Figure 2 : Trends in the annual cycle of temperature (T2m): SYNOP data of Hombori (central Sahel, 15°N) from 1950 to 2009. For each
month, the series of dots correspond to monthly-mean values from 1950 to 2009. A linear fit is added (for comparison, a 2 nd order fit is
displayed below). The trends over the period is also indicated for each month on the top-right (coloured bars). Dots are coloured red for
correlations > 0.4.
5. CMIP5 CLIMATE MODELS: CLIMATOLOGIES & TRENDS
▶ very large spread in annual cycles + smooth annual max [Fig 4a]
▶ large differences in Tmin (pb with nocturnal boundary layers)
▶ no consistency in magnitude & seasonality of warming trends
[Fig 4b] , ≠ temperature-humidity couplings (suggests ≠ feedbacks)
models
Temperature trend Relative humidity trend
(b)
IPSL-CM5A-LR
HadGEM2-ES
(a)
bcc-csm1-1
JJAS: daytime warm bias
CanESM2
also, not shown: (i) NCEP-CFSR cold bias in climatology, (ii) ∃ variables
affected by unphysical trends & discontinuities (e.g. MERRA rainfall)
Figure 3
JFMA: nighttime cold bias
(a)
(a) same as Fig. 2 except average
over 10°W-10°E, 10°N-20°N] from
1958 to 2009 for CRU (top) and
ERA-40 (bottom) [note ERAInterim is used after 2002 when
ERA-40 stops, with adjustement.
CMIP5
models
OBS
from
weaker to stronger
warming trend
NorESM1-M
CNRM-CM5
MRI-CGCM3
Figure 4 (a) monthly-mean diurnal cycles of temperature, observations (grey)
and models (pink) in the Sahel (cfSites files, Agoufou pt) (b) temperature and
relative humidity trends in CMIP5 climate models as a function of month,
period ~[1950,2010], models are ordered by increasing warm trends.
(b) as (a) except 1979 to 2009, for
CRU (top) and reanalyses (3 last
plots).
CSIRO-MK3-6-0
6. SUMMARY , PERSPECTIVES
References
Guichard et al. 2009: Surface thermodynamics and radiative budget in the Sahelian Gourma: seasonal and diurnal
cycles, J. Hydrology, 375, 161-177. doi : 10.1016/j.jhydrol.2008.09.007
Guichard et al. 2012 and Hourdin et al. 2012: deliverables ESCAPE (D1.1 and D3.2)
Hourdin et al., 2010 : AMMA-Model Intercomparison Project, Bull. Amer. Meteor. Soc., doi : 10.1175/2009BAMS2791.1
Roehrig et al. 2012: The present and future of the West African monsoon: a process-oriented assessment of CMIP5
simulations along the AMMA transect. J. Climate, submitted.
▶ Observations show a strong warming in the Sahel during the past 60 years, with:
- no clear warming during the dry cool season ~ JFM, ND
“dry” meaning “very low moisture”, not “no rain”
- stronger warming trend during warmer moist months ~ AMJ
- weaker warming during the full monsoon ~ JAS
▶ a large radiative impact of water vapour in Spring ? (monsoon flow), explore
coupling of temperature trends with water vapour changes? data & models
▶ an impact on the monsoon onset? on rainfall intensity? daily data needed
societal repercussions? (agriculture, health...)