Influence of large scale climate variations on the isotopic composition of tropical precipitation 1∗
1
Camille Risi , Sandrine Bony , Françoise Vimeux 1
LMD/IPSL, Paris (France), 2LSCE/IPSL, Paris (France), ∗contact:
[email protected]
Introduction
1.2 LGM simulation
Water stable isotopes (HDO, H218O) constitute a promising tool to reconstruct past climate variations. In the tropics, however, the interpretation of isotopic composition changes in terms of climate variations is debated. By analyzing simulations of a singlecolumn model run in radiative-convective equilibrium and of a general circulation model (LMDZ GCM) equipped with water stable isotopes, we investigate the relative contributions of regional dynamical changes and of coherent tropical temperature changes on the isotopic composition of the tropical precipitation, at the interannual time scale and in climate change. Notation: δ 18O and δD measures the enrichment in heavier isotopes. Deuterium excess d = δD − 8 · δ 18O measures the enrichment in HDO relatively to H218O.
We conducted a Last Glacial Maximum (LGM) simulation simulation forced by CLIMAP SST. The model simulates reasonably well the more depleted precipitation during the LGM at high latitudes, but underestimates the depletion observed ponctually in tropical ice cores, as most other isotopic GCM ([4, 6, 5]).
3. Relative contribution of dynamical changes in precipitation and global temperature changes At the regional scale, the amount effect dominates the precipitation δ 18O signal at seasonal, inter-annual and climatic scales. But is there a larger-scale isotopic signature of climate change, as suggested by the enhanced depletion in most tropical data available at the LGM? Fig 5 compares the relative impact on δ 18O of changes in tropical mean SST (T¯ ) and in regional precipitation rate P , for both SCM ([1]) and LMDZ simulations.
δ 18O LGM - present day change (h) simulated by LMDZ Cape Century −12.9 −1
Summit −6.6 −7
Renland −5 −5
Europe
England −1.2 −9
−15 to −40 −60 à +20
N. Africa
−20 to −80 0 to +20
US
−10 to −25 −50 to +20 Dunde −2 +3 Huascaran −6.3 0
Qinghai −5.4 −1.2
Sajama −5.4 0
Namibia +1.5 0
1.1 Evaluation for the present day
Vostok −3.5−7 observed δ 18 O (h) LMDZ δ 18 O (h)
observed δD (h) LMDZ δD (h)
a
Fig 3: LGM - present day change in δ 18 O in precipitation simulated by LMDZ, compared to some available observations from ice cores, speleothems or groundwaters (cited in [4, 6, 5]). d-excess (h) LMDZ
2. Are isotopes in tropical precipitation a good proxy for precipitation rate?
−40
c
−30 −20 −18
−12
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−6
−4
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−30 −20 −18
−12
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δ O (h) observations
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LMDZ, composites over tropical oceans 1D model with Ts = 29◦C and varying ω GNIP data, tropical island
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δ 18 0 (h)
Fig 2: Averaged amount effect simulated by LMDZ and a SCM ( [1]) over tropical oceans, compared with monthly data from tropical stations of the GNIP network ([3])
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d-excess (h) observations
−2
Fig 1: Annually weighted averaged δD and dexcess simulated by LMDZ (above) and from observations (below: GNIP network ([3]) and Antarctic data ([7])
4
a
• the amount effect is robust and similar at the inter-annual and climatic scales,
−6 −8 −10 −12 0
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precipitation rate (mm/day)
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• at first order and at the regional scale, δ 18O records precipitation changes.
−4 −6
−12 0
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20N
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10N eq
-4
10S 20S 30S 0
60E
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−0.1 −0.2 −0.3 −0.4 −0.5 −0.6 −0.7 −0.8 −0.9 −1 −2
d
LMDZ, inter-annual variability
20N
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10N eq
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10S
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20S
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30S 0
60E
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120E
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P ր
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δ 18 O ց
LMDZ, climate change experiments unsaturated downdraft
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surface evaporation
present day SST T¯ increased by 2K T¯ decreased by 2K T¯ decreased by 4K
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Ts − T¯ ր ωց (regional change)
low-level vapor
−4
T¯ ր (global change)
δ 18O ր Eր
−5
−4
−3
precipitation rate (mm/day)
30N
60W
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precipitation rate (mm/day)
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-9
LGM - today precipitation change reconstructed LMDZ δ 18 O (mm/day)
120W
2
−1
T¯ anomalies > 0.3◦C composite T¯ anomalies < 0◦ C composite
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-10 0
Ts − T¯ < 0
T¯
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120E
LMDZ δ 18 O LGM - present day change
b
Ts − T¯ > 0
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−10 0
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-8
0
P
−5
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precipitation rate (mm/day)
c
ω>0
−4
−10
30N
60W
ω
