ＭＭ echanisms of global warming impacts on roechanisms of global warming impacts on robustness of tropical precipitation asymmetrybustness of tropical precipitation asymmetry

全球暖化下熱帶降水不對稱現象一致性的機制研究

報告人：談珮華 博士國立嘉義大學史地學系副教授

日期： 2009 年 10 月 7 日

Global Warming ：溫室效應的加強

溫室效應• 溫室氣體• 例： CO2, CH4, N2O, HFCs, PFCs, SF6

• 聯合國氣候變遷綱要公約－　京都議定書

Global Warming impacts on temperature (1)

• Anthropogenic forcing of greenhouse gases (IPCC, 2007) 聯合國跨政府氣候變遷小組第四次評估報告

• A significant warming trend of globally averaged surface temperature (e.g., Santer et al. 1996; Trenberth et al. 2007).

• Tropospheric temperature also increases with time (Fu et al. 2004).

• For the future projection of climate models to the end of the 21st century, the warming becomes much stronger (e.g. Meehl et al. 2007).

• Globally warming trend: 0.6 C/100yr

• Future projection at 2100: 1.8 – 4 C

Global Warming impacts on temperature (2)

• Precipitation changes of climate model simulations reveal a much more complicated pattern, particularly on a regional basis.

• On a global scale, most model simulations show a positive trend of global-mean precipitation in the future, but this trend has not found in observations (Allen and Ingram 2002).

• On a regional scale, projected precipitation changes have great differences in many climate models (e.g. Neelin et al. 2006; Meehl et al. 2007).

Global Warming impacts on precipitation (1)

• Disagreement is found not only among climate models, but also between climate models and observations (e.g. Allan and Soden 2007; Zhang et al. 2007).

• This implies that precipitation changes have complex and distinct behaviors among different regions. complicated hydrological cycle (Allen and Ingram 2002) and the interaction between large-scale dynamics and tropical convection (e.g., Chiang and Sobel 2002; Neelin et al. 2003).

Global Warming impacts on precipitation (2)

• Similarities for tropical droughts over margins oSimilarities for tropical droughts over margins of convective regions, especially over Central Af convective regions, especially over Central American and the Caribbean Sea during summer merican and the Caribbean Sea during summer (Neelin et al. 2006).(Neelin et al. 2006).

• Tropical precipitation asymmetry in a hemispheTropical precipitation asymmetry in a hemispherical average (Chou, Tu and Tan, 2007).rical average (Chou, Tu and Tan, 2007).

Global Warming impacts on precipitation (3)

Goal

• Examine what mechanisms control asymmetric responses of tropical precipitation changes between hemispheres.

• Understand why this asymmetry is in good agreement among different coupled atmosphere-ocean climate models (CGCMs). 大氣海洋耦合的氣候模式

The Data

• Thirteen IPCC AR4 model simulations

• (2070-2099)-(1961-1990)

• Future － current climate = anomaly

• 距平或異常：比如溫度距平或溫度異常

Resultstropical precipitation tropical precipitation

asymmetry under global asymmetry under global warmingwarming

(2070-2099)-(1961-1990)

WCRP CMIP3

IPCC AR4

13-model ensemble mean

(a) P anomaly - ASO

(b) P anomaly - FMA

Tropical precipitation Tropical precipitation

asymmetry under global asymmetry under global

warming can be found in the warming can be found in the

13-model ensemble mean and 13-model ensemble mean and

also for each model.also for each model.

Zonal averages for precipitation anomalies of multi

model ensemble

ASO:

Monsoon 42%

FMA:

ENSO 43%

Effect on the magnitude of precipitation asymmetry

No effect on the scattering of its asymmetry

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哈德雷環流

Conclusions• 1) The trends of the hemispherically-averaged tropical precipit

ation vary strongly with season, which indicates an enhancement of the seasonal precipitation range, defined as differences between wet and dry seasons, and the hemispherical difference, defined as differences between summer and winter hemispheres.

• 2) Considering averages in different seasons and over different hemispheres, the hemispherically-averaged tropical precipitation anomalies exhibit a robust asymmetric pattern among climate models: increased precipitation over the summer hemisphere and slightly reduced precipitation over the winter hemisphere.

Conclusions

3) For the multimodel ensemble, the seasonal precipitation range is increased by about 12% relative to the current climate (0.4 mm day-1) and the hemispherical difference is increased by about 14% (0.45 mm day-1). These values are much larger than the change of the averages over the entire tropics (3% and 0.1 mm day-1), so both the hemispherical difference and the seasonal precipitation range might be useful indicators for detecting global warming impacts on tropical precipitation.

Conclusions• 4) Examining the spatial distribution of the precipitation

anomalies, the magnitude and area of positive precipitation anomalies are much greater and wider than those of negative precipitation anomalies in both ASO and FMA. Most positive precipitation anomalies are found in the convergence zones, while negative precipitation anomalies are found in the convective margins and the descending regions.

• 5) The vertical moisture transport associated with the mean Hadley circulation is asymmetric to the equator, which is a robust feature among the 13 climate models and a major process for inducing the tropical precipitation asymmetry.

Conclusions• 6) Under global warming, tropospheric water vapor incr

eases as the temperature rises and most enhanced water vapor is at the lower troposphere. The ascending motion of the Hadley circulation then transports more water vapor upward, i.e., anomalous moisture convergence, and enhances precipitation over main convection zones. On the other hand, the thermodynamic effect associated with the descending motion of the Hadley circulation, i.e., anomalous moisture divergence, reduces the precipitation over the descending regions.

全球暖化下熱帶降水南北不對稱現象一致性的機制研究 (1)

• 本文以十三個海氣耦合氣候模式來探討全球暖化下熱帶降水南北半球不對稱及季節不對稱現象。在北半球夏季晚期，系集模式顯示在熱帶降水距平的半球平均，北半球呈現明顯增加而南半球則是微弱減少的趨勢。另一方面，在南半球夏季晚期時，南北半球的趨勢就相反過來。這表示熱帶地區，夏半球會越溼而冬半球會有點乾。因此熱帶降水的季節範圍，以乾溼季降水差異來定義，是增加的。系集模式緯向平均的降水距平也有南北方向的移動。類似的不對稱現象不只在系集模式，本研究所使用的十三個氣候模式的個別模擬中也可發現。

全球暖化下熱帶降水南北不對稱現象一致性的機制研究 (2)

• 根據水氣收支分析，與平均環流相關的垂直水氣平流，是熱帶降水距平不對稱現象一致性的主要貢獻者。在全球暖化下，當溫度增高時，對流層水氣會變多，而大多數增加的水氣是集中在低對流層。哈德雷環流的上升環流會將更多的水氣往上傳送，即產生水氣輻合距平，使得主對流區降水會變多。換句話說，與哈德雷環流下沉運動相關的熱力效應，即產生水氣輻散距平，會使下沉區降水變少。

• Thank you for your attention.

參考資料

• Tan, P.-H., C. Chou, and J.-Y. Tu, 2008: Mechanisms of global warming impacts on robustness of tropical precipitation asymmetry, J. Climate, 21, 5585-5602.

• Chou, C., J.-Y. Tu, and P.-H. Tan, 2007: Asymmetry of tropical precipitation change under global warming, Geophys. Res. Lett., 34, L17708, doi:10.1029/2007GL 030327.

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