上新世中期古气候模拟研究

1. 上新世中期古气候模拟研究 Jiang, D., H. –J. Wang, Z. L. Ding, X. Lang, and H. Drange, Modeling the middle Pliocene climate with a global atmospheric general circulation model, Journal of Geophysical Research, 2005, 110, D14107, doi:10.1029/2004JD005639.

2. Observed annual temperature trend (IPCC, 2001)

3. Observed seasonal temperature trend (IPCC, 2001)

4. Motivation With the steady increasing concentrations of atmospheric greenhouse gasses, it is of particular interest to explore warm past climate intervals that may have some similarities to the expected near future climate state. The middle Pliocene (ca. 3 Ma BP), the most recent interval of geologic time during which global climate was significantly warmer than today (e.g. Crowley, 1996; Dowsett et al., 1996), may provides a unique climate interval to improve our understanding on the operation of a warmer-than-today climate regime.

5. Outline 1. Boundary conditions 2. AGCM and experiments 3. Results 4. Conclusions Discussions

6. 1. Boundary conditions The middle Pliocene surface conditions are the U.S. Geological Survey’s Pliocene Interpretations and Synoptic Mapping (PRISM2) 2ox2o digital data set, including six items, i.e. monthly SST and sea ice extent, coastline, continental topography, surface vegetation, and continental ice sheet. The RRISM2 data set represents a reconstruction within the time period 3.29-2.97 Ma BP according to the geomagnetic polarity timescale of Berggren et al. (1995). Atmospheric CO2 level is set to 315ppmv (ca.1958)

11. Features of the conditions Reconstruction data from various proxies indicate that the warm middle Pliocene climate was characterized by 1) warmer SST at mid to high latitudes 2) reduced extent and thickness of ice sheets and sea ice 3) a sea level rise of at least 25 metres 4) the presence of warmth or moisture loving vegetation at mid to high latitudes compared with the present (Dowsett et al., 1999, USGS Open file Report 99-535, and references therein)

12. 2. AGCM and experiments AGCM: a global atmospheric GCM 5o×4o (Lon by Lat), 9 vertical levels with top at 10 hPa (Zeng et al., 1987; Zhang, 1990; Bi, 1993; Liang, 1996)

15. For surface air temperature, spatial correlation coefficient is high up to 0.98, and rmse is 2.81oC between simulation and observation during 1980-1989 in the domain of 70-140oE and 15-60oN. For precipitation, they are 0.80 and 1.18 mm/day. Therefore, the model is quite representative in East Asia.

16. 2. AGCM and experiments Surface boundary conditions for experiments Exp. 1: Today configuration Exp. 2: PRISM2 dataset from USGS Exp. 3: As Exp2, but with today vegetation Exp. 4: As Exp2, but with today continental ice sheet Exp. 5: As Exp2, but with today monthly SST and sea ice extent All simulations were run for 12 years, and the results reported here are ensemble averages for the last 10 years.

17. 3. Results, for annual surface air temperature Exp2 minus Exp1: Middle Pliocene Exp2 minus Exp4: continental ice sheet Exp2 minus Exp3: vegetation Exp2 minus Exp5: SST and sea ice

18. 3. Results, for annual precipitation (Exp2 minus Exp1) Total precipitation (mm/day) Convective precipitation (mm/day) Large-scale precipitation (mm/day)

19. 3. Results, wind field at 850 hPa (Exp2 minus Exp1) Boreal summer wind differences East Asian summer monsoon intensity was notably decreased. Boreal winter wind differences East Asian winter monsoon intensity was slightly declined.

20. 3. Results, wind field at 200 hPa (Exp2 minus Exp1) Easterly wind anomalies prevail within subtropical zone in both hemispheres in summer. Boreal summer differences East Asian subtropical westerly jet, the most important atmospheric general circulation system in the upper troposphere in the region, weakens obviously from eastern China to the dateline both in winter and summer, consistent with the UKMO AGCM (Haywood et al., 2000) Boreal winter differences Easterly wind anomalies prevail within 40oN-40oS. m/s

21. 3. Results, annual mean Hadley cell (Exp2 minus Exp1) m/s by 10-4 hPa/s m/s by 10-4 hPa/s Today Middle Pliocene minus today Hadley cell is reduced at the middle Pliocene, consistent with the GISS AGCM (Chandler et al., 1994)

22. GISS (Chandler et al., 1994) NCAR-GENESIS (Sloan et al., 1996) UKMO (Haywood et al., 2000) IAP-AGCM Temperature (NH) (1.4) 3.6 1.9 2.6 (3.0) Precipitation (NH) (5.1%) 5.0% 6.0% 4.0% (-0.9%) 3. Results, Model-model comparison Globally annual mean surface temperature difference (C) and the precipitation fraction value at the middle Pliocene relative to today. As for globally annual mean temperature and precipitation, the above models generally accord with each other. However, regional discrepancies are also pronounced.

23. 3. Results, model-records comparison in East Asia Warmer and slightly wetter conditions at the middle Pliocene qualitatively agrees with the records (Han et al., 1997, 2002; Ding et al., 1998). Weaker-than-today East Asian winter monsoon at the Middle Pliocene is consistently reflected in the Chinese Red Clay and marine sediments (Ding et al., 1998, 2000; An et al., 2001; Tian et al., 2002, 2004; Wehausen and Brumsack, 2002). In general, proxies suggested a weak or similar East Asian summer monsoon intensity at the middle Pliocene (Sun et al., 1998; An et al., 1999, 2001; Ding et al., 1999, 2001; Tian et al., 2002, 2004; Wehausen and Brumsack, 2002). It is difficult to evaluate whether or not data-model is consistent before much more records are reported.

24. 4. Conclusions 1) At the middle Pliocene, globally annual mean surface temperature increased by 2.60oC compared with the present. The warming generally decreased towards low latitudes. In contrast, cooling conditions were registered over parts of the Tibetan Plateau, over most parts of the northern tropical Pacific and the East African rift zone, and at 80-90oS. 2) The reconstructed SST and sea ice extent are main reason for the middle Pliocene warmth as simulated by the IAP AGCM. The influences due to the PRISM2 reconstructed vegetation and continental ice sheet cannot be neglected in some regions at mid to high latitudes, such as over Greenland, over the Tibetan Plateau, and in the circum-Antarctic, although their individual effects are quite limited on a global scale.

25. 4. Conclusions 3) Slightly wetter climate dominated at the middle Pliocene. Globally annual precipitation increased on average by 4.0%, and terrestrial precipitation was 112.5% of the present value. The precipitation enhanced notably at high latitudes. On the contrary, drier conditions were registered over most regions in northern tropics. Annual precipitation was significantly reduced in much of East Asia and over the northern tropical Pacific. 4) At the middle Pliocene, both East Asian summer and winter monsoon systems were significantly decreased relative to the present. The IAP AGCM indicates the PRISM2 SST and sea ice extent itself can induce such changes. Therefore, particular attention should be paid to oceanic behaviors when exploring the Pliocene climate changes in East Asia.

26. Discussions In general, the middle Pliocene warmth is believed to mainly relate to the increased pole-ward ocean heat transports (e.g. Cronin, 1991; Rind and Chandler, 1991; Crowley, 1996; Dowsett et al., 1996; Raymo et al., 1996; Billups et al., 1998; Kim and Crowley, 2000; Bennike et al., 2002), to slightly increased atmospheric CO2 concentration (Crowley et al., 1996; Dowsett et al., 1996),to alterations in the cryosphere (Haywood and Valdes, 2004), or to combinations thereof. A combination of modeling studies and evidence from the Pliocene fossil flora indicate that the atmospheric CO2 concentration at 3 Ma BP was not too far from its current level (e.g. Rind and Chandler, 1991; Raymo et al., 1996), so it cannot be used to fully account for the middle Pliocene warmth. Therefore, the causal factors of the middle Pliocene warming and future climate change are undoubtedly different.

27. Discussions Modeling the middle Pliocene climate is of interest because it can: • provide insights into the nature of warmer-than-today climate and consequently improve our understanding of processes involved in a warm climate; • assess climate model’s ability to reproduce a warmer climate; • examine the sensitivities of climate models to different forcings and boundary conditions and the associated feedbacks that may be operating in a warm climate regime; • help to interpret the Pliocene geological records.

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