The Influence of Obliquity on Quaternary Climate

Transcription

1 The Influence of Obliquity on Quaternary Climate Michael P. Erb 1, C. S. Jackson 1, and A. J. Broccoli 2 1 Institute for Geophysics, University of Texas at Austin, Austin, TX 2 Department of Environmental Sciences, Rutgers University, New Brunswick, NJ 20 th Annual CESM Workshop, Breckenridge, CO, June 15-18, 2015

2 Introduction Changes in the tilt of the Earth s axis (obliquity) have a considerable effect on insolation, especially at higher latitudes. Despite this, many proxy records show little obliquity signal during the late Quaternary. 1) How does obliquity affect climate in AOGCMs? 2) Are proxy records consistent with these changes? NASA

3 Experimental design The climate response to obliquity is explored in three coupled AOGCMs. GFDL CM2.1: Low (~22 ) and high (~24.5 ) obliquity simulations NCAR CESM: Low (~22 ) and high (~24.5 ) obliquity simulations HadCM3: Obliquity isolated from 62 orbitally-forced snapshot simulations spanning ka (Singarayer and Valdes 2010) HadCM3 methodology: A multiple linear regression is conducted at every cell to determine the portions of the climate response that vary with obliquity, precession, and eccentricity. These models offer three estimates of the climate response to altered obliquity.

4 Effect on insolation Obliquity doesn t change total insolation, but redistributes it meridionally and seasonally. Zonal-mean insolation anomaly (Wm -2 ) over the course of a year for low (~22 ) minus high (~24.5 ) obliquity. Lowered obliquity: Annual-mean insolation is increased at low latitudes and decreased at high latitudes. Insolation seasonality is weakened.

5 Temperature response Despite zero global-mean change in insolation, all models show global-mean cooling. In some areas, cooling occurs despite local insolation increase. What contributes to the predominance of cooling?

6 Radiative feedbacks In response to low obliquity, feedbacks cool at high latitudes and are mixed at lower latitudes. High latitudes: Ice albedo: Increased sea ice (especially May-September) reflects additional sunlight. Lapse rate: The higher atmosphere does not cool as much as the surface. (Because much OLR comes from the higher atmosphere, changes in the lapse rate allow larger or smaller surface anomalies.) Low latitudes: Clouds: Changes in low cloud amount increase or decrease regional albedo. Total effect of radiative feedbacks (surface albedo, water vapor, lapse rate, and clouds) on net TOA radiation (Wm -2 ), calculated with Kernel method (Soden and Held 2006). Blue = decreased net downward radiation. Red = increased net downward radiation.

7 Energy transports Lower obliquity increases the annual-mean equator-to-pole insolation gradient. To partly balance this, meridional energy transport increases in both the atmosphere and ocean. More energy is transported away from the equator and toward the poles, where strong feedbacks can help cool the climate.

8 Comparing idealized simulations to proxies In the GFDL CM2.1, single-forcing experiments have been run for obliquity, precession, CO 2, and ice sheets. By scaling the modeled responses by time series of past forcings, model-based linear estimates of past climate can be made. Vostok temperature record (Petit et al. 1999) compared to a model-based reconstruction. On the whole, the model-based estimate does a decent job at Vostok. However, considerable mismatches appear to correspond with the obliquity signal. Should modeled responses be larger or smaller to best match the data?

9 Response scaling - Vostok Vostok (106.8 E, S; Antarctica) A Bayesian methodology is employed to select scaling factors for each response term, so that the reconstruction best matches the data. Top: Unfitted (default). Middle: Best-fit. Bottom: Posterior PDF of fitting parameters. This suggests that the obliquity response is smaller in the proxy than in the model.

10 Response scaling TR TR (90.95 W, 2.26 N; equatorial Pacific) At the location of TR163-19, the modeled temperature response to obliquity is small. The fitting methodology suggests a slightly larger (but still small) signal in the proxy.

11 Conclusions In all three models, lowered obliquity cools global climate by ~0.5-1 C despite no change in total insolation. Low latitudes: Modeled temperature changes are near zero in the tropics, consistent with the lack of obvious obliquity signal in the TR record. Increased energy is transported from low latitudes toward the poles, where climate can be cooled by strong feedbacks. SH high latitudes: Models produce sizeable cooling at high latitudes, which does not seem to be supported by the data. The fitting exercise suggests that Vostok has less than half of the modeled obliquity response. Uncertainties remain. The results may be affected by obliquity signals in other forcing records as well as potential non-linearities in the climate response. However, results suggest that models (using this linear assumption) can not explain all aspects of Southern Hemisphere high-latitude climate change.

12 Potential equilibrium issue: albedo In CESM, surface albedo in JJA changes greatly with obliquity, leading to large temperature anomalies. Is the CESM s albedo too high for snow and/or ice? English et al. 2014: However, CAM5 has compensating SW errors: Surface albedos over snow are too high while cloud amount and LWP are too low.

13 References English, J. M., J. E. Kay, A. Gettelman, X. Liu, Y. Wang, Y. Zhang, H. Chepfer, 2014: Contributions of clouds, surface albedos, and mixed-phase ice nucleation schemes to Arctic radiation biases in CAM5. J. Climate, 27, , doi: /jcli-d Petit, J. R., et al., 1999: Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature, 399, Singarayer, J. S., and P. J. Valdes, 2010: High-latitude climate sensitivity to ice-sheet forcing over the last 120 kyr. Quat. Sci. Rev., 29, 43-55, doi: /j.quascirev Soden, B. J., and I. M. Held, 2006: An assessment of climate feedbacks in coupled ocean-atmosphere models. J. Climate, 19, Data access CESM obliquity output is available on NCAR s Yellowstone at: /glade/p/cesm/palwg_dev/singleforcing/ Future idealized single-forcing experiments will be added: precession, CO 2, ice sheets. For climatologies or questions, contact me at merb@ig.utexas.edu. Thank you. Questions?