Xiaojuan Liu

1. The influence of orbital forcings of tropical insolation on the climate and isotopic composition of precipitation in South America

The precession of the earth's rotational axis changes the seasonal distribution of insolation received by the earth. These orbitally paced changes in insolation changed the climate, which was recorded in speleothems in some places of the world. Using an isotope-enabled GCM (ECHAM4.6) coupled to a slab ocean and δ18O records of speleothems, I studied how the climate and isotopic composition of precipitation in South America change on the precessional scale.  I found that:

(1) the δ18O signal in the speleothems can be reproduced by ECHAM 4.6; 

(2) the precession-induced insolation change induces a zonal dipole pattern of precipitation change in South America — less precipitation inland and more along the eastern coast; 

(3) This dipole pattern of precipitation change is induced by both the local and remote insolation forcing.

Reference: 

Liu, X. and D.S. Battisti, 2015. The influence of orbital forcing of tropical insolation on the climate and isotopic composition of precipitation in South America. J. Climate, 28, 4841– 4862. [link]

 

2. What determines the poleward energy transport? 

The atmosphere-ocean system transports energy poleward, balancing the energy surplus in the tropics with the deficit in the extratropics. The previous study argued that the magnitude of the annual mean poleward energy transport (PHT) is insensitive to the details of dynamics of the atmosphere-ocean system.  I studied the effect of cloud cover on the Earth’s PHT by performing a set of variable rotation rate experiments with an aquaplanet GCM (GFDL AM2) coupled to a slab ocean. I found that the spatial pattern of clouds is the leading control on PHT: surface temperature increases with increasing rotation rate, tending to decrease the PHT, but the spatial pattern of clouds changes in a way that the absorbed shortwave radiation increases by an amount that almost offsets the local increase in OLR. This relative constancy in PHT can also be explained in terms of the diffusion of near-surface moist static energy. 

Reference: 

1.  Liu, X., D.S. Battisti, and G.H. Roe, 2017. The effect of cloud cover on the meridional heat transport: Insights from varying rotation rate experiments. J. Climate, 30, 7465–7479. [link]

2. Stone, P. H., 1978: Constraints on dynamical transports of energy on a spherical planet. Dyn. Atmos. Oceans, 2, 123–139, doi:10.1016/0377-0265(78)90006-4.

 

3. The cross-equatorial energy transport and the ITCZ

The position of annual-mean zonal-mean ITCZ is closely linked to the cross-equatorial energy transport by the atmosphere, with a northward displacement of the mean ITCZ corresponding to a southward atmospheric energy transport across the equator because the meridional transport of moisture, sensible heat, and potential energy in the deep tropics is primarily accomplished by the Hadley circulation. Using 12 coupled climate models from PMIP3 and the energetic analysis framework, I studied the change in the annual-mean zonal-mean ITCZ in the mid-Holocene relative to today. I found that the mean ITCZ located more northward in the mid-Holocene relative to present. This is due to the enhanced cross-equatorial northward ocean heat transport which, in turn, is primarily accomplished by a change in the upper-ocean gyre circulation in the tropical Pacific acting on the zonally asymmetric climatological temperature distribution.

Reference: 

1. Liu, X., D.S. Battisti, and A.D. Donohoe, 2017. Tropical precipitation and cross-equatorial ocean heat transport during the mid-Holocene. J. Climate, 30, 3529–3547. [link]

2. Frierson, D. M. W., Y.-T. Hwang, N. S. Fučkar, R. Seager, S. M. Kang, A. Donohoe, E. A. Maroon, X. Liu, and D. S. Battisti, 2013. Contribution of ocean overturning circulation to tropical rainfall peak in the Northern Hemisphere. Nat. Geosci., 6(11), 940–944. [link]