Changes in Tropical Precipitation at the Mid-Holocene: Role of the Oceanic Heat Transport

There is ample geological and geochemical evidence that precipitation in the tropics is largely different from today at the mid-Holocene, an era roughly 6,000 years ago when the Northern Hemisphere summer (winter) insolation was stronger (weaker) than today. These insolation differences are caused mainly by the precession of the earth's rotational axis, or called "precessional forcing". Using the mid-Holocene experiments of PMIP3, we studied changes in the zonal mean tropical precipitation, and its associated change in cross-equatorial energy transport. A northward movement of the zonal mean precipitation in the mid-Holocene is seen in 10 out of 13 PMIP3 models, with a correspondingly anomalous southward atmospheric heat transport across the equator. The slope is 3.0º per PW, close to the estimate given by Donohoe et al. (2013). The changes in cross-equatorial atmospheric heat transport are dictated by changes in the hemispheric asymmetry of heating from the surface, which in turn are associated with changes in the cross-equatorial oceanic heat transport: an anomalous northward oceanic heat transport at the equator is seen in all of the PMIP3 models. Analysis on this anomalous oceanic heat transport reveals that changes in the wind-driven gyre in the Pacific Ocean are primarily responsible for the changes in cross-equatorial ocean heat transport. Specifically, stronger easterly anomalies north of the equator in the western Pacific drives an anomalous northward mass transport, and therefore accomplishes an anomalous northward heat transport across the equator by acting on the asymmetric mean-state zonal temperature. The wind anomalies responsible for this anomalous ocean heat transport are seen in every PMIP3 model, as well as an ECHAM4-slab ocean model, indicating that it is atmospherically driven and independent of the changes in ocean heat transport. It also explains the consistency of ocean heat transport change, and eventually the relative consistency of zonal mean precipitation change across the PMIP3 models. Although we focus on the change of the zonal mean precipitation, it is important to note that the dominant changes in climate associated with precessional forcing are zonally asymmetric and associated with changes in the monsoon and independent of changes in ocean circulation.