|Collaborative Project: Water isotope model-data comparison for Holocene climate variability|
Gavin Schmidt (Goddard Institute of Space Studies, NY)
A renewed focus on reconstructing the paleoclimate of the tropics underscores the dynamic response of the low-latitude hydrologic cycle to changes in the earth?s orbital geometry. Shifting perihelion from northern hemisphere summer in the early Holocene to northern hemisphere winter in the late Holocene has effected the seasonal cycle of incoming solar radiation. Since the early Holocene, the amplitude of the seasonal cycle of incoming radiation has decreased in the northern hemisphere and increased in the southern hemisphere. The increasing tilt of the Earth?s axis since the early Holocene also caused a symmetrical change in the distribution of the mean annual incoming solar radiation, with an increase in the low latitudes and a decrease in the high latitudes. A considerable body of work demonstrates that the low latitude summer monsoons have weakened in response to the Northern Hemisphere trend of decreasing summer radiation. In addition, the mean latitude of the Intertropical Convergence Zone (ITCZ) over Central/South America has migrated south over the course of the Holocene, also in response to weaker northern hemisphere summer insolation. Paleoclimate evidence is also mounting that Holocene orbital trends affected the intensity or frequency of tropical Pacific interannual variability (El Ni?o-Southern Oscillation; ENSO) has varied during the Holocene. These insolation-induced trends, as well as higher frequency variability in the low-latitude hydrological cycle, significantly influenced the distribution of precipitation on land, with consequences felt by early civilizations. Less clear is how changes in the tropical water cycle affected the distribution of fresh water within the global ocean.
We are comparing paleoceanographic and terrestrial data to output from AOGCM simulations. Currently, we are comparing differences in two runs, the "6k" run and the pre-Industrial control run (see Schmidt et al., 2006) to paleoceanographic data. Our goal is to understand how changes in insolation affect the tropical hydrologic cycle. For paleoceanographic data, we are using detailed, well-dated, paired Mg/Ca-oxygen isotope records. Terrestrial archives will include those that records water isotopes - ice cores, spleothems, groundwater.
Several additional experiments are planned.