Australsian MonsoonsCollaborators: Yair Rosenthal, Brad Linsley, Mahyar Mohtadi, Fern Gibbons, and Jessica Tierney
The warmest surface waters on earth are in the western Pacific warm pool. This region is a net exporter of water vapour to higher latitudes, most transported by monsoon winds. Changes in temperature of this region influence the amount of poleward water vapor transport. The monsoons, in turn, influence surface properties of the western Pacific. My colleauges and I, as well as other researchers, have found changes in surface water properties on orbital to millennial time scales.
We use stable isotopes and Mg/Ca of planktonic foraminifera to reconstruct changes in surface temperature and the oxygen isotope composition of sea water. We also use organic geochemistry to infer changes regional convection. Our most recent results are summarized in these two publications (with several others submitted, so check back soon):
Mohtadi M., Oppo D.W., Lückge A., De Pol-Holz R, Steinke S., Groeneveld J., Hemme N., Hebbeln D., 2011. Reconstructing the thermal structure of the upper ocean: insights from planktic foraminifera shell chemistry and alkenones in modern sediments of the tropical eastern Indian Ocean. Paleoceanography 26, PA3219, doi:10.1029/2011PA002132.
Mohtadi, M., D. W. Oppo, S. Steinke, J. W. Stuut, E. C. Hathorne, J. Groeneveld, D. Hebbeln, A. Lückge, Glacial to Holocene swings of the Australian-Indonesian monsoon, Nature Geosciences DOI: 10.1038/NGEO1209 (2011).
Tierney, J. E, D.W. Oppo, J. M. Russell, B. K. Linsley, Y. Rosenthal, Coordinated hydrological regimes in the Indo-Pacific region during the past two millennia, region during the past two millennia, Paleoceanography, doi:10.1029/2009PA001871, 2010.
Oppo, DW, Y Rosenthal, BK Linsley, 2000-year-long temperature and hydrology reconstructions from the Indo-Pacific Warm Pool, Nature, 460 113-160, doi:10.1038/nature08233, 2009.
Modeling the Hydrologic CycleCollaborators: Gavin Schmidt, Allegre LeGrande (GISS, Columbia University)
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. 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. 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 Niño-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.
Our first study focussed on the tropical Pacific. We compared differences in two runs, the "6k" mid Holocene run and the pre-Industrial control run (see Schmidt et al., 2006) to paleoceanographic data. Existing data suggested large regional salinity changes in the tropical Pacific, and our goal was to understand the mechanisms causing them.
We found that large changes in export of water vapor in and out of the Pacific Ocean, as well as changes in salt advection by currents, and monsoon-driven redistirbution of rainfall between the land and the ocean.
Oppo, D. W., G. A. Schmidt, and Allegra N. LeGrande, Seawater isotope constraints on tropical hydrology during the Holocene, Geophysical Research Letters, 34, doi:10.1029/2007GL030017, 2007.
Related PublicationsSun, Y., F. Wu, S. C. Clemens, D. W. Oppo, Processes controlling the geochemical composition of the South China Sea sediments during the last climatic cycle, Chemical Geology, 257. 243–249, 2008.
Xiang, R., Y. Sun, T. Li, D. W. Oppo, M. Chen, F. Zheng, Paleoenvironmental change in the middle Okinawa Trough since the last deglaciation: Evidence from the sedimentation rate and planktonic foraminiferal record, Palaeogeography, Palaeoclimatology, Palaeoecology 243, 378-393, 2007.
Dahl, K. D., D. W. Oppo, Sea surface temperature pattern reconstructions in the Arabian Sea, Paleoceanography, 21, PA1014, doi:10.1029/2005PA001162, 2006.
Oppo, D. W., Sun., Y., Amplitude and timing of sea surface temperature change in the northern South China Sea: dynamic link to the East Asian Monsoon, Geology; 33, 785-788; doi: 10.1130/G21867.1, 2005.
Sun, Y., D. W. Oppo*, R. Xiang, W. Liu, S. Gao, The last deglaciation in the Okinawa Trough: subtropical northwest Pacific link to northern and tropical climate, Paleoceangraphy 20, PA4005, doi:10.1029/2004PA001061, 2005. (*corresponding author).
Dahl, K. D., D. W. Oppo, T. I. Eglinton, K. A. Hughen, W. B. Curry, F. Sirocko, terrigenous plant wax inputs to the Arabian sea: implications for the reconstruction of winds associated with the Indian Monsoon, Geochimica et Cosmochimica Acta, 69, 2547-2558, 2005.
Beaufort, L., de Garidel-Thoron, T., Linsley, B., Oppo, D. and Buchet, N., Continental biomass burning and oceanic primary production estimates in the Sulu Sea record East Asian summer and winter monsoon dynamics for the last 380 kyr, Marine Geology, 53-65, 2003.
Oppo, D. W., B. K. Linsley, Y. Rosenthal, S. Dannenmann, and L. Beaufort, Orbital and suborbital climate variability in the Sulu Sea, western tropical Pacific, Geochemistry Geophysics Geosystems, 4, doi:10.1029/2002GC000260, 2003.
Dannenmann, S., B. K. Linsley, D. W. Oppo, Y. Rosenthal, and J.-L. Beaufort, East Asian Monsoon Forcing of suborbital variability in the Sulu Sea during Marine Isotope Stage 3: link to Northern Hemisphere climate, Geochemistry Geophysics Geosystems, 4, doi:10.1029/2002GC000390, 2003.
Rosenthal., Y. , D. W. Oppo and B. K. Linsley, The amplitude and phasing of climate change during the last deglaciation in the Sulu Sea, western equatorial Pacific, Geophysical Research Letters, 30, doi:10.1029/2002GL016612, 2003.