Global climate research: Paleoclimate

The importance of precessional signals in tropical climate

Orbital forcing refers to changes in sunshine resulting from variations in earth's orbit, including the ~100,000 year eccentricity cycle, the ~40,000 year obliquity cycle, and the ~20,000 precessional cycle. It is probably the largest natural climate forcing of the Pleistocene era (approximately the past 2 million years). Past research on climate response to orbital forcing has emphasized explaining worldwide signatures of the 100,000 year glacial-interglacial variations in ice volume, temperature, and the hydrologic cycle.  This emphasis may be misleading and inappropriate in the tropics, where the response to the precessional forcing is likely to be somewhat independent of the glacial-interglacial variations, particularly in variables relating to the hydrologic cycle. To illustrate this point, we used an atmospheric general circulation model coupled to a slab ocean model, performing experiments that quantify the tropical climate’s response to (1) opposite phases of precessional forcing, and (2) Last Glacial Maximum boundary conditions. Comparing the simulated responses to these precessional and glacial forcings allows us to assess quantitatively their relative importance in the tropics.

The glacially-forced tropical temperature changes are typically more than an order of magnitude larger than those arising from precessional forcing, confirming the importance of the 100,000 year cycle for tropical temperature variability. However this is not the case for hydrologic signals. The accompanying figure shows mean simulated precipitation (mm/day) during northern hemisphere winter, as well as precipitation anomalies when glacial and precessional forcing are imposed. The figure demonstrates that precipitation anomalies stemming from glacial and precessional forcing are comparable in magnitude. (The same result is seen during the other three seasons.) This is because hydrologic signals are driven by circulation changes stemming from changes in temperature gradients.  Precessional forcing induces significant zonal temperature gradients in the tropics of comparable magnitude to gradients resulting from glacial forcing. The precessional response is large because tropical land masses, with their small heat capacity, respond more strongly to the orbital forcing than adjacent ocean regions, resulting in large zonally-asymmetric changes in circulation (also shown in the figure as vectors representing upper tropospheric winds in m/s) and hydrology.   We also examined the simulated impact of obliquity forcing in the tropics, and concluded its effect on hydrology is small compared to both glacial and precessional forcings.  Finally, we found examples from the paleoclimate record that confirm the importance of precessional signals in past variations of the tropical hydrologic cycle.  

Download the publication (Clement et al. 2004) describing these results in more detail.

Amy Clement of the University of Miami, Alex Hall, and Tony Broccoli of the Rutgers University make up the team that performed this research.