HIGH-RESOLUTION MODEL OF OROGRAPHIC PRECIPITATION AND WATER ISOTOPE FRACTIONATION, AS APPLIED TO THE PATAGONIAN ANDES, WITH IMPLICATIONS FOR UNDERSTANDING PALEOTOPOGRAPHY AND CLIMATE-TECTONICS FEEDBACKS

Orographic precipitation is an important feedback between climate and topography. Isotopic fractionation of precipitation has also become an essential tools for estimating paleotopography. Geologic studies of rivers, glaciers, and erosion generally require information about climate, not weather. In other words, we need a good representation of the time-averaged precipitation field and the associated water isotope fields (δ2H and δ18O).

The “linear theory of orographic precipitation” (LTOP) model of Smith and Barstad (2004) is ideal for this purpose. It provides a full representation of the precipitation field created by steady atmospheric flow of fully saturated air over an arbitrary mountain topography at a spatial resolution of 1 km or better. The model is based on only 9 parameters: wind speed and direction, sea-level temperature, Brunt-Väisälä frequency, eddy diffusion, time delays for formation and fallout of hydrometeors, and the δ2H and δ18O of the first precipitation. We have added a full isotope fractionation calculation to the model. The program takes only 20 seconds on a laptop to calculate the full precipitation and water isotope fields for a 1000 x 1000 km region.

We demonstrate this model by fitting 185 measurements of modern δ2H and δ18O meteoric water, collected in a 500 km wide region straddling the Patagonian Andes from 40 S to 48 S. A non-linear inverse was used to find model parameters that best fit the water isotope data. The data are well fit by the model and the estimated parameters match well with known values from modern climate data. The highest precipitation rates are located over the Northern and Southern Patagonian Ice Sheets. An important advantage of the mean solution is that we can perturb the model to see how changes in climate and topography would change the precipitation and water isotope fields. This exercise shows that the precipitation rate and water isotope fractionation are linearly dependent on the size of the topography. The water isotope fractionation is also strongly dependent on sea level temperature. For example, the isotopic shift caused by a 5 C increase in sea surface temperature would be equivalent to that produced by 20 percent decrease in topography. Also notable is that the other model parameters have little influence on the isotopic fractionation.