Modeling topographic and climatic control of east‐west asymmetry in Sierra Nevada glacier length during the Last Glacial Maximum

Abstract

Glaciers draining westward from the Sierra Nevada divide, California, during the Last Glacial Maximum (LGM) were ∼7 times longer than east‐draining glaciers. We address the degree to which this difference may be attributed to the topographic asymmetry of the west‐tilted Sierran block and the climate asymmetry resulting from orographic modification of Pacific Ocean storms. We simulate kilometer‐scale glaciers within the 50 × 50 km Kings Canyon region of the southern Sierra by employing a two‐dimensional numerical model that is driven by simple, spatially variable climates and treats ice transport by deformation, sliding, and avalanching. In numerical experiments, we match simulated termini to LGM moraine positions to constrain the parameters of different climate scenarios. The 38‐km‐long LGM glacier in Kings Canyon was reproduced by a climate specified by an equilibrium line altitude (ELA) of 3170 m, a mass balance gradient of 0.01 m/yr/m, and a maximum positive balance of 2 m/yr. This climate generates much shorter (average ∼6 km long) east‐draining glaciers that, however, overshoot the LGM moraines by ∼1 km. Roughly 97% of the E‐W difference in glacier lengths can therefore be attributed to topographic asymmetry alone. A second experiment suggesting a 120‐m‐higher ELA of 3290 m east of the divide can explain the shorter east‐draining glaciers. An experiment in which orographic precipitation is explicitly simulated and melt is prescribed using a positive degree‐day algorithm matches both Kings Canyon and the average east‐draining glacier length with an LGM climate that was 5.6°C cooler and ∼2 times wetter than the modern Sierra Nevada.