Large-Scale Climate Influences on Interannual Variation in Onyx River Flow

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Karen Cozzetto
Diane McKnight
Balaji Rajagopalan

Large-scale climate features such as sea level pressures and sea surface temperatures have been shown to influence streamflows in regions around the world. We examined the influence of such features on total annual flows in a summertime glacial meltwater river, the Onyx River, in the ice-free McMurdo Dry Valleys, Antarctica. The 38-year Onyx record is the longest flow record for the continent. The study focused on the December-January climate features.

High flow summers were generally characterized by unusually high (+30 m) 500 mb geopotential heights and air temperatures (+1.2 ºC) over Antarctica and by anomalous 500 mb vector winds over West Antarctica moving air from the Berkner Island region around the Transantarctic Mountains, towards the Ross Sea and Dry Valleys.

In contrast during the low flow summers, the 500 mb geopotential height and air temperature anomalies were opposite to those during the common high flow summers and the magnitudes were greater, averaging -50 m and -1.8 ºC, respectively. The summers were also characterized by anomalous 500 mb vector winds over East Antarctica moving air from the interior of the continent out of the South Indian Ocean clockwise around the continent before descending towards the Ross Sea and the Dry Valleys.

The highest flow summer on record, 2001-2, was found to be different than the other high flow summers having 500 mb geopotential heights that were anomalously low over the Antarctic region by as much as 120 m. Instead, the high 2001-2 flows might be attributable to more localized climate factors. One such factor may have been stratospheric ozone concentrations. An ozone hole involving an extreme decrease in ozone concentrations occurs each year over the Antarctic continent. Maximum depletion typically occurs in October. However, during some years, levels of the radiation absorbing ozone may remain somewhat deficient into December. The 2001-2 summer was one year during which this happened directly over the valleys, and that could have contributed to incoming shortwave radiation levels that were among the highest on record. In addition, the 2001-2 summer was one during which the valleys experienced a greater number of katabatic wind events. During these events winds descend into the Dry Valleys from off the polar plateau, warming the air as they do so, often depositing sediment on glacier surfaces. Sediment deposition from the katabatics may have contributed to the much lower than normal outgoing shortwave radiation observed at glacier meteorological stations and also to the much warmer than normal air temperatures recorded at those stations. The unusually high incoming shortwave radiation, unusually low outgoing shortwave radiation, and the higher than normal air temperatures all seemed to have combined to result in the highest flow summer of the record.