The Influence of Flow Deflection on Leeside Isotope-Based Paleoaltimetry Estimates in the New Zealand Southern Alps

Abstract:

Topography develops as a result of the interaction between tectonics, surface processes, and climate and can yield insight into the evolution of a mountain range. Global δ¹⁸O data from meteoric water show a linear relationship between net elevation and Δδ¹⁸O on the windward side of a mountain range, where δ¹⁸O values decrease as net elevation increases. The development of relief may generate leeside rain shadows where the δ¹⁸O values are lower due to rainout on the windward side. The magnitude of lowering in isotopic values in paleo-δ¹⁸O sampled from the leeside of a mountain range should therefore be related to the elevation of the range. Several studies have used this relationship to constrain the topographic evolution of mountain ranges using leeside isotope records. One assumption in this technique is that the interaction between the mountain range and atmosphere is two-dimensional and that all air masses surmount and record the highest elevation. For some mountain ranges, the topographic evolution from leeside isotope records is complicated by flow blocking and deflection. This does not appear to be the case in the New Zealand Southern Alps. The record of δ¹⁸O collected in kaolinites on the leeside of the Southern Alps agrees well with other lines of evidence suggesting that the mountain range uplifted ~2 ± 1 km over the last 5 Ma. In this study we aim to address to what extent flow deflection around a topographic barrier influences leeside isotope-based paleoaltimetry for the New Zealand Southern Alps. Flow deflection can be quantified using the nondimensional flow parameter Nh/U, where N is the buoyancy frequency (1/s), h (m) is the mountain height, and U (m/s) is the wind speed. Using the Weather Research and Forecasting model to dynamically downscale NCEP 6-hourly reanalysis at 32 km grid spacing we run back-trajectory analyses for locations on the leeside of the Southern Alps. We also characterize storms that reach the leeside in terms of Nh/U to determine the degree of blocking during these events. Preliminary results suggest that flow deflection is relatively infrequent in the Southern Alps, at least in the modern climate, and that due to the relatively low elevation of the mountain range that leeside isotope-based paleoaltimetry records may not be as complicated by three-dimensional flow as other settings.