Controls on the Basal Water Pressure in Subglacial Channels Near the Margin of the Greenland Ice Sheet

Assuming a channelized drainage system in steady state, we investigate the influence of enhanced surface melting on the water pressure in subglacial channels, compared to that of changes in conduit geometry, ice rheology and catchment variations. The analysis is carried out for a specific part of the western Greenland ice-sheet margin between 66 degrees N and 66 degrees 30' N using new high-resolution digital elevation models of the subglacial topography and the ice-sheet surface, based on an airborne ice-penetrating radar survey in 2003 and satellite repeat-track interferometric synthetic aperture radar analysis of European Remote-sensing Satellite 1 and 2 (ERS-1/-2) imagery, respectively. The water pressure is calculated up-glacier along a likely subglacial channel at distances of 1, 5 and 9 km from the outlet at the ice margin, using a modified version of Rothlisberger's equation. Our results show that for the margin of the western Greenland ice sheet, the water pressure in subglacial channels is not sensitive to realistic variations in catchment size and mean surface water input compared to small changes in conduit geometry and ice rheology.

Physical and Biological Controls on the Latitudinal Asymmetry of Surface Nutrients and pCO(2) in the Central and Eastern Equatorial Pacific

Surface nutrients and dissolved inorganic carbon (DIC) in the central (CEP) and eastern equatorial Pacific (EEP) show much higher concentrations to the south than to the north of the equator. In this study, the physical and biological controls on this asymmetry are investigated using a coupled physical-biogeochemical model. Two numerical experiments are conducted to examine the effects of asymmetrical photosynthetic efficiency (a) due to asymmetrical iron supply about the equator. The experiment with asymmetrical photosynthesis produces improved results as compared with historical observations. A nitrate budget analysis suggests that in the EEP the divergence of upwelling waters controls the surface nitrate asymmetry with additional contribution from the South Equatorial Current (SEC) carrying nutrient-rich Peru upwelling water. The changes of a affect the surface nitrate distribution but not the overall asymmetry. The SEC further carries excess nitrate to the west and thus extends the asymmetry in the east to the CEP. In the CEP, however, stronger northward than southward transport tends to reduce the nitrate asymmetry, while the asymmetrical photosynthesis would help to maintain it. Similar processes also control the distributions of surface silicate and DIC in the equatorial Pacific, which is also affected by the air-sea CO(2) exchange. The asymmetrical photosynthesis influences the distribution of surface DIC, pCO(2), and the air-sea CO(2) flux, by redistributing about 20% CO(2) flux from the north to the south of the equator. Owing to the adjustment of air-sea CO(2) flux, however, the net surface DIC change is smaller than the direct change associated with primary production.