The knowledge base of Arctic hydrology has expanded due to a recent
study conducted by J. W. Weatherly and J. E. Walsh which focused on
modeling sea-ice and its dependence upon precipitation and river-runoff
(1996). The Arctic Ocean receives a surprisingly large amount of
river-runoff, approximately 10% of the world's total river discharge.
Precipitation over the Arctic's center averages 50-70 cm of snow annually
with localized maxima reaching approximately 700 cm. Consequently,
the Arctic plays an important role in the global energy balance and
the hydrologic cycle and deserves study.
To generate relevant data about the Arctic's role in the hydrologic cycle, specifically those variables related to sea ice, a coupled ice-ocean model was used. The basic features of the ice model include grid boxes of 110 x 110 km and time steps of one day. Three vertical layers were used in the ice model and the model was based upon dynamic and thermodynamic methods developed previously, with certain variations and advancements utilized of course. For example, many of the constants used in the new model were updated (such as albedos, geostrophic winds, etc.) as they were derived from recent measurements.
The ocean model was based upon a spherical coordinate system but was adapted to conform with grid boxes of 110 x 110 km. The vertical resolution of the ocean model is 15 layers, 5 times greater than the vertical resolution of the ice model. Fundamentally, the ocean model is a derivative of the modular ocean model (MOM) developed at NOAA's Geophysical Fluid Dynamics Laboratory. Coupling of the models can only be accomplished every other day, as the MOM employs an additional time-step process to filter spurious transient waves. This process allows for a larger time step. With the coupling of the models though, model initialization can proceed. To initialize the model, data from a previous experiment, as well as observational data gathered from 1980-1989, was used. Initially, all ocean currents were set to zero and the model was allowed to run through a 20 year simulation in order to "spin up" the model.
In order for the model runs which vary precipitation and river-runoff to be useful, a control run was necessary. The control run, along with all subsequent simulations, covered a ten year period. Ice thicknesses obtained from the control run were verified by data collected from submarine sonar data. Modeled results of ice coverage did not always agree with actual values. However, when the Arctic area is considered as a whole, the model ratio of maximum ice coverage to minimum ice coverage (due to seasonal variations) followed closely to a satellite derived ratio.
In addition to ice thickness, the control run also produced ice drift velocity, halocline/mixed layer, salinity, freshwater influx, ocean currents, and temperature data. More importantly the control run produced a stable sea-ice configuration with seasonality differences which were not simply climatological in nature.
Two precipitation scenarios were run following the successful completion of the control run. Doubling precipitation constituted the first variable run of the model, while the second run assumed zero precipitation. Results indicated that a doubling of the precipitation in the Arctic would only slightly increase the amount of sea-ice. Qualitatively, a 10% increase in sea-ice was observed in the simulation. Conversely, the zero precipitation simulation produced a significant decrease in sea-ice. Sea-ice coverage dropped 65% from values found in the control run. The doubled precipitation event produced sea-ice increases due to a decrease in summer ice melt and a greater amount of snow covered areas. Whereas the zero precipitation events allowed ocean salinity to increase (in upper layers) and hence greater mixing occurred as a result of decreasing salinity gradients--weakening the stability of the halocline. The subsequent increase in mixing brings warmer water from below and melts sea-ice.
The second phase of the experiment compared a doubled river-runoff situation and a zero river-runoff scenario with the control. Results indicated that doubling river-runoff caused virtually no change in sea-ice accumulation. However, a zero runoff scenario will reduce sea-ice by approximately 10%.
Clearly, there in a non-linear relation between precipitation amounts versus sea-ice accumulation, as well as river-runoff versus sea-ice accumulation. While this study has provided insight into the Arctic's role in the hydrologic cycle many more questions still remain.
Reference