October 24, 2006: Major problems in the modeling of the Arctic ice-ocean climate system
Hindcast simulations of the Arctic ocean-sea ice system for the last five to six decades indicate large fluctuations in sea ice volume and liquid fresh water storage. Both reservoirs exhibit maximum content in the 1960s and a long-term downward trend that continues today. Both temperature and wind forcing were responsible for the sea ice volume maximum in the 1960s. The long term trend is mostly due to increasing surface air temperatures. On the other hand, the liquid fresh water maximum followed a near-breakdown of exchanges through Fram Strait . The further development can be described as a slow adjustment back towards normal conditions. The simulation results depend sensitively on the treatment of the surface fresh water fluxes, an Achilles heal of virtually every ocean-sea ice hindcast.Over the whole simulation period, none of the reservoirs shows a trend with values in the 1950s being similar to those in the 1990s. Longer time series are necessary to better distinguish natural variability and externally forced trends. Reconstructed atmospheric forcing data are used to drive the NAOSIM ocean-sea ice model and the results are validated with historical observations of sea ice extent. A clear downward trend in ice volume exists over the 20 th century. This result is consistent with the vast majority of IPCC simulations for the 20 th century climate.