CSAS Findings

Research Efforts to Date

Our efforts have focused on two areas, evaluating and improving the ecosystem model performance in the arctic region, and modifying the model code to allow for iron accumulation in, and transport by sea ice. We have looked at the ecosystem/biogeochemical module performance in the Arctic region in a one degree coupled model simulation, focusing on comparisons with observed chlorophyll (satellite) and nutrient distributions (World Ocean Atlas climatologies). The model does a reasonable job in the Arctic region, but we are exploring ways to improve the performance, using the coarser resolution (3 degree) version of the ocean model, including modifications to a few key parameters that govern grazing rates, and the remineralization of sinking organic matter. Graduate student Shanlin Wang has been modifying the ocean model code to allow for passing dissolved iron between the sea ice and ocean models. This will permit transport of iron by sea ice, which we hypothesize influences productivity in some regions of the both Arctic and Antarctic. We are also in the process of modifying the sea ice model code in this regard in collaboration with Marika Holland and Dave Bailey at NCAR. These efforts are progressing and we anticipate our first simulations designed to look at sea ice iron-transport influences on ocean biogeochemistry will be conducted this summer.

Research Findings

The new Community Climate System Model, version 4 (CCSM4) has improved sea ice shortwave radiative transfer and new capabilities, including a melt pond parameterization and aerosol deposition and cycling. These have implications for the timing of seasonal triggers (such as melt onset, pond formation, freezeup) that can influence the shortwave absorption and albedo feedback. These improvements have been documented in a submitted manuscript (Holland et al., submitted) that assess the role of melt pond formation and aerosol (including black carbon) deposition on the sea ice and how this changes with a changing mean climate state. While the direct radiative forcing of the melt ponds and aerosol deposition is modest (about 1 W/m2), the surface albedo feedback amplifies the sea ice response and lead to a considerably different sea ice state.

An initial analysis of Arctic sea ice seasonal triggers (melt onset/freezeup) from Community Climate System, 4 (CCSM4) integrations has been performed. This includes a comparison of the timing of melt onset and freezeup in the model integrations to satellite derived observations (Markus et al., 2009). The simulated last 20th century CCSM4 climatology agrees well to the observations with the regionally averaged melt onset dates differing by only a few days (Jahn et al., submitted). The discrepancy in freeze-up dates is somewhat larger but still within the model-data comparison uncertainty. Given the good agreement in the timing of seasonal triggers simulated by CCSM4, we have started to analyze the changes in the timing of these triggers over the late 20th-early 21st century and their implications for the surface albedo feedback.
References cited:
Markus, T., J.C. Stroeve, and J. Miller, 2009: Recent changes in arctic sea ice melt onset, freezeup, and melt season length. J. Geophys. Res., 114, doi:10.1029/2009JC005436.