A SCIENTIFIC PAPER MADE EASY: impact of atmospheric forcing uncertainties on sea ice simulations
Sea ice is an important component of the polar climate system and has experienced dramatic changes during recent decades. In the Arctic, the total sea ice extent decreased dramatically over the satellite-observing period. In the Antarctic, the total sea ice extent changed very quickly in recent ten years and sea ice extent reached the lowest value in February 2023 ever recored (Fig. 1). These rapid changes have important implications for the weather and climate especially around polar regions.
Figure 1. Antarctic sea ice concentration on February 21, 2023. Figure credit: NASA Earth Observatory images by Lauren Dauphin, using data from the National Snow and Ice Data Center.
A good simulation of sea ice is crucial for improving model predictions and climate change projections under anthropogenic warming. However, limitations still exist in both fully coupled climate models and ocean–sea ice models. The interactions between atmosphere, sea ice and ocean are complicated. By performing sensitivity experiments with ocean–sea ice models, one can gain some insight into the origins of those biases. In our recent study, we detect how the sea ice simulation can be improved by changing atmospheric forcing fields in ocean–sea ice models.
Figure 2. 1980–2007 September and February mean Arctic (a–j) and Antarctic (k–t) sea ice concentrations from the NSIDC-0051 data (first column) and the differences between OSI-450 and NSIDC-0051 (second column), NorESM2-LM/C and NSIDC-0051 (third column), NorESM2-LM/J and NSIDC-0051 (fourth column) and NorESM2-LM/J and NorESM2-LM/C (fifth column). The black lines are contours of 80 % concentration (a, f, k, p). Figure credit: Xia Lin
The representation of Arctic and Antarctic sea ice in models participating in the Ocean Model Intercomparison Project Phase 2 (OMIP2, using the updated Japanese 55-year atmospheric reanalysis, JRA55-do) was significantly more realistic than in OMIP1 (forced by the atmospheric state from the Coordinated Ocean-ice Reference Experiments version 2, CORE-II). An example from NorESM2-LM model is shown in Fig. 2. Negative summer biases in high-ice-concentration regions (Figs. 2c and 2m), positive biases in the Canadian Arctic Archipelago (CAA) and central Weddell Sea (CWS) regions (Fig. 2c), winter Antarctic ice concentration near the ice edge (Fig. 2r) in NorESM2-LM/C are reduced in NorESM2-LM/J (Figs. 2d, 2n and 2s).
To understand why, we study the sea ice concentration budget and its relations to surface heat and momentum fluxes. The summer ice concentration simulation in both hemispheres can be improved with changed surface heat fluxes (Figs. 3a, b, i and j) and the winter Antarctic ice concentration near the ice edge are improved with changed wind stress (Figs. 3o and p). Net shortwave radiation fluxes provide key improvements in the Arctic interior, CAA and CWS regions.
Figure 3. 1980–2007 March–August and October–January mean Arctic (a–h) and Antarctic (i–p) net surface heat flux (first two columns) and surface stress (last two columns) on sea ice. The positive values indicate a surface flux downward. The first and third columns correspond to NorESM2-LM/C, and the second and fourth columns are differences between NorESM2-LM/J and NorESM2-LM/C. Figure credit: Xia Lin
The Arctic ice drift magnitude simulations near the ice edge and ice velocity direction simulations in the Beaufort Gyre and the Pacific and Atlantic sectors of the Southern Ocean in OMIP2 are also improved. The connections between the simulated ice drift and the ice concentration, the ice thickness and the wind stress are explored. The improvements in ice drift simulations are primarily owing to surface wind stress changes (see more details in the paper). Our findings suggest that attention should be paid to the radiation fluxes and winds in atmospheric reanalyses in polar regions.
Some aspects of the sea ice simulation are not improved by changing the forcing from CORE-II to JRA55-do, such as the winter Arctic ice concentration in the exterior region, summer Antarctic ice concentration inthe coastal regions. In these exterior and coastal regions, thermodynamic processes tend to compensate for the changes caused by dynamic processes, and properly combined contributions from thermodynamic anddynamic processes are needed to improve the simulations. Both atmospheric forcing and model physics of the sea ice growth and melt processes are crucial for an improved simulation in these aspects.
Written by Xia Lin