Characterizing melt ponds on sea ice in the Norwegian Earth System Model (NorESM2)

Abstract. Melt Ponds (MPs) are pools of open water that formed on Arctic sea ice during the warm months. They significantly affect the surface radiation budget of the Arctic Ocean and play an important role in the Arctic sea ice mass balance, climate and ecological systems. The second generation of the coupled Earth System Model developed by the Norwegian Climate Center, NorESM2, includes an explicit representation of MPs. To characterize the spatial distribution and the seasonal evolution of MPs, we introduced four key variables: melt pond fraction (MPF), relative melt pond fraction (RMPF), MP area (MPA), and pond area fraction (PAF). Using these variables, particularly PAF, we conducted a comprehensive evaluation of MPs in NorESM2 across the Pan-Arctic region and its four sub-regions: the Pacific, Atlantic, Laptev and Canadian sector. Additionally, we classified MPs base on ice type, distinguishing between first-year ice (FYI) and multi-year ice (MYI), over the period from May to September during 2002–2011. This study utilized observational data, including MPF from MODIS and MERIS, sea ice age from NSIDC, sea ice concentration from MODIS and AMSR-E, and surface downward solar radiation from SYN1deg, and reanalysis atmospheric data from ERA5. Our study revealed that while differences existed between MODIS and MERIS, these discrepancies were relatively small during May, June and September, which corresponds to the early melt season and the refreezing season. In contrast, larger discrepancies were observed during July and August, the peak melt season. These significant deviations are likely attributed to the challenges associated with retrieving MPF using coarse-resolution optical sensors. Despite these differences, the overall spatial patterns and temporal evolutions of MPs derived from MODIS and MERIS were largely consistent. MPs were found to form earlier and more extensively on FYI compared to MYI, leading to more abundance of MPs on FYI. NorESM2 successfully reproduced these general characteristics. However, the model consistently simulated the maximum PAF was in August (except for FYI in the Pacific region), which was typically one month later than in the observations. NorESM2 exhibited a systematic underestimation of MPs in May and June, not only in the pan-Arctic but also across the sub-regions, and on both FYI and MYI. Conversely, MPs on MYI in the Pacfic sector were overestimated, likely due to the higher prevalence of MYI in NorESM2 during these months in this region. The systematic underestimation of MPs in NorESM2 during the early melt season can be attributed to the too low surface downward solar radiation in NorESM2 and the too large retrieved MPF from MODIS and MERIS. The former attribution was further confirmed by the experiments conducted with the 1D sea ice model ICEPACK. Giving the critical role of MPs play in Arctic albedo and energy budgets, advancing the representation of MPs in the model has a profound implications for the Arctic system.