楚科奇海融冰期热收支的数值模拟

Heat budget analysis during the ice-melting season in the Chukchi Sea based on a model simulation

楚科奇海海冰剧烈变化对极地生态系统及碳循环过程具有重要影响.基于1/4°(经纬度)水平分辨率的北大西洋-北冰洋-北太平洋海洋-海冰耦合模式(简称NAPA1/4)在1994~2015年的模拟结果,本文估算了楚科奇海融冰期的热收支,并揭示了海(冰)面净热通量和太平洋入流热通量在楚科奇海融冰过程中的相对贡献.结果表明:楚科奇海融冰期间的海冰体积变化主要由热力过程调控的海冰融化导致,且冰底融化占优.在海冰快速融化的6~8月,海冰表、底融化速率的相对强度受海冰密集度影响存在空间差异,在密集度大于80%的海区,冰表融化占优,反之冰底融化速率远高于冰表.冰面净热通量是冰表融化的热驱动,平均约60%的冰面净热通量用于冰表融化.冰底融化消耗的热量则主要来自海面净热通量以及太平洋入流所携带的热量;融化初期和盛期,海面净热通量贡献占优;后期和末期则主要受太平洋入流热通量的影响,且该影响可持续作用至结冰早期,抑制冰底冻结速率.在整个融冰期,进入研究区域水体的总热输入中,约67%由太平洋入流贡献.总热输入中约58%的热量用于海水增温,冰底融化吸收的热量约占28%,向下游海区输运的热量仅占14%.年际变化的相关性分析表明,相对于融冰期海面净热通量,太平洋入流热通量对楚科奇海海冰面积年际变化的影响更为显著.

Rapid changes of sea ice in the Chukchi Sea are substantially influencing the marine ecosystem and carbon cycling in the region. In this study, sea ice variations during 1994–2015 are simulated with a coupled ocean and sea ice model covering the North Atlantic-North Pacific-Arctic Oceans(NAPA) with a nominal horizontal resolution of 1/4° in latitude/longitude.The model is based on version 3.6 of the Nucleus for European Modelling of the Ocean(NEMO) and version 3 of Louvainla-Neuve Sea Ice Model(LIM). The analysis is focused on the heat budget of sea ice and sea water in the Chukchi Sea during the ice-melting season, defined as the duration with positive net water flux from sea ice to sea water.Averaged over 1994–2015, sea ice melting in the Chukchi Sea occurs from mid-May to mid-October. The relative contributions of melting at the surface and bottom of the sea ice vary spatially in relationship to the variations of sea ice concentration, in particular from June to August when the ice melts relatively rapidly. The rate of melting at the ice surface is much higher in areas with ice concentration greater than 80%, while ice bottom melting is more important elsewhere.Bottom melting accounts for 68% of the total seasonal ice loss. Further analysis of the heat budget of the sea water column reveals the contributions of net heat flux at sea water surface(including open waters, ice leads, and under ice covered areas)and Pacific inflow heat flux to ice bottom melting. First, during the entire ice-melting season, the Pacific inflow heat flux contributes ~67% of the total heat input to the sea water in the study area. Among the total heat flux input to the sea water,about 58% is used to heat the sea water, 28% is used to melt ice bottom, and 14% is transported downstream. Secondly,during the early and peak stages of ice melting(from mid-May to early August), net heat flux at sea water surface makes the dominant contribution to ice bottom melting. During the late and final stages(from mid-August to mid-October), the Pacific inflow heat flux plays a leading role and continues to provide heat during the early freezing stage, acts to slow down the freezing rate, and extends the ice-free season. Finally, ~60% of the net heat flux at ice surface is used for ice surface melting. Very little of the net heat flux at ice surface could penetrate the sea ice into the sea water for ice bottom melting.During the ice-melting season of 1994–2015, the time-mean and standard deviation of interannual variations of the following three variables in the Chukchi Sea are:(3.16±0.67)×1020 J for the heat content variation in the Chukchi Sea,(3.65±0.69)×1020 J for the Pacific inflow heat flux, and(1.81±0.46)×1020 J for the net heat flux at sea water surface. Interannual variation of heat flux causing ice bottom melting is negatively correlated with the heat content variation in the Chukchi Sea(R=-0.62;P<0.01), and is positively correlated with the sum of the Pacific inflow and net heat flux at sea water surface minus the heat content variation in the Chukchi Sea(R=0.96;P<0.01). This suggests that the total heat input to sea water is first used to heat the sea water, and then the sea ice melts from its bottom by taking heat from the sea water. By comparison,interannual variation of the heat flux causing ice surface melting is solely caused by net heat flux at ice surface. The two have a perfect correlation(R=1.0;P<0.01). Since 2000, the Pacific inflow heat flux during the ice-melting season has been increasing by 6.06×1018 J per year, while the heat content variation in the Chukchi Sea has been increasing by 6.03×1018 J per year. Overall, the present model results suggest that variations upstream of the Pacific inflow play an important role in variations of sea ice area in the Chukchi Sea.