大气河对南极冰盖及海冰的影响

The impacts of atmospheric rivers on Antarctic ice sheet and sea ice

南极冰盖与海冰对全球气候具有重要影响。大气河作为高纬度地区经向水汽输送的重要途径,其对南极冰盖与海冰的影响在近年来愈发受到重视。南极大气河通常形成于高压脊(阻塞高压)与温带气旋之间的强向极经向输送带内。低频的大气河活动为南极带来强降雪,有利于冰盖质量增加。然而,强暖湿水汽侵入同时会导致表面融化、冰架崩解和极端高温,对冰盖质量存在潜在负贡献。大气河携带极端暖湿水汽与强风通过热力与动力过程导致海冰密集度下降。目前,大气河的识别算法仍不完善,其对液态降水的直接影响、与南大洋的相互作用等仍不清楚,需要进一步明晰大气河对南极冰盖与海冰的影响机制,以准确预估未来大气河对南极冰盖物质平衡与海冰变化的作用。

Atmospheric rivers(ARs)are characterized as long,narrow,and transient channels of strong horizontal water vapor transport.Previous studies have primarily focused on their impact on mid-latitudes,emphasizing the potential risks of deleterious hazards and financial losses.However,less attention has been given to ARs in the Antarctic region,despite they account for over 90%of moisture transport into the high latitudes.ARs typically originate in the robust poleward meridional transport flank within ridges(blocking highs)and explosive extratropical cyclones in the Antarctic region,facilitating the substantial moisture transport through a vigorous low-level jet.Three widely-used metrics for characterizing the moisture intrusion state are integrated water vapor,v-component of integrated water vapor and integrated water vapor.An AR is detected when the enclosed shape of the extremely high moisture intrusion path is adequately elongated.The frequency of ARs varies across different AR detection algorithms based on diverse metrics and distinct extremity thresholds.The annual frequency of ARs decreases with latitude,exhibiting a zonally asymmetric pattern that show higher seasonal frequencies in austral winter and spring.These spatial and temporal features are shaped by the geographical environment and the distribution of synoptic systems in the Antarctic region.The annual variability of AR frequency appears to be associated with dominant atmospheric modes in southern high latitudes,such as the Southern Annular Mode.Additionally,it is also modulated by the natural variability of sea surface temperature modes,including El Ni o Southern Oscillation and Indian Ocean Dipole.ARs have significant impacts on the Antarctic ice sheet and sea ice.ARs contribute both positively and negatively to the Antarctic ice sheet.On one hand,the intense snowfall during ARs constitutes a major portion of the total precipitation over the ice sheet,favoring its mass gain.Conversely,warm-moist air intrusions accompanying ARs induce surface melting and extremely high temperatures due to foehn winds and anomalously high net surface energy flux.Moreover,surface meltwater during ARs promotes hydraulic fracturing on the ice shelves and trigger their disintegration by removing sea ice through strong winds.These processes pose a substantial threat to the ice sheet mass balance.Meanwhile,the warm-moist air and strong winds during ARs thermodynamically and dynamically reduce sea ice.ARs lead to anomalously high temperatures and net surface heat flux,intensifying sea ice thermodynamically melting,especially in winter.The strong winds dynamically drift the sea ice onshore,accelerating ice breaking through powerful waves and further enhancing lateral melting.Though ARs in the Antarctic region have been subject to various analyses,certain questions persist.The evaluation on ARs impact on Antarctic ice sheet and sea ice remain contingent on the chosen detection algorithm,necessitating the development of a more universal and robust approach.Implementing machine learning to extract the spatial and temporal features of ARs could offer such an approach.Moreover,although the fact that most liquid precipitation is attributed to ARs,its influence on the ice sheet and sea ice is often overlooked,despite its potential to enhance melting and destabilize ice shelves.In addition,ARs may exert a profound influence on the ocean,subsequently providing feedback to the atmosphere.However,the interaction between ARs and the Southern Ocean is not well understood.Therefore,further research imperative to elucidate these mechanisms and evaluate the future changes in Antarctic ice sheet mass balance and sea ice influenced by ARs.