A Quasi-Linear Relationship between Planetary Outgoing Longwave Radiation and Surface Temperature in a Radiative-Convective-Transportive Climate Model of a Gray Atmosphere

In this study,we put forward a radiative-convective-transportive energy balance model of a gray atmosphere to examine individual roles of the greenhouse effect of water vapor,vertical convection,and atmospheric poleward energy transport as well as their combined effects for a quasi-linear relationship between the outgoing longwave radiation(OLR)and surface temperature(T_(S)).The greenhouse effect of water vapor enhances the meridional gradient of surface temperature,thereby directly contributing to a quasi-linear OLR-T_(S) relationship.The atmospheric poleward energy transport decreases the meridional gradient of surface temperature.As a result of the poleward energy transport,tropical(high-latitude)atmosphere-surface columns emit less(more)OLR than the solar energy input at their respective locations,causing a substantial reduction of the meridional gradient of the OLR.The combined effect of reducing the meridional gradients of both OLR and surface temperature by the poleward energy transport also contributes to the quasi-linear OLR-T_(S) relationship.Vertical convective energy transport reduces the meridional gradient of surface temperature without affecting the meridional gradient of OLR,thereby suppressing part of the reduction to the increasing rate of OLR with surface temperature by the greenhouse effect of water vapor and poleward energy transport.Because of the nature of the energy balance in the climate system,such a quasi-linear relationship is also a good approximation for the relationship between the annual-mean net downward solar energy flux at the top of the atmosphere and surface temperature.

“北极放大”现象驱动因素及其影响的研究进展综述

21世纪以来,受全球变暖的影响,北极近地表快速升温,升温速率可达全球平均水平的2倍,称为“北极放大”(Arctic Amplification,AA)现象。本文首先探讨了北极放大效应产生的原因:在局地气候反馈方面,海冰减少导致开阔水面增多,水体吸收太阳辐射增多使海-气相互作用增强,海冰进一步融化,形成正反馈;由于北极地区绝对温度较低,根据普朗克黑体辐射定律,普朗克反馈在北极为负反馈,促进北极放大效应;开阔水面和增温使北极地区大气湿度增加导致云量增多,从而使下行长波辐射增加,促使近地面层升温,这种水汽和云反馈对北极变暖的贡献有显著的季节性,主要发生在冬季;在北极以外地区水汽和热量的极向输送方面,温度的升高使大气柱增厚,高层气压梯度的变化引起纬向风变化,纬向分量减少,经向分量增加,形成槽脊,较低纬度地区可通过槽脊实现与北极的热量和水汽交换。此外,北极涛动(Arctic Oscillation,AO)与北大西洋涛动(North Atlantic Oscillation,NAO)也被证实与北极变暖有关。大西洋经向翻转环流(Atlantic Meridional Overturning Circulation,AMOC)异常曾被认为是北极变暖的驱动因素,而近期研究表明其更可能是北极变暖导致的结果。与此同时,“北极放大”现象会通过影响大气环流对中纬度地区甚至全球气候产生影响。北极变暖会导致东亚冬季严寒和北美东部冷事件的发生,形成“暖北极-冷欧亚”的格局并影响北美冬季气候波的形成。目前认为北极变暖与海冰减退可以通过两种机制影响中纬度地区:一是通过影响气压场的分布改变纬向风,使阻塞事件增加;二是通过对流层-平流层耦合作用影响北半球大气环流。但以上两种机制还存在争议。

The extreme Arctic warm anomaly in November 2020

2020年11月北极东部显著偏暖,表面气温暖异常为1979年以来第二强,且北极表层偏暖可以延伸至对流层上层.本文进一步研究了此次北极极端偏暖的可能原因.2020年秋季北极海冰大幅减少,11月从北大西洋向北极的水汽输送显著增加,且二者的变化幅度均超过了1979年以来的最高纪录,进而导致北极出现极端暖异常.此外,从中纬度向北极的Rossby波传播有利于向极水汽输送增加,且此过程可能与北大西洋中纬度海温异常有关.

北极放大效应原因的研究进展

近几十年来北极的增温幅度是全球平均的2倍以上,这种被称为"北极放大"的现象是全球气候变化的最显著特征之一。从北极局地气候反馈和北极外热输送2个方面总结了北极放大原因的最新进展。局地方面,海冰—反照率正反馈以及云和水汽增加导致的向下长波辐射增强是北极放大的重要原因,而较低的背景温度和相对稳定的大气层结使得温度反馈在北极为正,有利于变暖信号被放大。向极热输送方面,大气环流和洋流的输送作用对北极放大均有贡献,大西洋和太平洋海温的年代际变化和热带太平洋海温异常是驱动大气环流变化的主要原因。最后提出了尚需解决的关键问题。