海洋环流的长期变化和预估

Long term variation and projection of ocean circulation

IPCC第六次评估报告(AR6)于2021年8月在IPCC第一工作组第14次联合大会上得到审议通过,并得到了IPCC第54届全会接受和批准。文中主要对该报告第九章“海洋、冰冻圈和海平面”中与海洋环流的相关评估内容进行解读。与以前的IPCC报告相比,AR6进一步确认人类活动对海洋环流的影响,并基于最新的数值模式给出对未来变化预估的结果。报告指出,海洋各区域表层盐度梯度增加(基本确定),预估到21世纪末认为海水较淡的海洋区域将变得更淡,而咸的区域将变得更咸(中信度);至少自1970年以来在全球海洋绝大多数区域的上层海洋层结更稳定(基本确定),预估到21世纪末认为上层海洋的密度层结会继续增加(基本确定),而绝大多数区域的混合层深度在高排放情景下会变浅(低信度);自20世纪80年代以来海洋热浪的发生频次翻倍(高信度)且持续时间更久(中信度),预估结果认为海洋热浪发生频次将更高;在4个东边界上升流系统中,20世纪80年代以来,仅有加利福尼亚上升流系统经历了有利于上升流的风力增强,而其他3个上升流系统未出现(中信度),东边界上升流系统将以偶极子的空间型态变化,即低纬度减弱而高纬度增强(高信度);所有预估情景下大西洋经向翻转环流(AMOC)均将减弱(非常可能),虽然AMOC会减弱,但全球变暖不会导致AMOC在2100年之前突然停止(中信度);AR6增加了可分辨海洋中尺度涡旋的高分辨率数值模拟试验,结果显示高分辨率模式有效地改进了海洋表面温度(SST)、海气通量和动力海面高度变化等要素的模拟。

IPCC AR6 was formally approved at the 14th joint session of Working Group I of the IPCC and accepted by the 54st session of the IPCC on August 2021.Assessment content related to the ocean circulation of chapter 9“Ocean,cryosphere and sea level change”is synthesized in this paper.Compared to the former IPCC reports,AR6 further confirmed the influence of human activities on ocean circulation,and provided the projected results based on the newest numerical simulations.AR6 pointed out that,surface salinity contrasts are increasing(virtually certain),and fresh ocean regions will continue to get fresher and salty ocean regions will continue to get saltier in the 21st century(medium confidence).The upper ocean has stably stratified since at least 1970 over the vast majority of the globe(virtually certain),and the upper ocean stratification is projected to increase(virtually certain)while the mixed layer depth is projected to mostly shoal under high emissions scenarios(low confidence).The frequency of marine heatwaves has doubled since the 1980s(high confidence)and the duration has become longer(medium confidence),furthermore the projection shows such trend will continue.Of the four eastern boundary upwelling systems(EBUS),only the California current system has experienced upwelling-favorable wind intensification since the 1980s(medium confidence),and the EBUS will change with a dipole spatial pattern of reduction at low latitude and enhancement at high latitude(high confidence).Under all SSP scenarios,the Atlantic Meridional Overturning Circulation(AMOC)will decline over the 21st century(very likely).The decline will not involve an abrupt collapse before 2100(medium confidence).AR6 has added the high resolution numerical simulation experiments which could resolve mesoscale eddy,such experiments could effectively improve the simulation of Sea Surface Temperature(SST),air-sea flux and dynamic sea-level change.