IAP/LASG ，中国科学院 4 Jan 2008

《大洋环流和海气相互作用的数值模拟》 第十讲大洋热盐环流 IAP/LASG，中国科学院 4 Jan 2008 周天军 E-mail: zhoutj@lasg.iap.ac.cn http://web.lasg.ac.cn/staff/ztj/index_c.htm

主要内容 热盐环流基础知识热盐环流的气候作用热盐环流的数值模拟利用耦合模式开展气候变率研究的范例

2. 热盐环流的气候作用 • 盐分（水）平衡 • 热平衡 • 碳循环等物质平衡

2. 主要学术业绩 气候系统中的经圈环流系统示意图大气热带加热极地冷却海洋大洋热盐环流全球海气相互作用最为剧烈的区域 (After Webster)

大气和海洋中的经圈环流

气候系统的能量收支 吸收短波，射出长波

TOA年平均净辐射通量 TOA净短波、长波通量的纬向平均分布

实际估算的全球大洋环流的经向热输送 25% Ganachaud and Wunsch (2000)

实际估算的全球大洋环流的体积输送 Ganachaud and Wunsch (2000) Uncertainty 15-20%

实际估算的全球大洋经向热输送 (Trenberth, 2000)

SST of 4-9 November 2002 大西洋冬季海温较之同纬度北太平洋偏暖 50N Annual mean deviations from zonal mean (From NERC RAPID) 大西洋年均表层气温较之同纬度北太平洋偏高 Bodo (67N14 E) Jan -2ºC Nome (64 N147W) Jan -15ºC (Covey et al. 2003)

Bergen (62N)的冬天：阴雨连绵 Nov 2001 at Bergen

Meridional heat transport &surface heat flux Y 0 X -H

质量守恒 上层暖水向北输送、下层冷水向南输送; 西边界暖水向北输送、东部冷水向南输送

风生环流在经向热输送中的作用 NADW AABW

第二节热盐环流的气候作用 • 大西洋是全球大洋唯一的存在跨赤道的、均匀一致的向北热量输送的大洋，原因在于其包括热盐环流在内的经圈翻转环流的存在； • 大洋环流特别是大西洋经圈环流的经向能量输送，是深层洋流和表层洋流共同作用的结果，具体说来上层暖水向北输送、下层冷水向南输送，西边界暖水向北输送、东部冷水向南输送； • 对于全球气候系统而言，热带存在辐射盈余，极地则存在辐射亏损，为保持整个系统的能量平衡，在低纬与高纬之间，必须存在强的经向能量输送。强烈的跨赤道极向热输送，是大西洋热盐环流影响气候变化的重要途径。

主要内容 热盐环流基础知识热盐环流的气候作用热盐环流的数值模拟利用耦合模式开展气候变率研究的范例

热盐环流的数值模拟 主要科学问题： • 热盐环流变率气候突变与热盐环流崩溃

GOALS4模拟的THC及其与SLP的相关分布 强迫THC的大气环流型，表现为NAO模态 (Zhou,Zhang,Wang, 19999)

热盐环流与大气环流的耦合桥在边缘海 Simplified sketch of currents in the North Atlantic, showing the two main convection sites in the Greenland and Labrador Seas. Warm surface currents are shown in red; cold, deep currents in blue. Red-andblue circles, convection sites. (Rahmstorf,1999) 模式中热盐环流增强时的对流活动（周，2003）

热盐环流的变化 • 热盐环流的变化，受大气的影响； • 与热盐环流相联系的大气环流变化，在北半球表现为北极涛动（北大西洋涛动），在南半球表现为南极涛动 • 直接影响热盐环流强度的是水热通量

气候突变与热盐环流崩溃

气候突变的定义 An abrupt climate change occurs when the climate system is forced to cross some threshhold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause. (National Academy of Sciences, 2003)

热盐环流状态的变化乃至崩溃可能是触发气候突变的因子之一热盐环流状态的变化乃至崩溃可能是触发气候突变的因子之一

有、无热盐环流的两种平衡态 Manabe&Stouffer (1988) 开拓一个研究方向的著名工作

热盐环流强度变化影响气温

热盐环流历史档案：新仙女木事件(Younger Dryas Event)

Ice cores extracted from the two-mile thick Greenland ice sheet preserve records of ancient air temperatures. The records show several times when climate shifted in time spans as short as a decade. The Younger Dryas —about 12,700 years ago, average temperatures in the North Atlantic region abruptly plummeted nearly 5°C and remained that way for 1,300 years before rapidly warming again. The 8,200-Year Event —A similar abrupt cooling occurred 8,200 years ago. It was not so severe and lasted only about a century. But if a similar cooling event occurred today, it would be catastrophic. The Medieval Period —An abrupt warming took place about 1,000 years ago. It was not nearly so dramatic as past events, but it nevertheless allowed the Norse to establish settlements in Greenland. The Little Ice Age —The Norse abandoned their Greenland settlements when the climate turned abruptly colder 700 years ago. Between 1300 and 1850, severe winters had profound agricultural, economic, and political impacts in Europe. (R.B. Alley, from The Two-Mile Time Machine, 2000).

为什么称作“新仙女木事件”？ 仙女木花

为什么称作新仙女木事件？ 仙女木花：生长在高寒地带

新仙女木事件令欧洲几十年内变冷5C，寒冷气候持续1300多年新仙女木事件令欧洲几十年内变冷5C，寒冷气候持续1300多年上次冰期结束时，气候回暖，覆盖欧洲和北美大陆的冰盖快速融化，冰融化使得大量的淡水输送到北大西洋，结果在副极地大洋表面形成一个大的淡水帽，使海表变淡，抑制了海洋深对流的发生，表层海水下沉中止，使得北大西洋热盐环流中断。原本生长在高寒地带的仙女木花，在欧洲大陆盛开

新仙女木事件影响波及东亚？ • 中国黄土研究（安芷生） • 中国南海（汪品仙） • 南京石笋 (Wang)

Figure 19. Comparison of oxygen isotope records in a Greenland ice core (red) and a stalagmite from Hulu Cave, China (blue). The Younger Dryas event is well known as an abrupt cool event in the North Atlantic region (more negative values indicate colder conditions in Greenland). The significance of the Hulu Cave record is that a concident change occured half-way around the world in summer rainfall. More negative values for Hulu Cave are interpreted pimarily as an indicator of more summer monsoon rainfall relative to winter rainfall. Thus the east Asian summer monsoon was weaker during the Younger Dryas when the North Atlantic was cooler. The time 16,000 to 10,000 years before present spans the transition from the glacial to interglacial state.

海温距平梯度气候影响的数值模拟 气温 SSTA 降水 (Wang&Zhou et al. 2004)

热盐环流的今天：资料匮乏，难!

格陵兰冰原融化 1992和2005的冰原区域 NSIDC (above) Braithwaite: Univ. Manchester

北极海冰在减少

“狼”真的来了吗： 北大西洋的确在变淡 Subpolar seas bordering the North Atlantic have become noticeably less salty since the mid-1960s, especially in the last decade. This is the largest and most dramatic oceanic change ever measured in the era of modern instruments. This has resulted in a freshening of the deep ocean in the North Atlantic, which in the past disrupted the Ocean Conveyor and caused abrupt climate changes. (B. Dickson, et. al., in Nature, April 2002)

热盐环流的未来：前途未卜

IPCC报告给出的数值模拟结果：高度不确定

第三节热盐环流的数值模拟 • 大气环流与热盐环流存在耦合作用 • 热盐环流对淡水通量变化响应敏感 • 新仙女木事件：淡水强迫令热盐环流崩溃，是流行理论之一，但并非唯一原因 • 当今的热盐环流：有减弱的迹象，但证据不够充足 • 未来的热盐环流变化：全球变暖可能令其减弱，但存在模式不确定性

IAP/LASG ，中国科学院 4 Jan 2008