Reduction in Ocean Heat Transport at 26°N since 2008 Cools the Eastern Subpolar Gyre of the North Atlantic Ocean

Northward ocean heat transport at 26 degrees N in the Atlantic Ocean has been measured since 2004. The ocean heat transport is large-approximately 1.25 PW, and on interannual time scales it exhibits surprisingly large temporal variability. There has been a long-term reduction in ocean heat transport of 0.17 PW from 1.32 PW before 2009 to 1.15 PW after 2009 (2009-16) on an annual average basis associated with a 2.5-Sv (1 Sv equivalent to 10(6) m(3) s(-1)) drop in the Atlantic meridional overturning circulation (AMOC). The reduction in the AMOC has cooled and freshened the upper ocean north of 26 degrees N over an area following the offshore edge of the Gulf Stream/North Atlantic Current from the Bahamas to Iceland. Cooling peaks south of Iceland where surface temperatures are as much as 2 degrees C cooler in 2016 than they were in 2008. Heat uptake by the atmosphere appears to have been affected particularly along the path of the North Atlantic Current. For the reduction in ocean heat transport, changes in ocean heat content account for about one-quarter of the long-term reduction in ocean heat transport while reduced heat uptake by the atmosphere appears to account for the remainder of the change in ocean heat transport.

自2004年以来，一直在测量大西洋北纬26度的向北海洋热量输送。海洋热量输送量很大——约为1.25拍瓦（PW），并且在年际时间尺度上，它呈现出令人惊讶的大时间变率。从年度平均来看，海洋热量输送从2009年之前的1.32 PW长期减少到2009年之后（2009 - 2016年）的1.15 PW，减少了0.17 PW，这与大西洋经向翻转环流（AMOC）下降2.5斯维尔德鲁普（Sv）（1 Sv相当于10⁶立方米每秒）有关。AMOC的减弱使得从巴哈马到冰岛的墨西哥湾流/北大西洋暖流近海边缘沿线区域北纬26度以北的上层海洋变冷且变淡。在冰岛以南冷却达到峰值，2016年的表面温度比2008年低多达2摄氏度。大气的热量吸收似乎尤其受到北大西洋暖流路径的影响。对于海洋热量输送的减少，海洋热含量的变化约占海洋热量输送长期减少量的四分之一，而大气热量吸收的减少似乎占海洋热量输送变化的其余部分。