海底地热通量对海洋深层温度和环流的长期影响

doi:10.11867/j.issn.1001-8166.2014.10.1167

虽然地球海底地热通量在全球热能收支平衡中所占的比例非常低,在目前的海洋气候模式开发中也并没有将其包含在内,但是由于海底地热通量可以持续改变海洋的浮力而影响海水层结,进而影响海洋温度分布以及环流等海洋水文要素,并且可以进一步影响海水的化学性质、碳氮的分布循环以及生物分布等,因此其对海洋环流和气候变化长期影响的潜在可能性仍不能完全排除。在通用地球系统耦合模式(CESM)的基础上,通过在全球大洋中脊区域持续加入1 W/m2的地热通量的方式运行了长达5000年的数值模拟实验,模式结果显示：海底地热通量对深层海洋的物理性质和全球海洋环流的长期影响是不可忽略的;受地热通量的局地加热效应影响,大洋深层3000~3500 m 总体升温约0.4 ℃;在南大洋和北大西洋的深层水形成区域,海洋深层的增温信号可以影响到表层海洋。北大西洋深层水和南极底层水形成增强,并且模拟的北大西洋深层水的深度加深,更符合观测结果。

关键词: 地热通量, 海洋模式, 温度, 海洋环流, 气候变化

Although Geothermal Heat Flux (GHF) through the seafloor has a trivial contribution to the oceanic heat budget balance, and is excluded in the development of most climate models, its potential effect on long-term ocean circulation and climate change may not be ignored. The GHF could be as a continuously buoyancy forcing and changes ocean stratification, affects the ocean temperature distribution as well as the ocean circulation and other hydrological elements, and further affects the chemical properties of seawater, carbon and nitrogen cycle and biological distribution, etc. Here we presented a 5000-year sensitivity experiment with the Community Earth System Model (CESM) by adding geothermal heat flux of 1 W/m² at the seafloor near the Mid-ocean ridge. The numerical results suggested that the long-term impact of GHF on deep ocean circulation and physical characters was not to be neglected. Comparing to the control experiment, the local geothermal heating contributed to an overall warming of deep waters (between 3 000~3 500 m) by 0.4 ℃, with the maximum warming of 0.85 ℃ at the southeast Pacific Ocean. It then further decreased the stability of the water column, enhanced the formation rates of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW), and strengthend the corresponding Meridional Overturning Circulations (MOC) by 1.7 Sv and 3.7 Sv, respectively. In the sensitivity experiment, the penetration depth of NADW also increased to the depth of 3 000~3 500 m, which was closer to observations. At the deep water formation region of North Atlantic and Southern Ocean, the GHF-induced warming signals could even reach to sea surface.

Key words: Geothermal Heat Flux, Ocean Model, Temperature, Ocean Circulation, Climate Change.

中图分类号:

P731

图 1 地热通量的分布区域以及在各大洋中的深度分布图 (a) 全球海洋水深(颜色)分布以及加入地热通量区域(网格化区域,量值为1 W/m²);(b) 地热实验中各大洋的地热通量的深度分布图,包括全球(黑线)、大西洋(蓝线)、印度—太平洋(红线)以及南大洋(绿线)

Fig.1 Horizontal distribution of geothermal heat fluxes and vertical distribution at different oceans (a) Depth of global ocean (color) and distribution of geothermal heat fluxes (gridded area with 1W/m² added) at GHF run; (b) Vertical distribution of geothermal heat fluxes at different oceans. Shown are profiles for the global summation (black), Atlantic basin (blue) and Indo-Pacific basins (red) and the Southern Ocean (ACC)(green), respectively

图 2 地热实验与控制实验的全球海洋的体积平均温度差值的时间曲线

Fig.2 Time series of mean temperature difference of global ocean between GHF and CTRL runs

图 3 地热实验与控制实验不同海域平均温度差随深度的变化包括全球(黑线)、印度—太平洋(红线)、南大洋(绿线)以及大西洋(蓝线)

Fig.3 The vertical distribution of mean temperature difference between GHF and CTRL runs at different oceans Shown are profiles for the global oceans (black), Atlantic basin (blue) and Indo-Pacific basins (red) and the Southern Ocean (ACC) (green)

图 4 3 500 m深度上地热实验与控制实验平均海水温度差等值线间隔0.1℃,最大值0.82℃位于南太平洋(117°W, 23°S)和南大西洋(20°W, 33°S)

Fig.4 The temperature difference at 3500 m between GHF and CTRL runs Contour interval is 0.1℃. Peak value is 0.82℃ in South Pacific (117°W, 23°S) and South Atlantic (20°W, 33°S)

图 5 控制实验中经向翻转环流和纬向平均温度的垂向分布(a,b)及其与地热实验的差值的垂向分布(c,d) (a)控制实验中全球经向翻转环流和纬向平均温度的垂向分布,图中线条表示经向翻转环流,单位是Sv,间隔为3Sv,颜色表示纬向平均温度,单位是℃,间隔为2℃;(b)与(a)类似,但是在大西洋海域,线条(经向翻转环流)间隔为2Sv;(c)地热实验与控制实验的全球经向翻转环流和纬向平均温度的差值垂向分布,图中线条表示经向翻转环流差,单位是Sv,间隔为1Sv,颜色表示纬向平均温度差,单位是℃,间隔为0.1℃;(d)与(c)类似,但是在大西洋海域,线条(经向翻转环流差)间隔为0.5 Sv

Fig.5 The vertical distributions of meridional overturning circulation and zonally averaged temperature in CTRL run(a,b),and their differences between GHF and CTRL runs(c,d), respectively. (a) The vertical distributions of global meridional overturning circulation (contour, unit is Sv) and zonally averaged temperature(color, unit is ℃) in CTRL run. Contour interval is 3 Sv and color interval is 2℃. (b) the same as (a) except in the Atlantic Ocean and contour interval is 2Sv, (c) the differences of global meridional overturning circulation (contour, unit is Sv) and zonally averaged temperature(color, unit is ℃) between GHF and CTRL runs. Contour interval is 1 Sv and color interval is 0.1℃. (d) the same as c) except in the Atlantic ocean and contour interval is 0.5 Sv

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摘要

Long-term Impact of Geothermal Heat Flux on the Deep Ocean Temperature and Circulation