大西洋热盐环流年代际变化机制研究Ⅲ.北大西洋海气要素对热盐环流年代际振荡的影响

基于该系列文章前文研究中构建的海气耦合气候模式和所揭示的北大西洋热盐环流年代际振荡机制,针对海气要素对该振荡机制的影响问题进行了重点的探讨。为细致准确的研究北大西洋海洋要素同北大西洋热盐环流年代际振荡的关系,有针对性的定义了副极地海区表层密度指数和北大西洋暖流强度指数并对模式结果进行了全面分析。分析结果表明副极地海区表层密度变化领先大西洋径向翻转环流(MOC)变化7 a,北大西洋暖流的变化领先 MOC变化4 a,格陵兰－苏格兰海脊溢流水强度(包括丹麦海峡溢流水和法鲁海峡溢流水,是北大西洋深层水的重要来源)的变化领先 MOC的变化3 a;北大西洋大气要素变化对北大西洋热盐环流年代际振荡有非常重要的调制作用,当副极地流环和北大西洋暖流(NAC)达到最强的2 a之前,高纬度地区大气为气旋式环流异常,中纬度地区大气为反气旋式环流异常,海表热通量在大西洋副极地海区是负异常,这都有利于副极地流环和NAC的加强,更多高盐度的北大西洋水进入格陵兰-冰岛-挪威海(GIN)海域,由此可以导致GIN海域表层密度上升,使水体的层结稳定性减弱,有利于深层对流的发生,同时大气变化通过风应力旋度和海表热通量也直接影响GIN海域深层水的生成,进而导致格陵兰－苏格兰海脊溢流水的强度增加。     

大西洋热盐环流年代际变化机制研究Ⅲ.北大西洋海气要素对热盐环流年代际振荡的影响

基于该系列文章前文研究中构建的海气耦合气候模式和所揭示的北大西洋热盐环流年代际振荡机制,针对海气要素对该振荡机制的影响问题进行了重点的探讨.为细致准确的研究北大西洋海洋要素同北大西洋热盐环流年代际振荡的关系,有针对性的定义了副极地海区表层密度指数和北大西洋暖流强度指数并对模式结果进行了全面分析.分析结果表明副极地海区表...     

Hydrography of chromophoric dissolved organic matter in the North Atlantic

The distribution and optical absorption characteristics of chromophoric dissolved organic matter (CDOM) were systematically investigated along three meridional transects in the North Atlantic Ocean and Caribbean Sea conducted as part of the 2003 US CLIVAR/CO₂ Repeat Hydrography survey. Hydrographic transects covered in aggregate a latitudinal range of 5° to 62° north along longitudes 20°W (line A16N, Leg 1), 52°W (A20), and 66°W (A22). Absorption spectra of filtered seawater samples were collected and analyzed for depths ranging from the surface to ∼6000 m, sampling all the ocean water masses in the western basin of the subtropical North Atlantic and several stations on the North and South American continental slopes. The lowest surface abundances of CDOM (< 0.1 m⁻¹ absorption coefficient at 325 nm) were found in the central subtropical gyres while the highest surface abundances (∼0.7 m⁻¹) were found along the continental shelves and within the subpolar gyre, confirming recent satellite-based assessments of surface CDOM distribution. Within the ocean interior, CDOM abundances were relatively high (0.1–0.2 m⁻¹ absorption coefficient at 325 nm) except in the subtropical mode water, where a local minimum exists due to the subduction of low CDOM surface waters during mode water formation. In the subthermocline water masses of the western basin, changes in CDOM abundance are not correlated with increasing ventilation age as assessed using chlorofluorocarbon (CFC) concentrations and the atmospheric CFC history. But dissolved organic carbon (DOC) mass-specific absorption coefficients of CDOM increase with increasing ventilation age in the deep sea, indicating that CDOM is a refractory component of the DOC pool. The overall CDOM distribution in the North Atlantic reflects the rapid advection and mixing processes of the basin and demonstrates that remineralization in the ocean interior is not a significant sink for CDOM. This supports the potential of CDOM as a tracer of ocean circulation processes for subducted water masses.     

Impact of spatial resolution on simulated surface water mass transformations in the Atlantic

We analyze the water mass transformation in coarse (1°) and high (1/6°) resolution ocean simulations with the identical configuration of the CLIPPER model and interannual ERA15 forcing function. Climatological characteristics of surface water mass transformation in the two experiments are quite different. The high resolution experiment exhibits a stronger surface transformation in equatorial and tropical regions, in the Gulf Stream area and in the location of the formation of Subtropical Mode Water (STMW), associated with high levels of eddy kinetic energy. The coarse resolution experiment shows a better representation of the transformation rates corresponding to the densest subpolar mode waters and Labrador Sea Water (LSW). This is explained by the differences in lateral mixing procedures between high and coarse resolution experiments. The high resolution 1/6° run is eddy-resolving only in the tropics and mid-latitudes. In these areas eddies are found to enhance the process of water mass transformation compared to the isopycnal diffusion used to parameterized the eddies in the 1° model. Despite its 1/6° resolution, the high resolution model does not adequately represent eddies in the subpolar gyre and Labrador Sea. In these areas the high resolution model fails to correctly simulate water mass transformation because the lateral mixing (provided through the bi-harmonic sub-gridscale parameterization) of newly ventilated waters with surrounding waters is not efficient enough. In contrast in the coarse 1° resolution model, the strong lateral mixing and the unrealistically broad boundary currents imposed by the high diffusivity required for numerical stability mixes newly formed LSW waters with the warmer and saltier waters of the rim current. Finally, it results in a more effective representation of the surface water mass transformation in high latitudes in the 1° model. A possible impact of the increased lateral diffusion in high resolution experiment on the representation of re-stratification in the Labrador Sea was studied in sensitivity experiments with different lateral diffusion coefficients compared to the regional eddy-resolving 1/15° simulation in the subpolar North Atlantic. If the eddies are not resolved in subpolar latitudes (as in the case of 1/6° model), the GM90 parameterization with the coefficient close to 800 m² s⁻¹ provides the closest agreement with the solution of eddy-resolving 1/15° model.     

Impact of spatial resolution on simulated surface water mass transformations in the Atlantic

We analyze the water mass transformation in coarse (1°) and high (1/6°) resolution ocean simulations with the identical configuration of the CLIPPER model and interannual ERA15 forcing function. Climatological characteristics of surface water mass transformation in the two experiments are quite different. The high resolution experiment exhibits a stronger surface transformation in equatorial and tropical regions, in the Gulf Stream area and in the location of the formation of Subtropical Mode Water (STMW), associated with high levels of eddy kinetic energy. The coarse resolution experiment shows a better representation of the transformation rates corresponding to the densest subpolar mode waters and Labrador Sea Water (LSW). This is explained by the differences in lateral mixing procedures between high and coarse resolution experiments. The high resolution 1/6° run is eddy-resolving only in the tropics and mid-latitudes. In these areas eddies are found to enhance the process of water mass transformation compared to the isopycnal diffusion used to parameterized the eddies in the 1° model. Despite its 1/6° resolution, the high resolution model does not adequately represent eddies in the subpolar gyre and Labrador Sea. In these areas the high resolution model fails to correctly simulate water mass transformation because the lateral mixing (provided through the bi-harmonic sub-gridscale parameterization) of newly ventilated waters with surrounding waters is not efficient enough. In contrast in the coarse 1° resolution model, the strong lateral mixing and the unrealistically broad boundary currents imposed by the high diffusivity required for numerical stability mixes newly formed LSW waters with the warmer and saltier waters of the rim current. Finally, it results in a more effective representation of the surface water mass transformation in high latitudes in the 1° model. A possible impact of the increased lateral diffusion in high resolution experiment on the representation of re-stratification in the Labrador Sea was studied in sensitivity experiments with different lateral diffusion coefficients compared to the regional eddy-resolving 1/15° simulation in the subpolar North Atlantic. If the eddies are not resolved in subpolar latitudes (as in the case of 1/6° model), the GM90 parameterization with the coefficient close to 800 m² s⁻¹ provides the closest agreement with the solution of eddy-resolving 1/15° model.     

Analysis of an 80-Year Integration of a 1/3-Degree Ocean Model of the Subpolar North Atlantic

Previous work had examined an ocean model of the subpolar gyre of the North Atlantic Ocean that used the Gent and McWilliams parameterization with a variable eddy-transfer coefficient, and showed significant improvements to the model’s circulation and hydrography. This note examines an extended (80-year-long) integration of the same model and focuses on the adjustment of the intermediate and deep waters as well as on model stability. It is shown that the model is able to retain a good representation of the water masses, especially in the Labrador Sea, through the full integration. Labrador Sea Water dispersal is well simulated by the model in the western basin, with a good correspondence between the model and observational salinities on the σ₂ = 36.95 isopycnal surface. Labrador Sea Water dispersal to the eastern basin is not nearly as well represented, as this water mass has trouble passing over the Mid-Atlantic Ridge in the model. The variable eddy-transfer coefficient significantly improves the model representation of the Cold Intermediate Layer on the Labrador shelf by reducing spurious diapycnal mixing. Finally, the evidence in this note suggests that open boundary conditions do not generate significant model drift, even for integrations approaching a century in length.     

近惯性内波破碎混合方案对MOM4模式的影响

风生近惯性内波破碎引起的跨等密度面混合在海洋内部混合中起重要作用。然而其参数化对海洋模式的模拟影响仍有待进一步认识。本文给出的是在模块化海洋模式（MOM）中海洋表面边界层以下引入一个考虑风驱动近惯性内波破碎引起的跨等密度面混合参数化方案的研究工作。模拟结果显示,该方案有效改善MOM4模拟的上层1 000 m以上的温盐偏差,特别是在北太平洋和北大西洋的通风地区。数值试验表明,风生近惯性内波破碎有可能是维持海洋通风过程的重要机制之一,它使得海洋通风区的位温变冷,盐度变淡,整层等位密面加深。维持的通风过程使得北太平洋副极地大涡的影响延伸到副热带大涡。从而模拟的北太平洋中层水源头及其副热带大涡东侧的温盐更接近观测实际。同时,模拟的北大西洋经圈翻转环流强度也更为合理。     

Sensitivity of the Atlantic Ocean circulation to a hydraulic overflow parameterisation in a coarse resolution model: Response of the subpolar gyre

We investigate the sensitivity of a coarse resolution coupled climate model to the representation of the overflows over the Greenland–Scotland ridge. This class of models suffers from a poor representation of the water mass exchange between the Nordic Seas and the North Atlantic, a crucial part of the large-scale oceanic circulation. We revisit the explicit representation of the overflows using a parameterisation by hydraulic constraints and compare it with the enhancement of the overflow transport by artificially deepened passages over the Greenland–Scotland ridge, a common practice in coarse resolution models. Both configurations increase deep water formation in the Nordic Seas and represent the large-scale dynamics of the Atlantic realistically in contrast to a third model version with realistic sill depths but without the explicit overflow transport. The comparison of the hydrography suggests that for the unperturbed equilibrium the Nordic Seas are better represented with the parameterised overflows. As in previous studies, we do not find a stabilising effect of the overflow parameterisation on the Atlantic meridional overturning circulation but merely on the overflow transport. As a consequence the surface air temperature in the Nordic Seas is less sensitive to anomalous surface fresh water forcing.Special attention is paid to changes in the subpolar gyre circulation. We find it sensitive to the overflow transport and the density of these water masses through baroclinic adjustments. The analysis of the governing equations confirms the presence of positive feedbacks inherent to the subpolar gyre and allows us to isolate the influence of the overflows on its dynamics.     

Optimum multiparameter analysis of the water mass structure in the Atlantic Ocean thermocline

An analysis of the water mass structure of the Atlantic Ocean central layer is conducted by applying optimum multiparameter (OMP) analysis to an expansive historical data set. This inverse method utilises hydrographic property fields to determine the spreading and mixing of water masses in the permanent thermocline. An expanded form of OMP analysis is used, incorporating Redfield ratios and pseudo-age to correct for the non-conservative behaviour of oxygen and nutrients over large oceanic areas.Three water masses are considered to contribute to the central layer of the Atlantic Ocean. One of these is formed in each hemisphere of the Atlantic Ocean and the other advects around the southern tip of Africa from its formation region in the Indian Ocean. The Atlantic Ocean is analysed on a fine three-dimensional grid so that at every grid point the relative contributions of each water mass and the pseudo-age are determined.The model is remarkably successful in verifying many accepted circulation features in the Atlantic Ocean, including the large-scale circulations of the subtropical gyres, the zonal flows of equatorial currents at the equator, and a cross-equatorial flow of the water masses formed in the southern hemisphere near the western boundary. The inter-hemisphere flow is so important that almost half of the thermocline waters in the Caribbean Sea and the Gulf of Mexico are supplied by the two water masses formed in the South Atlantic and Indian Oceans. This provides support for an upper-layer replacement path for the formation of North Atlantic Deep Water. Further east, the sharp front at about 15°N between North and South Atlantic Central Waters is clearly discriminated throughout the thermocline. The central waters of the South Atlantic thermocline are found to be highly stratified, with central water formed in the Indian Ocean underlying the South Atlantic Central Water. At around 5°N a strong upwelling zone is identified in which the central water formed in the Indian Ocean penetrates towards the surface. The pseudo-age results allow pathways for the flow of water masses to be inferred, and clearly identify circulation features such as the subtropical gyres, the Equatorial Undercurrent, and the shadow zones in the eastern equatorial regions of the Atlantic Ocean. Water mass renewal in these shadow zones occurs on considerably longer time scales than for the well-ventilated subtropical gyres.     

Mixing and biological production at eddy margins in the eastern Gulf of Alaska

We performed a multi-day shipboard experiment in June 2001 to test whether combining water from within an anticyclonic mesoscale eddy in the eastern Gulf of Alaska with water from outside could result in enhanced phytoplankton growth and to determine how mixing might influence planktonic assemblages. Initially, the eddy had lower standing stocks of algal pigments (chlorophyll a [chl a] and accessory pigments), nutrients, phytoplankton, and particulate organic carbon/nitrogen compared to waters outside of the eddy. The eddy possessed a greater diversity and abundance of coastal diatoms while the outside waters had a greater proportion of oceanic species, including the endemic pennate diatom, Nitzschia cylindroformis. After one week of incubation, rates of primary production were significantly higher in the mixed water compared to both the eddy and outside treatments. Pigment concentrations (except chl c₃, alloxanthin, and zeaxanthin) and the proportion of large diatoms (mainly Pseudo-nitzschia spp.) and heterotrophic dinoflagellates were greater in the mixed water than would be expected from the simple combination of inside and outside waters. Nutrient limitation (most likely by trace metals) appeared to be less severe in the mixed water. Chl a was enhanced in the mixed water, particularly when compared to the eddy water. The mixing of eddy and outside water masses stimulated primary production by ∼20%, but more importantly, the mixing resulted in a distinct planktonic assemblage. The biomass enrichment was short-lived, indicating that the maintenance of elevated chl a would require further mixing events in a physical setting that also permits an accumulation of biomass. We note that submesoscale processes, including the intensification of ageostrophic circulation that elicits strong vertical mixing in the presence of strain, might explain observed patterns of high phytoplankton standing stocks at the inner edges of Haida eddies in the field.     

Water circulation in the kuril basin of the Okhotsk Sea and its relation to eddy formation

This paper describes the water circulation in the Kuril Basin and its role in the formation and seasonal variation in intensity of the large anticyclonic eddies which occur in the basin. Oceanographic data for the period June 1977 through June 1979 suggest that these eddies develop in summer and decay in winter. In summer, the eddy development is associated with a deepening of the isopycnals caused by the surface flow of the Soya Warm Current over the basin, and the deep advection of cold, less saline, oxygen-rich water from Terpenia Bay and the eastern continental shelf of Sakhalin Island. In winter, the eddy decay is caused by surface cooling and convective mixing downward of the warm, saline surface water, which causes the isopycnals to rise and leads to an attenuation of the eddies. This combination of the summer influx of water into the region, and the fall and winter cooling of the eddies leads to the annual variation in eddy intensity.     

A 450-kyr record of hydrological conditions on the western Agulhas Bank Slope, south of Africa

The Agulhas Bank region, south of Africa, is an oceanographically important and complex area. The leakage of warm saline Indian Ocean water into the South Atlantic around the southern tip of Africa is a crucial factor in the global thermohaline circulation. Foraminiferal assemblage, stable isotope and sedimentological data from the top 10 m of core MD962080, recovered from the western Agulhas Bank Slope, are used to indicate changes in water mass circulation in the southeastern South Atlantic for the last 450 kyr. Sedimentological and planktonic foraminiferal data give clear signals of cold water intrusions. The benthic stable isotope record provides the stratigraphic framework and indicates that the last four climatic cycles are represented (i.e. down to marine isotope stage (MIS) 12). The planktonic foraminiferal assemblages bear a clear transitional to subantarctic character with Globorotalia inflata and Neogloboquadrina pachyderma (dextral) being the dominant taxa. Input of cold, subantarctic waters into the region by means of leakage through the Subtropical Convergence, as part of Agulhas ring shedding, and a general cooling of surface waters is suggested by increased occurrence of the subantarctic assemblage during glacial periods. Variable input of Indian Ocean waters via the Agulhas Current is indicated by the presence of tropical/subtropical planktonic foraminiferal species Globoquadrina dutertrei, Globigerinoides ruber (alba) and Globorotalia menardii with maximum leakage occurring at glacial terminations. The continuous presence of G. menardii throughout the core suggests that the exchange of water from the South Indian Ocean to the South Atlantic Ocean was never entirely obstructed in the last 450 kyr. The benthic carbon isotope record and sediment textural data reflect a change in bottom water masses over the core location from North Atlantic Deep Water to Upper Southern Component Water. Planktonic foraminiferal assemblages and sediment composition indicate a profound change in surface water conditions over the core site approximately 200–250 kyr BP, during MIS 7, from mixed subantarctic and transitional water masses to overall warmer surface water conditions.     

北冰洋水体对格陵兰海混合增密对流的可能影响分析

格陵兰海内发生的等密度混合后产生的增密对流是重要的对流现象之一。北冰洋正在发生快速变化,其内水团变性以及环流系统的改变都将使格陵兰海等密度混合对流发生明显变化,继而对全球气候变化产生影响。以往关于等密度混合对流的研究很少,大都集中在对流发生海域。由于等密度混合的主体是大西洋回流水与北冰洋流出水体,本文目的是探讨北极内部不同海域的水体会对混合增密对流造成的可能影响。文中定义了有效对流速度,强调水平温度梯度和垂向层化强度是影响有效对流速度的决定性因素;水平温度差越大,垂向层化越弱,产生的对流越强。发生在东格陵兰极锋处的有效对流都是大西洋的水体,一部分是在格陵兰海回流的大西洋回流水;一部分是在北冰洋潜沉并回流的北极大西洋水,该水体在北冰洋循环的时间越长,温度差越大,产生的有效对流越强。而横越北冰洋的太平洋水因密度过低而不能参与等密度混合对流,加拿大海盆主盐跃层之上的水体也都不能参与对流。北冰洋几个海盆深层水的温度差异明显,有可能与格陵兰海深层水形成有效对流;但是,由于深层水流速低、湍流混合弱、水平温度梯度小,是否可以产生有效对流尚不清楚。     

Cessation and partial reversal of deep water freshening in the northern North Atlantic: observation-based estimates and attribution

Recent decadal salinity changes in the Greenland-Scotland overflow-derived deep waters are quantified using CTD data from repeated hydrographic sections in the Irminger Sea. The Denmark Strait Overflow Water salinity record shows the absence of any net change over the 1980s–2000s; changes in the Iceland–Scotland Overflow Water (ISOW) and in the deep water column (σ₀ > 27.82), enclosing both overflows, show a distinct freshening reversal in the early 2000s. The observed freshening reversal is a lagged consequence of the persistent ISOW salinification that occurred upstream, in the Iceland Basin, after 1996 in response to salinification of the northeast Atlantic waters entrained into the overflow. The entrainment salinity increase is explained by the earlier documented North Atlantic Oscillation (NAO)-induced contraction of the subpolar gyre and corresponding northwestward advance of subtropical waters that followed the NAO decline in the mid-1990s and continued through the mid-2000s. Remarkably, the ISOW freshening reversal is not associated with changes in the overflow water salinity. This suggests that changes in the NAO-dependent relative contributions of subpolar and subtropical waters to the entrainment south of the Iceland–Scotland Ridge may dominate over changes in the Nordic Seas freshwater balance with respect to their effect on the ISOW salinity.     

An isopycnal view of the Nordic Seas hydrography with focus on properties of the Lofoten Basin

Few basins in the world exhibit such a wide range of water properties as those of the Nordic Seas with cold freshwaters from the Arctic in the western basins and warm saline waters from the Atlantic in the eastern basins. In this study we present a 50-year hydrographic climatology of the Nordic Seas in terms of depth and temperature patterns on four upper ocean specific volume anomaly surfaces. This approach allows us to better distinguish between change due to variations along such surfaces and change due to depth variations of the stratified water column. Depth variations indicate changes in the mass field while property variations along isopycnals give insight into isopycnal advection and mixing, as well as diapycnal processes. We find that the warmest waters on each surface are found in the north, close to where the isopycnal outcrops, a clear indication of downward mixing of the warmer, more saline waters on shallower isopycnals due to convective cooling at the surface. These saline waters come from the Norwegian Atlantic Slope Current by means of a very high level of eddy activity in the Lofoten Basin.The isopycnal analyses further show that the principal water mass boundary between the waters of Arctic origin in the west and Atlantic waters in the east aligns quite tightly with the Jan Mayen, Mohn, Knipovich Ridge system suggesting little cross-ridge exchange. Instead, the main routes of exchange between the eastern and western basins appear to be limited to the northern and southern ends of ridge system: Atlantic waters into the Greenland Sea in the Fram St and Artic waters into the southern Norwegian Sea just north of the Iceland-Faroe Ridge.Analysis of a representative isopycnal in the main pycnocline shows it to be stable over time with only small variations with season (except where it outcrops in winter in the Greenland and Iceland Seas). However, two very cold winters, 1968–1969, led to greater than average heat losses across the entire Lofoten Basin that eroded away much of the Lofoten eddy and induced the greatest temperature anomaly in the entire 50-year record. Interannual variations in isopycnal layer temperature correlate with the NAO index such that waters in the Iceland Sea become warmer than average with warming air temperatures and conversely in the Lofoten Basin.     

Transport and transformation of surface water masses across the Mascarene Plateau during the Northeast Monsoon season

The Mascarene Plateau lies in the south-west Indian Ocean between the islands of Mauritius and the Seychelles Bank, and is characterised by a series of shallow banks separated by deep (>1 000 m), narrow channels. The plateau acts as an obstruction to the general ocean circulation in this region, separating the westward-flowing South Equatorial Current (SEC) into two branches downstream of the plateau. In this article, we present the results of a survey conducted along the entire Mascarene Plateau during the Northeast Monsoon, in October–November 2008. In addition, data from Argo floats were used to determine the origin of water masses entering this region. The plateau contains three gaps through which branches of the SEC are channelled. The northern, central and southern gaps receive 14.93 Sv, 14.41 Sv and 6.19 Sv, respectively. Although there are differences in water-mass properties to the west and east of the Mascarene Plateau due to mixing, the SEC acts as a sharp boundary between water masses of southern and northern Indian Ocean origin. Mixing occurs in the central gap between intermediate water masses (Red Sea Water [RSW] and Antarctic Intermediate Water [AAIW]) as well as in the upper waters (Subtropical Surface Water [STSW] and Indonesian Throughflow Water [ITW]). Through the northern gap, mixing occurs between Arabian Sea High-Salinity Water (ASHSW), ITW and Tropical Surface Water (TSW), while through the southern gap, mixing occurs between STSW and ITW. North Indian Deep Water (NIDW) is present in the region but the plateau appears to have no effect on it.     

Mid-depth Lagrangian pathways in the North Atlantic and their impact on the salinity of the eastern subpolar gyre

The role of Mediterranean Overflow Water (MOW) in creating subsurface salinity anomalies within the Rockall Trough, a gateway to high latitude areas of deep convection, has been examined closely in recent years. Eulerian investigations of high latitude property fields have suggested that these subsurface anomalies are likely the result of variability in the zonal extent of the eastern limb of the subpolar gyre: when expanded into the eastern North Atlantic, the gyre is presumed to limit the extent to which MOW is able to penetrate northward to subpolar latitudes. However, though the depth of the subsurface salinity anomalies in the Rockall Trough supports the hypothesis that the intermittent presence of MOW is involved in creating the anomalies, MOW pathways to the Rockall Trough have not yet been established. Here, Lagrangian trajectories from floats released in the eastern North Atlantic between 1996 and 1997 and synthetic trajectories launched within an eddy-resolving ocean general circulation model are used to demonstrate that two main density neutral transport pathways lead to the Rockall Trough. One pathway involves the transport of relatively fresh waters as part of the North Atlantic Current and the other involves the transport of relatively salty waters from the eastern reaches of the subtropical North Atlantic. The results from this study indicate that changes in these pathways over time can explain the subsurface salinity variability in the Rockall Trough.     

在大洋环流模型中对应于惯性内波破碎的垂直混合的一个参数化方案

Based on the theoretical spectral model of inertial internal wave breaking(fine structure) proposed previously, in which the effects of the horizontal Coriolis frequency component f-tilde on a potential isopycnal are taken into account, a parameterization scheme of vertical mixing in the stably stratified interior below the surface mixed layer in the ocean general circulation model(OGCM) is put forward preliminarily in this paper. Besides turbulence, the impact of sub-mesoscale oceanic processes(including inertial internal wave breaking product) on oceanic interior mixing is emphasized. We suggest that adding the inertial internal wave breaking mixing scheme(F-scheme for short) put forward in this paper to the turbulence mixing scheme of Canuto et al.( T-scheme for short) in the OGCM, except the region from 15°S to 15°N. The numerical results of F-scheme by using WOA09 data and an OGCM(LICOM, LASG/IAP climate system ocean model) over the global ocean are given. A notable improvement in the simulation of salinity and temperature over the global ocean is attained by using T-scheme adding F-scheme, especially in the mid- and high-latitude regions in the simulation of the intermediate water and deep water. We conjecture that the inertial internal wave breaking mixing and inertial forcing of wind might be one of important mechanisms maintaining the ventilation process. The modeling strength of the Atlantic meridional overturning circulation(AMOC) by using T-scheme adding F-scheme may be more reasonable than that by using T-scheme alone, though the physical processes need to be further studied, and the overflow parameterization needs to be incorporated. A shortcoming in F-scheme is that in this paper the error of simulated salinity and temperature by using T-scheme adding F-scheme is larger than that by using T-scheme alone in the subsurface layer.     

21世纪格陵兰冰川融化速率对海平面变化的影响

本文利用大洋环流模式POP研究RCP4.5情景下21世纪格陵兰冰川不同的融化速率对全球及区域海平面变化的影响。结果显示:当格陵兰冰川的融化速率以每年1%增加时,全球大部分海域的动力和比容海平面变化基本不变,主要是由于格陵兰冰川在低速融化时并不会导致大西洋经向翻转流减弱。当格陵兰冰川的融化速率以每年3%和每年7%增加时,动力海平面在北大西洋副极地、大西洋热带、南大西洋副热带和北冰洋海域呈现出显著的上升趋势,这是因为格陵兰冰川快速融化导致大量的淡水输入附近海域,造成该上层海洋层化加强和深对流减弱,导致大西洋经向翻转流显著减弱;与此同时,热比容海平面在北冰洋、格陵兰岛南部海域和大西洋副热带海域显著下降,而在热带大西洋和湾流海域明显上升;此时盐比容海平面的变化与热比容海平面是反相的,这是由于大量的低温低盐水的输入,造成北大西洋副极地海域变冷变淡、大西洋经向翻转流和热盐环流显著减弱,引起了太平洋向北冰洋的热通量和淡水通量减少,导致了北冰洋海水变冷变淡,同时热带大西洋滞留了更多的高温高盐水,随着湾流被带到北大西洋,北大西洋副极地海域低温低盐的海水,被风生环流输运到副热带海域。