Spatial and seasonal variability of global ocean diapycnal transport inferred from Argo profiles

The global diapycnal transport in the ocean interior is one of the significant branches to return the deep water back toward near-surface. However, the amount of the diapycnal transport and the seasonal variations are not determined yet. This paper estimates the dissipation rate and the associated diapycnal transports at 500 m, 750 m and 1 000 m depth throughout the global ocean from the wide-spread Argo profiles, using the finescale parameterizations and classic advection-diff usion balance. The net upwelling is ~5.2±0.81 Sv (Sverdrup) which is approximately one fifth in magnitude of the formation of the deep water. The Southern Ocean is the major region with the upward diapycnal transport, while the downwelling emerges mainly in the northern North Atlantic. The upwelling in the Southern Ocean accounts for over 50% of the amount of the global summation. The seasonal cycle is obvious at 500 m and vanishes with depth, indicating the energy source at surface. The enhancement of diapycnal transport occurs at 1000min the Southern Ocean, which is pertinent with the internal wave generation due to the interaction between the robust deep-reaching flows and the rough topography. Our estimates of the diapycnal transport in the ocean interior have implications for the closure of the oceanic energy budget and the understanding of global Meridional Overturning Circulation.     

利用全球温盐观测剖面计划资料估算太平洋上层跨等密度面混合

基于全球多年来积累的大量温盐观测剖面计划资料,利用精细尺度参数化方案估算了太平洋上层600米区域内的跨等密度面混合系数。结果表明,在不同季节跨等密度面混合系数的整体分布类似,但是细节上存在很大的不同。跨等密度面混合系数的加强与底部摩擦、海表面近惯性能量输入、赤道效应等因素密切相关。由于粗糙的地形,混合在东北太平洋门多西诺断裂带附近存在明显加强。同时,在南大洋西风带区域,由于大风引起的近惯性能量输入也导致了混合的强化。 与前人的研究结果相比,本文的估算具有更好的空间覆盖率以及更高的分辨率,更重要的是我们的结果体现了季节性变化。由于我们给出的是网格化产品,非常适合在数值模式中的应用。     

Argo floats revealing bimodality of large-scale mid-depth circulation in the North Atlantic

Analysis of Argo float trajectories at 1 000 m and temperature at 950 m in the North Atlantic between November 2003 and January 2005 demonstrates the existence of two different circulation modes with fast transition between them. Each mode has a pair of cyclonic - anticyclonic gyres. The difference is the location of the cyclonic gyre. The cyclonic gyre stretches from southeast to northwest in the first mode and from the southwest to the northeast in the second mode. The observed modes strongly affect the heat and salt transport in the North Atlantic. In particular, the second mode slows down the westward transport of the warm and saline water from the Mediterranean Sea.     

Argo浮标观测溶解氧数据的原理与质量控制

Argo剖面浮标已成为海洋溶解氧观测数据的主要来源,截至2018年7月已在全球范围内获取了超过13万条溶解氧剖面,然而目前其数据的质量控制仍然存在不足。本文首先介绍了Argo浮标上携带的溶解氧传感器的测量原理、主要误差来源以及两种校正方法:气候态校正法与空气测量校正法,并提出了目前在溶解氧传感器数据质量控制研究亟需解决的技术问题,包括:①由于传感器伸出水面的高度不够,一些浮标并不能有效测量空气氧含量,因为无法进行空气测量校正方法;②Argo观测的溶解氧数据库中目前存在一些异常测量剖面没有得到有效剔除,影响校正系数的计算;③使用平均值确定校正系数的方法容易受到异常剖面的干扰。     

Direct measurements of surface and mid-depth circulationin the Shikoku Basin by Argo profiling floats

The circulation in the Shikoku Basin plays a very important role in the pathway of the Kuroshio and the water exchange in the subtropical gyre in the North Pacific Ocean. The Argo profiling floats deployed in the Shikoku Basin are used to study the circulations and water masses in the basin. The trajectories and parking depth velocity fields derived from all Argo floats show an anticyclonic circulation at 2 000 m in the Shikoku Basin. There are inhanced eddy activities in the Shikoku Basin, which have large influence on the Shikoku Basin circulation patterns. The characteristics of temperature-salinity curves indicate that there are North Pacific Ocean tropical water (NPTW), North Pacific Ocean subtropical mode water (NPSTMW) and North Pacific Ocean intermediate water (NPIW) in the Shikoku Basin. The NPTW is only exists south of 32°N. In the middle part of the basin, which is 28°~31°N,133°~135°E, there is a confluence region. Water masses coming from the Kuroshio mix with the water in the Shikoku Basin.     

Evaluation of spatial distribution of turbulent mixing in the central Pacific

A long-term mean turbulent mixing in the depth range of 200–1000 m produced by breaking of internal waves across the middle and low latitudes (40°S–40°N) of the Pacific between 160°W and 140°W is examined by applying fine-scale parameterization depending on strain variance to 8-year (2005–2012) Argo float data. Results show that elevated turbulent dissipation rate (ε) is related to significant topographic regions, along the equator, and on the northern side of 20°N spanning to 24°N throughout the depth range. Two patterns of latitudinal variations of ε and the corresponding diffusivity (K_ρ) for different depth ranges are confirmed: One is for 200–450 m with significant larger ε and K_ρ, and the maximum values are obtained between 4°N and 6°N, where eddy kinetic energy also reaches its maximum; The other is for 350–1000 m with smaller ε and K_ρ, and the maximum values are obtained near the equator, and between 18°S and 12°S in the southern hemisphere, 20°N and 22°N in the northern hemisphere. Most elevated turbulent dissipation in the depth range of 350–1000 m relates to rough bottom roughness (correlation coefficient?=?0.63), excluding the equatorial area. In the temporal mean field, energy flux from surface wind stress to inertial motions is not significant enough to account for the relatively intensified turbulent mixing in the upper layer.     

基于卫星和浮标数据的副热带西北太平洋反气旋型中尺度涡旋的个例观测研究

The comprehensive three-dimensional structures of an anti-cyclonic mesoscale eddy(AE) in the subtropical northwestern Pacific Ocean were investigated by combining the Argo floats profiles with enhanced vertical and temporal sampling and satellite altimetry data. The AE originated near the Kuroshio Extension and then propagated westward with mean velocity of 8.9 cm/s. Significant changes and evolutions during the AE's growing stage(T1) and further growing stage(T2) were revealed through composite analysis. In the composite eddy core,maximum temperature(T) and salinity(S) anomalies were of 1.7(1.9)°C and 0.04(0.07) psu in T1(T2) period,respectively. The composite T anomalies showed positive in almost whole depth, but the S anomalies exhibited a sandwich-like pattern. The eddy's intensification and its influence on the intermediate ocean became more significant during its growth. The trapping depth increased from 400×10~4 Pa to 580×10~4 Pa while it was growing up, which means more water volume, heat and salt content in deeper layers can be transported. The AE was strongly nonlinear in upper oceans and can yield a typical mean volume transport of 0.17×10~6 m~3/s and a mean heat and salt transport anomaly of 3.6×10~(11) W and –2.1×10~3 kg/s during the observation period. The Energy analysis showed that eddy potential and kinetic energy increased notably as it propagated westward and the baroclinic instability is the major energy source of the eddy growth. The variation of the remained Argo float trapped within the eddy indicated significant water advection during the eddy's propagation.     

The variation of turbulent diapycnal mixing at 18°N in the South China Sea stirred by wind stress

The spatial and temporal variations of turbulent diapycnal mixing along 18°N in the South China Sea(SCS) are estimated by a fine-scale parameterization method based on strain, which is obtained from CTD measurements in yearly September from 2004 to 2010. The section mean diffusivity can reach ~10~(–4)m~2/s, which is an order of magnitude larger than the value in the open ocean. Both internal tides and wind-generated near-inertial internal waves play an important role in furnishing the diapycnal mixing here. The former dominates the diapycnal mixing in the deep ocean and makes nonnegligible contribution in the upper ocean, leading to enhanced diapycnal mixing throughout the water column over rough topography. In contrast, the influence of the wind-induced nearinertial internal wave is mainly confined to the upper ocean. Over both flat and rough bathymetries, the diapycnal diffusivity has a growth trend from 2005 to 2010 in the upper 700 m, which results from the increase of wind work on the near-inertial motions.     

Dissipation of baroclinic tidal energy and diapycnal mixing in the White Sea

The modeling results obtained using the original version of the three-dimensional finite-element hydrostatic model QUODDY-4 testify that the spatial distributions of dissipation of baroclinic tidal energy and the related coefficient of diapycnal mixing in the deepwater stratified subdomain of the White Sea (the Basin and Kandalaksha and Dvina bays together) are highly similar to those found for low- and midlatitude oceans. It is in the open part of the sea that their values remain equal to the minimum possible values determined by the molecular kinematic viscosity; at its lateral boundaries (not all boundaries, but only individual segments (sites of mixing)), their values increase. In the shallow homogeneous subdomain of the White Sea, the dissipation of baroclinic tidal energy is considerably larger than in the deep stratified subdomain. Accordingly, the vertical eddy viscosity in the first subdomain is a few orders of magnitude higher than the coefficient of diapycnal mixing in the second subdomain. This is caused by an increased tidal velocity due to reduced depths.     

Using satellite altimetry to correct mean temperature and salinity fields derived from Argo floats in the ocean regions around Australia

We present results from a suite of methods using in situ temperature and salinity data, and satellite altimetric observations to obtain an enhanced set of mean fields of temperature, salinity (down to 2000-m depth) and steric height (0/2000 m) for a time-specific period (1992–2007). Firstly, the improved global sampling resulting from the introduction of the Argo program, enables a representative determination of the large-scale mean oceanic structure. However, shortcomings in the coverage remain. High variability western boundary current eddy fields, continental slope and shelf boundaries may all be below their optimal sampling requirements. We describe a simple method to supplement and improve standard spatial interpolation schemes and apply them to the available data within the waters surrounding Australia (100°E–180°W; 50°S–10°N). This region includes a major current system, the East Australian Current (EAC), complex topography, unique boundary currents such as the Leeuwin Current, and large ENSO related interannual variability in the southwest Pacific. We use satellite altimetry sea level anomalies (SLA) to directly correct sampling errors in in situ derived mean surface steric height and subsurface temperature and salinity fields. The surface correction is projected through the water column (using an empirical model) to modify the mean subsurface temperature and salinity fields. The errors inherent in all these calculations are examined. The spatial distribution of the barotropic–baroclinic balance is obtained for the region and a ‘baroclinic factor’ to convert the altimetry SLA into an equivalent in situ height is determined. The mean fields in the EAC region are compared with independent estimates on repeated XBT sections, a mooring array and full-depth CTD transects.     

利用 Argo 数据计算吕宋海峡以东海域水文特性参数和流场 *

利用2006年Argo浮标资料分析吕宋海峡以东海域水团季节特性和混合层的月平均变化规律;并分别利用Argo多年季节平均资料与2006年资料,以秋季为例,基于P矢量方法计算该区域流场;同时考虑风生流的影响,将所得结果分别与利用Levitus和高度计资料计算的流场进行比较。结果表明,水团特性季节变化不明显,春冬季表层水团与夏秋季比较表现为低温高盐;次表层、中层和深层季节变化不大;混合层深度明显表现为冬季最深、夏季最浅的季节性变化。利用2002—2009年Argo季节平均资料基于P矢量方法能得到地转流场的基本结构,与Levitus资料的计算结果相比较,除可以反映黑潮,还可以反映一些涡旋结构;利用2006年秋季Argo资料计算流场与高度计资料计算的地转流场比较,其流场结构位置吻合得比较好,但存在流速偏小等不足,这可能与Argo资料较少且分布不均以及插值误差等有关,但其可以获得流场的三维结构,而利用高度计资料计算只能得到表层流场结构。     

Global mapping of 10-day differences of temperature and salinity in the intermediate layer observed with Argo floats

Journal of Oceanography - The authors examine small-scale spatiotemporal variability of the layer nearly 2000-m depth, which is the “bottom” of the present Argo observation system,...     

南海中部上层海洋湍流混合的空间分布特征及参数化模型

通过对2010年5月南海16°N和14.5°N断面的湍流微结构剖面观测资料分析,给出了南海海盆上层湍流混合空间分布特征:在16°N断面上,上层10~400m垂向平均湍动能耗散率ερ在东侧略大于西侧;相反,在14.5°N断面上,西侧ερ均值约是东侧ερ的4倍,其中,西侧110.5°~111°E的ερ的平均值为2.6×10-6 W/m3,东侧118.5°E的ερ仅为5.89×10-7 W/m3。通过分析细结构剪切和湍流混合的相关性,发现剪切是南海中部上层强湍流混合的主要驱动力,揭示了高模态内波破碎可能是湍流混合的主要机制。另外,研究了大洋中的3种参数化模型,发现适用于大洋近海的参数化MacKinnon-Gregg(MG)模型能较好地用浮频和剪切估算南海中部深海区上层湍流耗散率。     

南海北部中尺度反气旋涡的湍流混合空间分布特征

文章利用GHP细结构参数化方法和Thorpe-scale方法,分析水下滑翔机于2015年5月在南海北部采集的数据,估算了南海北部中尺度反气旋涡的湍流混合空间分布特征。结果显示该反气旋涡的混合具有明显的空间非对称性,混合率在其运动方向的后侧边缘明显增强达到O(10^-3 m²/s)量级;而在其运动方向的前侧边缘,平均混合率要小一个量级。这一混合非对称特征与中尺度的涡动能密切相关性。中尺度涡后侧边缘处存在高流速剪切,容易引起垂向剪切不稳定,可能是引起该处混合增强的主要因素。另外,中尺度涡后侧边缘发展的次中尺度过程同样导致了该处强混合。本研究结果有助于人们进一步认识南海北部的混合过程。     

Energetics of wind-induced turbulent mixing in the ocean

The pattern and magnitude of the global ocean overturning circulation is believed to be strongly controlled by the distribution of diapycnal diffusivity below 1000 m depth. Although wind stress fluctuation is a candidate for the major energy sources of diapycnal mixing processes, the global distribution of wind-induced diapycnal diffusivity is still uncertain. It has been believed that internal waves generated by wind stress fluctuations at middle and high latitudes propagate equatorward until their frequency is twice the local inertial frequency and break down via parametric subharmonic instabilities, causing diapycnal mixing. In order to check the proposed scenario, we use a vertically two-dimensional primitive equation model to examine the spatial distribution of “mixing hotspots” caused by wind stress fluctuations. It is shown that most of the wind-induced energy fed into the ocean interior is dissipated within the top 1000 m depth in the wind-forced area and the energy dissipation rate at low latitudes is very small. Consequently, the energy supplied to diapycnal mixing processes below 1000 m depth falls short of the level required to sustain the global ocean overturning circulation.     

Geographic distribution of the diapycnal component of thermohaline circulations in coupled climate models

Data archives from four global coupled ocean–atmosphere models are used to construct maps of diapycnal mass flux through selected isopycnal surfaces in the model oceans. The maps illustrate location and strength of the up and downwelling limbs of thermohaline-forced overturning loops whose stability in the face of rising atmospheric carbon dioxide (CO₂) concentrations is of major concern in century-scale climate prediction. The up and downwelling limbs simulated by the four models for present-day greenhouse gas concentrations are compared with observational estimates. Predicted changes in the overturning brought about by gradually rising atmospheric CO₂ content are compared model-to-model. While all four models predict some decline in the rate of Atlantic overturning during CO₂-induced global warming, the geographic layout of the overturning circulations in each model is found to be insensitive to the changing climate.     

Argo资料的全球上层海温年和半年周期振荡的空间分布特征

利用小波分析方法,对2003-2008年周平均的Argo(地转海洋学实时观测阵)海温资料进行了分析,给出了全球上层海温年周期和半年周期振荡的空间分布特征.结果表明,南北半球中高纬地区以表层海温的年周期变化为主,在低纬度地区,表层海温以半年周期为主,而温跃层附近海温既有年周期也有半年周期(赤道太平洋、东南印度洋和赤道西大西洋以年周期为主;赤道东、西印度洋以半年周期为主).南北半球中高纬的年周期海温和北半球中纬度的半年周期海温在表层范围最大,显著性最高,强度最强,位相最前.随深度的增加,范围减小,显著性降低,强度减弱,位相滞后.信号主要集中在水深50 m以上,影响深度在150m以浅;赤道附近的太平洋和热带东南印度洋的年周期海温以及赤道东、西印度洋的半年周期海温在水深100m范围最大,显著性最高,强度最强,位相最前,信号主要集中在温跃层附近,影响深度均可达500m.     

Effects of sea ice formation and diapycnal mixing on the Okhotsk Sea intermediate water clarified with oxygen isotopes

Processes relating to the formation of dense shelf water and intermediate water in the Okhotsk Sea were studied by examining oxygen isotope ratios (δ¹⁸O), salinity, and temperature. The salinity and δ¹⁸O of the cold dense shelf water on the northern continental shelf showed peculiar relationship. The relationship indicates that 3% of the mixed-layer water, having salinity of 32.6, froze and the remaining 97% became dense shelf water of salinities of more than 33.2 (σ_θ>26.7) during the sea ice formation. The salinity–δ¹⁸O relationship also shows that 20% of the Okhotsk Sea Intermediate Water at the σ_θ=26.8 level was derived from the dense shelf water. The remaining 80% came from the Western Subarctic Pacific water modified by diapycnal mixing of water affected by the surface cooling and freshening within the Okhotsk Sea. The mixing with dense shelf water contributes to only 26% of the temperature difference or 8% of the salinity difference between the original Pacific water and the Okhotsk Sea Intermediate Water at σ_θ=26.8. This result suggests that the cold and less saline properties of the Okhotsk Sea Intermediate Water are produced mainly by diapycnal mixing, rather than by mixing of the Pacific water with the dense shelf water.     

Quantitative evaluation of turbulent mixing in the Central Equatorial Pacific

Turbulent mixing in the central equatorial Pacific has been quantitatively evaluated by analyzing data from microstructure measurements and conductivity temperature depth profiler (CTD) observations in a meridionally and vertically large region. The result that strong turbulent mixing with dissipation rate ε (>O(10^?7) W kg^?1), continuing from sea-surface mixed layer to low Richardson number region below, in the area within 1° of the equator, shows that turbulent mixing has a close relationship to shear instability. ε > O(10^?7) W kg^?1 and turbulent diffusivity K _ρ  > O(10^?3) m² s^?1 were obtained from near-surface to 85 db at stations even southwardly beyond 3°S, where it is already far from the southern boundary (~2°S) of the Equatorial Undercurrent. Turbulence-induced heat flux and salinity flux were calculated, and both had their maxima in the equatorial upwelling region, though the former was downward and the latter was upward. Accordingly, vertical velocity in the upwelling region was estimated to be similar to the results derived by other methods. These fluxes and the vertical velocity suggest the critical importance of turbulent mixing in maintaining the well-mixed upper layer. Secondly, in the intermediate region (>500 db), turbulent eddies were investigated by applying Thorpe’s method to the CTD data. A large number of overturns were detected, with spatial-averaged K _ρ (700–1,000 db) being 3.3 × 10^?6 m² s^?1, and the corresponding K _ρ-max reaching to O(10^?4) m² s^?1 in the north (3°–13°N). The results suggest that, in the intermediate region, considerable turbulent mixing occurs and moderates the properties of the water masses.     

Geostrophic balance of the Kuroshio as inferred from surface current and sea level observations

Historical observations of the surface current and daily mean sea level during the period 1965–1985 are analyzed in order to examine the geostrophic balance of the Kuroshio current in the Tokara Strait and near the Izu Islands. The variation in the sea level difference across the Kuroshio is associated with a variation in surface current velocity as predicted by the theory of geostrophic balance. However, the slope of the linear relation between the current velocity and sea level difference is smaller than the theoretically predicted value by about 30%. This disagreement may be ascribed to the effects of the centrifugal force and the occasional rise in sea level due to storm surges.Absolute mean sea level differences between the tidal stations are estimated by making use of the empirical relationship between the surface current and sea level difference. Estimated differences are: 87.4±22.1 cm between Naze and Nishinoomote, 24.3±9.2 cm between Miyake and Minamiizu, 41.3±17.7 cm between Miyake and Mera and 45.1±8.8 cm between Hachijyo and Miyake. The absolute value of sea level difference between Miyake and Minamiizu and that between Miyake and Mera may be about 30 cm, since geodetic levelling tells us that the mean sea level at Minamiizu is nearly equal to that at Mera.