The role of quasi-stationary flaw polynya in formation of dense shelf waters in the wintertime and their subsequent slope cascading (the Laptev Sea case study)

Shelf areas in the region of the Severnaya Zemlya Archipelago in the Laptev Sea are characterized by existing quasi-stationary flaw polynya that periodically opens throughout the entire wintertime under the action of strong offshore winds, which occur during the passage of cyclones. In periods of the open water surface, a near-surface turbulent layer or forced convection layer is formed in the flaw polynya; the water in the layer formed undergoes intense salinization and its dense increases due to active volumetric frazil ice production. As a result of the gravity force action, intense three-dimensional convective circulation develops in the underlying layers. It leads to a fast convective adjustment of the entire water column, especially, in the late winter, when residual stratification in the area of polynya is weakened with the total action of salinization due to the background static ΣMs _back and periodical local frazil ice formation ΣMs _f . On the whole for the entire winter period ΣMs _f is 3.4 times greater than ΣMs _back, although, during one month, probable lifetime of polynya with open water surface is several days. However, in these periods, salt fluxes with frazil ice production exceed background salt fluxes in the congelation polynya and background salt fluxes under heavy ice (limiting the polynya) 10–80 times. Spreading outside the polynia, dense shelf waters form in the area of polynya mesoscale baroclinic circulation, first generating intense shelf cascading, then intense slope cascading, which is of a local and random character. Some estimates of elements of baroclinic circulation of a convective origin in the area of polynia were obtained from the laboratory modeling results and are confirmed by field observation data.     

Impact of anthropogenic CO 2 on the next glacial cycle

The model of Paillard and Parrenin (Earth Planet Sci Lett 227(3–4):263–271, 2004) has been recently optimized for the last eight glacial cycles, leading to two different relaxation models with model-data correlations between 0.8 and 0.9 (García-Olivares and Herrero (Clim Dyn 1–25, 2012b)). These two models are here used to predict the effect of an anthropogenic CO ₂ pulse on the evolution of atmospheric CO ₂, global ice volume and Antarctic ice cover during the next 300 kyr. The initial atmospheric CO ₂ condition is obtained after a critical data analysis that sets 1300 Gt as the most realistic carbon Ultimate Recoverable Resources (URR), with the help of a global compartmental model to determine the carbon transfer function to the atmosphere. The next 20 kyr will have an abnormally high greenhouse effect which, according to the CO ₂ values, will lengthen the present interglacial by some 25 to 33 kyr. This is because the perturbation of the current interglacial will lead to a delay in the future advance of the ice sheet on the Antarctic shelf, causing that the relative maximum of boreal insolation found 65 kyr after present (AP) will not affect the developing glaciation. Instead, it will be the following insolation peak, about 110 kyr AP, which will find an appropriate climatic state to trigger the next deglaciation.     

Submesoscale fronts observed by satellites over the Northern South China Sea shelf

Fronts are ubiquitous dynamic processes in the ocean, which play a significant role in the ocean dynamical and ecological environments. In this paper strong temperature fronts are investigated on the shelf of the Northern South China Sea using high resolution satellite data. These fronts have large horizontal gradients exceeding 1 °C km⁻¹ with spatial scales around several kilometers. The fronts generate meanders and eddies due to baroclinic instability, since these instabilities have spatial scales around the local first baroclinic mode deformation radius. The estimated Rossby number of the fronts is O(0.4), suggesting that the fronts tend to be ageostrophic and show submesoscale features. The Finite Size Lyapunov Exponent analysis of the generation mechanism indicates that the fronts are tightly related to the combined flow straining of geostrophic and Ekman currents.     

Shelf wave dispersion in a geophysical ocean

We consider subinertial, free waves trapped along three coastlines (i.e., shelf waves) in an ocean governed by a geophysical model in which stratification is explicitly obtained by taking the Vaisala frequency N much greater than the inertial frequency f. The behavior is generalized in terms of the parameter S = (N/f)a where a is the bottom slope of the trapping region. Only when $\text{S}\text{\$}\text{?}\text{0.2}$, are the predicted shelf waves like those predicted by Laplace's tidal equations (LTE) on an f-plane. When 0.2 ? S < 1, LTE are inappropriate because the shelf waves are only qualitatively like those predicted by LTE, and when S 1, the shelf waves are like baroclinic Kelvin waves in that they can occur at any subinertial frequency up to f (in qualitative disagreement with the predictions of LTE). Since N/f is usually a large number in the real ocean (of order 50–250), S is likely to be large unless the bottom slope is very gentle throughout the trapping region. Some applications to coastal current observations are discussed.     

Estimating the baroclinic Rossby radius of deformation in the Sea of Okhotsk

The first baroclinic Rossby radius of deformation (R ₁) in the Sea of Okhotsk is estimated using the hydrological datasets from the World Ocean Atlas (WOA) 2001 and WOA 2013. It is established that the maximum values of R ₁ are observed over the Kuril Basin (18–20 km), and its minimum values (1.5–2 km), over the northern shelf of the Sea of Okhotsk. In the central part of the sea R1 varies from 8 to 10 km. The seasonal variability of R1 for both datasets is characterized by the minima in winter and by the maxima in summer. It was found that on the eastern shelf of Sakhalin Island R ₁ reaches the maximum both in November (～6 km) and in April (～4-5 km). According to the obtained estimates of R ₁, the model grid resolution of 3 to 8 km should be used for the eddy-permitting numerical simulation of circulation in the Sea of Okhotsk, and the model grid resolution from 1.5 to 2 km, for the explicit simulation of mesoscale variability.     

The influence of geometry on recirculation and CO2 transport over forested hills

Flow distortion over a forested hill is asymmetric, forming a recirculation region on the lee slope that increases the complexity in understanding atmosphere–biosphere interaction. To understand the complexity, we examine the effect of the geometry of forested hills on recirculation formation, structure, and related CO₂ transport by performing numerical simulations over double-forested hills. The ratio (0.8) of hill height (H) to half length (L) is a threshold value of flow patterns in the recirculation region: below 0.8, sporadic reversed flow occurs; at 0.8, one vortex is formed; and above 0.8, a pair of counter-rotating vortices is formed. The depth of recirculation increases with increasing H/L. The contribution of advection to the CO₂ budget is non-negligible and topographic-dependent. Vertical advection is opposite in sign to horizontal advection but cannot exactly offset in magnitude. Height-integrated advection shows significant variation in fluxes across hills. Gentle slopes can cause larger advection error. However, the relative importance of advection to CO₂ budget is slope-independent.     

簡单斜压大气中热成风的建立和破坏(一)

本文第一部分利用两层模式討論了热成风的适应問題.如果起始非热成风流場的热成风涡度大于溫度場的热成风涡度时,促使上升运动加强,反之,促使下沉运动加强。对于250—750毫巴之間的斜压系統,适应的特征尺度L_0,是决定流場向溫度場調整,还是溫度場向流場調整的临界水平尺度。 第二部分根据溫度場和流場的平流作用,分析了非热成风产生的机制.并由热成风适应速率和破坏速率的对比,討論了准地轉运动和非地轉运动的形成。 最后,用热成风不断破坏和建立的观点和方法,分析了长波斜压不稳定发展的过程,并进一步揭示了它的物理本貭。     

On the validity of layered models of ocean dynamics and thermodynamics with reduced vertical resolution

With the purpose of studying the upper part of the ocean, the shallow water equations (in a `reduced gravity' setting) have been extended in the last decades by allowing for horizontal and temporal variations of the buoyancy field ϑ, while keeping it as well as the velocity field u as depth-independent. In spite of the widespread use of this `slab' model, there has been neither a discussion on the range of validity of the system nor an explanation of points such as the existence of peculiar zero-frequency normal modes, the nature of the instability of a uniform u flow, and the lack of explicit vertical shear associated with horizontal density gradients. These questions are addressed here through the development of a subinertial model with more vertical resolution, i.e., one where the buoyancy ϑ varies linearly with depth. This model describes satisfactorily the problem of baroclinic instability with a free boundary, even for short perturbations and large interface slopes. An enhancement of the instability is found when the planetary β effect is compensated with the topographic one, due to the slope of the free boundary, allowing for a `resonance' of the equivalent barotropic and first baroclinic modes. Other low-frequency models, for which buoyancy stratification does not play a dynamical role, are invalid for short perturbations and have spurious terms in their energy-like integral of motion.     

Response of Southern Ocean circulation to global warming may enhance basal ice shelf melting around Antarctica

We investigate the large-scale oceanic features determining the future ice shelf–ocean interaction by analyzing global warming experiments in a coarse resolution climate model with a comprehensive ocean component. Heat and freshwater fluxes from basal ice shelf melting (ISM) are parameterized following Beckmann and Goosse [Ocean Model 5(2):157–170, 2003]. Melting sensitivities to the oceanic temperature outside of the ice shelf cavities are varied from linear to quadratic (Holland et al. in J Clim 21, 2008). In 1% per year CO₂-increase experiments the total freshwater flux from ISM triples to 0.09 Sv in the linear case and more than quadruples to 0.15 Sv in the quadratic case after 140 years at which 4 × 280 ppm = 1,120 ppm was reached. Due to the long response time of subsurface temperature anomalies, ISM thereafter increases drastically, if CO₂ concentrations are kept constant at 1,120 ppm. Varying strength of the Antarctic circumpolar current (ACC) is crucial for ISM increase, because southward advection of heat dominates the warming along the Antarctic coast. On centennial timescales the ACC accelerates due to deep ocean warming north of the current, caused by mixing of heat along isopycnals in the Southern Ocean (SO) outcropping regions. In contrast to previous studies we find an initial weakening of the ACC during the first 150 years of warming. This purely baroclinic effect is due to a freshening in the SO which is consistent with present observations. Comparison with simulations with diagnosed ISM but without its influence on the ocean circulation reveal a number of ISM-related feedbacks, of which a negative ISM-feedback, due to the ISM-related local oceanic cooling, is the dominant one.     

Impact of terrain complexity on the turbulence drag coefficient: A case study from the Indian Himalayan region

The aerodynamic drag coefficient (C_D) is conjectured to change (or remains almost uniform) with the horizontal wind speed (U) over a flexible (or fixed) surface element, which is represented with the surface roughness (z₀). This conjecture is tested for the near neutral atmospheric turbulence (i.e. when surface stability z/L is almost equal to 0, where z is the measurement height and L is Obukhov length) of monsoon and winter season at an on-slope and a ridge-top site in the Indian Himalaya, wherein the ridge-top site is associated with a higher degree of sensitivity to the roughness element and terrain attributes. This hypothesis is successfully verified for two conditions, (i) the monsoon period observations of ridge-top site are found to have higher z₀ due to vegetative growth than the winter period for flows having similar terrain signature, and (ii) the monsoon and winter period observations of on-slope site are noted to have similar z₀ for flows having signature of steep terrain. Subsequently, constants (i.e. a and b) of the power-law relationships between C_D and U (i.e. C_D = aU^b), as a function of z₀, are optimized. It is noted that the relationship between C_D and U has higher sensitivity towards the terrain slope than the vegetative growth.     

Surface energy balance,parameterizations of boundary-layer heights and the application of resistance laws near an Antarctic Ice Shelf front

A study of the surface energy balance with turbulent fluxes obtained by the Monin-Obukhov similarity theory and a comparison with results for resistance laws are presented for the strong baroclinic conditions in the vicinity of the Filchner/Ronne Ice Shelf front. The data are taken from a field experiment in the Antarctic summer season 1983/84. For the first time in the coastal Antarctic region, this data set comprises synchronous energy balance measurements over the polynya and the ice shelf together with soundings of the boundary layer, yielding vertical profiles of the wind velocity and temperature over the ice shelf, at the ice shelf front and over the polynya.Over the ice shelf, the radiation balance is the largest component of the energy fluxes and is mainly compensated by the subsurface energy flux and the turbulent heat flux in the daily mean. Over the polynya, turbulent fluxes of sensible and latent heat lead to large energy losses of the water surface in the night-time and in situations of very low air temperatures.Different parameterizations for boundary-layer height are compared using tethered sonde and energy balance measurements. With the height of the inversion base over the polynya and the height of the critical bulk Richardson number over the ice shelf, external parameters for the application of resistance laws were determined. The comparison of turbulent surface fluxes obtained by the energy balance measurements and by the resistance laws shows good agreement for the convective conditions over the polynya. For the stably stratified boundary layer over the ice shelf with small amounts of the turbulent heat flux, the deviation is large for the case of a cold air outflow with a superposed inertial oscillation.     

1215号“布拉万”台风暴雨及降水非对称性分布的成因分析

利用中尺度非静力模式WRF对2012年第15号台风“布拉万”在中国东北地区造成的暴雨过程进行了数值模拟,结合观测资料对模拟结果进行了验证,利用模式输出的高分辨率资料,对“布拉万”台风造成的强降水及其非对称性分布的成因进行了诊断分析。结果表明,模式很好地再现了台风登陆过程中的路径、强度变化和降水分布,受中纬度西风槽带来的干冷空气影响,“布拉万”台风登陆后的降水和环流结构具有明显的不对称性,降水主要集中在台风中心西北侧的能量锋区附近。水汽散度通量和水汽螺旋度能够较好地描述强降水过程的发生、发展及其非对称性分布的时空特征,在强降水区,水汽散度通量表现为正值强信号,而水汽螺旋度表现为负值强信号,在非降水区和弱降水区,两者均表现为弱信号。等熵位涡分析显示,对流不稳定只是此次台风暴雨前期和初始阶段的不稳定条件,而湿位涡(MPV)的湿斜压项(MPV₂)则是暴雨增强和出现非对称性分布的主要机制。在暴雨形成过程中,由于冷空气侵入造成了在台风环流西北侧湿等熵面的陡立倾斜和水平风垂直切变的增强,导致了气旋性涡度的显著增强,气旋性涡度增强造成的强烈上升运动将降水区东南侧输送过来的暖湿空气向上输送,从而导致了暴雨的发生,这其中条件性对称不稳定是降水得以加强的一种重要不稳定机制。     

The temporal evolution of a geostrophic flow in a rotating stratified basin

A laboratory study in a rotating stratified basin examines the instability and long time evolution of the geostrophic double gyre introduced by the baroclinic adjustment to an initial basin-scale step height discontinuity in the density interface of a two-layer fluid. The dimensionless parameters that are important in determining the observed response are the Burger number S=R/R₀ (where R is the baroclinic Rossby radius of deformation and R₀ is the basin radius) and the initial forcing amplitude (H₁ is the upper layer depth). Experimental observations and a numerical approach, using contour dynamics, are used to identify the mechanisms that result in the dominance of nonlinear behaviour in the long time evolution, τ>2⁻¹ (where τ is time scaled by the inertial period T_I=2π/f). When the influence of rotation is moderate (0.25≤S≤1), the instability mechanism is associated with the finite amplitude potential vorticity (PV) perturbation introduced when the double gyre is established. On the other hand, when the influence of rotation is strong (S≤0.1), baroclinic instability contributes to the nonlinear behaviour. Regardless of the mechanism, nonlinearity acts to transfer energy from the geostrophic double gyre to smaller scales associated with an eddy field. In the lower layer, Ekman damping is pronounced, resulting in the dissipation of the eddy field after only 40T_I. In the upper layer, where dissipative effects are weak, the eddy field evolves until it reaches a symmetric distribution of potential vorticity within the domain consisting of cyclonic and anticyclonic eddy pairs, after approximately 100T_I. The functional dependence of the characteristic eddy lengthscale L_E on S is consistent with previous laboratory studies on continuously forced geostrophic turbulence. The cyclonic and anticyclonic eddy pairs are maintained until viscous effects eventually dissipate all motion in the upper layer after approximately 800T_I. The outcomes of this study are considered in terms of their contribution to the understanding of the energy pathways and transport processes associated with basin-scale motions in large stratified lakes.     

Inequality of eddy transfer coefficients for vertical transport of sensible and latent heats during advective inversions

Similarity of transport of water vapour and sensible heat was investigated within an advective inversion layer by measuring eddy fluxes together with gradients of temperature and humidity. The experimental site was a field of rice, grown under flood irrigation, which was situated in a semi-arid region. The fetch was about 300 m and local stabilities (z/L) over the rice ranged from 0 to 0.1. Results were expressed as the ratio of eddy transfer coefficients for sensible heat (K _H) and water vapour (K _w). Near neutral stability, K _H/K_W approached 1, but the ratio decreased to about 0.65 with increasing stability. Existing theory predicts the result qualitatively but accounts for only about one fifth of the decrease in K _H/K_W with stability.     

鲁西北一次持续性暴雨成因分析

利用地面自动气象站、NCEP 1°×1°和FY2E反演T_(BB)等资料,对2012年7月31日—8月1日发生在山东一次持续的区域性暴雨进行分析和研究。结果表明,(1)在副高稳定的形势下,低层切变线、地面冷锋是造成暴雨的主要天气系统。(2)低层锋生强迫激发了非地转垂直环流,上升气流加强,雨强增大,上升支的建立到强降水开始间隔约6 h。(3)暴雨区925 hPa湿斜压项在强降水时为负值,其增强应为低层锋生作用造成的。雨区范围和雨强随湿斜压项增强而增大。(4)对流单体的生成和加强影响降水落区和雨强,增强的单体合并时雨强最强。T_(BB)中心区位于低层切变线与锋面右侧,强降水位于两者之间。(5)地面辐合线与T_(BB)中心基本重合,辐合线左侧气旋性弯曲处与T_(BB)中心区后侧的等值线梯度大值区位置一致,强降水则出现在两者重叠处。(6)干线与辐合线是此次强降水的重要中尺度系统,干线始终落后于辐合线,其分布对暴雨落区有一定的指示意义。     

Simulation of glacial-interglacial cycles by simple relaxation models: consistency with observational results

The model of Paillard and Parrenin (Earth Planet Sci Lett 227:263–271, 2004) was modified to obtain a closer fit to δ¹⁸O and CO₂ time series for the last 800 kyr. The model performance can be improved if its CO₂ sensitivity to I₆₅ insolation is eliminated and if different response times are assumed for ablation/accumulation of ice. Correlations between simulated and experimental time series for CO₂ and ice volume V increase from 0.59 and 0.63 to 0.79 and 0.88, respectively. According to these models, terminations are produced by I₆₅ amplification through CO₂-T and T-CO₂ feedbacks, in synergy with an extra CO₂ contribution from the deep ocean. This contribution is strongly dependent on ice-sheet extent and ice volume (or alternatively, CO₂ concentration, which is a good proxy of Antarctic temperature) but is insensitive to Southern Ocean (SO) insolation on 21 February (I₆₀). Change of deep SO state may be the “order parameter” for nonlinear deglacial changes. According to these models, 100 kyr periodicity of glacial cycles arises from the characteristic time of Antarctic ice sheet advance to the continental slope.     

One-dimensional baroclinically unstable waves on the Gulf Stream potential vorticity gradient near Cape Hatteras

Application of linear baroclinic instability theory to the observed distributions of velocity, stratification, and potential vorticity in the Gulf Stream near 74° W is successful in predicting the time and length scales of the most rapidly growing disturbances. A continuously-stratified, one-dimensional model with realistic bottom slope predicts propagation speeds of 10–50 cm s⁻¹ associated with two regimes of rapid temporal growth centered at periods of 28 days and 5–7 days. This prediction is consistent with observations of the propagation and growth of Gulf Stream meanders derived from inverted echo sounder measurements in this region. The instability model also predicts that for realistic bottom slopes the baroclinic energy transfer should be weakly negative (eddy-to-mean) in deep water, but for low-frequency waves should change to significant positive (mean-to-eddy) transfer above depths of 1500 m, consistent with observations.     

Dynamics of the Leeuwin Current: Part 2. Impacts of mixing,friction, and advection on a buoyancy-driven eastern boundary current over a shelf

The boundary currents over the Western Australian continental shelf and slope consist of the poleward flowing Leeuwin Current (LC) and the equatorward flowing Leeuwin Undercurrent (LUC). Key properties of the LC are its poleward strengthening, deepening to the south, and shelfbreak intensification. The alongshore flow reverses direction below about 300 m, forming the LUC at greater depths. To investigate the processes that cause these features, we obtain solutions to an idealized, regional ocean model of the South Indian Ocean. Solutions are forced by relaxing surface density to a prescribed, meridionally varying density profile ρ^*(y) with a timescale of δt. In addition, vertical diffusion is intensified near the ocean surface. This diffusion establishes the minimum thickness over which density is well-mixed. We define this thickness as the “upper layer”. Solutions are obtained with and without a continental shelf and slope off Western Australia and for a range of values of δt and mixing parameters. Within this upper layer, there is a meridional density gradient that balances a near-surface, eastward geostrophic flow. The eastward current downwells near the eastern boundary, leading to westward flow at depth. The upper layer's meridional structure and zonal currents crucially depend on coastal processes, including the presence of topography near the eastern boundary. Kelvin waves inhibit the upper layer from deepening at the coast. Rossby waves propagate the coastal density structure offshore, hence modifying the interior currents. A comparison of the solutions with or without a continental shelf and slope demonstrate that topographic trapping of Rossby waves is a necessary process for maintaining realistic eastern boundary current speeds. Significant poleward speeds occur only onshore of where the upper layer intersects the slope, that is, at a grounding line. Its poleward transport increases when surface-enhanced vertical mixing is applied over a greater depth. When the timescale δt is sufficiently short, the poleward current is nearly barotropic. The current's spatial structure over the shelf is controlled by horizontal mixing, having the structure of a Munk layer. Increasing vertical diffusion deepens the upper layer thickness and strengthens the alongshore current speed. Bottom drag leads to an offshore flow along the bottom, reducing the net onshore transport and weakening the current's poleward acceleration. When δt is long, poleward advection of buoyancy forms a density front near the shelf break, intensifying poleward speeds near the surface. With bottom drag, a bottom Ekman flow advects density offshore, shifting the jet core offshore of the shelf break. The resulting cross-shelf density gradient reverses the meridional current's direction at depth, leading to an equatorward undercurrent.     

The influence of the coast on the dynamics of upwelling fronts: Part I. Laboratory experiments

We describe laboratory experiments on the instability and later evolution of a front in a two-layer rotating fluid. In particular, we focus on the influence of a nearby boundary on instability growth and eddy formation. The front is produced through the adjustment of a buoyant fluid initially confined within a bottomless cylinder. Typically a front in quasi-cyclostrophic balance establishes after two rotation periods, after which it becomes unstable. Measurements of the velocity and vorticity fields at the surface are made which provide detailed information on the evolution of the front as the instability grows to finite amplitude. We focus on the time evolution of the vorticity and distinguish between the cyclonic and anticyclonic components. The spatial averages of the cyclonic and anticyclonic vorticity first grow exponentially. This growth saturates when eddies form and are advected across the front. The growth rate depends upon two nondimensional parameters: the width W of the upwelling region in units of the internal radius of deformation and the depth ratio δ between the two layers. Measurements of the growth rates for the average of the cyclonic and anticyclonic vorticity are compared to the values inferred from a simplified model for baroclinic instability. A good agreement is obtained when the front develops far from the boundary (i.e. W1). However, the agreement is only qualitative when the front is near the boundary (i.e. W1). We find that, as W decreases, the growth of cyclonic eddies consisting of dense—“coastal”—water is enhanced compared to that of anticyclonic vorticity consisting of buoyant—“off-shore”—water. This crucial effect of the boundary with respect to the instability of the front has significant impact on exchanges across the front.