Salinity minimum in the North Pacific

Analyzed is the variability of seasonal salinity in the North Pacific. It is demonstrated that the formation and disappearance of salinity minimum in subsurface layers depends on the freshwater budget variability of different time scales. In general, the salinity minimum is a temporary phenomenon formed during the negative phase of freshwater budget, when the evaporation exceeds the precipitation. Seasonal variability of fresh water budget leads to the seasonal formation of salinity minimum on the boundary between the climatic zones. The existence of salinity minimum at intermediate depths in the tropics is caused by the negative phase of long-period variability of freshwater budget.     

On the role of the freshwater budget at the origin of the salinity minimum at intermediate depths in the northern Pacific

Seasonal variability of the salinity field shows that the salinity minimum at subsurface (intermediate) depths is a temporal phenomenon and not a water mass in the common interpretation of the term. This extremum in the vertical salinity distribution appears and disappears as the freshwater budget changes sign on different timescales. Its characteristics are a function of the freshwater budget; they change in time. The salinity minimum appears and disappears seasonally in the transition zone associated with an ocean basin where the freshwater budget changes sign on the seasonal timescale. The long-lasting salinity minimum at intermediate depths in the arid ocean zone is attributed to air-sea interaction on long-term (climate) timescales. Its existence is due to one of the phases of this interaction, the continuous salt flux from the ocean surface.     

Salinity minimum in the upper layer of the sea of Japan

Analyzed is the intraannual variability ofvertical distribution of salinity in the southern part of the Sea of Japan. It is demonstrated that the sign change in freshwater balance is the reason for the formation and disappearance of minima and maxima of salinity in the water column. The dynamics of variability of salinity minima parameters is demonstrated for the first time. The conclusion is made that the salinity minimum cannot be considered as a water mass because it is a temporary phenomenon.     

The influence of Hudson Bay runoff and ice‐melt on the salinity of the inner Newfoundland Shelf

Abstract

The present study examines sources of the interannual variability in salinity on the Newfoundland continental shelf observed in a 40‐year time series from an oceanographic station known as Station 27. Specifically, we investigate, through lag‐correlation analysis, the a priori hypotheses that the salinity anomalies at Station 27 are determined by freshwater runoff anomalies from Hudson and Ungava bays and by ice‐melt anomalies in Hudson Bay and on the Labrador Shelf. Interannual variations of summer runoff into Hudson Bay were significantly negatively correlated with salinity anomalies on the Newfoundland Shelf with a lag (9 months) that is consistent with expected travel times based on known current velocities in Hudson Bay and along the Labrador Shelf. Sea‐ice extent over the Labrador and northern Newfoundland shelves was significantly negatively correlated with salinity at a lag of 3 to 4 months, corresponding to the time of minimum salinity at Station 27. It appears that ice‐melt over the Labrador‐northern Newfoundland Shelf is primarily responsible for the seasonal salinity minimum over the Newfoundland Shelf. Interannual variability in runoff into Ungava Bay and ice‐melt in Hudson Bay were not correlated with interannual salinity variations on the Newfoundland Shelf.     

Interannual Salinity Variability in the Tropical Pacific in CMIP5 Simulations

Salinity variability and its causes in the tropical Pacific are analyzed using observations, reanalysis products and model simulations. The mixed-layer salinity(MLS) budget analyses from observations and reanalysis products indicate that its interannual evolution is closely related to ENSO and is predominantly governed by surface forcing and surface advection in the western-central equatorial Pacific. It is found that the observed MLS tendency leads Nin?o3.4 by about 12 months due to the effect of negative freshwater flux(evaporation minus precipitation). These observation-based analyses are used to evaluate the corresponding simulation using GFDL-ESM2 M. It is evident that the model can simulate the spatiotemporal variations of MLS with some discrepancies compared to observations. In the warm pool of the equatorial Pacific the MLS tendency in the model is sensitive to ocean dynamics, however model biases cause the tendency to be underestimated. In particular, the freshwater flux is overestimated while the ocean surface zonal current and vertical velocity at the base of the mixed layer are underestimated. Due to model biases in representing the related physics, the effects of surface forcing on the simulated MLS budget are overestimated and those of subsurface and surface advection are relatively weak. Due to weaker surface advection and subsurface forcing than observed, the simulated compensations for surface forcing are suppressed, and the simulated MLS tendency that leads Nin?o3.4 by 8–10 months, which is shorter than the observed lead time. These results are useful for the interpretation of observational analyses and other model simulations in the tropical Pacific.     

The impact of global freshwater forcing on the thermohaline circulation: adjustment of North Atlantic convection sites in a CGCM

On the time scale of a century, the Atlantic thermohaline circulation (THC) is sensitive to the global surface salinity distribution. The advection of salinity toward the deep convection sites of the North Atlantic is one of the driving mechanisms for the THC. There is both a northward and a southward contributions. The northward salinity advection (Nsa) is related to the evaporation in the subtropics, and contributes to increased salinity in the convection sites. The southward salinity advection (Ssa) is related to the Arctic freshwater forcing and tends on the contrary to diminish salinity in the convection sites. The THC changes results from a delicate balance between these opposing mechanisms. In this study we evaluate these two effects using the IPSL-CM4 ocean-atmosphere-sea-ice coupled model (used for IPCC AR4). Perturbation experiments have been integrated for 100 years under modern insolation and trace gases. River runoff and evaporation minus precipitation are successively set to zero for the ocean during the coupling procedure. This allows the effect of processes Nsa and Ssa to be estimated with their specific time scales. It is shown that the convection sites in the North Atlantic exhibit various sensitivities to these processes. The Labrador Sea exhibits a dominant sensitivity to local forcing and Ssa with a typical time scale of 10 years, whereas the Irminger Sea is mostly sensitive to Nsa with a 15 year time scale. The GIN Seas respond to both effects with a time scale of 10 years for Ssa and 20 years for Nsa. It is concluded that, in the IPSL-CM4, the global freshwater forcing damps the THC on centennial time scales.     

基于热含量探究PDO的三维结构及信号传播特征

基于1815—2013年SODA（Simple Ocean Data Assimilation）数据不同深度的温度数据资料,进行了热含量的计算,并通过EOF分解、功率谱分析等统计方法探究太平洋年代际振荡（Pacific decadal oscillation, PDO）的三维结构和周期性特征。结果表明,太平洋的年代际变化不仅仅存在于海洋表层,海洋300 m以浅均存在年代际变化特征,其中次表层（70 m左右）的年代际变化特征最为显著。功率谱分析的结果显示,北太平洋的年代际变化周期约为18 a。利用SODA数据的温度和盐度资料对北太平洋的Rossby（罗斯贝）波波速进行了计算,计算结果显示,Rossby波向西传播,其波速随着纬度的增大而减小。对300 m以浅水体的热含量时间序列与PDO指数做了超前滞后相关,在热含量序列滞后9 a时相关系数分布与同期相关反相。对不同层次的热含量与PDO指数进行了超前滞后相关,分析PDO的演变特征,结果表明,PDO在低纬度通过Rossby波向西传播,在传播过程中深度逐渐加深。     

A model-based study of ice and freshwater transport variability along both sides of Greenland

We investigate some aspects of the variability of the Arctic freshwater content during the 1965–2002 period using the DRAKKAR eddy admitting global ocean/sea-ice model (12 km resolution in the Arctic). A comparison with recent mooring sections shows that the model realistically represents the major advective exchanges with the Arctic basin, through Bering, Fram and Davis Straits, and the Barents Sea. This allows the separate contributions of the inflows and outflows across each section to be quantified. In the model, the Arctic freshwater content variability is explained by the sea-ice flux at Fram and the combined variations of ocean freshwater inflow (at Bering) and outflow (at Fram and Davis). At all routes, except trough Fram Strait, the freshwater transport variability is mainly accounted for by the liquid component, with small contributions from the sea-ice flux. The ocean freshwater transport variability through both Davis and Fram is controlled by the variability of the export branch (Baffin Island Current and East Greenland Current, respectively), the variability of the inflow branches playing a minor role. We examine the respective role of velocity and salinity fluctuations in the variability of the ocean freshwater transport. Fram and Davis Straits offer a striking contrast in this regard. Freshwater transport variations across Davis Strait are completely determined by the variations of the total volume flux (0.91 correlation). On the other hand, the freshwater transport through Fram Strait depends both on variations of volume transport and salinity. As a result, there is no significant correlation between the variability of freshwater flux at Fram and Davis, although the volume transports on each side of Greenland are strongly anti-correlated (−0.84). Contrary to Davis Strait, the salinity of water carried by the East Greenland Current through Fram Strait varies strongly due to the ice-ocean flux north of Greenland.     

Simulated global-scale response of the climate system to Dansgaard/Oeschger and Heinrich events

By using an Earth system model of intermediate complexity we have studied the global-scale response of the glacial climate system during marine isotope stage (MIS) 3 to perturbations at high northern latitudes and the tropics. These perturbations include changes in inland-ice volume over North America, in freshwater flux into the northern North Atlantic and in surface temperatures of the tropical Pacific. The global pattern of temperature series resulting from an experiment in which perturbations of inland ice and freshwater budget are imposed at high northern latitudes only, agree with paleoclimatic reconstructions. In particular, a positive correlation of temperature variations near Greenland and variations in all regions of the Northern Hemisphere and some parts of the southern tropics is found. Over the southern oceans a weak negative correlation appears which is strongest at a time lag of approximately 500 years. Further experimentation with prescribed temperature anomalies applied to the tropical Pacific suggests that perturbation of tropical sea-surface temperatures and hence, the tropical water cycle, is unlikely to have triggered Dansgaard/Oeschger (D/O) events. However, together with random freshwater anomalies prescribed at high northern latitudes, tropical perturbations would be able to synchronize the occurrence of D/O events via the mechanism of stochastic resonance.     

On the interannual variability of surface salinity in the Atlantic

The mechanisms controlling the interannual variability of sea surface salinity (SSS) in the Atlantic are investigated using a simulation with the ECHAM4/OPA8 coupled model and, for comparison, the NCEP reanalysis and an observed SSS climatology. Anomalous Ekman advection is found to be as important as the freshwater flux in generating SSS anomalies, in contrast to sea surface temperature (SST) anomalies which are primarily caused by surface heat flux fluctuations. Since the surface heat flux feedback does not damp the SSS anomalies but generally damps existing SST anomalies, SSS anomalies have a larger characteristic time scale. As a result, they are more influenced by the mean currents and the geostrophic variability, which dominate the SSS changes at low frequency over much of the basin. The link between SSS anomalies and the dominant patterns of atmospheric variability in the North Atlantic sector is also discussed. It is shown that the North Atlantic Oscillation generates SSS anomalies much more by Ekman advection than by freshwater exchanges. At least in the coupled model, there is little one-to-one correspondence between the main atmospheric and SSS anomaly patterns, unlike what is found for SST anomalies.     

The effect of Congo River freshwater discharge on Eastern Equatorial Atlantic climate variability

The surface ocean explains a considerable part of the inter-annual Tropical Atlantic variability. The present work makes use of observational datasets to investigate the effect of freshwater flow on sea surface salinity (SSS) and temperature (SST) in the Gulf of Guinea. In particular, the Congo River discharges a huge amount of freshwater into the ocean, affecting SSS in the Eastern Equatorial Atlantic (EEA) and stratifying the surface layers. The hypothesis is that an excess of river runoff emphasize stratification, influencing the ocean temperature. In fact, our findings show that SSTs in the Gulf of Guinea are warmer in summers following an anomalously high Congo spring discharge. Vice versa, when the river discharges low freshwater, a cold anomaly appears in the Gulf. The response of SST is not linear: temperature anomalies are considerable and long-lasting in the event of large freshwater flow, while in dry years they are less remarkable, although still significant. An excess of freshwater seems able to form a barrier layer, which inhibits vertical mixing and the entrainment of the cold thermocline water into the surface. Other processes may contribute to SST variability, among which the net input of atmospheric freshwater falling over EEA. Likewise the case of continental runoff from Congo River, warm anomalies occur after anomalously rainy seasons and low temperatures follow dry seasons, confirming the effect of freshwater on SST. However, the two sources of freshwater anomaly are not in phase, so that it is possible to split between atypical SST following continental freshwater anomalies and rainfall anomalies. Also, variations in air-sea fluxes can produce heating and cooling of the Gulf of Guinea. Nevertheless, atypical SSTs cannot be ascribed to fluxes, since the temperature variation induced by them is not sufficient to explain the SST anomalies appearing in the Gulf after anomalous peak discharges. The interaction processes between river runoff, sea surface salinity and temperature play an effective role in the interannual variability in the EEA region. Our results add a new source of variability in the area, which was often neglected by previous studies.     

A 20-year coupled ocean-sea ice-atmosphere variability mode in the North Atlantic in an AOGCM

In order to understand potential predictability of the ocean and climate at the decadal time scales, it is crucial to improve our understanding of internal variability at this time scale. Here, we describe a 20-year mode of variability found in the North Atlantic in a 1,000-year pre-industrial simulation of the IPSL-CM5A-LR climate model. This mode involves the propagation of near-surface temperature and salinity anomalies along the southern branch of the subpolar gyre, leading to anomalous sea-ice melting in the Nordic Seas, which then forces sea-level pressure anomalies through anomalous surface atmospheric temperatures. The wind stress associated to this atmospheric structure influences the strength of the East Greenland Current across the Denmark Strait, which, in turn, induces near-surface temperature and salinity anomalies of opposite sign at the entrance of the Labrador Sea. This starts the second half of the cycle after approximatively 10 years. The time scale of the cycle is thus essentially set by advection of tracers along the southern branch of the subpolar gyre, and by the time needed for anomalous East Greenland Current to accumulate heat and freshwater anomalies at the entrance of the Labrador Sea. The Atlantic meridional overturning circulation (AMOC) does not play a dominant role in the mode that is confined in the subpolar North Atlantic, but it also has a 20-year preferred timescale. This is due to the influence of the propagating salinity anomalies on the oceanic deep convection. The existence of this preferred timescale has important implications in terms of potential predictability of the North Atlantic climate in the model, although its realism remains questionable and is discussed.     

太平洋年代际振荡对中国冬季最低气温年代际变化的贡献

基于1961—2013年中国台站的均一化气温数据、NOAA月平均海温资料和CMIP5气候模式数据,利用气候统计手段,定量分析太平洋年代际振荡（PDO）对中国冬季最低气温年代际变化的贡献。结果表明：PDO的年代际序列与年代际滤波后的最低气温场在全国大部分地区呈显著正相关,即PDO负位相时中国冬季最低气温偏低,反之偏高。2006年后中国冬季最低气温变暖减缓,造成这一现象的主要原因是自然变率起到的降温作用,而自然变率又主要由PDO起主导作用,约占自然变率贡献的40%左右。PDO对温度的贡献呈现出明显的年代际变化,在变暖减缓期对升温有明显的负贡献,且负贡献逐渐增大至超过50%。     

理想热带气旋增强过程中的多尺度涡度收支特征分析

基于中尺度WRF模式,研究了无背景气流环境假设条件下理想热带气旋中低层大气多尺度涡旋运动的发展演变特征。精确的尺度分离是基于傅里叶变换实现的,且原始涡度场被划分为三个尺度范围：系统尺度(大于150 km)、中间尺度(50～150 km)、对流尺度(小于50 km)。研究结果表明：热带气旋的非轴对称本质主要是由于中间尺度和对流尺度上的运动造成的,且中间尺度涡度演变特征与热带气旋增强的阶段性有很好的对应关系,尤其是其快速增强阶段;全尺度涡度收支特征主要表现为两两抵消效应：STR/HAD和TIL/VAD,且前者的净贡献明显强于后者;系统尺度涡度收支特征与全尺度基本一致,但中间尺度涡度收支表现出明显不同特征：积分70 h之前,各收支项均表现出了与系统尺度相反的贡献,之后,各收支项的符号转变与系统尺度相同,但收支项净贡献明显大于系统尺度。总的来说,水平分辨率5 km下模拟的理想热带气旋的快速增强主要与中间尺度上STR/HAD净贡献的快速增长有关。此外,进一步研究了特定时段中间尺度涡度收支项的空间演变,结果表明：在热带气旋增强阶段,各收支项均在涡旋内核的轴对称化中有不可忽视的作用,且TIL在中心负...     

Oceanographic conditions at Ocean Weather Ship Bravo, 1964–1974

Abstract

Monthly averages of water temperature, salinity, and density anomaly at 11 depths between 10 and 1500 m, at Ocean Weather Ship Bravo (56°30'N, 51°00'W), between 1964 and 1974 are presented. Near‐surface salinity values between 1967 and 1971 were significantly lower than those between 1964–1967. Coincident with the lower salinity values, the winter‐time heat losses were less than normal. The combination of increased stratification with the low heat losses tended to limit the convectively mixed upper layer in winter to unusually shallow depths. It is suggested that the low salinity condition was indirectly due to an anomalously high atmospheric pressure cell over Greenland. This cell increased the anticylonic air flow around Greenland causing an increase in the proportion of low‐salinity polar water in the east Greenland and Labrador Currents and subsequently in the interior of the Labrador Sea.     

Variability of sea surface salinity in stochastically forced systems

The influences of horizontal advection and horizontal diffusion on the variability of sea surface salinity in stochastically forced systems are investigated. Basic ideas are developed using a two dimensional box model and then extended to a more realistic three dimensional ocean general circulation model. It is shown that, in the absence of advection and diffusion, the ocean response is essentially that predicted by Taylor's random walk model. Advection becomes important when the advective time scale is less than the response time of the mixed layer to the stochastic forcing. Advection of parcels from regions of upwelling into regions of downwelling limits their exposure time to the stochastic forcing and thus the maximum attainable variance in the system (variance increases linearly with time). Regions of upwelling and downwelling may be introduced through the thermohaline overturning circulation or by the wind driven Ekman transport, depending on the specific model configuration. Horizontal diffusion is found to be important when the diffusive time scale is less than the mixed layer response time. The primary role of diffusion is to reduce the effective stochastic forcing through rapid mixing of uncorrelated surface forcing events. Because sea surface salinity does not have a negative feedback with the atmosphere, it is more strongly influenced by weak horizontal processes than sea surface temperature (SST). Accurate knowledge of the stochastic forcing amplitude, decorrelation time, and length scale and distribution are critical to model the variance of sea surface salinity. Aspects of the ocean model which strongly influence the variability of sea surface salinity include the surface velocity, horizontal diffusivity, and the mixed layer depth. Implications on modeling of the ocean and coupled ocean-atmosphere systems are discussed.     

The freshwater transport of the labrador current

Abstract

A simple variant of the salt flux calculation is used to estimate the freshwater transport of the Labrador Current. A freshwater budget is then constructed for the Labrador Sea, comparing the summed inputs of fresh water with the fresh water lost in the Labrador Current. Our results indicate that Baffin Bay and Hudson Strait are the largest contributors to the freshwater flux of the Labrador Current. It is found that there is ample freshwater transport in the very low salinity waters to meet the required input of fresh water of northerly origin to the Middle Atlantic Bight.     

Sensitivity of the Atlantic meridional overturning circulation to South Atlantic freshwater anomalies

The sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) to changes in basin integrated net evaporation is highly dependent on the zonal salinity contrast at the southern border of the Atlantic. Biases in the freshwater budget strongly affect the stability of the AMOC in numerical models. The impact of these biases is investigated, by adding local anomaly patterns in the South Atlantic to the freshwater fluxes at the surface. These anomalies impact the freshwater and salt transport by the different components of the ocean circulation, in particular the basin-scale salt-advection feedback, completely changing the response of the AMOC to arbitrary perturbations. It is found that an appropriate dipole anomaly pattern at the southern border of the Atlantic Ocean can collapse the AMOC entirely even without a further hosing. The results suggest a new view on the stability of the AMOC, controlled by processes in the South Atlantic.     

热带太平洋盐度年际变化对海表温度异常作用比较：1997/1998、2014/2015和2015/2016年El Niño事件

海洋盐度变化为研究气候变化的机制提供了一个新的视角。本文通过对比1997/1998年、2015/2016年两次强厄尔尼诺（El Ni?o）事件和2014/2015年特殊El Ni?o事件,对盐度变化及其影响海表面温度异常（SSTA）的物理过程进行了比较分析。研究表明,El Ni?o和南方涛动（El Ni?o–Southern Oscillation, ENSO）发展的强弱与热带西太平洋大范围海表层盐度异常（SSSA）及其向东扩散的差异有明显关联。1997/1998、2015/2016年赤道东太平洋SSTA的增暖,对应两次强El Ni?o事件,在发生年4月,中西太平洋海域出现了明显的负SSSA,之后东移至日期变更线以西,SSSA引发的混合层深度（MLD）变浅、障碍层厚度（BLT）变厚,导致热带中—西太平洋表层升温增强,促使了赤道中太平洋的早期变暖;2014/2015年弱El Ni?o事件虽然在发生年4月,位于赤道中西太平洋出现了负SSSA,但没有发展东移,导致BLT的增厚过程减弱,对表层温度的调制作用减弱甚至消失。三次事件对应的盐度变化过程中,水平平流和淡水通量（FWF）引起的表层强迫是影响盐度收支的主要因子,水平平流影响盐度异常的前期变化,触发事件的发生;热带太平洋西部降水引起的FWF负异常的影响最为显著,对ENSO异常信号出现后SSSA的维持起决定性作用。相比较两次强El Ni?o事件,2014/2015年El Ni?o对应的早期FWF负异常没有发展和东移,并且之后迅速减弱,导致中西太平洋盐度负趋势减缓,MLD加深,BLT变薄,促使上表层海水冷却,抑制了赤道东太平洋的早期变暖和ENSO发展。研究结果表明,盐度变化与ENSO密切相关,热带中西太平洋海域早期表层盐度变化可能可以作为SSTA的指数。特别地,SSSA在调节SSTA时,不仅影响它的强度,而且可以作为判断ENSO是否发展及其强弱的前兆因子。     

Decadal climate variability simulated in a coupled general circulation model

A 1000 year integration of the CSIRO coupled ocean-atmosphere general circulation model is used to study low frequency (decadal to centennial) climate variability in precipitation and temperature. The model is shown to exhibit sizeable decadal variability for these fields, generally accounting for approximately 20 to 40% of the variability (greater than one year) in precipitation and up to 80% for temperature. An empirical orthogonal function (EOF) analysis is applied to the model output to show some of the major statistical modes of low frequency variability. The first EOF spatial pattern looks very much like that of the interannual ENSO pattern. It bears considerable resemblance to observational estimates and is centred in the Pacific extending into both hemispheres. It modulates both precipitation and temperature globally. The EOF has a time evolution that appears to be more than just red noise. Finally, the link between SST in the Pacific with Australian rainfall variability seen in observations is also evident in the model. Received: 29 August 1998 / Accepted: 31 July 1999