Interannual variations in the components of heat budget in the upper layer of the North Atlantic in different seasons

Seasonal variability of interannual fluctuations of the heat balance components of the upper quasi-homogeneous ocean layer (UQL) in the North Atlantic is analyzed by processing the reanalysis data set for the period of 1959–2011. It is shown that interannual variations in the components of the UQL heat budget are characterized by pronounced regional features in all seasons. In the tropics and subtropics, heat balance is quasistationary and is determined by the nonlocal processes, such as heat advection and horizontal mixing. In the subpolar latitudes, nonstationarity (in the spring) and heat fluxes at the UQL boundary (in the autumn and in the winter) are also important. A major role in the interannual variability of the UQL temperature in the vicinity of jet currents of the Gulf Stream type is played in all seasons by the fluctuations of horizontal heat advection. However, the contribution of interannual fluctuations of the individual components of the heat budget to variability of the UQL temperature varies considerably in different seasons. The interannual fluctuations of the local variation in the UQL temperature are characterized by the largest variability in the spring and the lowest one in the autumn. The greatest contribution of the variations in the horizontal heat advection to the change in the UQL temperature at the interannual scale is recorded in the winter, and the lowest one is in the summer. The contribution of the interannual variations in the heat fluxes at the UQL upper boundary to the variability of the UQL temperature is the highest in the summer and the lowest in the autumn. Fluctuations of the heat fluxes at the UQL lower boundary do not have a significant impact on the interannual variations in the UQL temperature for the whole water area. The exception is small areas in the region of the formation of the North Atlantic deep water in the autumn–winter period and in the vicinity of the Equatorial Counter Current in the spring–summer period.     

Evaluation of the heat balance constituents of the upper mixed layer in the North Atlantic

Different physical mechanisms which cause interannual and interdecadal temperature anomalies in the upper mixed layer (UML) of the North Atlantic are investigated using the data of ORA-S3 reanalysis for the period of 1959–2011. It is shown that the annual mean heat budget in UML is mainly caused by the balance between advective heat transfer and horizontal turbulent mixing (estimated as a residual term in the equation of thermal balance). The local UML temperature change and contribution from the heat fluxes on the lower boundary of the UML to the heat budget of the upper layer are insignificant for the time scale under consideration. The contribution of the heat fluxes on the upper UML boundary to the low-frequency variability of the upper layer temperature in the whole North Atlantic area is substantially less than 30%. Areas like the northwestern part of the Northern Subtropical Anticyclonic Gyre (NSAG), where their contribution exceeds 30–60%, are exceptions. The typical time scales of advective heat transfer variability are revealed. In the NSAG area, an interannual variability associated with the North Atlantic Oscillation dominates, while in the North Atlantic subpolar gyre, an interdecadal variability of advective transfers with periods of more than 30 years prevails.     

Stochastic study of the temperature response of the upper ocean to uncertainties in the atmospheric forcing in an Atlantic OGCM

The impact of errors in atmospheric forcing on the behaviour of ocean models is a fundamental issue for ocean modellers and data assimilation and one that has yet to be fully addressed. In this study, we use a stochastic modelling approach with 50 7-months (September–March) primitive equation eddy permitting (1/4°) integrations. We investigate the response of the oceanic circulation to atmospheric uncertainties, focusing principally on their impact on the upper oceanic temperature field. The ensemble is generated by perturbing the wind, atmospheric temperature and incoming solar radiation of the ERA40 reanalysis. Each perturbation consists of a random combination of the 20 dominant EOFs of the difference between the ERA40 and NCEP/CORE reanalysis datasets. The ensemble standard deviation of various interfacial and oceanic quantities is then examined in the upper 200 m of three distinct regions of the North Atlantic: in the Gulf Stream, in the Northern Tropical band and in the North East Atlantic. These show that even a very small perturbation of the atmospheric variables can lead to significant changes in the ocean properties and that regions of oceanic mesoscale activity are the most sensitive. The ocean response is driven by vertical diffusivity and eddy activity. The role of subsurface currents is also crucial in carrying the eddy signal away from the regions of mesoscale activity. Finally, the decorrelation time scale of the mesoscale activity is critical in determining the amplitude of the oceanic response.     

Towards a mesoscale eddy closure

A turbulence closure for the effect of mesoscale eddies in non-eddy-resolving ocean models is proposed. The closure consists of a prognostic equation for the eddy kinetic energy (EKE) that is integrated as an additional model equation, and a diagnostic relation for an eddy length scale (L), which is given by the minimum of Rhines scale and Rossby radius. Combining EKE and L using a standard mixing length assumption gives a diffusivity (K), corresponding to the thickness diffusivity in the [Gent, P.R., McWilliams, J.C. 1990. Isopycnal mixing in ocean circulation models. J. Phys. Oceanogr. 20, 150–155] parameterisation. Assuming downgradient mixing of potential vorticity with identical diffusivity shows how K is related to horizontal and vertical mixing processes in the horizontal momentum equation, and also enables us to parameterise the source of EKE related to eddy momentum fluxes.The mesoscale eddy closure is evaluated using synthetic data from two different eddy-resolving models covering the North Atlantic Ocean and the Southern Ocean, respectively. The diagnosis shows that the mixing length assumption together with the definition of eddy length scales is valid within certain limitations. Furthermore, implementation of the closure in non-eddy-resolving models of the North Atlantic and the Southern Ocean shows consistently that the closure has skill at reproducing the results of the eddy-resolving model versions in terms of EKE and K.     

Does the Atlantic meridional overturning cell really have more than one stable steady state?

The prevailing view regarding the oceanic meridional overturning cell (MOC) in the Atlantic is that, for a given North Atlantic freshwater flux, it has at least two stable states, one with a large surface (northward) mass flux and the other with a small flux. It has been argued that some abrupt paleoclimatic changes which occurred in the North Atlantic and the regions surrounding it might be related to a shift between these two distinctly different states. Here, we argue that, although the Atlantic MOC can indeed collapse due to a large freshwater flux, the actual ocean does not have multiple states for the same freshwater flux.The two state scenarios has its origin in the analytical box model of Stommel [1961. Thermohaline convection with two stable regimes of flow. Tellus 2, 244–230] and in a series of numerical models starting with that of Bryan [1986. High latitude salinity effects and interhemispheric thermohaline circulations. Nature 303, 301–304]. Using hybrid global analytical models involving both wind and density variations we demonstrate here that the application of Stommel's model to the North Atlantic yields multiple solutions because it considers the origin of the MOC upper limb to be a box whose export of water depends on its temperature and salinity which are not known in advance. When this origination box is replaced by a (observationally supported) Southern Ocean box whose surface water export depends solely on the wind, and when, together with this choice, the diapycnal diffusivities and eddy viscosities are taken to be as small as the usually observed values, the multi-solution scenario disappears and one gets only a single solution.Using the Uvic climate model, we re-confirm earlier results and argue that numerical models have multiple stable states and a resulting hysteresis because of the spuriously high eddy diffusivity that is typically used explicitly or implicitly. This is so because the diffusivity artificially introduces dense-to-light water conversion analogous to Stommel's origination box. Since we used a level model rather than a layered or isopynic model, the small vertical diffusivity limit still retains significant cross-isopycnal mixing due to the horizontal diffusivity, which is not supported by observations. Consequently, while our runs shows a tendency to no-hysteresis in the limit of small cross-isopycnal flow, we cannot actually reach that limit.     

Mechanism for the Formation of Temperature Anomalies in the Upper Layer of the North Atlantic

ORA-S3 oceanological reanalysis data for 1959–2011 is applied to analyze the role different factors play in forming advective heat transfer anomalies on an interannual–decadal scale in the upper mixed layer of the North Atlantic. Regions are revealed in which horizontal heat advection anomalies are determined by variations in current intensity, temperature gradients, and their joint influence. It is demonstrated that the contribution of different mechanisms responsible for advective heat transfer anomalies in the upper mixed layer to the total anomalies of advective origin varies fundamentally from one current to another in the North Atlantic. In the Gulf Stream area (after it separates from the continental slope), horizontal heat advection anomalies in the upper mixed layer result mainly from fluctuations in current intensity, while in the Caribbean Current and the Gulf Stream area (until its separation), they result from variations in the horizontal temperature gradients in the upper mixed layer. In the Labrador Current, both of these mechanisms have the same sign and approximately the same absolute values. In the East Greenland Current, they compensate each other. The contribution of anomalies in horizontal temperature gradients transferred by anomalous currents to the formation of heat transfer anomalies in the upper layer of the North Atlantic are, on the whole, relatively small throughout the water area. The areas of the North Atlantic and West Greenland currents are exceptions.     

台湾海峡及邻近海域海面热量平衡分布特征

本文利用1961—1985年历次台湾海峡及邻近海域水文气象调查资料,计算分析了该海域海面吸收辐射、有效回辐射、蒸发耗热、海-气感热交换的时空分布,给出了该海域热量平衡年平均状况和季节变化特征。     

北太平洋涡旋对基于细尺度参数化的海洋内部混合的影响

基于Vector Geometry方法对2016—2018年的高度计资料进行涡旋识别,并使用细尺度参数化方法和Argo数据计算了涡旋附近的海洋内部扩散率,分析了北太平洋的涡旋对海洋内部混合的影响。结果显示,研究区域在涡旋影响下的平均扩散率比无涡旋影响下的值大6%,并且气旋涡增强了600—1200m深度的混合,对600—900m深度的混合影响最大,可达18%;反气旋涡明显增强了300—900m深度的混合,但对900—1200m深度的混合没有明显影响。随着与涡旋中心距离的增大,涡旋外围混合扩散率缓慢减小,涡旋内部混合扩散率变化不明显,此结果与2014年3—10月在24°—36°N、132°—152°E区域的一个个例分析结果一致。此外,随着涡旋强度的增大,海洋内部混合明显增强。统计结果表明,在研究区域, 90%的扩散率值在10^-5.5—10^-4m²/s范围内。     

Towards the general circulation of the North Atlantic ocean

After reviewing the inverse method, we apply it to deducing the general circulation of the North Atlantic ocean. We argue that the method is purely classical in nature, being nothing more than a mathematical statement of the principles upon which nearly all previous circulation schemes have been based. The ‘smoothed’ solution is shown to represent the components of the flow field that are determinable independently of the initial reference level. We then produce two circulation schemes based upon two different initial reference levels — 2000 decibars and the bottom — called North Atlantic-1A and North Atlantic-1B respectively. The models share many features in common and are strikingly similar to several previous schemes, most notably those of Jacobsen and Defant in the region west of Bermuda. No simple level-of-no-motion emerges in the flow fields; rather the velocity sections exhibit a complex cellular structure. Zonally integrated meridional cells of models and of the uniquely determined components are very similar, showing a poleward movement of warm saline water compensated at depth by a return flow of cold, fresher water. The magnitudes of the implied polar sea overflows and the heat fluxes are in good agreement with previous estimates. Finally, it is argued that neither these model circulations nor any other circulation pattern based upon the existing data can be regarded as actually representing the true time average ocean circulation because the data are aliased in time; the frequency/wavenumber spectrum of the ocean is inadequately known to determine the resulting errors.     

2016与1998年春季北大西洋海表温度异常的差异及成因

利用再分析资料以及混合层海温诊断方程, 研究1997—1998与2015—2016年超级厄尔尼诺次年北大西洋海表温度异常(sea surface temperature anomalies, SSTA)的差异及成因。结果显示, 北大西洋SSTA在1998年春季呈明显正负正三极型式分布, 而在2016年呈弱的负正负型态。诊断热带北大西洋SSTA的影响因素表明, 1998年春季暖SSTA除了之前研究强调的海洋表面向大气的潜热输送异常减少, 以及吸收太阳辐射的增加外, 海洋动力过程即Ekman纬向漂流也起着重要的作用。热力过程与厄尔尼诺峰值后出现的北大西洋涛动(North Atlantic Oscillation, NAO)负位相有关, 其可引起亚速尔高压减弱, 产生西南风异常, 通过风-蒸发-海表温度(sea surface temperature, SST)反馈机制使热带北大西洋蒸发减弱, 海表增暖, 沃克环流下沉支的东移对这一增暖也有贡献。与1997—1998厄尔尼诺事件不同, 2015—2016厄尔尼诺事件没有强迫出负位相NAO, 而是出现弱NAO正位相, 热带北大西洋为弱的东风异常, 使海表发生一定的冷却, 形成2016春季北大西洋SSTA与1998年的明显差异。     

洋际通道的体积和热输运的年平均结构及季节变化：MOM4P1积分1400年的结果

The annual mean volume and heat transport sketches through the inter-basin passages and transoceanic sections have been constructed based on 1 400-year spin up results of the MOM4p1. The spin up starts from a state of rest, driven by the monthly climatological mean force from the NOAA World Ocean Atlas(1994). The volume transport sketch reveals the northward transport throughout the Pacific and southward transport at all latitudes in the Atlantic. The annual mean strength of the Pacific-Arctic-Atlantic through flow is 0.63×106 m3/s in the Bering Strait. The majority of the northward volume transport in the southern Pacific turns into the Indonesian through flow(ITF) and joins the Indian Ocean equatorial current, which subsequently flows out southward from the Mozambique Channel, with its majority superimposed on the Antarctic Circumpolar Current(ACC). This anti-cyclonic circulation around Australia has a strength of 11×106 m3/s according to the model-produced result. The atmospheric fresh water transport, known as P-E+R(precipitation minus evaporation plus runoff), constructs a complement to the horizontal volume transport of the ocean. The annual mean heat transport sketch exhibits a northward heat transport in the Atlantic and poleward heat transport in the global ocean. The surface heat flux acts as a complement to the horizontal heat transport of the ocean. The climatological volume transports describe the most important features through the inter-basin passages and in the associated basins, including: the positive P-E+R in the Arctic substantially strengthening the East Greenland Current in summer; semiannual variability of the volume transport in the Drake Passage and the southern Atlantic-Indian Ocean passage; and annual transport variability of the ITF intensifying in the boreal summer. The climatological heat transports show heat storage in July and heat deficit in January in the Arctic; heat storage in January and heat deficit in July in the Antarctic circumpolar current regime(ACCR); and intensified heat transport of the ITF in July. The volume transport of the ITF is synchronous with the volume transport through the southern Indo-Pacific sections, but the year-long southward heat transport of the ITF is out of phase with the heat transport through the equatorial Pacific, which is northward before May and southward after May. This clarifies the majority of the ITF originating from the southern Pacific Ocean.     

Recent changes in the North Atlantic circulation simulated with eddy-permitting and eddy-resolving ocean models

Three eddy-permitting (1/4°) versions and one eddy-resolving (1/12°) version of the OCCAM ocean model are used to simulate the World Ocean circulation since 1985. The first eddy-permitting simulation has been used extensively in previous studies, and provides a point of reference. A second, improved, eddy-permitting simulation is forced in the same manner as the eddy-resolving simulation, with a dataset based on a blend of NCEP re-analysis and satellite data. The third eddy-permitting simulation is forced with a different dataset, based on the ERA-40 re-analysis data. Inter-comparison of these simulations in the North Atlantic clarifies the relative importance of resolution and choice of forcing dataset, for simulating the mean state and recent variability of the basin-scale circulation in that region. Differences between the first and second eddy-permitting simulations additionally reveal an erroneous influence of sea ice on surface salinity, dense water formation, and the meridional overturning circulation. Simulations are further evaluated in terms of long-term mean ocean heat transport at selected latitudes (for which hydrographic estimates are available) and sea surface temperature errors (relative to observations). By these criteria, closest agreement with observations is obtained for the eddy-resolving simulation. In this simulation, there is also a weak decadal variation in mid-latitudes, with heat transport strongest, by around 0.2 PW, in the mid-1990s. In two of the eddy-permitting simulations, by contrast, heat transport weakens through the study period, by up to 0.4 PW in mid-latitudes. The most notable changes of heat transport in all simulations are linked to a weakening of the subpolar gyre, rather than changes in the meridional overturning circulation. It is concluded that recent changes in the structure of mid-latitude heat transport in the North Atlantic are more accurately represented if eddies are explicitly resolved.     

Eddy diffusivity and coherent mesoscale eddy analysis in the Southern Ocean

The spatial distribution of eddy diffusivity, basic characteristics of coherent mesoscale eddies and their relationship are analyzed from numerical model outputs in the Southern Ocean. Mesoscale fluctuation information is obtained by a temporal-spatial filtering method, and the eddy diffusivity is calculated using a linear regression analysis between isoneutral thickness flux and large-scale isoneutral thickness gradient. The eddy diffusivity is on the order of O (103 m2/s) with a significant spatial variation, and it is larger in the area with strong coherent mesoscale eddy activity. The mesoscale eddies are mainly located in the upper ocean layer, with the average intensity no larger than 0.2. The mean radius of the coherent mesoscale cyclonic (anticyclonic) eddy gradually decays from (121.2±10.4) km ((117.8±9.6) km) at 30°S to (43.9±5.3) km ((44.7±4.9) km) at 65°S. Their vertical penetration depths (lifespans) are deeper (longer) between the northern side of the Subpolar Antarctic Front and 48°S. The normalized eddy diffusivity and coherent mesoscale eddy activity show a significant positive correlation, indicating that coherent mesoscale eddy plays an important role in eddy diffusivity.     

A global heat and freshwater forcing dataset for ocean models

A global dataset based on the ECMWF Re-Analyses (ERA) is presented that can be used as surface boundary conditions for ocean models with sea-ice components. The definition of these conditions is based on bulk formulae. To study the mean ocean circulation, a mean annual cycle on a daily basis was constructed from ERA for all relevant parameters including wind stress. Continental runoff is considered by using information about the catchment areas of the rivers and about the main drainage basins. The bulk formulae were extended by using sea ice concentration.To estimate meridional heat transports (MHT) and to avoid any drift in ocean model simulations, the heat and fresh water budgets have been closed by applying an inverse procedure to fine-tune the fluxes towards observed transports. To improve the MHTs on the Southern Hemisphere the winds and the short wave radiation at southern higher latitudes should be corrected. Furthermore, tests were performed concerning short wave radiation which was increased in the tropics and decreased in the subsidence zones.The heat and fresh water fluxes are assessed by using a scheme of Macdonald and Wunsch based on hydrographic sections. The net heat fluxes of ERA and of the forcing dataset are consistent with the heat flux divergences and convergences estimated by this scheme except for parts of the South Atlantic and the Indian Ocean sector of the Southern Ocean where none of these datasets is consistent with these estimates. In the subtropical South Indian Ocean the forcing dataset is consistent with these estimates while ERA are not. The flux components of ERA and the forcing dataset were compared to several observational datasets (SRB, SOC, HOAPS, GPCP, and CMAP). For each component, at least one of these datasets (especially HOAPS) supports the effects of the inverse procedure and the bulk formulae almost globally with some regional exceptions: short wave radiation in the tropical oceans and the subtropical North Atlantic, latent heat flux at higher latitudes, and precipitation in the northern North Atlantic.Comparisons to the NCAR/NCEP Re-Analyses (NRA) (versions 1 and 2) and the ECHAM model in place of ERA lead to similar results. In the North Atlantic the net heat fluxes of the model based datasets approach the hydrographic estimate with increasing resolution. Applied to any ocean/sea-ice model and compared to ERA, the forcing dataset would induce only a relative small net sea-surface buoyancy loss.A comparison of the forcing dataset to measurements made using one buoy deployed in the western Pacific warm pool and five buoys deployed in the subduction region of the Northeast Atlantic shows that at the site of the first buoy the net heat fluxes of the forcing dataset are in poorer agreement than those of ERA. At the sites of two subduction buoys both datasets show the same level of agreement within the error bars specified. At the sites of the three remaining subduction buoys the forcing dataset shows a marginal improvement on ERA.     

Diapycnal mixing by meso-scale eddies

The mean available potential energy released by baroclinic instability into the meso-scale eddy field has to be dissipated in some way and Tandon and Garrett [Tandon, A., Garrett, C., 1996. On a recent parameterization of mesoscale eddies. J. Phys. Oceanogr. 26 (3), 406–416] suggested that this dissipation could ultimately involve irreversible mixing of buoyancy by molecular processes at the small-scale end of the turbulence cascade. We revisit this idea and argue that the presence of dissipation within the thermocline automatically requires that a component of the eddy flux associated with meso-scale eddies must be associated with irreversible mixing of buoyancy within the thermocline. We offer a parameterisation of the implied diapycnal diffusivity based on (i) the dissipation rate for eddy kinetic energy given by the meso-scale eddy closure of Eden and Greatbatch [Eden, C., Greatbatch, R.J., 2008. Towards a meso-scale eddy closure. Ocean Modell. 20, 223–239.] and (ii) a fixed mixing efficiency. The implied eddy-induced diapycnal diffusivity (κ) is implemented in a coarse resolution model of the North Atlantic. In contrast to the vertical diffusivity given by a standard vertical mixing scheme, large lateral inhomogeneities can be found for κ in the interior of the ocean. In general, κ is large, i.e. up to o(10) cm²/s, near the western boundaries and almost vanishing in the interior of the ocean.     

The effect of ocean tides on a climate model simulation

We implemented an explicit forcing of the complete lunisolar tides into an ocean model which is part of a coupled atmosphere–hydrology–ocean–sea ice model. An ensemble of experiments with this climate model shows that the model is significantly affected by the induced tidal mixing and nonlinear interactions of tides with low frequency motion. The largest changes occur in the North Atlantic where the ocean current system gets changed on large scales. In particular, the pathway of the North Atlantic Current is modified resulting in improved sea surface temperature fields compared to the non-tidal run. These modifications are accompanied by a more realistic simulation of the convection in the Labrador Sea. The modification of sea surface temperature in the North Atlantic region leads to heat flux changes of up to 50 W/m². The climate simulations indicate that an improvement of the North Atlantic Current has implications for the simulation of the Western European Climate, with amplified temperature trends between 1950 and 2000, which are closer to the observed trends.     

Comparative analysis of the North Atlantic surface circulation reproduced by three different methods

Calculation results are presented for long-term mean annual surface currents in the North Atlantic based on direct drifter measurements and numerical experiments with the ocean general circulation model using both climatic arrays of hydrological data World Ocean Atlas 2009 and Argo profiling data. The calculations show that the technique suggested for model calculations of oceanographic characteristics of the World Ocean with the use of Argo data significantly improves the climatic fields of the temperature and salinity even on a coarse grid. The comparison of the model calculation results with drifter data showed that the temperature and salinity fields found from Argo data with the use of data variational interpolation on a regular grid allow the calculation of realistic currents and can be successfully used as initial conditions in hydrodynamic models of the ocean dynamics.     

Baroclinic wind adjustment processes in the Mediterranean Sea

The wind-driven general circulation of the Mediterranean Sea is studied using a primitive equation model. The model uses a 0.25° horizontal resolution and eight or 16 levels in the vertical. The model uses the Mediterranean basin geometry, and the Strait of Gibraltar is closed. The vertical density structure is initialized with annual average data, and the temperature and salinity values are fixed at the surface to simulate perpetual annual mean conditions. The wind forcing consists of monthly mean climatological stresses.The results show that the general circulation of the Mediterranean Sea has a multiple time-scale character (seasonal excursions and steady state amplitudes are comparable) and it is composed by sub-basin scale gyres corresponding to the scale of the wind stress curl centers. The steady state circulation (annual mean average) is determined by a Sverdrup balacne modified by viscous effects.The unsteady vertically integrated transport circulation consists of sub-basin scale gyres similar to the steady state transport components, which amplify seasonally and the partial or total reversal of the currents in many subportions of the basin. The gyres can be stationary in position or propagating. This seasonal ocean response is partly constituted by Rossby modes due to the wind stress curl annual harmonic. The baroclinic circulation shows the seasonal shift of the North African Current from a position along the African coasts during winter to the center of the Balearic and Ionian basin during summer.     

Mass balance and heat transport in the equatorial Atlantic

The annual average mass and heat balances in the equatorial Atlantic as well as their seasonal variability are analysed using adaptation (initialization procedure) data. It is shown that correlations of temperature and the seasonal fluctuations of the meridional velocity component do not significantly affect the annual average meridional heat transport.Translated by Mikhail M. Trufanov.