Model of the Tropical Atlantic incorporating the upper mixed layer

We present a multilayer ocean model incorporating a non-local integral model of the upper mixed layer (UML). In the framework of a three-layer Tropical Atlantic model, we study the model's response to various closure hypotheses for heat fluxes at the UML lower boundary.Translated by V. Puchkin.     

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.     

Horizontal structure of the temperature field of the upper layer in the northwest part of the Tropical Atlantic

By using the data of observations over the spatial variability of the temperature field in the northwest part of the Tropical Atlantic carried out in a test range 400 × 400 miles in size with a horizontal resolution Δx ≈ 2 km and a vertical resolution Δ_z ≈ 0.5 m, we recorded quasiperiodic fluctuations of temperature with semidiurnal period in the subsurface layer. The internal baroclinic waves with the same period generated, most likely, on the northeast shelf of South America and propagating to the northeast are detected in the seasonal thermocline. The vertical fine structure of the temperature field has different intensities in the test range. The maximum levels of dispersions of temperature fluctuations are recorded on the boundary of the North Equatorial Countercurrent and the North Equatorial Current. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 6, pp. 44–52, November–December, 2006.     

Thermal advection in the tropical Atlantic upper layer

This paper focuses on the heat advection calculated for the tropical Atlantic upper mixed layer and a 0–200 m layer, using two different methods, specifically, (i) from the change in the heat content and the external heat balances and (ii) on the basis of the flows and horizontal seawater temperature gradients prescribed. Thermal advection in the upper mixed layer, in terms of its geographical distribution and the order of magnitude, coincides with that of the 0–200 m layer. This allows the deduction that basically the heat in the tropical Atlantic is transferred within the upper mixed layer. Seasonal variations of the heat advection prove to be appreciable; the annual course is well approximated by two principal waves, one having annual and the other semi-annual cyclicity, with the annual wave being predominant. Both waves reproduce 90% of the dispersion of monthly thermal advection.Translated by Vladimir A. Puchkin.     

Modelling of the upper turbulent layer in the ocean

A differential model of the upper turbulent layer in the ocean is considered. A closed system of equations includes equations of motion, balance, and dissipation of kinetic turbulence energy. Boundary conditions at the surface are determined using a solution of the atmospheric problem taking into account the interaction between the two media. The formulated algorithm allows for a relationship between turbulent energy dissipation and flux and the parameters of wind disturbance. The vertical profiles of turbulence and drift current characteristics are presented as well as parameters of the ocean-atmosphere interaction for various values of impulse jump within the limits of the wave layer with waves collapsing and not collapsing.UDK 551.456.152     

Thickness of the mixed bottom layer in the Northern Atlantic

The thickness of the mixed bottom boundary layer (BBL) has been analyzed based on the CTD data at transoceanic sections in abyssal waters of the Northern Atlantic. The measurements were carried out at two transoceanic sections approximately along 48° N (ASV-99) and 5° N (AI-2000) in 1999 and 2000. These data, and the WOCE data obtained at four zonal sections (AR7E and AR12 along 57° N, AR01 along 24.5° S, and A06 along 7.5° N), were used for the calculation of the statistical characteristics of the BBL??s thickness H _B . The probability distribution function F(H _B ) was close to lognormal. The mean value ??H _B ?? at different latitudes was in the range from 30 to 60 m. The averaged BBL thickness = 46.1 m. The BBL??s thickness was about 1% of the ocean??s depth D; the ratio H _B /D was the minimum (0.8%) near the equator and increased up to 1.6% in the polar latitudes.     

Thermostads and circulation in the upper layer of the Atlantic Ocean

There are three major permanent thermostads with roughly the same potential densities in the upper layer of the Atlantic Ocean. One is the thermostad of the 13°C Water in the equatorial Atlantic. The original type of the 13°C Water is formed in the thermocline in the eastern sector of the South Atlantic subtropical gyre by vertical mixing of dense, low-salinity water from the winter outcrop farther south and overlying less dense, high-salinity water. There might also be a lateral contribution of relatively high-salinity water from the Indian Ocean. The original 13°C Water thus formed is transported northwestward along the northern edge of the subtropical gyre and fed into the North Brazilian Current, which flows equatorward along the coast of Brazil. In the region of the equator, the Equatorial Undercurrent and the subsurface North and South Equatorial countercurrents branch off from the North Brazilian Current and carry the 13°C Water eastward to the thermostad region. Vertical mixing does not explain the development of the thermostad, but is found to be essential in determining the ultimate characteristics of the 13°C Water. The other two thermostads are those of the 18°C Water in the Sargasso Sea and the Subantarctic Mode Water in the western South Atlantic. Unlike the 13°C Water, both of these mode waters are formed as thermostads in the surface layer by winter convection, but vertical mixing in the subtropical gyres may play a role in determining their characteristics. All the three thermostads appear to be required to balance the system of flows in opposing directions.     

Influence of the upper mixed layer depth on Langmuir turbulence characteristics

The upper mixed layer depth (h) has a significant seasonal variation in the real ocean and the low-order statistics of Langmuir turbulence are dramatically influenced by the upper mixed layer depth.To explore the influence of the upper mixed layer depth on Langmuir turbulence under the condition of the wind and wave equilibrium,the changes of Langmuir turbulence characteristics with the idealized variation of the upper mixed layer depth from very shallow (h=5 m) to deep enough (h=40 m) are studi...     

Interannual variability of mixed layer depth and heat storage of upper layer in the tropical Pacific Ocean

By using the upper layer data(downloaded from the web of the Scripps Institution of Oceanography ),the interannual variability of the heat storage of upper layer(from surface to 400 m depth) and the mixed layer depth in the tropical Pacific Ocean are investigated. The abnormal signal of the warm event comes from the central and west Pacific Ocean, whereas it is regarded that the abnormal signal of the warm event comes from the east Pacific Ocean in the popular viewpoint. From the viewpoint on the evolution of the interannual variability of the mixed layer depth and the heat storage of the whole upper layer, the difference between the two types of E1Nino is so small that it can be neglected. During these two E1Nino/La Nina events( 1972/1973 and 1997/1998), other than the case of the heat storage or for the mixed layer depth, the abnormal signal propagates from the central and west Pacific Ocean to the east usually by the path along the equator whereas the abnormal signal propagates from the east to the west by the path northern to the equator. For the interannual variability, the evolution of the mixed layer depth corresponds to that of the heat storage in the upper layer very well. This is quite different from the evolution of seasonality.     

On the calculation of the depth of the upper mixed layer in the equatorial ocean

The equation for the turbulence kinetic energy balance as applied to the horizontally-inhomogeneous upper ocean layer encompassing the equatorial zone is analysed. A partial solution of the equation has been derived for the equilibrium conditions. Using the equatorial Atlantic as an example, the prime importance of considering the effect of horizontal heterogeneity in calculating the upper mixed layer thickness is shown.Translated by Vladimir A. Puchkin.     

人工神经网络方法估算海洋上混合层深度的初步研究

上混合层深度是海洋上层热力结构特征的重要参数.结合南海南部海区一连续温盐深观测站的实测资料和NCEP再分析风场资料,以海洋表层温度和风应力为输入,以温度差值判断方法计算所得上混合层深度为输出,采用BP神经网络和广义回归神经网络2种方法进行训练,建立了南海南部海区的上混合层深度人工神经网络计算模型.实验显示,两种模型仿真结果与温度差值方法计算结果均方根误差分别为3.58m、3.09m,线性相关分别达0.87、0.91,绝对误差分别为2.80m、2.37m,相对误差分别为9.40%、7.40%.这一结果表明,人工神经网络方法精度较高,是一种切实可行的上混合层深度估算方法.     

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.     

Hydrologic structure of waters in the north-western Tropical Atlantic

Analytical data based on hydrological observations (34th cruise of the R/VAkademik Vernadsky) are used to show that the basic element contributing to oceanic circulation is the well-developed North Equatorial countercurrent. The latter is considered as a frontal zone separating two structures of water mass. It has been demonstrated that in the salinity field—besides the meridional exchange of subtropical and Antarctic waters—the zonal advection of low-salinity waters also plays an essential role. The water masses have been specified, their parameters determined, and volumes calculated. We have found that the thermohaline indices in the cores, of tropical and west equatorial water masses have different salinity values.Translated by V. Puchkin.     

Hydrodynamic instability of the Equatorial Countercurrent in the Tropical Atlantic

Unstable wave disturbance parameters and their seasonal variability are considered using a multi-level quasi-geostropic model of a large-scale current. It has been postulated that hydrodynamically unstable processes become more intensive during the winter-spring period, with the dominating wavelength being 600 km and the period 350 days. The decay of the Equatorial Countercurrent in spring is related to a mixed type of hydrodynamic instability and to the generation of planetary waves. During the summerautumn period, when the Equatorial Countercurrent's hydrodynamic instability is developing, meandering occurs, with the lengths of the waves, slowly migrating across the ocean in an easterly direction, being 950–1500 km.Translated by V. Puchkin.     

C : N ratios in the mixed layer during the productive season in the northeast Atlantic Ocean

Redfield stoichiometry has proved a robust paradigm for the understanding of biological production and export in the ocean on a long-term and a large-scale basis. However, deviations of carbon and nitrogen uptake ratios from the Redfield ratio have been reported. A comprehensive data set including all carbon and nitrogen pools relevant to biological production in the surface ocean (DIC, DIN, DOC, DON, POC, PON) was used to calculate seasonal new production based on carbon and nitrogen uptake in summer along 20°W in the northeast Atlantic Ocean. The 20°W transect between 30 and 60°N covers different trophic states and seasonal stages of the productive surface layer, including early bloom, bloom, post-bloom and non-bloom situations. The spatial pattern has elements of a seasonal progression. We also calculated exported production, i.e., that part of seasonal new production not accumulated in particulate and dissolved pools, again separately for carbon and nitrogen. The pairs of estimates of `seasonal new production’ and `exported production’ allowed us to calculate the C : N ratios of these quantities. While suspended particulate matter in the mixed layer largely conforms to Redfield stoichiometry, marked deviations were observed in carbon and nitrogen uptake and export with progressing season or nutrient depletion. The spring system was characterized by nitrogen overconsumption and the oligotrophic summer system by a marked carbon overconsumption. The C : N ratios of seasonal new as well as exported production increase from early bloom values of 5–6 to values of 10–16 in the post-bloom/oligotrophic system. The summertime accumulation of nitrogen-poor dissolved organic matter can explain only part of this shift.     

Staircase and inversion structures of thermocline in the north-west Tropical Atlantic

The peculiarities of the vertical fine thermohaline structure of waters in the north-west Tropical Atlantic are considered on the data of STD surveys recorded in winter-spring 1984. The variability of the characteristics of staircase and inversion elements of stratification with depth is analysed over the horizontal as well as related to the mesoscale and large-scale dynamics of waters. The coefficients of horizontal turbulent exchange are estimated within the framework of Joyce's hypothesis on quasi-compensation of vertical and turbulent horizontal transport. The effects of double diffusion are considered to dominate in vertical transport.UDK 551.465.15Translated by Mikhail M. Trufanov.     

Specific features of the field of surface temperature in the Tropical Atlantic

On the basis of generalization of the data of many-year hydrological observations and the data of meteorological satellites accumulated in recent years, we characterize some specific features of the surface temperature in the Tropical Atlantic. The influence of solar radiation, local heat balance, and the advective and diffusion heat transfer on the temperature of the water surface is analyzed. The mechanism of formation of the thermohalocline and local sites of elevated temperature near the estuaries of large rivers (such as the Amazon, Orinoco, Mississippi, Congo, and Niger) is described. We also characterize the formation of the seasonal variability of the near-equatorial temperature maximum, equatorial temperature minimum, and equatorial divergence rate. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 6, pp. 28–38, November–December, 2007.     

Seasonal course of theT,S-characteristics in the north-eastern Tropical Atlantic

The problem of the seasonal variations of temperature and salinity in the intermediate zone located between the West African and the Gulf of Guinea upwellings is considered. The vertical distribution of the phase-amplitude characteristics of the annual and semi-annual variations is examined in good detail and their parameters are defined from the monthly means using the least-squares fit technique. The contribution of isopycnic advection to the formation of seasonal thermohaline variations is studied.Translated by Vladimir A. Puchkin.     

Physical and hydrological characteristics of climatic frontal zones in the Tropical Atlantic

Climatic frontal zones are selected in the thermohaline fields of the Tropical Atlantic by analyzing the many-year-average seasonal database reduced to the nodes of a one-degree grid. We determine physical characteristics of the frontal zones, study their spatial and temporal variability, and reveal basic regularities of the appearance of frontal zones in the fields of thermohaline characteristics. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

Parameterization of the depth of the upper quasi-homogeneous layer based on measurements in the North Atlantic (Section along 53° N)

The dependence of the variation in the depth of the upper mixed layer (MLD) on the governing parameters (the momentum flux, the buoyancy fluxes at the ocean surface, and the density gradient in the pycnocline) is considered. It is shown that, in the spring storm season, wind mixing dominates over convective mixing. In this case, the MLD is linearly correlated with the Ekman scale calculated from the friction velocity observed approximately 12 h before the measurement of the MLD.