EGU General Assembly 2014 Vienna /Austria

27 April – 02 May 2014 Session OS2.3

Numerical Analysis of the Hydrological Mode in the Upper Layer of the Black Sea for Spring Season

Diana Kvaratskhelia, Demuri Demetrashvili and Aleksandre Surmava M. Nodia Institute of Geophysics of Iv. Javakhishvili Tbilisi State Universsity, Tbilisi, Georgia.

diana_kvaratskhelia@yahoo.com

Contents

•Motivation and main object•The model description and method of solution• Key model parameters• The results of numerical experiment for April climatic condition: The general peculiarities of the Black Sea vertical circulation; About heat transport variability for the April climatic conditions;• Comparison with the January numerical results • Conclusion • References

Motivation and main objectThe changeability of the atmospheric processes developed above the Black Sea plays a significant role in spatial-temporal changes of the hydro and thermo-dynamical parameters in the upper layer of the sea. On the other hand, the hydro and thermo-dynamic processes in the Black Sea directly reflect on interaction processes between the Black Sea and atmosphere and significantly influence on the formation of the regional climate. Besides, the thermo-dynamical state of the upper layer of the Black Sea significantly influences on the living marine organisms. Therefore, a study of hydrological structure of the Black Sea upper layer in connection with variability of atmospheric processes is very actual for the Black Sea oceanography. The main object of the present paper is to investigate numerically the temporal variability of peculiarities of the vertical hydrological structure of the Black Sea within the depth 0-136 m for transitive seasonal conditions (April) in the inner-annual time scale. Generally this investigation is concentrated on the specification of the primary role between wind driven forcing and the thermohaline impact on the formation of the vertical structure of the Black Sea circulation for transitive climatic conditions. Besides, our attention is focused to the study of the thermohaline variability of the biologically acting 0-136 m layer of the black Sea in the same season to estimate heat transport variability on the horizons in the upper and lower layers. These heat transport processes developed in the upper layer of the Black Sea contribute an important role to the formation of the regional climate regime. The focus for April is caused by that fact, that this month represents the transitive period between warm and cold seasons, which is characterized with both the features of atmospheric circulating processes and variability of thermal interaction in the sea - atmosphere thermodynamic system.

The model description and method of solutionDespite of high adequacy of other numerical model’s [Korotaev et al., 2003; Staneva, 2005; Kara et al., 2005a] to achieve the specified goal, we used a 3-D, z-level BSM-IG developed by Kordzadze and Demetrashvil (2008). It should be noted that on the base of the high-resolution regional version of the BSM-IG the regional forecasting system www.ig-geophysics.ge is developed [Kordzadze and Demetrashvili, 2011], which is a part of the basin-scale Black Sea Nowcasting/Forecasting system [Korotaev et al., 2011]. The model equation system is written for deviations of thermodynamic values from their standard vertical distributions. This model takes into account quasi-realistic sea bottom relief, nonstationary atmospheric wind and thermohaline forcing, water exchange with the Mediterranean Sea and inflow of the Danube River, the absorption of short-wave radiation by the sea upper layer, space-temporal variability of horizontal turbulent exchange and diffusion [Zilitinkevich and Monin, 1971]. The model makes it possible to take into account wind driven forcing with alternation of different climatic wind fields and the atmospheric thermohaline action by both the Dirichlet conditions through setting the temperature and salinity at the sea surface and the Neumann conditions through setting the heat fluxes, evaporation, and atmospheric precipitation. Vertical turbulent viscosity and diffusion coefficients were chosen constant equal to 50 cm2s-1 and 10 cm2s-1 respectively. For solution of the problem a two-cycle splitting method was used [Marchuk, 1974].

Key model parameters BSM-IG with space resolution equal to 5 km is used. The quantity of points along axes x and y was 225 and 111, respectively. On a vertical the non- uniform grid with 32 calculated levels on different depths: 0, 2, 4, 6, 8, 12, 16, 26, 36, 56, 86, 136, 206, 306,..., 2206 m were considered. The types of atmospheric circulation for April were taken from [ “Atlas of excitement and a wind of the Black Sea”, 1969]. This data were observed to the period 1946-1962. The types of atmospheric winds differed from each other by direction, module and repeatability [Kordzadze at. al., 2000 ] .The multiyear monthly means of temperature and salinity at the sea used as the Dirichlet conditions for model equations and the depth profiles of the temperature and salinity averaged over the water area were obtained from Department of Oceanology of M. Lomonosov MSU. The main bulk of these data were correspond to the period 1955–1994.

The data of the heat fluxes, evaporation and precipitation borrowed from [Staneva at. al., 1998] were conveyed from the Marine Hydrophysical Institute (Sevastopol/Ukraine) within the framework of the EU project ARENA. All these products is reproduced in the model by using linear interpolation in the inner-annual time mode. In numerical experiments, the radiation absorption coefficient was assumed equal to 0.0023 m–1, which corresponds to usual ocean water, in which about 10% of the incident radiation reaches a depth of 10 m [Kondratiev at. al., 1987]. The time step is equal 1h. Values of other parameters are given by [ Kordzadze at al., 2008].

The climatic fields: temperature- (a), salinity- (b), heat fluxes-(c), precipitation – evaporation (d) at the Black Sea surface for April climatic conditions.

The climatic fields: temperature- (a), salinity- (b), heat fluxes-(c), precipitation – evaporation (d) at the Black Sea surface for January climatic conditions

Results of numerical experiment for April climatic conditions

The general peculiarities of the Black Sea vertical circulation. In the numerical experiment on simulation of transitive hydrological mode of the Black Sea the integration started on the 1st of January. As initial conditions annual mean climatic fields of current, temperature, and salinity was obtained by BSM-IG . In the present section investigations are carried out for April climatic conditions with 5 km resolution both the Dirichlet and Neuman boundary conditions. In order to illustrate the vertical changeability and transformation of the Black Sea circulation during the transitive period, we chose the time interval 2684-2860 hours (April, time is accounted from the 1st January), when the atmospheric circulation was reorganized as shown in the Table 1. In the same table the calculated locations of the homogenous sub-layers [ Demetrashvili at al., 2008] which are formed within upper 0-306 m layer of the Black Sea, are given with use of both conditions describing atmospheric thermohaline forcing.

Wind direction Wind speed, m/s

Time interval, h Sub-layers depth (m) with the both case: (a) Neuman and (b) Dirichlet conditions

Northwester - north 0-2 2684-2704 h 2-136, 206-306 m in both conditions

North-eastern 0-5 2704-2748h a) 2- 16m , 26- 136m,136—306 m;b) 2-16, 16-136,136-306 m.

North-western 0-2 2748- 2772h 2-136m, 136-306m in both condition.

Cyclonic 5-10 2772-2796h a) 2-12m, 16-136, 206-306m ;b) 2-12, 16-136m, 206-306m.

North –western 0-2 2796-2820h 2-36, 56-86., 136- transitive layer, 206-306m, in both condition

North- western 5-10 2820-2836h a)2-12, 12 and 16m transitivelayer, 16-36m, 56-136m , 206-306m.b) 2-8, 12-16, 26-36, 56-136, 136-306m

South-west 0-2 2836-2860 2-136, 136-306m in both conditions

Table 1.

The circulation patterns presented in these Figures(see next slides) characterize the general peculiarities of the Black Sea vertical circulation, which have place from the middle period of transitive season to the and of April according to both conditions, respectively. The mean features which are clearly expressed by results of the numerical experiment is that when the state of the atmosphere is close to calm conditions (0-2 m/c), the character of sea circulation practically does not change by depth within 2-136 m with use of both Neumann and Dirichlet boundary conditions. Current velocity does not importantly decreases from the sea surface to lower layer at 136m, under 136 m the mean current system gradually changes and it is characterized by the clear tendency to formation of several vortexes with very small sizes. That fact is clearly observed with use of both kinds of boundary conditions at t = 2704 h and 2860h when Northwestern-north and south- west winds with speed 0-2 m/s was operated.

Results show also, that the received circulating patterns with use of two kinds of boundary conditions qualitatively differ from each other, despite of their common feature - homogeneous character of vertical circulation for atmospheric calm conditions. There is a good correlation between circulation and salinity fields at both Neumann and Dirichlet conditions.

Besides above mentioned results, there are clearly observed that the structure of the circulation practically does not change at the time moments 2704 h and 2860 h, despite the operating wind types with obviously different direction . There is the difference only within both thermohaline meteorological conditions (see upper panel of the Salinity fields) , which defines the structure of the salinity fields in the Black Sea ( lower panel of the Salinity fields) and promote this phenomenon.In other hands the analysis of the numerical investigation, which shows a good correlation between circulation and salinity fields, confirms the known fact the general character of variability of the salinity field in the inner-annual time scale depends on some aspects, generally it is balance in system evaporation –precipitation , rivers inflow and the circulation characteristic. As this result shows in the transitive season the thermohaline conditions defines the structure of the circulation . This result allows us to note that in the case of the calm atmospheric conditions the double effects of the influence is developed in the Sea system, which defines the hydrological mode in the upper layer of the Black Sea.

The fields of wind tangential friction, wind types: (a)- Northwester-North (0-2m/s) , (b)- Cyclonic (5-10m/s) , (c)- North-Western (5-10 m/s) and (d) South Western (0-2 m/s) .

Calculated current fields (cm/s) , corresponding to the following wind types: (a)- northwester-north wind (0-2 m/s), (b)- Cyclonic wind (5-10m/s), (c)- North-Western wind (5-10 m/s), and (d) south western wind (0-2m/s) according to Neumann condition within depth 2-306m .

Calculated current fields (cm/s) at the same time: according to Dirichlet condition.

The Salinity (‰) fields correspond to Neumann and Dirichlet conditions respectively . (a), (b), ( c) and (d) at the sea surface; (a’), (b’ ) (c’) and (d’ ) calculated Salinity fields within depths 2-136m .

The Salinity field at the vertical section along 4205’ N.. (a), (b) and (c), (d) correspond to Neumann and Dirichlet conditions, respectively

The analysis of the circulating modes shows that during this time interval the circulation has cyclonic structure despite of their difference according to thermohaline conditions. Though, it should be note that several anticyclones vortex is observed outside of the meander current. Here, our attention is focused on the Batumi anticyclone eddy which obviously is expressed at use of Neuman termohaline conditions . The tendency to form the anticyclonic vortex is not clearly observed at use of dirichlet conditions. It is clear, the fields of heat fluxes and temperature at use of the both conditions define the thermal regime at the sea surface (see upper panel of the temperature fields in the next section) . It is interesting to note, that under analysis of the boundary data the Georgian coastal waters heat by intensive influence of the heat fluxes. On the other hand, this exchange between the sea and atmosphere affects the mode of evaporation-precipitation system and as a result the thermohaline condition (see upper panel of the salinity fields at use Neuman condition) has appeared favourable for formation intensive anticyclonic vortical structure in the Georgian coastal area. This phenomenon promotes the dominate role of thermohaline action too.

Climatic averaged data show that generally atmospheric winds with 5-10 m/s are dominated over the Black Sea for April [Kordzadze at. Al, 2000 ]. In this case two sub-layers are formed within 2-136 m with use of Neumann conditions and being tendency is formed three sub-layers - with use of Dirichlet conditions . This fact is illustrated in Figures well at the time t = 2796 h and 2836h. The location of sub-layers practically insignificantly depends on that fact what kind of boundary conditions are used – the Dirichlet or Neumann conditions. The uppermost sea layer with depth about 12 km is more sensible to wind driven forcing in any case of thermohaline action and is defined by wind type. In deeper layers below 26 m the influence of atmospheric wind forcing weakens and the structure of circulation gradually becomes similar to that, which was observed in case of calm atmospheric conditions at the time 2704h and 2860h . That fact shows also the primary role of thermohaline forcing in lower layers (see salinity fields according to Neumann and Dirichlet conditions respectively). Besides, in any case the level z = 136 m is transition level , which carries features of both sub-layers: 1-136 m and 136-306 m.

About heat transport variability for the April climatic conditions

Besides above mentioned, the heat transport variability on the horizons and the vertical formation of the thermal regime in the uppermost layers of the black Sea developed according to two case of the thermohaline impact are at the center of our attention. As the numerical investigations show the heat transport processes in the Black Sea surface layer are developed due to the nonstationary alternation of atmospheric circulation during the transitive season. This process plays one of the important roles in the formation of the thermal regime in the upper layer of the Black Sea.

During this period sea temperature patterns predicted by both conditions considerably differ from each other qualitatively. During this period the most part of the upper layer warms in the both case under the influence heat fluxes from atmosphere( see upper panel of the temperature fields). The effect of absorption of shortwave radiation resulted in small diurnal temperature variations, is assumed with result of the thermal fluxes and they hardly propagated below a depth of 11–15 m .

It should be note that the heat transport processes are more intensive in spring season. From this Figure it is clear that the main feature of the temperature patterns is that the heat transport on the horizon develops very actually in the entire depth 0-26 by using both condition . Generally, in any case the warm waters are observed in the Caucasus nearshore zone, where the stable Batumi anticyclonic eddy is developed. Despite penetration of cold waters from north part of the basin the warm water area is permanently extended. By means of horizontal penetration it transforms from the east to the west part along the Rim Current in the surface layer.

This process has permanently character in the chose time interval and it develops more fast in the south-west part of the Black Sea under influence of the warm waters located in the Bulgarian coastal zone. Besides, the warm deep flow enters into the Black Sea from the Mediterranean Sea through the Bosporus Strait which significantly contributes to the thermal regime of the south-west part of the Black Sea.

The Temperature fields correspond to Neumann conditions. (a), (b), ( c) and (d) at the sea surface; (a’), (b’ ) (c’) and (d’ ) calculated Temperature fields within depth’s 2-136m .

The Temperature fields correspond to Dirichlet conditions: (a), (b) , (c ) and (d) at the sea surface ; (a’), (b’) ,(c’) and (d)’ calculated Temperature fields at the same time

As results show, during the considered time period the turbulent mixied layer of the Black Sea predicted by using both Dirichlet and Neumann conditions is thinnest which does not reach approximately 4 m. Small thickness of the mixed layer in the spring is caused by action of week winds in this season. But here is obviously that these heat fluxes dominate and during the summer increased solar heating of the surface water leads to more stable density stratification. It’s effect riches in the lower layers , below of 4 m on the Thermocline. On the horizon z=56 m the cold intermediation layer (CIL) is well observed. The Circulating processes bring the contribution to the horizontal distribution of the temperature field not only by advection, also by vertical downward and upward moving of water masses. In the north peripheral area cold waters downward and in the central areas of cyclonic vortexes the open sea rise upwards.

The Temperature fields at the vertical section along 4205’ N. correspond to Dirichlet conditions.

The Temperature fields at the vertical section along 4205’ N. correspond to Neumann conditions.

Vertical distributions: (a) Turbulent Viscosity is divided by 100000 , (b) Temperature 0C, (c) - Salinity (%0) fields

(a) –deviation of Temperature , (b)-deviation of Salinity.

In order to illustrate the vertical changeability and transformation of the Black Sea circulation during the winter period, we chose the time interval 129-256 h (January), when the atmospheric circulation was reorganized as shown in Table 2.

It should be noted that as it in the Table 2 is indicated despite of calm conditions at time moments t = 176, 256 hours there are observed also several homogenous sub-layers in difference of the first case at time moment t=142h and the case at t= 207. The detail analysis of the results of the numerical experiment has shown that the reason of this fact is the effect of influence of a previous strong wind. In case of strong winds at time moments t = 163, 194 and 243 hours in the Black Sea there are formed several sub-layers within depts 2-136 m.

Wind direction Wind speed, m/s

Time interval, h

Sub-layers:

North-western wind

0-2 129-142 2-136; 136-306 m.

North easterly-western wind

15-20 142-163 2-12; 16-36;56-136; 136-306 m.

North easterly-North wind

0-2 163-176 2-36; 56-136; 136-306 m.

Eastern wind 10-15 176-194 2-12; 12-26; 36-136; 136-306 m.

North-easterly -Earth

0-2 194-207 2-86-136; 136-306 m.

North-eastern wind >20 207-243 2-8; 12-26; 36-136, 136-306m.

North wind 0-2 243-256 2-16; 16—86-136; 136-306 m.

The general peculiarities of the Black Sea vertical circulation.

Comparison with the January numerical results

Table 2

28 30 32 34 36 38 40

42

44

46

28 30 32 34 36 38 40

42

44

46

28 30 32 34 36 38 40

42

44

46

28 30 32 34 36 38 40

42

44

46

019.6

018.27

017.2

09.45

h=2m h=86m

h=136m h=306m

Fig.3. C alcu la ted current fie lds(cm /s) a t tim e: t=142 h (January), accord ing to the N orth-w estern w ind(1m /s)

Calculated current fields (cm/s) , (January ) corresponding to the following wind types: Northeasterly-western wind (15-20 m/s), Eastern wind (10-15m/s), North-eastern wind >20. They have same structure according to Dirichlet condition

Calculated current fields (cm/s) at time: t=142h, (January )corresponding to the North-western wind (0-2 m/s). It have same structure according to Dirichlet condition

Calculated current fields (cm/s) at time: t=256 h, (January ) corresponding to the North wind (0-1 m/s). It have same structure according to Dirichlet condition

Generally, it is well visible that salinity fields are in a good correlation with circulation features. Despite of that the salinity fields in the layer 1-26 m obtained with use of Neumann and Dirichlet conditions differ from each other, sea circulation is the same at any case of thermohaline boundary conditions within depth 2-136m. This specifies that in the upper layer the primary factor of formation of sea circulation vertical structure is wind driven impact in the cold season.

Vertical distributions: (a) Turbulent Viscosity is divided by 1000000, (b)Temperature 0C, (c) - Salinity (%0) .

The temperature variability for the January climatic conditions

(a) –deviation of Temperature , (b)-deviation of Salinity.

The analysis of the January numerical experiments showed that distribution of temperature and salinity fields on the horizons of the Black sea is connected generally with variability of atmospheric thermohaline forcing, but variations of the thickness of the mixed layer mainly depend on circulation processes developed during the year.

On example of January there is shown that in the cold season the thickness of the mixed layer is increasing. In this period the mixed is initiated by turbulence caused by the strong wind stress on the Sea surface. In January the mixed layer is approximately allocated between the 0-26 m. Besides, as the analysis of the climatic data shows in all most part of the Black Sea evaporation exceeds precipitation, which on the other hands plays the important role on the mixed layer forming.

ConclusionBy means of the 3-D baroclinic model of the Black Sea dynamics developed at M. Nodia Institute of Geophysics (Tbilisi, Georgia) some peculiarities of the forming of the vertical structure of the Black Sea hydrological regime for transitive (April) climatic conditions are investigated. Investigations are carried out with consideration of both the nonstationary atmospheric wind and thermohaline forcing. Herewith the atmospheric thermohaline action is tested in the model by both the Dirichlet conditions through setting the temperature and salinity at the sea surface and the Neumann conditions through setting the heat fluxes, evaporation, and atmospheric precipitation. In the performed numerical experiments wind driven action is reduces to alternation of different climatic wind fields.The performed numerical experiments have promoted the primary role of the thermohaline impact on formation of the vertical structure of the Black Sea circulation within upper 0 - 136 m layer for transitive climatic conditions. Besides the thermohaline action plays an important role on the horizontal heat transport intensity within upper 2-26 m layer. in the inner-annual time mode.

Acknowledgement: The research leading to these results has received funding from Shota Rustaveli National Science Foundation Grant R/373/9-120/1

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