Vasiliy Melnikov, Lidija Moskalenko and Natalija Kuzevanova

P.P.Shirshov Institute of Oceanology RF Academy of Sciences, Moscow, Russian Federation, Physical oceanography dept.

Vmelnikov@ocean.ru , Lidija-moskalenko@rambler.ru, Kuzevanova-nata@mail.ru

North-East Black Sea climate system decadal variability

 Introduction We present an analysis of meteorological and hydrophysical variability in the vicinity of Black sea North-East

coast with the use of satellite sea level anomaly (SLA), sea surface temperature (SST) databases, in-situ temperature measurements and meteorological stations standard data.

In addition to satellite data calibration, particular goal was to examine meteorological forcing on SLA and SST fields in order to study ocean-atmosphere interactions through descriptive elements of coastal weather system.

Despite the present study revealed several typical properties of the Gelendzhik coastal “weather machine”, there are essential opportunities for further combined meteorological and hydrophysical processes examination on the basis of satellite and in-situ measurements.

On the basis of the Gelendhzik coastal weather station(44.55_N, 38.05_E) long-term (1974-2010) observationaldata, climatic variability organization from a variety of synoptic conditions is considered. The mechanism ofevolution of fields from small to large time scales is the "universal" set of wind vector variations, which due totheir crucial role for the region deserves a special name "elementary cycle" (EC). Typical changes in the EC arecharacterized by a cyclic change in dominant wind from the south-east to north-east direction and vice versa. Thesimilarity of temporal EC variations at different time scales is regarded as a manifestation of wind variabilityfractality. It is shown, that the fractality is due to recurrence of basic regional baric synoptic fields. Three long-term EC in the period 1974-2010 constitute a decadal climatic "wave" – repeated wind vector variations, which due to rather simple appearance can be traced easily with the use of progressive vector diagram without any filtration. This type of long-term EC figure can be used effectively as a reference curve for the numerous climatic “events” and processes, considered in nowdays. Meridional component of wind velocity in the climatic wave, as well as the accompanying changes in temperature of air and water, are statistically associated with the atmospheric pressure East Atlantic-West Russia dipole. Effects of North Atlantic Oscillation are revealed in the air zonal transport changes.

As follows from the estimate of linear trends over the past 30 years, the background warming is 0.072 C/yearfor sea water and 0.051C/year for the air. Similar estimation for the 70-year time series yields 0.009C/yearand 0.011C/year, respectively. During this period, 43-year temperature cycle in 1947-1990 was followed by ahalf-cycle (incomplete) in 1990-2005 with a shorter period, and the amplitude of temperature long-term variationssince 1990 is clearly increased. For the other hydrometeorological parameters, amplitude and frequency oflong-term oscillations were also changing in the time course. Thus, the duration of the low-frequency sea levelcycles in the period 1995-2010 had been increased to 7 years as compared to 3 years during 1980-1995. Theaccording amplitude was increased from 5 to 10 cm. The reverse pattern is visible in the long-term changes ofatmospheric pressure and precipitation: the amplitude and period of recurrence in the second half of observationsat weather station were significantly decreased. The reasons for the above changes of the oscillation modes andtheir relationship with atmospheric circulation indices, has not yet been clarified.In general, the impact of winds on regional multiscale hydrophysical processes in the North-East Black Sea israther complicated. However, for the above relatively simple wind cycles the dominant response signals of themarine hydrosystem can be separated. The response to winds is due to the air temperature advection, wind strength, direction, duration and spatial inhomogenity. Currently, it is obtained, that the "quanta" of wind cycles produces sea-level fluctuations of different time and space scales, which adapt to the equilibrium by means of various dynamical processes including inertial waves, shelf upwellings/downwellings, local jet streams and various eddies.

RegionNorth-East Black Sea part under consideration extends in the square 43-45.5N, 36.5-39E and embraces Cemesskaya Bay, Blue Bay and Gelendzhik Bay (Fig. 1) along the shore. The shelf in this region is rather narrow, extending only five to seven nautical miles from the shore. To the north from the 44.7 N, the shelf becomes much broader. The continental slope is regular along the coast and very abrupt in the cross-shore direction, the depths being increasing from 100 m to 2200 meters. Along the shore moderate mountains (up to 600 m high) chains (with rare passages across chains) form a weak shelter against cold north air spreading. It is this along-shore Earth surface and bottom relief regularity (symmetry), which determines to a large extent the character of regional dynamical processes in the atmosphere and in the sea.

It is well-known, that besides the advection (horizontal and vertical), North-East Black Sea hydrophysical parameters variability in a broad regimes and scales are governed to a large extent by atmospheric influences, such as atmospheric pressure, wind stress, heat and moisture exchange. There are several interesting examples of atmospheric “events” and their hydrophysical consequences, such as wind-driven Black Sea Main Current intensification, eddies production enhancement, Novorossiysk bora far sea traces, local off-shore jets(Colchis) and larger atmospheric cyclonic (storms) field signatures, shelf-break upwellings, A.G.Zatsepin and M.V.Flint (2002).

Fig.1 The Black Sea North-East coast Earth surface topography 3-D view of the area under consideration. 1-minute resolution bathymetry data from Smith, W. H. F., and D.T. Sandwell (1997).

Fig.2 Bottom topography and orography in the vicinity of the Black and Azov Seas. From Smith, W.H.F. and DTSandwell (1997), Global seafloor topography from satellite altimetry and ship depth soundings, Science, v. 277, 1957-1962. Scale of heights and depths, in tens of meters, the resolution of the space -1 minute.

Global system of atmospheric circulation associated with the Jets -subtropical and polar jet streams in the upper troposphere, is transforming over the Caucasian highlands and forms a regional system. Regional climatic features of the Black Sea are the result of geographical location,orographic irregularities and smoothing effect of the sea water. Large seasonal variations are related to the inflows of cold air from the north, warm air from the south, as well as to the impacts of storm cyclones coming from the Mediterranean Sea.

The Data and Data ProcessingFor the purposes of our research, we divided the total North-East region into three parts: the nearest to the Blue Bay part includes shelf measurement sites, which data were used for different instrumentation comparison; the second area covers dynamically active domain over the continental slope; the third - embraces the whole region ( Fig.2). The following data were used:1. Mooring(44.57N,37.98E) sea surface temperature,1 hour sampling,1998- 2003.2. Coastal station (44.55N, 38.05E) meteorological parameters, including wind speed and directions, air temperature (sampling 3 hrs.), water temperature, sea level (sampling 6 hrs.), atmospheric pressure (daily), 1990-2009. 3. Satellite Black Sea surface temperature, nightly, of AVHRR Pathfinder SST v5 array, spatial resolution 4 km, 1985-2008.4. AVISO altimetry data Black Sea level anomaly, daily, spatial resolution 1/8, 2000- 2008. 5. Precise bottom topography “ETOPO-1” data base, spatial resolution 1‘.

The non-stationary records were detrended and filtered to remove high-frequency variations. Synchronized parts of time rows had been converted to a set of auto and cross spectra.

Fig.3 Measurements sites. Notations: 1)red star–Shirshov Institute pier (Blue Bay, 44.58N, 37.98E); 2)red square - Gelendzhik meteostation (44.55N, 38.05E); 3)green dots–satellite temperature data(1985-2008), nightly SST, AVHRR, NOAA; 4)pink squares-Black Sea SLA(sea level anomaly), AVISO products. Fuzzy blue lines-bottom topography isolines; orange lines-Earth surface orography (without heights); red dashed lines separate three polygons under analysis.

Wind systemThe basic features of the regional wind variability are governed by the relevant types of the large-scale

synoptic atmospheric processes, which depends upon the state of the global atmospheric circulation, their large-scale gyres and separate smaller vorticity cells. The following classification of the 4 types of annual wind regimes can be deduced on the basis of the multy-year meteo-station annual wind direction probability diagrams ( Fig.4). In particular, in 1990-1993 winds of North rhumb were dominated. In the course of 1994-2001, 2007-2008, there were frequent North–East winds. In the period of 2002-2006, we can distinguished persistent East winds. In 2004-2005 practically equally frequent East and South winds were observed. Related mean annual temperature characteristic of the above mentioned wind type periods are as follows. North wind regime (1993) was anomalously cold; North-East winds were accompanied by variety of temperature states: warm (1998, 1999, 2001, 2007), moderate-warm (1994-1996, 2000, 2008) and moderate-cold (1997), depending upon the characteristics of interacted air masses; East regime can be moderate-cold (2003), moderate-warm (2006) and warm (2002). Years of the East and South winds are predominantly moderate-warm (2004) and warm (2005).

сссв

св

всв

в

вю в

ю в

ю ю вю

ю ю з

ю з

зю з

з

зсз

сз

ссз

0 5 10 15 %

Fig.4 Wind direction frequency occurrence for the period 1980 - August 2010.Basic wind direction intervals: N(C) - 337.5-0-22.5; NE(CB)- 22.5-45-67.5; E(B)- 67.5-90-112.5;SE(ЮВ)- 112.5-135-157.5; S(Ю)- 157.5-180-202.5; SW(ЮЗ)- 202.5-225-247.5; W(З)- 247.5-270-292.5;NW(CЗ)-292.5-315-337.5.

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2007 - 2009

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Fig. 6 A- long-term wind variations; B- typical seasonal cycles(Elementary cycle- EC).

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Fig. 7 A- first phase of the EC(SE wind); B - second phase of the EC (the transitional period of weak winds in different directions), C - the third phase of the EC (NE winds), and E - transition period breeze fluctuations.

A

C

D

B

A B C

D E F

Fig. 8 Typical atmospheric pressure fields as related to basic types of wind conditions: A - Northern, B - North-east, C - East, D - South-east, E – South-west, F - North-west. B-High, H-Low pressure.

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Fig. 9. Characteristics time course : A – filtered smoothed velocity components of air transport of U (+ - to the east, the solid line), V (+ - to the north, dashed line), B - filtered winter (February) sea surface temperature (top curve - dashed) and air temperature (lower solid curve); C – filtered NAO index; D - filtered EAWR index .

№ Row Correlation coefficient

95% confidence interval

Zero correlation probability

(p-parameter)

Linear regression coefficient

1 NAO - U 0.58 0,25-0,79 2*10-3 0.69

2 NAO - V -0.093 -0.46-0,31 0.65 -0.153 NAO - Ta -0.15 -0.51-0,26 0.48 -1.41

4 NAO - Tw -0.28 -0.60-0,12 0.16 -1.325 EAWR - U 0.23 -0.18-0,56 0.26 0.37

6 EAWR - V -0.27 -0.60-0,13 0.18 -0.327 EAWR - Ta -0.37 -0.67-0,015 0.06 -4.348 EAWR - Tw -0.36 -0.66-0,025 0.07 -2.059 Ta-Tw 0.65 0.35-0,83 3*10-4 0.31

Table 1. The correlation coefficients of hydro-meteorological parameters with the indices of atmospheric circulation NAO and EAWR.

1980 1985 1990 1995 2000 2005 201011

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pera

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ars

Fig.10 Long-term variations (trends) of hydrometeorological parameters on the Gelendzhik weather station during 1980-2009. Thin solid lines – filtered values , the filter ½ year; solid line-filter 1 year, dotted and dashed lines are linear trends and the approximation of a polynomial of degree 10.

1940 1950 1960 1970 1980 1990 2000 201011

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17

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pera

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air temperature

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stru

ctur

e in

dex

NAOEAWREAPOL

Fig.11 Sea surface and surface air temperature long-term variations at the Gelendzhik weather station in the period 1938-2009, and atmospheric indices trends in the period 1950-2009 ; NAO (North Atlantic Oscillation), - EAWR (East Atlantic - West Russia), - POL (Polar / Eurasia Pattern), - EA (East Atlantic Pattern).

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Fig.12 A time-course: sea level (H-430 cm) - 1; SST (C) - 2; wind gust speed (m/s) - 3; wind speed (m/s) - 4; wind direction Azimuth (grad./100) - 5, during the development of strong upwelling on August 1, September 1 and September 28, 2008, B-average sea surface temperature at weather stations in Gelendzhik in the period 2007 to 2010.

A

B

Fig 13 IR image of the Black Sea from NOAA satellite, June 29,1998.

Fig.14 Fragments (15 km-20 km) of images obtained by radar ASAR satellite Envisat, showing the spiral small-scale eddies, typical for warm season: a - two cyclonic eddies with diameters of 3.75 km (A) and 3 km (B); b - a cyclonic vortex with diameter of 3.5 km (C); в - cyclonic eddy with a diameter of 2.5 km (D); r - cyclonic eddy with diameter of 5.3 km (E).

а1?

Геленджикская бухта

а1

Геленджикская бухта

а1

ц1

Геленджикская бухта

а2?

Геленджикская бухта

Fig.15 The velocity field in the coastal-shelf zone in the region of Gelendzhik according ADCP surveys of 27-30.09 (a) - (d). The dotted line denotes the position of the alleged sub -mesoscale anticyclonic eddies A1 and A2, as well as cyclonic vortex C1.

Conclusions

1. A typical feature of the regional climate system are the cyclic transitions from the north-east wind to south-east and back again.2. Several long-term Elementary cycles(EC) constitute climatic "wave" of annual variations in wind direction from the direction of the north-west (average wind - SE) to the direction to the southwest (average wind - NE).3. Resemblance of the temporal structure of wind variations for the EC of different scales is viewed as fractal variability of wind associated with the recurrence of the NE and SE types of synoptic atmospheric processes, prevailing in the region of the North Caucasian coast.

4. Meridional component of wind velocity in the climatic wave, as well as the accompanying variations in air and water temperatures associated with the atmospheric baric dipole EAWR. NAO influence is manifested in changes in the zonal transport of air.5. Bringing more air from the south to the north corresponds to the negative EAWR. phase. Accordingly, interannual air and water temperatures are increasing with an increase in the positive (from south to north) meridional air transport

6. Several different long-term cycles superimposed on parameters trends have been mentioned. 7. Typical winds give rise to a number of sea dynamic processes.of wide-range spatial and time scales. 

References

1. Zatsepin, A.G. and Flint, M.V. (Editors), (2002), “Multidisciplinary Investigations of the North-East Part of the Black Sea”, ”Nauka”, Moscow, 475 p. 2. Smith, W. H. F., and D.T. Sandwell (1997), “Global seafloor topography from satellite altimetry and ship depth soundings”, Science, v. 277, 1957-1962.

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