ISSN 1028334X, Doklady Earth Sciences, 2012, Vol. 444, Part 1, pp. 630–633. © Pleiades Publishing, Ltd., 2012. Original Russian Text © S.A. Kirillov, I.A. Dmitrenko, V.V. Ivanov, E.O. Aksenov, M.S. Makhotin, B.A. de Quevas, 2012, published in Doklady Akademii Nauk, 2012, Vol. 444, No. 2, pp. 212–215.
OCEANOLOGY
The Influence of Atmospheric Circulation on the Dynamics of the Intermediate Water Layer in the Eastern Part of the St. Anna Trough S. A. Kirillova, I. A. Dmitrenkob, V. V. Ivanova, E. O. Aksenovb, M. S. Makhotina, and B. A. de Quevasb Presented by Academician A.P. Lisitsyn November 19, 2011 Received November 30, 2011
Abstract—This paper discusses the results of unique direct observations over the current velocities in the east ern part of the St. Anna deepwater trough, made in the period from August 2009 until September 2010, and analyzes the physical ways how the temporal variability of currents is formed. The stable northward barotropic transport with an average annual velocity of about 20 cm/s was discovered. It was found that changes in veloc ity with a characteristic timescale of several weeks occurred synchronously in the entire water column and were determined by the deformation of the sea level field due to longperiod disturbances of the largescale field of ground wind above the northern parts of Barents and Kara seas. For the winds of the southwest and west directions, the sea level’s gradient is formed across the St. Anna Trough and the northward meridional water transport is intensified owing to the geostrophic adjustment. These are verified by the results of numer ical simulation. DOI: 10.1134/S1028334X12050121
The St. Anna deepwater trough, located in the northern part of the Kara Sea, is one of the most dynamically active regions of the Arctic Basin, and it is where water masses of different origins interact [1, 13]. The complicated water circulation regime in this region is determined by the influx of the Norwegian current’s Fram Strait Branch Water from the north, along the western slope of the trench, on the one hand, and by that of the Barents Sea Branch Water, supplied from the Barents Sea through the strait between Cape Zhelaniya and Franz Josef Land, on the other hand [5, 12]. In the southern part of the St. Anna Trough, two branches of the Norwegian current are joined and flowed into the Arctic Basin along the eastern slope of the trench (Fig. 1). Such a circulation regime is veri fied by both the analysis of the thermohaline structure of the region [8, 12, 13] and the numerical simulation results [3, 8]. The only published measurements of the current velocities, made on the eastern slope of the St. Anna
a
Arctic and Antarctic Research Institute, ul. Beringa 38, St. Petersburg, 199397 Russia b Leibniz Institute of Marine Sciences, ChristianAlbrechts University of Kiel, Kiel, Germany c National Oceanography Centre, Southhampton, University of Southhampton, United Kingdom email:
[email protected], idmitrienko@ifmgeomar.de,
[email protected]
Trough utilizing an acoustic profiler, were derived in 1996 [13]. An intensive northward current was found in the surface layer, down to the depth of 240 m, with the velocities of up to 30 cm/s; however, the short period of measurements (about three days) did not allow this information to be considered as reliable with respect to water transport and its temporal variability. Thus, detailed descriptions of the dynamical regime peculiarities in the St. Anna Trough are nearly absent in the literature. In August 2009, in the framework of the “Arktika– Kara–2009” program, a deepwater buoy station (mooring A1), equipped with current velocity record ers, was installed for the first time on the eastern slope of the trench (Fig. 1). The present communication is devoted to analysis of the results derived from the year long measurements made at this station and to identi fication of the principal ways how variability of water dynamics in the St. Anna Trough is formed. This study is based on the current velocity data, derived in the period from August 25, 2009, until Sep tember 22, 2010, utilizing two acoustic Doppler profil ers (RDI), installed at the point with the coordinates 81°02' N and 73°05' E, at a depth of 520 m as a part of an anchored oceanographic complex (Fig. 1). The profilers were placed at the depths of 128 and 370 m and performed everyminute measurements in the depth ranges of 134–238 and 376–472 m with a verti cal discretization of 4 m. The 30min averaged velocity
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components were later corrected with the magnetic declination taken into account. The data of atmospheric reanalysis (http://www. esrl.noaa.gov/psd/data/) were used to reconstruct the speed and direction of the ground wind, which were averaged for the area between 77.5°–82.5° N and 55°–85° E; this allowed us to take the vertical inte grated effect of wind on the surface circulation into account. Additionally, the 6h quantized sea level data from the meteorological station on Vize I. were used (Fig. 1). The current velocity records demonstrate the pre dominance of general northward water transport with a velocity of about 20 cm/s in the whole measurement range. The average annual values of the velocity’s northward component increases from 16 cm/s (at the depth of 134 m) to 22 cm/s (at the depth of 472 m). A similar increase was also noted in [13]: in the layer from 135 to 240 m, the average measured velocity of the northward component was about 28 cm/s, which is comparable to our results with their variability taken into consideration. Relatively small variations in the average velocity by depth and the small value of the eastward component (up to 4 cm/s) allow us to con sider the discussed current as a barotropic one directed mainly along the slope. Despite the presence of the stable northward trans port, the velocity values are characterized by a signifi cant temporal variability. A substantial contribution to the current’s variability is made by variations of several days to a month in period (hereinafter we analyze the northward component as the main one), and the gen eral flux and its fluctuations are homogeneous by depth (Fig. 2). The most likely way that explains the observed synchronous variability of the current is geo strophic adjustment of circulation due to change in the level inclination across the St. Anna Trough. Such an effect can be caused by the Eckman transport or by large scale change in the density field. Unfortunately, the absence of the data on the spatial structure of tem perature and salinity fields during the mooring opera tion period does not allow us to assess the contribution made by thermohaline processes to the dynamical regime, and anemobaric sea level variations due to dynamical effects at a substantial distance from the shallow water zone cannot be considered significant for periods of more than several days [2, 11]. To determine the relationship between the current velocity and the nearsurface wind, we calculated the correlation coefficients between the currents’ series, having different time averaging periods and different wind projections. The result with the highest correla tion factor was chosen as the optimal one. It was found experimentally that the best results are derived if an 11day sliding averaging is used (correlation factor is from +0.44 to +0.50 at different horizons), and the highest correlation factors are related to the wind pro jections in the directions from 34° to 52° (southwest wind). It should be noted that a high level of a statisti
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Fig. 1. The seafloor relief and the temperature distribution at the depth of 150 m based on the oceanographic survey in August 2009. Black points denote the positions of oceano graphic stations; arrows, the general scheme of circulation of Atlanticorigin modified waves; the vertical distribution of temperature along the 81° N section is in the inset. FSBW is the Fram Strait Branch Water, and BSBW is the Bering Sea Branch Water.
cal relationship was found on the basis of the entire length of the current velocity records, including the period of drifting ice presence in the northern part of the Kara Sea (this ice cover can reduce friction stress on the upper boundary of the water stratum by up to 50%) [14]. According to the classical theory of drift currents, stable southwest winds form eastward transport of waters in the surface layer, and this transport can lead to change in the sea level inclination across the St. Anna Trough, hence strengthening the northward geo strophic component of the barotropic flux. Analysis of the temporal variations in the sea level on Vize I. (Fig. 2) verifies the relationship between the variations and the current velocity. The greatest contribution to the high correlation factor (from +0.63 to +0.72) is made by the record portion of about 3.5 months long starting from the beginning of January.
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Fig. 2. Temporal variability in the sea level values at the Vize I. station (a), that of the northeast component of the wind at sea level (b), and that of the north component of current velocity at the depths of 142 m (solid line) and 444 m (dash line) (c). Thick solid lines show their sliding 11day average values.
Taking into consideration that the variations in the sea level based on the data from Vize I. do not reflect the spatial pattern of the level variations in the moor ing place due to the influence of local conditions, and in the absence of satellite altimetry data, we verified the suggested qualitative and quantitative relationship between the dynamical effect of wind, the change in the sea level inclination, and the strengthening of the alongslope current with the help of model calcula tions. The NEMO global zlevel oceanic model [10] was used; its high spatial resolution (about 12 km) allows us to simulate the main part of the ocean turbu lence spectrum. The model calculations were made for the period of 1958–2007 with the use of nearsurface wind, temperature, and humidity fields, and the short and longwave solar radiation field as well (all these data were derived from the oceanic reanalysis [9]).
records at the mooring (9–16 cm/s). Simultaneously with the increase in northward transport velocity from northward winds, the model shows formation of the sea level anomaly’s gradient from west to east; how ever, the model variations in the level in the area of Vize I. (~10 cm) are substantially lower than the real ones (Fig. 2), which is related to the insufficiently high resolution of the model.
The calculations showed the principal agreement between the suggested pattern of change of the sea level surface and the current velocity field depending on southwest winds. The differences between the aver age sea level values and current velocities at the depth of 150 m were calculated; velocities were derived sep arately for the periods with predominant southwest winds and those with northeast ones (Fig. 3). To deter mine the average values, we used only the episodes when the averaged wind speed for the 11day period was beyond one standard deviation from the average projection to the northeast. According to the model calculations, the change in the meridional transport velocity in the depth range from 134 to 472 m at the point of mooring placement was about 4–5 cm/s, that is, slightly less than the values derived based on the
The studies implemented allowed the following conclusions to be made. The direct observations over the currents on the eastern slope of the St. Anna deep water trough in the layer of 134 to 472 m, made in the period from August 2009 until September 2010, allowed us to detect the stable northward barotropic transport with an average annual velocity of about 20 cm/s. It was found that the observed variability of the barotropic flux event under the conditions of nearly constant presence of drifting ice cover is closely related to longperiod disturbances of the largescale field of ground wind above the northern parts of the Barents and Kara seas. This effect is related to the eastward transport of surface waters by the southwest and west winds, which leads to formation of the sea level’s gradient across the St. Anna Trough. Geo
In the mooring placement point, the sea level anomaly’s gradient is about 6 mm per 10 km from west to east (Fig. 3). Estimation of the change in the merid ional transport velocity at these sea level variations under the geostrophic approximation conditions yields the value of ~4 cm/s, which conforms well with the estimate derived above and allows us to establish a direct relationship between the sea level variations and strengthening of the current along the shelf slope.
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ACKNOWLEDGMENTS We thank I.V. Polyakov (International Arctic Research Center, University of Alaska Fairbanks, Fair banks, United States), T. Klagge and Ch. Kassens (Leib niz Institute of Marine Sciences (IFMGEOMAR) at the ChristianAlbrechts University of Kiel, Kiel, Ger many), and J. Hülemann and U. Schauer (Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany) for help in organization and implementation of the expedition studies, and I. M. Ashik (Arctic and Antarctic Research Institute, St. Petersburg, Russia) for the sea level data from Vize I.
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Fig. 3. The difference between the sea levels (cm) and the vector difference of current velocities (cm/s) at the depth of 150 m, derived based on the model calculations for the southwestward and northeastward winds. The differences values lower than 2 cm/s are not shown.
strophic adjustment intensifies the northward meridi onal water transport, and this is verified by the numer ical simulation results. Thus, the changes in the atmo spheric circulation regime of the northern parts of the Barents and Kara seas influences the intensity of water exchange between the St. Anna Trough and the deep water part of the Arctic Basin.
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