Oceanographic regime 1 page

This chapter expands upon the particular oceanographic regime of the White Sea. Previous observations have shown a salient feature of the White Sea thermo- dynamics is a permanent influence of strong ebb and tide currents originating in the Barents Sea. The sea level variations arising from this phenomenon may attain several meters, thereby significantly reducing the geostrophic balance there. In addition to these strong water movements, there is a wide spectrum of other motions such as Poincare and Kelvin waves, as well as shelf zone movements. Despite a long history of studies of the White Sea thermodynamics, there remain many uncertainties (e.g., the water exchange between the White and Barents Seas have not yet been investigated sufficiently, nor have the hydrodynamics in the largest bays of the White Sea). This chapter focuses on the White Sea hydrophysics and hydrodynamics as revealed primarily by multi-year field observations. These obser- vations are of great value per se as well as for the interpretation of remote sensing (Chapter 6) and numerical modeling of the hydrodynamics and marine ecosystem (Chapter 10).

Section 4.1 characterizes the optical properties of the White Sea. The currents and circulation features of the sea are described in Section 4.2. Section 4.3 describes the water masses and exchanges with the Barents Sea. Fronts and frontal zones in the White Sea are discussed in Section 4.4. Section 4.5 characterizes the variability of water temperature and circulation across a range of timescales. Section 4.6 is a modeling study of the currents and transport in the bays and estuaries of the White Sea. Sea level variability and tides are presented in Section 4.7. Finally, Section 4.8 describes the White Sea ice regime and winter hydrology.

4.1 TRANSPARENCY AND OPTICAL CHARACTERISTICS

Contemporary, comprehensive oceanographic studies of spatial and temporal dis- tributions of suspended and dissolved substances in the White Sea began in 1995

Table 4.1. Monthly mean values of variables characterizing the light climate and water temperature in the White Sea.

Leonov et al. (2004).

T -1.3 -1.7 -1.7 -1.4 0.2 3.5 6.7 8.4 7.6 4.7 1.8 -0.2

Voronka

SD 9.0 9.0 9.0 8.0 7.0 6.0 6.5 7.0 7.5 8.0 8.5 9.0

SD = Secchi Depth (m); E = diurnal incident irradiance (cal cm-2day-1); T = water temperature (o C).

(e.g., Dolotov et al., 2002), although the pioneering works in this area date back to 1964-1966 (e.g., Medvedev, 1972).

The water transparency (characterized by Secchi Depth, SD) has high seasonal variability throughout the sea (Table 4.1). This is controlled by both the input of suspended matter and dissolved organics from river discharge and land runoff. The

Sec. 4.1]

Transparency and optical characteristics 75

The transparency of the open waters of the White Sea reaches on average a value of 7-8 m, whereas in the regions conterminal with river deltas, it drops to 2-3 m and may be even as low as 1 m (Table 4.1). The Gorlo and Voronka waters are typically bluish-green, with blue waters at the boundary separating the White and Barents Seas. This is naturally explained by the remoteness of these regions from prolific sources of terrigenous suspended matter.

Lisytsyn et al. (2003) have shown that the suspended matter concentration (Csm, mg l-1) can be related to the light attenuation coefficient c(m-1) through a regression expression like:

Csm= A · c-B (4.1)

where A and B are regression coefficients. Their actual values were determined empirically, respectively, at 0.62 and 0.13 in 2001, and 0.90 and 0.01 in 2002. These data point to very substantial interannual and seasonal variability in the input of suspended matter into the White Sea.

From their measurements of remote sensing reflectance at 550 nm, Bourenkov et al. (2001) have established a regression parameterization relating the content of suspended matter with the coefficient of backscattering bbp(m-1):

Csm = 73.5bbp(550)+ 0.016 (4.2)

A suite of dedicated measurements has indicated that the so-called marginal filter mechanism (Lisytsyn, 1994) is also active in the White Sea. The prevailing part of the suspended matter settles down in the vicinity of river mouths within the isohalines ranging between 0 and 200. Such natural marginal filters are known to be of a cascade type with a consecutive passage from a gravitational to a physico- chemical and eventually to a biological precipitating component.

Qualitative and quantitative studies on spatial and temporal variations in dissolved and suspended organic matter are important not solely for revealing the processes controlling the life cycles of a marine ecosystem, but also for establishing the hydro-optical characteristics and eventually the aquatic-medium light climate (Pozdnyakov and Grassl, 2003).

Because the White Sea is essentially a mediterranean sea, almost completely encompassed by land, it is greatly impacted by organic matter sources located within the catchment. Hence, the levels of dissolved organic matter (dom) in the White Sea prove to be fairly high: generally, the gross concentrations of organic matter in the White Sea vary between 2.5 and 8.0 mgC l-1.

The spatial (both vertical and horizontal) distribution of dom is essentially controlled by the characteristic water movement patterns. In summer, within the Dvinskiy Bay the highest concentrations of dom (about 12 mgC l-1) are found along the Zimniy coast. The dom concentrations in the area of the Letniy coast vary between 5 and 11 mgC l-1(Agatova et al., 1994). Importantly, the concentra- tion of dom in the upper layers increase with the distance from the coast, whereas

near the bottom they are highest in the vicinity of the coastline. In the central part of the Dvinskiy Bay, the dom concentrations decline to about 3-4 mgC l-1, which is conditioned by the salinity front generated therein at that time of year. Within the shallow area of the Onega River estuary, the content of dom is nearly homogenous vertically. The highest amounts of dom (7-8 mgC l-1) are reported from the area of the Onega River mouth. In the offshore direction, the content of dom decreases to about 4 mgC l-1. In Kandalakshskiy Bay, the lowest concentrations of dom (3.5mgC l-1) are found in the shallow areas, but in deeper waters it increases up to 5-6 mgC l-1. Mezenskiy Bay is the least loaded with dom (ca 1.5mgC l-1), owing to the highly pronounced influence of the Barents Sea waters.

Generally, it can be emphasized that the concentrations of both suspended and dissolved organic matter are determined by the primary production level, strong runoff, and water exchange rate between the White Sea and the Barents Sea within the Gorlo area. It is exactly in this region that the major transformation of the Barents waters takes place within the upper 30-m layer. Relatively dom- impoverished waters of the Barents Sea become mixed in the region of the Gorlo with the waters of the White Sea appreciably loaded with the organic matter.

Importantly, there are numerous observations indicating that the content of dom

peaks nearly invariably just beneath the water surface throughout the entire sea.

Filters with pores as small as 0.2 µm do not extract the entire suspended organic matter in the samples of the White Sea water. This is a strong indication of an enhanced abundance of extremely small picophytoplankton. According to the results reported by Ilyash (1998), the picoplankton biomass in the first 10-day period of June (when the surface water temperature was about 6-7oC) reached a value of 1.2 gC m-2, the primary production rate was assessed at 0.1 gC m-2day-1. The number counts were as high as 5 x 107cm-3. In July, the picoplankton biomass increased to ca 170 mgC m-2.

These data are of critical importance for the interpretation of optical remote sensing data. Indeed, for comparing the water quality retrievals from satellite data with in situ measurements, the chlorophyll concentration is routinely determined using filters with larger pores. Therefore, in light of the above, the retrieved chlor- ophyll concentrations can be expected to be higher than the in situ measured ones. Returning to Table 4.1, it can be seen that the levels of water surface irradiance remain fairly low until April. It implies that in early spring, the light climate in the waters of the White Sea is such that the indigenous phytoplankton community should be rather sparse even given the species developing under the ice cover and in fairly cold waters (Berger et al., 2001). However, in April, while the sea is still ice- covered, the incoming solar radiation increases stepwise. In conjunction with the highest content of nutrients (accumulated over the wintertime), such conditions are conducive to the initial burst of phytoplankton growth/spring bloom. It is worth- while pointing out that nearly all early spring algae species first develop in the ice before they begin thriving in the water column. The phenomenon of extensive growth of sea ice algae in the Arctic Basin has been known since the early 1980s

(Horner, 1985).

As the surface water temperature increases and the White Sea gradually becomes

ice-free, the spring bloom occurs. It starts from the south-western part of the sea and propagates further to its north-eastern confines. This is reflected in a pronounced reduction in water transparency during the period April-May (Table 4.1). At the beginning of June, the algae bloom begins to decline giving place to the summer algae complex (July-mid-August). The autumnal algae abundance renaissance occurs in the middle of August. The growth period of algae in the White Sea terminates in late October, which is prompted by a depletion of nutrients and decline in the available solar radiation (Table 4.1), and a subsequent increase in SD. Together with the seasonal influxes of terrigenous matter (both of mineralogical and soil humus nature) due to river discharge and land runoff, the above-discussed seasonal variations in the abundance of algae determines the seasonal variations in the optical properties of the White Sea. As exemplified in Chapter 6 (Section 6.1.2), the aforementioned sequence in transitions of the White Sea hydro-optical conditions from one state to another is highly consequential in terms of choosing the right methodology of remote surveillance of the dynamics of water quality

parameters.

Indeed, the above data (high concentrations of dom, appreciable amounts of phytoplankton, inputs of suspended minerals) clearly indicate that the White Sea waters qualify as case II waters (A. Morel s classification, see Chapter 6 (Section 6.1.1)) (i.e., optically complex waters requiring other retrieval approaches than those widely used for monitoring open waters of the world oceans and marginal seas).

4.2 CIRCULATION PATTERNS AND CURRENTS IN THE SEA

There is a long history of investigations of the oceanography of the White Sea. Among the first workers who studied the inherent oceanographic features of the sea was M.F. Reinecke (1850). In the 1890s a prominent Russian scientist N.M. Knipovich studied the hydrological regime of water circulation in the White Sea and offered a concept of a two-layer nature of the White Sea waters. In the early 20th century, another Russian worker, K.M. Deriugin (1928), determined the White Sea boundaries and suggested a partitioning of the sea area into individual regions. He considered the southern confine of the Gorlo to be the White Sea boundary and did not incorporate the Gorlo and Voronka per se into the White Sea area.

The Russian scientists V.A. Beriozkin (1947) and V.V. Timonov (1950), who continued Deriugin s investigations, showed that the deep waters of the Bassein are actually the water masses formed in the Gorlo during the winter period. The Gorlo is where the flows of the feeding current coming from the Barents Sea and those of another, runoff current, are mixed with fresher waters of the White Sea proper (Figure 4.1).

The aforementioned currents were later named after the two scientists as the Deriugin and Timonov Currents (Figure 4.2). The intensive tidal mixing in the Gorlo mixes huge water masses coming from both the Barents and White Seas. As a result, the water temperature and salinity in the Gorlo are almost the same from the surface to the bottom, and a front is formed at the boundary with the Bassein. In summer,

Figure 4.1. Patterns of major currents in the White Sea.

From Timonov (1950).

the Timonov Current is rather strong and becomes an obstacle for the penetration of the Gorlo waters into the Bassein. In winter, the river runoff decreases, and the Timonov Current becomes somewhat weaker, then the water masses from the Gorlo get into the Bassein. In winter, the water masses of the Gorlo acquire negative temperatures and, being heavier, stream down to greater depths of the Bassein. Naumov and Fedyakov (1993), having summed up a wealth of available results, suggested a system of patterns of the White Sea currents shown in Figure 4.2. The general water circulation in the White Sea is determined by the river runoff, atmospheric processes, water exchange with the Barents Sea, the influence of ebb and

tide movements, and relief inhomogenieties of the bottom and shores.

A system of circulation patterns compiled according to the results obtained on board the research vessel (R/V) Ivan Petrov during the summer of 1991, is presented

Figure 4.2. Major water circulation patterns as established by Deriugin and Timonov.

From Naumov and Fedyakov (1993).

in Figure 4.3 (Sapozhnikov, 1994). Figure 4.3 displays the presence of a persistent current along the Zimniy coast caused by both the runoff of the Dvina River and the cyclonic circulation in Kandalakshskiy and Onezhskiy Bays. The basic difference is the existence of an anticyclonic circulation in the Bassein. Near the bottom, a current feeding to the Gorlo brings in saline water from the Barents Sea. It exhibits a distinct seasonal variability (e.g., in March the flow leaving the Gorlo turns westwards to Kandalakshskiy Bay, whereas in summer the main flow moves to the Dvinskiy Bay). This is due to the seasonal variability in the runoff of the Severnaya Dvina River, which peaks in May-June and subsides in February-March. The runoff current along the Zimniy coast and the anticyclonic eddy near the Letniy coast in July are equally displayed in Figure 4.3. It should also be noted that the current coming out from the Dvinskiy Bay subdivides into two branches in summer, the first of which heads to the Gorlo along the Zimniy coast, whereas the second reaches the Bassein and becomes entrained there into a cyclonic movement. The bottom currents in March and July are compensatory by nature (i.e., they have a direction opposite to the surface current). This is especially well pronounced in the Dvinskiy and Onezhskiy Bays, eastwards off the Solovetskiy Archipelago.

Figure 4.3. Water circulation patterns in the White Sea as established from on board the R/V

Ivan Petrov.

From Sapozhnikov (1994).

Based on the results of studies accomplished by a well-known Russian hydrol- ogist V.A. Babkov, a system of sea currents has been confidently established. It has much in common with the current patterns reported by Naumov and Fedyakov (1993) and Timonov (Figure 4.4). The similarities include, among other features, the presence of feeding and runoff currents, and cyclonic and anticyclonic systems in the Bassein.

One of the important factors determining the formation of water circulation patterns in the White Sea is the tidal effect. Semi-diurnal and diurnal tides prevail there. Ebb and tide-driven water movements certainly affect the water exchange between the White and Barents Seas. It is well known (Semenov and Luneva, 1999) that if tidal motions are taken into consideration, the inflow of the Barents Sea waters becomes less than if they are neglected. This is quite natural because the formation of the tidal bottom-boundary layer leads to a substantial increase in the energy dissipation (see Chapter 8).

During the high-water period, the currents are directed from the Gorlo to the Bassein. There is an outflow from Onezhskiy Bay to the Bassein. The coastal zone circulation in Dvinskiy Bay is cyclonic. At the same time, in the Bassein a tendency towards a cyclonic movement is apparent. The speeds of permanent currents in the sea are 10-15cm s-1, in narrows and near capes they are higher (30-40 cm s-1). In the Gorlo and Mezenskiy Bay they reach 250 cm s-1. In Onezhskiy and Kanda-

Figure 4.4. System of persistent surface currents in the White Sea.

From Babkov (1998).

lakshskiy Bays, the speeds are 80-100 cm s-1and 30-35cm s-1, respectively. In the Bassein, the speeds of tidal currents are generally rather low and comparable with the speeds of persistent currents (Dobrovolsky and Zalogin, 1965).

However, the above-mentioned systems of circulation patterns have been based on indirect observations of such variables as water temperature, water salinity, and some chemical and biological parameters. Measurements have been made irregularly from on board scientific research vessels and autonomous buoy stations. So far, there have not been enough long-term (e.g., several months) observations performed by autonomous buoy stations to allow compilation of water movement maps. For example, no such observations have been made in the Voronka and Gorlo to assess water exchanges between the Voronka and the Barents Sea, or between the Gorlo and the Bassein. Therefore, in order to study the currents and water exchange between the White and Barents Seas on an extensive timescale, some mathematical models have been developed (Elisov, 1997; Semenov and Luneva, 1999; Neelov and Savchuk, 2003).

4.3 WATER MASSES AND WATER EXCHANGE WITH THE BARENTS SEA

The water masses forming in the White Sea originate from saline waters coming from the Barents Sea and riverine freshwater. Mixing of fresh and saline water results in the generation of specific marine water masses. Purely freshwater can only be found in the apexes of bays. The Barents Sea saline water retains its identity in the Voronka. The Gorlo accommodates a vertically homogeneous water mass, which is a consequence of intensive turbulent mixing in this part of the White Sea.

The water masses residing in the Bassein have always been a subject of great interest to researchers. As far back as 1899, N.M. Knipovich studied the zone of a constant negative temperature in the deepest parts of the White Sea. He suggested that the water masses residing in the Bassein originated from waters with high salinity inherent in the Barents Sea. Having been cooled to a freezing point in winter, these waters move through the Gorlo towards deep regions of the Bassein. According to the data reported by K.M. Deriugin, the waters with a temperature of

-1.4oC and salinity of 300 are relevant to the central deep trough, which extends from the meridian running through Zhizhgin Island to the Sredniye Ludy region. During the warm period of the year, the water temperature below -1oC is already observed at a depth of about 50 m. K.M. Deriugin found that deep waters in the White Sea are not subject to stagnation, owing to which the living organisms indigenous to deep, cold waters find favorable conditions for themselves even in

... the deep pit of Kandalakshskiy Bay . Later K.M. Deriugin put forward a hypothesis of aeration of deep waters due to the horizontal currents coming to the Gorlo from the Barents Sea. Timonov (1950) found that in late winter, over the entire central part of the Bassein, at depths in excess of 50-60 m, there was a very thin intermediate warm layer, which was not subject to convection or any other kinds of mixing. Proceeding from this finding, he conjectured that appreciable ventilation of the deep layers took place due to the horizontal currents arriving from the Gorlo. Beklemishev et al. (1975) contributed substantially to the investiga- tion of the White Sea water masses by employing temperature-salinity (T -S) analysis to differentiate between waters having different properties. The latter authors distinguished three kinds of water masses (Figure 4.5).

In winter, the deep water mass is affinitive with the cold Arctic layer. The intermediate and surface water masses forming in the Gorlo are attributable to a moderate water mass, the genesis of which is related to thermo-hydrodynamic processes in the Gorlo.

In summer, the intermediate water mass is equally formed by the waters of the Gorlo when the water salinity is lower and the temperature is higher than they are in winter.

Based on his shipborne hydrological data, A.I. Babkov (1998) reports that in the open part of Dvinskiy Bay, at the boundary separating the bay from the Bassein, a salinity front has been found, most clearly pronounced in the surface layer. This front is characteristic of a combination of a relatively strong current and dynamic instability, which leads to the generation of mesoscale eddies on spatial scales

Figure 4.5. Types of water masses in the White Sea as they are displayed on (a) the White Sea transect, and (b) the T -S diagram.

(b) From Beklemishev et al. (1975).

Notation: I = surface water mass, II = intermediate water mass, III = deep water mass.

ranging from a few kilometers to tens of kilometers. In the central part of the bay, cyclonic eddies were observed, due to the formation there of a large-scale water circulation arising from cyclonic rotation. In the Bassein, owing to the cyclonic nature of the inherent currents, the so-called pole of cold was observed (Deriugin, 1928). Eastward from the cyclonic eddy, an anticyclonic eddy or the

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