Oceanographic regime 3 page

Lisytsyn et al. (2003) found that, on average, the concentration of suspended matter decreases by a factor of 5,000 in the immediate proximity to the outer boundary of river mouths. The river mouth boundary is the area of transition of hydrocarbonated water type to the chloride type, with a water salinity of about

0.50. In some areas of mixing sub-zones, the suspended matter concentration appears to be higher than in the river and the external sub-zone, and a so-called

mud jam forms. As Lisytsyn et al. (2003) state: under the impact of a mechanism of electrolytic solution in seawater, the coagulation of fine suspended matter - both mineral and organic - occurs. The flakes or floccules of estuary snow are formed, which often have a triple composition: clay particles glued by organic matter and hydroxides of iron. Suspended particulate matter takes up heavy metals, pollutants and nutrients.

As a result, in such areas the water transparency and photic layer vertical extent increase in the offshore direction, and an area of highest phytoplankton productivity is formed. This area is also called biological jam which is located on the offshore side of the mud jam .

In this zone, somewhat distanced from the river, dissolved chemical compounds and gases (e.g., carbon dioxide) are transformed into suspended matter. Moreover, the content of phytoplankton and the rate of primary production are much greater there than they are in the river. It is there that the dissolved forms of various substances (including pollutants), due to coagulation, turn into suspended matter, and eventually become bottom sediments. Lisytsyn (1994) calls this process bio- pumping of the first kind (phytoplanktonic).

In addition, crayfish (mainly copepods) are generally able to filter the entire volume of a marine estuary during 1-1.5 days. Plankton filtrators not only uptake the fine particles from the water, but also accumulate and transform them through compressing them into fecal conglomerates called pellets. Due to their relatively large size, pellets swiftly sink to the bottom (at a speed of 100-500 cm day-1).

However, marginal filters have not yet been thoroughly studied in seas with strong tides, such as the White Sea, for instance. That is why the term marginal filter is most appropriate from the viewpoint of the essence of processes taking part in the outlying regions of the continents.

At the river-sea barrier in Kandalakshskiy Bay, in the area of mixing of riverine and marine waters, the concentration of suspended matter nearly does not decrease as the water salinity rises from 0 to 200 - in fact it even slightly increases (Shevchenko et al., 2002). Therefore, the classical concept that the content of suspended matter, phytoplankton, and dom must gradually decrease in the offshore direction should be reviewed. The results obtained at the Northern Water Problems Institute (NWPI) in 2000-2001 from in situ (shipborne) measurements and satellite remote sensing data on chlorophyll-a explicitly indicate that there is a considerable inhomogeneity in spatial distributions of water quality parameters. Within the areas of a phytoplankton bloom, the highest concentrations of suspended mater (up to 5mg l-1) have been recorded in Onezhskiy Bay, whereas in shallow and warm Dvinskiy Bay, relatively low concentrations of this constituent were observed (1 mg l-1). In Kandalakshskiy Bay, the concentration of suspended matter was even lower.

4.5 VARIABILITY OF WATER TEMPERATURE AND CURRENTS

4.5.1 Large-scale variability

As mentioned before, pilot studies of water temperature variability began back in the 1920s and were continued by Beriozkin (1947) and Timonov (1950). In the analysis, data from diurnal and multi-diurnal station observations accomplished in the Gorlo and at the Solovkiy Archipelago were used. During the tidal cycle for the period June-September, the amplitude of water temperature variations was, on average, 0.5-0.6oC, occasionally reaching 2.3oC. Obviously, water temperature variations depend not only upon the tidal current speeds. This is confirmed by the contem- porary data from in situ observations. In the Gorlo, near Sosnovets Island and the

Intsy lighthouse, the amplitude of water temperature within the tidal cycle constituted between 0.1 and 0.8oC. The amplitude of water temperature fluctuations within the tidal cycle grows from May to August and declines from September to November, becoming practically undetectable during the cold season.

Analyses of observational data for the period up to the 1990s has made it possible to establish that linear trends for all stations involved indicate a decrease in water temperature and an increase in salinity. It implies that in the White Sea 11- year synchronous variations of these variables prevail. The data from deep-water stations have revealed large-scale processes that mainly influence the variability of the T -S fields in the vicinity of frontal zones. The impact of these processes increases downwards, and at the bottom it accounts for about 50% of T and S variability (Table 4.1). Thus, this finding indicates that T -S fields at the bottom reveal most reliably the climatic variability in the White Sea Basin.

Spectral analysis of temporal variations in the observational series reveal fluc- tuations with 11 and 18-year periods for temperature and 19-20-year periods for salinity (Berger et al., 2001). As a rule, quasi-cyclic variations of T and S in deep water occurs non-synchronously. Thus, although during 1945-1951 the increase in water salinity corresponded to a decrease in water temperature, a much more sig- nificant decrease of salinity in 1953-1963 took place against the background of higher water temperatures. This finding makes it possible to conclude that above the bottom, cold and saline waters are replaced cyclically by relatively warm and less saline waters.

Deep water can be formed during a short time period (about one month) after the polar anticyclone arrives at the White Sea Basin. The lack of a significant relationship between water temperature and the sum of days with the temperature below 0oC further confirms this conclusion (Berger et al., 2001). Meridional winds enhance the water exchange between the Barents and White Seas and intensify the inflow of high-salinity waters to the White Sea. In severe winters, as a rule, a decrease in water temperature at depth proves to be conjugated with a water salinity increase. However, the aforementioned changes are 4-5 times less significant than those caused by the interannual and quasi-cyclic variations discussed earlier. In the case of the N-type atmospheric circulation (see Chapter 3), a temperature decrease in the deep water occurs. The onset of the W -type circulation results in an increase of the deep-water temperature. The E-type atmospheric circulation results in a decline of deep-water salinity, whereas in the case of the N-type with predominant

winds the deep-water salinity becomes enhanced.

Now we shall briefly focus upon some specific features of large-scale variability in the hydrometeorological parameters pertaining to the White Sea. The earlier published data on the marine hydrological regime do not reflect any more the sea state during the last 5-7 years. But it is just during these last years that very pronounced changes in climatic variables were observed. Smirnova et al. (2001) indicate that both the variability in the thermohaline regime as well as sea level variations are generally indicative of a significant contribution of low-frequency components to the interannual variability. This also attests to the impact of global climatic processes upon the formation of long-term variability in the marine

Figure 4.13. Location of stations at which water temperature is routinely measured in the White Sea.

parameters. Up to the mid-1990s, such parameters as water temperature, salinity, and sea level exhibited a clear tendency to increase.

This is further confirmed by the latest data. To study seasonal and multi-annual variabilities of water surface temperature from the contemporary (1977-1999) data, we employed observational time series with a 3-hour sampling interval at the following stations: Kandalaksha, Gridino, Zhuzhmuy, Intsy, Chavanga, and Solovki. In addition, we also used decadal data from Chupinskiy Bay (point D-1) at six depths: 0 m, 10 m, 15m, 25m, 50 m, and 65m for the 1961-1999 period. In the analyses of air temperature variability, we have used the data obtained from the State Meteorological Network (SMN) stations located along the White Sea coast (there are stations and posts operating under the Federal Service of the Russian Federation on Hydrometeorology and Environmental Monitoring, viz. Kestenga, Engozero, Loukhi, Archangelsk, Onega, Kem-Port, Zasheyek, Kolezhma, Gridino, Niukhcha, Sheyeretskoye, and Muyezerka, Figures 3.1 and 4.13).

Figure 4.14 displays the temporal variations of the mean annual water surface temperature as revealed from the data collected at the stations along the Karelskiy coast. A very weak tendency towards warming (0.1oC per 60 years) is observed. Figure 4.15 and Table 4.3 illustrate the water temperature variability in the White Sea based on the mean annual data from six main stations located in various parts of the White Sea.

Figure 4.14. Variability of the water surface temperature in the White Sea according to the mean annual data from six stations from 1977-1998.

Figure 4.15. Surface temperature variations as revealed from the data of a hydrological survey in June-July 1991.

From Sapozhnikov (1994).

The only continuous water temperature series was that obtained from the Intsy station; therefore, it considered a baseline series. The coefficient of correlation (r) between all water surface temperature values varies from 0.90 to 0.98. From the analyses, the following conclusions can be drawn for individual hydrometeorological stations:

1 Gridino. The mean long-term value for the period is 3.69oC. A graphical pre- sentation of mean monthly water temperature variations reveals most distinctly a seasonal component, with a maximum of 17oC in 1997 and a minimum of 12oC in 1993. The maximum mean annual water temperature was observed in 1989 constituting 4.51oC, the minimum one was recorded in 1978 being as low as 3.15oC. On the whole, a minor increasing trend (0.5oC per period of observation) is evident.

2 Solovki. The mean long-term value is 3.68oC. The maxima values were recorded in 1989 (15.5oC) and in 1997 (17oC), the minimum value occurred in 1993 (13oC). The maximum mean annual water temperature (4.51oC) was recorded in 1989, the minimum one (3.14oC) in 1978. The plots of mean annual tempera- tures at the Gridino and Solovki stations practically coincide, pointing to the fact that their locations are in the same thermal region. The mean monthly water temperature variations recorded at the Solovki station also reveals a minor positive trend (+0.5oC).

3 Zhuzhmuy. The mean long-term value is 3.40oC, which is somewhat lower than the ones obtained at the first two stations. The maximum amplitude, recorded in 1984 and 1990 is 15oC, the minimum one, recorded in 1988, is 12oC. The maximum mean annual water temperature (4.45oC) was also recorded in 1989, and the minimum (2.62oC) occurred in 1993.

4 Intsy. The mean long-term value is 3.04oC, the lowest one in comparison with the other stations. The maximum amplitude (15oC) was recorded in 1990, and the minimum one (11oC) occurred in 1979. The maximum mean annual tem- perature (4.58oC) was recorded in 1989. The minimum temperature (2.29oC) was recorded in 1978. Owing to the higher latitude of the location of the Intsy station, the temperatures recorded there are generally much lower, but the annual variations and the entire 20-year variation is much smoother, with smaller amplitudes. Interestingly, this station exhibits the highest increasing trend, viz. nearly 1oC per 20 years. It can be assumed that the Severnaya Dvina River produces a warming and weather smoothing effect upon the region. It is known that the boundary of the gradient zone propagating from the south-west to the north-east across the Dvinskiy Bay and coming close to the Zimniy coast passes there. Therefore, the warmer river water is brought into the Gorlo with the currents.

5 Chavanga. The long-term mean value is 3.53oC. The maximum amplitude recorded in 1980 and 1981, constituted nearly 15oC; the minimum value (9.5 to 11oC) occurs with a period of 4-5 years, and was observed in 1979, 1983, 1988, 1993, and 1997. The temperature trend is hardly noticeable and does not exceed 0.2oC per 30 years. This station is within the zone of currents bringing the

Barents Sea water into the White Sea. The maximum mean annual temperature (4.49oC) was recorded in 1989, the minimum one (3.08oC) occurred in 1979.

6 Kandalaksha. The mean long-term value (maximum) is 3.87oC. The highest mean annual temperature (4.59oC) was recorded in 1989 and the minimum (3.37oC) was reported in 1977. The mean annual temperature variations here are rather smooth, and the water surface temperatures are the highest. The temperature increase may be due to the location of the station in the inner part of Kandalakshskiy Bay, most isolated from the Bassein. The water temperature there is under 0.1oC per 30 years.

Thus, a positive temperature trend is observed to some extent at all stations. Maximum values were recorded at all stations in 1989, which may be due to the regional manifestations of gradually strengthening western transport which had started in the mid-1970s and ended in the 1990s (Smirnova et al., 2001). These atmospheric circulation changes resulted in both the increase in freshwater runoff and the higher water level in the White Sea. The lowest surface water temperatures were recorded in the late 1970s, mid-1980s, and in 1993.

The probability analysis of the time series observations of air temperature, water surface temperature and salinity in the White Sea has led us to the following con- clusions. Seasonal variability prevails in temporal variations. Variables with time- scales of the order of 4-5 years stand out (Figure 4.14). Similar cyclic recurrences are revealed in the water temperature time series reported from the North Atlantic Ocean. The highest water temperatures were observed in the 1970s and 1990s, while the lowest ones occurred in the mid-1960s. The highest positive trend is observed at the Intsy station. The total water temperature rise during the period is in fair agreement with both the tendencies in the world s oceans water temperature variations and with the water surface temperature variability in the Barents Sea.

Water salinity in the White Sea is subject to seasonal and interannual variations. The observation data obtained at the Chupa station clearly indicates that during the last 40 years the water salinity somewhat increased in this region. The range of seasonal water salinity variations in Chupa Bay is between 0 and 280. Throughout a year, the water salinity variations near the bottom in Chupa Bay are minor in comparison with those at the surface. In addition, a fairly steady halocline hampers wind-induced mixing of water in the bay. There are several causal mechanisms of water temperature and salinity increase in this region of the sea, such as a highly pronounced variability of water exchange with the Barents Sea, unsteadiness of seawater circulation due to large-scale climatic processes, and variations in the fresh- water runoff flow into the White Sea. Nevertheless, our investigations have shown that the river runoff in the region varies albeit rather insignificantly (less than 10% per 30 years).

4.5.2 Mesoscale and synoptic variability

Most detailed water temperature and salinity spatial distributions have been obtained from the shipborne expeditions conducted by the Archangelsk Branch of Roshydromet (SEVGIDROMET) and the All-Russian Institute of Fishery Research

Time

Figure 4.16. Variability of both the water temperature near the bottom and sea level near the Kuzovskie Islands (12-15 July, 2000).

(VNIRO). A considerable inhomogeneity of the hydrological fields in the White Sea has been found. When compared to the data displayed in Figure 4.16, the data in question revealed the seawater parameters inhomogeneities on synoptic scales and mesoscales. The presence of some circulation movements and eddy-related inhomo- geneities on these scales are illustrated in Figure 4.1.

However, some of these mesoscale inhomogeneities may be due to the quasi- synchronous nature of the surveys performed during two synoptic periods. When the surveillance takes so much time, the effect of variable winds and tidal motions can be quite significant. Nevertheless, as is shown in Chapter 6, eddies and circulation patterns on such scales can certainly be realistic.

In the following, we consider the variability features of hydrological processes and fields in individual bays of the White Sea. Analyses of observations accom- plished in the Chupa, Kem, and Onezhskiy Bays have revealed certain regular features.

In Chupa Bay, a pronounced stratification of waters in the form of two separate layers has been revealed: there is an upper layer with a maximum water temperature and minimum salinity, and a lower one with a minimum water temperature and maximum salinity. Therefore, in compliance with the Pritchard classification, this region can be ascribed to the type of estuaries with a halocline. In the lower, deep layer, the minimum water temperatures (up to 1.5oC) and rather stable salinity (up to 26.0-26.50) were recorded across the entire bay. A strong mixing of the water column in the upper layer has been found in a most dynamic part of the region, namely, at the Bolshoy Keretsky roadstead. This phenomenon is further substan- tiated by a pronounced homogeneity of water temperature and salinity spatial distributions. As it was anticipated, an uppermost freshened water layer (from the surface to a depth of 3-5m) was observed closer to the estuary where the water salinity varied from 20.0 to 23.50.

During the field campaigns conducted in the estuary of the Kem River (Dolotov et al., 2002) during the time interval July-September, 2000-2003, the water surface temperature varied from 9.7 to 15.9oC, and at depth, closer to the bottom, it fluctuated from 8.7 to 11.6oC. Depths in this region do not exceed 18 m, and therefore at most stations the thermal stratification was weak. The highest tempera- ture was observed near the estuary of the Kem River. This is due to propagation of relatively warm river waters and an intensive insolation of shallow waters. As the distance from the estuary grew, the water temperature gradually decreased. The propagation of river water over the aquatic area of the estuary influenced the distribution of water salinity. Its lowest values were recorded near the estuary at the water surface and were in the range 8.1-170. The deeper the water layer, the higher was the water salinity: in near-bottom layers it varied between 18 and

260. At a number of stations a sharp halocline up to 2.5m thick was observed in the upper layer. For instance, in the 1-4-m layer, the difference in water salinity reached 100. Offshore, the salinity in the water surface layer increased to 26.20, and at some stations a reverse salinity stratification of waters was observed, with the difference between the surface and the bottom layers sometimes reaching 1.20.

The formation of water temperature and salinity fields in Onezhskiy Bay, and their spatial distributions, are controlled by the morphological features of the bay, inherent water dynamics, river runoff, and water exchange rates with the White Sea. The interaction between the bay water with the freshwater river runoff and inflow of relatively salty water from the White Sea leads to the formation and preservation of significant horizontal gradients of water temperature and salinity. Under the influence of both quasi-permanent currents, water transport over the bay took place, and as a result of strong ebb and tidal currents, and subsequent transforma- tion of the tidal waves in shallow waters, intensive water mixing occurs. During summer (notably in early July) a slight water stratification was observed to establish, but by early September the waters were already well mixed to the state of vertical homogeneity.

The lowest temperatures (less than 8oC) in the upper water layer (0-5m) were recorded southward and eastward off the Solovetskiy Archipelago. This is due to the inflow of cold and rather high salinity (over 270) water from the White Sea via the Solovetskiye Salmy Straits. An intensive mixing of water takes place in these straits. A comparison of water temperature and salinity profiles obtained for this area indicates that southward, off the Solovetskiy Archipelago, relatively cold and homo- geneous waters reside. Their temperature is lower than even that of deep-water layers in the Zapadnaya Solovetskaya Salma Strait. This makes it possible to hypothesize that the main flow of water to Onezhskiy Bay occurs via the Vostochnaya Solovets- kaya Salma Strait, which is in contrast to the previously adopted scheme of large- scale water circulation. Satellite assessments of water surface temperature obtained in July, 2001 are indicative of the formation (due to intensive mixing) of a frontal zone located northward, westward, and eastward off the Solovetskiy Archipelago. At the same time, the surface water temperature in the Bassein was much higher than it was both in the straits and the central part of Onegskiy Bay.

The water temperature and salinity spatial distribution patterns at horizons 0 and 5-m depths indicate that waters from the Bassein propagate down to Onezhskiy Bay flowing along the Liamitskiy. The distribution of these characteristics in Onezhskiy Bay are typical of the season, which may also be accounted for by the temporal stability of water dynamics, mainly due to tidal currents and the residual water circulation.

In early autumn a rapid cooling of water in the shallow parts of the bay were observed against the background of intensive vertical mixing of the entire water column. A concurrent temperature decrease in river water took place. A large amount of thermal energy had by then been accumulated in the waters of the Bassein. As a result, the spatial variability of water temperature in Onezhskiy Bay proper decreased. Water salinity as well as water temperature became constant with depth; however, its spatial distribution exhibited horizontal inhomogeneity.

Sea surface level measurements by an autonomous digital recorder WLR-5 reveal that the smallest amplitudes are observed in pelagic areas (e.g., near the Zayetskiye (55cm) and Kuzov (65cm) Islands. In the estuary of the Kem River, the sea level amplitudes increase to 95cm (near Rabocheostrovsk). The largest amplitudes of sea surface level variations have been recorded in Keret Bay at stations B1 and B2 (115cm). Tidal motions observed at the station in Chupa Bay have a regular nature, so that the time of increase of sea surface level is equal to the time of its decline. The difference in duration between the high and low tide phases, characteristic of semi-enclosed regions of the White Sea, has also been found. The increase of the sea surface level continues to increase during a 6-hour period, but its decrease takes 6 hrs and 25min.

All the field data obtained in the course of this experiment point to a predomi- nance of the semi-diurnal M2 tidal wave. At the same time, the diurnal S1 tidal wave is also present and reveals itself in the amplitude variations of semi-diurnal tides: the amplitude fluctuations can be 6 cm between successive tidal cycles. With an average tidal wave height of 115cm, the diurnal component reaches 8.6%.

Autocorrelation and spectral analyses of the field data obtained at these stations have been accomplished. The autocorrelation functions have revealed that the sea level variations have a semi-diurnal nature and the duration of the tidal cycle is 12 hrs 20 min, which closely corresponds to the theoretical value of 12 hrs 25min. Earlier in Kandalakshskiy Bay we recorded a diurnal component accounting for 8% of the total value, which is in conformity with the results of spectral analyses (performed using the maximum entropy method).

Measurements of the bottom layer water temperature, conducted simul- taneously with the registration of level variations, have revealed only weak coherence between them. The only exception here is the data from a station located in the vicinity of the Kuzovskie Islands, but there was a 2-hr lag in the water temperature variations (Figure 4.16). In the bottom-layer water, the water temperature variability, apart from semi-diurnal fluctuations, exhibited a number of relatively small-scale ones.

In order to characterize the statistical variability of tidal and intra-wave motions, a suite of time series of synchronous observations of water temperature

and sea level variations have been obtained and analyzed. To study synoptic and mesoscale motions, the observation data obtained with 10-min intervals were employed. The water temperature variability, revealed at various depths in con- ditions of water stratification, points to the presence of thermocline variations corre- sponding to a period of semi-diurnal internal tidal motions. Thus, maximum water temperature variations were observed in the upper layer (4-8 m thick), whereas in the case of a deeper location of the thermocline, they occurred at depths ranging between 11 m and 21 m. The presence of small-scale temperature variations can be an indica- tion of thermodynamic instability if the speed shear resides in the gradient and/or bottom layers.

Water temperature observations conducted by means of a profile recorder TR-1 within a 10-m layer in the region of the Kuzovskie Islands made it possible to conclude that there was a close relationship between water temperature variability and tidal phenomena. The water temperature difference between the surface and the 10-m depth did not exceed 0.5-1.0oC.

Spectral characteristics of the sea level and water temperature variability have been calculated (Figure 4.17). The drawn periodograms indicate that the sea level variation spectrum is typical of a purely periodic process with a single dominant harmonic at a frequency corresponding to a period of 12 hrs. The periodogram of temperature fluctuations reveals a predominance of 12-hr fluctuations in the presence of a few harmonics with multiple frequencies. A weak trend was revealed in the low-frequency range.

A reciprocal spectral analysis of selected time series reveals that at the frequency of the semi-diurnal tide, the coherence is 0.80 and the phase difference is equal to a quarter of the tidal cycle (i.e., the temperature intra-wave variations lag behind the tidal level fluctuations). At other frequencies, there is no statistical relationship because in the high-frequency region (timescales are equal or less than 12 hrs) there are no significant periodic level fluctuations.

The data on tidal movement directions and speeds, obtained by autonomous buoy stations, show that over a relatively small sea area the speeds did not exceed 5cm s-1and, as a rule, remain in the range of 2-4 cm s-1(at 5-m depths). At some stations in the Onezhskiy Bay, the speeds were as high as 17 cm s-1. In the Onezhskiy Bay, near the Zayetskiye Islands, maximum speeds of tidal currents were about 25cm s-1(at 3-m depths) and exceeded 75cm s-1(at 3-m depths) near the Kuzovskie Islands. An example of measurements of tidal current speeds and direc- tions in the estuary of the Kem River is given in Figure 4.18. As seen, the absolute tidal speed in the estuary during the low-tide phase was greater than during the high-tide period. Tidal phase changes lead to sharp current directions, but a characteristic counterclockwise rotation movement of water was also observed (Figure 4.18).

Date: 2016-03-03; view: 532