The Gulf of Ob is a classical positive estuary with a general seaward transport of freshened riverine water in the surface layer and a landward transport of saline seawater in the bottom layer. During the ice-free season, salinity in the Gulf of Ob is equal to 0 from surface to bottom in the shallow (<10 m) southern and central part of the estuary. Further northward, in the northern part of the gulf, the river flow detaches from the bottom and forms a freshened surface layer with depth ∼10 m (Lapin, 2011; Osadchiev et al., 2021a). Surface salinity steadily increases within the northern part of the gulf and is equal to 4–10 when it outflows to the open sea (Osadchiev et al., 2021a). Salinity of the bottom layer within the northernmost part of the gulf is 30–32 and decreases landward.

Circulation and mixing in river estuaries generally are governed by gravitational forcing (due to salinity difference in river water and seawater), tidal forcing, and wind forcing. The relatively shallow Gulf of Ob receives very large freshwater discharge, which results in strong longitudinal density gradient (Osadchiev et al., 2021a). On the opposite, tidal circulation in the gulf is relatively low, the maximal tidal velocities are 0.4–0.5 m/s (Pavlov et al., 1996; Vvedensky et al., 2017). In the absence of wind forcing, the estuarine dynamical and mixing regime determined by the buoyancy and tidal forcing can be characterized by two dimensionless parameters, namely, the freshwater Froude number Frf=URβgSH and the mixing parameter M=CDUT2wN0H2, where U_R is the inflowing river velocity, U_T is the amplitude of the depth-averaged estuarine tidal velocity, w is the estuarine tidal frequency, S is the ambient sea salinity, H is the depth of an estuary, g is the gravity acceleration, β is the saline contraction coefficient prescribed equal to 7.7 × 10^–4, C_D is the quadratic drag coefficient for wind stress parameterization prescribed equal to 10^–3, and N0=βgSH is the buoyancy frequency for maximum top-to-bottom salinity variation in an estuary (Geyer and MacCready, 2014). Values of these parameters for the Gulf of Ob are as follows: Fr_f ∼ 0.3/(7.7 × 10^–4 × 10 × 32 × 15)^0.5 ∼ 0.6, M ∼ 10^–3 × (0.4)²/[2.3 × 10^–5 × (7.7 × 10^–4 × 10 × 32/15)^0.5 × (15)²] ∼ 0.4. The obtained estimations demonstrate that the Gulf of Ob is a typical salt-wedge estuary with strong vertical stratification due to very large freshwater runoff during the ice-free season, while tidal forcing limitedly affects circulation and mixing in the gulf.

The influence of wind forcing on the estuarine processes in the Gulf of Ob was assessed using the dimensionless Wedderburn number We=τL△ρgH2, where τ is the along-estuary wind stress, L is the length of the considered segment of the estuary, and Δρ is the density difference over L (Monismith, 1986; Chen and Sanford, 2009; Lange and Burchard, 2019). Once the along-estuary wind speed is large enough so that We ∼ 1, the role of wind forcing is comparable with the role of gravitational circulation (Geyer, 1997; Chen and Sanford, 2009; Lange and Burchard, 2019; Lange et al., 2020). In case of the northern part of the Gulf of Ob with a two-layer stratification induced by saline inflows, We ∼ τ × 150 × 10³/6 × 10 × (20)² = τ × 6.25. According to in situ measurements of wind forcing at the meteorological station of Seyakha, the average monthly wind speed in the study area from July to October in 2005–2020 varies between 5.5 and 5.7 m/s, while the maximal registered wind speed is 20 m/s. No prevailing wind direction is registered during the warm period; the repeatability of all eight compass wind directions is 10–15%. Therefore, for the study area we consider the average along-estuary wind speed equal to 5 m/s. In this case, We ∼ 0.04 × 6.25 = 0.25, which shows that under average wind forcing the estuarine circulation is governed mainly by gravitational circulation.

Local wind forcing affects estuarine circulation and mixing in case of strong along-estuary wind speed ∼10 m/s because in this case We ∼ 0.16 × 6.25 = 1. In particular, strong up-estuary wind is expected to intensify estuarine circulation and increase stratification, while strong down-estuary wind tends to induce wind straining (Chen and Sanford, 2009; Lange and Burchard, 2019). Indeed, Osadchiev and Sedakov (2019) reported that strong northern winds reverse surface flow in the northwestern part of the gulf, i.e., change its predominant flow direction from northward to southward. However, the along-estuary wind speed exceeds 10 m/s during only 4% of days during the ice-free season; therefore, the local wind forcing significantly modifies general circulation and mixing in the Gulf of Ob only in rare cases.

While showing no modification in general estuarine circulation, moderate local wind forcing has a certain influence on circulation in the surface layer in the northern part of the Gulf of Ob during ice-free periods, which can be detected at optical satellite imagery (Figure 3). The general northward flow is observed within the southern and central part of the gulf occupied by fresh river water from surface to bottom. Circulation in the deeper and wider northern part of the gulf is more complex. According to the coastline and local bathymetry, riverine water propagates northward in this part of the gulf along its western shore as a narrow stream (10–25 km wide) (illustrated by solid black arrows in Figure 3b). Inflows of more saline water from the open sea to the northern part of the gulf occur along its eastern shore (illustrated by dashed black arrows in Figure 3b). Note that the estuarine—sea salinity difference is only several units of salinity, which is by an order of magnitude smaller than the salinity difference between the surface and bottom layer at this area. The anticyclonic circulation pattern is distinctly observed during moderate southern and eastern winds (Figures 3a–d). Northern and western winds, on the opposite, hamper the anticyclonic circulation and cause the formation of multiple mesoscale eddies and complex frontal zones at the periphery of the riverine stream and in the northeastern part of the gulf (Figures 3e,f). However, the influence of local wind forcing on the mesoscale structure of surface circulation in this area requires additional study.

The intensity of inflow of saline seawater to the Gulf of Ob (indicated by the location of the isohaline of five in the bottom layer) shows large variability during the considered field surveys (Table 1 and Figures 2, 4). In most cases, saline seawater reached the latitude of 72.0–72.3°N (Figure 4A). More distant inflow (i.e., distant from the open sea) was observed in September 2013 and July 2016 (71.6°N) (hereafter referred to as “moderate inflows”) (Figure 4B); the most distant inflow was registered in August 2019 (71.0°N) (hereafter referred to as “intense inflow”). Moreover, this distant inflow was observed in progress, on August 8–10, 2019 saline water was registered at the latitude of 71.7°N (Figure 4C), and 10 days later it reached 71.0°N (Figure 4D).

Previous studies of this process by Lapin (2011) and Lapin et al. (2015) based on three field surveys (August 2010, September 2010, and September 2014) reported significantly smaller variability of the salt-water intrusion to the gulf (72.0–72.3°N) and its association with the seasonal variability of river discharge. However, an analysis of more extensive in situ data demonstrates no direct dependence between the intensity of seawater inflow and the river discharge rate, including both instant discharge (Figure 5A) and average discharge rates from the beginning of summer freshet till the periods of oceanographic field surveys (Figure 5B).

The distances of seawater propagation to the gulf among different years with similar discharge conditions varied by ∼50 km in July (light brown and green squares in Figure 5), ∼150 km in August (yellow square and red circles), and ∼70 km in September (dark brown and gray squares). On the other hand, similar inflow distances (72.1°N) were registered in July 2019 (green square in Figure 5), August 2010 (orange square), and September 2007 (cyan square) with significantly different discharge conditions. Finally, two of the three cases with distant inflows of saline water to the gulf, namely, a moderate inflow in July 2016 (light brown square in Figure 5) and an intense inflow in August 2019 (red circles), were registered during high discharge conditions (25,000–30,000 m³/s). The moderate inflow in September 2013 (dark brown square in Figure 5), however, occurred during the drought period (11,000 m³/s) of the year with relatively low annual runoff. The correlation coefficient between the inflow intensity and river discharge is −0.1 (Figure 6A), and the correlation coefficient between the inflow intensity and average discharge from the beginning of summer freshet till the periods of oceanographic field surveys is 0.

As discussed earlier, local winds limitedly affect the circulation in the bottom layer due to strong gravitational forcing and vertical stratification in the northern part of the Gulf of Ob. The saline inflow distances also do not show any dependence on the along-estuary wind stress, and the related correlation coefficient is 0.2 (Figure 6B). To evaluate the influence of remote wind forcing on saltwater intrusion to the gulf, we calculated the time integrals of downwelling (W_d), upwelling (W_u), offshore (W_of), and onshore (W_on) wind stress. These wind speed integrals were calculated over the central part of the Kara Sea (73.75–75.5°N, 65.0–85.0°E). The integration is performed over 10 days preceding the middays of the field surveys. Note that the downwelling and offshore wind integrals are positive, and the upwelling and onshore wind integrals are negative. However, to compare all the four wind integrals, we operate with their absolute values, which are shown in Figure 7.

Among the four wind integrals, only the upwelling wind shows a good correlation (0.5) with the inflow distance, while the correlations of the downwelling (0.1), onshore (0.2), and offshore (0) winds were low. Large absolute values of the upwelling wind integral (and the respective low values of the downwelling wind integral) explain two of the three distant inflows, namely, in July 2016 and August 2019, which were preceded by strong eastern winds. On the other hand, the other wind forcing conditions (except September 2013) were not accompanied by distant inflows, saline waters in these cases propagated only till the latitudes of 72.0–72.3°N. The third registered distant inflow, which occurred in September 2013 (71.6°N), was not preceded by upwelling winds (Figure 7). However, it occurred during the drought period in September, and the discharge of the Ob River in 2013 was very low as compared to the other years (Figure 5). In particular, the total runoff during the summer freshet in 2013 (270 km³ or 17,000 m³/s) was by ∼20% lower than the average value in 2007–2019 (325 km³ or 20,500 m³/s). Therefore, we associate this moderate inflow with low discharge conditions, i.e., low gravitational forcing, as described by Lapin (2011).

Strong eastern winds in July 2016 and August 2019 induced upwelling circulation in the central part of the Kara Sea, including the area adjacent to the Gulf of Ob. This process is manifested by the formation of positive sea level anomaly in the central part of the Kara Sea northward from the Gulf of Ob, which is visible at satellite altimetry data (Figure 8). Offshore (northward) flow in the surface layer within the Ob–Yenisei plume and onshore (southward) flow in the bottom layer increased the estuary—sea pressure gradient, which resulted in intensified saltwater intrusion to the gulf. This mechanism of impact of upwelling/downwelling wind events over the open sea on estuarine—sea water exchange was reported earlier for many regions in the World Ocean (Stigebrandt, 1990; Aure et al., 1996; Monteiro and Largier, 1999; Hickey et al., 2002; Hickey and Banas, 2003; Gilcoto et al., 2007) and was explicitly described and analyzed by Giddings and MacCready (2017).

Generally, upwelling events strengthen the barotropic gradient, which increase the horizontal density gradient and the estuarine circulation, and strengthen the baroclinic gradient, which induce the upstream transport of increased salinities. The role of the barotropic effect in the increased saltwater intrusion will be more important because salinity in the bottom layer in front of the Gulf of Ob does not change significantly. The observed variability of salinity is 30–33, which corresponds to variability of density 1,024–1,027 kg/m³, i.e., ∼0.3% of relative variation. The role of the barotropic pressure gradient typically dominates the role of the baroclinic pressure gradient in water exchange processes in large estuaries (Osadchiev, 2017; Zavialov et al., 2020).

Coastal upwelling events are often manifested at optical and thermal satellite imagery by areas of cold (and sometimes turbid) water along the shore once the bottom layer penetrates up to the sea surface near the coast (Osadchiev et al., 2020c). However, during the upwelling events in the Kara Sea, the cold and saline bottom layer does not reach the surface layer near the Gulf of Ob because it is blocked by the Ob plume in this area. Nevertheless, the bottom layer reaches the surface layer eastward from the Gulf of Ob along the coast of Taymyr Peninsula. Therefore, the formation of a turbid and cold stripe in this area visible at satellite imagery (indicated by white arrows in Figure 9) is an indicator of general upwelling circulation in the central part of the Kara Sea. Note that no cloud-free satellite imagery is available for the beginning of August 2019; therefore, in Figure 9 we showed satellite imagery on July 2019, which are the closest dates to the period of field survey with cloud-free images.

In this study, we selected the integration period for the wind speed equal to 10 days because the best correlation between the wind and inflow conditions was provided. This period is a reasonable time lag between atmospheric forcing in the central part of the Kara Sea and the response of the estuarine circulation in the Gulf of Ob. In particular, the period of response of surface layer circulation to wind forcing in the central part of the Kara Sea is equal to several days (Osadchiev et al., 2017), similar temporal periods are registered in the Laptev and East-Siberian seas (Osadchiev et al., 2020c). The onset of the inflow of saline seawater to the gulf located 200–300 km southward from the central part of the Kara Sea requires approximately a week of additional time. The response period of estuarine inflow to remote upwelling winds reported in Giddings and MacCready (2017) is equal to 8 days, which is also consistent with our averaging period equal to 10 days.

In situ measurements performed in the study area confirm that an intense inflow to the gulf in the middle of August 2019 was induced by upwelling winds and was formed during a relatively short time period (Figures 4C,D). Variable wind conditions observed in the study area in the first half of July 2019 were accompanied by typical low-inflow conditions (72.1°N) registered on July 16–18, 2019. Then, strong eastern and northeastern winds dominated in the central part of the Kara Sea from July 30, 2019 to August 8, 2019 (Figure 8B). It induced an intense inflow of saline seawater to the gulf till the latitude of 71.7°N registered on August 8–10. After August 8 upwelling winds ceased; however, the inertial flow of saline water in the bottom layer in the Gulf of Ob reached the latitude of 71.0°N, which was registered on August 18–20. In July 2016, upwelling winds dominated the regional atmospheric circulation for 8 days in a row and ceased 4 days before the in situ measurements. Thus, in both cases distant inflows of saline water to the Gulf of Ob were formed during 8–10 days by the prevailing upwelling winds in the central part of the Kara Sea. However, the upwelling wind forcing was stronger in the first half of August 2019 (| W_u| = 0.4–0.6 N/m² × day) than in July 2016 (0.3–0.5 N/m² × day), as a result, the inflow was more intense in August 2019 (71.0°N) than in July 2016 (71.6°N).

The analysis of in situ data and wind forcing conditions shows that the moderate/strong upwelling winds in the central part of the Kara Sea induce the moderate/intense inflows of saline seawater to the Gulf of Ob. To estimate the frequency and duration of these inflows, we calculated the absolute value of upwelling wind stress integral | W_u| for every day for the ice-free seasons (July to October) in 1979–2020 using the ERA5 wind reanalysis. Based on the conditions 0.4 > | W_u| > 0.3 N/m² × day for moderate inflows and | W_u| > 0.4 N/m² × day for intense inflows, we assessed the periods of wind forcing favorable for the formation of distant inflows to the gulf (Figure 10).

Both moderate and intense inflows occurred on average during 26% of days from July to October steadily increasing from 17% in July to 24 and 28% in August and September and then to 35% in October. This feature is caused by intensification of atmospheric circulation in the study region in autumn as compared to summer conditions. The intense inflows have almost the same frequency (14% from July to October, 6% in July, 14% in August, 14% in September, and 22% in October) as the moderate inflows (12% from July to October, 11% in July, 10% in August, 14% in September, and 13% in October). Extremely intense inflows (|W_u|> 0.5 N/m² × day), which were not yet detected by in situ measurements, occur relatively rarely (5% from July to October, 1% in July, 6% in August, 5% in September, and 9% in October), but they can result in even more distant salt intrusion to the gulf than the observed extent to 71.0°N. The duration of the distant inflows varied from several days to 1 month. The total annual duration of periods of distant inflows varied from 67 days in 1979 and 64 days in 1993 to 5 days in 1999 and 9 days in 2009 due to significant inter-annual variability of local atmospheric circulation (Figure 11).

Phytoplankton populations in the Gulf of Ob were addressed in a number of studies during the previous years. Phytoplankton productivity strongly depends on hydrological regime in the gulf and, therefore, has distinct seasonal variability. The spring bloom, which is formed mainly by diatoms shortly after ice melting, shifts to a steady decrease of abundance and biomass till the ice formation period (Makarevich, 2008). Generally, three zones with different phytoplankton communities are distinguished, namely, the freshwater zone with maximal productivity, the frontal zone with a distinct two-layered structure and bottom salinities increasing from 5–6 to 18–20 characterized by an abrupt decrease of productivity, and the saline zone with highly variable phytoplankton characteristics (Druzhkov and Makarevich, 1996; Makarevich et al., 2003; Sukhanova et al., 2010, 2018). However, the field surveys, which are reported in these papers, were performed during different seasons and were limited to certain zones within the gulf, i.e., not covering all the three zones within one survey. As a result, the dependence of phytoplankton communities on highly variable salinity conditions in the gulf still remains unaddressed.

The intensity of propagation of saline seawater to the Gulf of Ob affects local phytoplankton communities and, therefore, modifies the biological productivity in the gulf. In this study, we compare the characteristics of phytoplankton communities observed during the same successional season in the central and northern part of the Gulf of Ob in September 2007, September 2014, and August 2019 during different seawater inflow conditions. In the first and second case, the inflow of saline seawater to the gulf was small, while the third case was accompanied by the development of an intense inflow. As a result, the distribution of phytoplankton in both 2007 and 2014 was significantly different from that in 2019.

Figure 12 illustrates the biomass of phytoplankton and number of cells at the stations along the transects in the gulf. In 2007 and 2014, the maximal biomass and the maximal number of cells were registered in the fresh zone of the gulf (in 2014 also the second maximum of biomass was registered ∼15 km northward from the fresh zone), while further northward the productivity of phytoplankton communities steadily decreases. In particular, in September 2007, the biomass of phytoplankton decreased by 10 times (from 207 to 21 mg/m³) with an increase of bottom salinity from 0 to 12. In August 2019, on the opposite, the maximal biomass of phytoplankton (1,200 mg/m³) was observed not in the fresh zone, but at bottom salinities of 10–18. Southward from this area, phytoplankton biomass steadily decreased to 400–600 mg/m³ in the fresh zone. The number of cells in August 2019 also decreased from 1,000–1,500 million at the latitudes of 71.6–72.2°N to 200–400 in the fresh zone at the latitudes of 70.8–71.4°N.

The qualitative characteristics of phytoplankton communities at different zones within the Gulf of Ob during the considered periods are illustrated by their taxonomical structure certain stations (Figure 13). We show the distribution of taxa Bacillariophyta, Dinophyta, and Chlorophyta, while Flagellatae determines a group of flagellated unicellular organisms comprising Cryptophyta and Euglenophyta. The other taxa in Figure 13 determine mainly Cyanobacteria and other minor taxa. The typical distribution of phytoplankton taxa in the study area described in previous studies was observed in September 2007 and September 2014. Freshwater diatoms, green algae, and cyanobacteria dominate in the fresh zone with the presence of Euglenophyta and a small amount of freshwater dinoflagellates. The genus Aulacoseira is the most abundant among the freshwater diatoms in the Gulf of Ob. It has a proportional number of cells and biomass and is registered in all seasons. The amount of green algae is decreased in the frontal zone, euglenids steadily disappear, and the amount of dinophytes steadily increases toward the open sea, while the diatoms dominate. At the same time, algae of the genus Aulacoseira, which are able to live in brackish water, still dominate in the phytoplankton community of the frontal zone together with other brackish and marine species of the genera Thalassionema, Thalassiosira, etc. The marine zone of the gulf is characterized by an increase in the amount of dinophytes, which dominate together with diatoms (genera Thalassionema, Thalassiosira, Chaetoceros, etc.). Phylum Cryptophyta also contributes to the phytoplankton community in the marine zone, while Chlorophyta, Euglenophyta, and Cyanobacteria are almost completely absent. The largest difference in the taxonomical structure of the phytoplankton communities between September 2007 and September 2014 consists of a significantly lower amount of large diatom algae in the later stage, which results in different ratios of number of cells and biomass (Figure 13).

The distribution of phytoplankton communities in the Gulf of Ob in August 2019 was significantly different from those observed in September 2007 and September 2014. Dominance of diatom algae was maximal in August 2019. Diatoms provided 80–98% of total biomass at the majority of stations with bottom salinities equal to 20–25, which were located at the latitudes of 71.6–72.4°N. This large share was associated with an increase in the amount of brackish water species of the genera Thalassionema, Thalassiosira, Melosira, etc. The share of species of Aulacoseira was 52–76%, while in 2007 this share at the same salinities was equal to 24–34%.

In accordance with the abovementioned successional cycle, the spring bloom after ice melting occurs due to the development of diatoms, as was observed at the end of July 2016 (Sukhanova et al., 2018). Blooming occurs due to diatoms of all ecological groups, namely, freshwater, marine, and brackish. It is followed by a decline in quantitative indicators and an increase of species ratio of freshwater algae due to Cyanophyta and Chlorophyta in the frontal zone, as was observed in September 2007 and September 2014. In September 2019, on the opposite, the number of diatoms remained high throughout seasonal, salinity and longitudinal distribution in the gulf, the same was observed for common indices of abundance and biomass. At the same time, the taxonomic diversity of the extended frontal zone decreases in contrast to euryhaline or brackish water diatom species (Olli et al., 2019). This feature can be caused by two factors. First, the distant advection of saline seawater results in the development of brackish water algae at a large area in the Gulf of Ob. Second, upwelling-advected seawater is especially rich in nutrients because active reproduction of diatoms at this season can occur only in the presence of high silicon content in seawater, which is non-typical for this area (Ardyna and Arrigo, 2020). The freshwater species are almost absent at the saline zone of the gulf in August 2019, which is typical for the open part of the Kara Sea. Dinophyta and Cryptophyta phylums together with marine diatom species dominated in this zone in August 2019. In September 2007 and September 2014, however, freshwater species of green and euglena algae were present in the surface layer of the saline zone.