To get a better understanding of the phenomena responsible for the formation, development and disappearance of ice rings we conducted dedicated field measurements in the regions where ice rings have been observed previously. In the framework of French‐Russian‐Mongolian cooperation every spring (March–April) since 2010, we conduct yearly field observations of ice cover in the central part of Lake Baikal and since 2014 in Lake Hovsgol. Measures include observations of ice thickness and snow depth (using a hand drill and a ruler), as well as ice structure and roughness. Since 2012 using the YSI CastAway CTD probe operating down to 100 m depth these observations have been complemented by vertical profiles of temperature (accuracy 0.05°C, resolution 0.01°C) and conductivity (accuracy 0.25% ± 5 μ S/cm, resolution 1 μ S/cm). Since 2014, we have also performed semi‐quantitative estimations of currents by defining direction and current strength (weak, moderate, or strong) from CTD cable inclination. The existing dataset contains more than 250 stations located along the tracks of radar altimetry missions (Kouraev et al. 2015 ), in the regions of known ice ring observations and other regions with interesting features. For the region near the Nizhneye Izgolovye Cape our sampling strategy is to do a transect(s) covering the region of ice ring locations known from the previous years, and for other regions of known ice rings observations to do at least one or several stations when possible. The winter of 2013/2014 was warmer than usual, the ice was thinner and already on 01 April 2014 an ice ring had been clearly identified on the MODIS image prior our field survey. Knowing the exact location of the ring we were thus able to perform 2 d later (03 April 2014–04 April 2014) a dedicated study with high spatial sampling.

Water structure and ice surface characteristics associated with ice rings

As a result we have now four datasets of field hydrographic measurements in the regions where ice rings has been observed (Table 2). For the region of the Nizhneye Izgolovye Cape, measures in 2012 and 2014 represent situations when an ice ring was already visible on satellite images. These datasets (Fig. 4) are the most detailed and provide two transects across each of the ring structures. Observations in 2013 (Cape Shartlay, Lake Baikal) and 2015 (Lake Hovsgol) (Fig. 5) are less detailed but still reveal water structure prior (20–35 d) to ice ring appearance.

Figure 4 Open in figure viewer PowerPoint Hydrographic measurements on 06 April 2012 (transects A–B and C–D, left panel) and on 03 April 2014 and 04 April 2014 (transects E–F and G–H, right panel) in the region of Nizhneye Izgolovye Cape, Lake Baikal. (a) maps of stations with their numbers (for 2014 the first number is the day—03 April or 04 April, and the next two numbers—station number), ice thickness (cm, in brackets), transects (black lines) and location of the ice rings (as identified from satellite images on 21 April 2012 and on 24 April 2014) shown in grey. Vertical sections of (b) water temperature (°C), (c) specific conductance (μSi/cm) and (d) density (kg/m3, TEOS‐10) are shown along the transects (vertical lines—station positions). All horizontal distances are to scale.

Figure 5 Open in figure viewer PowerPoint Same as Fig. 4, but hydrographic measurements on 04 April 2013 (Lake Baikal, Cape Shartlay) and on 31 March 2015 (Lake Hovsgol). For Lake Hovsgol, the map of stations shows also current strength and direction. Ring locations as identified on satellite images for 05 May 2013 (Cape Shartlay) and 21 May 2015 (Lake Hovsgol). The horizontal scales are different for each lake.

Table 2. Water structure parameters and ice cover properties for ice rings in 2012–2015. Year 2012 2013 2014 2015 Region N. Izgolovye C. Shartlay C. N. Izgolovye C. Hovsgol CTD date 06 Apr 04 Apr 03–04 Apr 31 Mar Ice ring first observed 06 Apr 07 May 01 Apr 20 May Thermocline depth outside of the ring, m 40–45 35–40 40–45 45 Undisturbed isopicnal level, m 45 55 50 30 Undisturbed isopicnal density, kg/m3 1000.17 1000.225 1000.22 1000.15 Typical water properties in the ring T (°C) 1.2–1.4 1.2–1.3 1.2–1.4 1–1.2 Sp. Cond. (μS/cm) 120.5–121.5 119.5–120.5 120.5–121.5 263–265 Difference between water column in the ring and outside T (°C), 0–30 m depth 0.6 0.3 0.4 0.1–0.2 T (°C), 80–90 m depth −1.3 −0.6 −1.1 −0.4 Sp. Cond. (μS/cm), 0–30 m depth 1 1 1 0.4 Sp. Cond. (μS/cm), 80–90 m depth −1 −1 −1 −1 Surface conditions 0–1 cm snow, ice crust on surface Transparent crystalline ice, with some patches of snow (0–2 cm) 0–1 cm snow, ice crust on surface 1–1.5 cm snow, up to 4–5 cm at station 7 Ice thickness (cm)—ring center 65–72.5 n.a. 49–65.5 n.a. Ice thickness (cm)—ring border 68.5–74 105 32–61 87–102 Ice thickness (cm)—outside 80–91 108.5–122 61–73 94–99 Ring radius 3 2.7 3.5 3.1 Baroclinic Rossby radius, km (ring center/outside of the ring) 2.8/3.25 2.34/2.55 2.95/3.04 2.9/3.16

All these hydrographical sections (Figs. 4, 5) reveal lens‐like structures (double‐convex form) in temperature, specific conductance and density fields. These lens‐like structures are located at depths (defined as position of undisturbed isopicnal level) of 45–55 m for Lake Baikal and 30 m for Lake Hovsgol with density values of 1000.15–1000.225 kg/m3 (Table 2) and correspond to the location of ice rings (already existing or yet to be manifested). The core of the lens‐like structures has fairly uniform values of temperature (1–1.4°C, which at depth 0–30 m is 0.1–0.6°C warmer and at depths 80–90 m 0.4–1.3°C colder than surrounding water) and specific conductance (119.5–121.5 μS/cm for Lake Baikal and 263–265 μS/cm for Lake Hovsgol). For Lake Hovsgol specific conductance values show a more complicated distribution than for Lake Baikal, indicating potential lateral intrusion of more mineralised (264–266 μS/cm) water at depths 30–50 m, although there are not enough observation points to draw a firm conclusion. Comparison of water structure in the ring and outside shows that upward and downward extension of isolines in the lens‐like structure region result in warmer (+0.1–0.6°C) and more mineralised (0.4–1 μS/cm) water in the upper 30 m below the ice cover, and colder (−1.3: −0.4°C) and less mineralised (−1 μS/cm) water at depths 80–90 m.

This three‐dimensional spatial structure of temperature, specific conductance and density is typical for oceanic anticyclonic lens‐like eddies (Dugan et al. 1982; Armi and Zenk 1984; McWilliams 1985; Kostianoy and Belkin 1989). Eddies in ice‐covered lakes have been observed previously. Forrest et al. (2013) observed a cyclonic eddy in the ice‐covered Lake Pavilion in British Columbia, Canada (maximal depth 61 m) that has a morphological structure similar to Lake Baikal: a long narrow shape, with three basins separated by narrow sills. Using an autonomous underwater vehicle they observed in February 2008 a cylindrical density anomaly with a radius of about 110 m, affecting depth up to 14 m. The authors state that this radius is smaller than the internal Rossby radius of deformation (200 m) and suggest that it is a result of cyclostrophic balance between centrifugal, Coriolis, and pressure forces. Cyclonic circulation resulted in a double‐concave structure (opposite to the one observed in Lakes Baikal and Hovsgol), i.e., a deepening of isolines in the upper layers (colder and more mineralised water compared with surroundings) and rising of isolines in the bottom part (warmer and less mineralised water).

In the northern part of Lake Kilpisjarvi, Finland (mean depth 19.5 m, maximum depth 57 m), in 2013 and 2014 horizontal density anomalies were observed below ice cover with vertically paired cyclonic and anticyclonic rotating circulations (Graves 2015; Kirillin et al. 2015). In 2013, they resulted in a warm anomaly in the central part of the lake, with radius of 350 m and height of 22 m. The authors hypothesise that this anomaly is linked to warmer and denser water flowing down the slopes, converging in the center of the lake and leading to upwelling of warmer water. The internal Rossby radius of deformation was estimated to be much smaller (about 160 m) and authors suggest that the warm anomaly was modified by the earth's rotation and was in either geostrophic or cyclostrophic balance.

Our observations for Lake Baikal in 2012 and 2014 show water structure similar to the one found below the ice ring in 2009 (Granin et al. 2015) for temperature (core values 1–1.25°C). However for the conductivity/salinity this similarity is only partial. Granin et al. (2015) found less mineralised water in the center than at the periphery of the ring (this is similar to our observations), but from the depth of 40 m downward their observations show no increase of salinity. This observation apparently led the authors to conclude that this anomalous structure results from upwelling of deep waters (with lower mineralisation and higher temperature) through the thermocline, and is potentially related to the presence of gas hydrates in bottom sediments. However, our observations of specific conductance for the four datasets (Figs. 4, 5) show no such feature; water in the eddy center always has higher mineralisation than water at lower depth and we observe upward and downward extension of water with specific conductance typical for the eddy core (same as for water temperature). A well‐developed eddy (such as the one in 2014, Fig. 4, right panel) may show homogeneous mineralisation in the upper 100 m in the central part of the eddy. This is not water coming from deeper layers, but less mineralised water pushed up‐ and downward from the undisturbed isopicnal layer. This, and the fact that rings were observed over relatively small depths and in regions without known gas sources (see “Ice ring detection and inventory” section) contradict gas‐related hypotheses of ice ring formation discussed in the Introduction.

Our observations of water structure for different transects along the ring regions in 2012 and 2014 has very similar pattern, showing that the observed lens‐like eddies have an isolated circular form and radial symmetry. The observed anomalous water structure both before (2013 and 2015) and during (2012 and 2014) ring appearance shows that such lens‐like structures are directly associated with ice rings, that they exist before and continue to exist during ice ring appearance and development, and that an ice ring is a surface manifestation of dynamical processes going on below the ice cover. This contradicts the hypothesis of ice ring formation related to ice structure heterogeneities (Bordonskiy and Krylov 2014).

Depending on ice formation in the beginning of winter, its drifting, deformation and consequent growth, ice thickness is not spatially homogeneous and ice melting related to dynamical processes going below ice cover will result in different rate of ice melting and thus different ice thickness. This is also illustrated by different manifestations of ice rings (such as open rings, diamond rings etc., see Table 1) related to different types of ice fields presented in the regions of ice ring formation. Measures of ice thickness in the region of the ice ring in Southern Baikal in 2009 (Granin 2009; Granin et al. 2015) show increased ice thickness in the ring center (74 cm) and outside of the ring (70 cm), and much thinner ice (43 cm) in the ring itself. Our datasets for 2013 and 2015 (Fig. 5) representing situations 20–35 d before ice ring manifestation are too coarse to draw definite conclusions on ice thickness spatial distribution inside and outside of ice rings. However our observations (Fig. 4) for 2012 (when ice ring has been faintly seen) and especially 2014 (when an ice ring was well manifested) clearly show the spatial distribution of ice thickness.

For all four datasets ice was black and crystalline (with some ice crust and/or snow on the surface, see Table 2) and without impurities. No water was observed on ice and no gas bubbles were presented in the upper water column. In 2012, the ice was thicker (80–91 cm) outside the ring, while in the ring and in its center it was already thinner (65–74 cm). In 2014, the ice was thick outside (61–73 cm) and less thick in the center of the ring (49–65 cm), while in the ring itself it was much thinner (32–61 cm), especially at the two stations of the northern part of the transect G‐H (32 cm for station 401 and 34 cm for station 402). At these two stations we observed large needle ice crystals (up to 12 cm long and 2–3 cm thick, Fig. 6a) at the bottom of the ice. Neighbouring stations 315 and 316 presented no needle ice on the bottom, but ice was water‐laden from a depth of 30–35 cm. This northern part of the ring represents an advanced stage of ice melting and metamorphism that will later on be typical for other regions of the ring. Further ice ring development will lead to thinner ice in the ring, metamorphism, fracturing, the appearance of leads and finally break‐up of the ice (Figs. 1i, 3).

Figure 6 Open in figure viewer PowerPoint (a) Needle ice crystals from ice bottom (hole diameter—11 cm, crystals size—10–12 cm long, 2–3 cm thick) at station 401, 03 April 2014; (b) currents strength and direction for ring near Cape Nizhneye Izgolovye on 03 April 2014–04 April 2014.

Currents in 2014 (Fig. 6b) and 2015 (Fig. 5a) confirm typical anticyclonic (clockwise) direction of eddy rotation, with currents oriented approximately 30–45° to the left of the ring direction. This is consistent with geostrophic flow subject to frictional stress although the nature and mechanisms of the associated frictional stress remain to be identified. Another issue could be influence of larger scale currents field. Currents in the center of the ring in 2014 are absent or weak, the strongest velocity is observed for northern part of the ring (especially for station 402—one of the two stations with minimal ice thickness). This indicates that current speed (and thus intense heat exchange between ice and water), rather than presence of warmer water in the ring center, is the main driver related to ice melting and metamorphism.