No abstract

We investigated upwelling events in the pelagic area of Lake Baikal that developed during summer stratification (July-November) using a combination of in situ and satellite observations. These upwellings appear in the centres of local cyclonic macrovortices with compensatory downwelling located on their periphery in coastal areas. The average duration of upwelling events was 5 weeks, with an observed maximum of 16 weeks. The most stable upwellings in Southern Baikal and over the Academician Ridge covered areas of up to 4,400 km 2 (59 % of Southern Baikal's surface) and 1,550 km 2 , respectively. Water was ascending in the upwelling zones at velocities of 1 9 10 -4 to 1.2 9 10 -2 cm s -1 . Temperature differences of 1-4°C and 2-13°C were observed between the downwelling and upwelling zones in the epilimnion and metalimnion, respectively. On the surface of the lake, water temperature can drop 4-7°C for water ascending from depths of 10-75 m, but the observed thickness of the layer within which water was ascending reached a depth of 280 m in August-September and 380 m in October; i.e. the ascending water included the entire upper layer (0-300 m). Geostrophic currents reached up to 24-38 cm s -1 on the boundary between up-and downwelling zones (usually 5-7 km offshore), but did not exceed 6-10 cm s -1 in the centres of upwelling zones.Comparison with other large lakes of the world is carried. This study is important for understanding the upwelling process that develops in Lake Baikal during summer stratification and can influence the heterogeneity of nutrients and primary production.

A large scale rise of deep water (upwelling) has a visible effect on thermal and dynamical processes in oceans and large lakes, since it causes heterogeneities in the distribution of physical-chemical and biologi cal characteristics, and the supply of biogenic ele ments required for phytoplankton development into the photic layer [1,2]. The main reasons why upwell ings occur are run down of surface water during sea ward (lakeward) winds, Ekman transport in along shore waters, and water rise in the center of cyclonic vortices. In Lake Baikal, under the conditions of its great depths [3], upwellings play a special role in lake processes, especially in the period of direct thermal stratification when they provide mainly thermal and mass exchange between the deep and upper water lay ers. However, there are still no generalizing data on this phenomenon in Baikal, and the observations are casual.The first description of upwelling during a storm wind blowing from the shore was made by G.Yu. Vereshchagin [4]. He estimated the depth of water raise: with the measured values of water temper ature T and oxygen content near the surface taken into account, it was 50-200 m and, with CO 2 used, 600 m. The latter value is highly questionable due to the high value of the CO 2 determination error (±20%) [5] and due to the fact that the data used on CO 2 depth distri bution prior to the storm were taken 200 km from the observation point. The further shipboard and satellite based observations and those with the use of buoy sta tions [6-9] yielded only the brief information about the local manifestation of upwelling. Relatively reli able results were acquired on the basis of multiannual studies of currents [10]. On the basis of the cyclonic circulation scheme, the velocity of water sinking near the shores was estimated (~10 -2 cm/s), as well as that of the rise in the central part of the lake (10 -3 cm/s) [10,11], and the likely areas where these processes develop were identified [12].The present work deals with investigation of the principal quantitative characteristics of the coastal and pelagic upwellings in the period of direct temperature stratification (July-mid November) based on repre sentative data of different years (Fig. 1). These data included the temperature and conductivity measure ments in July-October of 2002-2010 in the perma nent cross sections utilizing an SBE 25 probe (preci sion is ±0.002°С); based on them, the velocities of geostrophic currents in the upper water layer were cal culated, as well as water mineralization by the tech nique from [13] with a precision of ±0.01 mg/kg. In addition to this, we used the temperature measure ments (precision of ±0.02°С) in South Baikal: along the cross section settlement Listvyanka-settlement Tankhoi , the cross section in Listven nichnyi Bay (1957), and at the coastal station in the bay near the settlement of Listvyanka . The information on the upwelling distribution within the water area of the lake was acquired based on the day to day measurements of a NOAA/AVHRR satel li...