BALTICA Volume 23 Number 1 June 2010 : 13-24 Mixing by Langmuir circulation in shallow lagoons Irina Chubarenko, Boris Chubarenko, Elena Esiukova, Henning Baudler Chubarenko, I., Chubarenko, B., Esiukova, E., Baudler, H., 2010. Mixing by Langmuir circulation in shallow lagoons. Baltica, 23 (1), 13-24. Vilnius. ISSN 0067-3064. Abstract Field measurements and observations in shallow basins (Vistula Lagoon and Darss–Zingst Bodden Chain, the Baltic Sea) are reported, revealing characteristics of Langmuir circulation (LC) patterns during moderate winds. A system of large–scale rolls with horizontal axes is shown to be different in the open ocean as compared to shallow areas. CTD horizontal tows across the windrows, GPS registration of the cell’ width, videotape recording were used. Regular patterns of windrows, marking the roll-shaped circulation cells, develop within 5–10 min after the wind onset. The most probable distance between streaks is about double the local water depth, so that the width–to–depth ratio for the rolls is equal to 1; 78% of the rolls have a ratio of width to depth from 0.65 to 1.6, with peak values at 0.75, 1, 1.2, 1.4. It is shown, that in a shallow basin the pattern of windrows is fully developed, and the growth of the roll’ diameters are limited by the depth of the basin. Thorough analysis of video–records of the rows’ breakdown and reconstruction has revealed four possible kinds of Y-junctions, whilst in deep areas only one of them is reported to prevail. One more major difference of shallow water LC is caused by the presence of shores: eventually, the wind, waves and water current in a lagoon propagate in different directions. This makes the streak lines curved, and they drift in the direction of the cross–current component of the Stokes wave transport. Mathematical analysis of the behaviour of suspended particles has revealed, that the flow within the LC transports particles of different size and buoyancy along different trajectories, making mixing more effective. Keywords Water mixing, Langmuir circulation, shallow water, Darss–Zingst Bodden Chain, Vistula Lagoon. Irina Chubarenko [[email protected]], Boris Chubarenko [[email protected]], Elena Esiukova [elena_esi- [email protected]], P. P. Shirshov Institute of Oceanology of Russian Academy of Sciences, Atlantic Branch, prospect Mira, 1, 236000 Kaliningrad, Russia; Henning Baudler [[email protected]], Biological Station Zingst, Rostock University, Mühlenstrasse, 27, 18374 Ostseebad Zingst, Germany. Manuscript submitted 21 January 2010; ac- cepted 12 April 2010. INTRODUCTION subject of the present study––is the roll–structured wind–wave mixing in upper layer, well known as Water mixing, induced by wind and waves, is of the Langmuir circulation. In shallow closed basin it utmost importance for shallow lagoon water dynamics appeared to be much different from the open ocean and water quality. Since influence of both bottom case, being limited by water depth and deflected by and boundaries is very significant, the resulting local currents. We report our field measurements flow structure is much different from the principal and observations in two shallow Baltic lagoons – the picture, well known for deep and unbounded open Vistula Lagoon and Darss–Zingst Bodden Chain, ocean. Before the all, general wind driven flow is carried out during summer measurement campaigns often deflected by boundaries. Then, since basin is of 2000–2006 years. closed, compensating currents occur inevitably along On windy days, one can often observe on water the bottom depressions. And the third––and the main surface of large basins long parallel streaks of foam, 13 flotsam, algae, and called windrows. They have un- doubtedly been familiar to sailors since long ago and even used to categorize wind speed (e.g., Thorpe 2004). These windrows make visible a pattern of parallel pairs of large alternate left–handed and right–handed horizontal rolls, or circulatory cells, called Langmuir circulation (hereinafter, LC). The phenomenon is named after Irving Langmuir, who noticed during an- Atlantic crossing in 1927 that Sargasso weed aligned into nearly regular rows when wind exceeded 5 to 10 m/s, and that, when the wind suddenly shifted 90 degrees, the lines reformed within 20 minutes (Lang- muir 1938). This phenomenon is now considered as one of turbulent processes in upper layers of large water bodies, driven by wind and waves, influential in producing and maintaining the uniform surface mixed layer and in driving dispersion (see Leibovich Fig. 1. Sketch of the Langmuir circulation. Important to note 1983). Even though Langmuir himself continued his that the distance between convergence zones on the surface investigations in smaller basin (Lake George, NY), equals to two roll’ diameters. In lower part forces are shown, acting on the sediment particle suspended by the flow: the and established the essential kinematics of alternating dynamic pressure force Fflow, the Archimedean force FArch and horizontal roll–vortices aligned with the wind there, the gravity force mg; vector sum of the latter two gives the the most serious attention was paid historically to LC buoyancy force B of the particle. features in the open ocean. In order to emphasise the features of LC, which are most important for our study, Perhaps here is the reason why this phenomenon is not we just list here the most famous investigations. So, sufficiently investigated in such areas; a few references Stommel (1949) calculated particle trajectories based still can be mentioned here––laboratory work of Mat- on idealized roll–vortices, and showed that particles, sunaga and Uzaki (2004), and field measurements on which sink (such as phytoplankton), or rise (such as 15 m deep oceanic shelf, reported by Gargett and Wells micro–bubbles), are trapped within the cores of the (2007). Lack of information had motivated us to per- vortices. Laboratory experiments of Faller (1971), form a special field measurement campaign in shallow Faller and Caponi (1978) showed that both wind and and semi–enclosed areas––the Darss–Zingst Bodden surface waves are required to produce the rolls. As- Chain and the Vistula Lagoon of the Baltic Sea. Appar- saf et al. (1971) used aerial photography to observe ently, natural conditions in brackish lagoons are very streak patterns on the ocean surface, and reconfirmed much favourable for the instrumental investigations the existence of multiple scales, as noted originally of the specific features of LC: their waters have some in Langmuir (1938); three scales were seen in several vertical salinity and temperature gradients, with more photos, separated by just under an order of magnitude cold and saline sea waters at the bottom and freshened and ranging from a few to hundreds of meters between riverine waters at the surface. When LC arises in such streaks. Some attempts to explain how the circulations a system, it can easily be registered, for example, by are generated have been done by Garrett (1976), Craik CTD-towing in surface layer: water salinity and tem- (1977), and Leibovich (1977). perature will vary coherently, showing the temperature Theory suggests (Leibovich 1983), that LC is pro- minimum/salinity maximum at the divergence zones, duced by the interaction of wave orbital motions with and the temperature maximum/salinity minimum in depth–varying upper layer current. Consider for clarity zones of convergence. Further statistical data analysis the simplest case, when wind and waves go in the same shows the distribution of line spacing, amplitude of direction, what is typical for the open sea conditions. temperature and salinity variations etc. Here, for the problem under investigation, we use mostly the data of Taken separately, both wind and waves generate water GPS and echo–sounder, since they are in full agreement flows directed down–wind: the wind–induced shear with the results of statistical analysis of CTD data, but flow, decaying with depth, and the wave–induced are easier to handle with. Stokes’ drift. The resulting down–wind flow, however, becomes unstable and breaks down into long rolls, FIELD SITE AND MEASUREMENT aligned with the wind––what we see as appearance of TECHNIQUES windrows on water surface (Fig. 1). In a shallow and closed basin, the features of LC Field measurements and observations of LC have been (like roll’ spacing, direction, lifetime) are influenced performed in Vistula Lagoon (VL) and Darss–Zingst by local bottom topography and depth, coast geometry, Bodden Chain (DZBC) during summer field campaigns particular water density profile, mutual orientation in years 2001-2006 using small scientific boats of wind and coastline, probable near–shore currents. Ecolog of Atlantic Branch of P. P. Shirshov Institute 14 Fig. 2. Measurement sites: shallow Vistula Lagoon and Darss-Zingst Bodden Chain in the Baltic Sea. of Oceanology of Russian Academy of Sciences sortechnik GmbH and Idronaut). The CTD probe was and Gamarus of Field Biological Station Zingst of fixed aside, so that measurements have been performed Rostock University (Fig. 2). These coastal lagoons in undisturbed zone from the moving boat with a spa- have limited water exchange with the Baltic Sea (they tial resolution of CTD measurements of 0.5–1.0 m. are of the restricted and chocked hydrological types, The second technique used GPS and echo–sounding: correspondingly; Chubarenko et al. 2005). Maximum coordinates of crossings of the streaks by the boat length of 100 (60) km and average depth of 2.7 (2) were registered by GPS simultaneously with the lo- meters, respectively, allow winds easily mix water cal depth echo–sounding, while the boat was moving column down to the very bottom. The observations perpendicularly to the windrows. Thirdly, 45 min. long were performed in central parts of both lagoons at the movie was taken in order to analyze the dynamics of depths of about 4 and 2.5 m, under moderate winds row’ reconstruction process.