Increased frequency of wintertime stratification collapse events in the Gulf of Finland since the 1990s

Elken, Jüri; Raudsepp, Urmas; Laanemets, Jaan; Passenko, Jelena; Maljutenko, Ilja; Pärn, Ove; Keevallik, Sirje

doi:10.1016/j.jmarsys.2013.04.015

Cited by 28 publications

(25 citation statements)

References 39 publications

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“…The shift year 1989 was found for the majority of time series analysed in this study. This result is in good concordance with the previous investigations in the same region (Keevallik and Soomere, 2008;Elken et al, 2014;Soomere et al, 2015).…”

Section: Discussionsupporting

confidence: 93%

“…An increase in zonal circulation starting in 1987 was detected in February (Keevallik and Soomere, 2008). A similar shift at the end of the 1980s was found in the time series of cumulative wind stress calculated for December-January using data from Utö (Elken et al, 2014). Dynamics of wave properties of the Baltic Sea were analysed by Soomere and Räämet (2014), who simulated these using the WAM model.…”

Section: Introductionsupporting

confidence: 54%

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Trends and regime shifts in climatic conditions and river runoff in Estonia during 1951–2015

et al. 2017

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Abstract. Time series of monthly, seasonal and annual mean air temperature, precipitation, snow cover duration and specific runoff of rivers in Estonia are analysed for detecting of trends and regime shifts during 1951-2015. Trend analysis is realised using the Mann-Kendall test and regime shifts are detected with the Rodionov test (sequential t-test analysis of regime shifts). The results from Estonia are related to trends and regime shifts in time series of indices of large-scale atmospheric circulation. Annual mean air temperature has significantly increased at all 12 stations by 0.3-0.4 K decade −1 . The warming trend was detected in all seasons but with the higher magnitude in spring and winter. Snow cover duration has decreased in Estonia by 3-4 days decade −1 . Changes in precipitation are not clear and uniform due to their very high spatial and temporal variability. The most significant increase in precipitation was observed during the cold half-year, from November to March and also in June. A time series of specific runoff measured at 21 stations had significant seasonal changes during the study period. Winter values have increased by 0.4-0.9 L s −1 km −2 decade −1 , while stronger changes are typical for western Estonia and weaker changes for eastern Estonia. At the same time, specific runoff in April and May have notably decreased indicating the shift of the runoff maximum to the earlier time, i.e. from April to March. Air temperature, precipitation, snow cover duration and specific runoff of rivers are highly correlated in winter determined by the large-scale atmospheric circulation. Correlation coefficients between the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) indices reflecting the intensity of westerlies, and the studied variables were 0.5-0.8. The main result of the analysis of regime shifts was the detection of coherent shifts for air temperature, snow cover duration and specific runoff in the late 1980s, mostly since the winter of 1988/1989, which are, in turn, synchronous with the shifts in winter circulation. For example, runoff abruptly increased in January, February and March but decreased in April. Regime shifts in annual specific runoff correspond to the alternation of wet and dry periods. A dry period started in 1964 or 1963, a wet period in 1978 and the next dry period at the beginning of the 21st century.

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Section: Discussionsupporting

confidence: 93%

Section: Introductionsupporting

confidence: 54%

Trends and regime shifts in climatic conditions and river runoff in Estonia during 1951–2015

et al. 2017

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“…As a consequence, the stratification was greatly weakened, the current structure was altered and the hypoxic salt wedge disappeared during those events. Our observations revealed that differential advection in case of temporary estuarine reversal (anti-estuarine straining) is an important factor for the stratification disappearance, as suggested earlier by Elken et al (2003) and Elken et al (2013). Unfortunately, within the present study it was not possible to establish the balance of different advection and diffusion terms during rapid changes of stratification because of the long time interval between the along-thalweg mappings and missing information on transverse flow and density structures.…”

Section: Discussionmentioning

confidence: 52%

“…In contrast, northeasterly and northerly winds support standard estuarine circulation, resulting in stronger stratification (Elken et al, 2003;Liblik and Lips, 2011) and import of phosphorus-rich waters from the deeper sub-halocline layers of the Baltic Proper (Lips et al, 2008). Elken et al (2013) analyzed routine CTD measurements and wind data over several decades to investigate possible changes in the frequency of wintertime stratification collapse events. They found that the bottom-to-surface density difference becomes close to zero (note that the winter is already characterized by the weakest stratification in the Gulf, see Haapala and Alenius, 1994) during wind-induced strong estuarine circulation reversal events caused by up-estuary (southwesterly) wind forcing.…”

Section: T Liblik Et Al: Estuarine Circulation Reversals and Relatementioning

confidence: 99%

Estuarine circulation reversals and related rapid changes in winter near-bottom oxygen conditions in the Gulf of Finland, Baltic Sea

et al. 2013

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Abstract. The reversal of estuarine circulation caused by southwesterly wind forcing may lead to vanishing of stratification and subsequently to oxygenation of deep layers during the winter in the Gulf of Finland. Six conductivity, temperature, depth (CTD)+oxygen transects (130 km long, 10 stations) were conducted along the thalweg from the western boundary to the central gulf (21 December 2011-8 May 2012. Two bottom-mounted ADCP were installed, one near the western border and the second in the central gulf. A CTD with a dissolved oxygen sensor was deployed close to the western ADCP. Periods of typical estuarine circulation were characterized by strong stratification, high salinity, hypoxic conditions and inflow to the gulf in the near-bottom layer. Two circulation reversals were observed: one in DecemberJanuary and one in February. The first reversal event was well developed; it caused the disappearance of the stratification and an increase in the oxygen concentration from hypoxic values to 270 µmol L −1 (to 6 mL L −1 ) throughout the water column along the thalweg and lasted approximately 1.5 months. Shifts from estuarine circulation to reversed circulation and vice versa were both associated with strong longitudinal (east-west) gulf currents (up to 40 cm s −1 ) in the deep layer. The change from oxygenated to hypoxic conditions in the western near-entrance area of the gulf occurred very rapidly, within less than a day, due to the intrusion of the hypoxic salt wedge from the NE Baltic Proper. In the eastern part of the gulf, good oxygen conditions caused by reversals remained for a few months.

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“…This can be explained by decreased sea-ice cover (see Chap. 8), which favours wind mixing (Vermaat and Bouwer 2009) and by stronger south-westerly winds in winter that cause stratification collapse events, due to wind straining effects on estuarine gradients (Elken et al 2014).…”

Section: 3mentioning

confidence: 99%

Recent Change—Marine Circulation and Stratification

Elken

Lehmann

Myrberg

2015

Regional Climate Studies

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This chapter describes recent change in the circulation and stratification of the Baltic Sea. A recent warming trend in sea-surface waters has been clearly demonstrated by in situ measurements, remote sensing data and numerical models. Trends in sea-surface temperature (SST) for the past three to four decades based on remote sensing data generally agree with trends determined from in situ observations. Models suggest the current warming within the Baltic Sea lies within the range experienced during the past 500 years. The salinity and stratification of the deep waters are strongly linked to the major inflows of North Sea water that occur sporadically and bring high-saline water into the deep layers of the Baltic Sea. The major inflows normally occur during winter and spring and bring cold oxygen-rich waters into the deep basins. Since 1996, large inflows have also occurred during summer, bringing in warm low-oxygen water. IntroductionChanges in thermohaline characteristics and stratification of the water column are usually analysed on the basis of data from repeat hydrographic observations, originating from shipborne monitoring programmes and/or permanent coastal and offshore oceanographic stations. Such datasets are described in recent books by Feistel et al. (2008) and Leppäranta and Myrberg (2009). In situ observations are often irregular in space and time, especially for offshore areas; therefore, trends and variability in the ocean state can only be determined by careful pre-processing of the data to address issues such as data homogenisation and the suppression of aliasing errors. In recent decades, horizontal undersampling by in situ measurements has been partially offset by the remote sensing of sea-surface properties from satellites; with these data free of spatial aliasing errors due to their high horizontal resolution. Satellite-derived cloud-free products for sea-surface temperature (SST) have been reasonably accurate since the 1980s. Many of the papers published over the past decade that address trends and variability in the circulation and stratification of the Baltic Sea are based on remote sensing data, often in combination with in situ observations and the results of numerical modelling. Improvements in the physical and numerical features of the models, accompanied by model validation studies, have increased confidence in the realism of the model results. Models provide dynamically balanced gridded datasets over decades or longer, including information for 'noisy' variables such as currents, transport and mixing fluxes. This chapter analyses the recent peer-reviewed literature, extending the findings from earlier review (Box 7.1).

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Increased frequency of wintertime stratification collapse events in the Gulf of Finland since the 1990s

Cited by 28 publications

References 39 publications

Trends and regime shifts in climatic conditions and river runoff in Estonia during 1951–2015

Trends and regime shifts in climatic conditions and river runoff in Estonia during 1951–2015

Estuarine circulation reversals and related rapid changes in winter near-bottom oxygen conditions in the Gulf of Finland, Baltic Sea

Recent Change—Marine Circulation and Stratification

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