Elsevier

Estuarine, Coastal and Shelf Science

Volume 209, 30 September 2018, Pages 123-135
Estuarine, Coastal and Shelf Science

The role of nearshore slope on cross-shore surface transport during a coastal upwelling event in Gulf of Finland, Baltic Sea

https://doi.org/10.1016/j.ecss.2018.03.018Get rights and content

Highlights

  • Bathymetry of the offshore plays a major role in the onset of cross-shore jets.

  • Cross-shore upwelling jets likely start from steep nearshore segments.

  • The source depth of upwelled waters may be estimated from the extent of cross-shore jet.

  • This depth discloses the flux of nutrients (phosphates or nitrates) brought up to surface.

Abstract

The wind induced coastal upwelling process often contains a mid-phase that involves coherent long-living cross-shore surface jets of cooler water. These jets extend to 40–45 km from the coast and tend to start from particular coastal locations. We develop a simple method for evaluating the depth in the water column from where the upwelled water originates. We also test the hypothesis that the development of cross-shore surface jets may be triggered by some permanent characteristics such as the bathymetry or the slope of the nearshore seabed. The analysis is performed for a strong and well-documented upwelling in the Gulf of Finland, the Baltic Sea, using high resolution bathymetry data, satellite derived sea surface temperature, surface currents measured by in situ drifters, and properties of water masses in two sampling locations. The results indicate that the cross-shore jets originate exclusively from the shore sections with much steeper slopes (>0.0075) than in the rest of the study area. The cooler water most likely origins from intermediate water masses at depths between 15 and 30 m. The resulting identification of the source depth of the upwelled water and its spatial location assists in understanding the flux of nutrients during upwelling events and their link to the onset of cyanobacteria blooms.

Introduction

Wind-driven coastal upwellings serve as a core mechanism responsible for the vertical exchange of water masses and different substances in the World Ocean. Recent studies have also indicated that such phenomena (e.g., upwellings in the Southern Ocean) may even influence our climate and ecosystem globally (Anderson et al., 2009; Morrison et al., 2015). Among several interesting aspects of the upwelling phenomena, this study focuses on the role of the near-shore slope on the presence of distinct surface features during an upwelling event in the Gulf of Finland, Baltic Sea (Fig. 1). To isolate and quantify these features, it is important to recap that an upwelling is generally represented by two stages: an active phase and a relax phase (Gurova et al., 2013). The active phase develops when persistent winds stimulate offshore Ekman transport of surface waters. This subsequently generates an upward movement of denser and usually cooler water from deeper layers and an associated tilt of isopycnals. If the wind lasts long enough, cooler water surfaces and forms an elongated patch in the nearshore or an alongshore jet (Fig. 2a) (Bakun, 1990).

If the wind weakens or rotates so that offshore Ekman transport diminishes, the upwelling enters into so-called relax phase. The strong temperature and/or density gradients on the sea surface produced in the first phase, persist for some time (Zhurbas et al., 2008; Gurova et al., 2013). The nearshore patch of upwelled water (or the associated alongshore jet) eventually becomes unstable (Zhurbas et al., 2008) and usually develops various (sub)mesoscale phenomena such as jets, filaments, eddies, etc. (named differently by different scientists). Evidence of an upwelling event is often captured by satellite measurements. This decrease in sea surface temperature (SST) tends to have a strong signature compared to that of surrounding waters. As satellite images are often contaminated by cloud cover, the appearance and evolution of the different phases may not always be captured.

The gulf has two specific features that may impact the course and appearance of upwellings. Firstly, it is a shallow sea area with a mean depth of about 37 m. Secondly, its temperature regime has a strong seasonal variation. This area regularly hosts clearly detectable upwellings during the summer and autumn months when the water masses are relatively strongly stratified (Zhurbas et al., 2008; Gurova et al., 2013). Many studies have observed the dynamics of the different phases of upwellings in the gulf (Zhurbas et al., 2008; Laanemets et al., 2011). This process is crucial for the gulf ecosystem. During strong thermal stratification the surface layer is depleted of nutrients. On the one hand, upwellings supply the euphotic zone with nutrients sourced from deeper layers. Whilst, on the other hand, this process also supports the onset of cyanobacterial blooms (Laanemets et al., 2004).

Owing to the importance of this process for the Gulf of Finland, numerous studies have examined the upwelling phenomenon in this region using various methods (models, in situ, satellite) (Myrberg and Andrejev, 2003; Suursaar and Aps, 2007; amongst others). Due to the prevailing south-westerly and westerly winds the northern nearshore of the gulf hosts upwellings during some 30% of the time whereas the southern coasts often (during about 25% of the time) has downwellings (Myrberg and Andrejev, 2003).

Even though upwellings in the southern nearshore are less frequent and not as persistent as near the northern coast, strong upwellings also occur near the southern coast of the gulf. The changing wind patterns (Soomere et al., 2015) apparently have increased their frequency in the recent past. This change may substantially modify the functioning of the ecosystem of the gulf. In particular, it may cause a major shift of the feeding area of fish and associated wildlife.

This study focuses on an intense upwelling event near the south-western coast of the gulf that was triggered by a relatively infrequent system of persistent easterly winds. This event has an unprecedented coverage by in situ drifters that followed surface currents in the area directly affected by the upwelling (Delpeche-Ellmann et al., 2017). As it developed during a time interval with a reasonable number of at least 75% cloud free days, this event was also captured in a sequence of high-resolution satellite images.

This unusually wide coverage made it possible to establish several specific features and interesting developments of the upwelling events in the region. The analysis of satellite-derived SST and in situ drifters’ motions near the entrance of the gulf (Fig. 1) has demonstrated that on several occasions the upwelling process may contain an additional well-defined mid-phase as depicted in Fig. 4 of Delpeche-Ellmann et al. (2017). This phase becomes evident between the classic active phase (when cooler water upwells near the coast and optionally forms an alongshore jet at the surface) and the relaxation phase (when the jets start to disintegrate). This phase is characterised by the presence of jets of cooler water that gradually migrate offshore. The intensity of lateral mixing is low during this phase. After some time, in the relaxation phase, after the winds have decreased, these cross-shore jets lose their identity and develop into several filaments and mesoscale features. The most intense mixing occurs in this phase (Fig. 3b) (Zhurbas et al., 2008; Delpeche-Ellmann et al., 2017).

The focus of this study is on the characteristic feature of upwellings in this region. Namely, during the mid-phase the cross-shore jets tend to migrate offshore from distinct locations (Fig. 2b). This feature has been identified by several studies of upwelling events in this area (Suursaar and Aps, 2007; Laanemets et al., 2011; Kikas and Lips, 2016). It has an interesting (but not necessarily dynamically related) analogue in model simulations of Lagrangian transport of surface waters in the gulf. Namely, such simulations suggest that an intense net-transport of surface waters in the north-south direction systematically occurs in the vicinity of the two distinct locations where such cross-shore jets have been observed (Fig. 3a, c). This (albeit qualitative) match of the outcome of the two independent studies signals that two locations near the southern coast of the gulf may host certain permanent (e.g., geographic) features that enhance intense surface cross-shore transport.

The idea that bathymetry may have a strong influence on the appearance of the cross-shore jets is not new and has been repeatedly mentioned in previous studies (Zhurbas et al., 2004; Laanemets et al., 2009). However, this connection has not been quantified and the actual verification of an interrelation of the bottom slopes and the behaviour of cross-shore jets has remained unexplored. One of the main objectives of this study is to quantify, at least to a first approximation, the role of the sea bed slopes at certain locations on the development of cross-shore jets by utilising simple statistical analysis of satellite SST and in situ drifter data.

The evolution and transport of water masses and different substances in coastal upwellings are generated and influenced by a multitude of drivers. These drivers can be grouped into two categories. Firstly, permanent (temporally and spatially unchangeable) conditions such as geographic features (the shape of the nearshore, bathymetry, coastline undulations, etc.) that can potentially steer upwelling events. Secondly, a particular upwelling is shaped by several features that vary greatly on weekly to annual scales such as properties of winds and characteristics of water masses such as stratification, the associated internal Rossby radius, the local water level and interactions with other oceanic processes (waves, eddies etc.) (Preller and O'Brien, 1980; Narimousa and Maxworthy, 1985, 1987a; 1987b; Lentz and Chapman, 2004; Chen et al., 2013).

Extensive variability of some of these drivers affects the appearance of the upwelling and its transport properties both spatially and temporally (Miranda et al., 2013; Tim et al., 2015; Wang et al., 2015). Upwellings generally tend to appear in similar locations. Due to the short-term changes in wind and ocean properties, the appearance and intensity of single upwellings usually differ from each other (Suursaar and Aps, 2007). Globally and locally changing wind patterns evidently enhance such differences (Bakun, 1990; Soomere et al., 2015). In this light it is increasingly important to understand which properties of upwellings are invariant with respect to the listed changes. Solving this task obviously contributes to further clarification of the functioning of the ecosystem of the gulf. Ecosystem studies usually utilise a vast amount of resources from massive in situ data collection up to extensive computing facilities. As the upwelling events have usually intermittent nature, it is usually not possible to timely concentrate the necessary resources and simpler solutions may at times be required to study their properties. A feasible way is to exploit the satellite data in combination with limited in situ data.

Many studies have examined the role of coastline features and local bathymetry in the dynamics of upwellings since the 1980s (Preller and O'Brien, 1980; Narimousa and Maxworthy, 1985, 1987a; among others). It is likely that some concealed features of bathymetry such as the nearshore slope may steer, stabilise and even trap upwelling events (Kämpf, 2012). While coastline features seem to have very little effect on the upwelling dynamics, bottom topography tends to steer the upwelling maximum (Kämpf, 2012).

The upwelling dynamics is often examined using a steady two-dimensional theory (Lentz and Chapman, 2004; Choboter et al., 2011, among others). Its outcome was supported by observations of depth averaged velocity profiles and by numerical simulations with primitive equations. It is convenient to examine the cross-shelf momentum flux divergence relative to the wind stress using a dimensionless parameter – the Burger number. It indicates where the upwelled water may have originated from in the deeper layers (e.g., Fig. 2, Vi or Vb). The knowledge of interrelations between Vi (the onshore return flow) and Vb (the flow in the bottom boundary layer) makes it possible to a certain extent conclude whether nutrients, carbon, sediment parcels, or oxygen may be present in the upwelled waters.

Most of the relevant studies have focused on the onshore–cross-shore transport dynamics related to the deeper layers. Their main question has been whether the source of the cooler water is the intermediate layer (Vi) or the bottom layer (Vb).

A different viewpoint was adopted by Narimousa and Maxworthy (1985). They addressed the parameters that could be used to calculate the cross-shore extent of the upwelled water on sea surface (Fig. 2, Fig. 3a). They utilized the geostrophic balance between the Coriolis force and the horizontal pressure gradient forces. In this paper, we attempt to utilize the theory developed by Lentz and Chapman (2004) to determine where within the deeper layers the cooler surface water originated from. Further on, we apply the approach of Narimousa and Maxworthy (1985) to determine the extension of the upwelled water patch.

Even though the presence of the cooler upwelled water is evident in satellite SST data, its dynamics is largely unknown. Our intention is to combine SST data and information about bathymetry with in situ data about currents to: (1) identify a possible threshold for bottom slopes that may serve as a trigger for the cross-shore jets, (2) utilize the Burger number (Lentz and Chapman, 2004) to identify the depth (or layer) where the cooler surface water may have originated from, (3) exploit the concepts developed by Narimousa and Maxworthy (1985) to quantify the cross-shore extension of upwelling jets and (4) finally, combine an estimate of this extension and in situ data about stratification to approximately solve a sort of inverse problem – to derive an estimate of unknown parameters such as the thickness of the surface layer. These results would assist in identifying the transport of nutrients in the surface layer and their link to cyanobacterial blooms in the study area. As we mostly rely on nondimensional parameters, the outcome outlines a simple method for identifying some of the characteristics of the upwelling process, especially when limited resources are available.

Section snippets

Study area

The Gulf of Finland is an elongated bay at the north-eastern end of the Baltic Sea. It stretches from the main basin of the Baltic Sea between Finland and Estonia to the major city of Saint Petersburg in Russia. The gulf is about 400 km long and its width varies between 48 and 125 km (Fig. 1). The impact of saltier water from the Baltic Proper combined with a large amount of fresh water input from the eastern gulf results in a strong gradient and extensive spatio-temporal variability in

Stratification and chemical parameters

The upwelling in question was apparently triggered by persistent easterly winds on 21–24 May 2013 with an average speed of 7.6 m/s according to the data from the Kalbådagrund weather station. The SST decreased from 8 °C on 24 May to 4 °C on 28 May 2013. This event contained a clearly identifiable mid-phase (about 28 May–2 June) between the active and relax phases. During this phase the wind speed was still high, the upwelled water travelled almost straight across the gulf to a distance of

Discussion

Previous studies in the Baltic Sea have suggested that a strong relationship exists between the appearance of cooler water jets during upwelling events and bottom inhomogeneity (Zhurbas et al., 2004; Laanemets et al., 2009; Gurova et al., 2013). One of the central topics addressed in this paper was to demonstrate and quantify whether a robust connection exists between the shape of the seabed (in particular, its cross-shore slope) and the location of the cross-shore jets of cooler upwelled

Conclusions

  • A simple method that uses satellite derived sea surface temperature data and limited in situ data is developed to evaluate several properties of upwellings.

  • Steep bottom slopes of >0.0075 have a strong influence on the location and development of cross-shore jets observed during upwelling events in the Gulf of Finland.

  • The upwelled cooler water was most likely sourced from the interior of the water column during the upwelling event in May–June 2013. This feature suggests that the dynamics of

Acknowledgments

The research was supported by the institutional financing by the Estonian Ministry of Education and Research (Estonian Research Council grant IUT33-3) and partially by the ERA-NET+RUS project EXOSYSTEM (grant 4-8/16/1). Several underlying ideas are sourced from the project,“Infotechnological Mobility Observatory” (IMO), funded by European Regional Development Fund. The authors are deeply grateful to the Finnish Meteorological Institute for providing the wind data at Kalbådagrund, the Estonian

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