Coastal upwelling (CU), a phenomenon found in large stratified lakes, estuaries, and oceans [1
], is an important process for ecologically sensitive regions of the global ocean, such as the Baltic Sea, a large brackish water body that has a very limited water exchange with the open ocean through the narrow and shallow Danish straits [2
]. Its extensive coastlines oriented in many directions also mean that coastal upwelling can occur with sustained wind over the Baltic Sea in almost any direction [3
], which makes upwelling quite a common process [4
Coastal upwelling is one of the main factors affecting the circulation and the ecosystem of the Baltic Sea region. It is responsible for the mixing of water masses [5
] as well as for water exchange between the coastal zone and the open sea; the formation of frontal zones with strong cross-front property gradients [6
]; and the modification of biotic and abiotic conditions of marine ecosystems by bringing cold, nutrient rich waters from the depth to the euphotic layer [8
]. The formation of upwelling fronts also importantly modifies the vertical stratification and turbulent regime in the marine–atmosphere boundary layer (MABL), resulting in a change in the surface wind stress and direction in the coastal zone [3
Nutrient-rich waters brought up from deeper layers to the surface, particularly in the summer time, and the exposure of upwelled phytoplankton to surface radiation enhances the primary production and phytoplankton biomass during upwelling events [10
], hence influencing the coastal pelagic communities and higher trophic levels [12
]. Moreover, upwelling-related coastal ocean dynamics may eventually modify the spatial patterns of background algae blooms in the coastal zone by transporting them farther offshore.
During the summer-time holiday season, CU might also have a negative impact on particular tourist areas as a result of a rapid drop in water and air temperatures near the shore [2
]. Furthermore, bathing tourism depends on good water quality, while post-upwelling phytoplankton blooms might significantly reduce it, making coastal waters unattractive for recreation [13
The need to study coastal upwelling in the Baltic Sea is also essential in order to assess the regional variability of water and energy exchange, salinity dynamics, and the response of marine ecosystems to extreme events—some of the “Grand Challenges” of the Baltic Earth Science Plan [14
], established in 2016. Thus, the upwelling phenomenon is indeed of certain interest to researchers, fishermen, and coastal managers [15
]. The availability of wide spatial and temporal coverage multi-spectral satellite data comes in handy when gathering information on coastal upwelling properties, as conventional in situ monitoring methods are limited in space to resolve the full spatial patterns of upwelling properties, its dynamics, and implications to the coastal environment. Besides, the size and complexity of coastal waters makes it difficult to monitor them with ships and buoys alone, while satellites are proving to be cost-effective for observing large ocean and coastal areas [16
]. Our study is primarily based on the utilization of satellite infrared and visible band data, which is now traditionally used for such studies (e.g., [1
]), and has certain advantages over in situ measurements and models in that it provides the spatial details and dynamical features of the phenomena.
In general, Baltic Sea coastal upwelling has received considerable attention in the literature (e.g., [2
]). In this work, our focus is on the south-eastern (SE) part of the Baltic Sea, a frequent location of upwelling development under northerly winds (see e.g., [2
]). Yet, no comprehensive investigations of upwelling properties have been carried out so far in this region; most of the existing works have typically addressed CU across the entire Baltic Sea basin, providing limited information about its development and implications for the SE Baltic (SEB). Several model studies analyzed the upwelling generation and evolution process, and its vertical structure in the study region; however, their results appear to underestimate the horizontal upwelling parameters when compared to satellite-derived sea surface temperature (SST) maps [22
]. More details about the fine horizontal structure of CU front in the SEB are available from two case studies based on high-resolution synthetic aperture radar (SAR) measurements combined with infrared (IR) SST maps and models [9
]. Yet, these works have very limited temporal coverage and considered only a few upwelling events, and primarily addressed the mechanisms of upwelling manifestation in the satellite data.
Our aim here is to provide the first detailed record of coastal upwelling events and their statistical properties in the SE Baltic Sea during 16 years of satellite observations, from 2000–2015, with a special focus on a major upwelling event in the summer of 2006. As will be further shown, coastal upwelling has a significant influence on the environmental conditions not only of the SE Baltic Sea, but also in the shallow Curonian Lagoon. In Section 2
, the data and methods are addressed, while the main results of the satellite-based analysis and discussion follow in Section 3
. Section 4
contains a summary followed by conclusions.
4. Summary and Conclusions
This study provides a detailed analysis of wind-induced coastal upwelling and its properties in the SE Baltic Sea based on multi-mission satellite data. Upwelling events were detected during the thermally stratified season between April and September, with the earliest registered on 14 April 2010 and the latest on 23 September 2008. About 90% of events were observed between May and August, with a clear peak in July characterized by eight upwelling days on average. For the high upwelling season from May to August, the monthly average value is about seven upwelling days per month, or 20–25% of the month.
An evaluation of the Ekman-based upwelling index enabled the identification of 96 upwelling events, meaning that the satellite-based CU detection represents about 72% of the total of all possible upwelling events. The use of satellite data is more effective during May–August, allowing for the observation of about 87% of all UI-based upwelling events under relatively cloud-free summer conditions. In general, about one to three days of positive UI values are needed for the upwelling front to be manifested in the SST data. As has been found, the milder upwelling-favorable winds of a shorter duration are required to cause upwelling during the summer months when the seasonal thermocline is shallow and the vertical stratification of the water column is at its strongest, and a longer and stronger wind impulse is needed in the spring and early autumn.
The obtained results show that short-lived upwelling events that are two to six days long clearly dominate in the record, while about 27% of the events persist for seven to ten days. In some distinct cases, the coastal upwelling may last up to 23 days, as a result of a chain of consecutive upwelling events. The frequency of coastal upwelling ranged from one to several events per year, occurring about four times per season on average. The longest upwelling season was up to 57 days per year, covering about 30% of the warm period.
As has been further shown, no significant latitudinal differences in upwelling-induced ∆T values along the SEB coast were observed with maximum and median values of about 12 °C and 4 °C, respectively. At the same time, considerable spatial variations of the SST gradient were observed with a maximum value of about 1.5 °C km−1. In the summer and autumn months, higher SST gradients clearly prevail, while the highest values (more than 0.75 °C km−1) are observed more frequently in the summer than in the spring or autumn.
The most frequent values of the along-shore length of the upwelling front are about 300–350 km, covering almost the entire coastal zone of the SE Baltic Sea. In 57% of the cases, the upwelling extent is up to 3000 km2, covering mainly the Lithuanian and Latvian coastal waters. Yet, it may reach up to 16,000 km2 during some extreme events, extending over a significant part of Gdansk and the eastern Gotland Basins.
The cross-shore extent of the upwelling front in the SE Baltic Sea ranges between 5 and 70 km, yet during intensive long-lasting events, it may exceed 30–40 km everywhere, and reach 60–70 km in some locations. This is quite significant considering that the average width of the Baltic Sea is around 190 km. Furthermore, the generation of such filamentary patterns results in mixing between coastal and open waters, and enhances the exchange of biological properties between them. The maximum values of the cross-shore extent are recorded along the Latvian coast near Pape (70 km) and Liepaja (61 km). The generation of such cold-water jets and transverse filaments was reported earlier, but the length that was reported was twice as short as the one recorded here.
An analysis of the satellite optical data during the major upwelling event in the summer of 2006 clearly shows a clear three-fold decrease in the chl-a concentration in the coastal zone, relative to the ambient waters. Similarly, a strong chl-a gradient resulting from the inflow of upwelled water masses is identified in the northern part of the Curonian Lagoon, where the chl-a concentration drops down by an order of magnitude. As a result, the overall drop in the background chl-a concentration in the Curonian Lagoon before and after an upwelling event is ~15 mg m−3.
As has been further shown, the presence of a cold upwelling front might significantly alter the stratification of the marine atmosphere boundary layer and result in a significant drop of air temperature and near-surface winds in the coastal zone. As such, it is prone to affect the formation and frequency of sea fog, which might have implications for the Klaipeda Port activities as well as the offshore and near-shore wind farming that is planned in this area in the near future.
The study outcomes suggest that the satellite-derived SST, wind, and chl-a products could be used to analyze the intensity of the coastal upwelling and its biological and atmospheric response, and, moreover, yield significant information about sea–lagoon interaction.