6 The Gulf of Finland
195
predominant length of waves, the major mixing agent in this practically non-tidal
area. In front of the Saint Petersburg flood barrier, the mixing depth, even after
strong storms, is as small as about 10 m. The maximum vertical density gradient
(sometimes called the jump layer core) lies about 1–3 m below the lower boundary
of the mixed layer. Since the water depth is comparable with the typical depth of
the mixed layer, strong storms may cause mixing of the entire water column and at
times yield large, vertically homogeneous patches.
6.2.5 Upwelling and Turbulence
Other key elements of the stratification-related dynamics in the Gulf of Finland are
mechanically forced vertical motions of water masses called upwelling and downwelling. In both the gulf and the Baltic Sea upwelling events are predominantly
of coastal type and based on Ekman transport, resulting from the horizontal divergence of wind-driven motions in the surface layer. In essence, upwelling is the
penetration of dense, cooler and usually nutrient-rich water towards the sea surface. It not only affects dramatically the stratification but has an important effect
on redistributing nutrients and other substances in the vertical direction (see, e.g.,
Lips et al. 2009). Its complementary process—downwelling—takes warmer, usually
nutrient-depleted surface water to the lower layers but usually does not play such an
important role in sea dynamics as upwelling does.
In the Gulf of Finland upwelling is typically triggered by alongshore winds (Haapala 1994). Owing to the complex coastline and the presence of many islands, wind
from virtually any direction may cause up- or downwelling near a certain coast with
accompanying vertical mixing and displacement of water masses. The predominant
westerly winds normally cause upwelling in the Finnish nearshore and downwelling
at the Estonian coast. Easterly winds lead to a reversed pattern of these phenomena.
At the Finnish coast of the gulf upwelling occurs as frequently as 15–30 % of
the time (Lehmann et al. 2012). A wind event with a duration of about 60 hours or
a wind impulse of the order of 4000–9000 kg/(m s) (Haapala 1994) is usually required for such an event to develop. During summer and autumn, when the sea surface is warm, upwelling results in a local surface temperature drop of up to 10 °C,
between the upwelled water and the surrounding surface water, which is easily detected by infrared satellite measurements (Fig. 6.10). The horizontal gradient may
reach 1 °C/km and the vertical velocity 3 × 10 −5 m/s (Myrberg and Andrejev 2003).
The nutrient-rich waters gradually spread offshore via advection and horizontal diffusion in the form of so-called upwelling filaments (Zhurbas et al. 2007), which
are often steered by local topographical features. The major primary effects are observed in a 5–20 km broad coastal zone (sometimes from 5 km and/or up to 40 km)
in the entire Baltic Sea (Lehmann et al. 2002, 2012; Myrberg and Andrejev 2003;
Lehmann and Myrberg 2008; Myrberg et al. 2010b) (Fig. 6.10) and extend to about
100 km alongshore (Gidhagen 1987). Their lifetime spans from several days to several weeks.
195
predominant length of waves, the major mixing agent in this practically non-tidal
area. In front of the Saint Petersburg flood barrier, the mixing depth, even after
strong storms, is as small as about 10 m. The maximum vertical density gradient
(sometimes called the jump layer core) lies about 1–3 m below the lower boundary
of the mixed layer. Since the water depth is comparable with the typical depth of
the mixed layer, strong storms may cause mixing of the entire water column and at
times yield large, vertically homogeneous patches.
6.2.5 Upwelling and Turbulence
Other key elements of the stratification-related dynamics in the Gulf of Finland are
mechanically forced vertical motions of water masses called upwelling and downwelling. In both the gulf and the Baltic Sea upwelling events are predominantly
of coastal type and based on Ekman transport, resulting from the horizontal divergence of wind-driven motions in the surface layer. In essence, upwelling is the
penetration of dense, cooler and usually nutrient-rich water towards the sea surface. It not only affects dramatically the stratification but has an important effect
on redistributing nutrients and other substances in the vertical direction (see, e.g.,
Lips et al. 2009). Its complementary process—downwelling—takes warmer, usually
nutrient-depleted surface water to the lower layers but usually does not play such an
important role in sea dynamics as upwelling does.
In the Gulf of Finland upwelling is typically triggered by alongshore winds (Haapala 1994). Owing to the complex coastline and the presence of many islands, wind
from virtually any direction may cause up- or downwelling near a certain coast with
accompanying vertical mixing and displacement of water masses. The predominant
westerly winds normally cause upwelling in the Finnish nearshore and downwelling
at the Estonian coast. Easterly winds lead to a reversed pattern of these phenomena.
At the Finnish coast of the gulf upwelling occurs as frequently as 15–30 % of
the time (Lehmann et al. 2012). A wind event with a duration of about 60 hours or
a wind impulse of the order of 4000–9000 kg/(m s) (Haapala 1994) is usually required for such an event to develop. During summer and autumn, when the sea surface is warm, upwelling results in a local surface temperature drop of up to 10 °C,
between the upwelled water and the surrounding surface water, which is easily detected by infrared satellite measurements (Fig. 6.10). The horizontal gradient may
reach 1 °C/km and the vertical velocity 3 × 10 −5 m/s (Myrberg and Andrejev 2003).
The nutrient-rich waters gradually spread offshore via advection and horizontal diffusion in the form of so-called upwelling filaments (Zhurbas et al. 2007), which
are often steered by local topographical features. The major primary effects are observed in a 5–20 km broad coastal zone (sometimes from 5 km and/or up to 40 km)
in the entire Baltic Sea (Lehmann et al. 2002, 2012; Myrberg and Andrejev 2003;
Lehmann and Myrberg 2008; Myrberg et al. 2010b) (Fig. 6.10) and extend to about
100 km alongshore (Gidhagen 1987). Their lifetime spans from several days to several weeks.