270
J. Kjellsson et al.
The spreading rate of pairs of drifters in the uppermost layer of the Gulf of Finland was studied using surface drifters (called GPS/GSM drifters) following motions at a depth down to about 1.5 m (Soomere et al. 2011). The drifters were therefore constrained to 2D flow. However, they may be influenced by the 3D turbulence
of the ocean. Unlike for the SVP drifters, the inertial oscillations were not filtered
out. As the filter did little change to the relative dispersion for the SVP drifters, it
should not markedly influence the results from the GPS/GSM drifters either. Differently from a number of similar studies (Ollitrault et al. 2005; Döös and Engqvist
2007; Lumpkin and Elipot 2010) the experiments performed in the western and
central part of the Gulf of Finland in August–October 2010 were concentrated on
relatively small initial separations of the drifters (∼100 m). The deployments resulted in 7 pairs of drifter trajectories. As the signal from the drifters was lost at
times, it was not possible to adequately calculate the detailed statistics of the drift
but the recorded data still allowed quantification of the temporal evolution of the
pair separations.
The observed trajectories reflected a variety of phenomena characteristic of the
currents in the Gulf of Finland (Fig. 8.15): relatively small mesoscale eddies with a
diameter of about 5 km to the north of Naissaar, inertial oscillations in the open part
of the gulf, and relatively rapid and almost straight drift sections (cf. Kõuts et al.
2010; Verjovkina et al. 2010). While most of the trajectories were relatively short
(shorter than 50 km), one drifter covered more than 150 km during about two weeks
and left the Gulf of Finland to the Gotland Sea.
Contrary to the previously mentioned theoretical expectation, Richardson’s law
has been found to describe spreading properties for small distances fairly well while
Lin’s law has proven a better fit for larger distances (Döös and Engqvist 2007).
A probable reason for this counter-intuitive observation is that the character of
spreading is particularly complicated for 2D flows occurring on the surface of 3D
flows, which is often the case in strongly stratified environments. Velocity fields in
such flows may be highly compressible 5 and may exhibit a considerable decrease in
the exponent b in Eq. (8.12) compared to the pure 2D case (Bec et al. 2004; Kalda
2007). For a review of relevant laboratory experiments see Cressman et al. (2004).
For realistic geophysical flows one might expect quite a large variation in the range
of 1.5 ≤ b < ∞ of this exponent. Note that the Gulf of Finland is rather narrow and
the separation process of drifters eventually becomes affected by the boundaries.
The gulf is O(10 5 ) m wide, but the drifters were not deployed in the middle of the
it. There may therefore be some influence of the southern coast on the drift, e.g., the
drifters could not move as far south as in the other directions.
The typical spreading rate was almost constant for all the pairs within the first
10–15 hours, or until the drifters were separated by about 150 m, and increased considerably afterwards (Fig. 8.16). Although the estimates for initial distances below
5 The (flow) compressibility (equivalently, the compressibility of the associated velocity field) is
defined as the relative weight of the potential component in the decomposition of the net velocity
field into solenoidal and potential components. See a more detailed discussion of this quantity in
the context of the Baltic Sea in Giudici et al. (2012), Kalda et al. (2013).
J. Kjellsson et al.
The spreading rate of pairs of drifters in the uppermost layer of the Gulf of Finland was studied using surface drifters (called GPS/GSM drifters) following motions at a depth down to about 1.5 m (Soomere et al. 2011). The drifters were therefore constrained to 2D flow. However, they may be influenced by the 3D turbulence
of the ocean. Unlike for the SVP drifters, the inertial oscillations were not filtered
out. As the filter did little change to the relative dispersion for the SVP drifters, it
should not markedly influence the results from the GPS/GSM drifters either. Differently from a number of similar studies (Ollitrault et al. 2005; Döös and Engqvist
2007; Lumpkin and Elipot 2010) the experiments performed in the western and
central part of the Gulf of Finland in August–October 2010 were concentrated on
relatively small initial separations of the drifters (∼100 m). The deployments resulted in 7 pairs of drifter trajectories. As the signal from the drifters was lost at
times, it was not possible to adequately calculate the detailed statistics of the drift
but the recorded data still allowed quantification of the temporal evolution of the
pair separations.
The observed trajectories reflected a variety of phenomena characteristic of the
currents in the Gulf of Finland (Fig. 8.15): relatively small mesoscale eddies with a
diameter of about 5 km to the north of Naissaar, inertial oscillations in the open part
of the gulf, and relatively rapid and almost straight drift sections (cf. Kõuts et al.
2010; Verjovkina et al. 2010). While most of the trajectories were relatively short
(shorter than 50 km), one drifter covered more than 150 km during about two weeks
and left the Gulf of Finland to the Gotland Sea.
Contrary to the previously mentioned theoretical expectation, Richardson’s law
has been found to describe spreading properties for small distances fairly well while
Lin’s law has proven a better fit for larger distances (Döös and Engqvist 2007).
A probable reason for this counter-intuitive observation is that the character of
spreading is particularly complicated for 2D flows occurring on the surface of 3D
flows, which is often the case in strongly stratified environments. Velocity fields in
such flows may be highly compressible 5 and may exhibit a considerable decrease in
the exponent b in Eq. (8.12) compared to the pure 2D case (Bec et al. 2004; Kalda
2007). For a review of relevant laboratory experiments see Cressman et al. (2004).
For realistic geophysical flows one might expect quite a large variation in the range
of 1.5 ≤ b < ∞ of this exponent. Note that the Gulf of Finland is rather narrow and
the separation process of drifters eventually becomes affected by the boundaries.
The gulf is O(10 5 ) m wide, but the drifters were not deployed in the middle of the
it. There may therefore be some influence of the southern coast on the drift, e.g., the
drifters could not move as far south as in the other directions.
The typical spreading rate was almost constant for all the pairs within the first
10–15 hours, or until the drifters were separated by about 150 m, and increased considerably afterwards (Fig. 8.16). Although the estimates for initial distances below
5 The (flow) compressibility (equivalently, the compressibility of the associated velocity field) is
defined as the relative weight of the potential component in the decomposition of the net velocity
field into solenoidal and potential components. See a more detailed discussion of this quantity in
the context of the Baltic Sea in Giudici et al. (2012), Kalda et al. (2013).