Currents are large scale water movements which occur everywhere in the ocean, both on the surface and far below it. Tidal currents are the horizontal currents associated with the vertical rise and fall of the tide, while wind-generated currents are flow caused by the wind’s drag on the sea surface. Because of the Earth’s rotation, currents will be deflected to the right on the Northern Hemisphere, and variations in the bottom topography will affect the current direction. Friction between the sea bottom and the water masses causes a decrease in the current speed near the sea bottom. Eddies may occur over undersea mountains and in areas where two water masses meeting each other with different speed. The result is that the current is composed of many periodical and non-periodical movements and may vary from place to place and hour by hour.
Tides and tidal currents are two manifestations of the same process. They are most closely coupled. Tidal currents are the horizontal currents associated with the vertical rise and fall of the tide. The tidal current associated with a rising tide is called the flood, whereas the current associated with a falling tide is called the ebb. Slack water occurs during the short time interval between the end of the flood and the beginning of the ebb, or vice versa, when the water has no horizontal motion.
Tidal waves have periods of about 24 hours and 12 hours, corresponding to diurnal and semidiurnal tides and so their wavelengths are increased to many hundreds of kilometres. The wave speed is depth dependent and can be several hundred knots with a wavelength, some places, of 5000 nautical miles. The wave speed is lower in shallow water than in deep oceans. Although the connection between the tide and tidal currents is familiar it must not be supposed that the strength of the current, nor the times of slack water, at any locality necessarily coincide with the corresponding vertical changes of the tide. Under certain conditions, in fact, slack water occur midway between high and low tide, contrary to the commonly accepted notion that it occurs at high and low tide.
The oceanic tide propagates as a very long wave, and high tides correspond to the wave crests, and low tides to the wave troughs. Tidal currents in the ocean are the horizontal motions associated with the passage of this wave; floods correspond to the forward motions under the crests, and ebbs to the backward motions under the troughs. The speed of the tidal current is highest at high and low water. In the open ocean along the Norwegian coast from the west coast (Vestlandet) to the northern part (Finnmark) the tidal wave is propagating northwards, and the current speed northwards is highest at high water and the current speed southwards is highest at low water. In open ocean the tidal current has the same periodicity as the vertical oscillations, but tend to follow an elliptical path and do not normally involve a simple to-and-from motion. The sense of rotation of the ellipse may be either clockwise or anticlockwise, but rotations cum sole tend to be favoured if there are no constraining landmasses. The current will turn 360º during the course of a tidal period (12 hrs 12 min).
Mouth of the fjord
At the mouth of the fjord it is slack water at high and low tide and maximum current between high and low tide, i.e. the current moves in the fjord at rising tide and out the fjord at falling tide.
Near land, in straits and in fjords
Near land, in straits and in fjords the current can only run in two opposite directions, i.e. parallel with the shore.
The current speed follows approximately the tidal range, i.e. the speeds in the Skagerrak area is very low and along the west coast of Norway the speed is about 0.5 knots and is increasing northwards to about 1 knot at the northern part of Norway (Finnmark). In narrow straits and at the mouth of the fjords the current speed can be several knots.
In the North Sea the highest current speeds occur in the north-south direction, except for the southern part where the east-west direction is dominant. The current speed is increasing from the eastern part of the North Sea towards Britain where the speed can be about 2 knots. In the British Channel and in the Pentland Firth (between Scotland and the Orkney Islands) the current speeds are especially high.
The most famous tidal current in Norway is Saltstraumen in Nordland, not far from Bodø. Huge water masses passes through the narrow sound four times daily. The mean speed is estimated to 7-9 knots, with maximum speeds far above this. No reliable current records are available.
The current speed is dependent of the tidal range and the current speed is highest at spring tides (new moon or full moon) and with possible onshore wind. Huge whirlpools are formed; sometimes they can be as much as 10 metres in diameter and about 6 metres deep. When the current is at maximum it can be dangerous for boats to pass through the sound. During the centuries there have been several fatal accidents in Saltstraumen. .
Moskenstraumen (the Maelstrom) is one of the world’s strongest tidal currents. It is located between the Lofoten Point and the island Værøy southwest of the main chain of the Lofoten Islands. Moskenstraumen is about 4 kilometres across and 40 to 60 metres deep, and is considerably shallower than the surrounding sea.
The tide fills up the Vestfjord (inside Moskenstraumen) twice a lunar day (about 24 hours and 50 minutes), and a strong sea level gradient occur. This sea level gradient drives the current around Lofotodden and the other strong tidal currents in the narrow sounds between the Lofoten Islands further east. The current is said to run with speed up to 10-12 knots, but there are no current records available that can be used for estimates of current extremes.
The mariner has learned that tidal current often represents a considerable risk for marin activities. In some cases the tidal current in interaction with wind waves and swell may lead to dangerous wave conditions. The details of the tides at any given place are governed by the responses of the ocean to the gravitational forces, as modified by the local coastal and bottom topography. By using high resolution tidal models it is possible to calculate the tidal current fields for a given geographical area (in a grid, see figure on this webpage). It is important to have an accurate depth matrix for the simulations. Sea level and current measurements are needed for validation of the model.
Read more about Tidal models -Trondheimsleia and Tjelsundet