The condition of the sea changes just as often as the weather in the atmosphere. Motions in the water is given by the sum of the forces that act on a body of water. Currents are caused by winds, horizontal differences in pressure and by the tidal forces. The rotation of the Earth, effects of friction, coasts and bottoms affect the motion. The motion varies greatly in time and space and requires measurements on several occasions on various depths.
Currents in the oceans are like large rivers, limited by thermal and salinity layers in the water column and geographically concentrated due to the rotation of the Earth. The Gulf stream is the largest, which like an enormous river rushes through the North Atlantic. A permanent circulation can often be shown in coastal areas. However, this is affected strongly by the winds and therefore it varies greatly. The speed of the current is given in cm/s. A westerly current flows towards the west.
It is not easy to measure the mean speed and direction of currents in the sea. The motion varies over days, during the year, from place to place and with the depth, i.e. on different space and time scales.
Different kinds of waves are always occurring on the oceans. They can be caused by singular, temporary disturbances, such as volcanic eruptions, by periodic disturbances, such as tidal forces, or by permanent disturbances, e.g. wind and air pressure. In a wave the water particles move in closed orbits. As a result of this, waves with crests and troughs are formed in the surface (see pdf version). Short waves are formed when the depth of the water is greater than half the length of the waves. Long waves are formed if the length of the wave is greater compared to the depth. In the sea, waves are not only formed in the surface, but long internal waves can occur in temperatures or salinity clines between water masses. The internal waves can be very important for the exchange between surface and deep water. If the internal waves break, oxygen rich surface water will be mixed down to the bottom water and at the same time, nutritious water from the bottom is brought up to the surface.
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of Earth. The period of the tides around Sweden is approximately 12 hours. It is a so called half daily tide (semidiurnal) with two high tides and two low tides each day. SUBMON: Add information for the Mediterranean. The time between high and low tide is called ebb and the time between low and high tide is called flood. The amplitude is the deviation of the water level from the average water level. The North Atlantic is large enough, so that tides with an amplitude of 1 metre can form. The Baltic Sea is too small and has tidal oscillations with amplitudes of 17-19 cm. Tides in the Baltic Sea are mostly caused by sea breeze (wind), so that by strict definition, these are not tides.
The tide can easily be measured by mounting a dipstick in the water front, for example on a mountain side or a jetty. The sea level is read every hour for at least 12 hours. During this time, one high and one low tide will pass. The sea level is also affected by winds and air pressure, so that anticyclones give low sea levels, while cyclones give high sea levels.