During the summer months, when the wind blows from land or along the coast with land to the left of the wind direction, the warm surface water is transported away from the coast. This happens for example along the Swedish east coast when the wind blows from between south and west.
If the wind is strong enough and blows for long enough, the whole entire volume of the warmer surface water is blown out to sea, and the colder layer of water underneath (30 m) is brought to the surface.
The deeper water often has a temperature of 5-10 degrees and the water out at sea is warmer than at the coast. Upwelling is common along Sweden’s east coast, especially in the Hanö Bay and around Gotland, and because of the prevailing westerly winds is least common along the west coast.
Upwelling
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Wind has a great effect on the water temperature. A sea breeze leads to warmer water at the coast, while an offshore breeze leads to colder nearshore water. This is because when the wind blows towards the land (an onshore breeze) warm surface water is forced inshore. The opposite happens when the wind blows offshore: warm coastal water is moved offshore, and is replaced by colder water brought up from underneath. This phenomenon is called upwelling.