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Sea surface temperatures

Cape Town's coastal water ranges from cold to mild, and the difference between the two sides of the peninsula can be dramatic. While the Atlantic Seaboard averages annual sea surface temperatures around 13 °C (55 °F), the False Bay coast is much warmer, averaging between 16 and 17 °C (61 and 63 °F) annually. In summer, False Bay water averages slightly over 20 °C (68 °F), with 22 °C (72 °F) an occasional high. Beaches located on the Atlantic Coast tend to have colder water due to the wind driven upwellings which contribute to the Benguela Current which originates off the Cape Peninsula, while the water at False Bay beaches may occasionally be warmer by up to 10 °C (18 °F) at the same time in summer.

In summer False Bay is thermally stratified, with a vertical temperature variation of 5 to 9˚C between the warmer surface water and cooler depths below 50 m, while in winter the water column is at nearly constant temperature at all depths. The development of a thermocline is strongest around late December and peaks in late summer to early autumn. In summer the south easterly winds generate a zone of upwelling near Cape Hangklip, where surface water temperatures can be 6 to 7 °C colder than the surrounding areas, and bottom temperatures below 12 °C.

In the summer to early autumn (January–March), cold water upwelling near Cape Hangklip causes a strong surface temperature gradient between the south-western and north-eastern corners of the bay. In winter the surface temperature tends to be much the same everywhere. In the northern sector surface temperature varies a bit more (13 to 22 °C) than in the south (14 to 20 °C) during the year.

Surface temperature variation from year to year is linked to the El Niño–Southern Oscillation. During El Niño years the South Atlantic high is shifted, reducing the south-easterly winds, so upwelling and evaporative cooling are reduced and sea surface temperatures throughout the bay are warmer, while in La Niña years there is more wind and upwelling and consequently lower temperatures. Surface water heating during El Niño increases vertical stratification. The relationship is not linear. Occasionally eddies from the Agulhas current will bring warmer water and vagrant sea life carried from the south and east coasts into False Bay.