Why ocean temperatures have dropped dramatically along the Sydney and NSW coast

A stagnant weather pattern over the past week has triggered a dramatic drop in ocean temperatures along parts of the NSW coast, with Sydney and central NSW beaches seeing some of the biggest changes.

While warm northerly winds have pushed air temperatures towards 30°C, they've also driven a process known as coastal upwelling, bringing much colder water to the surface near the coast.

High pressure dominates coastal NSW weather

Over the past week, temperatures have gradually warmed across eastern NSW as a dominant high pressure system in the Tasman Sea edged eastwards. Warm northerly winds on the western edge of this high have strengthened, reaching a peak this Sunday, with the mercury expected to reach close to 30°C in Sydney.

Cool coastal waters during spring can lead to sea fog, as seen here in Dee Why, NSW
Image: Maximum temperatures forecast across NSW on Sunday, October 25, 2026. Source: Weatherzone.

Warm and humid conditions this weekend will cause people to flock to the beach, with a chilling surprise awaiting in the water.

Ekman transport and coastal upwelling of cold water

The warm northerly winds that have been blowing over the past week have driven upwelling of cold water from the sub-surface of the ocean into coastal parts of NSW.

Image: Sea Surface Temperature (SST) between Sydney and Brisbane compared between Monday, October 19, 2026 (left) and Saturday, October 24, 2026 (right), with regional estimates for Sydney, Seal Rocks and Port Macquarie. Source: OceanCurrent/IMOS

This upwelling is driven by a process known as Ekman transport. Periods of sustained winds start moving the ocean's surface layer. When winds maintain a consistent direction for more than a day or two over a body of water, the top layer of water starts moving in the direction of the wind. This broad movement of water is affected by Earth’s rotation, also known as the Coriolis effect, and deflects to the left of the wind direction (in the Southern Hemisphere).

For the East Coast of Australia, persistent and strong northerly winds deflect surface water away from the coast. To replace the displaced surface water, this deflected surface water is replaced by water from below the surface. Deep water is usually colder and more nutrient-rich than surface water. Coastal upwelling (and downwelling) becomes most notable the longer and stronger winds blow parallel to the coast.

Image: Winds blowing parallel to the coast can cause upwelling (or downwelling) through a process called Ekman Transport. Source: Bureau of Meteorology

During spring and summer, cold coastal waters can cause sea fog to form by rapidly condensing warm and humid air near the cold sea surface. Other impacts of coastal upwelling include algae and phytoplankton blooms, as well as shrinking and shrieking surprises for beachgoers entering the water.

Conversely, downwelling is the result of usually warm surface water being pushed into coastal areas. Southerly winds usually bring cooler weather to eastern NSW, but they can actually cause coastal water temperatures to rise through downwelling.