Sea temperatures

Sea surface temperatures

Oceans cover more than 70% of Earth's surface. Measuring sea surface temperature provides vital information on the health of our climate.

Sea surface temperature is also an essential part of weather prediction and atmospheric model simulations. It helps us track major climate drivers, such as the El Niño Southern Oscillation.

These factors affect sea surface temperatures in Australia's coastal waters.

Latitude

Sea surface temperatures depend strongly on the angle at which the sun's rays strike the Earth's surface. Seas are generally warmest at the equator and gradually cool toward the poles.

Time of year and day

Sea surface temperatures change throughout the year. This is due to changes in the amount of solar radiation reaching the ocean surface.

The ocean warms more slowly than the land. This creates a time lag between changes in the air temperature and water temperature. The lag is usually around three months, depending on location. Sea surface temperatures are usually lowest in early spring and highest in early autumn.

Temperatures also vary slightly during the day and tend to be highest in the late afternoon. Cloudiness may moderate water temperatures by reducing the amount of direct heating.

Currents

Australia's sea temperatures are influenced by ocean and coastal currents, including the East Australian and Leeuwin currents. These can move warmer water into cooler locations. Learn more about ocean currents.

Water depth

In shallow, calm waters, the sun warms the water quite efficiently. But it doesn't really warm water beyond the surface layer, so the deeper waters stay cooler.

Wind and water mixing

Periods of sustained winds can move warmer or colder water towards the coastline. Under the surface, upwelling and downwelling circulate water of different temperatures. Find out more about these processes on our Ocean currents page.

Tides can influence sea temperatures in the short term. They do this by moving warmer or colder water into and out of estuaries and bays:

• During high tides, cooler marine water mixes into warmer coastal areas.
• At low tide, rivers and streams flowing into the bays and estuaries can have a greater influence. Water temperatures may rise as the water becomes shallower.

River run-off, rainfall and snow melt

Waters close to the coast can be affected by changes in the:

• amount of fresh water flowing into the ocean
• extent to which this water is mixed by winds or tides.

Coastal waters near river entrances can cool as colder water flows in, after heavy rain or snow melt.

Marine heatwaves

Marine heatwaves happen when sea temperatures are warmer than normal for an extended period. The effect on our marine environment and the aquaculture industry can be devastating.

Heatwaves in the ocean often last much longer than on land – months or even years.

Definition of marine heatwave

A marine heatwave is commonly defined as temperatures being warmer than 90% of the previous sea surface temperature observations at the same time of year over a 30-year period, for at least 5 days in a row.

This means that the temperature of a marine heatwave is relative to location and season. For example, a heatwave that affects kelp forests in cooler waters is lower in temperature than one that affects coral reefs in warmer waters.

Causes of marine heatwaves

Like heatwaves over land, heatwaves in the ocean come about from a mix of factors.

On sunny days, sunlight passes through the atmosphere and heats the surface of the ocean. If there are weak winds this warm water doesn't mix with the cooler waters below. It sits on top and continues to heat.

Such warming can be:

• local, such as when a high-pressure system remains slow-moving for an unusual period of time, or
• large scale, covering much of an ocean basin. This can happen during El Niño and La Niña events.

Another contributor to marine heatwaves is warm water moving from one area to a cooler location. Ocean currents vary over time. They can shift around, speed up and slow down, and replace colder water with warmer water.

An example is when warm waters from the Western Pacific Warm Pool pass through the Indonesian Throughflow, and down the coast of northern Western Australia via the Leeuwin Current.

Climate change and sea temperatures

Australia’s weather and climate are changing in response to a warming global climate.

Average sea surface temperature in the Australian region has warmed by more than 1° C since 1900.

Warming of the ocean has contributed to longer and more frequent marine heatwaves.

More marine heatwaves, more often

More frequent marine heatwaves have permanent impacts on ecosystem health, habitats and species.

These impacts include:

• depleting kelp forests and sea grasses
• a poleward shift in some marine species
• increased occurrence of diseases.

Recent marine heatwaves are the main cause of mass coral bleaching and widespread damage to coral reefs around Australia.

For more detail about Australia's changing climate and our oceans, view the State of the Climate report. We produce this report with the CSIRO every 2 years.

Monitoring and forecasting sea temperatures

Our data and forecasts inform vital decisions about marine activities and environmental management.

We monitor sea surface and subsurface temperatures through a variety of observation platforms, including ocean buoys and floats, and satellites.

Our climate forecast system for weekly to seasonal and longer-range climate outlooks is called the Australian Community Climate Earth-System Simulator – Seasonal (ACCESS–S). Learn more about the ACCESS-S climate model.

We produce real-time ACCESS-S forecasts of sea surface temperature and anomalies for several months into the future. We provide these for both the Great Barrier Reef and the wider Australian region.

The ocean model component of ACCESS–S currently operates at a resolution of about 25 km in the Australian region. At this resolution, the model can identify small-scale currents and eddies.