If you’ve ever been for a dip in the ocean, you know the water temperature can vary dramatically.
What you’re actually feeling is the sea surface temperature.
Sea surface temperatures (SSTs) are important for swimmers and fish but they also impact the weather on land and hold the key to predicting larger-scale climate influences.
Understanding sea surface temperatures around Australia
Oceans cover about 70 per cent of the globe and moderate the temperature changes we feel on land by absorbing a lot of the sun’s incoming heat.
In general, sea surface temperatures are warmest at the equator and gradually get cooler as you head north or south.
Temperatures around the Tasmania coast are influenced by several factors.
For example, the amount of heat from the sun coming into the ocean at any location depends on the time of day, the time of year and latitude.
Local warming and cooling of the sea surface can be noticeable in shallow waters when conditions are calm.
But in deeper waters, massive amounts of heat from the sun are absorbed at the surface and redistributed horizontally and vertically by complex ocean circulation phenomena.
The vertical distribution of heat into the oceans means they take longer to respond to changes in solar radiation than the land.
This time lag means the coolest sea surface temperatures typically occur in early spring, which can mean the sea still feels a little chilly into early December.
The warmest sea surface temperatures usually occur around early March, and this is reflected in the cycle of sea ice around Antarctica which is then at its lowest level.
The flow-on effect of currents for Tasmania
There are some very important ocean circulation patterns that originate near the equator and bring warm water south, influencing the weather over Tasmania.
Originating in the Coral Sea, the East Australian Current (EAC) is wide and transports a considerable amount of heat between the tropics and mid-latitudes, with most of the water flowing into the Tasman Sea.
The Leeuwin Current, known as the Zeehan Current near Tasmania, causes sea surface temperatures off Tasmania’s west coast to be several degrees cooler than those along the east coast.
This current originates to the north-west of Western Australia and carries about a quarter of the EAC’s water volume.
It flows all the way down the west coast of Australia, before turning east into the Great Australian Bight and moving down the west coast of Tasmania, losing most of its warmth along the way.
How do the warmer and cooler SSTs impact Tasmania’s weather?
The variation in water temperatures to the west and east of Tasmania due to the Leeuwin and East Australian currents impacts the state’s rainfall records.
Dominated by locations in the west, records for the highest monthly and yearly rainfall for Tasmania reflect the persistent westerly winds, which bring consistent rain to the west of the state.
In contrast, the highest daily rainfall totals for Tasmania are dominated by sites in the north-east of the state.
This is because any east to north-easterly airstream over Tasmania will draw moisture from over the considerably warmer waters of the EAC, which evaporates high levels of moisture into the atmosphere.
These moisture-laden north-easterly winds push against the ranges of north-eastern Tasmania and produce heavy downpours, but typically only for a day or two.
The warmer waters of the east coast mean water vapour is evaporated into the atmosphere more easily, feeding into rain-bearing systems like the east coast low.
Sea surface temperatures and Australia’s climate drivers
Around the globe, sea surface temperatures help drive atmospheric patterns and play a significant role in the climate drivers that influence the weather across Australia.
Despite being a small island, Tasmania falls into the area of influence of three climate drivers: the El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Southern Annular Mode (SAM).
The difference in sea surface temperatures between the eastern and western halves of the Pacific and Indian oceans gives rise to ENSO and the IOD.
ENSO has three phases: El Niño, La Niña and neutral.
During the El Niño phase, warmer waters pool over the eastern Pacific Ocean and sea surface temperatures to the north of Australia cool, weakening the easterly trade winds across the Pacific Ocean.
These conditions combine to produce typically drier conditions for eastern Australia.
The opposite condition — where waters warm to the north of Australia and cool over the equatorial eastern Pacific — is called La Niña.
It tends to produce higher-than-average rainfall in northern and eastern Australia.
The IOD also has three phases; positive, negative and neutral, but these phases are dictated by the difference in sea surface temperatures between the western and eastern tropical Indian Ocean.
The difference in sea surface temperatures influences how much moisture is available from the Indian Ocean to fall as rain over central and south-eastern Australia.
Both ENSO and the IOD are natural cycles in ocean temperatures, and atmospheric wind and cloud.
For more information on sea surface temperatures, visit Sea temperatures and Currents; for information on climate drivers visit Long-range forecasts and climate drivers, Bureau of Meteorology.
Matthew Thomas is a senior meteorologist with the Bureau of Meteorology