Swellnet Analysis Why Does The Water Get Colder In Sydney After A Nor'easter?

In: Swellnet Analysis by Ben Matson 6 Comments Thu 2nd Jul '09

Tags: Ben Matson , Analysis , Ekman , Upwelling , Cold Water , Cool , Water

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Most Sydney-siders associate southerly winds with cool air temperatures, and northerly winds with warmth. But after a spell of nor'easters along the East Coast, the water temperature will often drop a couple of degrees. Why is this so?

The process that causes this is called 'Ekman Transport', and it refers to an upwelling effect that occurs under certain synoptic situations.

When the wind blows along the ocean's surface, the effects of the Coriolis force diverts the top layer of water to the left in the southern hemisphere (while in the northern hemisphere, the top layer of water is diverted the to the right). Therefore, a NE'ly wind blowing along the coast pushes the top layer of water out into the Tasman Sea.

As this water needs to be replaced, an 'upwelling' effect draws in cooler (and usually nutrient-rich) water from below. It usually takes a couple of days of nor'easters to see any noticeable change, and you'll also find that certain parts of the coastline tend to react more than others - usually due to the presence of underwater ridges and canyons that amplify the upwelling effect.

There are various spin offs from an upwelling event, depending on where you live - many parts of the southern coastline will see an increased risk of sea fog, if a warm and moist airmass settles on top of the cooler sea surface temperatures. Afternoon sea breezes may also be enhanced due to an increase in the temperature gradient between the land and the ocean. It can sometimes take weeks for these upwelling effects to ease - Newcastle had a spell of cool water in January of 2004 that took around three weeks to settle down.

This process works similarly in reverse. A southerly airstream in the Tasman Sea pushes the surface layer of water to the left (ie towards land), which will usually return near-shore sea surface temperatres to a more stable average.

Fisherman can really benefit from an upwelling event - the nutrient rich water dragged up from greater ocean depths is photosynthesised as it nears the surface layers, and this sharp increase in food source can rapidly build fish stocks.

The upwelling principle applies in most other parts of the country as well. Qld also reacts to a sustained nor'easter, but the southern states will see upwelling events under different wind directions. Just trace your finger along a map of the coastline, and imagine the surface layer of water diverting to the left - lengthy stretches of coastline (such as the Coorong under a SE'er, Tassie's West Coast under a SSE'er, and the Torquay-Apollo Bay stretch under an ENE'er) are susceptible to Ekman Transport.

So to avoid a cold shock next time you dive into the surf along the eastern seaboard, take note of those periods when we see three or more days of NE winds - this is an early warning that sea surface temperatures may shortly take a dive.

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