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. | 
