Farewell to Autumn

The low and high pressure air cells presently rotating over the Tasman Sea are operating like a pair of giant cog wheels to channel a stream of moist air on to the east coast.
As well as lifting moisture into the atmosphere this onshore wind is pumping up the sea swell.

 

  Dawn at the Heads. 

 

  

 

 South Reef.

  

 

 Derek casually sailing over the shoulder of an emerald tube peeling out of the Tasman Sea.

  

 

   

 

 When the eroded section of cliff at "The Gap" finally breaks through between the sea and the harbour (in about 66,000 years by rough guess-timation), South Head will be an island not unlike Bowen Island at the south head of Jervis Bay.

 

Back in the harbour and exploring the shoreline for crays.

(One seen but left for another day)

 

A week of solid late-autumn rain, sucked off the warm sea and deposited in-shore, finds its saturated way back into the harbour.

Water in motion

The CSIRO has published the following commentary on Sydney's sea surface temperature and currents:

May 18, 2010 - update on Sydney:

Beach water temperatures are still extremely high but the East Australian Current has now started to flow mostly away from the continental shelf at the latitude of Sydney, rather than towards the coast (as it has since late April), triggering the beginning of the end of the high temperatures.

The explanation is faily complex:

Between Sydney and Newcastle a clockwise-rotating, cold-core eddy has now become well established inshore of the EAC. Until the last few days, the EAC was flowing south around this cold-core eddy, feeding into a large pool of EAC water that is warmer than is normally seen off southern NSW at this time of year. The cold-core eddy has now grown so large that very little of the EAC is continuing south: most appears to be flowing offshore at the critical junction point at 34.5S, 152.5E. To complete this eddy-shedding process, the cold-core eddy off Newcastle is likely to merge with the larger cold-core eddy to the SE, thereby completely separating the two masses of warm EAC water: the parent body to the NE, and the newly-shed body to the south.

May 19, 2010 Update:

The eddy shedding is now complete.

Image from CSIRO

Paddling out through the Heads and into the last of the warm sea water for this season.

A Bonito tuna leaps aboard while, in the background, the saturated clouds of moisture forming over the "hot" sea current release some of their water back into the sea ...

... and onto the land.

At its edges the sea's motion is visible in crashing waves along the coastline. But it's the larger "un-seen" drama of massive tumbling vortexes of water moving around offshore that affects our local conditions so profoundly. With the conveyor belt of hot energy that is the EAC now declining in its capacity to push southward at the end of this year's annual cycle we will soon feel cold water on our paddling finger tips and soon see the stream of migrating whales, soaring albatross and curious seals of winter.

Sydney cray

Previously we have observed that kayaking and snorkelling are complimentary arts of the sea.

This morning Derek and Tony find a spot accessible by nimble kayak where a dive beneath the surface reveals an exceptional Sunday lunch ...

Tony earns bonus domestic-leave-kayaking-rights from his happy wife with this excellent result.

Offshore wind

At this time of year a range of subtle factors lead to the regular occurence of offshore winds in Sydney during the small hours of the night / early day. This can be graphically observed in the BOM wind rose (above) which describes the average wind speeds and directions recorded for Sydney during the month of May at 9am

One of the factors generating this offshore morning wind is that the sea surface temperature is significantly warmer than the land surface temperature. Warm and moist air rising in columns over the "hot" sea cells is replaced by cooler air rushing off the land to fill the vacancy.

 

In the CSIRO image above the southward flow of the East Australian Current is clearly visible, delivering sea surface temperatures of up to 25 degrees C to Sydney's eastern flank.

With clear skies at night the air temperature above the Sydney land basin is falling to between 5 and 10 degrees C. This air temperature differential between land and sea sets up the condition for Sydney's autumn offshore winds.

For kayakers this condition opens the door for many great mornings to be spent paddling in the lee of the sandstone cliffs and in the warming rays of an autumnal sun ... 

A sail on a kayak makes logical sense at this time of year to catch the morning westerly down the harbour.

South Head - where the westerly is driven temporarily upwards forming a wind shadow on the eastern face of the cliffs.

Out along the continental shelf, where the sea current is at its warmest, columns of moist air rise to form a chain of cloud cells.

Here's an interesting fact:

wind flowing over land which ends as a cliff will form a separation bubble of increased turbulence on the lee-side of the cliff, with wind direction reversal forming for a distance beyond the cliff of approximately 4 x the cliff height.

Returning to the harbour in autumn sunshine.