Analysis: Cold water and green seas
Regular Swellnet readers would be familiar with the yearly article explaining the phenomena behind the cold water upwelling that's most noticeable during East Coast summers.
It's a fairly simple process where persistent strong north-east winds transport the surface ocean water away from the coast. Though the wind blows towards the coast, the surface water counter-intuitively moves offshore due to the Coriolis Effect, or the spinning of the Earth on its axis.
As the warm surface water moves offshore it's replaced by deeper colder water in a process called coastal upwelling.
The phenomena is pronounced as it usually happens during bouts of hot weather weather when surfers get more than they bargained for. Keen for a refreshing session they're forced to freeze in boardies.
This cold water is usually upwelled from the continental shelf, but if the process continues for a sustained period we get even deeper colder water brought up from beyond the shelf break. When looking at Google Maps or any other map showing details of the ocean floor (bathymetry) you can identify areas where the shelf break comes close to the coast.
It's these areas that see the most pronounced upwelling events and concurrently the biggest drop in temperature.
Along the East Coast you can identify these regions as close to Jervis Bay, Narooma, Seal Rocks followed by the upper Mid North Coast and Ballina.
Upwelling is a major source of primary production, as the nutrient rich water is bought into the photic zone - where the sunlight impacts the water column - causing photosynthesis and the blooming of phytoplankton.
Phytoplankton are the base of the food chain and everything builds up from here.
Chlorophyll is needed for the photosynthesis process to take place and this is what we observe when the water starts turning that greeny colour.
The more concentrated the phytoplankton the more intense the colour but also the lower the visibility. This is why if you're a diver or spear fisherman you'd be best to avoid swimming after days of strong north-east winds on the East Coast as the visibility will likely be compromised. You may also notice in some areas the upwelling also causes corn-flake like red/brown floaties in the water.
The amazing thing is that the chlorophyll can be observed by satellite and if we look at the latest available data for the East Coast (18th December), where we've had nearly a fortnight of non-stop and at times strong north-east winds, we can see a few hot spots.
The main one is off Seal Rocks and this is one of the most productive zones for upwelling off the East Coast. Being close to the shelf break and having the coast bend away to the south-west lends the area to rapid and prolonged upwelling events.
The other is just south of Ballina from Evans Head to Yamba with the East Australian Current sitting further offshore.
Currently the sea surface temperatures (SST's) off the Mid North Coast are well below what's sitting just offshore in the East Australian Current. Steamers are a must with 18 deg water, while it's 24-25 degrees only 20-30km offshore.
For this warm water to move back in shore we need a strong southerly change and unfortunately this doesn't look to occur within the next fortnight at least across the northern NSW coast, with a brief burst of southerly winds Sunday across the southern NSW coast, bringing SST's up a touch temporarily.
Therefore we're set for a continued period of cold greeny water.
On the upside, at least the fish should be on the bite.