Ekman Transport phenomenon on display of the Coffs Coast in October 2013
Nobody who fishes the north coast of NSW would argue about the fact that we have our own localised weather effects. In fact, we have some pretty extreme stuff here from time to time, particularly as far as winds are concerned. To illustrate this, all you've got to do is look at the weather forecast and then compare that to the live weather readouts - it's not at all uncommon to see the actual wind blowing significantly stronger than predicted, and that's after the BOM adds it's usual +50% caveat. Winds are frequently double the forecast strength.
It's almost as if the BOM's central forecasting computers and the algorithms that drive them seem incapable of accounting for the localised and under-appreciated multiple influences that the unique proximity of the continental divide to the coast has right at Coffs Harbour.
At no time is this more pronounced than when the wind is blowing from the north, when it is frequently stronger than forecast right around Coffs and the Solitary Coast, due primarily to a pronounced macro venturi effect created by the dividing range.
While having to deal with stronger than anticipated wind is one of the drawbacks of fishing this coast (always outweighed by the overwhelming advantages mind you . . .), the problem takes on another dimension entirely when the wind is from the north, because if those northerlies persist for longer than a couple of days and become the predominant wind for periods longer than a week, the Ekman Transport phenomenon can kick in and temporarily trash the local game fishing for reasons that are not fully appreciated by some game fisherman.
Our oceans are affected by many macro and micro phenomena, and some of these work on a global scale, defining the climate over entire continents. Some of the huge oceanic gyres and dominant currents have such a strong influence that were they ever to stop or undergo some sort of radical change, the consequences in some cases could be dire.
That said, while the influence of prolonged northerly winds that can sometimes blow down our coast are more in the micro effect category along the stretch from Evans Head to Southwest Rocks, it can produce a strong localised Ekman Transport effect, and this really messes with the game fishing.
The satellite SST shot above illustrates the more extreme result of the Ekman Transport phenomenon that appeared in September/October 2013 off Coffs Harbour, when after a month during which (despite occasional southerly fronts coming through) strong northerly winds dominated the local weather.
The resulting and pronounced Ekman Transport process pushed the surface layers (to a depth of about 150m, or 85 fathoms) of the ocean out to sea to the east, and the sub-surface green-brown water that welled up and sat between the 1000 fathom line and the beach stopping what had been a promising start to the game fishing season in its tracks.
In the satellite shot, you can see the warmer water out to sea while the colder sub-surface layers of the ocean have welled up and completely dominate the coastline from Yamba to SWR from the beach to beyond the continental shelf.
In a nutshell, the Ekman Transport process that did this was the effect of a prevailing northerly wind running down the coastline.
It would be logical to assume that a strong wind from any direction would create relatively strong sympathetic movement of the surface layers of the ocean in the same direction, but this is not correct. So when the northerly winds blow down our coastline, the surface layer of the ocean does not move south as you'd expect - it in fact moves directly to the east. This takes it away from the coast, and counter-intuitively, directly across the wind's path.
This is a result of the force exerted by the earth's rotation, or Coriolis Force. Coriolis has the abilty to "bend" winds in the case of tradewinds, and bend water flows in the case of Ekman Transport, and cause water to move in another direction entirely, ninety degrees offset from the wind that's driving it.
In this case, the Ekman principle says that (in the Southern Hemisphere) when a northerly wind is blowing over the sea, the top layer of the ocean from the surface to about 85 fathoms will move directly eastwards. That's a lot of water.
So that leaves us with strong northerlies pushing the surface layer of the ocean away to the east, and of course something has to take its place, so the underlying layers of cooler, nutrient-laden (and therefore murky) sub-surface ocean water then well up to the top. We (and the marlin and other clean water loving pelagics) see this as the sudden unexplained replacement of good, warm, clean, blue water out there on the edge of the shelf with cooler, greenish-brown, lifeless looking water which signals the end of the inshore game fishing for as long as it remains.
In short, when the northerlies blow, the water quality and temperature all go to hell, followed immediately by the marlin. We've all more or less known this to be the case, but I doubt many of us actually knew what really caused it.
Incidentally, this also helps to explain why a prolonged period of southerly or southeasterly winds also cause the EAC to move in westwards onto the shelf and to concentrate into a narrower stream width.
It's almost as if the BOM's central forecasting computers and the algorithms that drive them seem incapable of accounting for the localised and under-appreciated multiple influences that the unique proximity of the continental divide to the coast has right at Coffs Harbour.
At no time is this more pronounced than when the wind is blowing from the north, when it is frequently stronger than forecast right around Coffs and the Solitary Coast, due primarily to a pronounced macro venturi effect created by the dividing range.
While having to deal with stronger than anticipated wind is one of the drawbacks of fishing this coast (always outweighed by the overwhelming advantages mind you . . .), the problem takes on another dimension entirely when the wind is from the north, because if those northerlies persist for longer than a couple of days and become the predominant wind for periods longer than a week, the Ekman Transport phenomenon can kick in and temporarily trash the local game fishing for reasons that are not fully appreciated by some game fisherman.
Our oceans are affected by many macro and micro phenomena, and some of these work on a global scale, defining the climate over entire continents. Some of the huge oceanic gyres and dominant currents have such a strong influence that were they ever to stop or undergo some sort of radical change, the consequences in some cases could be dire.
That said, while the influence of prolonged northerly winds that can sometimes blow down our coast are more in the micro effect category along the stretch from Evans Head to Southwest Rocks, it can produce a strong localised Ekman Transport effect, and this really messes with the game fishing.
The satellite SST shot above illustrates the more extreme result of the Ekman Transport phenomenon that appeared in September/October 2013 off Coffs Harbour, when after a month during which (despite occasional southerly fronts coming through) strong northerly winds dominated the local weather.
The resulting and pronounced Ekman Transport process pushed the surface layers (to a depth of about 150m, or 85 fathoms) of the ocean out to sea to the east, and the sub-surface green-brown water that welled up and sat between the 1000 fathom line and the beach stopping what had been a promising start to the game fishing season in its tracks.
In the satellite shot, you can see the warmer water out to sea while the colder sub-surface layers of the ocean have welled up and completely dominate the coastline from Yamba to SWR from the beach to beyond the continental shelf.
In a nutshell, the Ekman Transport process that did this was the effect of a prevailing northerly wind running down the coastline.
It would be logical to assume that a strong wind from any direction would create relatively strong sympathetic movement of the surface layers of the ocean in the same direction, but this is not correct. So when the northerly winds blow down our coastline, the surface layer of the ocean does not move south as you'd expect - it in fact moves directly to the east. This takes it away from the coast, and counter-intuitively, directly across the wind's path.
This is a result of the force exerted by the earth's rotation, or Coriolis Force. Coriolis has the abilty to "bend" winds in the case of tradewinds, and bend water flows in the case of Ekman Transport, and cause water to move in another direction entirely, ninety degrees offset from the wind that's driving it.
In this case, the Ekman principle says that (in the Southern Hemisphere) when a northerly wind is blowing over the sea, the top layer of the ocean from the surface to about 85 fathoms will move directly eastwards. That's a lot of water.
So that leaves us with strong northerlies pushing the surface layer of the ocean away to the east, and of course something has to take its place, so the underlying layers of cooler, nutrient-laden (and therefore murky) sub-surface ocean water then well up to the top. We (and the marlin and other clean water loving pelagics) see this as the sudden unexplained replacement of good, warm, clean, blue water out there on the edge of the shelf with cooler, greenish-brown, lifeless looking water which signals the end of the inshore game fishing for as long as it remains.
In short, when the northerlies blow, the water quality and temperature all go to hell, followed immediately by the marlin. We've all more or less known this to be the case, but I doubt many of us actually knew what really caused it.
Incidentally, this also helps to explain why a prolonged period of southerly or southeasterly winds also cause the EAC to move in westwards onto the shelf and to concentrate into a narrower stream width.
