HI everyone. just a general question regarding swell production.
say if you have a scenario where the winds been blowing from the SE for quite a few days and worked up a fully developed sea state, then the winds turn E or NE for a few days at the same wind speed that caused the original sea state. Is it now easier for the E/NE swell to develop? OR, is it harder due to it having to work against the original SE vector still in the water. Comparing this to a settled sea surface.
Thanks

Firstly the wind speed , will have an input making the waves easier to form a rhythm with the first swell .
But most importantly fetch will be the deciding factor , ie the longer distance the new wind is from ( in this example the east coast ) then the more likely the new wind direction could combine with and divert the old swell train . And yes there would be a ESE and then finally E swell .
If this is all happening less than a thousand km's from shore then NO , you'll just get two crossed up wind swells. And there would be little combination only the old swell , maintaining strongest appearance until the new would over power and replace it . This would require time as opposed to distance .
A perfect example of this , and taken to the next level .
Is a westerly swell running below tasmania gets redirected by a constant stationary provided by the western flank of a large low/frontal system hitting or centred on NZ . The Further North southerlies trailing a connected Trough keep bending it . With a High sitting roughly centred on Central NW NSW , we would see slight to moderate sth easterly eminating 500 -1000km's off Gold coast . Over a three to four day period a tropical low may push down increasing the strength of the Easterlies . So would see a long time scale of close to 1-2 weeks creating a circle path of Fetch that has seen a swell energy transfered completely in 180 degree . Direction .
The continantal shelves would also help with this refraction , along with eprhaps a coriolis effect ?
These events are referred to greater circle path swells . It most commonly rely's on lighter winds during the middle period of the event ( a high lingering ) .
PS . None of this is schooled analysis , just observations and third hand knowledge . Hence their could be inaccuracy's that I'd love to know about . ( Technically ) .

thanks SO,
I think more what Im asking is, can the current disturbed area where the swell was generated be easily redirected to produce new swell at a different direction. Not so much redirecting swell trains, just focusing on the area of genesis.

For example, I have been following a high thats been sitting over NZ for about 4 days. These tend to bobble and rotate themselves anticlockwise. It started out as blowing 35Knt SEer for 4 days over the same area. Now, it has bobbled as a front and low approach from the Tasman and is starting to turn the winds over the same area at the same speed to the E/NE. Will this swell from the E/NE be easily produced due to the active sea state, or be harder to produce due to having to redirect the sea state?

Yeah , I'm hearing you . ( It's more Physics )

But like my example , Swell builds faster when the weather system follows the direction of Travel ( helping it on its way ) hence you get periods
( wavelengths of over 16 seconds and approaching 22 ) in westerly originating easterly eminating swell . ( occasionally retrograding Cut off's and building TC's provide decent " snowballing effect " in the opposite direction but rarely.)

Really what I'm getting at is yes any swell/wind that flows over a particular area will be able to be redirected ( reasonably quickly/1-2 days ) if its below 45 degrees from the original direction of energy . Over this up to 90 degrees will take a little longer . And as mentioned , out in the middle of the Pacific direction changes of almost 270 degrees can be achieved . But as previously mentioned it involves travel and broad areas of influence .

Your particular case in hand , is more of a wind swell change , which is messier as the swell won't have reached a true state in the general location it was built from . So infact your forgetting that the original swell would have advanced from that region already . Yes an active/disturbed state of seas would be easier to build from but it would all come down to the rhythm of the swell , ie matching the wind speed . So you would need a change in wind direction and the speed to stay similar over a time period of atleast 6-8 hrs . But saying this you really are getting into scenarios which will be minute and indecipherable as depending exactly where your talking about there will be outside factors . Be it contributing swells originating far further east / the trades themselves / the blocking-shelter effect of NZ from the stonger and generally more predominate west/south originating swell trains . For instance a south easter will harness the SW - S swells coming from below Tassie and NZ , whilst there would have to be a gap in the Southern Swell trains for any sort of E-NE energy to last even a short period . Obviously NZ can block this but also distort it ....

I sort of get what your saying but, dont deep water swell trains(groundswell) travel relatively fast and show very low surface amplitude where the energy potential is more hidden within the period and it more susceptible to radial spread and refraction? Rather than just a chopped up seastate with no harmony in the direction and period whilst being easily manipulated by the new wind direction and speed and where theres no radial spread and refraction acts as a filter?

I guess what im looking for is trying to track bursts of short period(9-12s) peaky tradewind swell and their small direction changes, rather than south groundswells that tend to be blocked up on the sunny coast.

Look for breaks in the Southern Swell train . ie when Victoria goes flat two days later if there is winds ESE-ENE off NZ and beyond then the lack of Sth refracted swell rolling up the Coast will not cut down this energy . So A big stable high that gets looked into position centred on Eden . And no real cold fronts or better still a strong Northerly blowing off the Bight ... So looking furtehr afield can sometimes give you the heads up !

Swell trains are seen on the surface , even in 5000M of water . Yes your right that they feel the bottom in shallower waters and this slows them up / or have more energy under the surface . But if you think off the swell as rolling ball of energy you'll know that this deep water energy pushes down infront of the swell and rotates up behind it . So you'll see surfaec conditions break a swell up or down and sometimes redirect it .
At a standard measure the wavelength metres /or ( period in seconds ) actually correlates to the depth it reaches . So for instance a 16 sec swell interval in open/ deep ocean can reach a wave length of close to 90-100M and so forth . This is where refraction comes into it radials are more to do with coriolis .

I think radials are due to increased friction of the outer ends of the swell band, due to being weaker and less organised whilst trying to spread the swell horizontally continuing to thin out the energy at the ends whilst the stronger swell in the middle is moving faster.

Yeah , Agree .

Coriolis causes the ( Southerly storm tracks ) to turn out of the swell direction , so as you see a Eastward moving swell Pulse the originating front will deflect down to the Pole so you see radials spread in the opposite equator ward direction . Unless of course I'm missing something ?

BTW , are you going to look for gaps in the "Roaring Forties" swell trains to see sly swells rock up from the east in your neck of the woods .?

Yeah definitely. What have noticed so far is its a bit hit and miss and its quite hard to judge size lol. will keep my eyes peeled.

Links :

" www.bom.gov.au/products/IDT65015.shtml " Good for size , timing and general sea state , (more can be read from it once time has been cross referencing it with other sites .

" www.portofmelbourne.com/wavedata.asp " which gives another angle to how much swell will radiate and then what effect the above's sites surface conditions have generated into a swell . ( As tassie tends to be stuck within constant wind noise ).

Gone are the days of time looking at a Broad synoptic chart made you judge the sizes that will be generated , but being able to do both really sets you in good stead . sometimes the computers don't pick up swells , and thats what I think your angling at !

yeah you hit the nail. Im just trying to pick up slight angle changes which will allow me to pick my spots better. This was easy back home in nz as I had been doing it for 15 years but over here im starting from scratch again. like you say, just trying to pick up what the models(and everyones else) miss.