SW swell Tahiti shadow

[R B]

Frequently Southern California or maybe other parts of California are
in a shadow of Tahiti
on SW summer swells. I remember quite a few years back surfing spotty
inconsistent waves
here in Southern California on a SW swell that we knew was shadowed.
But what we didnt know
nor was it specifically explained , was the during this swell Santa
Cruz was going off from the SW.

I'm wondering if there are some SW shadowed Southern Calif. swells
where I could either head
to Santa Cruz area or maybe even to K-58 area and score substantially
better waves.
Of course, a forecaster would have to point out the general areas of
unshadowed swell on such swells?

[Nathan Cool]

Hey RB,

I can't recall any southern hemisphere swell that filled in better to
northern California than it did southern California. In fact, energy
should have diminished further due to the extended distance.

Shadowing may be a factor, but it could also be a matter of time and
place. For instance, if a swell were to hit SoCal on a Saturday, and
NCal would feel the full brunt on a Sunday, then someone could
experience a better swell in NCal on Sunday compared to SoCal.

Shadowing though would have two elements: proximal and distal. The
latter, based on the distance, would affect all areas in California.
For instance, if the swell was somewhat blocked by the islands of the
South Pacific, then all of California would be affected. The proximal
effect though is more likely, and would cause a difference in how the
swell fills into various breaks.

For proximal shadowing to have a noticeable influence on a swell, we'd
need to identify a land obstruction -- such as an island or
salient/extended portion of the coast -- that would block the incoming
swell to a particular area. In SoCal, we get blockage from islands, as
shown here:
http://groups.google.com/group/surf-forecasting/web/island-blockage

But this shadowing is mostly from our islands, and from westerly/NW
swells; however, Santa Barbara gets massive shadowing from the Channel
Islands. Thus, SoCal is not really affected much by proximal
shadowing.

Bearing this in mind, we now may have to consider whether it was "all"
of SoCal that was smaller than Santa Cruz on a given swell, or were
only certain surf spots negatively affected. If we can only determine
that "certain" south facing breaks didn't go off yet others did, then
we can add refraction to the possibilities. More on that here:
http://groups.google.com/group/surf-forecasting/web/refraction-and-diffraction

So in short, I suppose what I'm trying to say, is that it isn't likely
that Santa Cruz would see more "energy" from a southern hemi swell
compared to SoCal. There could be some kind of shadowing going on near
the coast, but that's unlikely as well. Instead, I'd be more inclined
to think that time and place were a major factor, as would be the
bathymetry/refraction factors at a given surf spot. And, of course, if
the disparity is based on personal accounts of others, then it could
just simply be anecdotal :)


Nathan Cool
http://www.NathanCool.com

"Strive not to be a success, but rather
to be of value." -- Albert Einstein

[R B RB]

It was within the last 2-3 years, 'SW swelll blocked by Tahit'i.
I surfed Trestles and Oceanside on the swell, head highish , inconsistent, dissapointing.
Surfline off that same swell had great video of Santa Cruz firing, overhead and being consistent.
Wam models at the time verified that the shadow was strong for Socal, but not NorCal.
However, Surfline did not mention this in advance. Wam models are good but I dont like basing
a trip off a simple graphic alone, hence the question thrown out to you.
 Been surfing since 71', experienced coastal traveler for waves.

I dont know how common this pattern can be, but it exists.

[NathanToddCool]

Still a puzzle to me. After tracking every swell headed to California
for the past 14 years, I can't recall something like that happening.
I'm not saying it didn't...just saying I can't recall it. Lots of
strange stuff can happen, but without having the archived data, none
of us will ever have the answer. Perhaps though this could happen
again, and we can all get a better look.

Some other things that come to mind:

1. How was the shadowing visible? Typically, it'll be evident on the
WAMs as to what blocked the swell (an island, etc.). If we can find
out what blocked the swell, then we can get to the root of the issue
-- if this is actual shadowing and not some other factor.

2. Another possibility is that perhaps SW was not the only swell in
the water. Perhaps there was some NW swell around that time as well?
If so, then this could have wrapped into Santa Cruz, and never
progressed much into SoCal. Santa Cruz may have had a sweet NW/SW
combo, but SoCal had just the spotty SW (and not the NW).

3. You mentioned that the last time this happened was a few years ago.
I'm somewhat surprised that it hasn't happened since. Storms in both
hemispheres take similar trajectories year after year, so it would
seem unlikely that we wouldn't see a similar system behave this way
again and produce the same shadowing. Next time it does though, we can
take a closer look to see what actually happened.


-Nathan

[NathanToddCool]

Hey RB, something else came to mind last night. Do you recall if this
southern hemi swell occurred around the 3rd week of June 2006?

-Nathan

[R B RB]

I dont recall exactly , but the timing would be about right.

[Nathan Cool]

Hey RB,

I think then I may have an explanation. Around June 20, 2006 there was
a significant southern hemi swell that hit SoCal. This was from a
storm that was intense when south of Easter Island, and formed into a
massive fetch with 45-foot seas, with some models estimates pointing
to 60-foot seas. It was a swell that made it into the papers. Peru
went off with 35-foot waves, and reports from Costa Rica observed 20+
foot face heights.

Shots of Malibu made it into our local paper here, showing DOH sets,
which you can see here:
http://www.toacorn.com/news/2006/0622/sports/086.html

But, a lot of breaks in SoCal didn't pull in this swell. Santa Cruz
would have though. Reason being: the angle.

This swell formed south of Tahiti, but grew in size once south of
Easter Island, which is where it was positioned when it took its most
significant trajectory to the north (when it threw the bulk of the
swell northward). This was angled from no more than 180 degrees
(170-180 actually), which would have been a tough angle for Oceanside,
which faces predominantly to the west, relaying on diffracted energy
from a southerly swell. Santa Cruz and Malibu point directly to the
south, so they would pull in a steep, southerly angle of 180 degrees.

I recall this swell vividly. A few members of the press contacted me
before hand, and I had floods of emails from Central America and
California readers days before the swell arrived. I was also in the
paper promoting my book on climate change that week, and one of the
backcover-quoters was in Peru at the time, telling me about the swell
down there.

If this is the same swell that you encountered, then angle could explain it.

-Nathan

[R B]

Thanks for trying Nathan.
Most likely not the swell I was thinking of.
My home is in Huntington Beach,1 mile from the beach, I speicifically
travel to Oceanside, Trestles when I know its a blocking SW.
Oh well, thems the breaks.
I appreciate the effort and thanks again.

Oh and for the record, we know Santa Cruz and S.F and SoCal are all
pretty much equidistant from Hawaii,
might not the same be true or roughly true on a big N.Z. swell that
stayed predominantly within the shadow of Tahiti in
relation to SoCal.

[Nathan Cool]

No problem RB. I guess the best thing to do is to keep an eye on it,
and when this happens again, let's take a closer look.

As for distance and swell arrival times, here are some southern hemi
numbers that might help:

Coordinates (approximate)
----------------------------------------
SoCal: 33N, 118W
Santa Cruz: 37N, 122W
Tahiti: 17S, 148W
Easter Isl: 27S, 109W
New Zealand: 35S, 180W

Distance and Time from Easter:
(for 16-second periods)
------------------------------------------
SoCal: 3640 nm, 6.3 days
SC : 3920 nm, 6.8 days
Difference = ~12 hours

Distance and Time from Tahiti:
(for 16-second periods)
------------------------------------------
SoCal: 3460 nm, 6.0 days
SC: 3560 nm, 6.2 days
Difference = ~6 hours

Distance and Time from NZ:
(for 16-second periods)
------------------------------------------
SoCal: 5370 nm, 9.3 days
SC : 5400 nm, 9.4 days
Difference = ~3 hours

Note also that NZ swells hit Costa Rica about the same time as the
California coast as well, given the westerly origination. Northern
hemi swells, if as you mentioned are near Hawaii, are fairly
equidistant from SoCal to NCal; however, since many NW swells
originate near the Aleutians, there is a difference of about 300 miles
(for a fetch at 45N 170W, just below the Aleutians), which causes a 12
hour difference in swell ETA.

I have a swell calculator that you can also use to figure this out as well at:
http://wavecast.com/calculator/

-Nathan