North Pacific El Niño Outlook
Yet one swell does not a season make, so with an El Niño signal bubbling away let's drill down into the specifics of El Niño and the seasonal outlook.
- Not every El Niño provides a great Hawaiian season. It depends where the pool of warm water is positioned across the equatorial Pacific Ocean.
- When the warm pool is positioned further east, the storm track is drawn down towards Hawaii leading to an active season.
- When the warm pool is near the Date Line, there's no major influence on the North Pacific swell season.
- A positive Indian Ocean Dipole also feeds into an overactive North Shore season
El Niño years are historically synonymous with an overactive North Pacific storm season which then results in large to extra-large surf for Hawaii and an active surf season across California.
However, not all El Niño events are alike, there's quite a bit of diversity, largely owing to the location and intensities of the equatorial warm pools that develop across the Pacific Ocean during El Niño.
Before we get into that, let's have a quick refresher about what exactly El Niño is.
While most readers are up to scratch regarding La Niña - i.e stronger than normal easterly tradewinds 'pile up' warm water around northern Australia as colder water upwells across the central and eastern Pacific Ocean - it helps to think of El Niño as the opposite. The tradewinds slacken, sometimes even reversing, which promotes warming across the eastern, equatorial Pacific Ocean, which then spreads slowly back to the west.
Remembering that warmer than normal sea surface temperatures warm the atmosphere directly above it (while also supplying additional water vapor), we then see increased convection, storms and rain over the these warm regions, and in turn lower air pressure.
The warm, rising air then flows back to the west at height, cooling and sinking in the western Pacific Ocean, creating high pressure.
This difference between high and low pressure feeds a return flow of westerly surface winds, strengthening El Niño further. This is known as the Pacific Walker Circulation and is the main driver for the corresponding Northern Hemisphere climate and storm track through their winter.
Though Peruvian fisherman coined the term El Niño hundreds of years ago, the scientific study of it is just decades old. Recently, more light has been shed on the diversity of El Niño so that El Niño events can be classified as either Eastern Pacific (EP) events, Central Pacific/Modoki (CP) events, or even a mixture of the two.
The distinction depends on where the majority of the warm surface water sits across the equator. If the warm pool is more pronounced to the east, (as explained above) it's considered the classic El Niño set up, the pattern Peruvian fisherman identified all those years ago, and can be classified as an EP event. Whereas, if the warm pool is focussed more towards the Date Line and the centre of the Pacific Ocean, it is classified as a CP event. See the image below for a graphical illustration*.
EP El Niño events are the ones we're interested in for bumper Hawaii surf seasons, as they draw the North Pacific storm track closer towards Hawaii, whereas more centralised, CP El Niño events don't seem to affect the jet-stream in any meaningful way.
But why is this the case..?
Thanks to the Walker Circulation, more easterly warm pool events (EP) produce cooler, sinking air and higher than normal pressure across the Western North Pacific - in the region directly south of Japan.
It helps to understand that climate drivers work like links in a chain: one element effects the next and so on.
As high pressure settles in the Western North Pacific, the next 'link' is the clockwise rotation bringing warm and humid south to south-west wind towards Japan. This is the seeding area for storms in the North Pacific, and introducing warm, unstable air into the path of cold air shedding off Siberia, increases the volatility. Further, the storms also follow the eastern flank of the high, tracking down towards Hawaii.
In short, the North Pacific storm track is strengthened while shifting further south and east than is normal. It's a perfect setup for storm generation and propulsion, especially when Hawaii is downwind of the storms path.
In contrast, when the warm pool is situated more in the central Pacific (CP events), the sinking air and high pressure anomaly is shifted further west towards the South China Sea, cutting off the in-feed of warm, tropical air into the seeding area off Japan. This results in less storm activity of strength, and a more subdued surf season, despite it ostensibly being an El Niño year.
If you're keeping up, then there's one more factor to consider.
The anomalous high pressure sitting in the Western North Pacific isn't solely connected to EP El Niño events. There's also the Indian Ocean Dipole (IOD). When a positive IOD event (as we currently have) is coupled with El Niño, we see the anomalous Western North Pacific high becoming more of a feature, hence reinforcing the bumper EP El Niño seasons.
So in short, during EP El Niño events there's a much greater chance of an overactive North Shore season, and even more so when coupled with a positive IOD, while CP events play less of an influence on the seasonal outlook.
No doubt everyone is wondering what type of El Niño event we will have this year.
If we take a look at the current state of sea surface temperatures across the Pacific, the warm pool is strongest to the east along with a well spread out tongue towards the central Pacific. It's not of Modoki (CP) flavour though, as the majority of the warm pool needs to be sitting towards the Date Line, flanked by cooler waters either side.
So we're looking at an EP event with a slight Modoki twist. Add in the positive IOD and we should see anomalous high pressure setting up in the Western North Pacific, feeding developing winter storms with warm, moisture laden tropical air.
Moreover, the seasonal long-term forecast charts are confident on lower than normal pressure developing north of Hawaii into January/February/March pointing at a bumper surf season following the Christmas and New Year's break (slower before hand).
It's very good news for Hawaiian surfers.
// CRAIG BROKENSHA
*Graph courtesy of 'Tropical and Extratropical Air-Sea Interactions - 4 - The El Niño Modoki'