Cape Fear Swell Forecast | Seasonal Overview
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By Ben Macartney and Katie Jackson.
Before delving into the seasonal outlook lets get clear about what elements need to come together for the Cape Fear event to run. The number one ingredient is swell – not just any swell – but well organised wave trains of four metres or more, carrying an east to southeasterly direction and strong peak wave periods in excess of ten seconds. The autumn month of May and the winter months of June and July see an above average number of large swell events originating from disparate parts of our swell window. Late season monsoonal activity can produce large easterly swells of tropical origins during late autumn and early winter. As winter progresses deep extratropical storms forming over the Southern Ocean can generate long interval southeast groundswells that also fit the bill. However, the primary driver of large swell events carrying the optimum, east to southeasterly swell direction are East Coast Lows.
East Coast Lows (ECLs)
So, what differentiates an East Coast Lows from other storms systems? ECLs are large-scale storms that form off the Eastern Australian coast all year round, but they’re most common in autumn and winter and most common in June. Unlike tropical cyclones, ECLs are sub-tropical in origin; exclusively developing between the latitudes 25S and 40S. They usually form close to the coast in response to an upper level low or trough developing over eastern Australia, but are also derived from ex tropical cyclones undergoing extratropical transition and can also spawn on fronts and troughs moving across Tasmania into the southern Tasman Sea. Whereas tropical cyclones feed off warm sea surface temperatures greater than 26 degrees Celsius and usually gradually develop over the period of a week or more, ECLs have shorter lifecycles and often rapidly develop to become very intense within a short time frame. Major ECLs can usually intensify overnight to set up vast swathes of gale to storm force winds at sea level in conjunction with adjacent high pressure systems. These wind-fetches are usually aimed directly at the Eastern Seaboard and often become established within close range of the coast, giving rise to large, storm-swells encompassing the ENE to SSE directional spectrum. ECLs are also notorious for their extensive damage to the coastal fringes, with flooding, coastal erosion and wind damage often following.
Pacific Ocean El Nino Southern Oscillation (ENSO): Neutral with warming expected
While there’s some positive correlation between El Nino events and the frequency of ECL formation, ENSO indicators are not considered overly useful in assessing the probability of ECL development in a given season. Never the less, there are emerging indications the current, neutral ENSO state will give way to a warming trend in sea surface temperatures over the Pacific Ocean that may reach El Niño thresholds this winter. According to one study there is also a positive correlation between big shifts in the ENSO state and the frequency and intensity of ECL development off the East Coast, so we’ll be keeping an eye out for major swings in the Southern Oscillation Index.
Sydney Swell Averages (Direction, Height)
In the absence of reliable atmospheric indicators, we’ve analysed a cross section of Sydney wave buoy data from the Manly Hydraulics Laboratory to look at the statistical probability of large E swell events in the late Autumn/ early winter time frame. These numbers show average swell conditions recorded during the months of May, June, and July over a near twenty-year period.
Swell Height: PERCENTAGE EXCEEDANCE % for Significant Wave Height
(m) MAY JUN JUL
0 100 100 100
0.5 99.65 99.59 99.72
1.0 80.63 78.36 79.43
1.5 51.46 50.16 47.14
2.0 28.8 28.88 26.31
2.5 13.96 16.6 14.25
3.0 7.31 9.81 7.42
3.5 2.97 5.83 3.91
4.0 1.29 3.3 2.37
4.5 0.69 1.77 1.38
5.0 0.53 1.02 0.58
5.5 0.33 0.61 0.28
6.0 0.18 0.24 0.17
6.5 0.13 0.03 0.06
7.0 0.10 0 0
7.5 0.04 0 0
8.0 0.03 0 0
8.5 0 0 0
Average 1.71 1.75 1.66
Maximum 8.43 6.87 6.96
Minimum 0.4 0.39 0.39
The table for swell heights shows the largest month on average is June with waves generally averaging 1.75m. However, when we look at particular maximum swell event, the largest single occurrences of swell happen to be in May. Tropical lows or troughs that translate into large scale East Coast Lows directly off the coast are most likely to occur early in the season, which statistically generate the largest single swell events. So if we look for largest easterly swell at Cape Fear, it would statistically show in May based on this data. However, if it does not happen in May, then June is a good bet to see the more steady run of increased large SSE wave conditions reaching our criteria to hold the event.
Swell Direction: Percentage Occurrence %
DIRECTION MAY JUN JUL
N 0 0 0
NNE 0.08 0.09 0.05
NE 1.36 0.97 0.8
ENE 6.11 3.26 3.56
E 9.57 8.36 9.22
ESE 10.56 10.43 12.29
SE 18.42 18.19 19.29
SSE 34.1 39.07 35.91
S 18.84 18.43 16.64
SSW 0.48 0.65 0.99
SW 0.1 0.08 0.3
WSW 0.03 0.08 0.13
W 0.05 0.13 0.27
WNW 0.09 0.08 0.27
NW 0.14 0.08 0.16
NNW 0.06 0.03 0.1
-SSE dominant direction on average
-E to NE swell most likely to occur in May (early season)
-Swells tend ESE to SE later in the season (Jun-Jul)
The table breakdown shows us waves tend to shift direction from the east to southeast as the swell season progresses with an average swell direction from the south-southeast. The component of east swell in May is due to the influence from the tropic, positioning storms in the north to central Tasman and generating east winds into the coast. As well, a south wind fetch can sometimes generate a close range south swell in May from either scenario:
1. A tropical low moves south and interacts with high pressure over Tasmania.
2. The tail end of an early season cold front can sweep north into close range of the coast.
As we progress through June and to July, cooler sea surface temperatures emerge and tropical lows from the north become less frequent. In June, there is a shift of focus to the southern hemisphere and the dominant swell source becomes low-pressure systems located near Antarctica. Winter southerly swells source from the deep polar lows that push up towards Tasmania. These storms generate south winds across the southern Tasman, leading to a stronger southeast swell generation
The other key ingredient is favourable winds. For Cape Fear that means wind blowing any direction from south to direct west. Otherwise very light/ variable breeze of 5 knots or less may also suite. Any north in wind direction tends to add chop on the face of the waves, transforming an already heavy situation into one that’s downright sketchy. Autumn is the ideal time of year for wind from the westerly quarter; primarily owing to inversion created between the cooling land temperatures and the still warm SST’s over the Tasman and Coral Seas. Warm air over the sea rises as land temperatures rapidly drop overnight, creating a circulation where cooling air over the land flows offshore to replace the rising air at sea, thereby blowing westerly across the East Coast. Likewise as the land warms during the day, there’s often a period of equilibrium between the land and sea temperatures that effectively cancels out any significant air movement over the coast – often resulting in light and variable winds mid morning and again during the afternoon/ evening.
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