Weathering the South Pacific

Weather-routing experts David Cannon and Amanda Delaney explain the major South Pacific weather patterns and how you can use them to your advantage.

So. Are you thinking of taking a trip soon? What comes to mind when you think of the prevailing weather patterns in the South Pacific?

Well tranquillity for sure, but is there more to it than that?

Unless you are a seasoned veterans, you may not be aware that there are weather dangers and concerns here as well. The South Pacific offers everything from the very tranquil to the very volatile, and we will examine the various patterns that typically make the weather what it is in this part of the world. We will also let you know of the best areas and times of year to travel. Weather is a very important part of trip planning, and it is our hope that this article will make those less familiar readers aware of how to plan ahead and avoid those weather trouble spots.

The southeast Pacific from the equator to 10 degrees south, east of the international dateline

Two weather features typically come into play here. We find a broad west-to-east oriented trough of low pressure, which covers much of the equatorial Pacific (near/north of 10 degrees south). To its south we also find a large ridge of high pressure, generally found as far north as about 10 degrees south. Over the course of the year we will see an overall northward/southward oscillation in the positioning of both the trough and ridge, further south during much of the latter half of the year (late July/August through December), with a northward progression during much of the first half of the year.

E-SE trades will tend to be higher the further south one travels and further away from the trough axis. Generally speaking, wind speeds will not exceed force 4-5, though merging of high-pressure ridges off to the south will bring general 1-3 day periods of winds as high as force 6, particularly during the period from May through October. Combined seas will generally be E-SE, no more than two metres or so, and will tend to be lower and longer-period, and mainly swell the further north one travels, where lighter winds are more common.

Very long-period S-SW swells can also propagate far enough north to reach this area. These are most common during the middle portion of the calendar year (May through September) when the effects from cold fronts further south and west are more far reaching, and when high pressure off to the south tends to be larger and stronger. Swell heights in these long-period sets typically do not exceed two metres.

Breaks in the E-SE trades will occur when slowing/stalling and weakening frontal systems further south and west yield more N-NE-E winds. This is most likely to occur near and west of 160 degrees west and from May through September, when cold fronts in the southwest Pacific tend to advance further east before any stalling/weakening occurs. N-NE-E winds are generally from force 3-5, with E-SE combined seas lowering and becoming long-period, mixing with N-NE-E sets of no more than 1.8m.

10 degrees south to 30 degrees south, east of the international dateline

High pressure is dominant here as well, especially from November through April, when cold fronts tend to be less prevalent (Figure 1). This means more E-SE trades, which will tend to be stronger, reaching gale to near gale force (force 7-8), during wind surges that occur when high-pressure ridges merge. E-SE combined seas tend to prevail as well, becoming longer in period as one travels further north, closer to 10 degrees south where wind surges are less severe (typically no more than force 6). E-SE sea heights will generally range from 1.5-2.7m across the area. However, during wind surges, larger sets can occur, especially across more southern waters (south of 15 degrees south to 20 degrees south), where wind surges are more severe and there is more of a shorter, wind-driven component to the seas.

Approaching cold fronts will bring a brief respite from the E-SE’ly trade winds. This will be most likely to occur in waters west of 140 degrees west during the period from May through September, with fronts eventually slowing, stalling, and weakening near/west of 140 degrees west (Figure 2). In this region, cold fronts and the resultant weakening of high pressure further east will veer winds to more of a NE-N-NW direction, speeds generally from force 3-5, with combined seas lowering and becoming longer-period, generally no more than 1.8m.

The passage of cold fronts off to the south will initially bring a period of more southerly winds and eventually a return to E-SE trade winds. Wind speeds increase once again toward the typical values for trades (generally up to force 6). Southerly swells will develop in the wake of cold fronts and propagate northward into the region as well. These will tend to become longer-period and lower the further north one travels, though sets of up to and in excess of three metres can often be found, especially after the passage of stronger cold fronts.

The tropical season for this region occurs primarily during the period from November into April and only in the extreme western portions. The peak months are January/February, when ocean waters are warmest, and during May through October tropical concerns are just about non-existent. One notable exception to all of this is when El Nino regimes are in place. During these times, warm ocean waters migrating eastward into the tropical eastern Pacific will allow tropical cyclones to develop more readily well east of the dateline, as far east as 150 degrees west. Once formation occurs, these rare tropical cyclones will tend to track in a general W-SW-S’ward motion, weakening as they get further south (south of 25 degrees south, in cooler waters and a more “hostile” environment in the mid to upper levels of the atmosphere).

30 degrees south to 50 degrees south and east of the international dateline

This area is certainly not for the faint of heart, nor for those not ready for a potentially “wild ride”. The weather here can be very active, especially further south. Frequent frontal passages (as often as every two to three days during May through August) occur, bringing gale to storm-force winds, both in pre-frontal passage W-NW winds, and in S-SW-W winds following fronts. Very large swells, often in excess of six metres, will occur for prolonged periods in S-SW sets. Ideal travel weather is tough to come by, with perhaps a 1-2 day break in higher winds, and a relative abatement of S-SW swells, though still reaching/exceeding three metres, especially further south, as summertime high pressure ridges move across the area.

Eastern Australia to the international dateline climatology

The southwestern Pacific has a greater variety of weather features that can become more of a challenge to plan a transit in this region. Winters are very volatile, particularly in the mid-latitudes, whereas the summer generally provides much calmer conditions. However, during the summer is when the tropics become more of a concern due to increased tropical cyclone development.

From May through October (Figure 4), the gale/storm track shifts to the north and by mid-winter the gales/storms are reaching 40 degrees south. Cold fronts extend northward through autumn and by mid-winter the tail end of these fronts will reach the southern Coral Sea and move over New Caledonia. These cold fronts will track offshore Australia every 2-3 days in the winter (approximately every three days during autumn and spring) and move eastward over New Zealand. The cold fronts eventually slow down and the tail end of the fronts will weaken prior to reaching Fiji due to a strong high to the east.

Generally south of 25 degrees south winds can reach as high as N-NW force 6-7 and swells will build N-NW 2.4-3.6m ahead of these fronts. Behind fronts, high pressure will generally move offshore eastern Australia and move eastward over the Tasman Sea before merging with a semi-permanent ridge generally found east of 160 degrees east and between 15 degrees south and 30 degrees south. Immediately behind the front as the high builds over the region, winds will shift and strengthen WSWSSW force 7-8 (force 9 is not uncommon particularly over the Tasman Sea and southern New Zealand) and swells will build S-SW 3-4.5m. Once the high becomes established approximately 24-48 hours later, the winds will lower and become more variable of force 3-5 and southwesterly swells will lower and become longer in period.

Gales will, at times, develop along these cold fronts over the northern Tasman Sea and track southeastward near/over New Zealand. These gales will enhance the winds further to sustained force 8-9 and southwest swells will build up to 4.5-6m (becoming long period and lowering 0.9-1.5m further north near 20 degrees south).

Between 10 degrees south and 25 degrees, the semi-permanent ridge will interact with the previously mentioned equatorial trough, which is generally defined on satellite imagery as clusters of showers and squalls moving east to west within the trough. Interaction between these two features generates strong E-SE trade winds south of 10S that will reach force 5-6 and E-SE swells 2.4-3.6m. A break in these winds will occur when a cold front passes south over the Tasman Sea and New Zealand, allowing for winds to lower a Beaufort force or two and swells lower 0.91.5m. On the contrary, a strong high building offshore eastern Australia will enhance these trade winds up to force 7-8 and swells E-SE 3-4m.

North of 10 degrees south, E-SE winds are generally lighter from force 2-4 and swells E-SE 1.2-2.1m. Scattered showers and squalls are more prominent in this region due to the proximity of the equatorial trough.

From November through April (Figure 5), the weather features described above shift further south and reach their southern-most zenith by mid-summer. The gale/storm track will move southward towards 50 degrees south. Cold fronts become weaker and move offshore southeastern Australia every 4-5 days. These cold fronts only reach as far north as 35 degrees south during the summer months. As a result, winds ahead of cold fronts are typically N-NW force 4-5 (though can reach force 6-7 in stronger squalls) and swells become N-NW 1.5-2.4m across the Tasman Sea and over New Zealand. Behind the fronts, winds will veer W-SW-S force 5-6 and swells S-SW 2.1-2.7m.

Further north, trade winds will subside to E-SE force 3-4 and swells E-SE 0.9-1.8m as the semi-permanent ridge of high pressure builds southward between 25 degrees south and 40 degrees and the equatorial trough shifts to the south between the equator and 15 degrees south. North of 10 degrees south, winds become mainly variable force 2-3 and swells are generally E-SE (although variable at times) 0.6-1.5m.

Although late spring through early fall usually provides calmer weather across the tropical southwest Pacific, we still have one concern: tropical cyclones (Figure 3). These cyclones generally develop from November through April when sea surface temperatures are warmest and the equatorial trough extends southward over the northern Coral Sea eastward to the north of Fiji.

Tropical cyclones usually form between 06 degrees south to 15 degrees south and typically either take a west to west-southwest track and move inland over eastern Queensland, or turn southwestward and eventually south upon reaching 20 degrees south to 25 degrees south, then turn more southeastward ahead of a cold front. Upon reaching cooler waters the tropical cyclone will transition into a non-tropical gale or storm, and generally accelerate to the southeast before either dissipating or becoming absorbed by a cold front.

Tropical cyclones will increase in frequency from November through early February and the season reaches its peak during February. Cyclone frequency declines in April and beyond as the equatorial trough begins moves north and sea surface temperatures cool across the tropical waters.

Tying it all together: the best times/areas for travel

Okay, we know the weather patterns. What does it all mean in terms of planning the best travel areas?

Well as one might surmise, the more northern waters of the southeast Pacific tends to be more benign, with lighter winds, longer-period swells, and generally E-SE trades and lighter winds to be found, especially near and north of 10 degrees south. Those on eastbound transits should try to remain further north as well, closer to the equatorial trough axis, where there is less interaction between this feature and ridging further south. Summer will be the best travel period, though if your plans take you through this area during winter, you will want to avoid more southern latitudes where cold fronts are more frequent and stronger, and the effects from such fronts are more severe.

Summer is normally your best travel period in the southwest Pacific, too, as any delays are generally at a minimum with weaker fronts confined to more southern areas. However, the tropics need to be monitored closely, especially during late January through March when tropical cyclones are near or at their climatological peak.

The best time to avoid tropical cyclones is typically during the late autumn or early spring. However, weather conditions can change rapidly because this is a time of transition from winter to summer or vice-versa. Trade wind surges are common, increasingly so during late autumn, and timing these surges and cold fronts becomes more critical.

While travelling across the midlatitudes, a more northern route is often best, to minimise stronger westerly winds and large swells.

So there you have it, a look at the South Pacific. Hopefully this has raised awareness of what Mother Nature can offer in this area, and has helped you plan ahead. Stay safe, stay informed, and enjoy!