Understanding South Pacific Weather
The South Pacific offers everything from tranquil to chaotic and everything in between. There are also differences in the weather patterns between summer and winter and where you are in the Pacific. Here we’ll look at the various patterns that typically produce weather and talk about the different seasons and when is the best time to make a passage. Weather is possibly the most important part of passage planning and can be the difference between a good experience and not!
The South East Pacific from the Equator to 10 Degrees South:
This region covers an area from Panama across the the 180 meridian to 10 degrees south latitude. The weather in this region will affect vessels making passage from Panama to Galapagos and for the first part of the passage from Galapagos to French Polynesia. It will also affect trans equatorial passages from West coast US ports, Mexico and Hawaii.
Two weather features typically come into play here. Firstly the intertropical low, which covers much of the equatorial Pacific (north of 10 degrees south). To its south the second feature is a ridge of high pressure which oscillates north during winter and south during the antipodean summer. This is the transition between the intertropical low and the migrating systems further south.
E-SE trades will tend to be stronger further south away from the intertropical low. Wind speed is dictated by the pressure gradient between the intertropical low and the high pressure systems migrating west to east further south with the time of year and how far north or south the high pressure systems are. As a general rule the SE trade winds are calmer in the early part of the year and accelerate during the period July to November trending to the east rather than SE.
It’s not unusual to see a long frequency swell from the S-SW in this region. 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 far reaching, and when high pressure systems 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 180 Meridian:
This region covers the cruising regions of French Polynesia as far south as the Australs and the Gambiers and west from French Polynesia toward the Southern Cooks.
The high pressure ridge is dominant here, especially from November through April, when cold fronts tend to be less prevalent which is also the Cyclone Season. The best time to cruise this region is May to November.
As the systems migrate from east to west with low following high following low, an approaching cold front will bring a brief respite from the E-SE trade winds often creating a wedge shaped area between the upper quadrants of the high pressures that has variable 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. 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, in this region once you see NW winds a trough or front is imminent.
The passage of cold fronts off to the south will initially bring a period of more southerly winds as the new high pressure establishes and eventually a return to E-SE trade winds. Wind speeds increase once again toward the typical values for trades. 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 they travel, 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 mainly in the extreme western portions. The peak months are January/March, when ocean waters are warmest, and during May through October tropical concerns are lower. 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
30 Degrees South to 50 Degrees South and East of the International Dateline:
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 — The Southwestern Pacific:
The Southwestern Pacific has a greater variety of weather features that can make planning a passage in this region more of a challenge. 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 September, 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 and move eastward over New Zealand. The lows and troughs slow down as winter comes to an end with the frequency out to between 5-7 days by mid October.
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. A break in these winds will occur when a cold front passes south over the Tasman Sea and New Zealand. On the contrary, a strong high building offshore eastern Australia will enhance these trade winds the strength determined by how far north the high centre pressure is and the intensity of the high, a rough rule of thumb is the peripheral winds on a high are likely to equate to the last twi digits of the centre pressure, ie. a high with a centre pressure of 1030 is likely to have 30kt winds on it’s periphery.
North of 10 degrees south, E-SE winds are generally lighter. Scattered showers and squalls are more prominent in this region due to the proximity of the equatorial trough and the South Pacific Convergence Zone.
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 5-8 days. These cold fronts only reach as far north as 35 degrees south during the summer months.
Further north, trade winds will subside to E-SE 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. 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/ March. Cyclone frequency declines in April and beyond as the equatorial trough begins to move north and sea surface temperatures cool across the tropical waters.
Passages in the equator to 10S are likely to be calm and particularly in the eastern Pacific where there is less risk of cyclonic conditions, can be made all year round.
Passages in the 10S to 30S region east of the 180 meridian the best time of year is April through October but a plan needs to be in place if you’re transiting through to the South Western Pacific where you’ll leave the boat for the cyclone season.
Passages in the SW Pacific to avoid the possibility of a cyclone should be made between the end of April and beginning of November and for those heading south to spend the summer in New Zealand departure from the islands mid October to mid November sees the frequency of lows and troughs slow down with the risk of cyclone activity still low.
For more in depth information on this region see the article Passage Planning in the South West Pacific.
John Martin, September 2019.