Bobgram 24 Nov

Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.
Compiled Sunday 24 November 2024

NIWA'S climate outlook for next three months for South Pacific

FROM A NEUTRAL NOVEMBER TO A LA NINA IN DECEMBER
These notes come from NIWA at their web site at
https://niwa.co.nz/climate-and-weather/island-climate-update
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There is a 60% chance that La Niña will develop by January 2025. Tropical Pacific trade winds will continue to push the ocean in a La Niña-like direction during December.
As of 19 November, the traditional 30-day Niño 1+2 Index anomaly was -0.12˚C, within the neutral range. The 30-day NINO3.4 Index anomaly (in the central equatorial Pacific) was -0.22˚C, also in the neutral range.
An alternative measure of central Pacific SSTs, called the relative oceanic Niño Index, has had an average anomaly of –0.84˚C over the last 30 days (to 19 November) and is more aligned with La Niña-like oceanic conditions.
The Southern Oscillation Index (SOI) was in the neutral range during September-November (+0.1), while the November value was -0.1 (in the neutral range), a decrease since last month.
The subsurface equatorial Pacific continues to be 2˚C to 4˚C cooler than average just below the surface in the east of the basin.
Above average upper oceanic heat content continues in western parts of the Pacific basin. The West Pacific Warm Pool is continuing to become more unusually warm, which is also reflective of the potential development of La Niña.
The South Pacific Convergence Zone was located slightly south of its climatological normal position during November.
During December-February, model guidance favours an enhancement in convective forcing over the western Pacific and Maritime Continent, consistent with a developing La Niña. This may lead to enhanced rainfall for some countries such as Palau, Federated States of Micronesia, Marshall Islands, and southern Papua New Guinea east to Fiji
Tropical cyclone season continues through April 2025. While no activity is forecast in the short-term, it is important to remain vigilant and prepared as tropical cyclone season is now underway.
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A LA NINA episode shifts the South Pacific convergence zone southwards, as shown in the expected rainfall map for the next few months

This shows the wettest area in the southern hemisphere, a proxy for the heart of the South Pacific Convergence zone, to be New Caledonia and south Vanuatu to Fiji.
So, the "juiciest" latitudes are likely to be between 15 and 20S, and cyclones forming in that latitude belt might not have enough time to mature before leaving the tropics.

TROPICS

Finally, the recent brush of cyclones seems to have come to an end.
Recently, at least 10 people died in storm-related accidents in the northern Philippines as Typhoon Man-yi became the sixth consecutive tropical system to hit the country in less than a month. Man-yi was also one of four November cyclones to exist simultaneously in the western Pacific—the first such occurrence since records began in 1951. • Tropical Storm Sara left at least four people dead from flash flooding across Nicaragua and Honduras. • Former Category-4 Cyclone Bheki skirted Mauritius and Réunion as a tropical storm.

The MJO, which boosts tropical activity is expected to make its way across northern Australia in early December and then into the Pacific by mid-December. This is likely to increase the risk of cyclone formation.
For a Time -Longitude prognosis from https://www.cpc.ncep.noaa.gov/



WEATHER ZONES
The wind accumulation shows a calm zone near northern New Zealand associated with the subtropical ridge. Also, it shows windy zones associated with lows L1, L2 and L3.
The South Pacific Convergence zone has a northern branch which extends from Solomons to Samoa to Northern Cooks, and a southern branch associated with Low L1 south of Tahiti and Low L2 near Fiji.
HIGHS and LOWS
HIGH H1 is staying quasi stationary near 40S well to southeast of Tahiti., with a subtropical Low L1 on its western shoulder.
HIGH H2 is up and down in strength and is quasi stationary to northeast of NZ with a strong ridge into the Tasman Sea. This should keep troughs away from northern NZ this week. It has a Low L2 on its NW shoulder forming near Fiji mid-week. As L2 come south its will be pushed by upper winds towards the west.
Low L3 I expected to cross the Aussie bight during the week and end up this weekend as a trough in South Tasman Sea.
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