BoB Blog 15 Sep

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 15 September 2024

The climate influencers

A quick review of the trends in the parameters we use to watch the main seasonal weather influencers. Thanks to bom.gov.au/climate

The Pacific

From the Pacific Ocean comes the ENSO (El Nino Southern Oscillation) parameters. La Lina is the name given to the period when sea surface temperature SST over the eastern equatorial Pacific is below normal.

In the atmosphere we monitor ENSO = El Nino/Southern Oscillation and its impact on the weather map by using. the Southern Oscillation Index SOI (30 day running mean). This is based on the standardized difference in the barometer readings between Tahiti and Darwin, in other words it counts the average number of isobars between them on the weather map. When the index stays above 7 (on the scale shown here) for more than a month we call it an LA NINA event.
This is showing that after neutral territory during winter , The SOI is no indicating a swing towards La Nina.

The Ocean:

The parameter used from the ocean is based on the sea surface temperatures in the equatorial eastern Pacific and called the NINO 3.4 SST anomaly.
International research Institute IRI have compiled the predictions for all the models as shown here from iri.columbia.edu. The consensus is that according to the Dynamic models there may be a weak LA NINA from October to February.

Expected impact in the South Pacific

The cooler than normal lounge of surface water across the east equatorial Pacific tends to nudge the South Pacific Convergence zone southwards. This makes the trade winds stronger than normal and shifts the subtropical ridge southwards. In spring this concentrates the disturbed westerly winds of the roaring forties. In summer the path taken by migratory Highs from south of Australia is across the Tasman Sea and along 40S, encouraging a northeast flow onto northern NZ. The peak cyclone prone area hugs Vanuatu.

However, each La Nina event is different and even a strong event only explains around 20% of the observed weather variance.

Tropics

The MJO Maden Julian Oscillation is the name given to a measurable increase in intensity that travels eastwards around the planet around once a month mainly along the Intertropical convergence Zone and affects tropical weather.

A good diagram of its current intensity and expected future for the next few weeks is at

www.cpc.ncep.noaa.gov/ --- in these maps BLUE is extra cloudiness associated with MJO and yellow is extra sunshine.

It is likely that with the MJO recently over Indonesia, it may have been associated with formation of Cyclone YAGI. MJO is expected to move east into the pacific Ocean over next few weeks and fade way.

Southern Ocean

For the Southern Ocean comes the SAM parameter (Southern Annular Mode). This measures the strength of the polar vortex, a ring of westerly winds circulating around Antarctica between 50 to 60S. When positive, polar outbreaks are few, and when SAM is negative polar outbreaks are encouraged.

After a long negative period, a few months ago, SAM is now positive, but is expected to drop over the next weeks. This is consistent with the snowy southerlies expected over SE Australia and South Island this week.

Rounding up the Climate Dogs

Agriculture Victoria have compiled some animated cartoons explaining six seasonal weather parameters as dogs with different characteristics see

agriculture.vic.gov.au/climate-and-weather/understanding-weather-climate-and-forecasting/the-climatedogs-the-six-drivers-that-influence-victorias-climate

and watch six climate docs: Ridgy, Eastie, ENSO, Indy, Sam and MoJo strut their stuff.

TROPICS
Around 200 people were killed in flash floods and mudslides triggered by Typhoon Yagi, which swept from South China's Hainan Island to northern Vietnam. Yagi was the most powerful storm the region had seen in decades. * Tropical Storm Bebinca is forecast to become a powerful typhoon before striking the Chinese coast near Shanghai this week. * Louisiana and parts of coastal Texas were battered by Hurricane Francine, which later caused flooding across the lower Mississippi Valley

WEATHER ZONES

The South Pacific Convergence zone is sitting over Solomon Islands and extends to Rotuma and Samoa and then to the southeast across Southern Cooks. A passing trough tonight west of Fiji is expected to travel east across Tonga and Niue early in the week and reach Southern Cook and Tahiti late in the week. This trough is expected to be accompanied by a lull and some squalls and followed by a day or two of SW swells.

HIGHS and LOWS

HIGH H1 remains slow moving well south of Tahiti.

A broad trough with many parts is crossing NZ area on Monday and Tuesday then is expected to deepen into a Low L1 near Chatham Islands. This maintains a cold strong southerly flow over NZ on Wednesday followed by a period of disturbed westerlies for the remainder of the week.

HIGH H2 is expected to travel across Australia and then from mid-week go east along 25S across northern Tasman Sea as a disturbed westerly flow spreads onto NZ

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If you would like more details about your voyage, then check metbob.com to see what I offer.
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Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).
Weathergram archive (with translator) is at weathergram.blogspot.co.nz.
Contact is bob@metbob.com or text 64277762212.
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