Bobgram

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 14 September
A more indicative measure of ENSO
ENSO stands for ElNinoSouthernOsciallation
The EN part of this parameter deals with sea temperature. We have been using the sea surface temperature measured in the Nino3-4 area as the parameter for deciding if the atmosphere is being driven by El Nino or La Nino or is in between the two in neutral gear. The SO part deals with the isobaric pressure difference between Darwin and Tahiti… located on this map:

However, climate change has been raising the world's average sea temperate in a roller-coaster fashion

Too try and remove this trend from the measurements, BoM have introduced a new measure called the RELATIVE NINO INDICIES.
As seen on their web site https://www.bom.gov.au/climate/enso/
Relative Niño indices
Traditional Niño index values were used at the Bureau of Meteorology until September 2025. From September 2025, the Bureau uses Relative Niño indices, which measure sea surface temperature anomalies in the tropical Pacific Ocean in the Niño regions, but calculated relative to the global tropical region temperature anomaly. This is to relate the indices more closely to the localised processes associated with ENSO, rather than larger-scale tropical SST features such as global warming.
Example: The Relative Niño3.4 index calculation:
Relative Niño3.4 = S x [(Niño3.4obs – Niño3.4clim) – (Tropical Meanobs – Tropical Meanclim)]
Where Niño3.4obs and Tropical Meanobs are the SST averages over the Niño3.4 region and the 20°S to 20°N tropical mean SST, respectively, while Niño3.4clim and Tropical Meanclim are the climatological values for the appropriate day/month depending on the dataset. S is a scaling factor applied so the variance of the relative Niño index matches that of the traditional index.
Sustained monthly Relative Niño3 or Niño3.4 index values above +0.8 °C as typical of El Niño conditions, with values of below −0.8 °C as typical of La Niña. These values are approximately one standard deviation from the long-term mean (e.g., around 70% of all monthly Niño3.4 values, lie between −0.8 °C and +0.8 °C).

This is what REL NINO 3-4 looks like going back to April 2021

It gives a good view of the El Nino in late 23/early 24
and the La Nina of late24/early 25 and shows a neutral trend in recent data.

BoM reports that there is now a negative IOD event and it is expected to continue during spring, until at least the next month. This is likely to bring wet conditions to parts of southern and eastern Australia.

TROPICS
The latest cyclone activity report is at zoom.earth and tropic.ssec.wisc.edu and Tropical Cyclone Potential is from https://www.ospo.noaa.gov/products/ocean/tropical/tcfp/

•A tornado spawned by Tropical Storm Peipah southwest of Tokyo tore roofs off homes and flipped cars.
• China's Guangdong province was drenched by Typhoon Tapah
• Hurricane Kiko weakened to a tropical storm just before skirting the northern Hawaiian Islands.
• MARIO is moving out to sea off south of Baja California.

Weather Zones
Mid-week GFS model showing isobars, winds, waves (purple), rain (red), MT (Monsoonal trough), STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) CZ (Convergence Zone)

Rain accumulation this week from Windy.com below shows well defined SPCZ between Solomon Islands and Funafuti and again east of Samoa. The eastern branch is forecast to shift south and form a depression L1 near the Southern Cooks late in the week.
Intense rain is forecast for Southern Alps in New Zealand especially with trough 2 on Tuesday and Wednesday.

Wind accumulation from windy.com below shows a windy area from Samoa to Southern Cooks late in the week with development of L1 from off the SPC. The disturbed westerlies south of 35S are typical of spring.

LOWS and HIGHS
There is a steady easterly migration with trough 1 followed by H1, then trough 2 and High2 then trough 3.

Trough 2 is expected over NZ on Tuesday and Wednesday preceded by NW winds, accompanied by rain heavy over the mountains and followed by a day or so of chilly SW winds. Then H2 crosses north island on Friday and Saturday. The sequence then repeats next week.
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Contact is bob@metbob.com or text 64277762212.
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