Bob McDavitt's ideas for sailing weather around the South pacific

12 March 2017

Bob Blog 12 March 2017

Compiled Sun 12 March 2017

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.

The state of the ENSO
The Atmosphere:
The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin. When the SOI is more than plus one (standard deviation from its mean) for more than a month we call it a LA NINA event, and when it stays more than minus one we call it an EL NINO event.
El Nino and La Nina are tropical influences on the weather: the La Nina shifts the subtropical ridge away from the equator and the El Nino draws the subtropical ridge closer to the equator. This affects the seasonal weather all around the planet.
From May 2015 to May 2016 we had an extreme El Nino. Then there was a weak La Nina in October 2016, and since then we have had only a weak SOI. In the past few weeks it has been near zero and slightly negative.
A weak SOI is seen at

The Ocean:
The eastern equatorial Pacific Ocean acts as a heat storage area during an El Nino, or becomes cooler than normal during a La Nina. This plays with the heat budget of the atmosphere and thus with the weather patterns.
There was a period of cooler than normal sea over the past few months, and now things are around normal.
as seen at

The International Research Institute of the Climate Prediction Centre compile data from several ENSO prediction model. This shows less than 10% chance of a La Nina this yea , and the current Neutral episode may be replaced by an El Nino event after July, but probability is only just above 50%, so not strong enough to change plans based on this forecast. See CPC/IRI predictions

For the next few months use monthly averages as seen at

The tropics
There is a tropical depression in the mid parts of the South Indian Ocean, otherwise things are quiet at present.

Rain from last week, compared with previous week, shows heavy rain about Madagascar (from TC ENAWO) and in the mid parts of the South Indian Ocean (from a tropical depression). This pulse of extra convection is expected to travel onto northern Australia during the next week or so and into the South Pacific later this month. Map also show as wet Tasman Sea/NZ area due to a cut-off low. This has locally been called the Tasman tempest.
The Intertropical Cyclone convergence zone is slowly rebuilding after a period of weakness. weak and broken into branches. And there is a convergence zone along 5S from 100 to 125W-this is due to directly overhead sun, and may last a few weeks, so I call it the equinoctial convergence zone.
Rain for the past fortnight from

SPCZ=South Pacific Convergence zone.
SPCZ is expected to remain much the same this week, rather diffuse and wide spread across the Coral Sea and may focused and narrow over Fiji and Tonga. A small tropical depression may form between New Caledonia and Fiji on weekend of 18/19 March and travel to southern Tonga early next week.
The "equinoctial convergence zone" is expected to hang around for a few weeks around the March 21 equinox and is a minor and normal annual weather feature Tropical accumulated rainfall for next week from

Subtropical ridge (STR)
New High is forming tonight in western Tasman Sea and pushing away the low that is crossing NZ. This High is expected to develop mostly in South Tasman Sea and spread around Southland on Wednesday then travel northwards across central NZ by Friday and split into two centres west and east of central NZ over the 18/19 March weekend.
This should produce moderate easterly winds and settled weather for the Westhaven Marina Open Day in Auckland on Sun 19 March see

Tasman Sea troughs.
Perhaps to make up for last week, there are no Lows expected in the Tasman Sea this week.

New this week (by popular request) Mexico to Marquesas:
The ITCZ is still recovering so rather weak and can be expected to bring some squalls between 10N and 5N. There may also be some squalls near 5S
Avoid the head current near 17N 110W, otherwise may as well go direct.
Should be a tail wind to 5N and a good sailing breeze from 15N to 5N, then light winds from 5N to Marquesas.

Panama to Galapagos
Looks good for departure until 22 March with northerly winds for starters. After that winds are lighter and may be southerly. Main convection is likely between 5N and 2N.
Prepare for light winds south of 3N, maybe head for 3N 84W as a waypoint for best wind and tail current.

Galapagos to Marquesas
To avoid the "equinoctial convergence zone" and ensure a nice SE wind for sailing, go to 10S 100W and then direct. Expect light winds between 3 and 7S, then SE 10 to 15 kt until 130W. Anticipate light NE/E winds west of 130W
See my website for more information
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