Bob Blog 23 Sep

WEATHERGRAM

YOTREPS

Compiled Sun 23 Sep 2018

 

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.

 

As an addendum to last week’s summary of time zone changes and national holidays:

Samoa also changes to Daylight saving on 30 Sep, from UTC +13 to UTC+14. This is so that Apia remains 1 hour ahead of Wellington all year round. (The only other place on the planet that uses time zone UTC+14 are the Line Islands, including Kiritimati, near equator 160W,almost south of Hawaii).

 

The state of the ENSO = neutral, with a weak hint of an El Nino

 

The Atmosphere:

El Nino and La Nina are opposite ends of the swing of an identifiable tropical influence on our seasonal weather. During La Nina we have cooler than normal seas along the equatorial eastern Pacific , and this shifts the subtropical ridge away from the equator. In El Nino, with warmer than normal seas, the subtropical ridge shifts closer to the equator- trade winds are weaker and the disturbed westerly wind so the Southern Ocean can penetrate further north. Each episode may last several months, sometime over a year, and so their status can be used to help forecast the weather for the coming season.

 

ENSO = El Nino/Southern Oscillation. The main parameter we watch from the atmosphere is the Southern Oscillation Index SOI (30 day running mean) as it sums up the whole weather pattern over the South Pacific in one number. It 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 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.

 

Since May we have mainly had a negative SOI, consistent with a weak but rather persistent El Nino signature.

BoM states that “model outlooks continue to indicate El Niño is possible from late spring 2018.”

Weak El Nino  conditions is seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly

(Note that in this graph on the vertical axis 10= 1 standard deviation)

 

The Ocean:

NINO3.4 is a region in the eastern equatorial Pacific Ocean that 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.

At the farmonline web site we can see the trend in the sea surface temperature in the NINO3.4 area. The diagram shows the weekly temperature anomalies since Jan 2015, with the El Nino of 2015 looking like a hump on a camel. Since then there has been a cool period late 2016/early2017, a warm period until July 2017, then a cool period until June 2018. Since then we have been having a warm period, but remaining less than 0.5 degrees above normal.

Weak El Nino is seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthly

 

The International Research Institute of the Climate Prediction Centre IRI/CPC compiles data from several ENSO prediction models. The model predictions for the Nino 3.4 SST anomaly is that the seas ae likely to gradually WARM during the rest of this year, but the mean of the predications has only warming to 0.9 above normal--- not enough to be called an El NINO event (but closer to it that we have been for a while).

CPC/IRI predictions are at iri.columbia.edu/our-expertise/climate/forecasts/enso/current/

 

Latest SST anomaly map shows a large blue pool of cooler water west of South America. Also, there are warmer yellow waters appearing around the Galapagos. Not much ice (white) left in the Arctic at the equinox.

Sea surface temperatures across the Pacific on 20 Sep are at www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

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TROPICS

The number of tropical features is reducing after that early September peak.

Map of current storms is at tropic.ssec.wisc.edu/

 

TRAMI is heading for Taiwan

And KIRK may skirt Venezuela

 

Looking at the weekly rain maps, last week doesn’t have the noticeable cyclone rain tracks that show in the previous week but shows more intense activity in the ITCZ across the Pacific and Atlantic. It also shows a resurgence of activity around the Bay of Bengal.  NZ had some passing fronts last week, but Australia… remains… dry.

See: trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ has been hovering around the region between Solomons Island and north of Fiji for a few weeks, and is expected this week to drift south across northern Vanuatu and across Fiji/Tonga. By the end of the week a low may form south of Niue and travel off southeastwards, taking a passing trough eastwards along 20S reaching Southern Cooks by local Friday/Saturday.

And over French Polynesia a convergence zone is expected to help form a low near 20S 135W that moves off Southeastwards.

 

Subtropical ridge (STR)

HIGH often over 1030hPa to travel east across Tasmania on Tuesday and linger over northern NZ from Thursday to Saturday then fade to NE of NZ next week. eats of NZ travelling east along 40S. Squash zone of enhanced winds on its northern side mainly north of 20S from Wednesday to Saturday.

 

Between Tropics and Tasman/NZ.

Avoid that SPCZ travelling south over Fiji by Wednesday and the squash zone that follows. SO, there are OK voyages from Tonga/Fiji on Monday, or for fast boats on Tuesday, then it’s a case of staying put.

Trough crossing NZ on Monday and then a Low deepening east of NZ by Tue moving off on Wednesday leaving NZ bathed in SW winds.

After the High, next trough for NZ is expected over southern NZ on Sunday/Monday, then a trough from the Tasman after Tuesday next week.

 

From Tahiti to Tonga

OK to depart after convergence zone ha gone. Anticipate a passing trough along the way late this week.

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

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Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

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