Bob Blog 10 March

WEATHERGRAM

YOTREPS

Compiled Sun 10 March 2019

 

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 = on watch for an El Nino, mixed messages.

ENSO stands for El Nino/Southern Oscillation.    El Nino is the name given when the ocean surface temperatures around the eastern equatorial Pacific rise warmer than normal.  This happens when the trade winds fade. El Nino events, with warmer than normal seas, draw the subtropical ridge closer to the equator. Their comings and goings can last several months, maybe over a year, and so their status can be used to help forecast the weather for the coming season.

During February we has an MJO and an extended monsoon trough and that weakened the trade winds and has indeed helped warm the target area for an El Nino.  This can be seen by looking at the latest Sea surface temperature SST anomaly map.  These trade winds have now returned to normal, suggesting that the SST may ease over the next few weeks.

 Sea surface temperatures can be seen at www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

Of note, there is also a warm stretch from Fiji to French Polynesia and another from Hobart to the central Tasman Sea.

Also of note; the Gulf Stream off the east coast of North America is much warmer than normal.  Overall, the yellow/red outweighs the blue.

 

The Ocean:

NINO3.4 is the parameter which measures the SST anomaly in the target region in the eastern equatorial Pacific Ocean for El Nino or La Nina. This plays with the heat budget of the atmosphere and thus with the weather patterns.

I have gone to the Bureau of Met page for latest NINO 3.4 data at www.bom.gov.au/climate/enso/indices.shtml, and it shows the weekly temperature anomalies since Jan 2015, with a cool period late 2016/early2017, and again in late 2017/early 2018. A warm period started in mid-2018 and briefly reached the El Nino threshold in late 2018, and the dropped and rose in Jan and Feb.  

 

The Atmosphere:

The SOI or Southern Oscillation Index (30 day running mean) comes from the atmosphere and 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.

Over the past year, the SOI has fluctuated up and down.  There was a period of negative SOI back in September/October 2018, but during December/January the SOI went positive, and in February, when the trade winds faded, it went negative. Trade winds have returned to normal , so maybe it will relax during the next few weeks,

 

SOI is seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly

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

 

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

The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. Sadly, during this time of the year, around the equinox and the following 6 weeks, the models usually do not perform as well as other times of the year.

The model predictions for the Nino 3.4 SST anomaly is that the seas ae likely to stay the same or slightly cool during the next few months, but some go for a slight increase. Mixed messages.

 

THE TROPICS

Latest cyclone activity as at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential as seen at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

TC HALEH has travelled into the South Ocean, and TC IDAI is in-between Madagascar and Mozambique.

The MJO wave of increased activity is travelling east across the Indian Ocean, and may reach northern Australia over next few weeks encouraging the formation of a monsoon trough, followed by another possible period of Tropical Cyclone formation in the South Pacific late in March.

Rain in the past week has faded in the South Pacific, but intensified across the equator around Papua New Guinea and along the ITCZ in western and central pacific.  See trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

As is usual at this time of the year, as the overhead sun travels from 5S to 3S, a “mirror Convergence Zone” of the ITCZ has formed just south of the equator in the eastern Pacific.

This “mirror CZ” may last for a month or two, affecting those sailing between Galapagos and Marquesas.

   

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is rebuilding over Papua New Guinea and around Fiji, and remains weak elsewhere.

There are some near equatorial westerly winds along the northern coasts of Papua New Guinea and western Solomons, and a Tropical Depression may form between PNG and Solomons by end of this week and drift southwards.

There is a weak convergence zone around southern parts of French Polynesia this week.

 

Subtropical ridge (STR)

HIGH over southern NZ is expected to travel off to northeast.

Next HIGH travelling eastwards across Bass Strait by Wednesday and then around southern NZ on Thursday and then off to the east. 

Next HIGH is likely to travel southeastwards across Tasmania around Sat/Sun.

 

Australia/Tasman Sea / New Zealand

Trough that crossed the North island on Friday developed a Low over Auckland on Saturday and this is expected to travel slowly southwards towards Chathams by mid-week then fade.

Trough from Southern Ocean expected to intensify over Tasman sea on Tuesday then cross South Island on Wednesday and North Island on Thursday followed by a brief southerly flow.

Low is expected to deepen off Sydney on Friday and then go southwards.

 

Panama to Marquesas

There should be good northerly wind for starters around Panama until local Wednesday and then light winds until late next week.  There is a good tail current to north end of Galapagos, then go to 6S 100W, to avoid the building “mirror convergence zone” along around 5 South,

 

Port Vallarta to Marquesas

Winds around Puerto Vallarta are likely to be best for departure around local Tuesday otherwise rather light for starters. Weak ITCZ likely between 6N and 3N, and then a “mirror CZ” near 3 to 5S.

 

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