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

31 May 2020

Bob Blog 31 May

The state of the ENSO = neutral

 

It has been months since we have looked at ENSO, so here goes.

 

El Nino and La Nina are opposite ends of the swing of an identifiable tropical influence on our seasonal weather. The La Nina, caused by cooler than normal seas along the equatorial eastern pacific, shifts the subtropical ridge away from the equator, and the El Nino, with warmer than normal seas, draws 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.

 

The Atmosphere:

 

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 a brief El Nino late in 2019 the ENSO has been neutral and the subtropical ridge line spent the Summer and Autumn moths mostly over central NZ, keeping troughs away from northern NZ (hence the droughts). This ridge line has recently become more variable and further north.

 

See  this 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:

The sea surface temperatures in the equatorial eastern Pacific has been warmish so far this year but now is showing a cooling ( La Nina) trend.

 

Sea temperatures, are seen at

www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf

 

Seas

 

Seas have been warmer than normal in North Pacific Ocean, in swaths across Indian Ocean and Atlantic Ocean, an d from Fiji to the east of NZ Indian. There is a slight cooling in the eastern equatorial Pacific.

 

Also, Waters just beneath the surface are slightly cooler than normal. These trends are towards a La Nina.

 

 

The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models.

The model forecasts neutral condition s during our winter but the chance of a la Nina may rises to (a rather meger) 31% in our Spring ( with neutral chance dropping to 40%,and El Nino chance around 25%)

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

 

So the outlook is for a Neutral window, but we on watch for La Nina trends.

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The Tropics

The latest cyclone activity report is at tropic.ssec.wisc.edu and Tropical Cyclone Potential is from www.ssd.noaa.gov/PS/TROP/TCFP/index.html

Quieter after MANGGA and a cold front combined over Perth early last week.

TD2 is affecting central America for now.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is active around Solomon Island, over northern Vanuatu, Tuvalu, Samoa, and Tahiti.

A Tropical low may travel south over or south of the Tahiti area late in the week.

 

Subtropical ridge (STR)

HIGH 1034 east of South Island expected to travel east along 45S and grow to 1038hPa (wow) on Monday to Wednesday. -1028hPa is expected over Tasmanian on Monday, and should cross central and southern NZ on Thurs

Next HIGH 1026hPa is expected to move into central Tasman on Friday and ease to 1020 north of NZ over next weekend. n Thurs and cross southern NZ on Friday and then build to 1036hPa well east of NZ.

 

Tasman Sea /NZ/Aus

Low from the sub-tropics to travel south over North island from now to end of Monday(Queens Birthday holiday ).

Another active trough is likely to cross NZ on Tuesday night/Wednesday dripping to 996hPa SE of South Island, and followed on Thu by a strong southerly with large swells in the eastern Tasman Sea on Thu

 

From Panama: Not good this week, and maybe for the coming months. SW winds in Panama, and the ITCZ is visiting at times. It is most active/squally to3N. I reckon that the best way to tackle these SW winds is to go SSE then west.

 

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