Bob Blog 14 July

WEATHERGRAM

YOTREPS

Compiled Sun 14 July 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.

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Status of the El Nino

The Atmosphere:

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.

 

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 March 2019 the SOI has been mostly negative, and in past few weeks it has been on the verge of an El Nino. The southern hemisphere subtropical ridge line has been further north normal, and trade winds have been inconsistent, and often interrupted by passing troughs

On the verge of an El Nino, as 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 that NINI3.4 has been warmer than normal for the past year , and verging on El Nino values since last October, but recently has been mostly trending downwards. This means the atmosphere and ocean are moving out of phase with each other.

Moving away from El Nino as seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthly

 

The warm waters in the NINO3.4 area are just near the surface, and a temperature/depth graph shows cooler waters near-by. This speaks against any swing to El Nino in the near future.

The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. The model predictions for the Nino 3.4 SST anomaly is that the seas are likely to stay as they are for the rest of this year. But one model is predicting a swing to the cool side.

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

 

Latest SST anomaly map shows a mix of cool and warm in the target area (around Galapagos). The Northern Hemisphere is mostly above normal. In the Sothern hemisphere there is a spread of cool water around eastern South Pacific. Cool water around Indonesia and Western Australia is linked to a positive phase of the Indian Ocean dipole (with stronger than normal trade winds), and is even cooler in the Australian Bight. Around New Zealand and Fiji, seas are continuing warmer than normal.  Sea surface temperatures may be seen at www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

 

THE TROPICS

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

TC BARRY has made in inland near New Orleans along with a large storm surge.

There is potential for another tropical system to form east of the Philippines.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ should stay over Solomons to Tuvalu/Tokelau/Samoa.

There is likely to be two passing troughs this week. The first should clear Tonga at 1012 on local Tuesday and then weaken by the time it reaches Niue on local Tuesday.

The second trough  at 1011hPa is expected to bring a swing to N then S winds around 20 knots, reaching Tonga on local Saturday, then Niue a day later on their local sat night, and going on to Rarotonga at 1015hPa by local Sun/Mon 21/22 July.

 

Subtropical ridge (STR)

There is a quasi-stationary High near 40S to south of French Polynesia this week. It should peak at 1038 on Tuesday UTC, but should be far enough away so s not to produce a squash zone.

Next High is staying over Australia until Sat and then expected to cross the Tasman and reach NZ next week.

 

Tasman Sea /NZ/Aus

No real Highs this week, but maybe 5 passing troughs, with deepest at 998 hPa on Tuesday night local.

This means it’s a stay-put week in NZ.

Mainly southerly winds between Australia and New Caledonia, good for going either way, but avoid 3+ swells on Tue to Friday

 

Tahiti to Tonga

Mostly easterly winds this week, so Ok to depart Tahiti on Monday, but this voyage is likely to encounter a passing trough near 169W on local Sunday 21st— can either sail around this or  maybe stop at Palmerston Island and wait out that trough.

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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 with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

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

Contact is bob@metbob.com or txt 6427 7762212

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