Bob Blog 23 Aug

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.

Compiled Sun 23 Aug 2020

 

Weather and climate drivers.

Seasonal and climate forecasting is more complicated than normal weather forecasting. The scope of the operation is wider and involves more variables. For Climate, one consideration is the amount of incoming energy (from the sun) and how this is distributed. As earth absorbs energy from the sun, it eventually emits an equal amount of energy to space Eventually. The (temporary) difference between incoming and outgoing can be measured and is given the name Radiative Forcing (RF). Volcanic dust is an example of one of the climate variables. Human-caused climate drivers, such as the amount of heat-trapping gases like CO2 and methane and the change in land-use have a trend, but at this stage, are not taken into account by seasonal forecasters. Seasonal forecasts use several regional based parameters to help decide what may happen in the next few months. ENSO, IPO, STR, SAM and MJO are the main parameters.

A comic way of reviewing and enjoying the effect of these on SE Australia is given at www.dpi.nsw.gov.au/climate-and-emergencies/seasonal-conditions/climatedogs

 

ENSO:

The El Niño–Southern Oscillation (ENSO) is currently neutral. However, in the OCEAN continued cooling of the tropical Pacific Ocean, changes in tropical weather patterns, and climate model outlooks shifting further towards La Niña, means that ENSO’s Outlook has moved to a La Niña ALERT.

The chance of La Niña forming in 2020 is around 70%— roughly three times the average likelihood. In the atmosphere, trade winds are stronger than average, while the Southern Oscillation Index (SOI) remains neutral but positive (indicating a La Nina trend). All international climate models surveyed suggest the tropical Pacific Ocean will cool further, with three of the eight models reaching La Niña thresholds in September, and two more in October.

For New Zealand this is likely to first bring the sub-tropical Ridge southwards with dry spells during the Spring, but it may encourage NE winds and wet spells in Northland during summer.

 

IPO:

The Indian Ocean Dipole (IOD) is currently neutral, despite the index dropping sharply recently. Large parts of the Indian Ocean are warmer than average, with some weak cool anomalies near the Horn of Africa. Half the six surveyed models indicate negative IOD thresholds could be met or exceeded during spring. This may increase the chance of rain in Australia but has little impact upon New Zealand.

 

SAM

The Southern Annular Mode (SAM) is currently negative and is expected to soon become weakly positive. A negative SM encourages southerly outbreaks.

 

MJO

The Madden–Julian Oscillation (MJO) is in the eastern Pacific and is expected to strengthen as it tracks eastwards towards Africa. At this time of year an MJO pulse in this region has little influence over NZ.

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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

There are two active storms in the Gulf of Mexico MARCO and LAURA and both seem to be heading to Louisiana, MRCO on local Sun/Mon then Laura on locl Tues/Wed Also BAVI is likely to make landfall over Korea. There are several areas of high potential for formation., especially west of central America.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

SPCZ is moderate and has shifted to north of normal near 10S from 150E to 160W.

Passing troughs over New Caledonia on Monday and over southern Tonga on Local Tuesday/ to Thursday.

 

Subtropical ridge (STR)

HIGH 1024hPa should get into Tasman sea at 30S n Thu and travel along 30S past northern NZ on Sunday over NZ at present is expected to travel east along 35S and build to 1040hP near 150W by mid-week, with a squash zone of enhanced trade winds along 25S with swells up to 4m from 145W to 155W.

 

Tasman troughs

Low off NSW likely to deepen to 988hPa over South Island on Mon wit a trough ove r North Island, then Low likely to go southeast and deepen to 980hPa. Low is followed by a vigorous West to SW flow over Tasman/NZ for remainder of the week with embedded weak troughs.

 

From Panama: ITCZ 5N to 10N, SW winds in Panama.W/SW winds from 7N 80W to 98W.

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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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