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

20 May 2018

Sam is going negative



Compiled Sun 20 May 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.


Weather regimes – SAM

Humans love finding patterns in things. Our orbit around the sun is so repeatable and predictable that we have “seasons”. Watching weather maps long enough, one can see that sometimes the same weather system (or extreme) may repeat in clusters. Last week I blogged about ENSO showing how flip-flops of the sea surface temperatures near the Galapagos are used to help decide if the weather pattern for the next season or so can be slotted into an El Nino or a La Nina regime. There are other cyclic anomalies on different time scales that we can use to help forecast if a weather regime may occur over the next few months.


Tonight, I think I’ll blog about SAM, the southern annular mode. This is a measure of the strength of the westerly winds in the Polar vortex - the ring of westerly winds that circle the planet between 50S and the Antarctic circle (66 S). The value of SAM alters the north-south movement of this vortex. A high positive value of SAM occurs when the air pressure over Antarctica are lower than normal, so that the westerly winds in the polar vortex are stronger than normal (note, the actual isobars over Antarctica are always higher than those in the polar vortex, but SAM works with the anomaly values, not the actual values). So, in a high positive SAM the polar vortex is shifted southwards, and pressures over NZ are higher than normal, with weaker than normal westerly winds and settled weather.


This can be seen at


However, when SAM is negative, the westerly winds in the polar vortex are weaker. This allows the polar vortex to spread outwards and thus northwards, so that west to southwest winds over NZ are stronger than normal.


SAM tends to flip-flop from positive to negative, and then to hold a phase for several weeks. When SAM jumps from positive to negative, it usually means that the polar vortex weakens, and the blob of cold air that has been sitting over Antarctica and getting colder over a number of weeks is able to burst outwards like a dam break. In other words: “a polar outbreak”.


I can not find any real time. or forecast data for SAM (which refers to differences  in pressure anomalies in the zone south of 50S), however a proxy of SAM is available, namely the AAO or Antarctic Annular mode (average 700hPa Z wind component 20S to 90S) as at


And forecast issued today shows that the AAO is about to have a negative jump later this week. This indicates a good chance of a polar blast somewhere (not necessarily affecting NZ). Looking at EC data of the surface air temperature forecast on windy and comparing Sunday with the forecast for Friday, it seems that the main change may occur around south and southeast of South America.



It was an interesting week with a tropical cyclone making landfall from the Red Sea onto Ethiopia. There’s also a tropical low in the Arabian Sea and it is expected to make landfall onto Oman, the horn of Africa, this week. And the Indian Monsoon is poised to strike a few days earlier than normal.


Next week a tropical low is expected to form in Gulf of Mexico and then make landfall on Florida next weekend.


If we compare the past week’s rain map with the previous week 

as at

we can see the increase in activity across the Indian Ocean. There are also been a build up in activity in the NE Pacific Ocean. And the South Pacific Convergence zone has weakened and split into two.



SPCZ=South Pacific Convergence zone.

No tropical depressions or troughs this week. The SPCZ is expected to weaken and retreat to be just between Solomon Islands and Tuvalu, maybe as far east as Tokelau. This provides a good weather pattern for yachts seeking to go westward this week.


Subtropical ridge (STR)

The STR is weak in the South pacific this week, and that allows a LOW to form in a passing trough near 30S 170W on Monday UTC and to deepen and it travels off to the SE reaching a peak 995hpa near 38S 140W on Wednesday, then fade and go east. May affect traffic between NZ and Papeete, avoid.

Next HIGH is still quasi stationary around Aussie Bight this week, but expected to poke out a tongue of a ridge along 30S from Wednesday, and this should expand be a High east of NZ from Friday. No squash zones.


Around Tasman Sea, NZ/Aus to tropics

Disturbed SW flow. One front travelling east over NZ on Tuesday/Wednesday., with SW swells reaching 7+m in eastern Tasman Sea.

Next front travelling east across NZ on sat/Sun 26/27 May. There is a chance that a HIGH may bud off from Southern Ocean and move into Tasman Sea after this front. In which case there may be a good weather pattern for departing from NZ to the tropics around Monday 28 May. Still too far away to be sure.

The SW swells from South Tasman sea should reach New Caledonia south coast with a burst over 3m from Thursday 24 May until early next week.


New Zealand to French Polynesia

Avoid departing when a front s near NZ as on Tue/wed or Sat/sun, otherwise Ok to go.


Panama to Galapagos /Marquesas

Panama area is surrounded by active shower activity this week, and there are SW winds, so nothing favourable on offer this week. If you do motor off, go SSE to 4N 79W then SSW to 2N 80W and then west to 2N 86W.

From Galapagos area to Marquesas, departure can be any time this week. Best path for wind and current is to motor/sail to 4 South 95W, then sail to 6S 127W and then go direct.



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