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=
Weather regimes - SAM
Humans love finding patterns in things. Our orbit around the sun is so repe=
atable and predictable that we have "seasons". Watching weather maps long e=
nough, 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 o=
f the sea surface temperatures near the Galapagos are used to help decide i=
f the weather pattern for the next season or so can be slotted into an El N=
ino 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 rin=
g of westerly winds that circle the planet between 50S and the Antarctic ci=
rcle (66 S). The value of SAM alters the north-south movement of this vorte=
x. A high positive value of SAM occurs when the air pressure over Antarctic=
a 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 value=
s, not the actual values). So, in a high positive SAM the polar vortex is s=
hifted southwards, and pressures over NZ are higher than normal, with weake=
r than normal westerly winds and settled weather.
This can be seen at blog.metservice.com/Southern-Annular-Mode
However, when SAM is negative, the westerly winds in the polar vortex are w=
eaker. 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 mea=
ns that the polar vortex weakens, and the blob of cold air that has been si=
tting 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 dif=
ferences in pressure anomalies in the zone south of 50S), however a proxy =
of SAM is available, namely the AAO or Antarctic Annular mode (average 700h=
Pa Z wind component 20S to 90S) as at www.cpc.ncep.noaa.gov/products/preci=
And forecast issued today shows that the AAO is about to have a negative ju=
mp 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 temp=
erature forecast on windy and comparing Sunday with the forecast for Friday=
, it seems that the main change may occur around south and southeast of Sou=
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. A=
nd 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 mak=
e landfall on Florida next weekend.
If we compare the past week's rain map with the previous week
as at trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif
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=3DSouth Pacific Convergence zone.
No tropical depressions or troughs this week. The SPCZ is expected to weake=
n and retreat to be just between Solomon Islands and Tuvalu, maybe as far e=
ast 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 fo=
rm in a passing trough near 30S 170W on Monday UTC and to deepen and it tra=
vels 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 expe=
cted to poke out a tongue of a ridge along 30S from Wednesday, and this sho=
uld 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 chanc=
e that a HIGH may bud off from Southern Ocean and move into Tasman Sea afte=
r this front. In which case there may be a good weather pattern for departi=
ng from NZ to the tropics around Monday 28 May. Still too far away to be su=
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 ar=
e 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.
If you would like more detail for your voyage, then check metbob.com to see=
what I offer.
Or Facebook at /www.facebook.com/metbobnz/
Weathergram text only (and translator) is at weathergram.blogspot.co.nz.
Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe=
email@example.com or txt 6427 7762212