Weather, Almost everyone we talked
to when we went into SE-Asia were a little confused when trying to
work out the reasons for the two differing wind directions for the
one monsoon season. The Intertropical Convergence Zone, or ITCZ, is the region that circles the Earth, near the equator, where the trade winds of the Northern and Southern Hemispheres come together. Sailing up in this area can be difficult at times, however if you can get a handle on the basics it really helps especially as some voyages will start in one hemisphere and end in another.
Indonesia
spans across the equator and you need to take this into account when
you are reading about which weather patterns fall in which month. We
didn't one time and as it turns out were not pleasantly surprised.
Indonesia doesn't get a summer or winter its either the wet season or
dry season the notation on the list below is more to denote the
hemisphere season. Also of note is the dry/wet is not the whole of
Indonesia in the same season.
Southern
Hemisphere Indonesia Northern Hemisphere Indonesia
N-West
monsoon Dec-Jan-Feb (S Summer) N-East monsoon Dec-Jan-Feb (N Winter)
First
transition Mar-Apr-May First transition Mar-Apr-May
S-East
monsoon Jun-Jul-Aug (S Winter) S-West monsoon Jun-Jul-Aug (N Summer)
Second
transition Sep-Oct-Nov Second transition Sep-Oct-Nov
 |
| Bruce's rough guide to the monsoon seasons on both side of the equator. |
Winds change through the year.
During December and January,
the Southern Hemisphere is heated more strongly by the Sun than the
Northern Hemisphere, so the hottest air — the air that rises in the
ITCZ — is found south of the equator. Winds from the Northern
Hemisphere blow across the equator toward the ITCZ.
During June and July, the
Northern Hemisphere is heated more strongly by the Sun, so the ITCZ
and its rising hot air lie north of the equator and winds blow from
the Southern Hemisphere across the equator to reach the ITCZ in the
Northern Hemisphere.
As the ITCZ changes location
through the year, the winds and rains and the location of monsoon wet
weather changes, too.
-----
Remember that the Coriolis
force changes direction on the equator: It turns winds toward the
right in the Northern Hemisphere (so a SE in the Southern Hemisphere
turn SW in the northern Hemisphere) Wind turns toward the left in
the Southern Hemisphere (so a NE in the Northern Hemisphere becomes
North West in the Southern Hemisphere).
So when air crosses the
equator as it flows from the cold winter hemisphere toward the ITCZ
in the summer hemisphere, it experiences a change in the Coriolis
force. This causes the trade winds to reverse direction and blow
toward the west in the winter hemisphere and to the east in the
summer hemisphere.
This seasonal reversal of the
winds was historically very important for trade between Africa and
Asia. Ships would sail from Asia to Africa in winter and then
undertake their return voyage when the summer monsoon changed the
wind from westward to eastward.
Winds within a couple of
degrees of the equator are generally “light and variable” all the
time. Weather forecasts are rarely accurate in these areas. Make sure
you have a cruising spinnaker and as a back up, an engine. Winds
are spasmodic
and
currents are/can be quite strong depending where you
are.
------------------------
SE
/ SW Monsoon Season
 |
| While the drawing I have done is a little rough I hope the fundamental idea makes sense. |
During the months of
June-September dry season trade winds blow consistently from the SE
and ESE throughout the Southern Islands of Indonesia. Trade winds are
stronger in Eastern Indonesia and areas around Timor and the Arafura
Sea than they are further to the West near Lombok, Bali, Java and
Sumatra.
Further North as SE trade
winds approach the equator they lose considerable strength and veer
from SE to a very light S. Then, once across the equator and into the
northern hemisphere, due to the Coriolis effect, these light S winds
again veer from S to SW and slightly increase in strength. It is for
this reason that we sometimes refer to this season as the SE/SW
monsoon season.
Precipitation on both sides of
the equator is at odds. While Bali and Java experience dry season
trade winds out of the SE, the areas of northern Sumatra, Kalimantan,
Sulawesi, Morotai and Thailand will be in their SW monsoon wet
season.
NE
/ NW Monsoon Season
During the months of
December-February NE winds blow in the northern hemisphere and across
the top of Indonesia. As these NE winds approach the equator they
lose strength and back to the N. Once across the equator, because of
the Coriolis effect, winds back further and increase in strength to
become the NW monsoon wet-season for much of Southern Indonesia.
Southern Sumatra, Southern Sulawesi, Java, Bali, Flores, Timor, and
Rote will all experience strong westerly winds and rainy conditions
for much of this period with the height of the wet season falling in
February.
Clear sky's and calms are
often experienced in between passing storm systems.
Cyclones can develop in the
Timor, Coral and Arafura Seas and this area should be monitored if
passing near Timor and Rote Islands during the NE/NW monsoon season.
Transitional Seasons
Some nice cruising can be had
during the changing of the seasons. Winds are often light to
non-existent with only short duration squalls passing through. Some
of the more exposed and windy areas of Indonesia are often calm and
pleasant and open to exploration at this time. With the sun directly
over the equator and calms likely, conditions can be very hot. The
ITCZ can move by as much as a thousand miles overnight or not move at
all and sometimes will appear in two places at once. With such
changing conditions occasional late or early season weather does
develop.
--------------------------------------------
We
have found that neither the GFS nor the ECMWF do a good job of
forecasting winds in Indonesia. Generally they get the direction
right, but wind strength is hit and miss. Weather forecasts (GRIB
files) in the Malacca Strait never seem to be right either, perhaps
this is just our observation because, we possibly never got far
enough off shore to get away from its effects.
Here
is a reasonable explanation of the monsoon seasons:
https://www.abc.net.au/news/2019-01-18/monsoon-explaining-the-complex-weather-phenomenon/10722716
Malacca
Strait
The weather description below
is for the Malacca Strait (Selat Melaka), being a passage between
Peninsular Malaysia and Indonesian Sumatra there are a couple of
anomalies. When using any of the Island anchorages, be prepared to
get underway at a moments notice. After being caught, we would work
out an action plan to get to the other side quickly. If we were
lucky, once the front passed there would be a calm for a couple of
hours after the storm, before the wind returns to its original
direction.
Northern
Hemisphere Melaka Strait
Northeast
Monsoon - November to March
This
is the dry season. The winds are predominantly from the NE from 10 to
20 knots, with occasional winds up to 30 knots. There are frequent
calms lasting from a few hours (often in the evenings and early
morning) to whole days or a few days duration. Rain is very rare and
the days are fine and relatively cool as the sun is at its furthest
south at this time (in December the sun is only 60 degrees above the
horizon at midday) - Very pleasant cruising weather and because the
wind is coming straight off the Malaysian peninsular there is no
swell and any seas that do come up during local strong winds drop
quickly when the wind drops.
Transition
period - April and May
This
is the height of the transition period between the NE and SW monsoons
and is marked by many calms, very light winds and flat seas. When
there is wind it can come from any direction and vary in strength and
direction, but is not common from the East through to South or from
the North. There is the odd rain squall that can last from five
minutes to an hour or two with winds sometimes up to 25 knots under
them.
Very
little chance of a "Sumatra" (see SW Monsoon). Hot, sultry
weather.
Southwest
monsoon - June to September (Some years into late October)
This
is the wet season. Hot, sultry weather as the sun is in the North and
the humidity is high. The winds are predominantly from the SW from.10
to 25 knots, with frequent calms and frequent rain squalls. The rain
squalls last from a few minutes to a few hours but typically last for
about 15 minutes with very heavy rain and either no wind or, more
frequently, winds up to 25 knots. These winds change strength and
direction depending on your position relative to the centre of the
squall.
Once
they have passed over, the skies clear and the wind returns to its
condition before the squall arrived. A squall can usually be seen
well before it arrives as an isolated large, dark, towering cloud
and, should you be close to land or rocks, gives you plenty of time
to either anchor or head for an open area before the heavy rain
causes a complete whiteout.
This
is the season for "Sumatra" winds.
"SUMATRA"
winds are a different matter altogether. In the day they can easily
be identified as a long rolling bank of very dark cloud extending
roughly North and South from horizon to horizon and coming from the
West from Sumatra, thus their name. At night, when they are most
common, the first warning is when they hit. They are most common in
the South Malacca Straits, but do occur occasionally around Penang
,and Langkawi, though they are rarer around Phuket. Typically there
will be two to four during this season in Penang and one or Penang
and one or two in Phuket. There is no avoiding the very strong (up to
60 knot)\ Westerly quarter wind that just precedes the cloud bank and
very heavy rain. They can last for five or more hours, though more commonly
last for from about 20 minutes to one hour and then end as quickly as
they began.
Transition period - October
Same
as the Transition period from April to May.
This
is the height of the transition period between the SW and NE monsoons
and is marked by many calms, very light winds and flat seas. When
there is wind it can come from any direction and vary in strength and
direction, but is not common from the East through to South or from
the North. There is the odd rain squall that can last from five
minutes to an hour or two with winds sometimes up to 25 knots under
them.
Very
little chance of a "Sumatra" (see SW Monsoon). Hot, sultry
weather.
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“Red
sky at night sailors delight; red sky in the morning sailors take
warning.”
There
is a simple explanation for this. When the sun is on the horizon, its
light, shining at an angle, must pass through more atmosphere.
Red
is the colour of the spectrum with the longest wavelengths and can
therefore reach all the way through to where water vapour and dust is
denser. Here it reflects off the vapour and dust projecting a red
light to our eye. In mid latitudes storms predominantly move from
west to east.
“Red
sky at night sailors delight" So when the sunset in the west
reflects off particles of a low pressure in the east, creating the
red lit sky, the weather has past making way for improved
(delightful) conditions.
"Red
sky in the morning sailors take warning" So then if the sunrise
in the east reflects off particles of a low pressure in the west,
making the red lit sky, we can assume that storms are approaching,
sailors take warning the weather should worsen.
