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ZCPS et autres curiosités météo

La météo n'est pas forcément plus compliquée dans la pacifique Sud mais la complexité n'est pas la même que par chez nous. Ici on est préoccupé par El Nino (et la Nina), par la Zone de Convergence du Pacifique Sud (ZCPS ou SPCZ) et bien d'autres choses.
Bob McDavitt est un gourou néozélandais de cette météo auprès des voiliers qu'il route en grand nombre. Il poste sur son blog un bulletin hebdomadaire avec les prévisions à moyen terme et généralement une explication sur un des phénomènes locaux. On ne comprend pas tout, mais cela donne l'impression d'être intelligent.
En voilà trois ou quatre ci-dessous.
Pour les recevoir (message de 7-10ko), on envoie un message to weathergram_short@cruisersat.net, avec le texte "SUBSCRIBE Boat name" sans guillemets.

The South Pacific Convergence Zone explained
Now that many yachts are about to travel west from Tahiti to Tonga, and are thus about to sail thru or around the SPCZ, as if it guards the eastern entrance to the South Pacific like a protective dragon, some have asked what is it, why is it there, how does it differ from the ITCZ and what makes it tick. Weather is a mix of pattern and chaos, and meteorological teaches concentrate on the pattern. In tropical meteorology the first idea given is the Hadley cell. Because the sun is most directly overhead at the equator, that's where the warmest seas are, and this causes rising air. Once the rising air reaches high enough it spreads outwards and sideways to the north or south, where it sinks at dries out. The sinking air reaches the surface again around 30N or 30S (subtropical ridge) and then recirculates back to the equator as surface winds know as trade winds. The trade winds from each hemisphere converge together in a zone, and this convergence narrows the zone of rising air into a feature called the Intertropical Convergence Zone or ITCZ But in the Southern Hemisphere, the Andes of South America cause a split in the trade winds. They block a HIGH near 30S around 90 to 110W (it is quasi stationary, just like the High between California and Hawaii, and has a gyre that is collecting a rubbish heap just as badly (see Henderson island). blogs.fco.gov.uk/lauraclarke/2018/04/10/henderson-island-plastic-pollution-i n-paradise/ There are easterly winds on the north side of this "Andes" High: they are dry due to continental outflow from off South America. And there are migratory Highs that travel east along the subtropical ridge from Australia to east of NZ, with a zone of south to southeast winds on their northern side. The easterly winds travel well to west of the dateline around 10 to 15S, and the South/SE winds come and go according to the migratory high and are usually found around 15 to 25S. The convergence zone between these easterly and Southeasterly winds is called the South pacific Convergence Zone, or SPCZ. It is typically located from the Solomon Islands southeastwards to the Southern Cooks, but sometimes may have large gaps or be very quiet. It is affected by many things: the PDO which takes many years to switch, by the El Nino/La Nina which last a year or so, and by the strong annual cycle which repeats each year, and by the MJO which comes for a week or so every six weeks or so. Read more about it at en.wikipedia.org/wiki/South_Pacific_convergence_zone

Red sky at night, sailors delight; red sky in the morning, sailors warning
 After my notes on lightning last week, one of my readers has asked me to try and have ago at explaining the old folk-lore expression "Red sky at night, sailors delight; red sky in the morning, sailors warning" (land dwellers change the sailor into a shepherd). Reading weather signs is one thing that humans have been doing ever since we have been human enough to detect pattern amongst the chaos. And this sign is a good one, often proving true. In Matthew 16.2, Jesus told the Pharisees that "When evening comes and the sky is red, you say the weather will be fair, and in the morning when the sky is red and overcast, you say today will be stormy-you know how to interpret the appearance of the sky, but not the signs of the times! "A Red sky at night is a delight." Explanation, it's as easy as 1,2,3: 1. The sun sets in the west, and, for many parts of the planet, 2 that's where tomorrow's weather is usually coming from. Usually the winds at cloud level above us are from northwest/west/or southwest, so weather features and clouds ride this steering field from west to east. If the western horizon is cloud-free, then there is a good chance that our tomorrow's weather may be cloud-free. 3. Sunlight shining slant-wise loses all but its red colouring. Air near the ground gets dirty: it accumulates particles of dust and salt and smoke that are small enough to float around (aerosols). Sun shining on these materials undergoes Mie scattering, whereby red light is mostly scattered forwards and blue light is mostly scattered laterally or backwards. As the sun sets, it beams through an increasingly longer path than at noon. It takes on a golden hue, or, if there is a lot of dust about, then the clouds above are side-lit in a rosy red. Colours can be seen to go through the spectrum and some colours are enhanced according to the aerosols of the day. Once the sun is set, red light from the aerosol layer mixes with the blue light scattered above it to form purple light. "A Red Sky in the morning is a warning" Explanation: Sun rises over the eastern horizon. If that's where the clear skies are, incoming light is rendered reddish by passing through cloud-free but dirty air. If there are clouds overhead, this usually means we are on the back end of a departing area of dry weather, and thus, as is the nature with weather, on the forward side of the next lot of rain approaching from the west. Dawn horizontally side-lights the overhead clouds with a reddish glow, enhancing our senses of pending gloom.

THUNDERSTORMS
WINDY.COM have added a new layer, called THUNDERSTORMS, which comes from the global ICON model from DWD as a combination of rain and lightning density. More resolution than the CAPE parameter.
See community.windy.com/topic/6269/windy-is-the-first-weather-service-in-the-wor ld-to-globally-forecast-thunderstorms.
Technical details are at confluence.ecmwf.int//display/FCST/45r1+new+parameters%3A+lightning+flash+de nsity
Lightning is static electricity. Any object that spikes upwards from an otherwise flat horizon is asking to be struck by lightning. When updrafts in a shower cloud exceed around 25 km/hr (13 knots), air and water molecules are rubbed together, creating static electricity (making lots of free electrons). In the updraft, rain, ice, and snow are jumbled, and the lighter particles are lifted to the cloud top, whilst freed electron tend to attach to the heavier particles at base of the cloud. Like charges repel, and so this reservoir of electrons in the base of the cloud causes the free electrons in the ground beneath the cloud to go elsewhere. This forms an area of positive charge in the ground under the cloud. When the cloud gets blown around, this positively-charged area follows the cloud like a shadow. When the reservoir of charge in the cloud gets large enough, an explosive discharge occurs.

First, groups of electrons leap outwards and downwards in all directions towards the positive area near the ground. This happens in jumps, called leaders, each taking around a nanosecond, and at the end of each jump the pause may break the leader into two. When one of these leaders is within about 100 metres of the ground a group of positively-charged particles streams up from the earth to meet the leader. These are called streamers and are best made from a spike or tower (or tree). When the streamer meets the leader, the stored energy (100 million volts) is released suddenly enough so that the air in the stream is ionized. It turns to plasma, 20,000C+, or 10 times hotter than the surface of the sun for a fraction of a second. This ionization works its way up the streamer (so the light starts at the bottom and ends at the top), and this is called the return stroke. On its way up all the places where the streamer has divided get lit, hence the spread-out pattern of some lightning photos. After the first strike, the main pathway between the ground and the cloud reservoir of charge can go through another ionization episode (or strike). This is the flicker seen in a lightning strike.

A normal lightning path or bolt may be many kilometres long and is only as thick as a thumb. The speed of sound is 1 kilometre in about 3 seconds or 1 mile in about 5 second, and you can use the time difference between the flash and the bang to measure the distance between you and the strike. Lightning may be seen when it is 60 kilometres away, but thunder is only audible for maybe 10 to 20 kilometres. In the 19th century, thunder was thought to be caused by the implosive collapse of the partial vacuum of the lightning bolt. Nowadays, the consensus is that it is the shock wave of the sudden thermal expansion as the bolt forms plasma (air breaking the sound barrier). The rolling sound after the initial clap is sound arriving later from parts of the lightning bolt that are further away. Over land, rolling thunder also echoes off surrounding terrain.

80% of lightning discharges occur from top to bottom of a cloud, or between two nearby clouds (C2C or cloud to cloud). The ones that reach the ground are called C2G or cloud to ground.

I handle weather forecasts for around 400 yachts each year, and in the past year two have lost their navigation instruments due to lightning damage (both in tropical waters). This seems to be a higher rate than the normal quote of 1 in a 1000 per year.

A yacht mast "spikes out from a flat surface" and thus tends to act like a lightning rod. Normally it is connected to the keel reasonably well by its rigging. Since lightning is static electricity it tends to flow on the outside of a surface, and so a metal mast and rigging should take most of the charge to the keel and surrounding seawater. Some extrapolate from that to say there is a cone of protection underneath the mast, but some say that's a myth (see www.lightningsafety.com/nlsi_pls/cone-of-protection-myth.html). Installing a proper lightning rod and connecting this via copper plate to the keel may just divert some of the lightning charge so that it blows thru the copper plate, holing the keel. Opinions differ on this point.

When you see a lightning squall coming, the standard advice is to get the sails down and the engine going on auto helm, and standby in the companion way or below deck and cross your fingers. I'd like to add that you protect your electronic equipment: Disconnect your antenna, and gather any mobile navigation equipment such as GPS, satellite phone, tablet, laptop, etc., and toss them into the galley oven (or a microwave if you have one) as an approximation to a Faraday cage.

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 blog.metservice.com/Southern-Annular-Mode

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  www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/new.aao_index_ensm.html

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.

ENSO (El Nino and La Nina) - May 2018 
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.

For the past year the SOI has been mostly around plus 0.5 to plus 1.0, consistent with a weak but rather persistent La Nina. The subtropical ridge line has been further from the equator than normal, and trade winds were stronger than normal, but are now close to normal.

However, over the past month, the SOI has settled into a near zero state, and has a NEUTRAL status.

Neutral conditions may be as seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly