Author:
David Sapiane BSc
These topics were covered on The Rag from Fiji (8173 at 1900 UTC) in 2012 The MJO, Tradewind Surge, Lows on the back of a High, Clouds in the tropics, ENSO,...
1. THE MJO CYCLE in the SOUTHWEST PACIFIC - 10 July 2012 as presented on The Rag The MJO or Madden- Julien Oscillation is a wave oscillation travelling eastward around the globe on an average of 40 days. It was discovered by Madden and Julian in 1971 who called it the 40 to 50 day oscillation.
Cycles have varied from 30 to 60 days. The MJO directly affects the weather for cruising yachts. When the MJO is overhead the yacht will expect light to heavy rain and squally conditions. The downflow winds can be gale force and may come from an inconvenient direction making a safe anchorage a lee shore. In addition to direct effects there is a major indirect effect for global weather. During heavy showers a tremendous amount of heat is released into the upper atmosphere and this affects what we call planetary Rossby Waves. These become the Long Wave pattern that affects global weather. If the MJO coincides with the Australian Monsoon it will increase the monsoon effects. If there is an existing quasi-stationary front or trough the MJO will exacerbate the feature. And perhaps most dangerous of all is the MJO’s interaction with Tropical Cyclones. The MJO begins its life in the Indian Ocean . It starts as a small area of precipitation south of the equator near 60 East. It then grows and moves eastward at about 8-12 knots. So, it becomes an eastward propagation of deep convection and rainfall from the Indian Ocean to the western Pacific.
The convective pulse and precipitation from a cruising yachts perspective affect the area between roughly 10 North to approximately 30 South when between the longitudes 150E and about 140W. As the pulse moves eastward from 140W it becomes more or less nondescript as it moves over cooler waters of the eastern Pacific, but later reappears over the tropical Atlantic and Indian Ocean as the cycle ends. The convective pulse is detected by usually two sources. The first is Satellite Infrared and Visual imagery. The second is also a satellite but one that uses radiometers and detects what we call OLR or Outgoing Longwave Radiation. As OLR decreases it’s an indication of increased cloud build-up. ( MJO prognostic charts on the internet typically use OLR with two colors, Blue and Red, with blue indicating the convective area.
Other prognostic models are used as well, one such is called the velocity potential at 200 hPa. These forecasts use colors as well; green being divergent flow at upper levels which corresponds to enhanced convection, while red is used for suppressed convection). There are many atmospheric cycles on earth, and they all affect our day to day weather. Many climatologists look dumb struck when appraised of them. Cycles of one sort can affect cycles of another sort. An example being strong MJO activity is often observed during weak La Nina years or during ENSO-neutral years, while weak or absent MJO activity is typically associated with strong El Nino episodes. This discussion revolves around the Southwest Pacific, but the MJO in actual fact affects global weather. A good example is a strong MJO pulse creates what is known as the ’Pineapple Express’ which brings several days of significant rain and flooding to Western North America. Yachties should be aware of the MJO. BoM offers forecasting available on the internet .
2.TRADE WIND SURGE - 10July2012 as presented on The Rag To start the topic I want you to visualize that air is a fluid and behaves in a similar way water does. Assume you have a beaker of water. If you pour it out straight down to a flat surface it splashes in all directions at once. An atmospheric analogy is a common phenomenon called microbursts. Nearly everyone has heard of what happens when a big Cb suddenly dumps a tremendous burst of air straight down. The wind goes in all directions at once and the strength of the downflow can cause, and has caused aircraft to crash. Okay, put this aside for a minute and look at another experiment. If you’re waist deep in water and flatten your hand vertically just below the surface of the water and move it rapidly from say right to left you’ll quickly see you’ve created an eddy. Your moving hand has created a mini whirlpool. At this point we now we have the basics of a common occurance in the Tropics, called the Trade wind surge.
These surges are often seen when a strong Anticyclone forms to our south. Typically whenever a big High, usually over 1030 central pressure moves in, we cruisers in the Islands not only get a blast of reinforced trades but the wind is accompanied by cloud and often rain and squalls. Where does the cloud and rain come from? Lets assume you are in Fiji and the prevailing wind is a light to moderate E or ESE and at the same time a strong High moves east perhaps at 35 to 40 degrees south. At this point the isobars tend to compress creating a tighter gradient as this mountain of Air, which is another way of looking at an Anticyclone, moves to your south. As these stronger winds reach your latitude they are of course faster than the winds directly to your north. If we go back to our hand in the water analogy these winds create an eddy. The wind to the south creates what is known as cyclonic shear and as it tries to turn right it collides with the mild wind already there. At this point there is no where for the clashing air to go but up and that’s exactly what happens. As the air ascends it reaches the dew point where cloud forms, and if the air rises high enough rain develops. Here is where the first analogy comes in.
The one where we pour the beaker of water down to a flat surface. When the cloud builds air rises, but at the same time an equal amount of air returns to the surface. That is why we have lulls and gusts when cloud is about. The downflow wind strikes the surface of the ocean and spreads out. But the only direction that means anything is the direction toward the existing trade wind. These flows collide and again, the air has to go somewhere, so all that’s left is up. The result of this is more cloud and rain. So now we can see how this becomes self sustaining. So that, in a nutshell, is the nature of Trade Wind Surges.
3 LOW FORMATION ON THE BACK END OF ANTICYLONES -10July 2012 as presented on The Rag . Many cruisers have noted that as a large and intense Anticyclone or High as we commonly refer to them moves eastward a small Depression often forms on the Highs left shoulder. Much of this depends on the shape of the back end of the High. This phenomenon is more apt to occur if the High has a notable ridge on the left side, so that the air flow makes a relatively abrupt turn from moving westward then suddenly turning southward. Some key ideas must be noted.
First we must accept that as an airstream makes a turn counterclockwise in the southern hemisphere it speeds up. This is because of coriolis force which is a function of the earth’s rotation. In other words wind speeds up going around a ridge and slows down when going around a trough. This is why when you estimate wind speed using geostrophic spacing you have to adjust downward or upward depending on whether flow is around a ridge or trough. If the isobars are straight no adjustment is needed.
If we were able to mark air parcels with a dye of some sort this effect would be clearly noticeable. Once the air parcels make the turn and then resume a relatively straight line the parcels must slow down and in this mode bump into the parcel in front of it. Much the same as a line of cars on a motorway; if the first few cars abruptly slow the ones behind bunch up very quickly, sometimes not quickly enough. Substitute cars for air parcels and what happens is also a bunch up but as the air has nowhere to go, it simply goes up. When the air goes up far enough where the temperature lowers to the dew point, cloud forms and precipitation may follow. Now, this is not quite enough to form a Depression but it’s the start of one.
Lets shift our thoughts to what follows the departing High. Usually it’s the next High in the progression of weather circling the globe. The departing High has wind flow going southward, but the new High has wind flow going northward. The air parcels of each High are vastly different in density. The southbound air is warm and moist, while the northbound air is cool with less moisture. Air parcels of different densities do not mix, they clash. So try to visualise this: Air going south clashes with air going north and spin is initiated. The spin is clockwise, the same direction that a depression spins. Now we have rising air creating cloud and rain and in addition we have cyclonic spin and so we’ve just created a depression.
This in a nutshell is how we find Low formation on the back end of Anticyclones. It is not necessarily a requirement to have a new Anticyclone following to have a low to form as we’ve just described. Often any large intense High can have a Low form somewhere on its equatorward side. This process starts with a fairly round isobaric shape of the intense High. The next thing we notice is somewhere in the middle of the Highs perimeter the isobars dip in a polar direction then swing back equatorward; this is termed a ‘polar dip’. The cause of this is generally a split in the upper levels, initiating a slight pressure drop at the surface. If we were to draw a line perpendicular to the dip to represent the dips axis the pressure on either side of this line would be higher. The only side that matters here is the upwind side or eastern side. The air flow accelerates as it initially turns left, then slows as it turns right and heads equatorward. This causes the air to pile up and rise, creating a small surface low.
Now to sustain either of these small depressions we must have a way to vent the rising air and a way to encourage it to continue. Therefore we need an upper level short wave sandwiched in between two upper level ridges which maintain our Highs, or in our second case the wave would be just equatorward of the surface high. If such a short wave is present then the Depression develops. If there is no short wave it doesn’t, and all that happens is a brief period of cloud and rain seemingly coming from nowhere and often not picked up on synoptic charts. If we’re unlucky enough to be under all this we get some miserable seemingly unexplained weather, and as the next High moves in it all goes away.
The lesson here is to be wary of the back end of any Large Anticyclone and in fact be wary of any intense Anticyclone particularly if over 1030hPa. The main point is to be aware of your synoptic map and never assume a nice High means nice weather.
4. CLOUD IN THE TROPICS - 10 July 2012 as presented on The Rag Today we will talk about clouds. What you see in the tropics and what they mean for weather you now have and what to expect for the future. For those of us under the influence of a large Anticyclone to our south we’ll generally see nice puffy Cumulus. They will be fairly white in colour and will be no taller than they are wide. They will appear to have the same altitude and the tops will all be the same level. This is because of the inversion layer common to an Anticyclone.
An analogy is a pot top on a stove. However if the tops climb high, are not even, and overall the cloud is taller than wide that is a message that the atmosphere is unstable and we should expect showers and possibly squalls in future. If we are experiencing a trade wind surge the cloud will certainly break thru the inversion layer and rain and windy conditions will prevail. The cloud bottoms will change colour from white to dark, almost black at times. If the top of the cloud leans that means there is strong wind in the middle layers of the atmosphere and is an indication of weather change. When cloud is made from updrafts there will always be downdrafts. A leaning cloud indicates the downdraft will not tend to snuff out the parent cloud but will encourage a new cloud to form as the downdraft collides with trade wind flow with the resultant updraft. Where does the nice fluffy cumulus come from in the open ocean? The basic mechanism is simple. The sun heats the surface of the water which in turn radiates water vapour. Air mixed with water vapour is less dense than air without, so it rises. When it reaches the dewpoint cloud forms and if the air rises far enough rain occurs. In time the falling rain scavenges cloud droplets and the cloud starts to disappear; its called ‘raining out’.
Should you see Altocumulus or Altostratus and you’re in the sub-tropics normally it means nothing. But in the tropics they could be the residue of rained out Cbs or it could indicate bad weather is approaching especially during cyclone season. If Alto stratus or even stratus turns dark and appears to be a grey thick cloud continuous rain may develop. This sort of cloud is found on the eastern side of upper level troughs. If your weather map indicates a cold front extending to the tropics signs of approach will be initially cirrus which gradually thickens and gives way to middle cloud, altostratus and altocumulus. At times cumulus will break through the inversion layer and grow very tall, but skinny.
This vertical development is called castellanus and indicates unstable air, and possible thunder activity later in the day. Cirrus cloud can be just residue from Cbs or it can indicate the jet stream. Lumpy cirrus is found on the poleward side of the jet stream and smooth cirrus on the equatorward side. If jet stream cirrus is moving from NW to SE it indicates you are under the east side of an upper level trough and the possibility of a depression forming is increased.
If its cyclone season and cirrus is heading your way, better take notice. If cloud above you is moving counter to the trade winds it is usually the result of an upper level trough or the cloud is under a jet max. A jet max can cause cloudy and rainy conditions not forecast on your weather maps. Extensive cloud cover with seemingly no explanation from your maps can also be the result of a shear line forming between two upper level ridges. It can also be the result of cyclonically flowing wind at 700 or 850 hpa. These sorts of conditions are seldom forecasted.
When you are on watch at night ever wonder why unexpected nasty squalls interfere with reading your book? Almost always at O Dark Thirty. It’s called radiative cooling. What happens is that the tops of your nice big fluffy Cumulus get cold, very cold because of the absence of the suns heating; while the surface of the water is quite warm. When we have cold on top and warm on the bottom air rises very quickly to create a nasty squall. The lesson is if there are lots of big cumulus around before dark you may want to think about putting in a reef. One more thing to be aware of. If you have a quasi-stationary trough over you in the tropics with low cloud it may be quite mellow with little wind or rain. But if a cold front passes you by to your south it tends to re-activate the trough. So suddenly, mellow changes to not so nice.
Lesson, pay attention to your weather charts. If you see a cloud band or Cb that seems to be moving with the trades; its not. The cloud actually moves with the mean wind between 5,000 to 10,000 feet or 850 to 700hpa. This is almost always to the left of the trade winds. So if you are at anchor and the cloud seems to be coming at you it will in fact move to your right as you are looking at it. So the cloud to worry about is the one to your left. And should the cloud actually move over you a wind shift will be guaranteed, because the gust from the cloud will be moving in the clouds direction.
Often when sailing in the trades the nice small puffy cumulus arrange themselves in ‘streets’. Rows and rows of streets. This occurs only in stable conditions, meaning there is a cap or inversion which prevents the cloud from further buildup. The streets are usually aligned with surface wind and when sailing under the cloud portion you’ll experience a drop in wind and when under the clear area, an increase in wind. The drop means some of the air is going up and robbing the tradewind a bit, while the gust is the return from the robbery. If your cruising takes you near the Solomons and across to Vanuatu, you’ll notice an increase in cloudy conditions. This entire area is considered the ‘hot pool’ of the South Pacific. The water temperature is very, very warm which causes evaporation. The less dense moist air rises, sort of an ‘upwelling’ of the atmosphere. As the air rises the surface pressure drops a bit and more air rushes in to replace it. This creates a large area of convergence which results in cloud, possible showers and squally conditions.
Lastly we have the SPCZ. The south pacific convergence zone. It’s not really a zone but the result of air from the semi-permanent High situated near South America, the flow generally from the East clashing with southerly or southeasterly flow from passing Anticyclones on their journey from the Tasman to the eastern pacific. The air streams are of different density and when they clash the colder air stream pushes the warmer stream up to create cloud and squalls. There is no set location for this unpleasant event.
5. ENSO AND WEATHER IN THE SOUTH PACIFIC - 15July 2012 as presented on The Rag The term ENSO stands for El Nino Southern Oscillation. It’s a catchy term that encompasses, La Nina, El Nino and Neutral states. Years ago researchers pinged onto the fact that when the barometric pressure over Easter Island became weaker than in Darwin for many months coincidently the sardine fishery in western South America failed. An index was created using barometric pressure, called the SOI or southern oscillation index. Today the pressure measurements are from Darwin and Tahiti.
In addition more emphasis is placed on sea surface temperature,and a new index was formed called the ONI. The prediction models today use both indexes and basically when the SOI is +8 or greater La Nina is possible; -8 or lower, El Nino. For ONI, El Nino events are defined as 5 consecutive months at or above 0.5C and -0.5C for La Nina. In the El Nino phase easterly trade winds weaken and SST (sea surface temperatures) in the eastern tropical Pacific become several degrees warmer than normal as a systematic shift of heat and moisture. Australia experiences higher air pressure and drought while NZ experiences more NE wind flow, higher temperatures and wetter conditions in the North and east of the country, but drought in the south. From a season standpoint during El Nino NZ has stronger and more frequent winds from the West (thus more dry in the east and more wet in the west).
La Nina brings more NE winds and that means more rain in the NE of the North Island. Looking at the Tropical Island groups El Nino generally brings weaker trade winds (notice the word ‘generally’). There is generally more cloud over the central equatorial regions. Fiji and Vanuatu experience drier and hotter conditions than normal from Dec to Feb (summer) followed by drier and cooler weather from Jun to Aug. A great deal of this is due to the SPCZ moving NE. Samoa and Tuvalu have an increased likelihood of drought. The eastern Islands have a greater chance for Tropical Cyclones. And in general the eastern Islands experience more rainfall activity.
During La Nina events most of the Islands experience greater rainfall ,troughs and generally cloudy conditions. This is a result of the SW movement of the SPCZ. There are more Tropical Cyclones west of the dateline. Going from La Nina to Neutral and possibly into El Nino we would expect to see more Tropical Cyclones east of the dateline: Samoa, Tonga, Cooks, French Polynesia. The impact for Fiji in this case would be a decrease in rainfall to below average. This can escalate into a mid-scale drought to a harder scale. The longer El Nino lasts the worse for the country. This general effect probably starts first in Australia then New Cal followed later by Fii and Tonga.
Fiji Met says there is often a time lag between changes in Fiji because Fiji is in a transition zone regarding the SOI. For example it can take 4-6 months for drought conditions to occur. An example is the 1997-1998 El Nino. Two months after the SOI dropped below 0, rainfall had just begun to decrease from normal levels. So what causes this? Why does water near Peru suddenly move west and why does warm water in the western Pacific decide to move east? The fact is not one researcher knows for sure. It has to do with what we call equatorial oceanic waves setting up a pulse in the deep oceans, but a tongue in cheek little story is to picture Neptune having a dip and some exercise in the his Pacific pool So, hands on Peru, legs and feet stretched out toward Australia and he starts his kicking exercise. Water moves west and starts to pile up over the western equatorial region. In fact it piles up 600mm in a strong La Nina episode. Thats right, water is 2 meters higher in the western pacific than the eastern. The displaced water near Peru gets replaced by upwelling. This upwelling water is very cold and cools the air in contact with it. Cold air is dense and creates a robust Anticylone and strong trade winds quickly develop.
In addition to bringing cold water to the surface nutrients follow and with the nutrients come the sardines and anchovies. At some point in time Neptune gets tired and stops kicking. The big pile of warm water near the Solomons rushes downhill, eastward, accompanied by very warm air which snuffs the big High near Peru. So the strong trades cease as does the upwelling and cold water. The anchovies take off and the fishermen fret. Now, some of our current and less gifted climatologists think this is proof of climate change. The problem with this dimwittedness is the fact these cycles have been occurring for millenniums. The next tactic is the implication that the cycles are more severe than in the past. I have a graph from 1960 to 2011 and I assure you there is an absolute balance of La Nina intensities to El Nino intensities while the degree of intensities has changed very little. What we are learning is there are more and more cycles being discovered. I won’t get into them but they include the MJO, PDO, AAO and the list goes on. The atmosphere is very complicated indeed. Lastly keep in mind there are at least 7 forecast sites for ENSO. I tend to look at POAMA which is from BoM in Australia. Regardless of which model you prefer if you are trying to rationalize where to spend a southern summer you can’t do it on ENSO alone. Take the forecast with a grain of salt. All that is certain, one cycle follows the other, but the unknown is the time frame and the modifiers.
FORECASTING IN THE SOUTH PACIFIC
Rules of thumb and general discussion
David Sapiane
FORECASTING SURFACE HIGHS AND LOWS
LOWS
1. Intensifying Lows tend to slow down and deflect more southward toward
colder air.
2. The speed of a Low seems to approximate the speed of the winds in the
warm sector and usually move slower than the associated cold front.
3. Look for surface Lows to develop where a strong jet crosses a frontal
boundary between different air masses.
4. A surge of cold air moving in behind a cold front can turbo-charge the Low.
5. A surge of warm moist air ahead of a cold front can intensify the Low.
6. Look for surface Lows to develop under cyclonically curved diverging
upper level contours and weaken under anticyclonically curved contours.
7. A temperature at 500mb of -30C is often associated with strong
deepening of a surface Low.
8. Lows moving toward colder air deepen, and weaken if moving toward
warmer air.
9. Lows move at about 50% of the 500mb wind speed and 70% of the 700mb
wind speed.
HIGHS
10.Highs build when they move under converging contours aloft and weaken
if they move under diverging upper contours
11.Highs strengthen when under anti-cyclonically curved contours aloft and
weaken when under cyclonically curved contours aloft.
12.Highs will avoid the heat of Australia during summer.
13.Traveling highs tend toward the equator and track with the 500mb winds.
14.Highs move at about 60-70% of the 700mb wind speed.
15.Blocking Highs usually intensify if they retrograde, and tend to weaken
when they progress eastward.
16.If there is strong low level (through 500mb) warm advection moving into
the western side of an upper ridge the surface High will build and possibly
retrograde.
17.There is a symbiotic effect that when an upper trough intensifies, it
intensifies the downstream ridge, and as a consequence, the surface High
also intensifies.
TASMAN SEA
18.Lows that form in the north Tasman , in winter, can be deep and slow
moving
19.Winter Highs moving eastward have a tendency to move equatorward
20.As a High in mid ocean collapses a new one tends to form somewhat to
the southwest.
21.Highs forming south of Tasmania are preceded by cold southerlies and
often move slowly.
22.Highs west of NZ can extend a ridge eastward. Then a new High center
forms east of NZ as the old center loses intensity. The cycle can repeat.
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23.An easterly dip is a trough that forms on the equatorward side of an
anticyclone near the east coast of Australia. It’s occasionally a precursor to
more vigorous cyclogenesis because of the warm/cold advection couplet
enhanced by the East Australian current.
ISOBAR AND CONTOUR CURVATURE GUIDELINES
1. Where the isobars or contours are cyclonically curved, expect cloudiness
and precipitation
2. Where surface isobars are cyclonically curved, and are supported by
upper level cyclonic curvature, the chances of precipitation are enhanced.
3. Anticyclonically curved isobars, with straight upper level height contours
are usually dry.
4. If the isobars are anticyclonically curved, but the upper level height
contours are cyclonically curved, cloudiness and some drizzle is possible.
5. If the upper level height contours and the surface isobars are anticyclonically
curved , expect fair or fine weather.
6. Typically a Northerly wind that blows straight or curves cyclonically, will
have clouds and some precipitation associated with it.
7. Typically Southerly quadrant winds produce clouds and showers only if
they are curved cyclonically; straight southerly quadrant winds produce
intermittent clouds and precipitation.
SOME ASPECTS OF THE 700MB CHART
1. If 700mb winds parallel a cold front the front is called an Ana front, is
active, and most of the precipitation is at and behind the front.
2. IF 700mb winds blow more perpendicular to the cold front it’s called a Kata
front, and most of the precipitation, including squall lines, occurs well
ahead of the front.
3. If 700mb winds cross a warm front and curve cyclonically or flow straight ,
precipitation is enhanced
4. If 700mb winds cross a warm front and curve anti-cyclonically precipitation
is minimal if at all.
SOME ASPECTS OF THE 500MB CHART
1. If the maximum winds in the long wave are oriented NW to SE expect
short wave production
2. Cold advection from the surface through 500mb entering the west side of
troughs will deepen the trough.
3. If the upstream ridge is too sharply curved for the wind speeds
approaching it, the wind will overshoot, possibly filling the trough to the
east of it.
4. Troughs move eastward more rapidly when the strongest winds round the
northern periphery or apex.
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5. Strong winds on the west side of a trough cause it to dig toward the
equator
6. When the strongest winds are on the east side of the trough the surface
low tends to elongate in the direction of the upper winds. The surface low
will usually last for another 24-36 hours.
7. When short waves get into phase the trough deepens considerably
8. Long wave troughs are hard to identify but look for lobe like extensions
toward the equator
9. Usually the surface storm track follows and is poleward of the 5640
contour
10.In general when the equator side of the trough axis points upwind surface
lows start to develop; as the axis rotates downwind the low deepens; and
once the axis swings downwind the surface low usually begins to fill and
weaken.
11.Positive or westward tilting troughs exist when the upper level trough is
west of the surface trough. Cyclogenesis is favored as a jet max and
subsequent CVA and temperature advection encourage development.
12.Negative or eastward tilted troughs exist when the upper trough is east of
the surface trough, and in general surface systems become weaker.
13.In the southern ocean zonal flow can last a fair while. It provides a ‘mobile’
pattern in contrast to a blocking pattern.
14.If an upstream ridge greatly intensifies and assumes an overlapping
orientation over the downstream trough, a cut-off low will form in the
trough
15.Upper level cut-off Lows typically last 3-4 days with surface conditions of
wind and rain prevailing. When the capping ridge moves on due to a
strong jet, the cut off dissipates.
16.Closed lows aloft move toward the greatest divergence aloft.
17.The short wave can be identified early by a kink or constriction of contours
within the overall large scale wave. This is most visible on 500 and 700mb
charts.
18. Numerical models have historically moved upper troughs too fast and not
deepened them enough but are steadily improving.
THICKNESS CHARTS
1. Thickness lines curve anti-cyclonically around warm fronts
2. Thickness lines curve cyclonically around cold fronts
3. Lows often develop on the right side of diverging thickness lines
4. Highs often develop to the left of diverging thickness lines
5. Lows often develop to the left of converging thickness lines
6. Highs often develop to the right of converging thickness lines
7. Thickness gradients are strongest behind cold fronts and the tighter the
gradient the stronger the front.
8. Thickness gradients usually parallel fronts
9. Expect upper mass convergence when winds are blowing in about the
same direction but slowing down, and when upper contours converge.
10.Expect upper mass divergence when winds are blowing in about the same
direction but speeding up, and when upper contours diverge.
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SOME ASPECTS OF THE 200MB CHART
1. Warm pools at 200mb are found above cold pools in lower level troughs
2. Cold pools are found above lower level warm pools in ridges
3. Diverging contours (diffluence) indicates divergence with cyclogenesis
likely in the area.
4. Diffluence, as stated, favors precipitation but divergence must also occur
(a net outflow of air).
5. Strong troughs at 200mb have temperatures from -40 to -45C (not valid for
cutoffs)
6. Strong ridges at 200mb have temperatures of -65C or colder.
7. Temperatures in the middle -50C’s indicate weaker systems
8. If the lower level troughs and ridges don’t reflect their existence at 200mb
the system is weak.
THE JET STREAM
1. In a series of lows of a cyclone family, each low is associated with a jet
max.
2. All lows have an associated jet max of varying magnitude.
3. All short waves are associated with a jet max of varying magnitude.
4. The jet parallels the warm sector of a surface low.
5. As a surface low is formed the jet moves equatorward pushing cold air into
the west side of the low.
6. When the surface low occludes the jet moves around the low center and
crosses the front at the occlusion
7. The sub tropical jet is found roughly above the 500mb -11C isotherm.
8. A mid level trough may deepen when the jet max is west of its axis.
9. A mid level trough may fill when the jet max is east of its axis.
10.A mid level ridge may intensify when the jet max is west of the ridge.
11.A mid level ridge may weaken when the jet max is east of the ridge.
BLOCKING HIGHS AND RIDGES
1. Strong warm air advection through at least 500mb into the west side of
ridges will build ridges.
2. Often there is cold advection at 200mb associated with lower level warm
advection
3. Blocks strengthen when they retrograde and weaken when they progress
eastward.
4. The most common cause of a block in the NZ area is a jet splitting into a
subtropical and high latitude branch
5. In general, if a long wave encompasses about 45 degrees of longitude
than a blocking high occupies the crest and/or a blocking low occupies the
adjacent trough. If greater than 45 degrees it may retrograde.
6. Blocking surface highs sometimes appear to have “left and right”
shoulders and lows may form on them. The left shoulder low tends to
behave like a blocking low and stall. It eventually will unravel into a trough.
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SOUTHERN HEMISPHERE CYCLONE CLASSIFICATION from Sinclair
Class Location of Jet Location of Low Upper flow
U upstream from upper trough Beneath jet exit Diffluent
E downstream from upper trough Beneath jet entrance Confluent
D downstream from upper trough Beneath jet exit varies
T equatorward of upper trough axis Directly beneath the Sharp
upper trough Trough.
Sinclair’s study categorizes southern hemisphere cyclones on the basis of
precursor upper jet and trough configurations. A feature common to all the
classes is the presence of a 300mb wind max exceeding 70 knots and most were
100 knots. The importance of jet max circulations inducing a cyclonic circulation
extending throughout the troposphere to the earths surface is discussed.
Class U. The dominant jet is upstream from both the surface low and the upper
trough. The max occurs in equatorward moving (digging) or zonal flow and is
upstream from the upper trough. Average 24hr pressure falls were 15mb and
some met the “bomb” criteria. Lows forming in diffluent flow can favor meridional
elongation of both the low and the cold front.
Class E. The low forms beneath the confluent entrance region of a jet max,
which lies downstream from the upper trough axis. Flow patterns are similar to
the “instant occlusion”. Pressure falls similar to class U. Strongest frontal strength
is on the warm front poleward and east of the low. At maturity the low processes
a strong warm front and weak cold front aligned almost perpendicular to the
warm front. These lows elongate zonally and have characteristics of the Shapiro-
Keyser model.
Class D. The low forms in the exit region of the jet max which also lies
downstream from the upper trough. The dominant feature is a northwesterly jet
which is sometimes on the eastern flank of an upper trough of considerable
meridional amplitude. The mature cyclone has the smallest and tightest
circulation of the four classes.
Class T. This category has the upper jet approximately 400 miles equatorward of
the low which forms almost directly under the upper trough. Most of the observed
cases formed off the east coast of Australia. It too, has a strong bent back front at
maturity.
While the above is a useful way of classifying cyclones one thing is certain, no
two cyclones or pattern of cyclogenesis is exactly the same. The significance of
the jet stream and jet maxes in southern hemisphere cyclogenesis cannot be
underestimated. No significant surface cyclone occurs without an accompanying
jet max. The ageostrophic flows about a jet max traveling through a long wave
are replicated downward where the “twisting” of height contours and temperature
contours are translated as a “short wave” which is most evident at 500 and
700mb. Thus mass removal occurs at upper levels as divergence, with pressure
falls at the surface. Coupled with this is air flow crossing isotherms creating warm
and cold advection, all of which is the perfect recipe for cyclogenesis.
The following provides further exploration of cyclogenesis in summary form.
1. Frontal Cyclogenesis. This is the closest to the Norwegian model with the
jet max lying to the west or cold side of an existing front. Satellite imagery
provides a view of the flow distortion as cyclonic-anticyclonic curvature of
the cloud band… the reverse S shape, or “baroclinic leaf”, with a sharp
edge on the cold side. The cold front may take the form as an Ana or Kata
structure. The final cyclone could resemble anything from the classical to
the Shapiro-Keyser model, or something in between.
2. Warm Influx Cyclogenesis. This involves a moist low level airmass of
tropical origin. A surface isobaric chart would show a broad High with a
polar dip in the isobars on the equatorial side. When a sub-tropical jet max
crosses the dip the system activates and “winds up”.
3. Cold Air Cyclogenesis. This is development within a cold air mass with
areas of shallow convection; usually found behind an established cold
front. Another jet max moving through the upper trough distorts the low
level flow and shapes the convective area into a vorticity center, or closed
isobar Low, with accompanying cold front.
4. Instant Occlusion. This essentially is the same as cold air cyclogenesis
except in this case the vorticity center, caused by a jet max, catches up to
the leading cold front, which buckles. The resulting new Low consists of
the original cold front, and as this front is twisted, a warm front, and trailing
from the warm front is the southern extension of the original cold front.
This is called an instant occlusion only because the vorticity center
“catches” up with the existing cold front.
The northern hemisphere conception of a continuous front extending around the
hemisphere separating polar and maritime air is classic. This “polar front” was a
cold front when it moved south and a “warm front” when it moved north;
otherwise it was quasi-stationary. In the South Pacific we seldom find a long
continuous “polar” frontal system extending east and west. More commonly we
find cold fronts extending northwards from Lows. Warm fronts are much less
common mainly because the air between cold fronts warms by descending, so
that while surface air ahead of a cold front is warm because its trajectory has
brought it from lower latitudes, the air is warm IN DEPTH. In addition, the warm
fronts are less abrupt in the southern ocean and satellite imagery does not show
pulses of warm advection as clearly as pulses of cold mainly because higher and
middle cloud obscures the surface warm front.
One further difference from our cousins in the north is the occlusion process. As
the cold front catches up with the warm front, and as each front slopes upwards
in opposite directions, warm air naturally is present in higher levels. This warm
wedge is held aloft and depending on temperature differentials on the surface
terms like ‘cold’ or ‘warm’ occlusion are used. However in the south pacific these
terms are seldom used mainly due to the difficulty in determining which type is in
existence. Thus it may be better to view occlusions as a process in which the
Low or vortex center becomes progressively separated from the warm sector of
the low, leaving a tongue of intermediate temperature air extending from the low
center to the warm sector. This tongue is the ‘occluded’ front.
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As a side note sometimes the surface chart shows an elongated occlusion front.
A satellite image would show a band of cloud, but with respect to surface
temperatures there is no air mass differential on either side; thus an ‘occlusion’ is
drawn or a convergence zone symbol in the tropics.
A still further departure from ‘classical’ is the Shapiro-Keyser ‘bent back’ warm
front type of cyclone which frequently occurs in the south pacific. It is also known
as the ‘T-bone’ because of this descriptive appearance. In one version and in
simple terms the cyclone or low starts out with a definitive warm and cold front.
The cold front slides along the warm front to give the appearance of a ‘T-bone’. A
second cold front forms just behind the first with the air between the two being
‘relatively’ warm, but it’s not the same air in front of the warm front. This
‘relatively’ warm air, which has minimal cloud, spirals into the vortex of the low
center and becomes completely surrounded by cold air and the clouds in the
comma head, thus creating an ‘eye’ of sometimes cloud free, and certainly warm
air. The pressure gradient near the eye can be enormous resulting in storm force
winds in some, but not all, systems. Another variation has the original cold front
‘fracturing’ from the warm front allowing warm air to rush through the gap and into
the vortex where it is wrapped up and surrounded by cloud and cold air.
A further variation is a cyclone which is also distinctive for having secluded air of
warm sector origin at its center coming from a branching warm conveyor belt
early in the seclusion process. However, there is no evidence of a traditional cold
conveyor belt, or an ‘occlusion’ process having occurred. The cold air, coming
from higher latitudes, also splits, with part wrapping around the vortex and warm
seclusion, and part paralleling the eastward moving branch of the warm conveyor
belt.
Another variation on cyclone development is a process quite common over the
southern ocean, occurring in relatively cold air and not interacting with warm
moist air from lower latitudes. This starts with a jet max west of an upper trough
axis. To the west of the max a cirrus band is evident and a cumulus cloud field to
the east. As the jet max migrates around the curved part of the upper trough
divergence downstream of the trough axis creates a comma shaped surface
depression. This development has no requirement for a pre-existing front and no
classical warm front is present.
COLD FRONTS
Processes regarding cold fronts are varied and intriguing as well. As mentioned
earlier there are two types, Ana fronts and Kata fronts.
An Ana front is quite active and has the warm conveyor belt paralleling the cold
front with rearward sloping ascent as descending dry air comes in beneath it. To
identify the Ana front look for:
1. The surface cold front is sharp with a significant drop in temperature and
wind shift.
2. There is little rain ahead of the front. The jet runs along the back of the
cloud band and the surface front is drawn on the eastern side of the visible
band.
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The Kata front is also known as a split front where one front is the surface cold
front and the other cold front is an upper level feature which forms as the
descending dry air, instead of undercutting the warm conveyor belt, rides over it
creating underneath it, and well in advance of the surface front, persistent rain
and squalls. There is only shallow precipitation at the surface front itself. This
type of front is common when very warm and moist tropical air is dragged south
and into a low forming in mid-latitudes like the Tasman Sea. Ideally the upper
front could be drawn using open triangles, but usually a surface trough in front of
the cold front is drawn. Sometimes this upper front pushes on poleward and the
wedge of warm air in front gets pushed along too and winds up spiraling into the
low center as a ‘Trowal’ or warm seclusion; however this seclusion is above
colder air and is unlike the Shapiro mode.
DOWNSTREAM DEVELOPMENT
Another phenomena is the concept of Downstream Development. The energy
released during strong cyclonic development takes two routes. One into the
storm itself, and the other into the jet stream where it’s rapidly transported
downstream. Arriving in the next system downstream, these huge amounts of
kinetic energy can have a significant domino effect by intensifying the next
cyclone or amplifying the next anticyclone. Explosive cyclogenesis upstream
releases enough energy to drive the jet poleward creating a blocking High and
further equatorward movement can rapidly turn cyclonically creating an upper cut
off Low. This is evident on satellite imagery as the developing upper cirrus has a
‘fountain’ shape with the fountain pointing westward. Explosive events in the
southern Indian Ocean can affect the south pacific.
Presented were ‘rules of thumb’ chosen to be useful guides. Many exceptions
occur which of course is the nature of such rules. The discussion is a general
overview and each topic merits deeper study.
Compiled by David Sapiane on sv CHAMELEON. chameleon4844@yahoo.com
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