El Niño explained

There’s been a lot in the news in recent months about El Niño. See, for example, the latest monthly outlook, and an ABC news story.

Here’s a ‘back-to-basics’ description of El Niño.

To start, the chart below shows a climatological average of air pressure at Mean Sea Level over the central and South Pacific. The chart is like isobars on an ordinary weather map, but instead of looking at a single time such as midday today or midnight last night, we’re averaging over a long duration – I’ve chosen 20 years.

Sea level pressure averaged over 18 October 1995 to 18 October 2015 (contours in Pa rather than hPa, so 102200 Pa = 1022 hPa). The spacing between the isobars is 1 hPa.

Image courtesy U.S National Oceanic & Atmospheric Administration Earth System Research Laboratory


In the chart we see a belt of high pressure along latitude 30 to 35S, an area of high pressure off the west coast of South America and relatively low pressure off Papua New Guinea. This is the average, or ‘background’ state of the atmosphere, if you like. This configuration of isobars drives a regime of persistent winds from the southeast direction in the Southern Hemisphere ­– these low-level winds are called the Trade Winds, named because of their historical importance to trading between continents.

In the discussion below I refer to easterly Trade Winds. The Trade Winds in the southern hemisphere are southeasterly winds and, in the northern hemisphere, northeasterly winds. But for the purpose of this explanation, we’re more interested in the east–west components, so I’ll leave out the north–south part in this discussion.

I’m also going to mention the thermocline. This is a layer in the ocean with a rapid temperature drop as you go down through it. The depth of the thermocline varies, but typically it’s a few hundred metres down below the ocean surface. (The thermocline is a bit like an oceanic version of a temperature inversion in the atmosphere.)


The ‘Neutral phase’

Under ‘normal’ conditions, neither one extreme nor the other, we have a situation referred to as a Neutral phase.

Low-level easterly Trade Winds (blowing from east to west) push the warm surface water away from South America towards tropical South-East Asia.

Trade Winds blowing from east to west at low-levels, and a return flow at upper levels.
Trade Winds blowing from east to west at low-levels, and a return flow at upper levels.


Around tropical South-East Asia the top layer of ocean water is deeper because the warm water has piled up there (see the cross-section below).

Conversely, on the other side of the Pacific, off the west coast of South America the depth of the warm equatorial top layer of the ocean has reduced, and the thermocline is nearer the ocean surface. This means that cold, nutrient-rich water has welled up into the upper parts of the ocean.

Schematic cross-section of the Neutral phase. For the purpose of illustration, the vertical scale is much exaggerated.
Schematic cross-section of the Neutral phase. For the purpose of illustration, the vertical scale is much exaggerated.

Around tropical South-East Asia, warm surface water evaporates into the air, and the air gets very moist. The moisture-laden air drives increased rainfall around Indonesia and neighbouring lands such as the Top End of Australia.

In contrast, over the eastern tropical Pacific and off the coast of Peru, dry air accompanied with cloud dissipation sinks, resulting in much drier weather there.


El Niño phase

The Trade Winds lose most of their strength in an El Niño phase, and may even reverse into a westerly wind (west to east) direction.

As a result, warm surface water flows from tropical South-East Asia along an equatorial corridor towards the coast of South America, where it increases the depth of the warm top layer of the ocean.

Schematic cross-section of the El Niño phase
Schematic cross-section of the El Niño phase

Off the coast of South America, the thermocline sinks. The cold deep water no longer wells up into the surface layer of the ocean. The source of nutrients is cut off, which has a subsequent effect on fishing stocks off South America. Nearby, over the eastern tropical Pacific, warm surface water evaporates. This moistens the air, driving more rainfall than usual there.

Conversely, on the other side of the Pacific, Indonesia and neighbouring countries are drier than usual during an El Niño.

Why is El Niño given this name? If you know some Spanish, you’ll be aware that it means “the boy”.

Early sailors off the coast of Peru noticed that the sea usually got warmer from about Christmas time, and they named this annual warming “the Christ child” or, in Spanish, El Niño. But sometimes the warming was more marked, coinciding with a poor local fishing season – and the term El Niño began to be used only for these events.

During periods of cooling, the opposite to El Niño occurred. In latter years, the term “the girl” or La Niña has been used.

The oscillation we get from one extreme to the other is simply called the El Niño-Southern Oscillation, or ENSO for short. The ENSO is a powerful interaction between the oceanic and atmospheric systems, that exerts a strong influence on the weather.

Typically, an El Niño (or La Niña) event will develop during the southern winter, intensify during the southern spring, and peak around Christmas. Usually, things weaken off during summer, and often (but not always) there is a return to neutral conditions during the southern autumn. In other words, the typical life cycle of an event lasts around 12 months. However, in some cases, the events will continue through 24 months, with the second phase of the event being weaker.

Earlier in this post we looked at the distribution of air pressure at Mean Sea Level. One way that ENSO is monitored is by measuring air pressure at each side of the Pacific, computing the difference and recording how this difference oscillates. The resulting index is called the Southern Oscillation Index (SOI), and it is most often based on the difference between the air pressures at Tahiti and at Darwin.

SOI_IncludingOct2015 (3)
A graph of monthly SOI values between January 2008 and October 2015. Large and sustained negative values denote El Nino conditions.

Another way of monitoring the characteristics and strength of ENSO is by the so-called ‘NINO’ indices. These average sea surface temperatures (actually, their departure from normal) in various boxes along the equator. This is the ‘centre of action’ with respect to abnormal sea temperatures caused by ENSO.



NINO3 4_IncludingOct2015 (3)
The NINO3.4 index is a measure of how abnormally cold/warm the seas are along the equator, over the central Pacific Ocean (5S to 5N, 170W to 120W). Values from January 1982 to October 2015 are shown. Large positive values denote El Nino conditions.

Looking at the NINO 3.4 index, we can see that the 2015 event ranks as the strongest El Nino seen since 1997/98. The El Nino of 1982/83 was also very strong.

Because of its scale, ENSO is felt in many parts of the world, including New Zealand.

New Zealand Impacts

Although each El Nino affects New Zealand differently, the general characteristics are:
• More southerly winds in winter (June-August), leading to a cooler than usual winter
• A stormy spring (September – November), with more frequent southwest winds
• For ALL regions of New Zealand, a large increase in the chances of a COLD spring
• Winds typically turn westerly in summer (December – February), with an increased risk of dry conditions for eastern regions of both Islands
• Cooler than usual seas around the New Zealand coastline across all seasons



You can monitor the state of ENSO by following the regular MetService commentary in the monthly outlook or by visiting the Australian Bureau of Meteorology website.

This blog was co-authored by Chris Webster and Georgina Griffiths.

Isolated showers

The Auckland forecast for Monday 12 October included mention of the following:

‘… isolated showers …‘ and ‘ … southwesterly winds …’.

From time to time you’ll hear the word ‘isolated’ in weather forecasts, so let’s see what it means with reference to observational data, some of which is available on www.metservice.com.


Upper air data

The meteorological balloon sounding from Whenuapai at midday on 12 October showed moist air in the low levels of the atmosphere and dry air above. There was a temperature inversion at 1800 metres (6000 ft) above the ground, and the air was very dry above that level. In this case we would expect no cloud above a height of 1800 m.

By the way, the surface temperature was 16°C when the balloon was released, cooling to 1°C before rising steeply to 3°C as the balloon climbed through the inversion. The temperature continued to cool above that, until it hit the tropopause.


Radar loop from 10:07 am – 12:15 pm Monday 12 October 2015

Animation of Auckland radar imagery

The time-stamp on the radar images is in UTC, and NZ is now in Daylight saving time so:

  • the loop starts at 2107 UTC = 10:07am on Monday 12 October, and
  • finishes at 2315 UTC = 12:15pm just after noon.

The radar loop comes from our weather radar sited on Mount Tamahanga, north of Auckland. The loop of imagery shows:

  • the nature of the showers, with small yellow dots moving across the Auckland area in the southwesterly flow,
  • most of the showers are light and brief in duration, but a few of the dots have blue centres indicating light to moderate intensity,
  • there is a faint yellow line flickering to the south of the radar (bottom middle of image) – this is an anomalous echo that is not generated by showers.


Satellite imagery loop from 10:10 am – 12:10 pm Monday 12 October 2015

Now let’s have a look at the corresponding images from Himawari-8 in visible wavelengths of light. Here all cloud shows as white or a shade of grey, and what we see is sunlight reflected off clouds and the Earth’s surface.

Satellite loop
Visible satellite animation, courtesy of JMA

The imagery shows:

  • lenticular lee-waves being generated at low-levels by the Kaimai ranges,
  • a lot of cloud over and east of Northland – this is stratocumulus cloud,
  • large areas of clear sky upwind (over Tasman Sea), so most of the cloud over Auckland and Waikato is forming as the low-level moist air moves onto the land,
  • over Auckland the clouds are generally a little lumpier in texture, which is consistent with the showers appearing on the radar. These clouds are cumulus, and there is evidence of some ‘streeting’ as some of the cumulus line up into bands parallel with the wind flow (see my previous post for another example of cloud streets).

The quality of satellite imagery is quite amazing, especially when you consider that the satellite sensors are rotating around the Earth 36,000 km above the equator!


Showers are common around Auckland, and showers can occur even in situations where you might expect a nearby anticyclone to suppress them.

Earlier in this post I mentioned the temperature inversion. The detailed nature of the inversion has a strong influence on the showers. Today the inversion has not been strong enough to stop the showers forming.

Typically, as a ridge of high pressure advances onto the region, the inversion gets strong enough to suppress any showers. This is usually the reasoning behind the forecast mentioning showers becoming isolated in southwesterly flows, and/or a clearance of the showers.


New satellite data

The Japan Meteorological Agency recently launched a new geostationary weather satellite called Himawari-8. “Himawari” means sunflower, and the name has been given to a new series of satellites that we can look forward to in coming years. “Geostationary” means the satellite rotates “in sync” with the Earth, always above the same point over the equator. We, in New Zealand, are now starting to receive early data from this satellite.

One of the important advances of the new sensors on board the satellite will be the capability to produce images for operational meteorologists every 10 minutes, compared with the previous standard hourly imagery. This frequency is close to that of our companion weather radar images.

Here is an early example of the InfraRed (IR) satellite loop on the morning of Wednesday 2nd September.

InfraRed satellite animation, courtesy of JMA.
InfraRed satellite animation, courtesy of JMA.


The time-stamp in the top-left is in “UTC”, currently 12 hours behind NZ time, so the loop:

  • starts at 2100 UTC = 9:00am on Wednesday 2 September, and
  • finishes at 2340 UTC = 11:40am on the same morning.

The InfraRed imagery in this animation is similar to familiar satellite images on TV, metservice.com and newspapers. It shows high cold clouds as red or white, and warm low clouds, the Earth’s surface and the sea as dark grey or black (see the description of the InfraRed “enhancement” in this earlier post).

The horizontal resolution of the images is 2 km, so it’s possible to discern more detail than from previous geostationary satellite sensors. Here’s an explanation of what we can see:

  • The cloud is moving from the northwest towards the southeast. The speckled texture of the cloud over western parts of North Island and the Tasman Sea is typical of convective showery conditions. These clouds are puffy cumulus clouds, towering cumulus clouds or very big cumulonimbus clouds (with cold tops).
  • Skies are clear over parts of North Island where the colouring is black, and the shading of these areas darkens as the morning progresses and the land heats up.
  • There is evidence of banding in the grey clouds over the east of North Island; this is particularly noticeable over Gisborne. These are lenticular clouds in the lee of the mountain ranges.
  • There are different bands aligned with the flow over Northland and Bay of Plenty – these are cloud streets, similar to those structures discussed in an earlier post.

Now let’s have a look at the corresponding satellite images in Visible wavelengths of light. These are at 0.5 km resolution.

Satellite animation in Visible light, same period as previous, courtesy of JMA.

Here all cloud shows as white or a shade of grey, and what we see is sunlight reflected off clouds and the Earth’s surface. There is much more detail than in the IR images.

  • The coastlines become clearer later in the morning as the sun rises higher in the sky and the land brightens up.
  • The cloud streets and lee-wave clouds are very clear. If you have eagle eyes, you can just make out a single cloud street generated by White Island (off Bay of Plenty) towards the end of the sequence. Over Gisborne, towards the end of the animation, the two processes of lee-waves and cloud streets seem to be super-imposing.
  • The lumpy shower clouds tend to have wispy edges, and these are generated by high cirrus clouds (composed of ice crystals) that spread out from the cumulonimbus clouds.
  • There are also some interesting cloud shapes around Mount Taranaki as the flow is disrupted as it flows over and around the mountain.

The hectoPascal and Air Pressure

The hectoPascal and Air Pressure

In meteorology, the quantity pressure is an important driver of physical processes in the atmosphere. Pressure is the force applied over a unit of area, so it can be increased by having more force acting over a smaller area. Pressure is measured in Pascals, named after the French mathematician and physicist Blaise Pascal (who also devised the famous “Pascal’s triangle”). The abbreviation for Pascal is Pa.

An example of where air has high pressure is the inside of an inflated tyre. For typical values of air pressure in a tyre, it’s best to measure pressure in units of kilopascals (kPa, thousands of Pascals). But for the range of air pressures that occur naturally in the atmosphere we normally use hectoPascals (hPa, hundreds of Pascals) rather than kPa. At the Earth’s surface the air pressure of the atmosphere is usually within the range 980 to 1030 hPa.

The hectoPascal is the modern replacement unit for the millibar:

one hPa = one millibar = one thousandth of a “bar”.

The millibar was introduced by the British meteorologist Sir Napier Shaw in the early 1900s. Shaw also created an important thermodynamic diagram that is still used extensively today in modern meteorological services worldwide.

In 1930 Shaw sent his Christmas greetings to the then director of the NZ Meteorological Service, Dr Edward Kidson. A copy of the card is reproduced below.

Card from Sir Napier Shaw to Dr Edward Kidson, NZ Meteorological Service, 1931
Card from Sir Napier Shaw to Dr Edward Kidson, NZ Meteorological Service, 1930

The text on the reverse side read as follows:

Dear Kidson

              This is in illustration of a new view (so far as I am concerned) of the relation between velocity and pressure gradient namely that it is the velocity that controls the gradient. If it increases, the gradient will increase so motion round an area means the creation of high pressure over the area. The picture is for the northern hemisphere.

What is fine weather for you? Anticyclonic as here, or cyclonic? I have just sent off the last of M.S. for Vol iv : shall have time to write soon.

                                         Sir Napier Shaw

(The reference to “Vol iv” relates to a manual of meteorology that Shaw had been working on for some time. “Aeolian” relates to Aeolus, the Greek god of wind)


I can most readily illustrate the importance of air pressure to weather by referring to today’s weather map:

Mean Sea Level analysis, 6am 3 July (18UTC 2 July) 2014
Mean Sea Level analysis, 6am 3 July (18UTC 2 July) 2014

The thin solid lines are isobars, which means lines connecting places with equal air pressure (note the “bar” in “isobar”). These lines give a good idea of what the broad-scale winds are.

Let me explain by describing what happens when you release the valve on a tyre. The air ejects from the region of much higher pressure within the tyre to the region of lower air pressure outside the tyre. In the Earth’s atmosphere, when the air accelerates from high to low pressure, the rotation of the Earth deflects the out-flowing air towards the left in our southern hemisphere (the opposite deflection occurs in the northern hemisphere). This is why the air around an anticyclone (High pressure area) rotates anticlockwise in our hemisphere. A previous blog-post describes the anticyclonic outflow of anticyclones, together with the descent that is characteristic of Highs.

At the time of the chart above, NZ was mostly affected by a southwesterly air-stream (i.e. coming from the southwest, and therefore cold). In this chart the spacing between the isobars is 4 hPa.

If we remove the fronts from the chart and increase the number of isobars by drawing them at 1 hPa spacing, we get an increased level of detail about the pressure field, as below:

Chart at same time as previous, but fronts removed and intermediate isobars added (in grey)
Chart at same time as previous, but fronts removed and intermediate isobars added (in grey)

Note, for example, the extra detail about the structure of the High over New South Wales and Victoria.

Thinking about the tyre again, when you initially release the valve the difference in air pressure between each side of the valve is large – the air rushes out with a hiss. If you were to continue deflating the tyre, the difference in air pressure would reduce and the air-flow would become much gentler. Similarly, in the atmosphere, when the isobars are close together the air rushes out faster and gets deflected more – thus the winds are stronger when the isobars are close together.

Less obvious is how air pressure changes as you go up in the atmosphere. For example, if you were flying to Australia the ambient air pressure as you cruised across the Tasman Sea would be about a quarter of what you’d experience at the sea surface. You can read more about these vertical variations at this blog post: http://blog.metservice.com/2011/11/up-and-away/.


Maths, Physics and Meteorology

When recruiting and training people to become meteorologists, MetService requires that trainees hold a university science degree in maths and physics. Why maths and physics?

First and foremost, meteorology is a science, and we need people in our National Forecast Centre who are capable of applying the scientific thinking that they’ve developed at university to understand the state of the atmosphere.

Let’s look at specific examples of why maths and physics are important to meteorology.


The physics of fog

Forecasting for aviation generally has a low profile as far as the general public is concerned, but it is critical for airlines and pilots for safe and efficient air travel. There are many meteorological phenomena that impact flying, e.g. thunderstorms, turbulence, air frame icing and cross winds on landing. One of the most critical is fog because of the severe reduction in visibility it can cause.

It is a fundamental property of moist air that (invisible) water vapour changes into (visible) water when the vapour content reaches a threshold value that depends critically on temperature.

The existence of this threshold makes fog forecasting one of the hardest of all weather phenomena to predict. Fog formation can sit on a knife edge. For example, under suitable conditions fog can form when the air temperature cools to, say, 7 deg C, but if it were to cool to only 8 deg C there’d be no fog. Almost a binary situation, and the difference in how far you can see could drop from 50 km to 500 metres with the extra degree of cooling.

When our aeronautical meteorologists forecast fog at New Zealand airports they apply maths and physics, including the principles of thermodynamics, radiative processes, graphs (of cooling rates), and statistics (of airport climates). A basic example of a cooling rate graph is below.

Actual (red) and predicted (orange) temperature, Christchurch International Airport, for overnight hours of 20 and 21 March 2013 (blue line is dew point temperature). The horizontal lines indicate fog formation thresholds. UTC time is 13 hours earlier than NZ Daylight Time, so 20 05 is 6pm local time, 20 15 is 4am and so forth.

Here, the forecaster needs to assess what the overnight temperature will in fact cool to, to form this type of fog (there are other types) based on the available guidance, and decide if there will be enough water vapour present to form the fog.


Probability and Statistics

The basic concepts of probability and statistics are introduced in maths at school. Meteorology is an inexact science and, every day in the National Forecast Centre, scientists are forming hypotheses about future weather outcomes around NZ. The concept of hypothesis testing is usually introduced in Year 13 mathematics with statistics, and further developed at university.

Currently most public weather forecasts in New Zealand state a single expected outcome rather than the range of viable outcomes. This is great in terms of delivering a short forecast but, unfortunately, misses out valuable information about the other viable outcomes that depend on the state of the atmosphere at the time. Some of our forecast users address this by obtaining information expressed as statistical probabilities, for example, as a probability distribution. In the future, I think it’s likely that public weather forecasts will adapt to an increasing need for probabilistic information.



There are many beautiful shapes in geometry, e.g. the cardioid. Another is the spiral. Spirals turn up in nature in shells and plants. A special type of spiral, the logarithmic spiral, is relevant to meteorology. It is used in a technique to determine the intensity of a tropical cyclone, based on satellite imagery near the eye of the cyclone.



The meteorologist assesses cloud-top temperatures in segments of the spiral, providing an objective gauge about the cyclone’s intensity.

Remember trigonometry from school? This branch of maths is important in many sciences. An example related to meteorology is spherical trigonometry, in which any location on Earth can be specified in terms of latitude, longitude and height above the Earth’s surface. Our weather radars make extensive use of the principles of trigonometry to determine the location and movement of precipitation targets.


Rates of change (differentiation)

The concept of rate of change occurs all the time in meteorology. It occurs on many scales, e.g. in less than an hour with the fall of temperature during the passage of a cold front, or over a few days with the decrease of air pressure in an intensifying  depression.

In an earlier blog post (Year 12 maths) I gave an example of the importance of maths to meteorology. In mathematics, a rate of change is called a differential or derivative (ask someone doing Year 12 maths!). To describe the motion of air using the techniques of fluid mechanics (a branch of physics and applied maths), many equations containing differentials crop up. It turns out that, because of chaos theory, there are no easy ways to solve these equations. But all is not lost…


Numerical analysis

A hundred years ago, a brilliant Englishman called Lewis Richardson found a way to approximate solutions to the equations of air motion. Nowadays, with the benefit of a new branch of mathematics called numerical analysis and increasingly powerful computers, we can solve the equations accurately up to several days ahead.  Just how accurately depends on the state of the atmosphere at the time, but sometimes it’s possible to predict the development and path of a depression before it has formed.

The computers that are required are extremely powerful. The best ones, called super-computers, are found in overseas meteorological centres. Data transfer between continents isn’t a big issue these days, so data from these centres is routinely sent to us so that NZ forecasts benefit from the fruits of Richardson’s pioneering work.


Sir Isaac Newton’s laws of physics

I referred to fluid mechanics earlier. Some of the most important equations in fluid mechanics are tailored versions of Newton’s second law of motion (Force = mass  x acceleration). Physics again!

Here’s one of the equations of motion based on Newton’s work: eqn_of_motion

The symbols are: eqn_of_motion_terms

The rate of change of the eastward wind is the acceleration, and the terms on the right-hand side of the equation are the forces acting on the air mass.


International standards of training

The World Meteorological Organisation (WMO) prescribes the educational standards for meteorologists (WMO 1083). A good grounding in both mathematics and physics is essential to achieve this internationally recognised standard.

At MetService we train university graduates to become meteorologists through our graduate training programme (trainee meteorologist career page), which is run in conjunction with the School of Geography, Environment and Earth Sciences at Victoria University of Wellington.

If you or someone you know of is thinking of becoming a meteorologist, and wondering what they need to study, tell them to keep up their maths and physics at school, and continue with them through to university.

Convergence lines

In the English language we have many words in common usage that have related but more specific meanings in a scientific and mathematical context. The key word of this blog post, “convergence”, is a good example of this.

We sometimes hear of people’s views on some matter initially disagreeing and then, at a later time, coming together or “converging”. In fluid dynamics we’re often interested in regions where different air flows come together. We call this type of flow convergence, and say that the air is converging. It’s an important concept in meteorology because convergence often has a big effect on weather conditions, driving where the cloud is (or isn’t). Sometimes, in suitable conditions, it leads to heavy showers and thunderstorms. Let’s take a closer look at convergence.

In its purest form, air that’s coming together would look like the graphic below, where the arrows show how the air is moving. Everywhere in this graphic the air flow is converging.

A field of pure convergence.


For example, in the centre where the arrows are all pointing inwards, the air flow is clearly coming together. More subtly, if you’ve got a good spatial imagination you might be able to spot another kind of convergence … run your eyes along one of the radial lines coming in towards the centre. A longer arrow indicates stronger wind and, even when there’s no change in direction, at each point along the radial the flow is convergent because there’s more air flowing in than flowing out. That is, convergence can be caused by speed effects as well as directional effects. Send me a comment if you’d like me to explain this further :-)

The opposite of convergence is divergence. In air flows aloft (in the upper atmosphere), meteorologists are often interested in upper divergence because it draws air upwards from underneath. Such regions usually have a big influence on the kinds of weather systems that bring stormy weather to NZ. But that’s another story; let’s get back to convergence.

Sea-breezes form on clear summer days when the land heats up more than the adjacent sea. The result is a cooling wind that flows at low-levels from the sea onto the land.

Around Northland and Auckland, we can have these sea-breezes occurring along the entire coastline, both west and east coasts. Because the land mass separating these coastal regions is quite narrow, convergence occurs when sea-breezes coming from opposite directions meet, as in the picture below.

Converging see-breezes on a clear summer day over Northland and Auckland.

Where the air comes together, it has to go somewhere. In the atmosphere, when air converges at low-levels, it gets pushed upwards. Provided that there’s enough moisture in the air, the uplift will generate cloud and potentially precipitation too. If the air is unstable (see Predictability & popcorn) the convergence can generate convection in the form of cumuliform cloud and showers.

As for previous figure, with the effect of rising air included.

Here’s a good example from 31 January 2012. The animation of satellite imagery below shows the distribution of cloud from mid-morning till early evening that day. As the sea-breezes developed and came together, a very prominent line of cloud formed along the middle of Northland and the Auckland isthmus.

MTSAT-2 visible satellite images, each an hour apart, from 9am to 6pm NZST on Tues 31 January 2012. Images courtesy Japan Meteorological Agency.


There are other ways that air can come together in the atmosphere.  The earlier post on Cloud Structures over NZ on 26 July showed the effect of air that was channeled by the topography. Air was flowing westwards through Cook Strait and the Manawatu Gorge area. As the low-level flow spread out downstream, a line of convergence was created where the air came together over the sea west of Manawatu. A similar effect generated a line of convergence over Bay of Plenty.

Convergence also occurs on bigger scales. The chart below shows typical low-level wind flows over the southwest Pacific during summer. Where the Trade winds come together there’s a zone of convergence called the Inter Tropical Convergence Zone (ITCZ).  A secondary zone of convergence runs from near Vanuatu east-southeastwards through the Cook Islands, and is called the South Pacific Convergence Zone (SPCZ).

Typical low-level flows over the southwest Pacific during summer.

Both of these zones are associated with increased cloudiness and showers of varying intensity, depending on the state of activity of the zone at any time. The positions of the zones shift around too in response to various influences. As we approach the summer months its pertinent to point out that Tropical Cyclones form over the southwest Pacific Ocean from disturbances associated with tropical convergence zones.

The inward flow in depressions causes convergence and lifting of air over a wide area above depressions. As stated above, rising air favours cloud and precipitation. This is why depressions (Lows) are generally associated with cloudy and often rainy or showery weather.

At the beginning of this post I referred to how convergence drives where cloud is and isn’t. A nice example of the latter comes with the sea-breeze. Over the sea where the air starts flowing towards land, the low-level air is divergent. This drives the air aloft downwards towards the water, and creates generally cloud free conditions. A useful tip if you’re an aircraft pilot wanting to avoid cloud on a cross-country flight in the vicinity of a coastline.

I hope you can see why meteorologists are interested in where and when convergence occurs!