Disconnect in springtime ozone

How come there's so much here when so little there?

But before that …

The question of seasons discussed in my last posts is even more interesting than I thought. New Zealand mainstream media just announced their first of spring last Thursday (1 September). Another colleague (thanks Ben) pointed me to an excellent discussion here, and noted that the definition I used dates back to ancient Roman Times with Phiny the Elder - around 50 AD. Last week’s humble pie is tasting better all the time …

But that’s not what this is about.

In those last posts, you may have been puzzled about those higher ozone levels in spring. What about the Antarctic ozone hole I hear you ask? I’m glad you did.

Starting in the late 1980s there was media frenzy each spring as the recurring Antarctic ozone hole developed. Although less newsworthy nowadays, it continues to develop every spring and the UV there still gets very intense. As you can see here, it has already started to develop again this spring and I expect - like previous years - it will continue to grow until late September or early October.

Perversely, although New Zealand is one of the closest countries to Antarctica, our own ozone levels are healthy at that time of year. In fact, nowhere on Earth has more ozone then than around southern New Zealand. You can see what I mean in the NASA image below which shows the average distribution of ozone for latitudes pole-ward of about 30S for the month of October. Different colours refer to different ozone amounts, with the lowest ozone amounts shown in purple (less than 100 DU), and the highest shown in orange areas (around 450 DU). The landmasses are shaded in light grey. It’s a bit hard to see New Zealand, so I’ve drawn a red circle around it.

NASA’s mean October ozone map from satellites looking down on the South Pole. Average from 1979 to the present.

In October, the highest ozone amounts present are just pole-ward of New Zealand, and over New Zealand they are comfortably over 400 DU, much higher than the global average of around 300 DU.

The ozone hole isn’t directly centered directly over the south Pole either. It tends to be displaced away from New Zealand’s quadrant, and the only populated region that’s directly affected in spring is the southern tip of South America, at latitudes well pole-ward of the southern tip of New Zealand.

In case you were wondering about parts of the world not visible in the image above, I show below the corresponding ozone map centred over the Arctic in October (their autumn). As you can see, there’s much less ozone at all latitudes pole-ward of 30N, as expected from last week’s post. The largest October-mean ozone value there is about 340 DU.

NASA’s mean October ozone map from satellites looking down on the North Pole. Average from 1979 to the present.

The equatorial band from 30S to 30N is still missing from these maps, but as I’ve discussed before, there’s always less ozone in that latitude range.

Now to the point at issue …

How come our ozone is so high, when right next door - so to speak - it’s so low?

The conditions that set up the ozone hole over Antarctica also help to keep ozone levels high over New Zealand. The ozone hole is restricted to the Antarctic region because (with occasional exceptions) that’s the only place where the air gets cold enough - during the Antarctic Polar night - for certain ice crystals to form in the stratosphere. It’s reactions on the surfaces of those ice crystals that cause the problem. If air from higher latitudes intruded, that would warm the stratosphere enough to stop those ice crystals forming. That’s essentially why there’s (usually) no ozone hole over the Arctic. The wind patterns there are more complicated because of the more complex underlying topography, with tongues of land and ocean extending to high latitudes. The topography and land-sea temperature differences set up waves in wind flows that bring lower-latitude air in and out of the Arctic Polar night, tending to increase temperatures. The topography is much more symmetrical in the southern hemisphere. Deflections in winds caused by the Earth’s rotation (the so-called ‘Coriolis Effect’) leads to an isolated region of strong circumpolar westerly winds (it’s called the Polar Vortex). They act as a barrier for warmer, ozone-rich air to intrude from lower latitudes, so ozone-rich stacks up at latitudes between 40 and 60 S. Just the right place to help keep kiwis safer from UV damage.

But, as I reminded you last week, protection is still required. Especially later in the spring and in the summer and autumn …

The springtime Antarctic ozone hole was long and persistent in the last couple of years. As a result, the peak UVI there was large. You can keep an eye on its development this year here.

A little extra for the science buffs …

For completeness, I’ve added below the corresponding ozone maps for April (spring in the south and autumn in the north). Notice how much more ozone there is in April over the northern hemisphere. And by comparing the right image below with the first one above, you can see that the northern spring has much more ozone than the southern spring.

NASA’s mean ozone maps for April from satellites looking down on the South Pole (at left) and the North Pole (at right). Average from 1979 to the present.

It’s the same in summer. Ozone amounts are lower in the south.

NASA’s mean summer ozone maps from satellites looking down on the South Pole (at left) in January and the North Pole (at right) in July. Average from 1979 to the present.

And the same in winter too. Differences are even larger …

NASA’s mean winter ozone maps from satellites looking down on the South Pole (at left) in January and the North Pole (at right) in July. Average from 1979 to the present.

You can check out all months (and much more) at the NASA Ozone Watch page