IRI’s Climate Briefing for March

The striking picture shown here came in yesterday’s update from NASA’s Earth Observatory. It shows La Niña’s fingerprint in southern Africa.

How does La Niña—a cooling of the eastern Pacific Ocean—affect plant growth on the other side of the globe in Africa? La Niña occurs when strong trade winds blow across the Pacific Ocean. The winds push sun-warmed surface water west towards Australia. Cool water rises to replace the surface water in the east. As a result, the Pacific Ocean is cooler than normal in the east off South America and warmer in the west off Australia. Warm, moist air rises over the pool of warm water in the western Pacific, where it generates abundant rain in eastern Australia and Indonesia. The rising air travels east in the upper atmosphere, drops as cool, dry air over the eastern Pacific, and then blows west as the strong trade winds that drive La Niña.

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This circulation pattern is so large that it influences circulation the world over. Echoing the circulation over the Pacific, an identical pattern drives wind and rain in the Indian Ocean. Warm, moist air rises in the west, while cool, dry air sinks in the east. The effects of the Indian Ocean circulation pattern are evident in these images. The warm, moist air dropped heavy rain over southern Africa, where plants responded with gusto.

Reason I start with this is that for the last few months the IRI team has been forecasting a strong-to-moderate continuation of the phenomenon. This image, and one of Australia, also featured in NASA’s story, helps us to understand the real world effects of teleconnections. In Africa at least, the effects aren’t limited to greener landscapes. The increased chance of rains worried the World Health Organization enough to issue an alert in January for higher-than-normal number of malaria outbreaks in the region. You can read more about climate and malaria here.

Anyway, the briefing is starting. Let’s see what Tony and the gang have for us this month….

Brad starts it off. Wants to talk a bit about how difficult it is to communicate science topics, as they relate to climate.

Ecuador has been flooding, but it is it La Niña related? How about the southwest U.S., where there has been above average rainfall as well?

We don’t usually associate these with La Niña. And yet, for example, in a recent situation report issued by the United Nations Office for the Coordination of Humanitarian Affairs, the first line says

Due the La Nina phenomenon, over the past weeks, floods in Ecuador have increased, affecting 11 provinces or 50% of the territory.

Yowsa. This isn’t a very accurate statement. El Niño is usually responsible for such flooding. So he did a little back-of-the-envelope calculation. Brad looked at the past 50 years to see when it has been wet in this area and how this coincided with ENSO/La Niña. He found nine La Niña events that showed dryness in this region, and five that showed above average rains. So the current situation isn’t really that far from the norm in terms of the last half century.

Another story talks about looming drought in East Africa. So he looked at the climate outlook forum results from the region, and saw that the forecast showed predominantly near normal-conditions for the next three months (MAM). To take it a little further, we know the forecast skill is quite high for this region for OND. But for MAM, it’s not so good. So it seems a bit of a stretch to be making headlines like this.

Judy was at the COF where the forecast was given. She says the scientists didn’t say anything about drought in the region. But there was a large number of press present. In their minds, the yellow on the map meant very dry, very hot. Green means wet, lush. And that’s the message they took home. So it’s a problem of graphical interpretation, it’s a problem for us who provide the climate information, she says.

Now Tony steps up to the mic. His first slide says it all. “Moderate/Strong La Niña begins to weaken”.

La Niña is causing enhanced probability for below normal number of tropical cyclones in the northeast Pacific.

Visit the IRI’s net assessment page for the full spectrum of forecasts.

• IRI’s Climate Briefing for February (0)
• A decade of multiseasonal climate forecasts (0)