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Tuesday, May 12, 2015

About El Niño

Sou | 10:17 PM Feel free to comment!

With the Australian Bureau of Meteorology declaring an El Niño, and for anyone who is not familiar with the phenomenon, here is a short version of what happens. I wrote a longer article last year. There are more references down below as well.

About ENSO and El Niño

El Niño is one of the phases of the El Niño Southern Oscillation, or ENSO. There are three distinct phases, plus an in-betweener. These are known as:

  • El Niño - the phase where the ocean releases warm air to the surface, usually resulting in an increase in global surface temperature
  • La Niña - the phase where the ocean cools the surface, usually resulting in a decrease in global surface temperature - though with climate change it usually means a lesser increase, not a decrease
  • Neutral - when there is neither an El Niño or a La Niña
  • Modoki - which is a part way El Niño. It doesn't affect as much of the Pacific Ocean, and has different teleconnections (ocean-atmosphere links over a distance). You could think of it as an in-betweener that can't make up its mind whether to turn into a full blown event or not. (Don't tell a climate scientist that I said that - they'd correct me.) 

There are at least three main things happening in the ocean and the air:
  1. The Walker Circulation, which is a pattern of circulation in the atmosphere, which are called trade winds at the surface.
  2. Warming and cooling of the ocean surface and sub-surface changes
  3. Equatorial ocean waves

First the Walker Circulation, which is the circulation of air over the tropical Pacific up into the troposphere. At the surface, this is known as trade winds. They typically blow from east to west at the surface from an area of high pressure in the east to an area of low pressure in the west. The air warms as it passes over the tropical Pacific, and rises, then falls again in a circular pattern. The entire circulation shifts either east or west, affecting the surface of the Pacific. It either:
  • straddles the Pacific, sitting around the middle, blowing from east to west with the ocean neither excessively warming or cooling the surface;
  • gets stronger, pushing more warm water from the east to the west (where it piles up), and pulling up cold water in the east. This results in a La Niña and a cooling of the surface, or
  • dropping back and weakening, sometimes even reversing the circulation pattern, with the warm water that was piled up in the west, spreading back eastward. Because the trade winds (Walker Circulation) are no longer pulling up the cold water in the east, the sea surface in the east gets a lot warmer. The results in an El Niño, and warmer surface temperatures.
Here are three images I've adapted from BoM diagrams, to illustrate what happens. (BoM has since changed the diagrams. They've put up new ones here.)

The first image is La Niña. The ocean to the west is warmer than than normal, with the trade winds pushing the warm tropical surface water way across to the west, and cold water being dragged up behind from the depths in the east.

Eventually it dissipates and there is usually a neutral period, when the Walker Circulation and trade winds lose some of their strength and the sea surface returns to its normal warm tropical self, neither anomalously warm nor anomalously cool.

After a bit, the Walker Circulation may slow right down and even stop, or reverse. A weakening of the trade winds is one of the signs meteorologists use to decide whether or not an El Niño is forming.  With the winds being a lot weaker, the Walker Circulation shifts back toward the east. The winds from the east aren't doing much, so warm water isn't pushed westward, it spreads out in the east. In the west, the winds from the Walker Circulation over the Pacific are now blowing from west to east at the surface, which means still more warm water piles up toward the east and the warm spot around Indonesia and northern Australia becomes cooler than normal. This results in the seas heating up the surface, (which you'll see exaggerated in temperatures in the lower troposphere).

Here is an animation of what happens, showing how the Walker Circulation strengthens, weakens and drops back to the east, and what is happening in the ocean at the same time.

BTW - I'm not saying that the Walker Circulation drives ENSO.  It's all part of the pattern of events that makes up ENSO.

You can see what's happening in the sea now in the images and animations my previous article here. Here's a snapshot of the sea surface temperature anomaly from that article (courtesy of BoM).

Sea Surface Temperature - 4 to 10 May 2015
Source: Bureau of Meteorology annotated by HotWhopper

I won't go into the special case of the Modoki. You can read about the El Niño Modoki here.

Madden-Julian Oscillation and Kelvin Waves

Other things you'll read about are Kelvin waves and the Madden-Julian Oscillation (MJO), which are related phenomenon (ie to each other) and related also to the start and end of an ENSO event. These occur frequently at much shorter time scales than ENSO events. BoM describes the MJO as:
...the major fluctuation in tropical weather on weekly to monthly timescales. The MJO can be characterised as an eastward moving 'pulse' of cloud and rainfall near the equator that typically recurs every 30 to 60 days.
Although the MJO operates at a much greater frequency than ENSO (recurring over a matter of weeks), it does play a role in triggering an ENSO event when the atmosphere and ocean are primed for one. I won't go into details of the MJO and associated Kelvin Waves (or Rossby Waves) in this article. Maybe next time. It looks to me that they are still a warm to hot topic in the research world. There's a relatively simple (by comparison!) website by Kyle MacRitchie that will give you a bit of a picture of what they are all about. I've also put a couple of references down below - but they are quite technical.

Pacific Decadal Oscillation and ENSO

Finally, the Pacific Decadal Oscillation deserves a mention (and the Interdecadal Pacific Oscillation or IPO). The PDO is an oscillation that recurs on a longer time scale than ENSO, but has not dissimilar characteristics in some sense. The PDO has warm and cool phases, though one can't tell which is operating for some time after a phase switch. It's measured by an index that's based on sea surface temperature patterns in the Pacific.  The IPO is similar, but longer term, and measured over a larger area of the ocean.

When the PDO is in a cool phase, La Niña events tend to dominate and El Niño events, when the occur, tend to be not as strong. The reverse happens during what is known as the warm phase of the PDO.

Here's a chart of the PDO that I put together some time ago, now updated to include the full year 2014. You can see that the index has turned positive. Whether that means a change to a warm phase is probably not yet determined.

Pacific Decadal Oscillation and GISTemp Global Surface Temperature
Data Sources: GISS NASA and Nate Mantua

By the way, the PDO index has been very high the last four months. At 2.51, December was in the top ten months since 1900, and January and February weren't that far behind. March was a bit lower, but still high (2).

Some further reading

El Niño Southern Oscillation (ENSO) - a good intro to ENSO from the Australian Bureau of Meteorology

El Nino Modoki - Research Institute for Global Change, JAMSTEC

Equatorial Wave Theory | Shallow Water Theory: (Intro to MJO and Kelvin Waves) Kyle MacRitchie, Chaotic Considerations

Hendon, Harry H., Brant Liebmann, and John D. Glick. "Oceanic Kelvin waves and the Madden-Julian oscillation." Journal of the atmospheric sciences 55, no. 1 (1998): 88-101. doi:<0088:OKWATM>2.0.CO;2 (open access)

Seo, Kyong‐Hwan, and Yan Xue. "MJO‐related oceanic Kelvin waves and the ENSO cycle: A study with the NCEP Global Ocean Data Assimilation System." Geophysical research letters 32, no. 7 (2005).  DOI: 10.1029/2005GL022511 (open access)

Some of the related articles at HotWhopper:

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