Monday, August 9, 2010

An Introduction To El Nino/Southern Oscillation, Atlantic Multidecadal Oscillation, and the Pacific Decadal Oscillation – Part 1

On July 15 we posted "Commodities Surging on Russian Drought Devastation: Harvest Estimates Being Lowered Daily" which linked to some older pieces:
Combined with the plummeting SST anomalies in the El Nino/Southern Oscillation Index, my best guess is that we have started another string of negative anomalies in the PDO.
This could be very bad. As I commented in April '08's "Climate Change and the Pacific Decadal Oscillation":
You might want to look up the word famine. And store a couple tons of wheat in a vermin proof room. The risk of a major crop failure somewhere in the world over the next ten years just went up. My best guess (wild-ass variant) would be northeastern [note: should have been Northwestern i.e. bordering Ukraine -ed] Russia/Ukraine. Which could get interesting...
Jan. '09's "Corn Prices May Enter Decade-Long Slump, Agency Says":
Watch that Pacific Decadal Oscillation. A New York Times archive search for the term "crop failure" returns 1950 hits, with a preponderance of stories written during the cool phase of the PDO. With the interconnectedness of the world's grain markets, a failure anywhere would raise prices everywhere.
(more on this follows the headline story)...
That post has some other links that may be of interest or you could use the 'search blog' box, keyword: PDO....
A basic understanding of this stuff can be money in your pocket. Here's an example on a MarketBeat post, "Loonie to Benefit from Wheat Price Surge?" I added:

  • Another potential beneficiary is the Australian dollar.
    Oz is being counted on to perform a ’swing producer’ role in wheat exports.
    If La Nina is a good girl she might bump their harvest by 1-2 Mil. MT.

    Word of warning though,
    AUD/USD has run from .81 to just under .915 in the last month and is approaching the level where it stalled out twice in the last nine months, the dreaded triple top
    Dear God help me, I’m beginning to hyperbolize in the manner of Clusterstock.
Four days later the headlines read:
From Australian Broadcasting:
Asian Mills Eye Aussie Wheat as Traders Scrap Deals
and from the WSJ:
Wheat Market Benefits Australia

Here's the tutorial from Climate Observations, definitely worth a bookmark:
This two-part series of posts is an introduction to three sea surface temperature-based indexes that are commonly referred to during discussions of global weather and climate. The three are (1) El Niño-Southern Oscillation (ENSO), which is typically expressed as El Niño and La Niña events, (2) Atlantic Multidecadal Oscillation (AMO), and (3) Pacific Decadal Oscillation (PDO). The intent of these posts is to provide the reader with a basic understanding of the phenomena. They are not intended to be single-source all-inclusive references.

Part 1 discusses El Niño and La Niña events.

An El Niño event is a natural warming of the central and eastern tropical Pacific that is caused by an occasional change in coupled ocean-atmosphere processes. The phrase “coupled ocean-atmosphere processes” refers to the many climate factors that all interact. The interacting components include sea surface temperature, surface winds, surface and subsurface ocean currents, atmospheric circulation, precipitation, cloud cover, sea level pressure, sea surface height, etc. How a few of those factors interact will be covered in this post.

Figure 1 includes three cross-sectional views of equatorial Pacific Ocean temperatures from 300 meters in depth to the surface and from Indonesia in the west to the South American Coast in the east. Illustrated are a “Normal” December (Cell a) in 1996, an El Niño December (Cell b) in 1997, and a La Niña December (Cell c) in 1998. “Normal” is listed as “ENSO Neutral” in the illustration, which means the conditions do not reflect an El Niño or a La Niña event. The month of December is shown because El Niño and La Niña events normally peak during the months of November, December and January.

Note how in the “Normal” state (Cell a), from the surface to 200 meters in depth, the warmer subsurface waters are in the west and the cooler waters are in the east. This is caused by trade winds pushing warm surface waters from east to west. The westward moving warm surface waters run into Indonesia so they accumulate in the western Pacific in an area called the Pacific Warm Pool. Just as important: the trade winds blowing across the surface from east to west also cause the waters to be about ½ meter higher in the west than in the east. I tried to show that (with limited success) by adding slopes to the ocean surfaces of the Figure 1. During the El Niño phase, the trade winds first slow, then reverse. Since the trade winds are no longer “holding” the water in place in the western Pacific, gravity causes the warm water to slosh to the east. Referring again to Figure 1, note how during the El Niño (Cell b) the warm subsurface waters have extended east to the South American coast. The shift in the location of that warm water causes changes in precipitation and atmospheric circulation patterns, which raise surface temperatures globally.
Figure 1

Cell c in Figure 1 shows the equatorial Pacific temperature profile during a La Niña event. Close inspection will allow you to note that the sea surface temperatures in the east are cooler than during “Normal” ENSO-neutral conditions shown in Cell a. This is caused by a strengthening of the trade winds as the coupled ocean-atmosphere processes attempt to return to “Normal” conditions. The processes overshoot in their attempt. The higher-than-normal trade winds return the warm surface waters to the western Pacific and also expose more of the cool waters in the east than “normal”.

El Niño events are an important part of climate on Earth. They discharge heat from the tropical Pacific. Atmospheric circulation and ocean circulation then carry that heat poleward, where it can be more easily radiated into space. In doing so, El Niño events help to reduce the temperature difference between the tropics and the poles that would exist without them.

La Niña events are just as important because they recharge the heat that had been discharged during the El Niño before it. They accomplish this by reducing cloud cover over the tropical Pacific. During a La Niña, the strength of the Pacific trade winds rise above “Normal” conditions, which reduce cloud cover over the central and eastern tropical Pacific. The reduced cloud cover allows more downward shortwave radiation (visible light) to warm the tropical Pacific. The stronger-than-normal trade winds then push this water that has been warmed by the sun back to the Pacific Warm Pool and the Western Pacific.

Note again that El Niño events discharge heat from the tropical Pacific and La Niña events recharge it. The above paragraph briefly explained how a La Niña recharges the heat that had been released during an El Niño. Now for a quick discussion of how an El Niño releases more heat than normal from the tropical Pacific. Figure 2 illustrates the Sea Surface Temperatures of the Pacific Ocean during the three ENSO phases. Cell a shows the ENSO Neutral or “Normal” phase. The sea surface temperatures greater than 28 deg C are shown in red. Note how the warmer surface waters are located in the western Pacific, in an area called the Pacific Warm Pool. During the El Niño (Cell b) more of the tropical Pacific sea surface is covered by the warm water. The increase in surface area covered by warmer water allows more heat to be released from the tropical Pacific to the atmosphere.
Figure 2

Of the three phases of El Niño-Southern Oscillation, the El Niño is the truly unusual phase. It involves the transport of warm water from the western tropical Pacific to the east. These changes are the opposite of the “Normal” and La Niña conditions. A La Niña, on the other hand, is simply an exaggeration of “Normal” conditions. As noted above, trade winds during the La Niña rise above normal conditions. These stronger trade winds increase the strength of the ocean currents moving from east to west....MUCH MORE