Ag Weather Forum

Interesting Facet of La Nina

Bryce Anderson
By  Bryce Anderson , DTN Senior Ag Meteorologist
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Elements leading to more precipitation in the western Pacific and a drier pattern in the eastern Pacific tend to reinforce each other during a La Nina event. (NOAA graphic)

Everybody's talking about La Nina right now. The sustained cool phase of the El Nino/Southern Oscillation (ENSO) cycle is in place. La Nina's expected drier impact on the U.S. Southern Plains and in central Argentina is already getting factored into crop yield forecasts.

Such interest makes the following excerpt from a La Nina blog by NOAA climate scientist Michelle L'Heureux very timely. This excerpt has details on the importance of a feature known as the Bjerknes feedback. The term refers to a reinforcing atmosphere-ocean loop first identified by meteorologist Jacob Bjerknes in 1969. (The Bjerknes name is associated with some of the foundation concepts of weather science.)

Here is that excerpt:

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We often don't know exactly what initiates the feedback loop -- sometimes sea surface temperatures start to cool and then the trade winds begin to strengthen, or vice versa. But we have to start somewhere ... start with the ocean cooling. This means the difference (or gradient) in temperature between the relatively warmer western Pacific and the cooler eastern Pacific becomes even larger than normal. Over the western Pacific, warmer temperatures cause air near the surface to rise, moisten, and become convective, leading to thunderstorms and more rain (just like how warm air rises into an attic, though thankfully, thunderstorms won't form in a house). Over the cooler eastern Pacific, air begins to sink more and dry out (also why basements are colder). With the increases in sinking motion over the eastern Pacific and rising motion over the western Pacific, the trade winds that typically blow from east-to-west along the equator become even stronger.

These stronger-than-normal winds, in turn (here's the feedback!), then help to reinforce the ocean temperature anomalies by increasingly pushing water away from the eastern Pacific to the western Pacific Ocean. This allows deeper colder water, which is residing below the surface of the surface water, to upwell and cool the surface even more. These same winds help to transport surface water, heated by the sun along the way, to the western Pacific, which literally piles up warmer water around Indonesia (sea level height also increases). What do these warmer waters do? They help the air to rise even more over Indonesia (leading to increased convection/rainfall in the region) and, over the central and eastern Pacific Ocean, the colder surface waters result in diminished convection/rainfall. In summary, the initial stronger-than-normal trade winds and east-west contrast in surface temperatures become even stronger because of this positive Bjerknes feedback.

The Bjerknes feedback also explains why there are so many different ways to measure ENSO. There are dozens of time series, or indexes, of various types (composed of temperature, pressure, rainfall, winds, etc.) that are strongly related, or as we say, correlated (a perfect correlation has a value of 1). For instance, the Southern Oscillation index (based on sea level pressure differences) has a correlation of approximately plus 0.9 with the Nino-3.4 index (measures sea surface temperatures in the east-central equatorial Pacific Ocean). The strong links between different types of meteorological variables is evidence that ENSO is one very large and interconnected system!

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The full blog by Michelle L'Heureux is at this link: https://climate.gov/…

More on Jacob Bjerknes is available here: https://en.wikipedia.org/…

Bryce Anderson can be reached at bryce.anderson@dtn.com

Follow him on Twitter @BAndersonDTN

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