La Nina Pattern Begins in the Pacific Ocean

A weak La Nina atmosphere-ocean pattern has fully developed in the Pacific Ocean. This phenomenon is part of the cool phase of the El Nino Southern Oscillation (ENSO). It is characterized by abnormally low surface pressure in the Western side of the Pacific Basin and abnormally high pressure on the Eastern side. This causes an enhancement of the easterly trade winds, causing significant upwelling of cold water along the equatorial coast of South America, with a build up of very warm water in the Western Pacific.

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Schematic of La Nina oceanic ocean-atmosphere pattern in the Pacific and expected jet stream behavior and temperature/precipitation impacts in the US/Canada during a La Nina winter.

ENSO patterns, as shown above can cause noticeable changes in the seasonal weather patterns over North America, particularly during the winter months. The jet stream can become more amplified, leading to a Pacific jet producing cooler and wetter than normal conditions over the Pacific Northwest, extending into the northern tier states. Meanwhile, the “Sun Belt” of the US can see abnormally warm, dry conditions.

The caveat of all this is is the strength of the La Nina versus the degree of influence other atmospheric patterns have on the seasonal climate variability. Other patterns include the North Atlantic Oscillation (NAO-surface pressure variability between the semi-permanent Icelandic Low and Azores High), Arctic Oscillation (AO-pressure anomalies between Arctic and mid-latitudes, closely related to NAO), and the Madden-Julian Oscillation (which can speed the development and enhance the effects of a El Nino or La Nina phase), among others on various timescales.

So what does it mean for our winter in the US? Well, as of now, the NWS Climate Prediction Center is generating winter temperature/precipitation forecasts accounting for the development of La Nina, with a strong latitudinal effect on temperature and precipitation. Below/above in the north and above/below in the south, respectively.

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In the meantime, long-range forecasts show the North Atlantic Oscillation becoming “negative” later in November (characterized by a south to north pressure gradient between the Azores high over Portugal and the Icelandic to the north). This pattern is favorable for an amplified upper-level jet stream wave pattern over North America and the North Atlantic and intrusions of cold air deep into the eastern half of the US. So in the shorter term colder than normal conditions may be possible for these areas this month (as has already occurred this week).

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When it comes to these “teleconnections”…the various cycles of variability within the annual climate regime of Earth…they can most definitely give us a head’s up on to what to expect in general. A canvas of how the weather may be behave over the course of days to weeks and months. But we must keep track of how these different cycles interact with each other and how they vary individually in terms of strength and mode. One curiosity is the strength and persistence of the La Nina. If it was fairly weak, it is more likely to be dominated by other teleconnections at times during the course of the winter, versus if it intensifies and produces more persistent effects on the upper-level air patterns.

Overall, the expected winter pattern is good news for drought-stricken areas in the northern tier such as Montana and the Dakotas. We will have to watch areas along the southern tier for potential further drought development. And as mentioned, November and at least early December could feature a more amplified jet stream so that even areas in the Southeast which may end up with an above average winter overall may see serious impacts from cold because of Arctic intrusions (something for citrus growers to watch out for in Florida, for example).

–Meteorologist Nick Humphrey

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