Atmospheric Modes of Variability: NAO

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Let's shift focus to three key atmospheric patterns that play a significant role in shaping weather and climate: the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO), and the Antarctic Oscillation (also known as the Southern Annular Mode, or AAO). These oscillations represent important modes of internal variability, each with distinct impacts both locally and across the globe.

Let's start with the NAO (we'll turn to the other two soon). The NAO is particularly influential when it comes to weather patterns in Europe and North America. Meteorologists have been relying on the NAO for decades to improve the accuracy of their weather forecasts and climate predictions in these regions. These oscillations are driven by a complex interplay of atmospheric pressure systems, sea surface temperatures, and other environmental factors, illustrating the deeply interconnected nature of the Earth's climate system.

The North Atlantic Oscillation (NAO)

“Late in the 18th century, a missionary who had traveled back and forth across the Atlantic Ocean for several years noted that mild winter conditions in Greenland often coincided with severe winter conditions in Denmark, and vice versa. The severe-versus-mild phenomenon he described is now recognized as an impact of the North Atlantic Oscillation, or NAO.”

The North Atlantic Oscillation (NAO) describes the fluctuations in sea level pressure between two key pressure systems: the Icelandic Low (a low-pressure zone near Iceland) and the Azores High (a high-pressure zone near the Azores islands). When the NAO is in its positive phase, both the Icelandic Low and the Azores High intensify, which increases the pressure difference between the subtropics and mid-latitudes. Ah, but we've learned about the pressure gradient force! Recall that an increased pressure differential leads to faster winds. In contrast, during the negative phase of the NAO, this pressure contrast weakens. It's important to note that the NAO is typically more pronounced in the winter months, meaning its climatic impacts are also stronger during that season.

By influencing pressure patterns over the North Atlantic, the NAO directly affects the strength and direction of westerly winds and storm tracks, which in turn alters temperature and precipitation patterns. While its effects can be felt globally, the NAO has the most significant impact on weather and climate across the North Atlantic region and nearby continents, particularly in Europe and North America. During a positive NAO phase, stronger winds (because of the increased pressure difference and, therefore, stronger pressure gradient force!) carry warm, moist air from lower latitudes to northern Europe, resulting in warmer, wetter winters. Meanwhile, southern Europe often experiences cooler, drier conditions. On the other hand, a negative NAO phase brings colder, snowier winters to northern Europe and milder, wetter winters to the south. The eastern coast of North America is also affected, with the NAO influencing winter cold air outbreaks and snowfall. Hopefully, the schematic below will help put the pieces together. Spend a few minutes digesting it!

A striking example of the NAO's influence occurred between December 28, 2009, and January 13, 2010. An extremely negative NAO led to record-breaking cold temperatures across the Eastern US and northern Europe. Anomalous northerly winds pushed Arctic air southward into cities like Washington D.C. and even Miami. The D.C. area received a remarkable 72 inches of snow that winter, while Miami recorded a low of 36°F (2.22°C) on January 11, 2010, breaking a decades-old record. This event highlights how the NAO can drive extreme weather conditions, particularly during its negative phase!

The exact mechanisms behind the North Atlantic Oscillation (NAO) remain complex and are not yet fully understood. The most widely accepted explanation involves the interaction between the jet stream and atmospheric eddies, which drives the NAO as a largely self-sustaining mode of internal atmospheric variability. However, the NAO is far from being an isolated phenomenon. It actively interacts with the ocean, creating a dynamic relationship between the atmosphere and the sea surface.

For instance, shifts in the NAO phase leave a noticeable impact on ocean surface temperatures, which in turn can influence future NAO pressure patterns by altering the temperature gradient* of the North Atlantic Ocean. Beyond its shorter-term variability, the NAO also exhibits changes over decadal to multidecadal timescales. This longer-term variation is thought to be linked to broader climate patterns like the Atlantic Multidecadal Oscillation (AMO) and the Atlantic Meridional Overturning Circulation (AMOC), suggesting that the NAO is part of a much larger system of interconnected climate processes.