Where does the wind come from? Does it have an unknown source, or does it arise randomly in the atmosphere? Climate models suggest that the flap of a butterfly’s wings in Brazil could trigger a hurricane in America.

In the 1800s, Zöppritz developed a model showing that wind could initiate vertical mixing in the ocean, all the way down to great depths. This mixing between cold and warm ocean currents leads to changes in the ocean’s surface temperature. With this understanding, climate and sea temperature came to be seen as a series of random events, since no one can know where the wind truly comes from.

This line of thought reinforced the perception that climate and marine ecosystems exist in a natural state characterized by random variations. The uncertainty surrounding wind theory led me to ask a crucial question: Does the wind have a signature? If it does, that means the wind originates from a source that imparts predictable patterns. If it does not, then the wind stems from an unknown and random source.

NAO as a Climate Indicator

One day I came across an article in Nature about the NAO index. NAO (the North Atlantic Oscillation)describes the pressure relationship between the low-pressure system over Iceland and the high-pressure system over the Azores. It serves as an important indicator of wind direction and climate.

The index provides information on whether Northern Europe is in a period dominated by low pressure from the south or cold winds from the north. At the same time, it exhibits a number of seemingly random variations.

Nature reported that no one had found a clear connection in the changes over time since the first measurements in 1822. This made the NAO index an important research topic. It could represent the characteristics of the wind and give insight into past weather and wind patterns. The question was: Does the NAO index have a signature with periodic properties, or is it just a series of random events?

NAO’s Signature 1822–2022

Figure 1: NAO winter index for the years 1822 to 2022.

The NAO index variations appear random, with large changes from one year to the next. However, we can also observe that the overall trend exhibits fluctuations throughout the series.

Figure 2: A statistical wavelet analysis of the NAO index from 1822 to 2022

reveals that the NAO exhibits periodic variations. The study uncovered a clear signature with cycles of 8, 24, and 74 years. The same signature was identified in Earth’s rotation, in the Arctic Ocean’s water circulation, and in the extent of Arctic sea ice in the Greenland Sea.

This means that the wind has predictable, periodic patterns that can be traced back to Earth’s rotation. The explanation is that Earth’s rotation creates resonance in the water masses of the Arctic Ocean. This circulation leads to periodic variations in the extent of Arctic ice in the Greenland Sea, which in turn affects wind direction.

Ocean-Driven NAO Index

Figure 3: The progression of a 74-year cycle for the NAO index (blue) and NAW (green).

These signatures provide insight into how ocean temperature and wind together shape the climate in Northern Europe. The NAO was negative between about 1940 and 1980, with a minimum around 1960.

The North Atlantic surface water temperature (NAW) affects the extent of Arctic ice. The ice extent is determined by the rate at which NAW is supplied to the Arctic Ocean. This means that variations in ice extent are time-lagged relative to NAW’s temperature trend. Variations in the ice extent then directly influence the signature of the NAO index. This shows that it is the ocean temperature that drives the wind – not the other way around.

NAO was negative from around 1940 to 1980, with a minimum around 1960. This period was characterized by cold winters with high pressure from the north, as well as dry and warm summers. At the same time, NAW saw its steepest decline around 1960, when the NAO was at its minimum. The temperature (NAW) reached its lowest point in 1979, when the NAO was increasing most rapidly.

NAW and NAO exhibit a phase difference of π/2 radians: the NAO reaches its peak values at the same time that NAW has its highest rate of increase. This interplay contributed to a cold climate period from 1960 to 1980. It also led to a warm climate period from 1997 to 2017 (Figure 3).

In this warm period, NAW led to increased ocean temperatures, while NAO brought low-pressure systems from the south, warm winds from the west, mild winters, and rainy summers. Extrapolating these signatures suggests a new cold climate period from around 2035 to 2055 (Figure 3).

NAO-Driven Climate in Europe

For a long time, the role of wind in climate change was regarded as random and unpredictable. However, the discovery of a moon-driven NAO signature has challenged this view. This discovery was published in 2006 and is now cited in around 100 international publications [1].

The study showed that wind-driven climate in Northern Europe has followed predictable patterns since 1822. This insight is of great importance for food production and societal planning in Northern Europe.

Post song: The wind’s ballet

Referanse

1. Yndestad H. (2006). The influence ot the lunar nodal cykler on Arctic climate. ICES Journal. ICES Journal od Marine Science, 63: 401-420 (2006), doi:10.1016/j.icesjms.2005.07.015.