When La Niña melts the ice | Polarjournal
Arctic sea ice plays an essential role in winter atmospheric circulation and cold waves across the northern hemisphere continents. Image: Julia Hager

The Pacific currents influence those of the Atlantic during their extreme phases. Their effects are felt as far as the Arctic, the Barents Sea and the Kara Sea.

From 1990, the abnormally small or anomalous large area covered by the Artic ice pack in the Barents and Kara Seas in winter reflects the instability of the current climate. A Chinese research team has just found the source of this anomaly in the Barents Sea and the Kara Sea. It traces the mechanism at work during the fluctuations of the ice pack. This discovery will improve the prediction of ice cover, which is useful for maritime traffic in the Barents and Kara Seas.

These are the extreme phenomena known in the Pacific that influence the retreat of the ice pack: La Niña and El Niño. La Niña influences oceanic and atmospheric circulations in the Atlantic, stimulating the intrusion of warm and humid air currents in winter in the Arctic. The opposite occurs with El Niño.

The frequency of these two phenomena has increased by 20% during the last decade, since then, the ice limits in the Barents and Kara Seas oscillate more frequently from one year to the next.

In black, the La Niña current and the Walker atmospheric circulation, in purple, the Hadley circulation, in red, the Atlantic temperature variations (left) and the Ural atmospheric circulation (right), in blue, the influence of the Azores high, in green, the humid air, and in orange, the warm water transport. Image: Binhe Luo & Al, Nature,2023.

When the waters of the South Pacific are abnormally cold, due to La Niña, the atmospheric currents rise less strongly over South America and fall gently on the Atlantic side through the so-called Walker circulation. As a result of such weakening, the main tropical circulation in the Atlantic (the Hadley cell) is strengthening. The air rises above the equator and falls further north, beyond the Tropic of Cancer. This event coincides with the Azores High being strong and overtaking the Icelandic lows, allowing currents of warm, moist air to flow up towards the Arctic.

The opposite is observed when La Niña disappears in favor of El Niño. The latter strengthens Walter’s circulation which weakens Hadley’s cell. Icelandic lows block the upwelling of warm air during the winter.

However, the actions of La Niña and El Niño on the sea ice have a time lag of 10 to 15 years. This is mainly due to the instability of Atlantic temperatures: since 1990, the variation between high and low Atlantic temperatures has increased by 59%. It favors the circulation of warm waters from the Atlantic to the north and atmospheric currents around the Urals that channel the warm air upwards towards the Arctic zone. When this instability is low, the opposite occurs.

The relationship between sea ice surface and the El Niño phenomenon is more complex, as the authors of the study point out. According to them, a recent publication suggests that El Niño could influence the melting of ice in summer. On the other hand, global warming would free the Arctic of ice seasonally from the end of the 21st century.
Furthermore, forecasting models show that an ice-free Arctic would lead to a strengthening of the El Niño phenomenon. In this case, the Arctic could in turn influence the climate phenomena of the Pacific.

Camille Lin, PolarJournal

Link to study: Binhe Luo, Dehai Luo,YaoGe, Aiguo Dai, LinWang, Ian Simmonds, Cunde Xiao, Lixin Wu & Yao Yao, Nature, 2023, Origins of Barents-Kara sea-ice interannual variability modulated by the Atlantic pathway of El Nino-Southern Oscillation, doi.org/10.1038/s41467-023-36136-5.

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