The North Atlantic jet stream

The North Atlantic jet stream is a narrow, fast-flowing current of air found in the upper troposphere, typically at altitudes between 7 and 12 kilometres (approximately 23,000 to 39,000 feet). It forms at the boundary between cold polar air and warmer subtropical air, driven by the strong temperature gradient and the Coriolis effect. Wind speeds within the core of the jet stream commonly range from 100 to 200 knots (approximately 185 to 370 km/h), but can exceed 250 knots (460 km/h) in extreme cases.

Globally, there are two main jet streams in each hemisphere: the polar jet and the subtropical jet. The North Atlantic jet stream is primarily the polar jet, which is stronger and plays a more direct role in mid-latitude weather. Its position is not fixed; it meanders north and south, and its strength varies seasonally and day-to-day. The jet stream is a key component of the general atmospheric circulation, acting as a steering current for weather systems across the Atlantic basin.

The vertical and horizontal shear associated with the jet stream is significant. For example, a typical jet stream might have a core speed of 150 knots at 9 km altitude, decreasing to 50 knots at 7 km and 11 km altitude, and to much lower speeds at the surface. This strong wind shear is a critical factor in the development and movement of weather systems.

The position of the North Atlantic jet stream is the primary determinant of the track of Atlantic depressions. Low-pressure systems, which bring strong winds and precipitation, tend to follow the path of the jet stream, often forming and intensifying in regions of strong upper-level divergence associated with the jet.

• Northerly jet position: When the jet stream is displaced to the north, typically across Scotland or Scandinavia, Ireland tends to experience more settled, often drier, and sometimes colder conditions. The main storm tracks pass to the north, reducing the frequency and intensity of direct impacts on Ireland.
• Southerly jet position: When the jet stream is displaced to the south, passing directly over or south of Ireland, it acts as a conveyor belt for depressions. This leads to an increased frequency of active low-pressure systems crossing the country, resulting in windier conditions, more precipitation, and often milder weather. This pattern is often associated with a higher risk of named storms.

For example, during the winter of 2013/14, a persistently strong and southerly-displaced jet stream contributed to a series of intense storms and record rainfall across Ireland and the UK. Conversely, periods with a northerly jet can lead to prolonged dry spells.

The North Atlantic Oscillation (NAO) is a large-scale atmospheric phenomenon that significantly influences the position and strength of the North Atlantic jet stream. The NAO is defined by the difference in atmospheric pressure between the Icelandic Low and the Azores High.

• Positive NAO phase: Characterised by a stronger-than-average Icelandic Low and Azores High. This results in an enhanced pressure gradient across the North Atlantic, leading to a stronger and more northerly-displaced jet stream. For Ireland, this typically means milder conditions, more precipitation, and increased wind in the north, but often drier and calmer conditions in the south. The storm track is directed towards northern Europe.
• Negative NAO phase: Characterised by a weaker Icelandic Low and Azores High, or a reversed pressure pattern. This leads to a weaker and more southerly-displaced jet stream, often with more meridional (north-south) flow. For Ireland, this commonly brings colder, drier conditions, with a higher chance of easterly winds and snow in winter. Storms may track further south, impacting southern Ireland or the Mediterranean.

The NAO index is a useful tool for understanding long-term climate variability and seasonal predictions, though its day-to-day influence on specific weather events is less direct than the immediate jet stream position.

Within the broader jet stream, there are regions of even stronger winds known as jet streaks. These are areas where the wind speed within the jet core is particularly high. Jet streaks are crucial for the development and intensification of mid-latitude cyclones, a process often referred to as explosive deepening or bombogenesis.

As air enters and exits a jet streak, specific patterns of divergence and convergence are created in the upper atmosphere. In particular, the left-exit region and the right-entrance region (relative to the direction of flow in the Northern Hemisphere) of a jet streak are associated with upper-level divergence. This divergence aloft reduces the atmospheric pressure at the surface, leading to rising air and the formation or intensification of low-pressure systems.

For example, if a surface low-pressure system moves into the left-exit region of a strong jet streak, the upper-level divergence can cause the surface pressure to fall rapidly. A commonly cited threshold for explosive deepening is a drop in central pressure of at least 24 hPa in 24 hours. This rapid pressure fall leads to a significant increase in the pressure gradient force, resulting in very strong surface winds and often heavy precipitation. This mechanism is frequently observed in the development of severe Atlantic storms that affect Ireland.

The persistence of certain jet stream patterns can lead to prolonged periods of specific weather conditions. For Ireland, a consistently strong and southerly-displaced North Atlantic jet stream during winter often results in a series of deep low-pressure systems tracking across or close to the island. This brings extended periods of high winds and significant precipitation.

Such patterns are often linked to the positive phase of the North Atlantic Oscillation, which enhances the westerly flow across the Atlantic. The increased frequency of depressions means more frontal systems crossing the country, leading to higher-than-average rainfall and increased storminess. For instance, the winter of 2015/16, influenced by a strong positive NAO, saw Ireland experience one of its wettest and stormiest winters on record, with multiple named storms impacting the country.

Conversely, a persistent northerly jet stream can lead to calmer, drier winters, or if combined with a negative NAO, colder conditions with a higher likelihood of easterly winds and snow. The long-term variability of the jet stream is a key factor in Ireland's climate, contributing to the year-to-year differences in winter weather severity.

The position and strength of the North Atlantic jet stream are fundamental drivers of monthly weather anomalies in Ireland. An 'anomaly' refers to the deviation of a meteorological variable (like temperature, rainfall, or wind speed) from its long-term average for a given month.

Consider a scenario where the jet stream is consistently further south than its climatological mean for a particular month. This would likely lead to:

• Positive wind speed anomalies: More frequent and stronger low-pressure systems tracking over or near Ireland would result in mean monthly wind speeds being above the long-term average.
• Positive precipitation anomalies: The increased passage of frontal systems would lead to higher total monthly rainfall compared to the average.
• Positive temperature anomalies: The predominant south-westerly flow associated with a southerly jet would bring milder air from the Atlantic, resulting in above-average monthly temperatures.

Conversely, a month with a persistently northerly jet stream would likely show negative wind and precipitation anomalies, and potentially negative temperature anomalies if continental air masses are allowed to dominate. Met Éireann's monthly climate statements frequently attribute observed anomalies to the prevailing jet stream pattern, demonstrating its direct link to Ireland's weather variability.

While the influence of the North Atlantic jet stream on Ireland's weather is clear, accurately predicting its precise position and strength beyond a few days remains a significant challenge for meteorologists. This limitation directly impacts the skill of seasonal weather predictions.

Numerical weather prediction models can forecast the jet stream's behaviour with reasonable accuracy out to about 7-10 days. Beyond this, the chaotic nature of the atmosphere means that small initial errors can amplify, leading to divergence in model outputs. Ensemble forecasting, which runs multiple model simulations with slightly varied initial conditions, helps to quantify this uncertainty. The spread among ensemble members for jet stream position increases significantly beyond the medium range.

Seasonal forecasts (e.g., for the coming winter) often rely on predicting large-scale atmospheric patterns like the NAO, which provides a statistical likelihood of certain jet stream behaviours. However, these are probabilistic outlooks, not deterministic forecasts of specific events. They indicate a tendency (e.g., 'higher probability of a mild, precipitation-rich winter') rather than guaranteeing it. The Wind Agent's ensemble fan and plume charts illustrate this uncertainty, showing the range of possible outcomes for surface wind speeds, which are ultimately driven by these larger atmospheric dynamics.