The subtropical jet stream (STJ) is a fast, narrow band of strong westerly winds in the upper troposphere, typically near 30° latitude in both hemispheres, forming at the poleward edge of the Hadley cell.

It sits higher (often around 200 hPa or ~12 km / 40,000 ft) and is generally weaker and more stable in position than the polar-front jet stream (around 50°-60° latitude).

Formation and Basic Dynamics

The STJ arises from the global atmospheric circulation and Earth’s rotation:

Hadley cell circulation: Warm air rises at the equator (Intertropical Convergence Zone), moves poleward aloft, and descends around 30° latitude. As this air flows poleward in the upper troposphere, it conserves angular momentum. Because Earth’s rotational speed decreases with latitude (farther from the equator), the air accelerates eastward, creating strong westerly winds.

Thermal wind balance: Horizontal temperature gradients (warmer tropics vs. cooler mid-latitudes) drive vertical wind shear. Stronger gradients strengthen the jet. The STJ is primarily thermally direct, linked to the descending branch of the Hadley cell, with a thermally direct circulation across its axis.

Coriolis effect: Deflects poleward-moving air to the right (Northern Hemisphere) or left (Southern), enhancing the westerly component.

Typical speeds reach 70 m/s (~160 mph or more) in winter cores, especially over Africa and Asia. The jet is stronger and more continuous in winter due to larger equator-pole temperature contrasts.

It meanders, splits, or merges with the polar jet at times and is influenced by land-sea contrasts, mountain ranges (e.g., Tibetan Plateau), and tropical convection.

Rossby Waves and Waveguide Effect

The STJ acts as a waveguide for Rossby waves (planetary waves), channeling their energy and enabling teleconnections over long distances.

• Rossby waves form due to the conservation of potential vorticity and the Coriolis parameter’s variation with latitude. They appear as large meanders in the jet.
• In strong zonal flow like the STJ, waves propagate eastward efficiently as “wave trains.” This creates quasi-stationary or slowly moving patterns of highs and lows.
• Interference and amplification: Upstream disturbances (e.g., from the North Atlantic Oscillation/NAO) can excite waves that propagate along the jet, especially when background conditions like potential vorticity gradients favor trapping. This links distant patterns, as seen in the Japan winter study (NAO + Indo-Pacific convection modulating waves toward East Asia).

Disturbances along the jet influence downstream weather via wave energy dispersion. The East Asian STJ, for example, is a key waveguide in boreal winter, linking to monsoon variability, precipitation, and cold outbreaks.

The Siberian High

The Siberian High (also called the Siberian Anticyclone) is a massive, semi-permanent high-pressure system that dominates the winter climate over northeastern Eurasia. It forms from September to April, centered near Lake Baikal, and reaches peak strength in mid-winter with surface pressures often exceeding 1,040 hPa (and record highs around 1,083 hPa). Central temperatures frequently drop below -40°C, making it one of the coldest and strongest surface anticyclones on Earth

The Siberian High exemplifies how a regional pressure system drives continental-scale climate while being modulated by global teleconnections. Its effects are most pronounced in East Asia’s winter weather, directly tying into Japan’s cold waves and snowfall patterns discussed earlier. Ongoing research focuses on improving seasonal predictions of its strength for better extreme weather preparedness.

A study from the University of Tsukuba (published in 2026 in the Quarterly Journal of the Royal Meteorological Society) shows that the severity of Japan’s winters—particularly cold waves, heavy snowfall, and temperature extremes—is strongly shaped by the interaction of two distant climate systems.

Researchers analyzed 76 years of global atmospheric data and used numerical simulations to demonstrate this interaction.

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How interference between the North Atlantic Oscillation and the tropical Indo-Pacific convection modulates wave trains along the subtropical jet: Impacts on the Asian winter climate

The 2026 paper by Yuki Asazuma, Masaya Kuramochi, and Hiroaki Ueda (University of Tsukuba) in the Quarterly Journal of the Royal Meteorological Society) precisely addresses this mechanism.

It explains how the interaction (interference) between the North Atlantic Oscillation (NAO) and tropical Indo-Pacific convection modulates Rossby wave trains along the subtropical jet (STJ), with significant impacts on Asian winter climate, including Japan.

Japan’s winters are already influenced by regional features like the Siberian High (cold, high-pressure system over Eurasia) and the Aleutian Low, which drive cold northwest winds across the country. This brings heavy snow to Sea of Japan-facing coasts and clearer (but still cold) conditions on the Pacific side.

Core Mechanism: Constructive vs. Destructive Interference

The wintertime Rossby wave train along the Asian subtropical jet extends from Western Europe toward East Asia. Its strength and eastward extent are not simply additive but depend on the phase relationship between two distant forcings:

1. NAO-related vorticity forcing (over Western Europe/North Atlantic-European sector): The NAO influences upper-level vorticity anomalies (cyclonic or anticyclonic). Positive or specific NAO phases inject vorticity that can excite or seed Rossby waves.
2. Tropical Indo-Pacific convection / anomalous Walker circulation: Enhanced convection (e.g., anomalous ascent and upper-level divergence over the tropical Indian Ocean/western Pacific, often linked to the Walker circulation) generates divergent wind anomalies, particularly meridional components over South China/Southeast Asia. This acts as a tropical Rossby wave source.

• Constructive interference (in-phase alignment): When NAO-related cyclonic vorticity injection over Western Europe coincides with anomalous meridional divergent winds from enhanced Indo-Pacific convection, the forcings reinforce each other. This amplifies the subtropical wave train, allowing it to propagate farther eastward along the STJ waveguide. Result: Significant low-pressure anomaly develops in the Far East, strengthening cold air advection, cold surges, and heavier snowfall in East Asia/Japan.
• Destructive interference (out-of-phase): When the NAO vorticity and tropical convective signals oppose each other, wave excitation weakens. Propagation is suppressed downstream of South Asia, leading to a shorter/weaker wave train with minimal impact on East Asian climate (milder winters).

The coherency of these forcings determines the eastern extent of the wave train and thus the downstream climate response. The STJ acts as an efficient waveguide for these planetary waves due to its strong zonal winds and potential vorticity gradients.

Evidence and Methods

Observational analysis: 76 years of global atmospheric reanalysis data, examining composites, regressions, and wave activity flux.

Numerical simulations: Model experiments to isolate and confirm the interference effects (e.g., imposing vorticity/divergence sources separately and together).

Key diagnostics: Upper-level geopotential height, vorticity, velocity potential, divergent winds, and Rossby wave source terms.

Impacts on Asian Winter Climate

East Asia/Japan: Stronger wave trains (constructive case) link to colder surface temperatures, intensified Siberian High influences or cold outbreaks, increased heavy snowfall (especially Japan Sea- side), and more extreme cold waves.

Broader Asia: Altered precipitation patterns, temperature variability across China, Korea, and beyond. The mechanism helps explain why some strong NAO or ENSO-like events have outsized (or muted) effects on the region.

Variability: This interference contributes to the non-stationary and complex teleconnections observed in Asian winter monsoon strength.

This builds on prior knowledge of individual teleconnections (NAO influencing the Asian jet via wave trains; tropical convection as a Rossby source) by highlighting their nonlinear interaction via interference.

It underscores global atmospheric interconnectedness: conditions over the North Atlantic and tropics thousands of km away can constructively or destructively shape East Asian extremes through the STJ waveguide.

Practical value: Improved understanding supports better subseasonal-to-seasonal forecasting of Asian winter extremes by monitoring both NAO phase and Indo-Pacific convective activity (e.g., via outgoing longwave radiation or velocity potential fields).

The full open-access paper is available at DOI: 10.1002/qj.70222.

It includes schematics of constructive/destructive cases and composite anomaly maps.

This research fits into the broader context of STJ dynamics and Siberian High influences discussed previously.

Published: Quarterly Journal of the Royal Meteorological Society (2026)

DOI: 10.1002/qj.70222

Provided: University of Tsukuba

Authors: Yuki Asazuma, Masaya Kuramochi, Hiroaki Ueda

Abstract

The Rossby wave train along the wintertime subtropical jet over the Asian continent extends from Western Europe to Japan, and it significantly influences the East Asian climate.

Two primary excitation sources of the subtropical wave train are the European vorticity anomaly related to the North Atlantic Oscillation (NAO) and tropical anomalous convection associated with the Indo-Pacific Walker circulation.

The wave trains excited solely by the European vorticity anomaly exhibit large amplitudes near Europe but diminish downstream. However, as they reach the Asian domain, the waves can amplify through vorticity advection linked with convective activity in the Indo-Western Pacific.

The coherency of the NAO-related European vorticity and the anomalous Indo-Pacific Walker circulation plays a key role in the excitation of the subtropical wave train.

To investigate their interference, statistical analyses were carried out based on reanalysis data, and additional numerical experiments were performed using a linear baroclinic model.

Results show that when the cyclonic vorticity injection associated with the NAO in Western Europe coincides with the anomalous meridional divergent wind over South China associated with the enhanced Indo-Pacific Walker circulation, constructive interference occurs, leading to a significant low-pressure anomaly in the Far East and colder winters.

On the other hand, when the NAO-related vorticity forcing and tropical convective activity are out of phase, destructive interference arises, suppressing the propagation of Rossby waves downstream of South Asia and ensuing a weaker impact on the East Asian climate.

The findings reveal that the phase combination of the NAO-related European vorticity and Indo-Pacific Walker circulation determines the eastern extent of the wave train along the subtropical jet and its associated climate impacts on the broader Asian region.