The East Asian Subtropical Westerly Jet (EASWJ) and the East Asian Summer Monsoon (EASM) are two pivotal components of the East Asian monsoon system, shaping the precipitation distribution and climate over East Asia. Whether the co-evolutionary EASWJ–EASM relationship remains consistent under different climatic backgrounds has been a key question in both modern and paleoclimate research.
Over the past millennium, the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA), and the Present Warm Period (PWP) stand out as the three most representative typical climate periods. Investigating the co-evolutionary relationship between the EASWJ and EASM during these three periods not only deepens our understanding of the physical mechanisms of the monsoon system but also provides a vital scientific basis for future projections.
Based on these considerations, the paleoclimate simulation team from Nanjing Normal University utilized the Community Earth System Model (CESM-LME) to investigate the interannual co-evolutionary EASWJ–EASM relationship during these three typical periods (MCA, LIA, PWP).
The results, recently published in Atmospheric and Oceanic Science Letters , show that, on the interannual time scale, the northwest–southeast displacement of the EASWJ and the intensity of the EASM maintain a stable anti-phase relationship across all three periods: a northward shift of the jet corresponds to a stronger monsoon, and vice versa. However, the study identified a significant “mode shift” in the second MV-EOF mode: during the MCA and LIA, the dominant feature is an out-of-phase (in-phase) relationship between EASWJ and EASM intensities in the midlatitudes (subtropics), whereas during the PWP it is characterized by a zonal displacement of the EASWJ, with an eastward (westward) shift associated with a stronger (weaker) EASM.
“This finding suggests that natural external forcings (such as solar activity and volcanic activity) and anthropogenic external forcings (such as greenhouse gas emissions and land-use change) may have fundamentally different modulation effects on the East Asian monsoon system. This implies that we may not be able to simply extrapolate climate variability patterns from past natural warm periods to future anthropogenic warming scenarios,” says Prof. Mi Yan, corresponding author of the study.
This study systematically compares the co-evolutionary relationship between the EASWJ and the EASM under different climatic backgrounds, revealing how this relationship shifts under the influence of external forcings and oceanic internal variabilities. In future work, the research team plans to further investigate the joint impacts of the three oceans on the East Asian monsoon system, as well as the role of atmospheric internal variability, to provide a more robust physical foundation for future climate projections.
Atmospheric and Oceanic Science Letters