Pacific La Niña-like warming and northeastward shift of western Pacific subtropical high are major responsible for the TC track change
Long-term changes in tropical cyclone (TC) activity have a significant impact on the socio-economic development of coastal countries and regions. The detection and attribution of the long-term changes in the position of TC activity have received great attention in the past decades in the context of global warming. The densely populated East Asia coastal countries including China face increasing TC threats, emphasizing the critical importance of understanding how the TC poleward migration occurs. However, there are still controversies and uncertainties about TC poleward migration and its causes. TC poleward migration is mainly evidenced by both the TC genesis and lifetime maximum intensity (LMI) positions, and there are inconsistent long-term trends between TC LMI and genesis latitudes, where the poleward trend of genesis is significantly larger than that of LMI (Figure 1).
Timeseries of the basin mean (a) genesis latitudes, (b) lifetime maximum intensity (LMI) latitude, and (c) genesis-to-LMI latitude difference. The dashed lines indicate the trend lines, with shaded areas denoting the 95% confidence intervals.
Academician Zhemin Tan’s team investigated inconsistent change between the LMI and genesis positions as well as its physical causes in terms of relative frequency changes in various TC tracks. The authors grouped TCs into 5 track types using cluster analysis (Figure 2). The relative distances between the mean genesis and LMI positions in each track are significantly different, especially in the latitudinal direction (Δφ). The change of this distance defines the consistency of TC genesis and LMI positions, and the distance is closely related to its track type. Based on this, the authors make the following mathematical decomposition of the mean genesis and LMI latitudes (see the original paper for the derivation and detailed description of the decomposition).
The tropical cyclone (TC) tracks (black curves) over the western North Pacific during 1980–2019 in each of the five typical tracks. The red lines denote the regressed mean curves of the TCs in each track. The green and yellow dots indicate the mean genesis and lifetime maximum intensity positions, respectively.
The trend of the genesis latitude and the genesis-to-LMI latitude distance (G2LD) are both contributed by five linear and five nonlinear terms based on Equation 2 and Equation 3. The linear term represents the relative frequency change of TCs in five tracks, and the nonlinear term represents the joint change in the relative frequency and mean position of TCs in each track (the product of the two). Equation 2 and Equation 3 thus directly give the quantitative contribution of each track to the poleward migration of typhoon activity. The results show that the NWL and RCE tracks together contributes the most to poleward migration of the WNP TC genesis latitude. NWL and RCE track exhibit relative low genesis latitudes and show significant decreasing trends of TCFRs. It means that the decrease of low-latitude formed TCs contribute much more to the poleward migration of TC genesis latitude than that of the increase of high-latitude formed TCs. Therefore, TC track change is the direct cause of the inconsistent poleward migration of typhoon activity.
The authors further analyze the physical context in which the above-mentioned typhoon tracks change. As the five basic categories are jointly determined by their genesis positions and track patterns, the authors focus on the analysis of the large-scale environmental fields associated with typhoon genesis and migration. The results show that the long-term cooling of the eastern Pacific Ocean and the long-term warming of the western Pacific Ocean (La Niña-type SST trend), as well as the eastward and northward retreat of the western Pacific subsurface are the main causes of the above typhoon track changes. However, these two factors may be intrinsically linked under the impacts of global warming and natural forcing, so further research is needed to fully clarify the underlying causes of typhoon poleward migration.
Journal of Geophysical Research: Atmosphere published a paper titled “Influence of track change on the inconsistent poleward migration of typhoon activity”. This work is jointly supported by the National Natural Science Foundation of China (42192555 and 42075003), and the National Key Research and Development Program of China under Grants 2017YFC1501601. Associate Professor Yi Peng Guo is the first author of this paper, and Academician Zhemin Tan is the corresponding author.
Guo, Y.-P., and Tan Z.-M., 2022: Influence of track change on the inconsistent poleward migration of typhoon activity. Journal of Geophysical Research: Atmospheres, 127, e2022JD036640. https://doi.org/10.1029/2022JD036640