Abstract
The average location of observed western North Pacific (WNP) tropical cyclones (TCs) has shifted north over the last several decades, but the cause remains not fully understood. Here we show that, for the annual average, the observed northward migration of WNP TCs is related to changes in TC seasonality, not to a northward migration in all seasons. Normally, peak-season (July–September) TCs form and travel further north than late-season (October–December) TCs. In recent decades, related to less frequent late-season TCs, seasonally higher-latitude TCs contribute relatively more to the annual-average location and seasonally lower-latitude TCs contribute less. We show that the change in TC seasonality is related to the different responses of late-season and peak-season TC occurrence to a stronger Pacific Walker Circulation. Our findings provide a perspective on long-term trends in TC activity, by decomposing the annual-average statistics into seasonal components, which could respond differently to anthropogenic forcing.
Highlights
The average location of observed western North Pacific (WNP) tropical cyclones (TCs) has shifted north over the last several decades, but the cause remains not fully understood
We first calculate linear trends of the annual-mean of TC latitudes using a variety of metrics: (1) all sixhourly points along tracks, tropical cyclogenesis, lifetime-maximum intensity (LMI) and cyclolysis; (2) the net transit distances of TC track and all-trackpoints, and the net transit distances of developing (LMI minus genesis) and dissipating lifecycle phases
It remains controversial how much of these observed changes are attributable to anthropogenic forcing, partially because in attribution studies contemporary general circulation model (GCM) disagree on the spatial pattern of Pacific sea surface temperatures (SSTs) response to anthropogenic forcing[24,26,32,33]
Summary
The average location of observed western North Pacific (WNP) tropical cyclones (TCs) has shifted north over the last several decades, but the cause remains not fully understood. In the WNP, the annual mean TC formation (cyclogenesis) location has shifted poleward[3] These migration trends are primarily based on satellite-era TC Best Track observations. For WNP TCs, climate models with anthropogenic forcing do not fully reproduce the observed poleward migration rate in the satellite era, with either no poleward migration or a much slower migration rate than observed[2,4,5,9] This suggests that GCMs might miss or underrepresent some processes that are crucial to the observed longterm trend in average TC location, provided that anthropogenic forcing is the cause. We show that this northward migration can largely be attributed to a significant change in TC seasonality, rather than to a seasonally uniform migration This indicates that the poleward migration of WNP TC tracks solely due to a seasonally uniform poleward shift of environmental conditions may be small, and may not be statistically identifiable during the modern observational period. We further show that the change in seasonality is associated with a stronger Pacific Walker Circulation, which has seasonally varying effects on WNP TC occurrence
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