Water vapour transport by tropical cyclones over East Asia

By Liang Guo

When talking about tropical cyclones (TCs), people tend to think about gusty winds and heavy rain. These weather phenomena impress us due to the immense impacts on our surroundings. However, these weather phenomena are short-lived. Most TCs reduce to much weaker weather systems in several days after making landfall.

Therefore, when discussing water vapour transport (WVT) in a larger and longer context, the contribution of TCs is often neglected. Especially over East Asia (EA), where the summer monsoon (EASM) is strong and dominates the WVT. However, according to observations (i.e., International Best Track Archive for Climate Stewardship, IBTrACs), EA and the adjacent north-western Pacific is also the most active TC basin. Therefore, quantitatively measuring WVT by the EASM and TCs will help us understand TCs’ role in long-term large-scale WVT.

Figure 1 shows the mean seasonal cycle (averaged between 1979 and 2012) of WVT by the EASM and TCs via two boundaries which are defined within the figure. The first obvious difference is the peaking period. The WVT peak of TCs is in late summer and early autumn, which is later than that of the EASM. This is because TC genesis is sensitive to the sea surface temperature (SST), and SST is higher in late summer and early autumn. As shown in Figure 1, in most of the time, the WVT by EASM is about one order of magnitude larger than that of TCs. However, with the EASM retreating from EA landmass in August, the contribution from TCs is as important as the EASM, and is even larger in September and October.

Figure 1. Seasonal cycle of monthly mean vertically integrated moisture flux passing through the southern (blue) and eastern (red) boundaries. The mean-flow moisture fluxes are shown as solid lines (using the left-hand vertical axis) and TC eddy moisture fluxes as dashed lines (using the right-hand vertical axis). The inner panel shows the definition of the southern and eastern boundaries. Positive values indicate moisture is transported into the EA landmass, and negative values indicate moisture is transported out of the EA landmass. Note that the scale of the right-hand vertical axis is an order of magnitude smaller than the left-hand vertical axis.  Units: kg/s.

The second difference is the direction of WVT. The WVT by the EASM is imported via the southern boundary and is exported to the Japan and the Korean peninsula via the eastern boundary, while the WVT by TCs is in the reverse direction. The WVT import by TCs via the eastern boundary is mainly due to the westward-moving TCs, which bring warm and moist air from the Pacific. The WVT export by TCs via the southern boundary, on the other hand, is smaller and is linked to the northward moving TCs. The small magnitude is due to the air flow coming from land and is weaker and drier after the northward moving TCs making landfall.

An interesting detail of the WVT via the southern boundary is its sudden sign change in September, due to the reverse of the meridional gradient of the mean specific humidity over the EA landmass. During the EASM (June-August), the maximum of the mean specific humidity is located over south China (about 25°N, therefore, a southward gradient). However, with the retreat of the EASM in late August, the maximum of the mean specific humidity also rapidly retreats to near the equator (becomes a northward gradient).

As we can see, due to the difference in the season cycle compared to EASM, TCs’ role on WVT is non-negligible over EA.

Reference

Guo, L., Klingaman, N. P., Vidale, P., Turner, A. G., Demory, M.-E., & Cobb, A., 2017. Contribution of Tropical Cyclones to Atmospheric Moisture Transport and Rainfall over East Asia. Journal of Climate, DOI: 10.1175/JCLI-D-16-0308.1, in press.

 

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