Possible feedback of tropical cyclone on climate variability

It has been know for a long time that the interannual and intraseasonal variation in the large-scale circulation can modulate the genesis and track of tropical cyclone. On the other hand, the possible feedback of tropical cyclone to climate variability has been hardly explored. In this study, we remove tropical cyclones in the western North Pacific from the ERA40 data to assess the impact of this artificial process on the computed climate variability. The 850-hPa vortex associated with each tropical cyclone was subtracted, following the procedure proposed by Kurihara and Wu (200?), from the 850 hPa vorticity field for the June-October period from 1951-2002. Variances for the intraseasonal and interannual time scale were then computed for the original and TC-removed vorticity fields. The results indicate a 40-50 percent reduction in both intraseasonal and interannual variance after the4 removal of tropical cyclone. This large reduction can be explained as follows. While the low-frequency, large-scale circulation has clustering effect on tropical cyclones, the latter with a large positive vorticity, which tend to occur in the positive vorticity background flow, significantly enhance the total strength of the positive vorticity. Conversely, when tropical cyclone does not exit or when a synoptic anticyclone exists, the contribution to the total vorticity field from the synoptic- and meso-scale disturbances is relatively small. This contrast leads to the enlargement of the mean and variance of vorticity. This effect is seen in both intraseasonal and interannual variability in this study. In reality, the relationship between tropical cyclone and the large-scale circulation is nonlinear. Bearing this in mind, our study took an artificial approach to remove tropical cyclone from the circulation to demonstrate that even a simple arithmetic approach can significantly affect the computed variance. In real world, the tropical cyclone, though moving fast, often has a life time longer than 10 days. With a moving large and long-lived energy source like tropical cyclone, the large-scale circulation is likely induced and emanates energy to remote regions. This effect can leave strong footprint on the climate variability. Most of the general circulation models used to simulate past climate suffer from the poor simulation of the climate variability in the western North Pacific during boreal summer. Our study suggests that the inability to resolve and simulate tropical cyclone may be one of the weaknesses of the GCM leading to this poor simulation. Using this type of coarse-resolution GCM can lead to inaccurate prediction/projection of future climate in both interannual and climate change time scales.