Conclusions and Discussion

This study has revealed the strong ISO-TC in-phase relationship in the tropical western North Pacific during JJASO 2004. The repeated appearance of the ISO during JJASO resulted in the fluctuation of the EA monsoon trough and the Pacific anticyclone, which in turn modulated the TC activity and led to the spatial and temporal clustering of TCs. While TCs occurred in groups during the cyclonic phase of the ISO, the clustering of these strong TC vortices significantly increased the overall amplitude of positive vorticity during the cyclonic phase of the ISO. On the contrary, the overall amplitude of negative vorticity during the anticyclonic phase of the ISO remained unaffected, because of poorly organized TC activity in the anticy-clonic phase of the ISO. The ISV was therefore enlarged with the occurrence of TCs. This result reveals that a significant percentage (larger than 50%) of the observed ISV in the tropical western North Pacific was contributed by TCs along the clustered TC tracks. This large contribution, which has not been well recognized, was clearly present in the 2004 typhoon season and is likely to have occurred in other years as well. Hsu et al. (2008) reported similar results, based on a study using multiyear datasets. A series of 24-hour hindcast numerical experiments with various designs were performed to evaluate the effect of TC-like vortices on the simulated ISV. Results of both the regional model and the GCM indicated that the experiments with larger TC-like vortices produced a stronger ISV and better simulations in terms of both spatial distribution and magnitude. This finding is consistent with the conclusion inferred from the empirical results.

The extraordinarily strong ISV and its coupling with TCs have been clearly demonstrated. This unusual phenomenon apparently led to the record-breaking number of typhoon landfalls in Japan. However, what mechanism led to these unusual features (e.g. the largest ISV in 54 years, the strong coupling between ISO and TC, the location of strong ISV) is still unknown. One may suspect that the warm SST might have an effect on the ISO and TC since it was an El Nino summer. On the other hand, the El NiƱo was a minor one and the SST anomaly was not particularly strong. It appears that there should be other unknown mechanisms that were responsible for the unusual phenomena in the western North Pacific during the summer of 2004.

An unconventional analysis procedure was carried out in this study to contrast the variance with and without TCs. The estimated variance difference is not likely to be the real contribution from TCs, but can probably be viewed as a quick estimation of the TCs' contribution to the ISV. The results shown above indicate that the contribution of TCs has to be taken into account to correctly estimate and interpret the ISV, especially during those years when TCs are strongly clustered by the ISO. Traditional wisdom usually assumes that the fluctuations on a shorter-time scale can be removed by time averaging or low-pass filtering. This may be true when the fluctuations in the positive and the negative phase are symmetric in both amplitude and recurrent frequency. Such a practice may be problematic for the intraseasonal variability analysis in the tropical western North Pacific, where TCs are the strongest cyclonic vortices and may not be canceled out by much weaker anticyclonic vortices. It is likely that the occurrence of TCs leaves footprints in the intrasea-sonal variability through the clustering effect.

The procedure adopted in this study has been used by the TC simulation and prediction community to remove the inadequate representation of TCs in the global analysis and plant an idealized vortex to represent TCs in the model. On the other hand, it is also true that TC circulation, although underestimated and sometimes unrealistic, does exist in the global analysis. It is this TC-like component that this study would like to evaluate its possible contribution to the large-scale climate variability, such as those on the intraseasonal time scale. Although the TC circulation is likely underestimated in the global analysis, the results presented above indicate that this inadequately represented TC component already contributes a significant amount of intraseasonal variability. If the TCs are accurately resolved in the global analysis, its contribution will likely be even more significant. This information will be valuable for the intrasea-sonal variability study, and perhaps even for the climate variability study on other time scales, as shown by Hsu et al. (2008).

In the tropical western North Pacific, the tropical cyclone often has a life span longer than

10 days. As the moving tropical cyclones have a strong and long-lasting energy source, large-scale circulations are likely to be induced, followed by energy emanation to remote regions. This feedback may affect the environmental flow, such as the subtropical anticyclone and monsoon trough, and leave notable footprints in the intraseasonal variability in the regions away from TC tracks. This potential effect cannot be estimated by the methodology adopted in the present study and will be explored in future works. Another problem is the coarse resolution of the datasets used in this study. Although the TCs are reasonably represented in the global analysis in the qualitative sense, the strength of the TCs is underestimated and the spatial structure is not as sharp as in the real world. As a result, the actual contribution of TCs to the ISV cannot be accurately estimated. Moreover, the contribution reported in this study is probably underestimated. Despite not being able to exactly quantify the effect, this research explains in a qualitative sense why the contribution from TCs cannot be overlooked.

Most of the general circulation models used to simulate past climate suffers from the poor simulation of the climate variability in the tropical western North Pacific during the boreal summer (Wang et al., 2004). The results reported here imply that the inability to resolve and simulate TCs may be one of the key weaknesses of the GCM leading to poor simulation. Using the coarse-resolution GCM may lead to inaccurate climate simulation and prediction on the intraseasonal, interannual, and perhaps even climate change time scales. High-resolution models that are able to reasonably simulate the ensemble effect of TCs, at least in the statistical sense, seem necessary for resolving the mul-tiscale interaction and producing better simulations of the ISV in TC-prone regions, such as the tropical western North Pacific. Although this study does not reveal any mechanism relating to the TC-ISO interaction (if there is one), it seems to imply the following. For numerical models that cannot explicitly simulate TCs in the tropical western North Pacific, the simulated ISV is probably underestimated. Whether an improved TC simulation would improve the ISO simulation in models is an interesting issue for further study. Another implication is that the clustering effect and the TC contribution to the ISV imply a possible two-way interaction between the TC and the ISO in the tropical western North Pacific. Well-designed numerical experiments and theory development are needed to quantify and understand this process.

Our finding also raises a question about the definition of the ISV in the TC-prone regions. Traditionally, the ISV is interpreted as the variability of intraseasonally filtered perturbations. Our results, however, point out that these intra-seasonal perturbations may contain the contribution from TCs, which fluctuate in much shorter periods and on a smaller spatial scale. In a region such as the tropical western North Pacific, where the mesosynoptic-scale and large-scale systems are closely intertwined, and the multiscale interaction is likely one of the key processes affecting the climate variability, the contribution from severe weather systems like TCs has to be taken into account in order to understand the mechanisms leading to the intra-seasonal (and climate) variability. The present results suggest that the TC and the ISO be viewed as an integrated system to improve our understanding of the intraseasonal variability in the tropical western North Pacific.

0 0

Post a comment