A good model ensemble simulates a suitable forcing–response mechanism for the WNP and East Asia, which is mainly due to the synoptic-scale atmospheric conditions and the minor effect of local SST. However, the predicted rainfall intensity and spatial pattern were different in CMIP5 good models and CMIP5 poor models. The simulated results were obtained by multi-model ensemble simulation of 20 CMIP3 models and 44 CMIP5 models, respectively.
Time series of precipitation anomaly (mm·day-1) and SST anomaly (oC) over 20-30oN, 120-140oE in (a) observed strong EASM years, (b) observed weak EASM years, (c) good model ensemble of strong EASM years, (d) good sample composition of weak EASM years, (e) poor sample composition of Strong EASM years, and (f) poor sample composition of weak EASM years.
Introduction
One reason for the failure is the neglect of air-sea interaction in the warm Indo-Pacific oceans (Gadgil and Sajani, 1998). For the boreal summer intraseasonal oscillation (BSISO) and the climatological intraseasonal oscillation (CISO) of the mid-Asian summer monsoon, the coupled air–sea system provides a more realistic simulation than the atmosphere-only approach (Fu et al., 2002; Fu and Wang., 2004). Nevertheless, studies are still needed on the comparison of CMIP5 models with CMIP3, which represents the air–sea correlation in the WNP.
In section 3, the study on the relationship between the WNP air–sea interaction and the prediction of the East Asian summer monsoon is carried out.
Relationship between Precipitation and Sea Surface Temperature in the Western North Pacific
The ASI reproduced by GCMs participating in CMIP3 and CMIP5 are analyzed in this section. This negative ASI then turns positive in the central Pacific above 160oE (not shown). To compare the performance of 20 CMIP3 global models and 44 CMIP5 global models for simulating WNP air-sea interaction, the produced ASI of each model is calculated and then compared with the observation (Figure 2.1(b), (c)).
There is no overall negative API in the WNP, but regions with positive API greater than 0.1 are represented. However, it is not enough to prove actual changes or improvements between models, because the average value of all models can cancel the result of each model. First, in both CMIP3 and CMIP5 cases, the set of bad models shows that precipitation tends to increase as SST increases (positive ASI) in most areas in the WNP (Figure 2.3(a), (b)).
The difference between the bad ensemble of CMIP3 and the bad ensemble of CMIP5 is reduced by the positive ASI in the SCS. On the contrary, the good model ensemble of both CMIP3 and CMIP5 exhibits negative ASI much more clearly than a bad model result (Figure 2.3(c), (d)). Focusing on the core area, precipitation rate and SST are inversely correlated over the WNP.
ASI between May–August SST and precipitation produced by multi-model ensemble of a) CMIP3 bad models, b) CMIP5 bad models, c) CMIP3 good models, and d) CMIP5 good models. In this context, it is determined that the models participating in CMIP5 are developed compared to the CMIP3 models in terms of simulating the WNP overall air-sea interaction.
Relationship between the WNP Air-Sea Interaction and the East Asian Summer Monsoon Prediction
The intensity of WNPSM in the bad model ensemble is weaker than the observation, but it is comparable. In weak EASM years, weakened EASM precipitation is prominently captured in the good model ensemble. Both biases for strong EASM years and weak EASM years are smaller in the good model ensemble than those in the bad model ensemble.
In strong EASM years, compared to the climatology, the low southwesterly over East Asia is stronger, and the WNPSH spreads further westward. On the other hand, the poor model ensemble reproduces the distorted synoptic-scale atmospheric conditions in both strong EASM and weak EASM years. A clear WNPSH, which is supposed to be located over East Asia and the WNP, is not shown in strong EASM years.
The SST anomaly and the precipitation anomaly of a good model ensemble are inversely related to the ASI of -0.41 and -0.55 in the strong EASM years and the weak EASM years, respectively. With good model simulation, the overall strong negative ASI around the core region becomes weaker in the early future compared to the historical period. The ASI is larger in the early future and becomes smaller in the late future due to the poor model ensemble.
In the early future, the simulation of the SST of a good model ensemble increases mainly with increasing precipitation. However, the rate of change in precipitation becomes smaller in the late future than in the early future. In good model simulation, precipitation increases by 3.13% in the early future and by 10.90% in the late future compared to the precipitation in the historical period.
In poor model simulation, precipitation increases by 6.69% in the early future and 17.20% in the late future.
Summary and Conclusion
In both good and bad models, precipitation and SST increase in the early future, and in larger amounts in the late future. However, the different rates of precipitation increase and SST are associated with different aspects of ASI change in the early and late future during the WNP. Good model simulation in the early future, the overall negative ASI becomes weaker, because the precipitation tends to increase sharply in the early future.
However, the ASI becomes stronger in the late future of the good simulation as the rate of increase in precipitation is smaller than in the early future. The poor model simulation simulated positive ASI over the WNP consistently, but with varying strength according to the rate of change of precipitation and SST. The positive ASI becomes stronger in the early future, as precipitation is increasing rapidly with increasing SST, but the signal becomes weaker in the late future, because the increase in precipitation is weaker compared to that in the future early.
GCMs with different performances simulating the WNP-air-sea interaction produce precipitation with different characteristics. Both in the early future and in the late future, the total precipitation in the poor model simulation is greater than that in the good model simulation. The study evaluated the simulated performance of CMIP5 models for the WNP air-sea interaction and identified elements of global climate models that need to be developed to predict the EASM.
This can contribute to the understanding of WNP atmospheric phenomena using CMIP5 models and can help direct performance comparisons between models. The underlying causes of the differences in the air-sea interaction simulation over the WNP with the CMIP5 models are not addressed in this research.
Acknowledgement
연구자로서의 첫 걸음인 석사과정을 마치는 지금, 그동안 저를 응원해주시고, 모든 과정에서 도움을 주신 많은 분들께 깊은 감사의 말씀을 전하고 싶습니다. 학부 3학년부터 석사학위를 취득할 때까지 전공 변경에 애를 먹던 5년의 긴 시간 동안 늘 믿음직스럽게 지도해주신 차동현 교수님께 진심으로 감사의 말씀을 전하고 싶습니다. 교수님께서 저에게 쏟으신 시간이 헛되지 않도록 앞으로도 더 나은 사람이 되도록 열심히 노력하겠습니다.
논문을 위해 많은 지도와 시간을 내어주신 이명인 교수님, 송창근 교수님 감사드립니다. 바쁜 와중에도 제 연구를 발전시킬 수 있도록 도와주신 덕분에 저는 연구를 수행하는 방법과 연구자로서 앞으로 나아가는 자세를 배울 수 있었습니다. 그리고 오랜 시간 함께해주신 기상재난연구소 식구들에게도 깊은 감사를 드립니다.
늘 넘치는 에너지로 격려해주시고, 제가 질문할 때마다 반갑게 맞아주시고 조언도 해주셔서 정말 큰 도움이 되었습니다. 항상 힘들게 해서 미안하지만 표현도 제대로 못하고 내 의지에만 의지하는 것 같다. 연구실 초창기부터 창립까지 함께 애써주신 선배님들, 하나뿐인 동기들, 현재 저보다 훨씬 더 잘하고 있는 후배들에게 진심으로 감사의 말씀을 전하고 싶습니다.
우리 재난기상연구소는 우리 연구원들의 노력과 열정 덕분에 앞으로도 더욱 탄탄하게 성장할 것이라고 믿습니다. 이 짧은 글을 통해 제 마음이 잘 전달되었으면 좋겠습니다.
