研究計畫

氣候變遷研究聯盟總計劃下共有三個子計畫

    1. Asian Monsoon
        • The onset of the East Asian summer monsoon is closely associated withthe first transition of the Asian summer monsoon, which was explainedby the interaction between the large-scale circulation, convection, andlower boundary forcing. During the sharp-onset years of the East Asiansummer monsoon, the first transition is characterized by strongeastward-propagating intraseasonal oscillation from Africa to the SouthChina Sea. After the onset, the rainfall in Taiwan increases dramatically.In addition, the Mei-Yu seasonal rainfall tends to above normal duringthese years. (Hung and Hsu, 2008, Journal of Climate)

        • Interannual variability of the East Asian summer rainfall is characterizedby a zonally-elongated, sandwich-like pattern. This pattern is stronglymodulated by the heating over the Tibetan Plateau. (Hsu and Liu, 2003,Geophysical Research Letter) Our recent study also indicates that thewet phase of the pattern is influenced mostly by perturbation originatingfrom the tropics, while the dry phase is influenced mostly byperturbation from the extratropics. (Hsu and Lin, 2007, Journal of Climate)

        • A climate regime shift of the East Asian summer climate in the early1950s is identified. It was characterized by the warming in Taiwan, theSouth China Sea, south Japan, and the Philippine Sea, and the cooling inChina, Korea, and northern Japan. It is proposed that this change was3affected with the evolution of the Pacific Decadal Oscillation from themid-1940s to 1950s. (Lo and Hsu, 2008, Climate Dynamics)
        • Our recent studies on the factors affecting cold surge in Taiwan clearly
        • identify the effect of upstream perturbation originated in the Atlantic,Europe and Sahara. The perturbation propagates through the Eurasianjet stream, which acts as a wave guide, to result in the farthersouthward-penetrating cold surge to affect Taiwan. This effect is bothseen in interannual and decadal time scale. The decadal fluctuation ofthe cold surge number in Taiwan is correlated with the decadalfluctuation of the North Atlantic Oscillation. Papers will soon appear inTAO and GRL (Hong et al. 2008a, b).

        • Recent study revealed the teleconnection between the 2010 Europeanblocking (Russian heat wave) and the Pakistan flood. A rarecombination of extratropical disturbances, due to the Rossby waveenergy dispsersion from blocking region) and the monsoon surgestriggered by Madden-Julian Oscillation. (Hong et al., to appear in GRL)

 

    1. Intraseasonal variability
        • Intraseasonal oscillation is a tropical phenomenon that has great impacton the climate in both tropics and extratropics at sub-seasonal timescales. Its importance is comparable to the El Niño/Southern Oscillation.Hsu and collaborators proposed mechanisms to explain the propagationof the intraseasonal oscillation in the Philippine Sea, South Asia, andalong the equator (Hsu and Weng 2001, Hsu et al. 2004, Hsu and Lee 2005; all in Journal of Climate). These three studies contributedsignificantly to the understanding of the intraseasonal oscillationpropagation mechanisms. Because of these contributions, H.-H. Hsuwas invited to write a review article about the intraseasonal variabilityin East Asia for a monograph on the intraseasonal variability (Hsu 2004,2011). The tropical intraseasonal oscillation is strongly affected by thetopography and land-sea contrast. Our recent studies (Hsu and Lee 2005,Wu and Hsu 2009, both in Journal of Climate) have revealed theseunique characteristics, which has been largely neglected.

 

    1. TC, climate variability, and multiscale interaction in the Western NorthPacific
      • It has been know for a long time that the interannual and intraseasonalvariation in the large-scale circulation can modulate the genesis and4track of tropical cyclone. On the other hand, the potential feedback oftropical cyclone to climate variability has been hardly explored. Ourrecent study reveal the TC’s contribution to the climate variability canbe as large as 40-60 percents in time scales ranging from intraseasonalto interannual. This study indicates the necessity to re-define thedefinition of ‘climate variability’. It also suggests that the inability toresolve and simulate tropical cyclone may be one of the weaknesses ofthe GCM leading to this poor simulation of the precipitation in the western North Pacific. Using this type of coarse-resolution GCM canlead to inaccurate prediction/projection of future climate in bothinterannual and climate change time scales. (Hsu et al., 2008a, J.Climate, Hsu et al. 2008b in the NTU/AS 50th Anniversary bookpublished by the World Scientific) .

 

      • A sub-monthly fluctuation in the Western North Pacific during Julyand August was identified. Tropical cyclones often embed in thecyclonic circulation of this pattern, which originated in the easternPhilippine Sea, and propagates northwestward toward the East ChinaSea with the pattern. It is suggested that this type of TC is part of thelarge-scale wave pattern, and the TC cannot be treated as an isolatedstrong vortex as we often do. A further study finds that both thesub-monthly wave pattern and TC are modulated by intraseasonaloscillation in the western North Pacific. The sub-monthly wave and TCare more active in the westerly phase and less active in the easterly ofthe ISO. The background mean flow advection played a dominant roleleading to the pattern propagation. The eddies tended to grow wherethe zonal wind gradient was the largest and were oriented in aparticular direction to efficiently extract kinetic energy from thebackground flow. The results suggest that the TC/submonthly wavepattern occurring in the confluent zone between the monsoon troughand the anticyclonic ridge, where kinetic energy can be efficientlyextracted, and with a circulation circuit in the northeast-southwestorientation, were the patterns to have optimal growth and propagation.It is suggested that the amplitudes, shape and propagation of eddies inEast Asia and the Western North Pacific were strongly constrained bythe configuration of the monsoon trough and the anticyclonic ridge.This is likely the reason why the northwestward propagating andnortheast-southwest orienting TC/submonthly wave pattern wasfrequently observed in the Western North Pacific during the borealsummer. The TC feedback had a significant effect on the large-scalesubmonthly wave pattern. While TCs developed in a development favorable background flow provided by the submonthly wave pattern,they in turn enhanced the amplitudes of the vorticity and kinetic energyof the TC/submonthly wave pattern by as much as 50% and helpedextract significantly more (40%) energy from the background ISOcirculation. On the other hand, the TCs had little effect on thepropagation mechanism. This study revealed that TCs and large-scalecirculation in the Western North Pacific were mutually intertwinedthrough rigorous eddy-mean flow interaction and therefore should betreated as an integrated multiscale system. (Ko and Hsu, 2006, J. of Meteorological Society of Japan; Ko and Hsu, 2009, J. Climate; Koand Hsu 2011, submitted).



  1. Decadal-multidecadal variability
      • Widespread abrupt warming in the extratropical Northern Hemisphereoccurred in the late 1980s and was associated with the switch of thedominant decadal mode from the PDO-like pattern to the AO-likepattern. The AO-like pattern has had a dominant influence on theNorthern Hemisphere mean temperature since the late 1980s, while theinfluence of the PDO has weakened. The AO-like mode appears as anatural mode in the CMIP3/IPCC climate models. However, itsemergence in the late 1980s was not simulated by most models with orwithout the observed increasing greenhouse effect in the 20th century.(Lo and Hsu, ASL 2010)

      • Decadal to bi-decadal rainfall variation in the Western Pacific duringJuly-October in the second half of the 20th century was identified in this study.This 10-20-year quasi-periodic oscillation was found associated with theleading sea surface temperature (SST) pattern in the South Pacific, which iscalled the 10-20-yr South Pacific (inter) Decadal Oscillation (SPDO). It issuggested that the 10-20-year fluctuation of the SPDO resulted in significant10-20-year rainfall variation along the western Pacific coast. The anomalousdivergent circulations were likely driven by the SSTA (SST anomaly) andresulted in the anomalous rainfall in Eastern Australia and the MaritimeContinent. It is conjectured that the SSTA in the Western South Pacific led toan anomalous Hadley-like circulation in the Western Pacific and indirectlyaffected the convection activity in the Philippine Sea, which in turn impactedthe rainfall in the Philippines, Taiwan and Korea. (Hsu and Chen, 2011, GRL)

    • A multi-decadal teleconnection pattern of geopotential height in theupper troposphere of the extratropical Northern Hemisphere is6identified. The pattern is characterized by a zonal band of geopotentialheight between 35N and 65N, which extends from the North Atlanticto the west coast of North America. The anomaly is positive before1967 and after 1986, and negative in between. The existence of thispattern results in the multi-decadal fluctuation of zonally-averagedgeopotential height in the extratropical Northern Hemisphere and theEurasian jet stream. This pattern is independent from the globalwarming mode, but is in phase with the large warming trend since late1980’s. The pattern exhibits an equivalent barotropic vertical structurewith the largest amplitude in the upper troposphere, and is closelyassociated with the multi-decadal fluctuation of surface temperature inthe Eurasian continent. This pattern fluctuates coincidentally with theAtlantic Multi-decadal Oscillation and the Northern Hemisphere meansurface temperature since the mid 20 Century. Numerical simulationresults indicate that the pattern was likely an atmospheric response tothe AMO-like SSTA. (Hsu and Lee, in preparation)

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