Jien-Yi Tu

Evaluating the “Rich-Get-Richer” Mechanism in Tropical Precipitation Change under Global Warming

Abstract Examining tropical regional precipitation anomalies under global warming in 10 coupled global climate models, several mechanisms are consistently found. The tendency of rainfall to increase in convergence zones with large climatological precipitation and to decrease in subsidence regions—the rich-get-richer mechanism—has previously been examined in different approximations by Chou and Neelin, and Held and Soden. The effect of increased moisture transported by the mean circulation (the “direct moisture effect” or “thermodynamic component” in respective terminology) is relatively robust, while dynamic feedback is poorly understood and differs among models. The argument outlined states that the thermodynamic component should be a good approximation for large-scale averages; this is confirmed for averages across convection zones and descent regions, respectively. Within the convergence zones, however, dynamic feedback can substantially increase or decrease precipitation anomalies. Regions of negative...

The abrupt shift of typhoon activity in the vicinity of Taiwan and its association with western North Pacific-East Asian climate change.

Bayesian analysis is applied to detect changepoints in the time series of seasonal typhoon counts in the vicinity of Taiwan. An abrupt shift in the typhoon count series occurs in 2000. On average, 3.3 typhoons per year have been noted before 2000 (1970‐99), with the rate increasing to 5.7 typhoons per year since 2000 (2000‐06). This abrupt change is consistent with a northward shift of the typhoon track over the western North Pacific‐East Asian region and an increase of typhoon frequency over the Taiwan‐East China Sea region. The northward shift of the typhoon track tends to be associated with typhoon-enhancing environmental conditions over the western North Pacific, namely, the weakening of the western North Pacific subtropical high, the strengthening of the Asian summer monsoon trough, and the enhanced positive vorticity anomalies in the lower troposphere. Based on observational analysis and model simulations, warm sea surface temperature anomalies over the equatorial western and central Pacific appear to be a major factor contributing to a northward-shifted typhoon track.

Interannual Variability of the Western North Pacific Summer Monsoon: Differences between ENSO and Non-ENSO Years

Abstract The interannual variability of the western North Pacific (WNP) summer monsoon is examined for the non-ENSO, ENSO developing, and ENSO decaying years, respectively. The ENSO developing (decaying) year is defined as the year before (after) the mature phase of ENSO, and the non-ENSO year is defined as the year that is neither the ENSO developing year nor the ENSO decaying year. A strong (weak) WNP summer monsoon tends to occur during the El Nino (La Nina) developing year and a weak (strong) WNP summer monsoon tends to occur during the El Nino (La Nina) decaying year. In all non-ENSO, ENSO developing, and ENSO decaying years, the strong (weak) WNP summer monsoon is associated with the positive (negative) rainfall anomalies, cold (warm) sea surface temperature anomalies, warm (cold) upper-tropospheric temperature anomalies, low (high) surface pressure anomalies, and a low-level cyclonic (anticyclonic) circulation anomaly over the subtropical WNP. The 850-hPa wave train associated with the WNP and east...

El Niño impacts on precipitation in the western North Pacific-East Asian sector

Abstract In this study, the western North Pacific–East Asian (WNP–EA) rainfall anomaly induced by the strong El Nino in 1982–83, 1991–92, and 1997–98, and its association with the mean state, are examined. Over the northern part of the WNP–EA region (north of 20°N), which is dominated by southwest–northeast tilting frontal systems, positive rainfall anomalies from the fall before the El Nino peak phase (year 0) to the first wet period after the peak phase (year 1) are affected by low- and midlevel horizontal moisture convergence anomalies induced by low-level anticyclonic circulation anomalies over the WNP region that are associated with El Nino. Over the southern part of the WNP–EA region (south of 20°N), which is dominated by tropical convection, positive precipitation anomalies in the first and second wet periods of year 0 and negative precipitation anomalies from the fall of year 0 to the second wet period of year 1 are associated with the variation of the net energy into the atmosphere, which is main...

Regional tropical precipitation change mechanisms in ECHAM4/OPYC3 under global warming

Mechanisms of global warming impacts on regional tropical precipitation are examined in a coupled atmosphere–ocean general circulation model (ECHAM4/OPYC3). The pattern of the regional tropical precipitation changes, once established, tends to persist, growing in magnitude as greenhouse gases increase. The sulfate aerosol induces regional tropical precipitation anomalies similar to the greenhouse gases but with opposite sign, thus reducing the early signal. Evidence for two main mechanisms, the upped-ante and the anomalous gross moist stability (M) mechanisms (previously proposed in an intermediate complexity model), is found in this more comprehensive coupled general circulation model. Preferential moisture increase occurs in convection zones. The upped-ante mechanism signature of dry advection from nonconvective regions is found in tropical drought regions on the margins of convection zones. Here advection in both the atmospheric boundary layer and lower free troposphere are found to be important, with an additional contribution from horizontal temperature transport in some locations. The signature of the M mechanism—moisture convergence due to increased moisture in regions of large mean vertical motion— enhances precipitation within strong convective regions. Ocean dynamical feedbacks can be assessed by net surface flux, the main example being the El Nino–like shift of the equatorial Pacific convection zone. Cloud–radiative feedbacks are found to oppose precipitation anomalies over ocean regions.

Changes in precipitation frequency and intensity in the vicinity of Taiwan: typhoon versus non-typhoon events

The hourly rainfall at 21 ground stations in Taiwan is used to investigate changes in the frequency, intensity, and duration of rainfall, which can be divided into typhoon and non-typhoon rainfall, in the period of 1970‐2010. As a whole, the frequency of rainfall shows a decreasing trend for lighter rain and an increasing trend for heavier rain. Also, the typhoon rainfall shows a significant increase for all intensities, while the non-typhoon rainfall exhibits a general trend of decreasing, particularly for lighter rain. In rainfall intensity, both typhoon and non-typhoon rainfall extremes become more intense, with an increased rate much greater than the Clausius‐Clapeyron thermal scaling. Moreover, rainfall extremes associated with typhoons have tended to affect Taiwan rainfall for longer in recent decades. The more frequent, intense and long-lasting typhoon rainfall is mainly induced by the slower translation speed of the typhoons over the neighborhood of Taiwan, which could be associated with a weakening of steering flow in the western North Pacific and the northern South China Sea.

An abrupt increase of intense typhoons over the western North Pacific in early summer

The frequency and intensity of typhoons have been a focus in studying typhoon-related climate changes. In this study, we focus on a seasonal cycle of intense typhoons (category 4 and 5) over the western North Pacific, particularly changes in the number of intense typhoons in early summer. In general, 81% of intense typhoons occur in July–November (JASON), with maxima in September and October. Our analysis shows that intense typhoons have tended to occur more frequently in May since the year 2000. Before 2000, intense typhoons seldom occurred in May, with a frequency of around once per decade. After 2000, however, the frequency of intense typhoons has become much higher in May—almost once per year. We have also examined changes in the large-scale environment in the past few decades. The results show that the large-scale environment did become more favorable for intense typhoons in the 2000s, which is consistent with a larger tropical cyclone genesis index. The changes include warmer sea surface temperature, higher sea surface height, larger upper-ocean heat content, weaker vertical wind shear, increased tropospheric water vapor, and greater water vapor in the mid-troposphere. The last two might be more important than the others.

Annual Cycle of Rainfall in the Western North Pacific and East Asian Sector

The annual cycle of precipitation over the western North Pacific and East Asian (WNP‐EA) sector has five major periods: spring, the first and second wet periods, fall, and winter. In this study, processes that induce precipitation in each period are examined from a large-scale point of view. The wet phase over this sector has two distinctperiods,whicharedominatedbytheAsiansummer monsooncirculation inducedbytheland‐ocean contrast of net energy into the atmospheric column (F net ). In the first wet period, the pre-mei-yu/mei-yu rainband is directly associated with a moisture flux convergence caused by the southwesterly Asian summer monsoon flow and the southeasterly trade winds, and indirectly associated with a dynamic feedback induced by this horizontal moisture convergence. The tropical convection, in the meantime, is associated with a rising motion that is induced by positive F net . In the second wet period, the WNP summer monsoon gyre dominates the rainfall of this region, which is partially associated with warmer local sea surface temperature (SST) via positiveF net . The land‐sea contrast of F net and the atmosphere‐ocean interaction alsoplayanimportant role in establishing the monsoon gyre. The dry phase over the WNP‐EA region is the winter period in which precipitation is associated with winter storm activities and large-scale lifting associated with a pressure surge. In the two transition phases, due to a difference in heat capacity, the atmosphere and ocean have distinct impacts on precipitation, albeit similar solar insolations during the two periods. In the spring period, the atmospheric condition is favorable for convection, while the ocean surface is relatively colder, so the horizontal moisture advection associated with the westward extent of the Pacific subtropical high, which is different from a typical winter frontal system, is a major source for the spring rain. In the fall period, however, the atmospheric conditions dominated by the Asian winter monsoon circulation suppress convection, while relatively warmer SSTstill maintainstropical convectionover thesouthern partoftheWNP‐EA region.Over thenorthernpartof the WNP‐EA region, the fall precipitation is associated with frontal systems, similar to those in winter.

Evaluating real-time air-quality data as earthquake indicator.

A catastrophic earthquake, namely the 921-earthquake, occurred with a magnitude of M(L)=7.3 in Taiwan on September 21, 1999, causing severe disaster. The evaluation of real-time air-quality data, obtained by the Taiwan Environmental Protection Administration (EPA), revealed a staggering increase in ambient SO(2) concentrations by more than one order of magnitude across the island several hours prior to the earthquake, particularly at background stations. The abrupt increase in SO(2) concentrations likely resulted from seismic-triggered degassing instead of air pollution. An additional case of a large earthquake (M(L)=6.8), occurring on March 31, 2002, was examined to confirm our observations of significantly enhanced SO(2) concentrations in ambient air prior to large earthquakes. The coincidence between large earthquakes and increases in trace gases during the pre-quake period (several hours) indicates the potential of employing air-quality monitoring data to forecast catastrophic earthquakes.

Possible Impacts of Global Warming on Typhoon Activity in the Vicinity of Taiwan

Typhoons are one of the most extreme natural events over the western North Pacific–East Asian (WNP–EA sector). Typhoons often affect the spatial distribution of regional precipitation in summer since they are a major source of rainfall over this region. For example, in 2004, 10 typhoons occurred in Japan and brought more than usual precipitation, causing widespread damage, whereas drought occurred in the Philippines and southern China (Kim et al., 2005; Levinson et al., 2005; Wu et al., 2005). Typhoon-related climate studies often focus on the variation of typhoon intensity, frequency, and track in multitemporal scales ranging from intraseasonal to interdecadal (Chan, 1985, 2000; Chia and Ropelewski, 2002; Chu, 2004; Ho et al., 2006; Matsuura et al., 2003; Wang and Chan, 2002). In recent years, the influence of global warming on the intensity of tropical cyclones has received much attention. Emanuel (2005), Hoyos et al. (2006), and Webster et al. (2005) found increasing trends in the western Pacific and Atlantic based on some available besttrack datasets. Chan and Liu (2004) and Klotzbach (2006) found small or no trends using alternate analysis techniques. Other studies (e.g., Landsea et al., 2006) have shown opposite trends to those found by Emanuel in the west Pacific by examining other best-track datasets. Besides observations, model simulations also show that intense tropical cyclones will be more frequent in the future warmer climate, while the total number of tropical cyclones tends to decrease (Bender et al., 2010; Emanuel et al., 2008; Zhao et al., 2009). In addition to the increase in tropical cyclone intensity, one study also found a detectable shift of the typhoon track over the WNP–EA in the past four decades (Wu et al., 2005). Such a change also affects regional precipitation (Ren et al., 2006). For a future climate projection, the typhoon track over the WNP–EA region may potentially be affected by global warming (Wu and Wang, 2004). In our study, we also focus on the variation of typhoon tracks, 5