Extreme Rainfall in Taiwan Henny, Lexi; Thorncroft, Chris D.; Hsu, Huang-Hsiung ...
Journal of climate,
06/2021, Letnik:
34, Številka:
12
Journal Article
Recenzirano
Taiwan regularly experiences precipitation extremes of hundreds of millimeters per day, especially between May and September. In this study, Taiwan’s extreme rainfall (ER) is analyzed over a 56-yr ...time period in different seasons and geographic regions, using a recently released, high-resolution gridded rainfall dataset. ER is defined using a seasonally and geographically varying 99th-percentile threshold to better resolve the characteristics of the most intense rainfall seen in different locations and times of year. The resulting monthly ER rates are largest in typhoon season and smallest in fall, winter, and spring. ER is spatially homogeneous in the mei-yu and typhoon seasons and concentrated in northern Taiwan during the rest of the year. A trend analysis revealed a positive trend in island-mean ER for the winter, spring, and typhoon seasons. In winter and spring, these trends are most pronounced in the north. In the mei-yu season, ER has increased most over the southwestern mountain slopes; in typhoon season, ER has increased consistently over much of Taiwan. These changes often exceed 1% yr−1. In many areas, typhoon season accounts for the largest fraction of the observed annual ER trend. TCs produce most of the observed typhoon season ER and ER trend, with nearly half of the typhoon season ER trend being associated with increases in TC frequency and duration around central and northern Taiwan. Certain regional changes in ER characteristics, particularly in areas with low sample size or complex seasonal contributions, merit further investigation in future work.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Unprecedented atmospheric circulations with extreme weather were observed in the extratropical Northern Hemisphere during the winter of 2013–2014. The anomalous circulations were the manifestation of ...the Pacific pattern or the North Pacific Oscillation/Western Pacific pattern but with extremely large amplitude. Simulation results suggest that the anomalous atmospheric circulations were constructively induced by anomalous sea surface temperature in the tropical Pacific and extratropical North Pacific, as well as the low sea ice concentration in the Arctic. Natural variability played a major role in inducing the anomaly pattern, whereas the anomalously warm sea surface temperature and low Arctic sea ice concentration in the Bering Sea contributed to the intensity. If the anthropogenic warming has a significant impact on causing the synchronization of the aforementioned anomalies in sea surface temperature and sea ice concentration and this trend continues, severe winters similar to that in 2013–2014 may occur more frequently in the future.
Key Points
Observed anomaly were the manifestation of a known teleconnection
Perturbations were forced by the Pacific SST and Arctic sea ice
Natural variability and anthropogenic warming both contributed to the event
The ocean–atmosphere coupling in the northeastern subtropical Pacific is dominated by a Pacific meridional mode (PMM), which spans between the extratropical and tropical Pacific and plays an ...important role in connecting extratropical climate variability to the occurrence of El Niño. Analyses of observational data and numerical model experiments were conducted to demonstrate that the PMM (and the subtropical Pacific coupling) experienced a rapid strengthening in the early 1990s and that this strengthening is related to an intensification of the subtropical Pacific high caused by a phase change of the Atlantic multidecadal oscillation (AMO). This PMM strengthening favored the development of more central Pacific (CP)-type El Niño events. The recent shift from more conventional eastern Pacific (EP) to more CP-type El Niño events can thus be at least partly understood as a Pacific Ocean response to a phase change in the AMO.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The western North Pacific subtropical high (WNPSH) in boreal summer shows a remarkable enhancement after the early 1980s. Whereas the sea surface temperature (SST) in the North Indian Ocean (NIO) and ...the equatorial eastern Pacific had been noted to have remarkable local or remote effects on enhancing the WNPSH, the influence of the Atlantic SST, so far, is hardly explored. This article reports a new finding: enhanced relationship between the tropical Atlantic (TA)‐SST and the WNPSH after the early 1980s. Regression study suggests that the warm TA‐SST produced a zonally overturning circulation anomaly, with descending over the equatorial central Pacific and ascending over the tropical Atlantic/eastern Pacific. The anomalous descending over the equatorial central Pacific likely induced low‐level anticyclonic anomaly to the west and therefore enhanced the WNPSH. One implication of this new finding is for predictability. The well‐known “spring predictability barrier” (i.e., the influence of El Niño–Southern Oscillation (ENSO) falls dramatically during boreal spring) does not apply to the TA‐SST/WNPSH relationship. The TA‐SST shows consistently high correlation starting from boreal spring when the ENSO influence continues declining. The TA‐SST extends the predictability of the WNPSH in boreal summer approximately one season earlier to boreal spring.
Key Points
The relationship between the TA‐SST and the WNPSH was enhanced after the early 1980sThe ENSO's impact on the TA‐SST has been weakening since the early 1980sThe warm TA‐SST may improve the predictability of the boreal summertime WNPSH
Changes in extreme precipitation in Taiwan's Mei‐yu season Henny, Lexi; Thorncroft, Chris D.; Hsu, Huang‐Hsiung ...
Quarterly journal of the Royal Meteorological Society,
July 2023 Part A, 2023-07-00, 20230701, Letnik:
149, Številka:
754
Journal Article
Recenzirano
Odprti dostop
High‐resolution Taiwan Climate Change Projection Information and Adaptation Knowledge Platform (TCCIP) gridded precipitation data are used to characterize days in the Mei‐yu season with the most ...extreme precipitation (EP). These “EP days” are grouped into weather types based on the presence of features such as tropical cyclones (TCs) and atmospheric rivers (ARs), then analyzed from the perspective of weather type frequency and synoptic changes. During the 1979–2019 period, EP days associated with ARs were associated with significant increasing trends in season‐total precipitation. These AR‐related precipitation increases are due to four events in 2005, 2006, 2012, and 2017 which had long duration and unusually intense precipitation, and which were anomalous even within the longer 1960–2019 time period. Meanwhile, TC‐related EP days contribute less precipitation than they did in the 1980s due to decreased frequency of TCs on EP days and in the Mei‐yu season climatology. Over the 1979–2019 period, the AR‐related and TC‐related trends combine to produce EP increases in western Taiwan and decreases in eastern Taiwan. Mei‐yu season southwesterly integrated vapor transport (IVT), wind speed, and specific humidity have all increased in association with these extreme events. Low‐level winds appear to the primary factor influencing the IVT increase, with increased moisture also contributing. The wind trends are consistent with climatological pressure increases south of Taiwan and decreases over the East Asian landmass, which facilitate a strengthened circulation in a corridor extending from the southern China coastline over Taiwan during this season.
In the Mei‐yu season, Taiwan experiences an abrupt onset of heavy precipitation associated with intense atmospheric river‐like moisture fluxes such as those represented in this composite. During 2005–2019, events of this type were more extreme than in 1979–2004 (or 1960–2004), with larger integrated vapor transport (IVT), stronger southwesterly winds, and a warmer upstream environment. These changes are also seen in season mean values. Higher precipitation totals and intensified moist flow are associated with four unusually extreme events occurring in 2005, 2006, 2012, and 2017.
An effective way of investigating the effects of tropical cyclones (TCs) on different spatial/temporal-scale environmental fields is to contrast the original circulation with the circulation from ...which the TCs have been removed. Although dynamical balance is required for analyzing TC contributions, the dynamic balance of TC-removed fields obtained by the existing TC removal method (which is widely used in the TC bogus procedure) is often ignored. In this paper, a TC removal method incorporating the potential vorticity (PV) inversion technique is proposed and its application to climate study is demonstrated. This method objectively detects the TCs’ positive PV disturbance, which is strong with a deep structure and overwhelmingly dominates the relatively weak and thin negative PV disturbance. The TC-removed field is well-balanced due to the dynamic balance consideration in the PV inversion framework. This approach isolates TC vortices, which are stronger and have a wider range of impacts compared with the TC components derived by the existing removal methods. The TC-removed fields obtained by the existing and the proposed methods are profoundly different, especially in dynamic balance. The TC contribution to intraseasonal variance and seasonal mean circulation in the tropical western North Pacific is examined. The existence of TCs enhances the amplitude and propagation of intraseasonal oscillation and strengthens the seasonal mean circulations such as the low-level monsoon trough and upper-level anticyclone in the region. Whereas the existing and the proposed TC removal techniques yield consistent results, the proposed technique yields larger TC contributions to the seasonal-mean circulation and the amplitude and northward propagation tendency of intraseasonal oscillation.
The monsoon trough and subtropical high have long been acknowledged to exert a substantial modulating effect on the genesis and development of tropical cyclones (TCs) in the western North Pacific ...(WNP). However, the potential upscaling effect of TCs on large-scale circulation remains poorly understood. This study revealed the considerable contributions of TCs to the climate mean state and variability in the WNP between 1958 and 2019, characterized by a strengthened monsoon trough and weakened subtropical anticyclonic circulation in the lower troposphere, enhanced anticyclonic circulation in the upper troposphere, and warming throughout the troposphere. TCs constituted distinct footprints in the long-term mean states of the WNP summer monsoon, and their contributions increased intraseasonal and interannual variance by 50%–70%. The interdecadal variations and long-term trends in intraseasonal variance were mainly due to the year-to-year fluctuations in TC activity. The size of TC footprints was positively correlated with the magnitude of TC activity. Our findings suggest that the full understanding of climate variability and changes cannot be achieved simply on the basis of low-frequency, large-scale circulations. Rather, TCs must be regarded as a crucial component in the climate system, and their interactions with large-scale circulations require thorough exploration. The long-term dataset created in this study provides an opportunity to study the interaction between TCs and TC-free large-scale circulations to advance our understanding of climate variability in the WNP. Our findings also indicate that realistic climate projections must involve the accurate simulations of TCs.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This study evaluates the performance of the Coupled Model Intercomparison Project Phases 6 models (CMIP6) in simulating the seasonal evolution and extreme precipitation indices in the western North ...Pacific and East Asia region (WNP-EA), and compare the results with those from CMIP Phases 5 (CMIP5). In the ensemble of CMIP6 models, the seasonal evolution simulation demonstrates improvements in seasonal northward migration of the rain band from spring to summer and more intense precipitation, resulting in a higher skill score in the CMIP6 ensemble than the CMIP5 one. In general, the skill scores for the spatial pattern of simple daily intensity (SDII), total rainfall occurrence (Totfq), and consecutive dry days (CDD) are higher than those of extreme precipitation intensity (R99p). The CMIP6 ensemble mostly gains higher skill scores for extreme precipitation indices in the wet season. However, the improvement is limited in extreme indices during spring and fall. The probability distributions for maximum 1-day precipitation (RX1day), maximum 5-day precipitation (RX5day), and R99p in the CMIP6 models demonstrate a more realistic shape and stronger intensity, indicating the improvement over the CMIP5 models. However, the biased distributions of the overestimated (underestimated) occurrence for lighter (heavier) SDII and shorter (longer) duration CDD cases remain as a problem in CMIP6 model simulations. The higher skill scores for the spatial pattern of SDII, Totfq, and CDD are likely due to the compensation between the biased distributions mentioned above, implying further improvements are needed for correcting the deficiency in simulating the precipitation occurrence.
Change in extreme events in climate projections is a major concern. If the frequency of dry events is expected to increase in a warmer climate (thus, the overall number of wet days will decrease), ...heavy and extreme precipitation are also expected to increase because of a shift of the precipitation spectrum. However, the forecasts exhibit numerous uncertainties.
This study focuses on the Asian region, separated into the following three subregions: the East Asian region, the Indian region, and western North Pacific region, where the summer monsoon can bring heavy rainfall. Particularly emphasized herein is the reliability of the projection, using data from a large ensemble of 30 models from phase 5 of the Coupled Model Intercomparison Project. The scattering of the ensemble enables obtaining an optimal estimate of the uncertainties, and it is used to compute the correlation between projected changes of extreme events and circulation changes.
The results show clear spatial and temporal variations in the confidence of changes, with results being more reliable during the wet season (i.e., the summer monsoon). The ensemble predicts changes in atmospheric circulation with favorable confidence, especially in the low-level moisture flux convergence (MFC). However, the correlation between this mean change and the modification of extreme events is nonsignificant. Also analyzed herein are the correlation and change of MFC exclusively during these events. The horizontal MFC exerts a nonnegligible influence on the change in the intensity of extremes. However, it is mostly the change in vertical circulation and moisture advection that is correlated with the change in frequency and intensity of extreme events.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
The Northeast Pacific (NEP) had two record-breaking marine heatwave events in the winters of 2013–2015 and summer of 2019, which had a detrimental impact on the fisheries, marine ecosystems, ...and climate in North America. Here, we investigated the cause of sea surface temperature (SST) variability in NEP during late spring–summer of 1981–2020. The regression circulation anomalies to the principal component of leading empirical orthogonal function mode suggested that the warm NEP SST were characterized by a cyclonic circulation anomaly in the midlatitude North Pacific and a warming SST center in the Gulf of Alaska. We noted that this cyclonic circulation anomaly, attributable to a barotropic atmospheric wave originating from the tropical central Pacific (CP) in the preceding spring, reduced the surface heat flux loss from the ocean to the atmosphere in the NEP and led to the warm SST anomalies in summer. This finding was confirmed by not only empirical diagnosis but also long-term numerical simulations forced by the observed SST perturbations in the tropical CP. Our results highlight the role of the tropical CP SST in driving the summertime North Pacific SST variability through the atmospheric bridge in recent decades.
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