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Yi-Peng Guo
,
Zhe-Min Tan
, and
Xu Chen

Abstract

Tropical cyclone (TC) translation speed (TCS) over the western North Pacific (WNP) has experienced a long-term decreasing trend. To date, however, little is known about the multidecadal variability of TCS and its possible influence on this trend. This study investigated the multidecadal variability of the WNP TCS and the underlying physical mechanisms. Results show that the WNP TCS presents robust multidecadal variability during the past seven decades, which is dominated by the TCS over the extratropics. Further analysis shows that the Atlantic multidecadal oscillation (AMO) is responsible for the TCS multidecadal variability. AMO positive (negative) phases lead to favorable (unfavorable) large-scale environmental conditions for maintaining TCs over the extratropics, which results in longer (shorter) residence time for TCs having been accelerated by the midlatitude westerlies, thus, leading to higher (lower) TCS. The TCS phase shift strongly offsets its slowdown trend, leading to the inconsistent trends during past decades. This inconsistency may also relate to the influence of extratropical transitioned cyclones without being totally excluded. These cyclones may be inhomogeneously recorded due to the absence of satellite observation before the 1980s. Our results indicate that internal variation such as AMO may dominate TCS low-frequency variations over the past several decades. Previous studies have attributed the inconsistent trends of TCS during different subperiods to data inhomogeneity. This study shows that AMO can modulate the TCS trends in different subperiods with phase shift, thus providing new evidence for the recent controversial TCS slowdown.

Restricted access
Chia-Wei Lan
,
Chao-An Chen
, and
Min-Hui Lo

Abstract

Between 1979 and 2021, global ocean regions experienced a decrease in dry season precipitation, while the trend over land areas varied considerably. Some regions, such as southeastern China, the Maritime Continent, eastern Europe, and eastern North America, showed a slight increasing trend in dry season precipitation. This study analyzes the potential mechanisms behind this trend by using the fifth major global reanalysis produced by ECMWF (ERA5) data. The analysis shows that the weakening of downward atmospheric motions played a critical role in enhancing dry season precipitation over land. An atmospheric moisture budget analysis revealed that larger convergent moisture fluxes lead to an increase in water vapor content below 400 hPa. This, in turn, induced an unstable tendency in the moist static energy profile, leading to a more unstable atmosphere and weakening downward motions, which drove the trend toward increasing dry season precipitation over land. More water vapor in the low troposphere is because of higher moisture convergence and moisture transport from ocean to land regions. In summary, this study demonstrates the intricate elements involved in altering dry season rainfall trends over land, which also emphasizes the importance of comprehending the spatial distribution of the wet-get-wetter and dry-get-drier paradigm.

Significance Statement

This study found that global land precipitation during the dry season slightly increased from 1979 to 2021, while precipitation over oceans declined. Moist static energy analysis showed a trend toward less stability in areas with increased dry season precipitation and the opposite trend in regions with declining precipitation. Water vapor content trends and dynamic components were the primary controlling mechanism for precipitation trends. Furthermore, the hotspots with pronounced increases or decreases in dry season precipitation reflect local circulation changes influenced by anthropogenic or natural factors.

Open access
Arshdeep Singh
,
Sanjiv Kumar
,
Liang Chen
,
Montasir Maruf
,
Peter Lawrence
, and
Min-Hui Lo

Abstract

This study examines the effects of land use (LU) change on regional climate, comparing historical and future scenarios using seven climate models from Coupled Model Intercomparison Phase 6 – Land Use Model Intercomparison Project experiments. LU changes are evaluated relative to land use conditions during the pre-industrial climate. Using the Community Earth System Model version 2 Large Ensemble (CESM2-LE) experiment, we distinguish LU impacts from natural climate variability. We assess LU impact locally by comparing the impacts of climate change in neighboring areas with and without LU changes. Further, we conduct CESM2 experiments with and without LU changes to investigate LU-related climate processes.

A multi-model analysis reveals a shift in LU-induced climate impacts, from cooling in the past to warming in the future climate across mid-latitude regions. For instance, in North America, LU's effect on air temperature changes from −0.24±0.18°C historically to 0.62±0.27°C in the future during the boreal summer. The CESM2-LE shows a decrease in LU-driven cooling from −0.92±0.09°C in the past to −0.09±0.09°C in future boreal summers in North America.

A hydroclimatic perspective linking LU and climate feedback indicates LU changes causing soil moisture drying in the mid-latitude regions. This contrasts with hydrology-only views showing wetter soil conditions due to LU changes. Furthermore, global warming causes widespread drying of soil moisture across various regions. Mid-latitude regions shift from a historically wet regime to a water limited transitional regime in the future climate. This results in reduced evapotranspiration, weakening LU-driven cooling in future climate projections. A strong linear relationship exists between soil moisture and evaporative fraction in mid-latitudes.

Restricted access
Yi Peng
,
Yi-Peng Guo
,
Jiuwei Zhao
,
Zhe-Min Tan
,
Xu Chen
, and
Xiangbo Feng

Abstract

Current coupled climate models contain large biases in simulating tropical cyclogenesis, reducing the confidence in tropical cyclone (TC) projection. In this study, we investigated the influence of sea surface temperature (SST) biases on TC genesis in the Coupled Model Intercomparison Project phase 6 simulations from 1979 to 2014. Positive TC genesis biases were found over the tropical central North Pacific (CNP) in most of climate models, including the high-resolution models. Compared to coupled models, TC genesis density (TCGD) simulations over CNP in uncoupled models forced by observational SST improved obviously. A warm SST bias over the tropical CNP in the coupled models is the main cause of TC genesis biases. The SST bias–induced diabatic heating leads to an anomalous Gill-type atmospheric circulation response, which contributes to a series of favorable environmental conditions for TC formation over the CNP. Numerical experiments were also performed with HiRAM to demonstrate the influence of SST biases on the TCGD simulation, further confirming our conclusion. The current results highlight the importance of improving TC simulation in state-of-the-art climate models by reducing SST simulation bias.

Restricted access
Lin Chen
,
Gen Li
,
Bo Lu
,
Yanping Li
,
Chujie Gao
,
Shang-Min Long
,
Xinyu Li
, and
Ziqian Wang

Abstract

The spring tripole sea surface temperature (SST) anomalies in North Atlantic are an outstanding regional mode of interannual variability. Based on the observed and reanalyzed datasets during 1979–2019, this study reveals the relationship and linking mechanism between the spring tripole North Atlantic SST anomalies and the central China July precipitation (CCJP). Results show that the tripole SST anomalies, especially the warm SST anomalies in the tropical North Atlantic (TNA) and the subpolar North Atlantic (SNA), often cause surplus CCJP through the tropical and extratropical pathways. On the one hand, the spring TNA SST warming induces a pan-tropical climate response with the cooling in the central equatorial Pacific and the warming in the Indo-western Pacific until the following July through a series of air–sea interactions, helping maintain an anomalous anticyclone over the northwest Pacific and transport more warm humid flows to central China. On the other hand, the spring TNA and SNA SST warming persist into the following July and then emanate a wave train extending from the SNA throughout the Eurasian continent to East Asia, which induces an anomalous anticyclone over North China with its southeast flank transporting more cold air to central China. The warm humid flows from the south against the cold air from the north are conductive to the local ascending motion, favoring the increased CCJP. Our results highlight both the tropical and extratropical teleconnection pathways of the North Atlantic SST anomalies affecting the CCJP. This suggests an important seasonal predictor of the regional climate.

Significance Statement

July is the peak rainy month of central China, with heavy precipitation occurring frequently and often causing serious impacts on the local production and livelihood of millions of people. This study finds that the spring tripole sea surface temperature anomalies in North Atlantic induced by the North Atlantic Oscillation can exert significant impacts on the following July precipitation over central China through both the tropical and extratropical pathways. This improves our understanding of the causes of the surplus July precipitation over central China and has important implications for the seasonal predictability of the regional climate.

Free access
Huimin Chen
,
Bingliang Zhuang
,
Jane Liu
,
Shu Li
,
Tijian Wang
,
Xiaodong Xie
,
Min Xie
,
Mengmeng Li
, and
Ming Zhao

Abstract

Black carbon (BC) aerosol is a significant and short-lived climate forcing factor. Here, the direct effects of BC emissions from India (IDBC) and China (CNBC) are investigated in East Asia during summer using the state-of-the-art regional climate model RegCM4. In summer, IDBC and CNBC account for approximately 30% and 46% of the total BC emissions in Asia, respectively. The total BC column burden from the two countries and corresponding TOA effective radiative forcing are 1.58 mg m−2 and +1.87 W m−2 in East Asia, respectively. The regional air temperature increases over 0.3 K at maximum and precipitation decreases 0.028 mm day−1 on average. Individually, IDBC and CNBC each can bring about rather different effects on regional climate. IDBC can result in a cooling perturbation accompanied by a substantially increased cloud amount and scattering aerosol loading, resulting in a complex response in the regional precipitation, while CNBC can lead to regional warming, and further induce a local flood in northern China or drought in southern China depending on the opposite but significant circulation anomalies. CNBC plays a dominant role in modulating the regional climate over East Asia due to its higher magnitude, wider coverage, and stronger climate feedback. The direct effect of the total BC from both countries is not a linear combination of that of IDBC and CNBC individually, suggesting that the regional climate responses are highly nonlinear to the emission intensity or aerosol loading, which may be greatly related to the influences of the perturbed atmospheric circulations and climate feedback.

Open access
Paul A. Levine
,
James T. Randerson
,
Yang Chen
,
Michael S. Pritchard
,
Min Xu
, and
Forrest M. Hoffman

Abstract

El Niño–Southern Oscillation (ENSO) is an important driver of climate and carbon cycle variability in the Amazon. Sea surface temperature (SST) anomalies in the equatorial Pacific drive teleconnections with temperature directly through changes in atmospheric circulation. These circulation changes also impact precipitation and, consequently, soil moisture, enabling additional indirect effects on temperature through land–atmosphere coupling. To separate the direct influence of ENSO SST anomalies from the indirect effects of soil moisture, a mechanism-denial experiment was performed to decouple their variability in the Energy Exascale Earth System Model (E3SM) forced with observed SSTs from 1982 to 2016. Soil moisture variability was found to amplify and extend the effects of SST forcing on eastern Amazon temperature and carbon fluxes in E3SM. During the wet season, the direct, circulation-driven effect of ENSO SST anomalies dominated temperature and carbon cycle variability throughout the Amazon. During the following dry season, after ENSO SST anomalies had dissipated, soil moisture variability became the dominant driver in the east, explaining 67%–82% of the temperature difference between El Niño and La Niña years, and 85%–91% of the difference in carbon fluxes. These results highlight the need to consider the interdependence between temperature and hydrology when attributing the relative contributions of these factors to interannual variability in the terrestrial carbon cycle. Specifically, when offline models are forced with observations or reanalysis, the contribution of temperature may be overestimated when its own variability is modulated by hydrology via land–atmosphere coupling.

Open access
Jee-Hoon Jeong
,
Hans W. Linderholm
,
Sung-Ho Woo
,
Chris Folland
,
Baek-Min Kim
,
Seong-Joong Kim
, and
Deliang Chen

Abstract

The present study examines the impacts of snow initialization on surface air temperature by a number of ensemble seasonal predictability experiments using the NCAR Community Atmosphere Model version 3 (CAM3) AGCM with and without snow initialization. The study attempts to isolate snow signals on surface air temperature. In this preliminary study, any effects of variations in sea ice extent are ignored and do not explicitly identify possible impacts on atmospheric circulation. The Canadian Meteorological Center (CMC) daily snow depth analysis was used in defining initial snow states, where anomaly rescaling was applied in order to account for the systematic bias of the CAM3 snow depth with respect to the CMC analysis. Two suites of seasonal (3 months long) ensemble hindcasts starting at each month in the colder part of the year (September–April) with and without the snow initialization were performed for 12 recent years (1999–2010), and the predictability skill of surface air temperature was estimated. Results show that considerable potential predictability increases up to 2 months ahead can be attained using snow initialization. Relatively large increases are found over East Asia, western Russia, and western Canada in the later part of this period. It is suggested that the predictability increases are sensitive to the strength of snow–albedo feedback determined by given local climate conditions; large gains tend to exist over the regions of strong snow–albedo feedback. Implications of these results for seasonal predictability over the extratropical Northern Hemisphere and future direction for this research are discussed.

Full access
Chu-Chun Chen
,
Min-Hui Lo
,
Eun-Soon Im
,
Jin-Yi Yu
,
Yu-Chiao Liang
,
Wei-Ting Chen
,
Iping Tang
,
Chia-Wei Lan
,
Ren-Jie Wu
, and
Rong-You Chien

Abstract

Tropical deforestation can result in substantial changes in local surface energy and water budgets, and thus in atmospheric stability. These effects may in turn yield changes in precipitation. The Maritime Continent (MC) has undergone severe deforestation during the past few decades but it has received less attention than the deforestation in the Amazon and Congo rain forests. In this study, numerical deforestation experiments are conducted with global (i.e., Community Earth System Model) and regional climate models (i.e., Regional Climate Model version 4.6) to investigate precipitation responses to MC deforestation. The results show that the deforestation in the MC region leads to increases in both surface temperature and local precipitation. Atmospheric moisture budget analysis reveals that the enhanced precipitation is associated more with the dynamic component than with the thermodynamic component of the vertical moisture advection term. Further analyses on the vertical profile of moist static energy indicate that the atmospheric instability over the deforested areas is increased as a result of anomalous moistening at approximately 800–850 hPa and anomalous warming extending from the surface to 750 hPa. This instability favors ascending air motions, which enhance low-level moisture convergence. Moreover, the vertical motion increases associated with the MC deforestation are comparable to those generated by La Niña events. These findings offer not only mechanisms to explain the local climatic responses to MC deforestation but also insights into the possible reasons for disagreements among climate models in simulating the precipitation responses.

Open access