 Subjects -> METEOROLOGY (Total: 106 journals)
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- On the signatures of local and regional dynamics in the distribution of
lower stratospheric water vapour over Indian region using balloon-borne
and satellite observations-
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Abstract: Abstract Balloon-borne cryogenic frost-point hygrometer (CFH) observations over two tropical stations Trivandrum (8.53° N, 76.87° E) and Hyderabad (17.47° N, 78.58° E) [2014–2017] along with Microwave Limb Sounder (MLS) observations [2011–2017] are used to examine the signatures of local and regional dynamics in the distribution of lower stratospheric water vapour over the Indian region. The column-integrated water vapour in the lower stratosphere (IWVLS) varies in the range 2.5–5 g/m2 with low values during winter and high values during summer monsoon and post monsoon seasons. About 50–75% of IWVLS lies in the lower regime of the lower stratosphere, the region from cold point tropopause (CPT) to 21 km (LS1). Hygropause is very near to the CPT in winter and pre-monsoon and 2–3 km above the CPT in summer-monsoon and post-monsoon. The CPT and the minimum in saturated mixing ratio (SMRmin) altitudes show a positive correlation, with SMRmin mostly occurring below the CPT. Though water vapour mixing ratio (WVMR) at CPT increases with increase in SMRmin, it is mostly less than the corresponding SMRmin value. CFH observations showed that the tape recorder signal in the LS1 is disturbed by the local/regional dynamics. While the amount of water vapour entering the lower stratosphere is higher over Hyderabad in winter and summer monsoon, it is higher over Trivandrum in the other two seasons. Hydration at the tropopause level is mainly determined by the overshooting/deep convection, monsoon dynamics and horizontal transport and dehydration is determined by temperature variations in the tropical tropopause layer (TTL).
PubDate: 2023-03-16
- Impacts of a weakened AMOC on precipitation over the Euro-Atlantic region
in the EC-Earth3 climate model-
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Abstract: Abstract Given paleoclimatic evidence that the Atlantic Meridional Overturning Circulation (AMOC) may affect the global climate system, we conduct model experiments with EC-Earth3, a state-of-the-art GCM, to specifically investigate, for the first time, mechanisms of precipitation change over the Euro-Atlantic sector induced by a weakened AMOC. We artificially weaken the strength of the AMOC in the model through the release of a freshwater anomaly into the Northern Hemisphere high latitude ocean, thereby obtaining a ~ 57% weaker AMOC with respect to its preindustrial strength for 60 model years. Similar to prior studies, we find that Northern Hemisphere precipitation decreases in response to a weakened AMOC. However, we also find that the frequency of wet days increases in some regions. By computing the atmospheric moisture budget, we find that intensified but drier storms cause less precipitation over land. Nevertheless, changes in the jet stream tend to enhance precipitation over northwestern Europe. We further investigate the association of precipitation anomalies with large-scale atmospheric circulations by computing weather regimes through clustering of geopotential height daily anomalies. We find an increase in the frequency of the positive phase of the North Atlantic Oscillation (NAO+), which is associated with an increase in the occurrence of wet days over northern Europe and drier conditions over southern Europe. Since a ~ 57% reduction in the AMOC strength is within the inter-model range of projected AMOC declines by the end of the twenty-first century, our results have implications for understanding the role of AMOC in future hydrological changes.
PubDate: 2023-03-16
- Seasonal cycle of marine heatwaves in the northern South China Sea
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Abstract: Abstract The seasonal properties and heat budget of marine heatwaves (MHWs) on the northern South China Sea (SCS) continental shelf are investigated. The winter MHWs are generally the warmest, gradually weakening in the following seasons. In winter and spring, all events have a temperature elevated by more than 1 °C while approximately half of the MHWs in summer and fall have temperatures elevated by less than 1 °C. The MHW duration is longest in winter, second longest in fall, and shortest in spring. Three types of MHWs have been defined based on the main heat source; i.e., air–sea heat flux dominant, ocean advection dominant, and mixed types. Air–sea heat flux is the dominant source for more than 80% of MHWs in winter and fall, ~ 70% of MHWs in spring, and ~ 50% in summer. There are slightly more ocean advection dominant events than Mixed-MHWs. Ocean advection always weakens an air–sea heat flux dominant MHW and enhances other types of events, so that the net heat supply is smallest for air–sea heat flux dominant MHW types. Ocean advection is mainly modulated by cross-slope water exchanges along the continental shelf edge. The offshore (onshore) cross-slope flow induces a negative (positive) contribution to MHW development. These results provide new insight into the seasonal cycle of MHWs on the northern SCS continental shelf.
PubDate: 2023-03-15
- Evolution of trends in North Atlantic dynamic sea level in the
twenty-first century-
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Abstract: Abstract This study looks at the evolution of trends in the dynamic sea level (DSL) component of sea-level rise for the North Atlantic (18°–67° N), primarily over the twenty-first century but with some analysis extending to 2300. We use an 18-member climate model ensemble of projections under medium and high greenhouse gas scenarios. We find that the long-term trend pattern (1950–2099) tends to be indicated in the early twenty-first century: 14 (11) of the 18 models have pattern correlations between this period and 1993–2022 of > 0.5 under the medium (high) greenhouse gas scenario. Whether a particular 1993–2022 DSL trend indicates the long-term trend in our ensemble can be assessed from concurrent changes in the Atlantic Meridional Overturning Circulation (AMOC) at 30° N: those cases with larger AMOC weakening trends have high DSL-trend pattern correlations. This suggests that the observed AMOC (or a proxy for that) could indicate whether observed DSL trends are likely to be a constraint on projections, provided suitable AMOC trend indicators can be determined. More generally, North Atlantic DSL and the AMOC show a similar evolution of measures of their successive 30-year trends (with these being projection-onto-a-pattern and the ratio with the long-term trend, respectively), both in the ensemble mean and for individual models. Model-to-model differences in this evolution indicate that the changing DSL and AMOC trends are more interconnected than solely the common influence of external forcings associated with the climate-change scenario. Model simulations extending to 2300 indicate that the twenty-first century trend patterns do not generally persist into subsequent centuries, as has been shown in previous studies.
PubDate: 2023-03-15
- Causes of positive precipitation anomalies in South China during La
Niña winters-
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Abstract: Abstract The effects of El Niño-Southern Oscillation (ENSO) on winter precipitation in South China are considerably asymmetric. A positive correlation is significant between the sea surface temperature anomalies (SSTA) in the equatorial middle-eastern Pacific and the precipitation in South China during El Niño winters, whereas no significant relationship instead of a negative correlation between them during La Niña winters. The possible causes of abnormally positive precipitation in South China during La Niña winters remain a puzzle. The analysis indicates that positive precipitation anomalies in South China during La Niña winters are closely related to the strengthening of southerly winds over the South China Sea and convective activities over South China. Moreover, during La Niña winters, the low-level southerly wind anomalies over the South China Sea are modulated by an anomalous positive local zonal sea surface temperature gradient (SSTG) between the South China Sea and the tropical western Pacific, which strengthens in late autumn and maintains through the following winter. This positive local zonal SSTG is independent of La Niña and leads to the weakening of Walker circulation. The strengthened downward motion over the tropical western Pacific further promotes the weakening of local Hadley circulation. Weakened local Hadley circulation, with strengthened southerlies over the South China Sea in the lower troposphere, is conducive to the enhancement of water vapor transport from the South China Sea to South China. Besides, the weakened local Hadley circulation also shows strengthened upward motion over South China, which provides more advantageous dynamic conditions for precipitation. Therefore, the positive local zonal SSTG simultaneously promotes the two most important conditions affecting precipitation, resulting in positive precipitation anomalies over South China during La Niña winters. These results advance the understanding of prediction of winter precipitation in south China.
PubDate: 2023-03-14
- Influence of compound zonal displacements of the South Asia high and the
western Pacific subtropical high on Meiyu intraseasonal variation-
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Abstract: Abstract Meiyu shows substantial intraseasonal variation at periods of 10–30 days and 30–60 days, which often leads to extreme precipitation and disastrous flooding over the Yangtze River basin. Monitoring and prediction of the intraseasonal variation of Meiyu is crucial for disaster prevention and mitigation. Here, we proposed two sets of novel indices for Meiyu real-time monitoring and prediction based on the compound zonal displacements of the South Asia high (SAH) and the western Pacific subtropical high (WPH) at 10–30-day and 30–60-day period, respectively. For the 10–30-day period of Meiyu, the zonal displacement of the SAH is associated with a mid-latitude Eurasian Rossby wave train, whereas the WPH is related to the second mode of the boreal summer intraseasonal oscillation. On the 30–60-day timescale, the zonal displacement of the SAH and the WPH are both associated with the first mode of the boreal summer intraseasonal oscillation. The subtle differences in zonal displacement of the SAH and the WPH determine eight type configurations, corresponding to distinct influences on Meiyu. Meiyu intraseasonal variation can be well reconstructed by using the relationship between these two indices and rainfall anomalies pattern over China. Given that the ECMWF S2S model is more skillful in forecasting upper- and lower-level circulation than in directly forecasting precipitation, a hybrid dynamical–statistical model is conducted to subseasonal prediction of Meiyu using the ECMWF model forecast indices. The hybrid model outperforms the ECMWF model in subseasonal prediction of the Meiyu variation at 17–40-day lead times.
PubDate: 2023-03-14
- The compound impacts of sea surface temperature modes in the Indian and
North Atlantic oceans on the extreme precipitation days in the Yangtze
River Basin-
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Abstract: Abstract Owing to the East Asian summer monsoon, extreme precipitation occurred frequently over the Yangtze River Basin (YRB), leading to flooding and secondary disasters. Therefore, understanding the physical mechanism and seeking predictability sources of extreme precipitation in the YRB are of scientific and practical importance. The present study examines the independent and compound impacts of leading sea surface temperature (SST) modes in the Indian Ocean and the North Atlantic Ocean on summer extreme precipitation days (EPDs) over the YRB. The Indian Ocean basin-wide uniform SST mode (the Indian Ocean Basin Mode) influences the EPDs over the YRB by inducing a Kelvin wave response and the Pacific–Japan pattern, whereas the two leading SST modes of the North Atlantic Ocean show different meridional tripole patterns with different climate impacts on East Asia. The North Atlantic SST tripole southern mode (NATS) induces quasi-stationary Rossby wave trains over mid-latitude Eurasia and the tropical waves that influence the EPDs in the YRB. The North Atlantic SST tripole northern mode (NATN) impacts the circulation anomaly over Northeast Asia through inducing different Eurasian quasi-stationary Rossby wave trains. When the IOBM and the NATS are both in the positive phase, enhanced EPDs occur over the YRB. On the one hand, the IOBM induces a Kelvin wave response, which strengthens the western North Pacific anomalous anticyclone (WNPAC). On the other hand, the NATS stimulates the mid-latitude quasi-stationary Rossby waves and results in the Northeast Asia anomalous cyclone (NEAC). The warm, moist air over the northwestern flank of the WNPAC and the cold, dry air over the southern flank of the NEAC converge in the YRB, leading to more EPDs in the region. When the IOBM and the NATN are out of phase, the Kelvin wave response in terms of the WNPAC induced by the IOBM warming is modulated by the negative phase of the NATN via quasi-stationary Rossby wave trains over mid-latitude Eurasia, resulting in more EPDs in the YRB. Based on the compound effect of different SST modes in the two ocean basins, the year-to-year EPDs over the YRB can be reconstructed reasonably well, which provides useful predictability sources for the seasonal prediction.
PubDate: 2023-03-14
- The role of semiannual cycle in modulating seasonality changes of surface
air temperature over China and its mechanism-
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Abstract: Abstract Taking the semiannual cycle of surface air temperature (SAT) into account in fitting the finer structure of the seasonal cycle, this study reveals the specific role of the semiannual cycle in modulating variations of the season onsets and lengths for the first time. The time-varying amplitudes and phases of the annual and semiannual SAT cycle for the period 1964 to 2017 in China are extracted from daily observations based on harmonic analysis. The results show that the phase differences between the two harmonics remain essentially unchanged, while the amplitude ratio of the semiannual to annual harmonics tends to increase after 1988. This enhanced semiannual cycle together with the global warming and annual cycle changes jointly led to a much longer summer with an average speed of 7.45 days/10 years and shorter spring, autumn, and winter with speeds of − 2.43, − 1.63, and − 4.26 days/10 years, respectively. Such obvious asymmetry cannot be found in fitting seasonal cycles by only annual harmonics. The average absolute percentages of the linear trends of season onsets and lengths caused by the intensification of the semiannual cycle are 52.63% and 78.66%. It indicates that season onsets and length not only depend on the phase of annual and semiannual harmonic, but also highly related to the strength of semiannual cycle. The time evolutions of semiannual cycle strength in China are found to be significantly correlated to the meridional temperature advection. The anomalous stronger southward wind in December can result in colder temperature and hence intensify the semiannual component.
PubDate: 2023-03-14
- Quantifying the processes of accelerated wintertime Tibetan Plateau
warming: outside forcing versus local feedbacks-
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Abstract: Abstract The Tibetan Plateau (TP) has experienced an accelerated wintertime warming in recent decades under global warming, but consensus on its causes has not yet been reached. This study quantifies the processes of the warming through analyzing surface temperature budget and surface energy balance. It is found that increased diabatic heating (73%) and warm advection (27%) by an anomalous anticyclone southeast of TP are two primary processes determining the surface air warming. The former is caused by a significant increase of the TP skin temperature which warms the near surface atmosphere through increasing upward surface sensible heat flux. The land surface warming is attributed to increased absorbed radiation fluxes in which three processes are identified to be major contributors. While outside forcing which is primarily due to increased anthropogenic emissions of greenhouse gases contributes to the warming by 25% through increasing downward longwave radiation, two types of local positive feedbacks which are triggered by the land surface warming are found to contribute to most of the warming. One is the snow-albedo feedback which accounts for 47% of the surface warming by increasing surface absorption of incident solar radiation. The other is the moisture process feedback which accounts for 28% of the surface warming. The surface warming which works with increased soil water due to increased precipitation in the preceding seasons tends to promote surface evaporation and moisten the atmosphere aloft over the eastern TP, which, in turn, tends to increase downward longwave radiation and cause a further surface warming.
PubDate: 2023-03-13
- Multivariate bias correction of regional climate model boundary conditions
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Abstract: Abstract Improving modeling capacities requires a better understanding of both the physical relationship between the variables and climate models with a higher degree of skill than is currently achieved by Global Climate Models (GCMs). Although Regional Climate Models (RCMs) are commonly used to resolve finer scales, their application is restricted by the inherent systematic biases within the GCM datasets that can be propagated into the RCM simulation through the model input boundaries. Hence, it is advisable to remove the systematic biases in the GCM simulations prior to downscaling, forming improved input boundary conditions for the RCMs. Various mathematical approaches have been formulated to correct such biases. Most of the techniques, however, correct each variable independently leading to physical inconsistencies across the variables in dynamically linked fields. Here, we investigate bias corrections ranging from simple to more complex techniques to correct biases of RCM input boundary conditions. The results show that substantial improvements in model performance are achieved after applying bias correction to the boundaries of RCM. This work identifies that the effectiveness of increasingly sophisticated techniques is able to improve the simulated rainfall characteristics. An RCM with multivariate bias correction, which corrects temporal persistence and inter-variable relationships, better represents extreme events relative to univariate bias correction techniques, which do not account for the physical relationship between the variables.
PubDate: 2023-03-10
- Projections of synoptic anticyclones for the twenty-first century
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Abstract: Abstract Synoptic anticyclones are a common feature of subtropical and midlatitude climate and are associated with descending air and clear conditions, while associated anticyclonic circulation anomalies can contribute to temperature extremes. When anticyclones are tracked in both the ERA5 reanalysis and 10 global climate models from the 5th Coupled Model Intercomparison Project (CMIP5) using a common grid, the CMIP5 models consistently underestimate the observed frequency of anticyclones in the southern hemisphere, while overestimating anticyclone frequencies in the northern hemisphere. Under a high emissions scenario, the overall frequency of anticyclones is projected to decline over the twenty-first century. Declines are largest in the southern hemisphere subtropics, where projected changes in anticyclone frequency can be linked to the projected poleward shift in the Southern Annular Mode. Stronger and more robust declines are projected for the subset of quasi-stationary anticyclones that move less than 4° over 24 h. Using the Australian region as a case study, regionally downscaled models show very similar projected changes to the driving CMIP5 models, adding little additional value for understanding projected changes in anticyclones.
PubDate: 2023-03-10
- Signals of northward propagating monsoon intraseasonal oscillations
(MISOs) in the RegCM4.7 CORDEX-CORE simulation over South Asia domain-
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Abstract: Abstract Northward propagating summer monsoon intraseasonal oscillations (MISOs) in the Indian Ocean region remain poorly understood and difficult to predict. Here we examine a free-running high-resolution regional atmospheric model (RegCM4.7 with 25 km resolution), forced distantly at the boundaries by atmospheric observations (ERA-Interim, 0.75 \(^{\circ }\) ) and forced locally by observed sea-surface temperature (SST) over the period 1979–2016, to assess its ability to reproduce key aspects of these MISOs. We find that the model MISO exhibits spatial structures and northward propagation characteristics broadly similar to observed MISO when confining the analysis to the 25–90 day period band. The MISO precipitation anomalies are then shown to be consistent with previously known observed relationships to broad-scale sea-level pressure patterns, Inter-Tropical Convergence Zone (ITCZ) positioning, and changes in the regional Hadley Cell component. The total simulated seasonal (JJAS) rainfall anomalies over India are not significantly correlated with observations, indicating that intrinsic variations in the regional model atmosphere dominate most of the precipitation variability. However, the bandpass-filtered MISO anomalies surprisingly exhibit a significant correlation (0.61) with observations. This suggests that instabilities in the regional broad-scale atmospheric circulation, e.g., linked to the ITCZ position or strength, may be partly controlled by the large-scale atmospheric flows specified at the domain boundaries and/or that specified local SST anomalies may help to guide some fraction of the developing model MISO to follow observations. This result motivates further research on MISO initiation and development using this type of regional atmospheric model.
PubDate: 2023-03-09
- Contemporary oceanic radiocarbon response to ocean circulation changes
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Abstract: Abstract Radiocarbon (14C) is a valuable tracer of ocean circulation, owing to its natural decay over thousands of years and to its perturbation by nuclear weapons testing in the 1950s and 1960s. Previous studies have used 14C to evaluate models or to investigate past climate change. However, the relationship between ocean 14C and ocean circulation changes over the past few decades has not been explored. Here we use an Ocean-Sea-ice model (NEMO) forced with transient or fixed atmospheric reanalysis (JRA-55-do) and atmospheric 14C and CO2 boundary conditions to investigate the effect of ocean circulation trends and variability on 14C. We find that 14C/C (∆14C) variability is generally anti-correlated with potential density variability. The areas where the largest variability occurs varies by depth: in upwelling regions at the surface, at the edges of the subtropical gyres at 300 m depth, and in Antarctic Intermediate Water and North Atlantic Deep Water at 1000 m depth. We find that trends in the Atlantic Meridional Overturning Circulation may influence trends in ∆14C in the North Atlantic. In the high-variability regions the simulated variations are larger than typical ocean ∆14C measurement uncertainty of 2–5‰ suggesting that ∆14C data could provide a useful tracer of circulation changes.
PubDate: 2023-03-08
- Evaluation of temperature and precipitation from CORDEX-CORE South America
and Eta-RCM regional climate simulations over the complex terrain of
Subtropical Chile-
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Abstract: Abstract An ensemble of three Regional Climate Models (RCMs) is evaluated over the current climate conditions with the aim of assessing RCMs’ skills and limitations in reproducing the near-surface temperature and precipitation over the Subtropical Chile complex terrain region (25 ºS–45 ºS). The simulations driven by ERA-Interim and by GCMs historical scenarios are compared against observational gridded products and ERA5 reanalysis for high and low elevations separately. The RCMs simulate reasonably well the main spatio-temporal characteristics of temperature and precipitation, such as latitudinal climate gradients, orographic uplifts, phase of the seasonal cycle, and realistic inter-annual variability. Although the simulations driven by ERA-Interim show better skills than those driven by GCMs, especially in the case of precipitation, none of the RCMs/simulations performs best or worst in all sub-regions. RCMs and ERA5 have a common prominent cold bias at high elevations north of 35 °S, which is particularly strong above 2000 m.a.s.l. This bias is associated with a strong overestimation of precipitation and an overestimation of surface albedo, likely related to an overestimated snow cover. Because of scarce in-situ observations, this region is also associated with inherent observational uncertainty. Our results emphasize the necessity of improving the density and quality of observational networks over the complex terrain of Subtropical Chile for an accurate assessment and tuning/calibration of RCMs. The assessment of a state-of-the-art RCMs ensemble provided by this study can help in interpreting, bias-correcting, and using the regional climate projections performed with these RCMs for downstream applications and impact studies.
PubDate: 2023-03-08
- Regional climate simulation of tropical cyclone at gray-zone resolution
over western North Pacific: with/without cumulus parameterization-
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Abstract: Abstract Tropical cyclones (TCs) simulated at gray-zone resolution (9 km) in the Advanced Research version of the Weather Research and Forecasting Model (WRF-ARW) are evaluated over the Western North Pacific (WNP) during 11-year (2008–2018) TC seasons (June–November). The effects of the cumulus parameterization (CPS) are tested with two sets of numerical experiments: one involving CPS (ICPS) and one without (NICPS). Through comparisons with the observation and reanalysis, it is shown that the ICPS experiment yields good skills in simulating TC frequency due to more realistic large-scale mean states, including stronger low-tropospheric circulation, wetter mid-tropospheric environment, and stronger ascending motion over the TC main development region (MDR). On the other hand, the NICPS experiment is found to better reproduce intense TCs (Saffir–Simpson hurricane categories 3, 4 and 5) in terms of intensity, accumulated cyclone energy (ACE), and inner-core size. NICPS can also reasonably capture TC inner-core structure with stronger radial inflow in the boundary layer, rising motion around the eyewall and outflow in the mid to upper troposphere. Nevertheless, NICPS significantly underestimates TC outer size due to rapidly decay of TC outer wind field, suggesting limited severe convective activities in the outer region.
PubDate: 2023-03-07
- Mechanisms for African easterly wave changes in simulations of the
mid-Holocene-
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Abstract: Abstract The mid-Holocene was a warm period with significantly amplified precipitation in North Africa, and a northward shifted Western African Monsoon during boreal summer. We conduct simulations for the pre-industrial and mid-Holocene periods to investigate the connection between summer rainfall variability and changes of African easterly waves (AEWs) during the mid-Holocene. Summer rainfall increases and migrates northward during the mid-Holocene, but the magnitude of change fails to reconcile the discrepancy with mid-Holocene proxy evidence, possibly due to no prescribed vegetation change in our simulations. The spectrum of summer rainfall over the Sahel and West Africa reveals enhanced synoptic time scale (3-to-6 days) variability during the mid-Holocene, which is consistent with the enhanced AEW activity influence. Specifically, the southern AEW track strengthens and migrates poleward during the mid-Holocene period, which modulates summer rainfall over the Sahel and West Africa. By comparison, the northern AEW track changes less and produces a minor contribution to rainfall changes in those regions. We find enhanced baroclinic and barotropic instabilities to promote the AEW activity during the mid-Holocene, with a doubling of the eddy kinetic energy of the meridional wind from that in PI, and baroclinic energy conversion plays a more important role. Stronger low-level meridional thermal gradients increase moisture flux from the Atlantic Ocean to inland.The amplified AEW activity, together with promoted moist convection and increased precipitation, results in a northern shift of the summer rainfall band during the mid-Holocene.
PubDate: 2023-03-07
- Record-breaking pre-flood rainfall over South China in 2022: role of
historic warming over the Northeast Pacific and Maritime Continent-
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Abstract: Abstract In 2022, South China suffered severe flooding due to unexpected persistent heavy rainfall in the pre-flood season. The in situ cumulative precipitation amount during May–June in South China broke the record established in 1981. The present study ascribed this record-breaking event to extreme warm sea surface temperature anomalies (SSTAs) in the Northeast Pacific (NP) and the Maritime Continent (MC) region. In May, the NP warming associated with La Niña and the negative phase of the Pacific Decadal Oscillation intensified extratropical diabatic heating and induced a cyclonic anomaly over Northeast Asia, which shifted the East Asian trough (EAT) southward. Subsequently, the westerly wind accelerated to the south of the anomalous EAT and strengthened the western North Pacific subtropical high, which led to increase in rainfall over South China. In June, the rainfall over South China remained above normal owing to anomalies of the meridional monsoon circulation manifested as a response to the development of a warm SSTA in the MC region. The anomalous warming in the deep tropics suppressed the Asian summer monsoon convection through anomalous atmospheric descent over South Asia, accompanied by enhanced atmospheric ascent and increased water vapor convergence over South China. Numerical sensitivity experiments validated the above processes and indicated that historic warm SSTAs in the NP and MC regions could increase the probability of extreme persistent above-normal pre-flood rainfall over South China. The results support improved predictability of pre-flood rainfall over South China on the interannual timescale outside the equatorial central–eastern Pacific.
PubDate: 2023-03-07
- Future projection of the African easterly waves in a high-resolution
atmospheric general circulation model-
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Abstract: Abstract Simulating the features of the African Easterly Waves (AEWs), such as their westward propagation off the east Atlantic coast, is challenging for coarse-resolution climate models. In this study, we use High-Resolution Atmospheric Model (HiRAM) to simulate AEWs and analyze their future projections by the end of the twenty first century. The simulations are performed globally at a horizontal resolution of \(\sim \) 25 km. The model uses shallow convective parameterization for moist convection and stratiform cloudiness. Future projections are conducted using representative concentration pathway 8.5. The AEWs are separated with respect to their periods as 3–5- and 6–9-day period AEWs, and bandpass filtering is used to filter the waves from the mean flow. HiRAM simulates structure and propagation of the waves well; however, it tends to overestimate the associated precipitation. In the future, the AEW precipitation and intensity of the circulation will considerably increase. The northward extent of the AEW track also shows a significant increase in the future. Enhanced baroclinic overturning and eddy available potential energy generated due to diabatic heating is also observed in the future.
PubDate: 2023-03-06
- Impact of anomalous Eurasian blocking activities on the East Asian Meiyu
rainfall-
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Abstract: Abstract An extreme East Asian Meiyu rainfall in both amount and duration occurred along Yangtze River valley during June–July of 2020, however, possible mid-high latitude signatures causing this super Meiyu have not been well identified. This study explores the cause of the Meiyu rainfall from the aspect of anomalous Eurasian blocking activities with a two-dimensional blocking index, using the Japanese 55-yr Reanalysis for 1979–2020. The major findings are as follows. Variabilities of the Eurasian blocking activities are primarily characterized by a tripole pattern with three centers over the Baltic Sea, the Ural Mountains, and the Sea of Okhotsk, respectively. The tripole pattern is associated with two zonally-oriented Rossby wavetrains which may originate from the rainfall anomaly in central Europe. Corresponding to a positive phase of the tripole pattern, the northern wavetrain through energy dispersion tends to induce an anomalous anticyclone (i.e., enhanced blocking) over the Sea of Okhotsk, while the southern wavetrain tends to induce an anomalous cyclone over the Sea of Japan which is conducive to a southward intrusion of more high-latitude cold airs. Consequently, a meridional dipole anomalous circulation pattern over the northeastern Asia is formed, favoring more East Asian Meiyu rainfall. In 2020, the Eurasian blocking activities exhibit a significantly positive phase of the tripole pattern and considerably contribute to the super Meiyu. The results identified in this study highlight the importance of the Eurasian blocking activities in the East Asian Meiyu and provide a new perspective for the prediction of the Meiyu rainfall with mid-high latitude signatures.
PubDate: 2023-03-06
- Impact of tropical cyclones over the eastern North Pacific on El
Niño–Southern Oscillation intensity-
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Abstract: Abstract Most tropical cyclones (TCs) generated over the eastern North Pacific (ENP) do not make landfall. Consequently, TCs in this basin have received less attention, especially those that occur away from the mainland. Furthermore, there have been few studies of the climatic effects of ENP TCs. This study explores the feedback relationship between ENP TCs and the intensity of the El Niño–Southern Oscillation (ENSO), including El Niño and La Niña events, from the perspective of accumulated cyclone energy (ACE). Observational and modeling results indicate that the ENP ACE 3 months earlier can still affect the intensity of El Niño and La Niña events, although the SST persistence is main contributor. Thereinto, the impact of ENP TCs on El Niño appears to be approximately equal to that on La Niña. Moreover, this impact is independent of the persistence of the sea surface temperature (SST) in the Niño 3.4 region and the Madden–Julian Oscillation. Generally, the greater the ENP ACE, the stronger the El Niño, and the smaller the ENP ACE, the stronger the La Niña; this is especially the case for those TCs that develop over the July‒September period. In addition, results show that the ENP TCs modulate ENSO intensity by changing anomalous zonal wind at the low-level atmospheric layer. And the joint impacts of the low-level zonal wind anomalies on the Walker circulation and the east–west thermocline gradient lead to the time characteristics that ENP TCs lead ENSO intensity by about 3 months.
PubDate: 2023-03-06
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