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The mid‐latitude extreme weather disasters are often associated with explosively developing cyclones (ECs). Based on different vertical development characteristics, 4,608 ECs identified over the North Pacific in the cold season of 44 years of NCEP‐CFSR reanalyzes are divided into four types of upward development and four types of downward development categories. ECs with vertical upward (downward) development follow a northeastward (nearly eastward) path, mainly explosively developing over the Northwest Pacific (Asia continent and Pacific). Furthermore, utilizing the piecewise potential vorticity inversion method reveals the synergetic forcing of the turbulent heat transport and baroclinicity in the lower troposphere, the latent heat release in the middle levels, the upper‐level jet stream, and the downward intrusion of stratospheric potential vorticity on the ECs. Different configurations of these influences from the troposphere to the stratosphere result in the occurrences of eight types of ECs in the cold season over the North Pacific. Plain Language Summary: Since the late twentieth century, there has been a significant variation in the frequency of explosively developing cyclones (ECs), which bring extreme weather disasters to the mid‐latitudes. This study classifies winter North Pacific ECs into four upward and four downward developing categories. The upward developing ECs include those enhanced only in the lower troposphere (UL‐EC), those developing upward to the middle troposphere (UM‐EC), those developing upward to the upper troposphere (UU‐EC), and those developing upward into the stratosphere (US‐EC). In contrast, the downward developing ECs can be classified as those intensifying only in the upper troposphere (DU‐EC), those developing downward to the middle troposphere (DM‐EC), those developing downward to the lower troposphere (DL‐EC), and those developing downward from the stratosphere (DS‐EC) to the lower troposphere. Furthermore, the piecewise potential vorticity inversion results show that the explosive development of UL‐EC and UM‐EC are dominated by the thermodynamical processes in the middle‐to‐lower troposphere, exhibiting a baroclinic structure with opposite anomalous geopotential height between the troposphere and stratosphere. However, the other types have a consistent stratospheric‐to‐tropospheric negative anomalies, which is determined by the upper‐layer dynamical processes. The downward intrusion of stratospheric potential vorticity dominates the explosive development for the US‐EC and DS‐EC. Key Points: The North Pacific explosively developing cyclones during cold season are divided into eight up‐ and down‐ward vertically developing typesThe explosively developing cyclones are proved to be an important link between the stratosphere and troposphere in the mid‐high latitudesUsing the piecewise potential vorticity inversion method, the quantitative forcings on the explosively developing cyclones are explored [ABSTRACT FROM AUTHOR]

We identify for the first time an Indian Easterly Jet (IEJ) in the mid‐troposphere during the pre‐monsoon using reanalysis data. The IEJ is weaker and smaller than the African Easterly Jet over West Africa, with a climatological location of 10°N, 60–90°E, 700 hPa, and strength 6–7 m s−1 during March–May. The IEJ is a thermal wind associated with low‐level meridional gradients in temperature (positive) and moisture (negative), resulting from equatorward moist convection and poleward dry convection. The IEJ is associated with a negative meridional potential vorticity gradient, therefore satisfying the Charney‐Stern necessary condition for instability. However, no wave activity is detected, suggesting that the potential for combined barotropic‐baroclinic instability is not often realized. IEJ strong (weak) years feature increased (reduced) near‐surface temperatures and drier (wetter) conditions over India. This study provides an introduction to the IEJ's role in pre‐monsoon dynamics, and a platform for further research. Plain Language Summary: Jets, or concentrated regions of fast‐moving winds, can form due to temperature gradients working together with the Earth's rotation. Typically these form in the east/west direction, such as the African Easterly Jet, which is well known over tropical West Africa. Using observation‐based data of the atmosphere, we set out to explore whether such a jet exists near India during spring‐time (the "pre‐monsoon"). The Indian Easterly Jet (IEJ), like its counterpart in West Africa, forms in the lower atmosphere between a south‐to‐north increasing low‐level temperature gradient and a reversed temperature gradient at higher altitudes. But unlike in West Africa, the IEJ breaks down once the summer monsoon begins. We use established methods to show that the IEJ winds have the potential to amplify small cyclonic disturbances, which can develop into larger storms. However, we find that this does not occur commonly. Meanwhile, years in which the IEJ is unusually strong tend to be associated with spring‐time conditions in India that are warmer and drier than normal. This first research into the IEJ paves the way for further analysis on its role in the development of heatwaves, severe storms and the monsoon onset. Key Points: The Indian Easterly Jet is a thermal wind in the mid‐troposphere over India during the pre‐monsoon, identified in reanalysis dataThe jet resembles the African Easterly Jet, but is smaller and weaker, and is not associated with significant easterly wave activityInterannual variability in jet state is linked to variability in pre‐monsoon weather, including surface temperature and precipitation [ABSTRACT FROM AUTHOR]

We perform a climatological analysis of summer‐time Arctic cyclone structure in reanalysis data using a phase space approach. A classification scheme for Arctic cyclones is proposed, dependent on whether vorticity structure during development is low‐level‐dominant (LLD) or upper‐level‐dominant (ULD). During growth, LLD cyclones (65.5%) exhibit warm‐core asymmetric structures, whereas ULD cyclones (34.5%) have cold‐core asymmetric structures. LLD cyclones typically have greater thermal asymmetry during growth. However, a transition to a persistent cold‐core axisymmetric structure after maturity is characteristic of summer‐time Arctic cyclones, regardless of structure during growth. LLD cyclones are typically stronger and preferentially track on the Russian coastline where there is high baroclinicity, whereas ULD cyclones tend to be longer‐lived and preferentially track in the Pacific sector, where they can interact with tropopause polar vortices. This study provides a platform for further research into different classes of Arctic cyclones and associated hazardous weather, and ultimately for developing conceptual models. Plain Language Summary: We investigate the structure of cyclones, large‐scale low‐pressure systems, in the summer‐time Arctic, using observations‐based data of the atmosphere. A phase space is used to represent two aspects of cyclone structure, specifically (a) the low‐level horizontal temperature contrast (i.e., whether there is a distinct warm sector), and (b) the vertical change in wind. We classify Arctic cyclones based on their vertical structure as they develop, as either low‐level‐dominant (LLD), where winds decrease with height, or upper‐level‐dominant (ULD), where winds increase with height. During growth, LLD cyclones (65.5%) exhibit warm‐core structures whereas ULD cyclones (34.5%) have cold‐core structures, both with strong low‐level temperature contrasts. LLD cyclones tend to have a stronger warm sector contrast during development. However, after maximum intensity, when a cyclone matures and begins to weaken, both subsets of cyclones typically evolve to have long‐lasting cold‐core structures without a warm sector. LLD cyclones are typically stronger and preferentially track on the Russian coastline, whereas ULD cyclones tend to be longer‐lived, and preferentially track in the Pacific sector of the Arctic. This study provides a platform for further research into different classes of Arctic cyclones, and ultimately for developing conceptual models, which are key for anticipating associated hazardous weather. Key Points: Arctic cyclones in summer are classified by their vorticity structure during growth: low‐level‐dominant (LLD) or upper‐level‐dominant (ULD)LLD cyclones (65.5%) exhibit more warm‐core baroclinic structures, preferentially track on the Russian coastline, and are typically strongerULD cyclones (34.5%) exhibit more cold‐core axisymmetric structures, preferentially track in the Pacific sector, and tend to be longer‐lived [ABSTRACT FROM AUTHOR]

Attributing sources of tropical cyclogenesis (TCG) bias to large‐scale circulation in global circulation models is challenging. Here, we propose the use of empirical orthogonal functions as an approach to understand model bias of TCG. Two leading modes of large‐scale wind circulations in the West Pacific can explain the TCG frequency and location in both climate reanalysis and the MetUM model. In the reanalysis, the two modes distinguish the summer monsoon trough position and the strength of the north Pacific subtropical high. However, in the model, the wind circulations are biased toward the positive phase of simulated modes thus overestimating TCG in the entire Main Development Region. This bias is further related to the north‐eastward shifted monsoon trough and a weakened subtropical high, and overly strong tropics‐subtropics connections. This approach could be deployed more widely to other basins and models to diagnose the causes of TCG bias. Plain Language Summary: General Circulation Models (GCMs) have bias in tropical cyclogenesis (TCG). This bias largely reduces the credibility of GCMs. In this study, we suggest an empirical orthogonal function‐based approach to trace the sources of TCG bias in the western North Pacific more rigorously back to large‐scale wind circulation patterns. We find that TCG bias in both basin‐wide and regional scales is a combination of the two biased circulation patterns, associated with too strong tropics‐subtropics connections. The two biased circulation patterns are also featured with a north‐eastward shifted monsoon trough and a weakened subtropical high. We choose the Met Office climate models to demonstrate the approach. But, it can be applied to other GCMs and basins to better understand TCG bias and inform further model development. Key Points: Multivariate empirical orthogonal functions are used to diagnose model bias of tropical cyclogenesis (TCG) in the West PacificTwo leading modes of large‐scale wind circulations can explain most of TCG variability in both ECMWF fifth generation climate reanalysis and MetUMIn MetUM, positive TCG bias is related to large‐scale wind circulations biased toward the positive phase of the simulated modes [ABSTRACT FROM AUTHOR]

Predicting the peak‐season (July–September) tropical cyclones (TCs) in Southeast Asia (SEA) several months ahead remains challenging, related to limited understanding and prediction of the dynamics affecting the variability of SEA TC activity. Here, we introduce a new statistical approach to sequentially identify mutually independent predictors for the occurrence frequency of peak‐season TCs in the South China Sea (SCS) and east of the Philippines (PHL). These predictors, which are identified from the preseason (April‐June) environmental fields, can capture the interannual variability of different clusters of peak‐season TCs, through a cross‐season effect on large‐scale environment that governs TC genesis and track. The physically oriented approach provides a skillful seasonal prediction in the 41‐year period (1979–2019), with r = 0.73 and 0.54 for SCS and PHL TC frequency, respectively. The lower performance for PHL TCs is likely related to the nonstationarity of the cross‐season TC‐environment relationship. We further develop the statistical approach to a hybrid method using the predictors derived from dynamical seasonal forecasts. The hybrid prediction shows a significant skill for both SCS and PHL TCs, for lead times up to four or 5 months ahead, related to the good performance of models for the sea surface temperatures and low‐level winds in the tropics. The statistical and hybrid predictions outperform the dynamical predictions, showing the potential for operational use. Plain Language Summary: Tropical cyclones (TCs) have huge impact in Southeast Asia (SEA). Seasonal forecasts of SEA peak‐season (July–September) TCs remains challenging for both statistical and dynamical models. In this paper, we introduce a novel statistical approach to sequentially identify independent predictors tailored for the frequency of SEA peak‐season TCs. The predictors are routinely constructed from far‐field sea surface temperatures and low‐level winds in the preceding‐season. The main concept in this approach is that each predictor represents a unique cluster of TC formation and propagation in the peak‐season. We also develop the approach to a hybrid prediction method by building the predictors from dynamical seasonal forecasts. The seasonal predictions based on the new approach perform much better than the current statistical and dynamical models. Such predictions can provide useful warning service for SEA TC activity 4–5 months ahead. Key Points: A new approach to identify independent predictors of Southeast Asia (SEA) tropical cyclone (TC) frequency is introducedThe preceding‐season predictors represent different clusters of TC formation and propagation through a cross‐season effectThe new method outperforms existing statistical and dynamical predictions for SEA TCs [ABSTRACT FROM AUTHOR]

Cut‐Off Lows (COLs) in the Southern Hemisphere and the mechanisms involved in their development are investigated in detail using the eddy kinetic energy (EKE) budget applied to data from the ERA‐Interim reanalysis. This approach considers the most important processes that are typical for the evolution of midlatitude disturbances such as the baroclinic (BRC) and barotropic (BRT) conversions, and the ageostrophic flux convergence (AFC), known as downstream development. Composites of the volume‐integrated EKE and its components are evaluated based on the 200 most intense SH COLs (>98th percentile) observed in a 36‐year period. Results show that the AFC together with the BRC conversion are the most important contributors to the EKE growth for the COLs, characterizing the downstream baroclinic development. The AFC plays an important role in genesis and intensification phases of the COLs, while the BRC conversion is important for the system maintenance. The dissipation of the COLs occurs due to dispersive fluxes (ageostrophic flux divergence) together with other processes not directly computed in the EKE equation, such as friction and latent heat release which are problematic in reanalysis datasets. The BRT conversion contributes negatively to the COL development by transferring EKE to the zonal flow kinetic energy, though this is not enough to dampen the intensification. Regional differences were found in the energetics, indicating that COLs originating upstream of the continents are clearly dominated by ageostrophic fluxes, while the systems over the Australian region are mostly driven by baroclinic processes. [ABSTRACT FROM AUTHOR]

North Atlantic tropical and post‐tropical cyclones impact midlatitude regions, but the inhomogeneous observational record of the latter stages of tropical cyclones precludes many climatological analyses. The frequency of tropical‐origin storms basin‐wide is projected to increase under anthropogenic climate change, so establishing confidence in our knowledge of their historical variability and lifecycles—against which climate model simulations may be evaluated—is important. We used a Lagrangian feature‐tracking algorithm to identify tropical cyclones that impacted Northeast North America and Europe in seven global reanalysis data sets, distinguishing systems that retained warm‐core structures or underwent warm seclusion from those that underwent extratropical transition, acquiring cold‐core, frontal structures. Over the last four decades, ∼25% and ∼10% of tropical‐origin cyclones made landfall across Northeast North America and Europe, respectively, as warm‐core systems, with, on average, higher wind speeds than cold‐core systems. Historical warm‐core and cold‐core landfalls also exhibit distinct tracks, likely responding to differing steering flow and midlatitude conditions. Plain Language Summary: Tropical cyclones and their impacts are not confined to the tropics. In the North Atlantic, the historical record of observed tropical cyclones is probably better than anywhere else, yet even this record is unreliable for storms that originate in the tropics and migrate to the midlatitudes, a particular danger to North American and European coastal regions. Operational collection of pre‐satellite aerial and ship‐based observations has changed over time, and the record of storm landfalls outside the United States is incomplete. These data issues matter because the frequency of tropical‐origin storms across the midlatitudes is expected to increase under anthropogenic climate change, and the basis of any assessment of future risk must be a robust historical understanding. We therefore turned to reanalyses (gridded climate data sets) and an objective storm‐tracking algorithm to identify tropical cyclones that impacted two regions: Northeast North America and Europe. We examined the lifecycles of these storms and their nature and intensity at landfall. Over the last four decades, approximately one in four landfalls over Northeast North America exhibited tropical‐cyclone characteristics, and such storms are associated with high wind speeds, posing a greater risk. For Europe, that number is one in 10, a surprisingly high proportion. How this may change in future will affect populous regions on both sides of the North Atlantic. Key Points: North Atlantic cyclones originating in the tropics examined across seven reanalysesReanalyses show consistent interannual variability, seasonality, and structural evolution of tropical‐origin cyclonesIntensity evolution and tracks are distinct between warm‐core and cold‐core cyclones [ABSTRACT FROM AUTHOR]

Post‐tropical cyclones (PTCs) extend many hazards associated with tropical cyclones (TCs) to the midlatitudes. Despite recent high‐impact cases affecting Europe such as Ophelia, little research has been done to characterize the risk of PTCs. Here we compare the climatologies and intensity distributions of midlatitude cyclones (MLCs) and PTCs in the North Atlantic and Europe by tracking cyclones in the ERA5 reanalysis. Considering hurricane season cyclones impacting Northern Europe, PTCs show a significantly higher mean maximum intensity than MLCs, but make only a small contribution to total windstorm risk. Our results show that a disproportionately large fraction of high‐intensity cyclones impacting Europe during hurricane season are PTCs. The fraction of PTCs impacting N Europe with storm force (>25 m s−1) winds is ~10 times higher than that for MLCs. Less than 1% of cyclones impacting Northern Europe are identified to be PTCs. This rises to 8.8% when considering cyclones which impact with storm force winds. Plain Language Summary: Ex‐hurricanes (post‐tropical cyclones; PTCs) can bring hazardous weather such as damaging winds and extreme precipitation to the midlatitudes. The importance of these cyclones for European wind and flood risk is still an open question. By tracking cyclones through 39 years of data, we show that on average, the maximum intensity of PTCs over Europe is significantly higher than that for European windstorms (midlatitude cyclones, MLCs). The difference between the maximum intensity of PTCs and MLCs is larger across Northern Europe than Southern Europe. Our results show that a disproportionately large fraction of high‐intensity cyclones impacting Northern Europe during hurricane season are PTCs. The fraction of PTCs impacting Northern Europe with storm force winds is ~10 times greater than for MLCs. Less than 1% of cyclones impacting Northern Europe during the North Atlantic hurricane season are PTCs. This rises to 8.8% when only considering cyclones which impact Northern Europe with storm force winds. Key Points: Post‐tropical cyclones are significantly more intense than midlatitude cyclones at their maximum intensity over EuropeThe fraction of post‐tropical cyclones impacting northern Europe with storm force winds is 10 times higher than that for midlatitude cyclonesA disproportionately large fraction of high‐impact European windstorms forming in hurricane season are caused by post‐tropical cyclones [ABSTRACT FROM AUTHOR]

Southern African tropical lows are synoptic‐scale cyclonic vortices that propagate westward across southern Africa in the Austral summer. They strongly influence local rainfall and aggregate in the climatological December, January, and February mean to form the Angola Low. In this study, tropical lows are identified and tracked using an objective feature tracking method. The statistics of tropical low tracks over southern Africa are presented and compared across three reanalysis products. Findings are compared to the literature of tropical low‐pressure areas elsewhere in the world, where it is found that most tracking statistics compare well but that the tendency of tropical lows to become semistationary over Angola is unique to southern Africa. The hypothesis that tropical lows in Angola have a causal relationship with Tropical Temperate Troughs is tested, and a correlation between occurrence frequencies is found at interannual but not daily time scales. Precipitation is attributed to the tropical lows, and it is found that tropical lows are associated with 31% of rainfall across tropical southern Africa, based on gridded precipitation products. The interannual variability of the number of tropical lows that form per year (σ=6 events/year) is linked to El Niño–Southern Oscillation (ENSO) and the tropical easterly jet. The mean latitude of tropical lows is shifted northward during El Niño and southward during La Niña. Much of the interannual precipitation variability maximum in Angola is attributed to rainfall associated with tropical lows. These results provide insights into the southern African response to ENSO and into the mechanisms of rainfall in the southern African tropical edge. Plain Language Summary: Tropical lows are storms that are a bit like very weak tropical cyclones. They occur in summer in tropical regions. This paper tracks individual tropical lows across southern Africa in weather records from the last 39 years. We then look at how they behave, how much they rain, whether they interact with other storms, and how they vary from year to year. About 33 tropical lows are found each year, and they tend to cluster in eastern Angola. We find that 31% of rainfall in the region surrounding Angola and Zambia comes from tropical lows. The number of tropical lows that form in a summer is really important for whether that summer will be unusually dry or wet, particularly along the northern border of Namibia. The number of tropical lows that form in each year is related to the El Niño–Southern Oscillation and also to the winds high up in the atmosphere. In the El Niño phase of El Niño–Southern Oscillation cycle, which usually causes drought in southern Africa, tropical lows form less frequently and further north. In the La Niña phase, which is usually wet in southern Africa, tropical lows form more frequently and further south. Key Points: Vorticity tracking and rainfall attribution algorithms are applied to reanalysis tropical lows in southern AfricaTropical lows track north (south) in El Niño (La Niña); this behavior dominates the ENSO rainfall signal west of 30°EOn a synoptic time scale tropical temperate troughs and Angola tropical lows have statistically independent timing [ABSTRACT FROM AUTHOR]

We examine the merit of atmosphere–ocean coupled models for tropical cyclone (TC) predictions in the western North Pacific (WNP), where accurate TC predictions remain challenging. The UK Met Office operational atmospheric global numerical weather prediction (NWP) model is compared with two trial coupled configurations, in which the operational atmospheric model is coupled to a one‐dimensional mixed‐layer ocean model and a three‐dimensional dynamical ocean model. Reforecasts for the 2016 TC season show that the coupled models outperform the NWP model for TC location predictions, with a systematic improvement of 50–100 km over the seven‐day forecasts, but the coupled models amplify the underestimation of TC intensity in the NWP model. Nearly identical TC predictions (for both location and intensity) are found in the two coupled models, indicating the dominance of thermodynamic processes at the air–sea interface for TC predictions on these timescales. The improved prediction of the TC position in the coupled models is associated with an enhanced Western North Pacific Subtropical High (WNPSH), which introduces an anticyclonic steering flow anomaly that shifts TC tracks further west in the southern part of the region and further east in the northern part. Based on sensitivity experiments, we show that these improvements in the coupled models are due mainly to colder initial sea‐surface temperatures (SSTs). Air–sea feedbacks do not change the WNPSH or TC tracks noticeably. Apart from the effect of the initial SSTs, tropical ocean warming due to air–sea interaction in the coupled forecasts can also reduce the predicted TC intensity, presumably due to a stronger regional Hadley circulation with increased subtropical subsidence. [ABSTRACT FROM AUTHOR]

Using a recently developed methodology, namely the multiscale window transform (MWT) and the MWT‐based theory of canonical transfer and localized multiscale energetics analysis, we investigate in an eddy‐following way the nonlinear eddy–background flow interaction in the North Pacific storm track, based on the ERA‐40 reanalysis data from the European Centre for Medium‐Range Weather Forecasts. It is found that more than 50% of the storms occur on the northern flank of the jet stream, about 40% are around the jet centre, and very few (less than 5%) happen on the southern flank. For storms near or to the north of the jet centre, their interaction with the background flow is asymmetric in latitude. In higher latitudes, strong downscale canonical available potential energy transfer happens, especially in the mid‐troposphere, which reduces the background baroclinicity and decelerates the jet; in lower latitudes, upscale canonical kinetic energy transfer intensifies at the jet centre, accelerating the jet and enhancing the middle‐level baroclinicity. The resultant effect is that the jet strengthens but narrows, leading to an anomalous dipolar pattern in the fields of background wind and baroclinicity. For the storms on the southern side of the jet, the baroclinic canonical transfer is rather weak. On average, the local interaction begins about 3 days before a storm arrives at the site of observation, achieves its maximum as the storm arrives, and then weakens. The nonlinear eddy–background flow interaction in the North Pacific storm track is investigated in an eddy‐following way. It is found that overall the storm lies in the northern part of the jet; this relative location determines a unique asymmetric interaction pattern, with a downscale baroclinic energy transfer (baroclinic instability) in the north part of the storm and an upscale barotropic energy transfer in the south. As a result, the jet strengthens but becomes narrower. [ABSTRACT FROM AUTHOR]

Cyclones play an important role in the coupled dynamics of the Arctic climate system on a range of time scales. Modeling studies suggest that storminess will increase in Arctic summer due to enhanced land-sea thermal contrast along the Arctic coastline, in a region known as the Arctic Frontal Zone (AFZ). However, the climate models used in these studies are poor at reproducing the present-day Arctic summer cyclone climatology and so their projections of Arctic cyclones and related quantities, such as sea ice, may not be reliable. In this study we perform composite analysis of Arctic cyclone statistics using AFZ variability as an analog for climate change. High AFZ years are characterized both by increased cyclone frequency and dynamical intensity, compared to low years. Importantly, the size of the response in this analog suggests that General Circulation Models may underestimate the response of Arctic cyclones to climate change, given a similar change in baroclinicity. [ABSTRACT FROM AUTHOR]

The potential changes in tropical cyclone (TC) activity, and the associated large-scale environmental conditions, for Vietnam and the South China Sea, posed by climate change, are examined using a high-resolution regional climate model system, PRECIS 2.1, to downscale the Met Office Hadley Centre CMIP5 model, HadGEM2-ES, to a resolution of 25 km (PRECIS_Had2). The ERA-Interim reanalysis is also downscaled to the same resolution (PRECIS_ERAI) for comparison. An objective algorithm is used to identify and track the TCs. The PRECIS_Had2 is evaluated for the period of 1990-2005 by comparing with the PRECIS_ERAI and the observed best-track data of TCs. Compared to PRECIS_ERAI, PRECIS_Had2 represents the TC-associated large-scale environments reasonably well but shows stronger vertical wind shear over the South China Sea and during the summer southwesterly monsoon. For TCs, PRECIS_Had2 is capable of capturing the TC distribution but shows a notable underestimation of very intense TCs. The analysis of the influence of global warming on the TC activity reveals that PRECIS_Had2 simulates a pronounced seasonal shift of TC activity in a warmer climate for both the RCP4.5 and RCP8.5 scenarios, with an increase in TC activity during winter related to the more favourable large-scale conditions, while a decrease in TCs associated with less favourable large-scale conditions is projected in summer. [ABSTRACT FROM AUTHOR]

The ability of a newly developed non-hydrostatic regional climate model ( RCM) based on the Unified Model of the UK Met Office ( MetUM) at a resolution of 12 km is examined for the simulation of tropical cyclone ( TC) activity affecting the South China Sea and is compared with the current version of the MetUM RCM at the resolution of 25 km. The results show that both the 25 and 12 km models can reasonably simulate the TC-associated large-scale environments, while the 12 km model has a better ability to simulate the South Asian monsoon. Compared with the 25 km model, the 12 km model generally improves the simulation of track density and the radial wind structure of TCs. However, the annual cycle of simulated TCs show that both models tend to overestimate the TC frequency in May and November-January while underestimating the frequency in June-September. Compared with the 25 km model, the 12 km model produces fewer intense TCs with 10 m maximum wind speeds >30 m s−1. It is also found that both the 12 and 25 km models reproduce the observed modulation of TC activity associated with different phases of the El Niño/Southern Oscillation ( ENSO) such as the reduced track density and accumulated cyclonic energy during El Niño events, while the 12 km model better captures the TC-ENSO response including the track density and the large-scale environments than the 25 km model. [ABSTRACT FROM AUTHOR]