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In this study, we investigate the climatology and trend of explosive cyclones (ECs) over the Southern Ocean (50oS–70oS) during 1980–2020 by combining a method that is most suited for identifying and tracking cyclones in the Southern Ocean and a latest climate reanalysis. On average, approximately 50 ECs are generated annually over the Southern Ocean, with a significant increasing trend of 2.3 per decade during the studying period. This increasing trend is dominated by the trend of strong ECs, particularly in autumn. We analyze the dynamical and thermodynamical effects associated with multiple deepened strong ECs in autumn over an identified key region in the southern Pacific Ocean sector (155oW–170oW, 50oS–65oS), where the density of the initiation of ECs shows the largest increasing trend in autumn. The composite analysis reveals the general patterns and duration of the effects on the atmosphere, ocean, and sea ice associated with multiple ECs in the southern Pacific Ocean. The results indicate that the deepened strong ECs are associated with significant changes in meridional winds, downward longwave radiation, and sensible and latent heat fluxes. These changes lead to cold sea surface temperature anomalies in the northeast of the key region, reaching a maximum 5–7 days after the EC deepening, and the increased sea ice cover south of the key region, peaking 4–5 days after the EC deepening. Highlights: Explosive cyclones (ECs) over the Southern Ocean (50oS–70oS) have a significant increasing trend of 2.3 per decade during 1980–2020. The increasing trend is dominated by the trend of strong ECs, especially in autumn. The anomalies of sea surface temperature and sea ice cover can sustain about 2 weeks following the generation of strong ECs. [ABSTRACT FROM AUTHOR]

Extreme precipitation events induced by tropical cyclones have increased frequency and intensity, significantly impacting human socioeconomic activities and ecological environments. This study systematically examines the spatiotemporal characteristics of these events across Hainan Island and their influencing factors using GsMAP satellite precipitation data and tropical cyclone track data. The results indicate that while the frequency of typhoon events in Hainan decreased by 0.3 events decade−1 from 1949 to 2020, extreme precipitation events have increased significantly since 2000, especially in the eastern and central regions. Different typhoon tracks have distinct impacts on the island, with Track 1 (Northeastern track) and Track 2 (Central track) primarily affecting the western and central regions and Track 3 (Southern track) impacting the western region. The impact of typhoon precipitation on extreme events increased over time, being the greatest in the eastern region, followed by the central and western regions. Incorporating typhoon precipitation data shortened the recurrence interval of extreme precipitation in the central and eastern regions. Diurnal peaks occur in the early morning and late evening, primarily affecting coastal areas. Typhoon duration (CC_max = 0.850) and wind speed (CC_max = 0.369) positively correlated with extreme precipitation, while the pressure was negatively correlated. High sea surface temperature areas were closely associated with extreme precipitation events. The atmospheric circulation indices showed a significant negative correlation with extreme precipitation, particularly in the western and central regions. ENSO events, especially sea surface temperature changes in the Niño 1 + 2 region (−0.340 to −0.406), have significantly influenced typhoon precipitation characteristics. These findings can inform region-specific disaster prevention and mitigation strategies for Hainan Island. [ABSTRACT FROM AUTHOR]

Ockhi was a devastating tropical cyclone that struck parts of Sri Lanka, India, and the Maldives in November 2017. The storm began as a low-pressure system over the Bay of Bengal and rapidly intensified into a very severe cyclonic storm, causing havoc along its path. Interestingly, Ockhi was a rare storm to have rapid intensification over the Arabian Sea basin during its cyclogenesis stage. In this analysis, we examine the factors that influenced Ockhi's genesis and rapid intensification. It is known that the warm core eddies have a potential role in the rapid intensification of tropical cyclones. However, in our study, it was observed that no warm core eddies were found along the track of tropical cyclone Ockhi during its rapid intensification. Thus, additional atmospheric and oceanic features that contribute to the development and intensification of tropical cyclones have been taken into account. A very high tropical cyclone heat potential of 60–90 kJ cm−2, an exceptionally high sea surface temperature anomaly of above 1–1.5 °C, and a high moist static energy of 340 × 103 J kg−1 or greater were observed to be persisting over the region where the intensification has taken place. The brightness temperature was < 200 K, indicating strong and very deep convective activity over the region of intensification. Along with favourable ocean and atmospheric features such as high tropical cyclone heat potential, sea surface temperature, and moist static energy, the presence of deep convection persisting for a prolonged time can eventually intensify the system. [ABSTRACT FROM AUTHOR]

East coast cyclones (ECCs) provide an essential reprieve from dry periods across eastern Australia. They also deliver flood‐producing rains with significant economic, social and environmental impacts. Assessing and comparing the influence of different types of cyclones is hindered by an incomplete understanding of ECC typology, given their widely variable spatial and temporal characteristics. This study employs a track‐clustering method (probabilistic, curve‐aligned regression model) to identify key cyclonic pathways for ECCs from 1950 to 2019. Six spatially independent clusters were successfully distinguished and further sub‐classified (coastal, continental and tropical) based on their genesis location. The seasonality and long‐term variability, intensity (maximum Laplacian value ± 2 days) and event‐based rainfall were then evaluated for each cluster to quantify the impact of these lows on Australia. The highest quantity of land‐based rainfall per event is associated with the tropical cluster (Cluster 6), whereas widespread rainfall was also found to occur in the two continental clusters (clusters 4 and 5). Cyclone tracks orientated close to the coast (clusters 1, 2 and 3) were determined to be the least impactful in terms of rainfall and intensity, despite being the most common cyclone type. In terms of interannual variability, sea surface temperature anomalies suggest an increased cyclone frequency for clusters 1 (austral winter) and 4 (austral spring) during a central Pacific El Niño. Furthermore, cyclone incidence during IOD‐negative conditions was more pronounced in winter for clusters 1, 2, 3— and clusters 4 and 5 in spring. All cyclones also predominantly occurred in SAM‐positive conditions. However, winter ECCs for clusters 1 and 3 had a higher frequency in SAM‐negative. This new typology of ECCs via spatial clustering provides crucial insights into the systems that produce extreme rainfall across eastern Australia and should be used to inform future hazard management of cyclone events. [ABSTRACT FROM AUTHOR]

This study investigates the impact of future climate warming on tropical cyclones (TC) and extratropical cyclones (ETC) using the database for Policy Decision-Making for Future Climate Change (d4PDF) large ensemble simulations. Cyclone tracking was performed using the neighbor enclosed area tracking algorithm (NEAT), and TC and ETCs were identified over the western North Pacific Ocean (WNP). For cyclone frequency, it was revealed that, although a slight underestimation of the total number of TCs and ETCs in both the WNP and near Hokkaido, Japan, exists, the d4PDF reproduced the spatial distribution of both TC and ETC tracks well when compared with observations/reanalysis. The 4-K warming scenarios derived from six different sea surface temperature warming patterns showed robust decreases in TC frequency in the tropical WNP and a slight reduction in ETCs near Japan. Next, precipitation characteristics for TCs or ETCs in the vicinity of Hokkaido were examined using 5-km-mesh regional climate ensemble simulations. Four representative cyclone locations near Hokkaido are identified using K-means clustering and revealed distinct precipitation characteristics between clusters, with higher TC-associated precipitation than ETC-associated precipitation and the heaviest precipitation in the southern portion of the prefecture. The 4-K warming scenarios revealed increased precipitation for all cyclone placements for both TCs and ETCs. Last, average cyclone intensity, translation speed, and size were examined. It was shown that TCs in future climates are more intense, propagate more slowly, and are smaller in terms of enclosed vorticity area as they approach Hokkaido. For ETCs, mean intensity does not change much; they travel slightly faster, and become smaller. [ABSTRACT FROM AUTHOR]

Generations of climate models exhibit biases in their representation of North Atlantic storm tracks, which tend to be too far near the equator and too zonal. Additionally, models have difficulties simulating explosive cyclone growth. These biases are one of the reasons for the uncertainties in projections of future climate over Europe, and the underlying causes have yet to be determined. All three biases are shown to be related, and diabatic processes are pointed to as a likely cause. To demonstrate this, two hemispherically symmetric storm tracks forming downstream of an idealized sea surface temperature (SST) front on an aquaplanet are examined using the seamless weather and climate prediction model ICOsahedral Non‐hydrostatic and its grid refinement capabilities. The analyzed perpetual boreal winter has a global grid spacing of 20 km, two bi‐directionally interacting grid nests over the Northern Hemisphere that refine the grid to 10‐km spacing over much of the stormtrack and further to 5‐km spacing near the SST front. In contrast, no grid refinement is performed for the Southern Hemisphere. Feature‐based cyclone tracking shows that the poleward propagation in the NH is enhanced, so the high‐resolution storm track is less equatorward and less zonal; explosive deepening rates are more frequent and precipitation rates are amplified. The implication is that resolving diabatic processes on the storm scale improves all three intersecting biases in the representation of storm tracks. While new challenges arise at cloud resolving scales, much improvement for the representation of storm tracks will be gained once climate models resolve the meso‐γ scale. Plain Language Summary: Most people in the mid‐latitudes will experience climate change through changes in their daily weather. Much of the daily weather variability is determined by the propagation of extratropical low pressure systems. The direction of propagation of these systems dictates regional precipitation patterns, and an accurate representation of the track is important to reduce uncertainties in future projections. However, climate models simulate tracks that are too zonal (i.e., east–west), too close to the equator and too weak. Using an idealized simulation, this study shows that individual tracks will propagate more poleward, intensification rates will increase and the tracks become less zonal at storm‐resolving model resolution, helping to reduce this well‐known circulation bias in climate models. Key Points: Generations of climate models simulate storm tracks too zonal, too equatorward and insufficiently represent explosive cyclone growthA 10‐year simulation with a local 5‐km grid nest suggests that all three biases are interrelated, because poleward storm propagation is inherently connected to intensificationEvidence is presented that diabatic processes are an important contributor to all three biases and convection‐resolving simulations have the potential to improve the storm track representation [ABSTRACT FROM AUTHOR]

A high-resolution ocean circulation model is customized over the region (80°E–95°E; 10°N–23°N) in the Bay of Bengal to study the changes in upper ocean properties as a result of super cyclone Phailin. The model when forced with high-resolution reanalysis winds, which are scaled on the observed along track wind data, gives realistic simulation of upper ocean (surface as well as sub-surface) variability during the cyclone. The cyclone causes significantly high decrease (increase) in sea surface temperature (salinity) to the right side of the cyclone track. A steep reduction in the sea surface height of the region is also noticed. The vertical stability analysis indicates the presence of high instability and turbulence during the cyclone. It is found that relatively higher (very low) energy is required for mixing during the cyclone (post-cyclone) period. The heat (salt) budget analysis suggests that strong temperature decrease (salinity increase) in the surface and sub-surface (i.e., at 50 m) occurs mainly due to the advection and diffusion (i.e., storm-induced mixing of colder and high-salinity waters from below) processes. [ABSTRACT FROM AUTHOR]

Analyzing the spatial-temporal changes in tropical cyclone (TC) tracks in the east China coastal ocean (ECCO) to quantify the magnitude of poleward and landward migration of TCs is of significant importance for coastal disaster mitigation and planning due to its susceptibility to the impacts of TCs. In this study, the TCs that affected the ECCO from 1949 to 2022 are classified into three typical types of tracks using the k-means clustering method, mass moments, and track interpolation based on TC location, shape, and intensity information. Type 1 is a northwestward track, Type 2 is a northwest to northeast-turning track, and Type 3 is a northwest to northeast-turning offshore track. Type 1 tracks mainly make landfall in southern China, while Type 2 predominantly makes landfall in eastern China. Moreover, the proportion of Type 1 decreases while their landfall percentage increases over time, and the proportion of Type 2 tracks is increasing. The probability of TC effects on the eastern and northern parts of the ECCO is increasing, and the boundary where the TC center reaches after landfall is shifting landward. During the period from 1994 to 2022, there has been a significant migration in TC tracks, with the mean centroid of the TCs affecting the ECCO shifting westward by 0.66° in longitude and northward by 1.26° in latitude, which means the magnitude of the poleward shift is about twice that of the landward shift. This migration appears to have been pre-conditioned by a combined influence of a weakening westward steering flow, reduced vertical wind shear, and warmer sea surface temperature Our findings provide valuable insights into the longitudinal and latitudinal migration of TC tracks and have important implications for disaster prevention, mitigation planning, and the adjustment of crucial coastal protection zones in the ECCO and similar regions around the globe. • The TCs that affected the ECCO from 1949 to 2022 are classified into three typical types. • The poleward and landward migration of the TCs were significantly greater than the earlier period and poleward twice the landward. • The probability of TC effects on the eastern and northern parts of the ECCO is increasing, and the distance the TC center penetrates after landfall is shifting landward. [ABSTRACT FROM AUTHOR]

The track of tropical cyclones (TCs) formed in the South China Sea (SCS) can be divided into eastward and westward directions. Significant decadal variation during 1980–2020 only exists in the number of eastward‐moving TCs, especially during July–September, with 47% TCs moving eastward during 1994–2004 (Period II), 22% during 1980–1993 (Period I) and only 15% during 2005–2020 (Period III). This decadal change is related to the zonal shift of Western Pacific Subtropical High (WPSH). An eastward‐retreated WPSH during 1994–2004 leads to upward motion and westerly flow anomaly over the northern SCS, and therefore favors TC genesis and eastward motion. The eastward‐retreated WPSH is associated with a warm sea surface temperature anomaly over the tropical western‐central Pacific which induces a cyclonic flow and weakens the WPSH. With the weaker modulation of WPSH, stronger intraseasonal oscillation (ISO) in the SCS during Period II favors eastward‐moving TCs due to the westerly flow associated with the ISO. Plain Language Summary: Tropical cyclones (TCs) formed over the South China Sea (SCS) can move either eastward or westward. Eastward‐moving TCs can make landfall along the eastern coast of China, the Philippines, and Taiwan Island, or entering western North Pacific. Meanwhile, westward‐moving TCs usually make landfall along the coasts of southern China and Vietnam. Our study reveals a significant decadal variation of eastward‐moving TCs. More TCs moved eastward during 1994–2004 due to enhanced westerly and convective activity associated with an eastward‐retreated Western Pacific Subtropical High, which is caused by the Gill‐pattern response to the warm sea surface temperature anomaly over the western‐central Pacific. The other reason for a decadal variation of eastward‐moving TCs over the SCS is the stronger intraseasonal oscillation activity during 1994–2004, which induces the convective activities and westerly flow anomaly over the main TC genesis region. Key Points: The number of eastward‐moving tropical cyclones (TCs) in the South China Sea has experienced a decadal variation since 1980The zonal shift of Western Pacific Subtropical High (WPSH) is the main factor of the decadal variation of eastward‐moving TCsThe eastward‐retreated WPSH together with strong intraseasonal oscillation favors TCs moving eastward during 1994–2004 [ABSTRACT FROM AUTHOR]

We used the top age of storm‐relocated coral blocks (SRCBs) from Xisha Islands and Huangyan Island, South China Sea (SCS), in combination with existing paleotempestology records and instrumental climatic data, to reconstruct the spatio‐temporal changes in tropical cyclone (TC) activity over the past millennium, and to project its future trends in the Northwestern Pacific region. We found a positive correlation between the frequencies of TCs, typhoons, and super‐typhoons and the NINO3.4 on the interannual timescale during 1945–2019. TCs latitude has a significant positive correlation with sea surface temperature, with a northward migration rate of 80 ± 11 km per decade during 1951–2019. Coral δ18O can well indicate the month of paleo‐TCs occurrence. The frequency of SRCBs (and by inference, TCs) is relatively low during relatively warm periods (RWP) but high during relatively cold periods (RCP) in the SCS. The spatio‐temporal pattern inferred from paleotempestology records further supports the notion of North‐South migration in TC activity as revealed by the instrumental record in this study. TC activity mainly affected relatively low latitudes (e.g., Philippines, Vietnam, and Hainan, Guangdong, Taiwan) during the RCP (AD 1450–1900), but it migrated from low latitudes towards higher latitudes during the RWP (AD 1000–1450), affecting locations such as Zhejiang, Jiangsu, and Shandong, southwestern Japan, the Korean Peninsula, and even Northeast China. Fujian and northern Guangdong may be in a transition zone of paleo‐TC activity between the North and the South in the past millennium. As global warming continues, the mean tracks and landfall locations of TCs will continue to migrate northward in this region. Key Points: Tropical cyclones (TC) latitude and sea surface temperature are positively correlated; TCs had a northward migration rate of 80 ± 11 km per decade during AD 1951–2019The coral δ18O can indicate the month when the paleo‐TC event happened; SRCBs appeared more frequently during relatively cold periods in the South China SeaPaleo‐TCs mainly affected relatively high latitudes during climatically warm periods, but lower latitudes during cold periods in the Northwestern Pacific region [ABSTRACT FROM AUTHOR]

The lack of observational data in the northern part of the Bay of Bengal (BoB) makes it challenging to investigate the upper oceanic responses to tropical cyclones. To overcome this challenge, a high-resolution Regional Ocean Model (ROMS) is set up with horizontal resolution of 0.03° × 0.03° and 50 vertical layers for the northern BoB, with the daily output of the First Institute of Oceanography surface wave-tide-circulation coupled ocean model (FIO-COM) as the boundary condition, and hourly ERA5 data as atmospheric forcings. This regional model is systematically validated and then utilized to reconstruct the upper ocean response due to the super cyclonic storm Amphan over the BoB from May 16 to 20, 2020. The upper ocean responses to Amphan in the northern BoB is well reconstructed with this regional model. On the right side of the cyclone track, sea surface temperature (SST) cooling (4 °C) and increased sea surface salinity (0.5 psu) are well reproduced. The primary oceanic triggering forces intensified the cyclone through the extraordinarily high SST (>31 °C) and deep isothermal layer depth. Tropical cyclone heat potential was high (over 100Kj cm−2) during the early stages of the cyclone, which aided the transformation of a depression into a super cyclonic storm. Vertical entrainment and horizontal advection had a major influence in the pronounced cooling within the mixed layer. • Upper ocean responses to the super cyclonic storm Amphan were simulated • ROMS model was systematically validated with available datasets • ROMS model successfully simulated the SST cooling and increased sea surface salinity on the right side of the cyclone track • Horizontal advection and vertical entrainment dominantly reduced the mixed layer temperature. [ABSTRACT FROM AUTHOR]

This study presents an assessment of the ability of the Hadley Centre Global Environment Model version 2—Earth system configuration (HadGEM2-ES)—in simulating the mid-latitude storm tracks over the Southern Hemisphere (SH). The storm tracks are primarily assessed using cyclone tracking using data from a 4 member ensemble of 27-year simulations of HadGEM2-ES over the historical period, and the European Centre for Medium-Range Weather Forecasts Interim Reanalysis. Both winter and summer periods are considered and contrasted. Results show that the storm track (ST) climatology of HadGEM2-ES presents similar patterns to those of the reanalysis. However, the model tends to represent the austral winter ST position with an equatorward bias and a zonal bias in the spiral towards the pole. The main differences were found during the austral winter, with large track density biases over the Indian Ocean indicating a poor representation of the ST in this specific region. This was found to be related to two factors. First, the large negative genesis biases over South America, Antarctic Peninsula and the Antarctic coast. Second, the model resolution and the representation of the Andes Mountains in South America. The link between STs and the large-scale circulation is examined and shows at upper levels an equatorward jet position bias of the subtropical jet and a negative bias in the eddy-driven, associated with a large cold bias over the extratropical and polar regions. The analysis of the large-scale circulation shows that the split jet during winter has problems in the model linked to these biases, including geopotential anomaly and sea surface temperature biases. Consequently, in general the track densities over the Southern oceans are underestimated in the austral winter. During summer, the results show the STs move poleward and there is a single eddy-driven jet, which is represented relatively well compared with the winter situation. These factors tend to reduce the differences seen in the cyclone track distribution biases. Although the model has biases in the ST behaviour in the SH it is still considered that these do not preclude this model being used for perturbation and future projection studies. [ABSTRACT FROM AUTHOR]

In this study, the effect of severe tropical cyclone Ockhi on the physical and biological changes in ocean properties has been investigated. Winds from SCATSAT-1 was used for estimating the cyclone induced Ekman pumping. The role of ocean stratification, cyclonic wind stress, eddies and cyclone translation speed on the post cyclone oceanic response has been investigated. 8-day composite data of winds, sea surface temperature (SST), chlorophyll and sea level anomaly (SLA) have been analyzed for pre-cyclone, during cyclone and after cyclone period to understand the effect of wind forcing on the ocean variables. It was observed that there was a significant phytoplankton bloom like concentration along with 2-3 °C drop in SST after the passage of Ockhi. Thermocline displacements of up to 56 m were observed along the cyclone track. SLA data analyzed for the period shows formation of a cyclonic cold core eddy and weakening of anticyclonic warm core eddies. The entire cyclone track was divided into 12 track points based on the 12 hourly position and the change in SST and chlorophyll analyzed. The maximum change in chlorophyll and SST was observed where the cyclone was slowest and mixed layer shallow. [ABSTRACT FROM AUTHOR]

The Northwest Pacific features strong sea surface temperature (SST) gradients providing favourable conditions for wintertime cyclone intensification in the midlatitudes. To estimate the relative contribution of the SST front to the evolution of cyclones and identify the mechanisms for cyclone intensification, we track individual cyclones and categorise them depending on their propagation relative to the SST front. We focus on cyclones remaining on either the cold or warm side of the SST front, as well as those crossing the SST front from the warm to the cold side. Cyclones crossing the SST front or remaining on its warm side propagate near the left exit region of the jet and are associated with higher precipitation, consistent with higher moisture availability and cyclone intensity. Comparing the different cyclone categories, there is no direct effect of the SST front on cyclone intensification. However, the SST front contributes to the climatological low‐level baroclinicity, providing a conducive environment for cyclone intensification for the cyclones crossing the SST front. Compared with the Gulf Stream region, the land–sea contrast plays a less prominent role for the low‐level baroclinicity in the Kuroshio region. [ABSTRACT FROM AUTHOR]

The Northwest Atlantic is a region of strong temperature gradients and hence is a favourable location for wintertime cyclone intensification co‐located with the storm track. The temperature gradient is associated with both the sea surface temperature front along the Gulf Stream and the land–sea contrast. To understand the respective influences of the sea surface temperature (SST) front and land–sea contrast in the Gulf Stream region, as well as the role of upper‐level forcing on cyclone development, we track individual cyclones and categorise them depending on their propagation relative to the SST front. We concentrate on cyclones staying either on the cold (C1) or warm (C2) side of the SST front, and on cyclones that cross the SST front from the warm to the cold side (C3). Comparing these categories, we find that the land–sea contrast is more important for supplying baroclinicity to cyclones in C1, while the strong low‐level baroclinicity in C3 is also partially attributable to the SST front. The propagation of cyclones in C1 and C3 near the left exit region of the North Atlantic jet explains the higher intensification and precipitation. [ABSTRACT FROM AUTHOR]

This study focuses on a single Mediterranean hurricane (hereafter medicane), to investigate its response to global warming during the middle of the 21st century and assesses the effects of a warmer ocean and a warmer atmosphere on its development. Our investigation uses the state-of-the-art regional climate model WRF to produce the six-member, multi-physics ensembles. Results show that our model setup simulates a realistic cyclone track and the transition from an initial disturbance to a tropical-like cyclone with a deep warm core. However, the simulated transition occurs earlier than for the observed medicane. The response of the medicane to future climate change is investigated with a pseudo global warming (PGW) approach. This is the first application of the PGW framework to medicanes. The PGW approach adds a climate change delta (defined as difference between future and present climate) to WRF's boundary conditions which is obtained for all prognostic variables using the mean change in an ensemble of CMIP5 simulations. A PGW simulation where the climate change delta is added to all prognostic variables (PGW ALL) shows that most of the medicane characteristics moderately intensify, e.g. surface wind speed, uptake of water vapour, and precipitation. However, the minimum sea level pressure (SLP) is almost identical to that under present climate conditions. Two additional PGW simulations were undertaken; One simulation adds the projected change in sea surface and skin temperature only (PGW SST) while the second simulation adds the PGW changes to only atmospheric variables (PGW ATMS); i.e. we use present-day sea surface temperatures. These simulations show opposing responses of the medicane. In PGW SST , the medicane is more intense than PGW ALL as indicated by lower SLP values, the stronger surface wind, and the more intense evaporation and precipitation. In contrast, the medicane in PGW ATMS still transitions into a tropical-like cyclone with a deep warm core, but the PGW ATMS medicane weakens considerably (SLP, surface wind, and rainfall decrease). This difference can be explained by an increase in water vapour driven by the warmer ocean surface (favourable for cumulus convection). The warmer and drier atmosphere in PGW ATMS tends to inhibit condensation (unfavourable for cumulus convection). The warmer ocean and warmer atmosphere have counteracting effects which leads to only a modest enhancement of the medicane by global warming. The novel approach in this study provides new insights into the different roles of warming of the ocean and atmosphere in medicane development. [ABSTRACT FROM AUTHOR]

A statistical comparative analysis of tropical cyclone activity over the Arabian Sea (AS) and Bay of Bengal (BoB) has been conducted using best-track data and wind radii information from 1977 to 2018 issued by the Joint Typhoon Warning Center. Results show that the annual variation in the frequency and duration of tropical cyclones has significantly increased over time over the AS and insignificantly decreased over the BoB. The monthly frequency of tropical cyclones over the AS and the BoB shows a notable bimodal character, with peaks occurring in May and October-November, respectively. The maximum frequency of tropical cyclones occurs in the second peak as a result of the higher moisture content at mid-levels in the autumn. However, the largest proportion of strong cyclones (H1-H5 grades) occurs in the first peak as a result of the higher sea surface temperatures in early summer. Tropical cyclones over the AS break out later during the first peak and activity ends earlier during the second peak, in contrast with those over the BoB. This is related to the onset and drawback times of the southwest monsoon in the two basins. Tropical cyclones over the AS are mainly generated in the eastern basin, whereas in the BoB the genesis locations are meridionally (zonally) distributed in May-June (October-November) as a result of the seasonal movement of the low-level positive vorticity belt. The Arabian Sea is dominated by tropical cyclones that track west and northwest, accounting for about 74.6% of all the tropical cyclones there, whereas the tropical cyclones with a NE track account for only 25.4%. The proportions of the three types of tracks are similar in the BoB, with each accounting for about 33% of the tropical cyclones. The mean intensity and size of tropical cyclones over the AS are stronger and larger, respectively, than those over the BoB and the size of tropical cyclones over the North Indian Ocean in early summer is larger than that in the autumn. The asymmetrical structure of tropical cyclones over the North Indian Ocean is affected by topography and the longest radius of the 34 kt surface wind often lies in the eastern quadrant of the tropical cyclone circulation in both sea areas. [ABSTRACT FROM AUTHOR]

A conventional empirical orthogonal function (EOF) analysis is performed on summertime (May-October) western North Pacific (WNP) tropical cyclone (TC) track density anomalies during 1970-2012. The first leading EOF mode is characterized by a consistent spatial distribution across the WNP basin, which is closely related to an El Niño-Southern Oscillation (ENSO)-like pattern that prevails on both interannual and interdecadal time scales. The second EOF mode is represented by a tripole pattern with consistent changes in westward and recurving tracks but with an opposite change for west-northwestward TC tracks. This second EOF pattern is dominated by consistent global sea surface temperature anomaly (SSTA) patterns on interannual and interdecadal time scales, along with a long-term increasing global temperature trend. ObservedWNP TC tracks have three distinct interdecadal epochs (1970-86, 1987-97, and 1998-2012) based on EOF analyses. The interdecadal change is largely determined by the changing impact of ENSO-like and consistent global SSTA patterns. When global SSTAs are cool (warm) during 1970-86 (1998-2012), these SSTAs exert a dominant impact and generate a tripole track pattern that is similar to the positive (negative) second EOF mode. In contrast, a predominately El Niño-like SSTA pattern during 1987-97 contributed to increasing TC occurrences across most of theWNPduring this 11-yr period. These findings are consistent with long-term trends in TC tracks, with a tripole track pattern observed as global SSTs increase. This study reveals the potential large-scale physical mechanisms driving the changes of WNP TC tracks in association with climate change. [ABSTRACT FROM AUTHOR]

Ocean surface phytoplankton responses to the tropical cyclone (TC)/storms have been extensively studied using satellite observations by aggregating the data into a weekly or bi-weekly composite. The reason behind is the significant limitations found in the satellite-based observation is the missing of valid data due to cloud cover, especially at the time of cyclone track passage. The data loss during the cyclone is found to be a significant barrier to efficiently investigate the response of chl-a and SST during cyclone track passage. Therefore it is necessary to rectify the above limitation to effectively study the impact of TC on the chlorophyll-a concentration (chl-a) and the sea surface temperature (SST) to achieve a complete understanding of their response to the TC prevailed in the Arabian Sea. Intending to resolve the limitation mentioned above, this study aims to reconstruct the MODIS-Aqua chl-a, and SST data using Data Interpolating Empirical Orthogonal Function (DINEOF) for all the 31 cyclonic events occurred in the Arabian Sea during 2003-2018 (16 years). Reconstructed satellite retrieved data covering all the cyclonic events were further used to investigate the chl-a and SST dynamics during TC. From the results, the exciting fact has been identified that only two TC over the eastern-AS were able to induce phytoplankton bloom. On investigating this scenario using sea surface temperature, it was disclosed that the availability of nutrients decides the suitable condition for the phytoplankton to proliferate in the surface ocean. Relevant to the precedent criterion, the results witnessed that the 2 TC (Phyan and Ockhi cyclone) prevailed in the eastern AS invoked a suitable condition for phytoplankton bloom. Other TC found to be less provocative either due to less intensity, origination region or the unsuitable condition. Thereby, gap-free reconstructed daily satellite-derived data efficiently investigates the response of bio-geophysical parameters during cyclonic events. Moreover, this study sensitised that though several TC strikes the AS, only two could impact phytoplankton productivity and SST found to highly consistent with the chl-a variability during the cyclone passage. [ABSTRACT FROM AUTHOR]

This study focuses on a single Mediterranean hurricane (hearafter medicane), to investigate the medicane response to global warming during the middle of the 21st century and assess the contradictory effects of a warmer ocean and a warmer atmosphere on its development. Our investigation uses the state-of-the-art regional climate model WRF with the optimum combination of physical parameterizations based on a sensitivity assessment study. Results show that our model setup can reproduce a realistic cyclone track and the transition from initial disturbance to tropical-like cyclone with a deep warm core although the transition is earlier than for the observed medicane. To investigate the response of the medicane to future climate change, a pseudo global warming (PGW) approach has been used. This approach adds the projected change of atmospheric and ocean variables obtained by an ensemble of CMIP5 models to the boundary conditions for the regional climate model. A PGW simulation where all variables (PGWALL) are incremented shows that most of the medicane characteristics moderately intensify, e.g., surface wind speed, uptake of water vapour and precipitation. However the maximum depression of sea level pressure (SLP) is almost identical with that under present climate conditions. Two additional PGW simulations were undertaken; One simulation adds the projected change in sea surface and skin temperature only (PGWSST) while the second simulation adds the PGW changes to only atmospheric variables (PGWATMS) i.e. we use present time sea surface temperatures. These simulations show opposite effects on the medicane. In PGWSST, the medicane is reinforced more vigorously than PGWALL: much deeper SLP depression, stronger surface wind, and more intense evaporation and precipitation. In contrast, the medicane in PGWATMS weakens considerably (SLP, surface wind and rainfall decrease) still converts into a tropical-like cyclone with a deep warm core. This difference can be explained by an increased water vapour driven by the warmer ocean surface (favourable for cumulus convection) and the warmer and drier atmosphere in PGWATMS tends to inhibit condensation (unfavourable for cumulus convection). As a result of these counteracting effects of warmer ocean and atmosphere, the medicane is enhanced only modestly by global warming. [ABSTRACT FROM AUTHOR]

How future tropical cyclone (TC) activity could change under global warming is enormously important to society, which has been widely assessed using state‐of‐the‐art climate models. However, these models were predominantly based on projection of an El Niño‐like warming pattern. Recent studies suggested that a La Niña‐like warming pattern is also possible. Here we compare the responses of TC track density (TCTD) over the western North Pacific and North Atlantic to the two distinct global warming patterns. We find that the La Niña‐like warming pattern reduces western North Pacific TCTD except in the South China Sea and along China coast and increases NA TCTD, while the El Niño‐like warming pattern generally reduces TCTD in both basins. This is due to different responses of large‐scale dynamic/thermodynamic conditions to the distinct zonal sea surface temperature gradients associated with the two warming patterns. These results help better understand potential future change in TC tracks. Key Points: The La Niña‐like warming reduces (increases) TC track density in the main WNP (NA), while the El Niño‐like warming reduces it in two basinsThe different responses are due to the different tropical zonal SST gradients associated with the two warming patternsChina and North America would experience more landfalling TCs in La Niña‐like warming pattern than in El Niño‐like warming pattern [ABSTRACT FROM AUTHOR]

The Bay of Bengal (BOB) region of Indian Ocean is affected by numerous tropical cyclones during pre- and post-monsoon seasons when various eddies are generated in the central and western bay. Here, numerical simulations of few tropical cyclones (Aila, Laila, Phailin, Hudhud and Madi) that occurred in different seasons are carried out using an ocean atmosphere coupled numerical model, consisting of Weather Research and Forecasting (WRF) and Regional Ocean Modeling Systems (ROMS), to analyse the influence of eddies on tropical cyclone intensifications over the bay. The model is able to track the tropical cyclones with less error, while the central pressure and wind speed are underestimated. Also, the coupling process steer the cyclones to the observed track compared to standalone WRF model. The variations in simulated sea surface temperature and salinity are in agreement with the satellite and in situ measurements. During the passage of Phailin and Hudhud over the eddies, the cyclones intensified almost twice of their initial central pressure and wind (before interacting with eddies), which the model captured very well. The eddy feedback factor, which estimate the influence of warm eddies on tropical cyclones, is about 152% for Phailin and 90% for Hudhud. In the case of the pre-monsoon cyclones, Aila and Laila, the eddy feedback factor is estimated up to 36% and 21%, respectively. Madi cyclone lost its intensification up to 80% because of its interaction with a cold eddy. These predictions are comparable with that of the observations during the cyclone passage. Also it is noted that the warm eddy regions resulted in the maximum transfer of latent heat resulting in strong intensifications of the cyclones. Our study reveals that the eddies play an important role in the intensification and dissipation of BOB cyclones and understanding their nature can help in estimating the track and intensity of cyclones. [ABSTRACT FROM AUTHOR]

A regional‐scale fully coupled data assimilation (DA) system based on the ensemble Kalman filter is developed for a high‐resolution coupled atmosphere‐ocean model. Through the flow‐dependent covariance both within and across the oceanic and atmospheric domains, the fully coupled DA system is capable of updating both atmospheric and oceanic state variables simultaneously by assimilating either atmospheric and/or oceanic observations. The potential impacts of oceanic observations, including sea‐surface temperature, sea‐surface height anomaly, and sea‐surface current, in addition to the observation of the minimum surface pressure at the storm center (HPI), on tropical cyclone analysis and prediction are examined through observing system simulation experiments of Hurricane Florence (2018). Results show that assimilation of oceanic observations not only resulted in better analysis and forecast of the oceanic variables but also considerably reduced analysis and forecast errors in the atmospheric fields, including the intensity and structure of Florence. Compared to weakly coupled DA in which the analysis update is performed separately for the atmospheric and oceanic domains, fully coupled DA reduces the forecast errors of tropical cyclone track and intensity. Results show promise in potential further improvement in tropical cyclone prediction through assimilation of both atmospheric and oceanic observations using the ensemble‐based fully coupled DA system. Plain Language Summary: Air‐sea interactions are critical to tropical cyclone (TC) development. However, oceanic state variables are still poorly initialized and are inconsistent with atmospheric initial fields in most operational coupled TC forecast models. In this study, the ensemble Kalman filter is used to develop a regional‐scale air‐sea fully coupled data assimilation (DA) system for coupled atmosphere‐ocean prediction. The potential impacts of oceanic observations, including sea surface temperature, sea surface height anomaly, and sea surface current on TC analysis and prediction, are examined through observing system simulation experiments of Hurricane Florence (2018). Results show that, using the flow‐dependent covariance within and across the oceanic and atmospheric domains, the fully coupled DA system is capable of updating both atmospheric and oceanic state variables simultaneously by assimilating either atmospheric and/or oceanic observations. The track, intensity, and sea surface height forecasts of Florence are improved by assimilating oceanic observations. When compared to weakly coupled DA, in which the analysis update is performed separately for the atmospheric and oceanic domains, fully coupled DA reduces the forecast errors of TC track and intensity. It shows great promise in potential further improvements in TC prediction through using the ensemble‐based fully coupled DA system. Key Points: A convection‐permitting air‐sea‐coupled ensemble data assimilation (EDA) system for improving tropical cyclone prediction is developedThe coupled EDA system is tested through observing system simulation experiments (OSSEs) of Hurricane Florence (2018)Assimilation of oceanic observations using the coupled EDA system improves the analysis and prediction of Florence [ABSTRACT FROM AUTHOR]

Cyclone Phailin, which developed over the Bay of Bengal in October 2013, was one of the strongest tropical cyclones to make landfall in India. We study the response of the salinity-stratified north Bay of Bengal to Cyclone Phailin with the help of hourly observations from three open-ocean moorings 200 km from the cyclone track, a mooring close to the cyclone track, daily sea surface salinity (SSS) from Aquarius, and a one-dimensional model. Before the arrival of Phailin, moored observations showed a shallow layer of low-salinity water lying above a deep, warm ''barrier'' layer. As the winds strengthened, upper-ocean mixing due to enhanced vertical shear of storm-generated currents led to a rapid increase of near-surface salinity. Sea surface temperature (SST) cooled very little, however, because the prestorm subsurface ocean was warm. Aquarius SSS increased by 1.5-3 psu over an area of nearly one million square kilometers in the north Bay of Bengal. A one-dimensional model, with initial conditions and surface forcing based on moored observations, shows that cyclone winds rapidly eroded the shallow, salinity-dominated density stratification and mixed the upper ocean to 40-50-m depth, consistent with observations. Model sensitivity experiments indicate that changes in ocean mixed layer temperature in response to Cyclone Phailin are small. A nearly isothermal, salinity-stratified barrier layer in the prestorm upper ocean has two effects. First, near-surface density stratification reduces the depth of vertical mixing. Second, mixing is confined to the nearly isothermal layer, resulting in little or no SST cooling. [ABSTRACT FROM AUTHOR]

The sensitivity of the October 1996 Medicane in the western Mediterranean basin to sea surface temperatures (SSTs) is investigated with a regional climate model via ensemble sensitivity simulations. For 11 SST states, ranging from -4 K below to +6 K above the observed SST field (in 1 K steps), 24-member ensembles of the medicane are simulated. By using a modified phase space diagram and a simple compositing method, it is shown that the SST state has a minor influence on the tracks of the cyclones but a strong influence on their intensities. Increased SSTs lead to greater probabilities of tropical transitions, to stronger lower- and upper-level warm cores and to lower pressure minima. The tropical transition occurs sooner and lasts longer, which enables a greater number of transitioning cyclones to survive landfall over Sardinia and re-intensify in the Tyrrhenian Sea. The results demonstrate that SSTs influence the intensity of fluxes from the sea, which leads to greater convective activity before the storms reach their maturity. These results suggest that the processes at steady state for medicanes are very similar to tropical cyclones. [ABSTRACT FROM AUTHOR]

Although the performance of the high-resolution global ocean forecasting system, which is based on First Institute of Oceanography surface wave-tide-circulation coupled ocean model (FIO-COM) has been demonstrated to have reasonable accuracy in global context, especially in tropical oceans and some special cases, the assessment needs to be extended in different parts of the global oceans. As such, given the significance of a regional Ocean Forecasting System (OFS) in the world's prominent freshwater dominated basin, the northern Bay of Bengal, a groundwork is crucial for the modeling community. Thus, performance of FIO-COM analysis and forecasting results is assessed by utilizing available moored buoy, satellite, and reanalysis datasets. The present study compares the thermohaline structure related variables (e.g., temperature, salinity) from FIO-COM with aforementioned datasets on a seasonal and daily basis. We focused on evaluating the FIO-COM outputs during a very severe cyclonic storm named Titli that formed in October, 2018. The root-mean-square errors (RMSEs) for sea surface temperature (SST) ranges from 0.47 to 0.71 °C, and for sea surface salinity from 0.62 to 0.83 psu; while vertical profile RMSE ranges from 0.27 to 1.0 °C for temperature, and 0.22 to 0.56 psu for salinity between FIO-COM products and observations. Daily variations in different thermohaline structure related variables at two RAMA positions are also well captured by the FIO-COM outputs. Both model and satellite data show pronounced SST cooling (approximately 2.0–2.5 °C) and increased sea surface salinity (~ 1 psu) on the right side of the cyclone track. High SST, TCHP and deep isothermal layer depth were the main oceanic triggering forces to intensify the cyclone Titli. • To evaluate the upper ocean response and forecasting ability of FIO-COM during the severe cyclonic storm 'Titli' • FIO-COM can capture pronounce SST cooling and increased sea surface salinity on the right side of the cyclone track • FIO-COM has reasonably simulated the seasonal responses of temperature, salinity, thermocline depth and mixed layer [ABSTRACT FROM AUTHOR]

A soft-computing cyclone intensity prediction scheme ( SCIPS) is introduced using an artificial neural network ( ANN) approach and adding ocean heat content, as an additional predictor to the normally used atmospheric parameters, to predict tropical cyclone intensity change in the western north Pacific Ocean. We used 1997-2004 data to develop and validate this scheme. The ANN-based estimations have been compared with observations and estimations using the multiple linear regression ( MLR). SCIPS performance improves upon MLR as the lead hour increases from 12 to 120 h and also for high intensifying cyclones. [ABSTRACT FROM AUTHOR]

A large meridional shift of the sea surface temperature front occurs off the south coast of Japan associated with transitions between the large-meander and straight paths of the Kuroshio. Most extratropical cyclones generated in winter near the Kuroshio in the East China Sea pass through the region where the Kuroshio takes either the meander or the straight path. To examine whether such cyclones change their tracks and intensities according to the two states of the path, a new dataset of winter cyclone tracks derived from surface weather charts from the period 1969/70-2008/09 was produced. The composite analysis of cyclone tracks with respect to the meander and straight path states reveals the following: the cyclone track axis for the meander path state is located away from the south coast of Japan with a dispersive tendency, while that for the straight path state is attached to the south coast with a long extending feature. A difference in track between these two states also occurs to the east of Japan over the North Pacific. In addition, this behavior of the cyclone track is shown to be independent of the wintertime atmospheric circulation anomalies around Japan. The development rate of cyclones is 41% faster for the straight path state than the meander path state. Snowfall in Tokyo caused by south-coast cyclones is more frequent for the meander than the straight path state because the former state can act to decrease air temperature in Tokyo. [ABSTRACT FROM AUTHOR]

In this study, we compare two different cyclone-tracking algorithms to detect North Atlantic polar lows, which are very intense mesoscale cyclones. Both approaches include spatial filtering, detection, tracking and constraints specific to polar lows. The first method uses digital bandpass-filtered mean sea level pressure (MSLP) fieldsin the spatial range of 200-600 km and is especially designed for polar lows. The second method also uses a bandpass filter but is based on the discrete cosine transforms (DCT) and can be applied to MSLP and vorticity fields. The latter was originally designed for cyclones in general and has been adapted to polar lows for this study. Both algorithms are applied to the same regional climate model output fields from October 1993 to September 1995 produced from dynamical downscaling of the NCEP/NCAR reanalysis data. Comparisons between these two methods show that different filters lead to different numbers and locations of tracks. The DCT is more precise in scale separation than the digital filter and the results of this study suggest that it is more suited for the bandpass filtering of MSLP fields. The detection and tracking parts also influence the numbers of tracks although less critically. After a selection process that applies criteria to identify tracks of potential polar lows, differences between both methods are still visible though the major systems are identified in both. [ABSTRACT FROM AUTHOR]

The spatial and temporal variability of the partial pressure of carbon dioxide (pCO 2) over the northern tropical Indian Ocean was estimated based on satellite-derived sea surface temperature (SST), sea surface salinity (SSS), and chlorophyll concentration (Chl-a). The distribution of sea surface pCO 2 is controlled by various physical processes, such as mixing, entrainment, upwelling, and advection, and by associated biological processes, such as phytoplankton blooms. The results showed lower surface water pCO 2 over the Bay of Bengal (BoB) (< 300 μatm) as compared to the Arabian Sea (AS), which could be attributed to low salinity during the northeast monsoon season (December–January) in the BoB. High pCO 2 values (> 500 μatm) were found near the Somali coast during the southwest monsoon season (June–August) caused by intense coastal upwelling. The pCO 2 over BoB (400–450 μatm) was more than that of the AS in the fall inter-monsoon season (September–November) as a response to the relatively warmer SST over the BoB. The highest spatiotemporal variability of surface pCO 2 was observed in the western AS near to the coast and over the northern BoB as a response to the higher variability in biological productivity and salinity, respectively, over these regions. The effect of tropical cyclones on pCO 2 variability over the BoB was also assessed in this study. The sea surface pCO 2 values were found to be much smaller along the cyclone track after the passage of cyclones because of the strong SST drop and the sudden phytoplankton blooms caused by the entrainment and vertical mixing of subsurface cold and nutrient-rich water with surface water. The cyclone-induced surface cooling and the associated decrease in surface water pCO 2 were found to be higher after the passage of the Madi cyclone compared to the Hudhud cyclone. • The spatiotemporal variability of surface pCO 2 in the BoB is dominated by the influx of riverine freshwater. • SST and chlorophyll are the decisive factors governing the variability of pCO 2 in the AS. • Tropical cyclone induced surface cooling and biogeochemical processes, affecting the pCO 2 distribution in the BoB. [ABSTRACT FROM AUTHOR]

In the recent literature, considerable attention has been paid to the relationship between climate signals and tropical cyclone activity. Much of the research has focused on Atlantic Ocean basin activity while less attention has been given to landfall frequency and the geographic distribution of risk to life and property. However, recent active seasons like 2004 and 2005 and the resulting damage and economic loss have generated significant interest in the relationship between climate and landfall risk. This study focuses on sea surface temperatures (SST) and examines modulation of landfall activity occurring in anomalously warm-SST seasons. The objective of the study is to evaluate the effect of warmer ocean conditions on U.S. landfall risk. The study is broken into two parts–—statistical and physical. The statistical analysis categorizes historical hurricane seasons as either warm or cool and then estimates shifts in landfall frequency under these two climate modes. The analysis is carried out for overall U.S. landfall risk and then for logical subregions along the U.S. coastline. The climatological behavior for warm-SST conditions is developed across the intensity spectrum, from weak tropical storms to major hurricanes, using wind speed as an intensity measure. The analysis suggests that landfall risk is sensitive to SST conditions but that sensitivity varies by region and intensity. The uncertainty associated with these estimates is discussed. The physical analysis is carried out to understand better why landfall risk is not affected uniformly along the U.S. coastline and to reinforce the reasonability of the statistical results. The study involves a detailed examination of the complete life cycle of historical storms. Results indicate that storms making landfall along the East Coast have different genesis and intensification characteristics relative to storms making landfall along the Gulf Coast. As SSTs warm, the genesis pattern shifts, greatly influencing regional landfall risk. Further, hurricane landfalls may react not only to warm-SST conditions, but also to the effect of ocean temperature anomalies on the atmosphere’s general circulation. There are implications that complex feedback mechanisms play a role in modulating the probability of landfall, especially from certain parts of the Atlantic basin. Such physical theories provide added confidence in statistical estimates of elevated risk for certain breeds of tropical cyclones. [ABSTRACT FROM AUTHOR]

A set of numerical simulations is relied upon to evaluate the impact of air-sea interactions on the behaviour of tropical cyclone (TC) Bejisa (2014), using various configurations of the coupled ocean-atmosphere numerical system Meso-NH-NEMO. Uncoupled (SST constant) as well as 1D (use of a 1D ocean mixed layer) and 3D (full 3D ocean) coupled experiments are conducted to evaluate the impact of the oceanic response and dynamic processes, with emphasis on the simulated structure and intensity of TC Bejisa. Although the three experiments are shown to properly capture the track of the tropical cyclone, the intensity and the spatial distribution of the sea surface cooling show strong differences from one coupled experiment to another. In the 1D experiment, sea surface cooling (∼1 °C) is reduced by a factor 2 with respect to observations and appears restricted to the depth of the ocean mixed layer. Cooling is maximized along the right-hand side of the TC track, in apparent disagreement with satellite-derived sea surface temperature observations. In the 3D experiment, surface cooling of up to 2.5 °C is simulated along the left hand side of the TC track, which shows more consistency with observations both in terms of intensity and spatial structure. In-depth cooling is also shown to extend to a much deeper depth, with a secondary maximum of nearly 1.5 °C simulated near 250 m. With respect to the uncoupled experiment, heat fluxes are reduced from about 20% in both 1D and 3D coupling configurations. The tropical cyclone intensity in terms of occurrence of 10-m TC wind is globally reduced in both cases by about 10%. 3D-coupling tends to asymmetrize winds aloft with little impact on intensity but rather a modification of the secondary circulation, resulting in a slight change in structure. [ABSTRACT FROM AUTHOR]

This study investigates long‐term trends in tropical cyclones (TCs) over the extratropical western North Pacific (WNP) over a period of 35 years (1982–2016). The area analyzed extended across 30–45°N and 120–150°E, including the regions of Korea and Japan that were seriously affected by TCs. The northward migration of TCs over the WNP to the mid‐latitudes showed a sharp increase in early fall. In addition, the duration of TCs over the WNP that migrated northwards showed an increase, specifically in early to mid‐September. Therefore, more recently, TC tracks have been observed to significantly extend into the mid‐latitudes. The recent northward extension of TC tracks over the WNP in early fall was observed to be associated with changes in environmental conditions that were favorable for TC activities, including an increase in sea surface temperature (SST), decrease in vertical wind shear, expansion of subtropical highs, strong easterly steering winds, and an increase in relative vorticity. In contrast, northward migrations of TCs to Korea and Japan showed a decline in late August, because of the presence of unfavorable environmental conditions for TC activities. These changes in environmental conditions, such as SST and vertical wind shear, can be partially associated with the Pacific decadal oscillation. [ABSTRACT FROM AUTHOR]

Between 19 and 22 January 2014, a baroclinic wave moving eastward from the Atlantic Ocean generated a cut-off low over the Strait of Gibraltar and was responsible for the subsequent intensification of an extra-tropical cyclone. This system exhibited tropical-like features in the following stages of its life cycle and remained active for approximately 80 h, moving along the Mediterranean Sea from west to east, eventually reaching the Adriatic Sea. Two different modeling approaches, which are comparable in terms of computational cost, are analyzed here to represent the cyclone evolution. First, a multi-physics ensemble using different microphysics and turbulence parameterization schemes available in the WRF (weather research and forecasting) model is employed. Second, the COAWST (coupled ocean–atmosphere wave sediment transport modeling system) suite, including WRF as an atmospheric model, ROMS (regional ocean modeling system) as an ocean model, and SWAN (simulating waves in nearshore) as a wave model, is used. The advantage of using a coupled modeling system is evaluated taking into account air–sea interaction processes at growing levels of complexity. First, a high-resolution sea surface temperature (SST) field, updated every 6 h, is used to force a WRF model stand-alone atmospheric simulation. Later, a two-way atmosphere–ocean coupled configuration is employed using COAWST, where SST is updated using consistent sea surface fluxes in the atmospheric and ocean models. Results show that a 1D ocean model is able to reproduce the evolution of the cyclone rather well, given a high-resolution initial SST field produced by ROMS after a long spin-up time. Additionally, coupled simulations reproduce more accurate (less intense) sea surface heat fluxes and a cyclone track and intensity, compared with a multi-physics ensemble of standalone atmospheric simulations. [ABSTRACT FROM AUTHOR]

Arctic climate change is strongly influenced by changed surface conditions like declining sea ice cover and rising sea surface temperatures. These changes are not confined to the Arctic, but interact with the global climate system. We performed time-slice model experiments with changed sea ice and sea surface temperature conditions. These experiments are analyzed in relation to reanalysis data aiming at a separation between the effects of sea ice and sea surface temperature and focusing on linkages between the Arctic and mid-latitudes in winter. The diagnostics involve cyclone tracking, blocking analysis and vertical wave propagation analysis. This framework allows to further characterize the relevance of tropospheric and stratospheric pathways. Results show that reanalysis results are best reproduced with the combined effect of sea ice and sea surface temperatures. Anyway, the reproduction is far from perfect and additional insights on further experiments and model improvements will be given considering the boundary layer as well as the stratosphere. [ABSTRACT FROM AUTHOR]

Tropical cyclone (TC) is the most destructive natural hazard in East Asia due to the associated storm surge, flood, and strong wind. It can lead to significant casualty and property damage in East Asian countries. About 25 to 30 TCs occur per year in the western North Pacific (WNP) basin, among them 5 to 8 TCs result in direct or indirect damages to Korea and Japan.We analyzed recent changes in the tropical cyclone (TC) track over the western North Pacific (WNP) for the period between June to September for 36 years (1979-2016). The study area (30–45 °N, 120–150 °E) covered the regions that TCs mainly affect on Korea and Japan. In recent years, the northward migrations of TCs over the WNP to the mid-latitudes increased in early fall. The increasing rates of TCs frequency and the Power Dissipation Index (PDI) within the study area were greatest in September because of favorable environmental conditions such as increasing sea surface temperature, decreasing vertical wind shear, shrunken subtropical high, and increasing relative vorticity. We also found that the negative phase of the Pacific Decadal Oscillation in recent years can be associated with the increasing migration of TCs to the mid-latitudes. [ABSTRACT FROM AUTHOR]

Jet streams exhibit large variability over a range of time scales, influencing both global climate and regional weather. They steer cyclones in the storm tracks, altering the exchange of heat with the underlying ocean as well as precipitation and evaporation patterns. The North Atlantic jet is mainly eddy-driven, with characteristic north-south fluctuations on synoptic time scales as captured in part by the North Atlantic Oscillation (NAO) index.However, during special periods, the dynamics of the North Atlantic jet can change - for example, during the 2009/2010 winter, it took on the attributes of a merged jet, which is both eddy-driven and thermally driven. The merged North Atlantic jet is extremely zonal, shifted equatorward by about 8° latitude relative to its climatological mean position, and is linked to cold and dry conditions over Northern Europe.In this study, we bridge the weather and climate perspectives and show that such altered dynamical behaviour of the North Atlantic jet occurs not only on longer (monthly or seasonal) time scales, but also on synoptic time scales. We identify weaker eddy forcing and slightly increased persistence in the North Atlantic sector during merged jet periods, associated with changes in the storm track, cold air outbreaks, and atmosphere-ocean heat exchange. In addition, we find indications of enhanced heating over warmer sea surface temperatures in the tropical Pacific that may act as remote source of thermal driving.These results help provide context for interpreting both observed North Atlantic variability from synoptic to seasonal time scales and simulated jet changes under global warming. Our findings also have potential implications for understanding biases in climate models, many of which simulate an overly zonal and extended North Atlantic jet. [ABSTRACT FROM AUTHOR]