Anomalous Anticyclonic Circulation Research Articles

Summer Drought Patterns in the Middle-Lower Reaches of the Yangtze River Basin and Their Connections with Atmospheric Circulation before and after 1980

The three summer drought patterns of the middle-lower reaches of the Yangtze River basin (MLRYRB) and their associated atmospheric circulation were investigated before and after 1980. For the whole-basin wide drought pattern during 1961–1979, the anomalous high ridge over Japan blocked the northerly flow from Siberia to southern China. Further, the western Pacific subtropical high (WPSH) was weaker than normal and shifted eastward. For the southern drought and northern flood pattern during 1961–1979, the zonal circulation was straight and an anomalous anticyclonic circulation was located over Japan. Less moisture was transported to southern China associated with the weakened WPSH. During 1980–2013, the WPSH extended westward and controlled the southern part of the MLRYRB, and an anomalous cyclonic circulation was centered over Japan. For the southern flood and northern drought pattern during 1961–1979, the meridional circulation was obvious, and the WPSH was weaker than normal. The anomalous southwesterly moisture transport appeared to southern China. However, during 1980–2013 the continental high pressure impacted northern China. The WPSH shifted eastward and the anomalous northeasterly moisture transport presented over eastern China.

Characteristics of long-term change in heatwave days in Jeju

This study analyzed the frequency of heatwave occurrences in Jeju Island in summer season (June to September) for the recent 46 years (1968-2013). The frequency showed an increasing trend up until now and we applied statistical change-point analysis in order to determine whether this increasing trend had a climate regime shift. As a result, it was discovered that frequency of heatwave occurrences has rapidly increased since 1990. Thus, to determine the causes of rapid increase in frequency of heatwave occurrences in Jeju Island since 1990, a difference in mean values of June to September between a period of 1990-2013 and a period of 1968-1989 was analyzed. The analysis result of differences between two periods about stream flows showed that large anomalous anticyclonic circulations were strengthened in the subtropical western Pacific and the center of the small anomalous anticyclone, which was branched westwardly out of the anticyclonic circulations was located within the southern part of the Korean Peninsula. This strengthening of the anomalous anticyclone in the Korean Peninsula was also shown at the middle and upper troposphere. The anomalous anticyclonic circulations, which were expanded to the Korean Peninsula from the subtropical western Pacific in recent years was due to strengthening of the western North Pacific subtropical high (WNPSH) in the Korean Peninsula. Hot and humid air can be introduced into the Korean Peninsula due to the strengthening WNPSH in the Korean Peninsula. Furthermore, a difference in daily maximum temperature at 2m height as well as sensible heat net flux between two periods were analyzed in order to determine the degree of the heat at the ground. There was positive anomalies found along the coast in East Asia and the center of the positive anomalies was linked to the Korean Peninsula via the northern part of China. Thus, the Korean Peninsula including Jeju area had more heat than usual in recent years.

Latitudinal Change of Tropical Cyclone Maximum Intensity in the Western North Pacific

This study obtained the latitude where tropical cyclones (TCs) show maximum intensity and applied statistical change-point analysis on the time series data of the average annual values. The analysis results found that the latitude of the TC maximum intensity increased from 1999. To investigate the reason behind this phenomenon, the difference of the average latitude between 1999 and 2013 and the average between 1977 and 1998 was analyzed. In a difference of 500 hPa streamline between the two periods, anomalous anticyclonic circulations were strong in 30°–50°N, while anomalous monsoon trough was located in the north of South China Sea. This anomalous monsoon trough was extended eastward to 145°E. Middle-latitude region in East Asia is affected by the anomalous southeasterlies due to these anomalous anticyclonic circulations and anomalous monsoon trough. These anomalous southeasterlies play a role of anomalous steering flows that make the TCs heading toward region in East Asia middle latitude. As a result, TCs during 1999–2013 had higher latitude of the maximum intensity compared to the TCs during 1977–1998.

The assessment of droughts in Northern China and Mongolian areas using PDSI and relevant large‐scale environments

ABSTRACTThis study assessed drought conditions in northern China and Mongolian areas using the Palmer drought severity index (PDSI). Droughts in this area were intensified throughout all seasons. In particular, this intensifying trend was pronounced in the summer and autumn. In the summer, the PDSI showed an overall increase from the early 1990s to the late 1990s, and then, it rapidly decreased after the late 1990s. Therefore, this study focused on summer droughts and analysed mean differences before the late 1990s (9098) and after the late 1990s (9905). Regarding differences in 850 hPa stream flows between the two periods, anomalous anticyclonic circulations were strengthened in northern China and Mongolia that showed strong negative anomalies in the PDSI. These anomalous anticyclones were formed as a positive North Atlantic Oscillation pattern formed in the European region spread eastward in the form of wave trains. The anomalous anticyclones formed in northern China and Mongolia led to reduced total cloud cover. As a result, this region exhibited high sensible heat net fluxes. Consequently, warm and dry anticyclones may have recently been strengthened in this region, thereby intensifying droughts. As this shows, recently strengthened warm and dry anticyclones in northern China and Mongolia were associated with recently reduced snow depths in this region during the preceding spring. In recent years, the frequency of Asian dust has also increased due to warm and dry air conditions.

An Assessment of the South Asian Summer Monsoon Variability for Present and Future Climatologies Using a High Resolution Regional Climate Model (RegCM4.3) under the AR5 Scenarios

We assessed the present and future climatologies of mean summer monsoon over South Asia using a high resolution regional climate model (RegCM4) with a 25 km horizontal resolution. In order to evaluate the performance of the RegCM4 for the reference period (1976–2005) and for the far future (2070–2099), climate change projections under two greenhouse gas representative concentration pathways (RCP4.5 and RCP8.5) were made, the lateral boundary conditions being provided by the geophysical fluid dynamic laboratory global model (GFDL-ESM2M). The regional climate model (RCM) improves the simulation of seasonal mean temperature and precipitation patterns compared to driving global climate model (GCM) during present-day climate conditions. The regional characteristic features of South Asian summer monsoon (SASM), like the low level jet stream and westerly flow over the northern the Arabian Sea, are well captured by the RegCM4. In spite of some discrepancies, the RegCM4 could simulate the Tibetan anticyclone and the direction of the tropical easterly jet reasonably well at 200 hPa. The projected temperature changes in 2070–2099 relative to 1976–2005 for GFDL-ESM2M show increased warming compared to RegCM4. The projected patterns at the end of 21st century shows an increase in precipitation over the Indian Peninsula and the Western Ghats. The possibilities of excessive precipitation include increased southwesterly flow in the wet period and the effect of model bias on climate change. However, the spatial patterns of precipitation are decreased in intensity and magnitude as the monsoon approaches the foothills of the Himalayas. The RegCM4-projected dry conditions over northeastern India are possibly related to the anomalous anticyclonic circulations in both scenarios.

On the weakened relationship between spring Arctic Oscillation and following summer tropical cyclone frequency over the western North Pacific: A comparison between 1968–1986 and 1989–2007

This study documents a weakening of the relationship between the spring Arctic Oscillation (AO) and the following summer tropical cyclone (TC) formation frequency over the eastern part (150°–180°E) of the western North Pacific (WNP). The relationship is strong and statistically significant during 1968–1986, but becomes weak during 1989–2007. The spring AOrelated SST, atmospheric dynamic, and thermodynamic conditions are compared between the two epochs to understand the possible reasons for the change in the relationship. Results indicate that the spring AO leads to an El Ni˜no-like SST anomaly, lower-level anomalous cyclonic circulation, upper-level anomalous anticyclonic circulation, enhanced ascending motion, and a positive midlevel relative humidity anomaly in the tropical western–central Pacific during 1968–1986, whereas the AOrelated anomalies in the above quantities are weak during 1989–2007. Hence, the large-scale dynamic and thermodynamic anomalies are more favorable for TC formation over the eastern WNP during 1968–1986 than during 1989–2007.

Linking interannual variability in extreme Greenland blocking episodes to the recent increase in summer melting across the Greenland ice sheet

ABSTRACTAtmospheric blocking commonly occurs over the high latitudes of the Northern Hemisphere, resulting from the development of persistent areas of high pressure that lead to warmer‐than‐average surface temperatures west of the high centre. While the variability and trends in anticyclonic circulation patterns (including blocking) over Greenland have been previously documented, an analysis of the most extreme blocking events within the observational record is lacking. In this study, a historical climatology of extreme Greenland blocking episodes (GBEs) from 1958 to 2013 is examined within the context of anomalous anticyclonic circulation patterns over the North Atlantic region during recent years. Based on a combination of the ERA‐40 (1958–1978) and ERA‐Interim (1979–2013) reanalysis data sets, the Greenland Blocking Index (GBI) is used to quantify 500 hPa geopotential height anomalies for the identification of extreme GBEs. The annual rate of extreme blocking days has doubled since 1958, reaching an average of approximately 20 days per year by 2013. The frequency and, to some extent, duration of extreme GBEs were unprecedentedly high from 2007 to 2013 compared to the 56‐year period of record, with a majority of the increase occurring during the spring (MAM) and summer (JJA). A multiple linear regression analysis reveals that interannual variability in extreme blocking and the Atlantic Multidecadal Oscillation (AMO) are the two predominant drivers of surface meltwater production across the entire Greenland ice sheet (GrIS), but Arctic sea ice extent and North Atlantic cyclone activity can also influence the extent of summer melting over portions of the GrIS. Thus, in addition to the larger‐scale atmospheric and oceanic variability, smaller‐scale features such as extratropical cyclones can play a significant role in modulating GrIS surface melting each summer.

Effects of urban land-use change in East China on the East Asian summer monsoon based on the CAM5.1 model

The effects of urban land-use change in East China on the East Asian summer monsoon (EASM) are investigated using a Community Atmosphere Model Version 5.1. The results show that the urban land-use change in East China causes spatially-varying changes in surface net radiation and heat fluxes, atmospheric circulation, and water budgets. It results in significant surface warming (cooling) and precipitation decrease (increase) in a large region north (south) of 30°N. Urban expansion agglomerated in (29°–41°N, 110°–122°E) alters the surface energy budget and warms the surface, resulting in strengthened southwesterly airflow south of 25°N and increased convergence below the mid-troposphere between 20° and 30°N. A concomitant northward downdraft associated with the increased convection generates an anomalous high pressure north of 30°N. Meanwhile, the downdraft not only produces adiabatic warming but also inhibits the dynamic condition for precipitation formation. The anomalous high pressure formed in North China prevents the southwesterly airflow from advancing northward, leading to increase the convergence and precipitation in South China. These changes reduce the meridional temperature gradient in the mid-lower troposphere and weaken the westerly airflow near 30°N. In addition, horizontal transport of vorticity north of 35°N weakens significantly, which leads to an anomalous barotropic structure of anticyclonic there. As a result, the anomalous anticyclonic circulation and descent north of 30°N are strengthened. At the same time, the anomalous cyclonic circulation and ascent south of 30°N are enhanced. These process induced by the thermal state changes due to urbanization weakens the EASM.

On the significance of the interannual relationship between the Asian‐Pacific Oscillation and the North Atlantic Oscillation

AbstractUsing monthly data from the European Centre for Medium‐Range Weather Forecasts 40 year Re‐Analysis, this study investigated the interannual relationship between the Asian‐Pacific Oscillation (APO) and the North Atlantic Oscillation (NAO) for the period of May to August. The NAO is represented by the leading Empirical Orthogonal Function of sea level pressure anomalies over the Atlantic sector. The difference of upper tropospheric eddy air temperature (T′) between Asia and the North Pacific is used to measure the APO. Results clearly show that the APO and the NAO correlated very well on the interannual time scale, with the positive (negative) phase of the APO corresponding to the strong (weak) NAO. Such a relationship is well supported by the atmospheric dynamic process. When the APO is in the positive phase (characterized by the positive T′ anomaly over Asia and the negative T′ anomaly over the North Pacific in the upper troposphere), the upper tropospheric geopotential height increases over Asia and decreases over the North Pacific. Accordingly, anticyclonic circulation and cyclonic circulation anomalies are introduced, respectively, over Asia and south of the North Pacific, and a zonal teleconnection wave train is excited from Asia to the North Atlantic. As a consequence, an anomalous anticyclonic circulation prevails in the low level of the North Atlantic and an anomalous cyclonic circulation appears to its north, which corresponds to the atmospheric pattern of strong NAO. Further study proposes that the quasi‐stationary wave pattern emanating from the western North Pacific region may serve as a bridge for this linkage.

Decreasing trend of tropical cyclone genesis frequency in July-August over the western North Pacific in the last 20 years

This study analyzed the frequency of tropical cyclones (TCs) that occurred in each month of July to September, which is when most TCs occur during a year, in the western North Pacific for the last 20 years. Since the mid-1990s, the TC genesis frequency has tended to decrease during July to August but to increase during September. Therefore, the time series of the TC genesis frequency during July to August was analyzed. The average TC genesis frequency during July to August was 10TCs. Until 2006, a greater number of years had a TC genesis frequency more than 10TCs; however, from 2006, a greater number of years had a TC genesis frequency less than 10TCs. To determine why the TC genesis frequency showed this change in the mid-2000s, the difference between the average of July and August 2007 to 2014 (0714) and that of July and August 1995 to 2006 (9506) was analyzed. With regard to the TC genesis frequency, TCs occurred largely in the eastern sea of the Philippines during 0714 and in the distant eastern sea of the Philippines during 9506. With regard to the TC passage frequency, TCs came ashore at the southeastern area of China via the South China Sea from the Philippines during 0714 and tended to go north toward Korea and Japan through the East China Sea from the distant eastern sea of the Philippines during 9506. The cause of the differences in TC tracks between the two periods was verified through an analysis of the 850- and 500-hPa streamlines. During 0714, anomalous anticyclonic circulations were strengthened in most areas of the western North Pacific. Therefore, the TC genesis frequency during 0714 was lower than that during 9506 in the tropical and subtropical western North Pacific; in particular, in this area, anomalous easterlies were strengthened, which caused TCs to go toward the southeastern area of China.

Quiescence of Asian dust events in South Korea and Japan during 2012 spring: Dust outbreaks and transports

This study examined the quiescence of Asian dust events in South Korea and Japan during the spring of 2012, presenting a synoptic characterization and suggesting possible causes. Synoptic observation reports from the two countries confirmed that spring 2012 had the lowest number of dust events in 2000–2012. The monthly dust frequency (DF) in March 2012 over the dust source regions, i.e., deserts in northern China and Mongolia, indicated a significant decrease compared to the 12 year (2000–2011) March climatology. The DF in April 2012 was comparable to the 12 year climatology values, but in May 2012 it was slightly lower. The daily Ozone Monitoring Instrument Aerosol Index and the Navy Aerosol Analysis and Prediction System simulations revealed stagnant dust movement in March and May 2012. Anomalous anticyclones north of the source regions decreased the dust outbreaks and enhanced the southeasterly winds, resulting in few dust events over the downwind countries (i.e., South Korea and Japan). By contrast, in April 2012, a strong anomalous cyclone east of Lake Baikal slightly increased the dust outbreaks over northeastern China. However, the major dust outbreaks were not transported downwind because of exceptional dust pathways, i.e., the southeastward pathway of dust transport was unusually blocked by the expansion of an anomalous anticyclonic circulation over the Sea of Okhotsk, with dust being transported northeast.

최대 강도 태풍의 북상 경향에 대한 종관분석

Regarding the tropical cyclone (TC) genesis frequency, TCs between 1999 and 2013 were generated more frequently in the northwest waters of the tropical- and subtropical western North Pacific than TCs between 1977 and 1998. TCs over the period from 1977-1998 showed a northward track trend generated mostly from the distant sea in east of the Philippines via the mainland of the Philippines and the South China Sea to the west toward Indochina or from the distant sea in east of the Philippines to the distance sea in east of Japan. TCS over the period from 1999-2013 showed a northward shift pattern to the mid-latitude region mostly in East Asia. Therefore, TCs over the period from 1999-2013 tended to move to much higher latitudes than TCs over the period from 1977-1998, which also resulted in the high possibility of maximum TC intensity occurred in higher latitudes during the former period than the latter period. In the difference of 500 hPa streamline between two periods, the anomalous anticyclonic circulations were strengthened in whereas the anomalous monsoon trough was placed in north of the South China Sea, which was extended to the east up to . The mid-latitude in East Asia is affected by the anomalous southeasterlies due to the above anomalous anticyclonic circulations and anomalous monsoon trough. The anomalous southeasterlies play a role in anomalous steering flows that directed TCs to the mid-latitude regions in East Asia, which made the latitudes of the maximum intensities in TCs over the period from 1999 - 2013 further to the north than those in TCs over the period from 1977-1998.

Asian summer monsoon onset in simulations and CMIP5 projections using four Chinese climate models

The reproducibility and future changes of the onset of the Asian summer monsoon were analyzed based on the simulations and projections under the Representative Concentration Pathways (RCP) scenario in which anthropogenic emissions continue to rise throughout the 21st century (i.e. RCP8.5) by all realizations from four Chinese models that participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5). Delayed onset of the monsoon over the Arabian Sea was evident in all simulations for present-day climate, which was associated with a too weak simulation of the low-level Somali jet in May. A consistent advanced onset of the monsoon was found only over the Arabian Sea in the projections, where the advanced onset of the monsoon was accompanied by an increase of rainfall and an anomalous anticyclone over the northern Indian Ocean. In all the models except FGOALS-g2, the enhanced low-level Somali jet transported more water vapor to the Arabian Sea, whereas in FGOALS-g2 the enhanced rainfall was determined more by the increased wind convergence. Furthermore, and again in all models except FGOALS-g2, the equatorial SST warming, with maximum increase over the eastern Pacific, enhanced convection in the central West Pacific and reduced convection over the eastern Indian Ocean and Maritime Continent region, which drove the anomalous anticyclonic circulation over the western Indian Ocean. In contrast, in FGOALS-g2, there was minimal (near-zero) warming of projected SST in the central equatorial Pacific, with decreased convection in the central West Pacific and enhanced convection over the Maritime Continent. The broader-scale differences among the models across the Pacific were related to both the differences in the projected SST pattern and in the present-day simulations.

Regional Structure of the Indian Summer Monsoon in Observations, Reanalysis, and Simulation

Abstract Regional variations in seasonal mean Indian summer monsoon rainfall and circulation for the period 1979–2009 are investigated using multiple data products. The focus is on four separate regions: the Western Ghats (WG), the Ganges basin (GB), the Bay of Bengal (BB), and Bangladesh–northeastern India (BD). Data reliability varies strongly by region, with particularly low correlations between different products for the BB and BD regions. Correlations between regions are generally not statistically significant, indicating rainfall varies independently in these four regions. The diagnosed associations between rainfall, circulation, and sea surface temperatures can be sensitive to the choice of rainfall product, and multiple precipitation products may need to be analyzed in this region to ensure that the results are robust. Enhanced precipitation in the BD region is associated with anomalous anticyclonic circulation at 850 mb and westerly anomalies along the foothills of the Tibetan Plateau, while precipitation in the other regions is associated with cyclonic flow and easterlies. These associations provide a dynamical explanation for previously reported weak, negative correlations between BD and the other regions. In addition to observed products, atmosphere-only simulations made using the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) during Project Athena are analyzed. While the simulations do not reproduce the observed interannual variations in rainfall, the fidelity of the simulated precipitation and circulation structure is comparable to or even outperforms the different state-of-the-art reanalysis products considered. Accuracy in representing interannual variability and regional structure thus appears to be independent.

Relationship between Winter Rainfall in South China and Preceding Heat Content Anomaly over Western Pacific Warm Pool and Its Possible Mechanism

By using the historical SST data provided by Japan Meteorological Agency,NCEP / NCAR reanalysis data,SST data from Hadley center,and the daily precipitation at wintertime in 1979-2010 from 753 stations in China,the relationship between winter rainfall of South China and the preceding heat content anomalies over western Pacific warm pool is studied. The results showthat winter rainfall in South China is negatively related to the heat content anomalies at 0 ~ 200 m over the key area of 6. 5°N-11. 5°N,160. 5°E-172. 5°E in June and July,thus the heat content over the key area could be regarded as a precursor of the winter rainfall. The difference field of the atmospheric circulation in winter between the cold years and the warm years resembles the regressed atmospheric circulation field against the sign reversal of heat content index defined within warm pool. It is found that there is an anomalous teleconnection on the East Asian coast in the 500 h Pa height field,with anomalous lowin South China accompanied by cyclonic circulation on 850 h Pa and divergence in upper troposphere,and with anomalous anticyclonic circulation near Philippines which significantly enhances water vapor transport to South China. It indicates that the winter rainfall in South China is more( less) in cold( warm) years. In summer,the strong westerly anomalies near the equator in the northern hemisphere results in the development of cold( warm) SST to the west( east) of key area. In late autumn,an anomalous anticyclone centered in Philippines is formed on the west of the cold SST anomalies,which is strengthened during the winter. The anomalous enhanced warm air on its northwest side transport water vapor to South China. At the same time,the anomalous Walker circulation,excited by the warm SSTA in the central and eastern Pacific,has sinking branch in the equatorial western Pacific,strengthening the Philippines anticyclone and rising motion in South China. It is the above processes that lead to more( less) winter rainfall in South China in the years of cold( warm) SST over the key area.

The observed and simulated major summer climate features in northwest China and their sensitivity to land surface processes

Northwest China (NWC) is a typical arid and semi-arid region. In this study, the main summer climate features over NWC are presented and the performance of an atmospheric general circulation model (NCEP GCM/SSiB) over this region is evaluated. Satellite-derived vegetation products are applied in the model. Based on comparison with observational data and Reanalysis II data, the model generally captures major features of the NWC summer energy balance and circulation. These features include: a high surface temperature center dominating the planetary boundary layer; widespread descending motion; an anticyclone (cyclone) located in the lower and middle (upper) troposphere, covering most parts of central NWC; and the precipitation located mainly in the high elevation areas surrounding NWC. The sensitivity of the summer energy balance and circulation over NWC and surrounding regions to land surface processes is assessed with specified land cover change. In the sensitivity experiment, the degradation over most parts of NWC, except the Taklimakan desert, decreases the surface-absorbed radiation and leads to weaker surface thermal effects. In northern Xinjiang and surrounding regions, less latent heating causes stronger anomalous lower-level anticyclonic circulation and upper-level cyclonic circulation, leading to less summer precipitation and higher surface temperature. Meanwhile, the dry conditions in the Hexi Corridor produce less change in the latent heat flux. The circulation change to the north of this area plays a dominant role in indirectly changing lower-level cyclonic conditions, producing more convergence, weaker vertical descending motion, and thus an increase in the precipitation over this region.

Relationship between SST in the equatorial Eastern Pacific and TC frequency that affects Korea

This study analyzed that there was a strong negative correlation between sea surface temperature (SST) in the equatorial Eastern Pacific and tropical cyclone (TC) frequency that affects Korea in summer season in 1951 to 2012. Then, 12 years that had the highest region-averaged SST values in the equatorial Eastern Pacific (warm SST years) and 12 years that had the lowest SST values (cold SST years) were selected, followed by analyzing average difference between two groups to identify any causes of possible correlation between SST in the equatorial Eastern Pacific and TC frequency. In the warm SST years, not only TC frequency that affected Korea but also TC genesis frequency was low, as well as TC intensity. A difference of 500 and 850 hPa streamlines between two groups showed that anomalous northwesterlies were strengthened in Korea due to anomalous cyclonic circulations developed in the mid-latitude region in East Asia, so that such anomalous steering flows played a role in preventing TCs from moving to Korea. Furthermore, anomalous anticyclonic circulations were strengthened in the east of Taiwan, which was related to weakening of the monsoon trough, thereby causing lowering TC genesis frequency in the warm SST years. In addition, as anomalous cyclonic circulations were strengthened in the southeast quadrant of the tropical and subtropical western North Pacific, it created high TC genesis frequency in the sea in the warm SST years. Low TC frequency, low TC genesis frequency, and low TC intensity that affected Korea in the warm SST years were clearly seen by the analysis of differences between two groups with respect to 500 and 850 hPa air temperature, 600 hPa relative humidity, precipitable water, 200–850 hPa vertical wind shear, and sea surface temperature.

Large‐scale vapor transport of remotely evaporated seawater by a Rossby wave response to typhoon forcing during the Baiu/Meiyu season as revealed by the JRA‐55 reanalysis

AbstractThe modulation of large‐scale moisture transport from the tropics into East Asia in response to typhoon‐induced heating during the mature stage of the Baiu/Meiyu season is investigated using the Japanese 55‐year reanalysis (JRA‐55), aided by a Rayleigh‐type global isotope circulation model (ICM). We highlighted the typhoons that migrate northward along the western periphery of the North Pacific subtropical high and approach the vicinity of Japan. Anomalous anticyclonic circulations to the northeast and southeast of typhoons and cyclonic circulation to their west become evident as they migrate toward Japan, which could be interpreted as a Rossby wave response to typhoon heating. These resultant anomalous circulation patterns form moisture conveyor belt (MCB) stretching from the South Asian monsoon region to East Asia via the confluence region between the monsoon westerlies and central Pacific easterlies. The ICM results confirm that the well‐defined nature of the MCB leads to penetration of the Indian Ocean, South China Sea, Philippine Sea, and Pacific Ocean water vapors into western Japan. The typhoons have the potential to accumulate large amounts of moisture from distant tropical oceans through the interaction of their Rossby wave response with the background flow. In the case of a typical typhoon, the total precipitable water around the typhoon center as it approaches Japan is maintained by the moisture supply from distant oceans rather than from the underlying ocean, which indirectly leads to the occurrence of heavy rainfall over western Japan.

Precessional forced extratropical North Pacific mode and associated atmospheric dynamics

AbstractUsing transient accelerated simulations of the Community Climate System Model version 3 and an Earth System Model of Intermediate Complexity as well as equilibrium experiments of the Community Earth System Model, we identified a response of the extratropical air‐ocean coupled system to the precessional insolation changes at orbital timescales and named this extratropical response pattern as the North Pacific mode (NPM). Corresponding to the increased/decreased boreal winter/summer insolation at 22 ka (relative to 10–8 ka), the NPM is characterized by a western warm‐eastern cold seesaw pattern of sea surface temperature (SST) over the extratropical North Pacific from November to April, a weakened winter Aleutian low and an anomalous anticyclonic circulation throughout the troposphere. This feature forms a barotropic warm‐ridge response of tropospheric temperature and geopotential height to the precessional insolation. At the surface, rainfall increases over East Asia and the Northwest Pacific, which indicates a weakened East Asian winter monsoon, while drier conditions appear over the Northeast Pacific and the western coasts of North America. Associated with a negative phase of NPM is a weaker warming over the equatorial Pacific during winter. The increased winter insolation at precessional band not only induces the in‐phase SST warming over the Northwest Pacific and the tropical Pacific, but also explains those extratropical atmospheric changes associated with NPM. The latter might be associated with the warm SST‐induced tropospheric downstream ridge response through transient eddy activities. Besides the vital role of air‐ocean interactions, the decreased summer insolation is also essential to the zonal SST seesaw of NPM at precessional band.

Scale interaction between typhoons and the North Pacific subtropical high and associated remote effects during the Baiu/Meiyu season

AbstractThe interaction between typhoons and the North Pacific subtropical high and the associated remote impact on East Asian and North Pacific anomalous weather during the Baiu/Meiyu season have been investigated using the Japanese long‐term Reanalysis project data aided by the Japan Meteorological Agency Climate Data Assimilation System. The typhoons that appeared in July have been categorized into two primary tracks, the Hainan Island course (HC) and the Okinawa Island course (OC). A typhoon gives rise to negative absolute vorticity advection along its eastern periphery, which locally reinforces the western ridge of the North Pacific subtropical high, whereas the resultant anomalous high stimulates the westward (northward) migration of the HC (OC) typhoon through its combination with the background flow. A combined effect of the typhoon and its induced anomalous anticyclonic circulation increases the transportation of moisture into the Baiu/Meiyu frontal zone in the vicinity of Japan. Over the East China Sea and the Sea of Japan, northward or northeastward moisture flux is pronounced along the western periphery of the typhoon‐induced anticyclonic circulation anomaly in the HC category, triggering heavy rainfall on central Japan's Sea of Japan coast. Similar remote effects also operate in the OC category, which is responsible for the occurrence of extremely heavy rainfall along the Pacific coast of western Japan. When an OC typhoon approaches the Asian jet, it is capable of giving rise to anticyclonic vorticity within the jet, leading to the downstream development of stationary Rossby wave packets via the North Pacific waveguide.