Pacific Summer Research Articles

Incorporating the Effect of Large‐Scale Vertical Motion on Convection Through Convective Mass Flux Adjustment in E3SMv2

AbstractRecent observational studies suggest that the large‐scale dynamical forcing (vertical motion) plays important roles in deep convection development. In this study we propose a convective mass flux adjustment (MAdj) approach to represent the dynamical effects of large‐scale vertical motion on convection in the Department of Energy's Energy Exascale Earth System Model version 2 (E3SMv2). With MAdj, convection is enhanced (suppressed) when there is large‐scale ascending (descending) motion at the planetary boundary layer top. The coupling of convection with large‐scale circulation significantly improves the simulation of climate variability in E3SMv2 across multiple scales from the diurnal cycle, convectively coupled equatorial waves, to the Madden‐Julian Oscillation (MJO). The standard E3SMv2 tends to simulate overly weak diurnal amplitude of precipitation and overly weak variance of convectively coupled equatorial Kelvin and westward inertio‐gravity (WIG) waves. It also fails to simulate the essential characteristics of the MJO: continuous eastward propagation. With MAdj, the amplitude of diurnal cycle of precipitation is systematically increased and its probability density distribution is much closer to observations. The MAdj can also simulate more realistic eastward propagation of the MJO and much stronger convectively coupled Kelvin and WIG waves. Moreover, the MAdj approach slightly improves the climatology simulations in precipitation, cloud, radiation, circulation, temperature, and moisture fields, with overall root‐mean‐square error (RMSE) of major climatological fields reduced by about 2%. The MAdj approach suppresses excessive grid‐scale precipitation, reducing precipitation wet biases over South China Sea, Philippine Sea, Himalayas, and South Pacific Convergence Zone in western Pacific in summer.

Variations in Summer Extreme Hot–Humid Events over Eastern China and the Possible Associated Mechanisms

Abstract This study aimed to explore the changing characteristics of summer, compound, extreme hot–humid events over eastern China in recent decades and the associated underlying mechanisms. Results showed that the summer extreme hot–humid events over eastern China have tended to increase but with strong interannual variability in recent decades. Further analyses indicated that the summer sea surface temperature (SST) anomalies over the northwest Pacific (NW Pacific) and western tropical Pacific, and the May Arctic sea ice concentration (SIC) anomalies are the key elements that are closely connected to the variations in extreme hot–humid events over eastern China. The positive SST anomalies over the NW Pacific in summer, on the one hand, could strengthen the local circulation patterns that lead to the anomalous anticyclonic circulation over eastern China. On the other hand, the positive SST anomalies over the western tropical Pacific excite an anomaly pattern similar to the Pacific–Japan pattern that weakens the westerly jet and enhances the anticyclone circulation anomalies over the upper level of eastern China. Both of them provide favorable conditions for extreme hot–humid events over eastern China with increased moisture and temperatures. The sensitivity experiments by the Community Atmosphere Model, version 5 (CAM5), with SST forcing yield similar influencing processes to the observations. Additionally, the effect of SIC may be potentially responsible for the hot conditions in extreme hot–humid events over eastern China. The signal of declining May SIC could persist to summer and then influence the SST anomalies to its south, which could also produce a beneficial background for extreme hot–humid events to increase over eastern China by exciting the Rossby wave train propagating southeastward across the Mongolia–Lake Baikal region.

Formation Mechanism of the ENSO-Independent Summer Western North Pacific Anomalous Anticyclone

Abstract The western North Pacific anomalous anticyclone (WNPAC) is the key circulation modulating the East Asian summer climate. In this study, the formation mechanism of the summer WNPAC that is independent of El Niño–Southern Oscillation (ENSO) is investigated. Although ENSO has a significant relationship with WNPAC, except for the super El Niño years, the WNPAC index remains almost unchanged after removing ENSO’s impact, suggesting the possibility of other origins of the WNPAC apart from ENSO. An Atlantic-to-Pacific two-step mechanism is proposed for the formation of ENSO-independent summer WNPAC. In boreal spring, diabatic heating induced by the positive sea surface temperature anomalies (SSTAs) over the tropical Atlantic could stimulate a stationary equivalent barotropic Rossby wave train that travels across the Eurasian continent and ends in the tropical North Pacific. At the end of the Rossby wave train, the lower-level anomalous anticyclone advects negative moist enthalpy into the equator, which suppresses the local convection over the tropical North Pacific and equatorial central Pacific, and thus triggers the lower-level equatorial easterly anomaly to its west. During boreal summer, the lower-level easterly anomaly leads to the zonal dipole SSTA pattern with a negative center in the tropical central Pacific and a positive one in the Maritime Continent. Then, this dipole SSTA pattern over the Pacific exerts a relaying effect that further reinforces and westward shifts the dipole convection anomaly pattern, generating the WNPAC as a Gill-type response. This study underpins the independent role of Atlantic oceanic forcing through the extratropical route in the formation of ENSO-independent summer WNPAC. Significance Statement By linearly removing ENSO’s impact on summer monthly sea surface temperature anomalies (SSTAs), this study explores the sources of summer western North Pacific anomalous anticyclone (WNPAC) apart from ENSO. It is found that the high correlation between WNPAC and ENSO largely depends on the super El Niño events, and the origin of the ENSO-independent WNPAC is rooted in the tropical Atlantic oceanic forcing. The positive tropical Atlantic SSTAs in spring can independently induce atmospheric teleconnection from the North Atlantic to the North Pacific through the extratropical route, which triggers anomalous easterlies over the equatorial Pacific via suppressed local convection caused by the dry air advection over the North Pacific. The equatorial easterlies enhance the dipole anomalous SST and convection pattern over the Maritime Continent and tropical western/central Pacific in summer, which finally stimulates the WNPAC. The result emphasizes the independent role of tropical Atlantic SSTAs in the formation of summer WNPAC, thus providing a new perspective for the seasonal prediction of East Asian summer climate.

Tropical Cyclone Activities Over the Western North Pacific in Summer 2020: Transition From Silence in July to Unusually Active in August

In the summer of 2020, tropical cyclone (TC) activities experienced a contrastive transition over the western North Pacific (WNP), from silence in July to unusually active in August. Furthermore, the generation location of TCs was further northwestward in August 2020, resulting in more typhoons landing, and three TCs successively moving northward, which is rare in history. Based on diagnoses with the total genesis potential index (GPI) in July and August 2020, it is suggested that the variation of mid-tropospheric relative humidity and upward convective motion is the major factor for the transition of TC genesis in summer 2020, while the changes of SST, low-level vorticity, and vertical wind shear anomalies played a secondary role. The exceptional variation of the Madden–Julian Oscillation (MJO) activity from July to August 2020 contributed to the transition of the environmental conditions over WNP. In July 2020, MJO was restricted in the Indian Ocean, thus generating an anomalous low-level anticyclone over WNP that intensified the WNP subtropical high. While in early August, MJO propagated eastward to enhance convective activities over the South China Sea and the Philippine Sea, favorable to TC genesis. Thus, MJO activity is a potential predictability source for intraseasonal variation of TC genesis anomalies in WNP.

Impacts of Sea Surface Temperature and Atmospheric Teleconnection Patterns in the Northern Mid-Latitudes on Winter Extremely Cold Events in North China

The frequency distribution of winter extreme cold events (ECEs) in North China and the influences of mid-latitude sea surface temperature anomalies (SSTAs) in the Northern Hemisphere are studied. The results show that (1) the frequency of single station ECEs (SSECEs) in winter increases from southeast to northwest, with a decrease before 2008 and then a significant increase. This trend abrupt change occurs in late winter. (2) When the SST in the North Pacific shows an “El-Niño-like” anomaly in winter, it triggers the negative Arctic Oscillation (−AO), positive Pacific North America (+PNA), and positive Eurasia Pacific (+EUP) atmospheric teleconnection patterns in the mid-lower troposphere. As a result, the ridge to south of Lake Baikal becomes stronger. Meanwhile, SST in the North Atlantic shows a “reversed C” negative anomaly with North Atlantic Oscillation (+NAO), (+PNA)-like and (+EUP)-like patterns, and the ridge to southwest of Lake Baikal becomes stronger. Furthermore, both cause the Siberian High to become weaker in the north and stronger in the south. With the weaker East Asia subtropical jet and stronger East Asia winter monsoon, these factors lead to a significant increase of SSECE frequency in North China. (3) When the SSTA shows an “El Niño-like” developing pattern from summer to autumn in the North Pacific, the winter SSECE frequency will be higher. (4) The purported mechanism between the mid-latitude SSTA and the winter SSECE frequency in North China is the following: the SSTA in the North Pacific in summer and autumn excites atmospheric teleconnection wave trains, and the Atlantic stores these anomaly signals. In winter, the interaction between the SSTAs in the North Pacific and the North Atlantic enhances the Eurasian teleconnection wave train. With the upstream fluctuation energy dispersing downstream, the wave train centers move eastward with the season, resulting in an increase in the frequency of the SSECEs.

Variations in northeast Asian summer precipitation driven by the Atlantic multidecadal oscillation

AbstractBy analysing a 113–year (1900–2012) observational dataset, it is shown that the interdecadal fluctuation of the summer precipitation over Northeast Asia differs from that in central East Asia during the 20th century, and has experienced three interdecadal shifts in the 1920s, mid‐1960s and late 1990s. That fluctuation coincides well with the multidecadal fluctuation of the sea surface temperature in the North Atlantic, known as the Atlantic Multidecadal Oscillation (AMO). The AMO affects Northeast Asia via a circumglobal teleconnection pattern extending from the North Atlantic to North America. Corresponding with the positive phase of this teleconnection pattern, a tripolar pattern emerges in the Asian–North Pacific sector, with anomalous low pressure over Northeast Asia and high pressure over Lake Baikal and the mid‐latitude western North Pacific. These results suggest that the positive phase of the AMO favours the occurrence of cold vortex over Northeast Asia and anomalous highs over Lake Baikal and the mid‐latitude western North Pacific in summer, which enhances the East Asian summer monsoon and southward intrusion of high‐latitude cold air, and eventually increases the summer precipitation in Northeast Asia. Furthermore, initialized decadal prediction simulations using the CCSM4 model reproduce well the observed variations of the AMO and its associated atmospheric teleconnection but with slightly shifted geographic locations. The simulated anomalous Northeast Asia cold vortex and strong East Asian summer monsoon, lead to above‐normal summer precipitation over Northeast Asia. The modelling results confirm that the multidecadal variability in the North Atlantic can cause the observed interdecadal variations of Northeast Asia summer precipitation. Our results suggest that to understand and predict interdecadal climate change over Northeast Asia, it is important to consider the key role of the AMO.

Extremely Active Tropical Cyclone Activities over the Western North Pacific and South China Sea in Summer 2018: Joint Effects of Decaying La Niña and Intraseasonal Oscillation

In summer 2018, a total of 18 tropical cyclones (TCs) formed in the western North Pacific (WNP) and South China Sea (SCS), among which 8 TCs landed in China, ranking respectively the second and the first highest since 1951. Most of these TCs travelled northwest to northward, bringing in heavy rainfall and strong winds in eastern China and Japan. The present study investigates the impacts of decaying La Nina and intraseasonal oscillation (ISO) on the extremely active TCs over the WNP and SCS in summer 2018 by use of correlation and composite analyses. It is found that the La Nina episode from October 2017 to March 2018 led to above-normal sea surface temperature (SST) over central-western Pacific, lower sea level pressure and 500-hPa geopotential height over WNP, and abnormally strong convective activities over the western Pacific in summer 2018. These preceding oceanic thermal conditions and their effects on circulation anomalies are favorable to TC genesis in summer. Detailed examination reveals that the monsoon trough was located further north and east, inducing more TCs in northern and eastern WNP; and the more east-ward WNP subtropical high as well as the significant wave train with a “ − + − +” height anomaly pattern over the midlatitude Eurasia-North Pacific region facilitated the northwest to northward TC tracks. Further analyses reveal that two successively active periods of Madden-Julian Oscillation (MJO) occurred in summer 2018 and the boreal summer intraseasonal oscillation (BSISO) was also active over WNP, propagating northward significantly, corresponding to the more northward TC tracks. The MJO was stagnant over the Maritime Continent to western Pacific, leading to notably enhanced convection in the lower troposphere and divergence in the upper troposphere, conducive to TC occurrences. In a word, the extremely active TC activities over the WNP and SCS in summer 2018 are closely linked with the decaying La Nina, and the MJO and BSISO; their joint effects result in increased TC occurrences and the TC tracks being shifted more northwest to northward than normal.

The relationship of frequent tropical cyclone activities over the western North Pacific and hot summer days in central-eastern China

Tropical cyclones (TCs) formed over the western North Pacific (WNP) often make landfalls over East Asia and Southeast Asia, causing strong winds and torrential rain in the coastal countries. However, they also bring cool weather on hot summer days and mitigate drought impacts. The present study demonstrates that TC activities over the WNP can strongly modulate extreme summer weather events in eastern mainland China, i.e., frequent TC activities would indirectly lead to more hot days in central-eastern China along the lower and middle reaches of Yangtze River, besides compensating the decreasing of hot days induced by the direct impact of TCs. Such indirect impact is largely determined by the feedback effect of TCs on pressure system, especially on the western Pacific subtropical high, resulting in an abnormal anticyclonic circulation band that dominates a large area from central-eastern China to the Pacific Ocean to the east of Japan in the middle and lower troposphere.

Seasonal and Interannual Variation Characteristics of Low-Cloud Fraction in Different North Pacific Regions

In this study, we use the long-term satellite data to investigate seasonal and interannual variation of low-cloud fraction (LCF) and the associated controlling factors over the eastern and western North Pacific. On the seasonal time scale, the enhanced LCF over the eastern North Pacific in summer is actively coupled with strong estimated inversion strength (EIS) and 700-hPa relative humidity, and the LCF over the western North Pacific in winter is large and mainly caused by increased sensible heat flux and tropospheric low-level cold advection. On the interannual time scale, the increased LCF over the eastern North Pacific in summer is associated with increased EIS and decreased sea surface temperatures, in which the El Niño plays an important role; the enhanced LCF over the western North Pacific in spring and winter has a positive correlation with enhanced sensible heat flux (SHF) and tropospheric low-level cold advection, which can be partly explained by the subpolar frontal zone (SPFZ) intensity.

Relationship between the Future Projections of Sahel Rainfall and the Simulation Biases of Present South Asian and Western North Pacific Rainfall in Summer

Confident model projections of regional climate, in particular precipitation, could be very useful for designing climate change adaptation, particularly for vulnerable regions such as the Sahel. However, there is an extremely large uncertainty in the future Sahel rainfall projections made by current climate models. In this study, we find a close relationship between the future Sahel rainfall projections and present rainfall simulation biases in South Asia and the western North Pacific in summer, using the historical simulations and future projections of phase 5 of the Coupled Model Intercomparison Project (CMIP5). This future–present relationship can be used to calibrate Sahel rainfall projections since historical simulation biases can be much more reliably estimated than future change. The accordingly calibrated results show a substantial increase in both precipitation and precipitation minus evaporation in the future Sahel, in comparison with the multimodel ensemble (MME) result. This relationship between the historical rainfall bias and future Sahel rainfall projection is suggested to lie with the different schemes of convective parameterization among models: some schemes tend to result in both overestimated (underestimated) historical rainfall in South Asia (the western North Pacific) and enhanced future Sahel rainfall projection, while other schemes result in the opposite.

Impact of the North Pacific subtropical sea surface temperature front on El Niño‐Southern Oscillation

ABSTRACTThe North Pacific subtropical front (NPSTF) of sea surface temperature (SST) is an important subtropical feature in the North Pacific. In this study, we reveal that the inter‐annual variability of the spring NPSTF has a robust correlation with the El Niño‐Southern Oscillation (ENSO) in the following winter over the period of 1961–2016. When there is a strong spring NPSTF, anomalous cyclonic and anticyclonic circulations in the low troposphere occur to its north and south, respectively. The northeasterly wind anomaly associated with the anticyclonic circulation increases the climatological northeasterly wind over the subtropical Northeast Pacific, resulting in SST decrease via the wind–evaporation–SST feedback. The SST and wind perturbations propagate southwards from the subtropical to the eastern central equatorial Pacific in summer and grow via the Bjerknes feedback from summer to winter, eventually leading to a La Niña event in winter.A strong spring NPSTF induces an anomalous anticyclonic circulation to its south through both transient eddy activity and atmospheric heat source anomaly. The enhanced air temperature gradient due to a strong NPSTF increases the atmospheric baroclinicity, favouring an enhanced transient eddy activity over the NPSTF and to the north. The enhanced transient eddy activity triggers a significant negative height anomaly to the north via high‐frequency transient eddy feedback forcing, which is conductive to an anticyclonic circulation over the subtropical North Pacific through the downstream effect of Rossby waves. On the other hand, the convergence and increased atmospheric baroclinicity related to the strong NPSTF act to enhance precipitation over the NPSTF and to the north, corresponding to the increased atmospheric heat source, which can also excite an anomalous anticyclonic circulation over the subtropical North Pacific.

Contributions of the North Pacific Meridional Mode to Ensemble Spread of ENSO Prediction

Abstract Seasonal prediction of El Niño–Southern Oscillation (ENSO) employs the ensemble method, which samples the uncertainty in initial conditions. While much attention has been given to the ensemble mean, the ensemble spread limits the reliability of the forecast. Spatiotemporal coevolution of intermember anomalies of sea surface temperature (SST) and low-level winds over the Pacific is examined in ensemble hindcasts. Two types of evolution of intermember SST anomalies in the equatorial Pacific are identified. The first features an apparent southwestward propagation of the SST spread from the subtropical northeastern Pacific southeast of Hawaii to the central equatorial Pacific in boreal winter–spring, indicative of the precursor effect of the North Pacific meridional mode (NPMM) on ENSO variability. Extratropical atmospheric variability generates ensemble spread in ENSO through wind–evaporation–SST (WES) in the subtropical northeastern Pacific and then Bjerknes feedback on the equator. In the second type, ensemble spread grows in the equatorial Pacific with a weak contribution from the subtropical southeastern Pacific in summer. Thus, the extratropical influence on ENSO evolution is much stronger in the Northern Hemisphere than in the Southern Hemisphere. The growth of Niño-4 SST ensemble spread shows a strong seasonality. In hindcasts initialized in September–March, the Niño-4 SST spread grows rapidly in January–April, stabilizes in May–June, and grows again in July–September. The rapid growth of the Niño-4 SST spread in January–April is due to the arrival of NPMM, while the slowdown in May–June and rapid growth in July–September are attributable primarily to the seasonality of equatorial ocean–atmosphere interaction. NPMM contributes to the ensemble spread in equatorial Pacific SST, limiting the reliability of ENSO prediction.

Estimation of the abundance of the sei whale Balaenoptera borealis in the central and eastern North Pacific in summer using sighting data from 2010 to 2012

The abundance of the sei whale Balaenoptera borealis in the central and eastern North Pacific (north of 40°N, south of the Alaskan coast including both the US and Canadian Exclusive Economic Zones between 170°E and 135°W), from July to August, was estimated by the line transect method using sighting data obtained during the 2010–2012 International Whaling Commission-Pacific Ocean Whale and Ecosystem Research cruises. The probability of detecting whales at a perpendicular distance from the transect was estimated using two different models: hazard rate and half-normal models. Because the difference in Akaike’s information criterion between the two models was small, the Akaike weighted average of the two models was taken, which gave an estimated abundance of 29,632 (coefficient of variation, 0.242; 95% confidence interval, 18,576–47,267). This is the first abundance estimate of sei whales in this region based on systematic sighting survey data, which contributes to an understanding of the current status of this species.

Association of the Poleward Shift of East Asian Subtropical Upper-Level Jet with Frequent Tropical Cyclone Activities over the Western North Pacific in Summer

The NCEP–NCAR reanalysis dataset and the tropical cyclone (TC) best-track dataset from the Regional Specialized Meteorological Center (RSMC) Tokyo Typhoon Center were employed in the present study to investigate the possible linkage of the meridional displacement of the East Asian subtropical upper-level jet (EASJ) with the TC activity over the western North Pacific (WNP). Results indicate that summertime frequent TC activities would create the poleward shift of the EASJ through a stimulated Pacific–Japan (PJ) teleconnection pattern as well as the changed large-scale meridional temperature gradient. On the contrary, in the inactive TC years, the EASJ is often located more southward than normal with an enhanced intensity. Therefore, TC activities over the WNP are closely related to the location and intensity of the EASJ in summer at the interannual time scale.

A Dynamic Index for the Westward Ridge Point Variability of the Western Pacific Subtropical High during Summer

Based on the ridge line of the western Pacific subtropical high (WPSH) and the theory of gradient wind approximation, a dynamic index for the westward ridge point (WRPI) of the WPSH is defined. Owing to its definition, the new dynamic index can be used to analyze the evolution of the WPSH at various time scales over most isobaric surfaces. The WRPI comprises two dimensions labeled ZWRPI and MWRPI, which depict the zonal and meridional movement, respectively, of the westward ridge point of the WPSH. The rationality and reliability of the dynamic index were validated using reanalysis atmospheric circulation, outgoing longwave radiation, surface air temperature, and rainfall data. The WRPI series revealed that the westward ridge point of the WPSH generally advances poleward while withdrawing eastward. Furthermore, there were close relationships between the WRPI, atmospheric circulation, outgoing longwave radiation, and precipitation over East Asia and the western Pacific in summer. The significant correlation coefficients indicated that the ZWRPI and the MWRPI can reflect the impact of the zonal and meridional movement of the WPSH on the climate over East Asia and the western Pacific. The ZWRPI has no significant linear trend at the interdecadal time scale, indicating that the WPSH did not significantly extend westward in summer. The slight decrease of the MWRPI suggests that the WPSH moves southward but with an insignificant trend. Compared with indices proposed in previous studies, the WRPI showed advantages in objectivity, reliability, predictability, practicability, and therefore extensive potential for application.

Monsoon intra-seasonal variability in a high-resolution version of Met Office Global Coupled model

Intra-seasonal oscillation (ISO) is a key ingredient of the East Asia and western North Pacific (EAWNP) summer monsoon and particularly important for seasonal forecast. This paper evaluates the seasonal means and ISOs of the EAWNP summer monsoon simulated by the latest version of the Met Office Global Coupled Model (HadGEM3-GC2) with two different atmospheric model resolutions at ~130 and ~25 km coupled to a same 0.25° × 0.25° resolution ocean model. Results show that the mean states of sea surface temperature (SST), low-level specific humidity and the western Pacific subtropical high are all improved in HadGEM3-GC2 with higher atmosphere resolution. Moreover, although ISO variance is overestimated over the western North Pacific, the model has good fidelity in characterising ISO basic features over the EAWNP including the dominant EOF structure, northward propagation and cycle evolution, as well as the zonal displacement of western Pacific Subtropical High and South Asian High associated with the northward propagating ISOs. Increasing atmosphere model resolution yields improvements in most aspects of the Monsoon ISO over the EAWNP, especially for its northward propagation. Further analysis indicates that this improvement is mainly due to the better description of ISO-related air–sea interaction in higher resolution experiment, as evidenced by the enhanced intra-seasonal SST variance and more coherent northward propagation of rainfall, SST, and the associated surface dynamic and thermodynamic variables in the higher resolution model.

How distinct are the two flavors of El Niño in retrospective forecasts of Climate Forecast System version 2 (CFSv2)?

Two different flavors of El Nino-Southern Oscillation (ENSO, canonical east Pacific type and Modoki/central Pacific type) are reported in the recent decades and are found to influence the global climate in different ways. The success of a seasonal prediction system is dependent on its ability to capture these two ENSO flavors accurately, together with associated teleconnections. The present study analyses the ability of Climate Forecast System version 2 (CFSv2) in simulating the two El Nino flavors and their teleconnections. The study uses two versions of CFSv2 in which the atmospheric model horizontal resolutions are different (T126 at 100 km and T382 at 38 km) and are initialized from a calendar month, ranging from February to June. The canonical ENSO pattern is captured as prominent mode of tropical Pacific sea surface temperature (SST) by both resolutions of CFSv2. Even though the tri-polar structure of ENSO Modoki is simulated as second mode, it has some disagreement with observations. The canonical El Nino induced SST, rainfall and atmospheric circulation in the tropical Pacific in summer and fall seasons are comparable with observations in both models. Meanwhile, the teleconnections in the tropical Indian Ocean and Indian monsoon regions are close to observations in T382 only. Teleconnections associated with El Nino Modoki are proper in T382 hindcasts, in which SST bias in Indian Ocean is slightly warm and the cold bias in central Pacific is marginal (mainly for Feb IC hindcasts). The present study indicates that the distinction of ENSO flavors in summer is the major reason for the higher skill of Indian summer monsoon rainfall (ISMR) in CFSv2 T382 Feb IC hindcasts. However, teleconnections associated with two flavors of ENSO are not distinguishable in fall and winter seasons, even in higher resolution model due to the presence of strong cold SST bias in central Pacific and warm SST bias in extreme east Pacific. Thus present study confirms that, higher resolution CFSv2 is required to differentiate the flavors of ENSO and their teleconnections properly. It has better prediction skill for ISMR at a lead time of 4 months, which is significant for seasonal prediction of Indian summer monsoon.

Asymmetric association of rainfall and atmospheric circulation over East Asia with anomalous rainfall in the tropical western North Pacific in summer

AbstractIt is well known that precipitation anomalies in the tropical western North Pacific (WNP) significantly affect circulation and rainfall in East Asia during summer. In this study, the authors further investigated this relationship, by examining the anomalies associated with positive and negative precipitation anomalies in the tropical WNP. The results suggest these anomalies are asymmetric between the enhanced and suppressed WNP rainfall. Positive precipitation anomalies over the WNP, in comparison with their negative counterparts, are more closely related to the meridional teleconnection pattern and rainfall anomalies along the East Asian rain belt. The implications of the results are discussed.

Satellite tracking of Bryde’s whales Balaenoptera edeni in the offshore western North Pacific in summer 2006 and 2008

Movements of two individual Bryde’s whales (Balaenoptera edeni) were recorded using satellite-monitored radio tags in the offshore western North Pacific where no such data had been recorded. One individual was recorded for 13 days 4 h 57 min from 13 to 26 July 2006. The total traveled distance of the individual was 917.3 km with a mean speed of 2.9 km/h. The other individual was recorded for 20 days 5 h 5 min from 24 July to 13 August 2008. The total traveled distance of the individual was 2649.7 km with a mean speed of 5.5 km/h. It has been documented that the subarctic-subtropical transition area (around 40°N) is one of the feeding areas of Bryde’s whales in summer. However, the results revealed that some individual Bryde’s whales moved from the subarctic-subtropical transition area to the subtropical area even in summer. The observation indicated Bryde’s whales did not stay in a feeding area persistently throughout summer. This study provides the first information regarding the continuous movement of Bryde’s whales in the offshore western North Pacific in summer which enhances understanding of their life history.

Impacts of decaying eastern and central Pacific El Niños on tropical cyclone activities over the western North Pacific in summer

We investigate the influences of the decaying eastern Pacific El Nino (EP-El Nino) and central Pacific El Nino (CP-El Nino) on tropical cyclone (TC) activities in the western North Pacific (WNP) during July, August, and September (JAS). During this period, TC geneses and tracks are reduced in the central and eastern WNP. However, TC tracks reaching the Philippines increase, and more TC geneses appear west of 145°E during EP-El Nino. During CP-El Nino, tracks reaching the South China Sea (SCS) and southeast coast of China increase, and positive anomalies of TC genesis are found in the southern part of the central WNP and southern SCS. It is possible that the different variations of the anomalous anticyclone over east of the Philippines in the WNP induced by El Ninos are instrumental to the different TC variations in the two types of decaying El Ninos during JAS. Compared with EP-El Nino, strengthening and northward expansion of the anomalous anticyclone during CP-El Nino cause a westward shift of the western Pacific subtropical high in summer, which is responsible for more westward TC tracks over the SCS and southeast coast of China. This northward expansion can cause the center of suppressed TC geneses in the central WNP to migrate further north during CP-El Nino. A decreased magnitude of vertical shear dominates the southern part of the central WNP and southern SCS, which enhances TC formation in these regions during CP-El Nino.