Western North Pacific Research Articles

Emergent constraints on the future East Asian winter surface air temperature changes

Abstract In East Asia, the climate variability in boreal winter is dominated by the East Asian winter monsoon, one of the most energetic monsoon systems that can lead to disasters. The key variable, the East Asian winter surface air temperature (SAT), has significantly changed over the past century and has substantially impacted agriculture, ecosystems, economics, and public health. However, its projections are limited by considerable uncertainties. Here, we identify the first leading mode that explains almost 32.1% of the inter-model spread in future SAT change. Our research delves into the evolution of present-day biases under future scenarios and their consequential impact on the SAT. Models with stronger western currents in the North Pacific exhibit a warmer North Pacific at mid-latitudes during historical periods. Additionally, these models consistently demonstrate stronger western currents in the future, contributing to the amplified warming of the western North Pacific, thereby warming Eurasia via the weakened trough and subtropical jet through barotropic responses to the warm North Pacific. Incorporating observational sea surface temperature (SST) constraints reduces uncertainties by 15.6%, revealing a more reliable SAT change pattern by the end of the 21st century.

Recent shortening of the mature tropical cyclone stage over the western North Pacific

AbstractThis study investigates long‐term trends related to the mature stage duration of tropical cyclones (TCs) over the western North Pacific (WNP) during June–November between 1982 and 2021. There is a significant shortening in the mean duration of the WNP mature TC stage, which is defined as the period when a TC is within 5 kt of its lifetime maximum intensity. This shortening is induced by a significant (weak) poleward migration in the starting (ending) location of the WNP mature TC stage, which can be further explained by changes in environmental conditions. From 1982 to 2021, there have been significant increases in maximum potential intensity and 700–500‐hPa relative humidity over most of the WNP, which has broadly expanded the TC‐favorable area poleward. Consequently, WNP TCs can start their mature stages and reach their lifetime maximum intensities at higher latitudes. By contrast, there are only weak changes in 850‐hPa relative vorticity and 850–200‐hPa vertical wind shear (VWS). Given the dominant role of VWS in modulating the TC weakening rate, the TC‐suppressive area over the subtropical WNP has shown lesser changes, thus leading to insignificant changes in the ending location of the mature TC stage.

Influence of Spring Precipitation over Maritime Continent and Western North Pacific on the Evolution and Prediction of El Niño–Southern Oscillation

Influence of Spring Precipitation over Maritime Continent and Western North Pacific on the Evolution and Prediction of El Niño–Southern Oscillation

Interspecific and intraspecific difference in egg size of two mackerel (Scomber spp.) species in relation to sea surface temperature in the western North Pacific: A new approach to species identification

AbstractChub mackerel (Scomber japonicus, Scombridae) and blue mackerel (Scomber australasicus) are two important fishery resources in the western North Pacific that spawn eggs during the same season. Although estimating the total egg production (TEP) is important for evaluating the spawning stock biomass of mackerel species, it is difficult to accurately identify formalin‐preserved eggs collected during ichthyoplankton surveys. Hence, a new identification criterion that incorporates the effects of water temperature on egg size was developed in this study. The diameter of 37,304 mackerel eggs collected over 16 years (2006–2021) was measured, and frequency distributions of egg size across sea surface temperature (SST) ranges at 1°C interval were constructed. The frequency distributions were classified into two groups using a Gaussian mixture model; based on the results of DNA analysis, the small group was chub mackerel, while the large group was blue mackerel. The SST at the sampling stations and the mean egg size of both groups were negatively correlated. The new identification criterion, incorporating the relationship between SST and egg size, provided reasonable estimates of the TEP of the two mackerel species compared with the conventional criterion. The new species identification approach is applicable to other fish taxa in the western North Pacific.

An ensemble-based data assimilation system for forecasting variability of the Northwestern Pacific ocean

AbstractAn adjoint-free four-dimensional variational (a4dVar) data assimilation (DA) is implemented in an operational ocean forecast system based on an eddy-resolving ocean general circulation model for the Northwestern Pacific. Validation of the system against independent observations demonstrates that fitting the model to time-dependent satellite altimetry during a 10-day DA window leads to substantial skill improvements in the succeeding 60-day hindcast. The a4dVar corrects representation of the Kuroshio path variation south of Japan by adjusting the dynamical balance between amplitude/wavelength of the meander and flow advection. A larger ensemble spread tends to reduce the skill in representing the observed sea surface height anomaly, suggesting that it is possible to use the ensemble information for quantifying the forecast error. The ensemble information is also utilized for modification of the background error covariance (BEC), which improves the accuracy of temperature and salinity distributions. The modified BEC yields the skill decline of the Kuroshio path variation during the 60-day hindcast period, and the ensemble sensitivity analysis shows that changes in the dynamical balance caused by the ensemble BEC result in such skill deterioration.

Using regional relaxation experiments to understand the development of errors in the Asian summer monsoon

Abstract. We describe the use of regional relaxation (“nudging”) experiments carried out in initialised hindcasts to shed light on the contribution from particular regions to the errors developing in the Asian summer monsoon. Results so far confirm previous hypotheses that errors in the Maritime Continent region contribute substantially to the East Asia summer monsoon (EASM) circulation errors through their effects on the western North Pacific subtropical high. Locally forced errors over the Indian region also contribute to the EASM errors. Errors arising over the Maritime Continent region also affect the circulation and sea surface temperatures in the equatorial Indian Ocean region, contributing to a persistent error pattern resembling a positive Indian Ocean dipole phase. This is associated with circulation errors over India and the strengthening and extension of the westerly jet across southeast Asia and the South China Sea into the western Pacific, thereby affecting the Asian summer monsoon (ASM) circulation and rainfall patterns as a whole. However, errors developing rapidly in the deeper equatorial Indian Ocean, apparently independently of the atmosphere errors, are also contributing to this bias pattern. Preliminary analysis of nudging increments over the Maritime Continent region suggests that these errors may at least partly be related to deficiencies in the convection and boundary layer parameterisations.

Footprint of the air-sea momentum transfer saturation observed by ocean wave buoy network in extreme tropical cyclones

Tropical cyclones are one of the most destructive natural phenomena, causing tremendous coastal disasters worldwide. The maximum intensity of tropical cyclones is determined by momentum and heat transfer at the air-sea interface. Momentum transfer corresponds to the momentum loss of tropical cyclones and, consequently, to the underlying ocean's momentum gain causing extreme ocean waves and storm surges. Air-side observations of wind profiles and ocean-side observations of ocean subsurface currents show a slowdown of momentum transfer under high tropical cyclone wind speeds in the previous studies. However, there is still disagreement regarding the slowdown owing to lack of data. Here, we show momentum transfer under high wind speed conditions by observing ocean waves. Although ocean wave observations are highly spatially limited, we deployed a fleet of drifting ocean wave buoys covering the active area of tropical cyclones in the Western North Pacific. The buoy fleet captured extreme waves near the eye of the strongest category 5 tropical cyclone, indicating an ocean wave footprint of the momentum transfer saturation above surface wind speeds of 25 m/s. Our approach from ocean surface wave observation is a unique contribution to determination on air-sea momentum transfer slowdown under extreme wind speeds, which can compensate for the knowledge from conventional air and ocean-side observations. This finding advances tropical cyclones, extreme ocean waves, and storm surge modeling.

Did recent sea surface temperature warming reinforce the extreme East Asian summer monsoon precipitation in 2020?

We analyzed the possible effects of recent sea surface temperature (SST) warming on the extraordinary East Asian summer monsoon (EASM) precipitation in 2020 summer. The dynamic and thermodynamic impacts of SST are examined by conducting regional climate model experiments with observed SST and cold SST where the 22-year SST trend is removed. In the presence of warm SST, precipitation increases in low latitudes but decreases in the EASM region. This dipolar precipitation change pattern opposes the precipitation anomalies in 2020 summer, indicating that the extraordinary 2020 EASM precipitation is not likely driven by recent SST warming. The warm SST suppresses the western North Pacific subtropical high expansion and weakens the southwesterly from the South China Sea toward the EASM region. In terms of large-scale atmospheric circulations, SST-induced wind changes strengthen the local Walker circulation in the South China Sea and the Philippines and the local Hadley circulation across the EASM region. These support the reduced EASM rainfall in the control experiment compared to the cold SST experiment and imply that the precipitation reduction by dynamical effects could exceed the precipitation increase by thermodynamic effects in the EASM region under warm SST.

Correction: Formation of long-lasting inactive and active multiple tropical cyclone events in the western North Pacific

Correction: Formation of long-lasting inactive and active multiple tropical cyclone events in the western North Pacific

Multiscale drivers of catastrophic heavy rainfall event in early August 2022 in South Korea

On 8–11 August 2022, South Korea experienced a catastrophic heavy rainfall event (HRE) with 14 fatalities. To elucidate its driving mechanisms, the present study performs a multiscale analysis by hierarchically delineating the synoptic and large-scale characteristics of the HRE. Its synoptic condition was featured by the confrontation of the western North Pacific subtropical high and the continental cyclone in the north of the Korean Peninsula. At their interface, a tremendous amount of moisture was transported in an elongated shape (i.e., atmospheric river) along with strong frontogenetic activity. This provided a favorable environment for potential instability. The continental cyclone was maintained throughout the HRE period, while a transient cyclone was superposed contributing to more intense rainfall in the early stage of the HRE. This persistent cyclone in the north of the Korean Peninsula originated from a far-upstream-originated cutoff low that became a part of the quasi-stationary wave train along the Asian subtropical jet. A linear model experiment suggests that the quasi-stationary wave train was excited by the enhanced tropical convection related to the boreal summer intraseasonal oscillation. The anomalously strong subtropical jet also acted as an effective waveguide. These results suggest that the integration of synoptic and large-scale processes is essential to understand this unprecedented HRE.

Assessing N2 fixation flux and its controlling factors in the (sub)tropical western North Pacific through high‐resolution observations

AbstractThe (sub)tropical western North Pacific is potentially an area of intense nitrogen (N2) fixation in the global ocean, despite limited understanding of the flux and controlling factors. We conducted high‐resolution observations from 2016 to 2021 in this region and used machine learning algorithms to simulate N2 fixation flux. Models estimated an N2 fixation flux from 5.72 to 6.45 Tg N yr−1, with strong seasonal variation and peak rates in summer. The western North Pacific Subtropical Gyre and the Kuroshio Current contributed more to N2 fixation flux than did the adjacent areas. Models suggested that sea surface temperature, photosynthetically available radiation, and nutrient supply were most strongly correlated with seasonal and spatial variations in N2 fixation. This study provides an improved estimation of N2 fixation in the western North Pacific and advances our understanding of its role in ocean productivity.

Siberian vegetation growth intensifies monsoon precipitation in southern East Asia in late spring and early summer

Regional hydrological cycle responding to rising temperatures can have significant influences on society and human activities. We suggest a new perspective on East Asia’s enhanced precipitation amount that emphasizes the role of Siberian surface warming. Increased vegetation greenness in late spring and early summer in eastern Siberia, which may be a response to global warming, acts to warm the surface by reducing the surface albedo with an increase in net absorbed shortwave radiation. Subsequently, eastern Siberia warming leads to the strengthening of anti-cyclonic atmospheric circulation over inner East Asia as well as the subtropical western North Pacific high via thermal forcing and the enhanced land-sea thermal contrast, respectively. Consequently, the anticyclonic circulation over inner East Asia transports much drier and cooler air to southern East Asia. This leads to favorable conditions for increased precipitation in combination with an increased tropical moisture flux from the subtropical western North Pacific high. Therefore, continuous Siberian vegetation growth has a potential influence on the future precipitation amount in the subtropics through vegetation–atmosphere coupled processes.

Future Projection of Tropical Cyclone Genesis in the Western North Pacific Using High-resolution GCMs and Genesis Potential Indices

Future Projection of Tropical Cyclone Genesis in the Western North Pacific Using High-resolution GCMs and Genesis Potential Indices

Unstable interdecadal relationship between the Pacific Decadal Oscillation and eastern China summer precipitation in simulated warm interglacial epochs and twenty-first century

The modern connection between the Pacific Decadal Oscillation (PDO) and eastern China summer precipitation (ECSP) at the decadal scale has been extensively studied. However, whether this correlation is stable during the warmer backgrounds in the past and future remains unclear. Clarifying this issue may yield meaningful insights into past PDO reconstruction and decadal climate projection in eastern China (EC). Based on the FGOALS-f3-L, which performs best in reproducing the observed PDO pattern and the decadal PDO–ECSP relationship among 10 models under the framework of the coupled Model Intercomparison Project phase 6 (CMIP6), we find an unstable relationship between the PDO and ECSP at the decadal scale during the last Interglacial (LIG), mid-Holocene, and the twenty-first century under the Shared Socioeconomic Pathway (SSP) SSP5–8.5 scenario. The PDO-related ECSP roughly shows a west–east dipole pattern with excessive precipitation in the east during the LIG, whereas the mid-Holocene witnesses a generally decreased ECSP, except for South and Northeast China. Under the SSP5–8.5 scenario, the PDO-related decadal ECSP exhibits excessive precipitation in the west but a deficit in the east, especially in South and Northeast China. This instability is ascribed to changes in the vertical motions and moisture divergence associated with various configurations of large-scale atmospheric circulations from the lower to upper troposphere. Furthermore, these diverse atmospheric circulation patterns relate to the variations in the convective heating over the western North Pacific and the propagation of extratropical Rossby waves, which are connected with the different PDO patterns and the mean state differences in the westerly jet stream locations during the past and future warm periods

The contrasting interannual variation and mechanism of the first rainy season precipitation before and after the South China Sea summer monsoon onset

The first rainy season (FRS) is usually considered as a whole when its precipitation variability and the underlying mechanism are discussed. This strategy does not consider the South China Sea summer monsoon (SCSSM) onset occurring within the FRS, which implies an abrupt change in nature and, thereby, the possible characteristics and mechanism of precipitation. To overcome this deficiency, this study divides the FRS into two stages before and after the SCSSM onset, revisits their interannual precipitation variability, and finds remarkable differences in the two FRS stages. During the FRS stage before the SCSSM onset, enhanced precipitation is accompanied by an anticyclone centered on the Indian Peninsula and extending further eastward to the Philippines, active synoptic-scale disturbances in the upstream regions of South China and an anomalous anticyclone in the mid-latitudes of East Asia. This configuration reinforces the warm-moisture air transportation from the Bay of Bengal and the South China Sea and cold air invasion from the mid-latitude regions to South China. It shapes the pattern of initial ascending motions over South China by both dynamical and thermal forcing. During the FRS stage after the SCSSM onset, in contrast, enhanced precipitation is accompanied by an anomalous anticyclone in the western North Pacific and the mid-latitude Silk Road pattern. This configuration reinforces warm-moisture air transportation from the tropical Western Pacific and cold air from the northwest direction to South China and shapes the initial ascending motion mainly by thermal forcing. The interannual precipitation variability is tied to a tropical Pacific sea surface temperature (SST) dipole for the FRS period before the SCSSM onset but not after. Such a difference is possibly due to the changes in the basic thermal-dynamical state of the atmosphere near the Maritime Continent and the tropical Northwestern Pacific and their different responses to the SST forcing before and after the SCSSM onset.

An environment-driven basin scale tropical cyclone model

This paper presents an environment-driven tropical cyclone (TC) model for the Western North Pacific basin, which comprises a revised Poisson regression genesis model, a tailored beta-advection track model, and a fast intensity model. The TC model reproduces the temporal and spatial distributions of genesis events, the motion pattern of tracks, as well as the intensity evolutions along tracks. Risk analyses for Hong Kong and along the southeast coastline of mainland China demonstrate that this model can simulate extreme TC events with high fidelity. And the Gaussian mixture model outperforms the Frank Copula in approximating the joint distributions of the annual maximum wind speeds and the corresponding wind directions. This model is driven by a set of environmental variables including relative vorticity, relative humidity, sea surface temperature, vertical wind shear, potential intensity, sub mixed layer depth stratification, mixture layer depth and so on. This enables the model to not only reproduce historical records, but also make predictions for future TC behaviors under climate change with combination of global climate models. Besides, the computational efficiency of the TC model is comparable to traditional purely statistical models. The proposed model can also be coupled with other natural hazard models to conduct multi-hazard analysis.

The Predictability of a Heavy Rainfall Event during the Summer of 2022 Using an All-sky Radiance Assimilation Experiment

AbstractThis paper presents the results of the recent development of the all-sky radiance assimilation system in the Korean Integrated Model (KIM). In the cycled analysis and forecast experiments, the increased coverage of radiance data in cloudy regions improved the quality of initial fields for mass variables, temperature and humidity. The experimental period covered the record-breaking heavy rainfall event on August 9, 2022. We examined the simulation accuracy of the western North Pacific subtropical high (WNPSH) in both clear- and all-sky experiments. In the clear-sky experiment, northward propagation of the WNPSH was restricted. A humid bias exists with clear-sky radiance assimilation over the WNPSH region. Since humid air is lighter than dry air, in this situation, the geopotential height (GPH) should be lower to achieve the same pressure, and a low-pressure bias occurs. All-sky radiance assimilation dries the moisture field, which helps elevate the GPH over the WNPSH region. The expansion of the WNPSH yielded a steeper confrontation in the air between the land and ocean around the southeastern sea of the Korean Peninsula to predict the strength of rainfall events more accurately. A more accurate simulation of the jet stream outlet was also demonstrated in an all-sky experiment. This study shows that the all-sky radiance assimilation can help to more accurately predict extreme rainfall events via proper simulations of large-scale fields.

El Niño–Southern Oscillation-Independent Regulation of Western North Pacific Tropical Cyclone Genesis

El Niño–Southern Oscillation-Independent Regulation of Western North Pacific Tropical Cyclone Genesis

Skilful multiweek predictions of tropical cyclone frequency in the Northern Hemisphere using ACCESS‐S2

AbstractThe skill of subseasonal (multiweek) forecasts of tropical‐cyclone (TC) occurrence over the Northern Hemisphere is examined in the Australian Bureau of Meteorology's (BoM) multiweek to seasonal prediction system, ACCESS‐S2. ACCESS‐S2 shows a good representation of the spatial distribution of TCs in the Northern Hemisphere; however, TC track frequency is generally underpredicted in the western North Pacific to the east of the Philippines and in the eastern North Pacific. The reduced activity relative to observations could be due to a significant positive bias in 850–200‐hPa wind shear in both of these regions, as well as a significant negative sea‐surface temperature (SST) bias in the eastern North Pacific. Despite biases in climatological TC frequency, the observed change in TC track frequency across the Northern Hemisphere with the phase of the Madden–Julian Oscillation (MJO) is well captured by ACCESS‐S2. Changes in the large‐scale environment (e.g., precipitation, 600‐hPa relative humidity, 850‐hPa absolute vorticity and 850–200‐hPa wind shear) are also well represented, with the location and size of the anomalies comparable to ERA‐Interim, apart from SST which shows a different response during some phases. ACCESS‐S2 shows skill relative to climatology for multiweek predictions of TC occurrence out to week 5 in the western North Pacific, eastern North Pacific and North Atlantic; and out to week 2 for the North Indian Ocean. Assessment of real‐time forecasts for Typhoon Rai (December 2021) showed that ACCESS‐S2 provided good guidance of the development and potential landfall of a TC in the Philippines at four weeks lead time.

Characteristics of the East Asian Summer Monsoon Using GK2A Satellite Data

In East Asia, where concentrated summer precipitation often leads to climate disasters, understanding the factors that cause such extreme rainfall is crucial for effective forecasting and preparedness. The western North Pacific subtropical high (WNPSH) is a key driver of summer precipitation variability, and therefore, its monitoring is critical to predicting the wet or dry periods during the East Asian summer monsoon. Using the Geo-KOMPSAT 2A (GK2A) satellite cloud amount data and ERA5 reanalysis data during the years 2020–2023, this study identified three leading empirical orthogonal function (EOF) modes and investigated the associated WNPSH variability at synoptic and subseasonal scales. The analysis includes a linear regression of meteorological fields onto the principal component (PC) time series. All three modes play a role in the spatiotemporal variability of the WNPSH, exhibiting lead–lag relationships. In particular, the second mode is responsible for its northwestward shift and intensification. As the WNPSH moves northwestward, the position of the monsoon rain band also shifts, and its intensity is modulated mainly by the moisture transport along the WNPSH boundary. Our results highlight the potential of high-resolution, real-time data from the GK2A satellite to elucidate WNPSH variability and its impact on the East Asian summer monsoon. By addressing the variability of the WNSPH using GK2A data, we pave the way for the development of a real-time monitoring framework with GK2A, which will improve our predictability and readiness for extreme weather events in East Asia.