Western Pacific Sea Surface Temperature Research Articles

A Zonal See‐Saw Variation of Tropical Cyclogenesis Over the Arabian Sea and Bay of Bengal‐South China Sea

AbstractThe variation of tropical cyclone (TC) genesis over the North Indian Ocean (NIO) and South China Sea (SCS) during the post‐monsoon season is explored during 1979–2020. A zonal see‐saw variation of TC genesis is detected between the Arabian Sea (AS) and Bay of Bengal (BOB) –SCS (BOBSCS for brevity). More (less) TCs over the AS tend to concur with fewer (more) TCs over the BOBSCS. Analyses of both the observations and the Coupled Model Intercomparison Project Phase 6 outputs show that tripole sea surface temperature (SST) anomalies in the tropical Indian and Pacific Oceans play a crucial role in the zonal see‐saw variation of TC genesis. Positive western Indian (WI) Ocean and tropical central and eastern Pacific (CEP) Ocean SST anomalies and negative western Pacific (WP) SST anomalies induce anomalous ascending and descending motions and high and low humidity transportation to the middle troposphere over the AS and BOBSCS, respectively. Meantime, anticyclonic vorticity anomalies appear in the BOBSCS region and negative anomalies of vertical wind shear are induced over the NIO and SCS. The combined positive contributions of upward motion, mid‐level humidity increase, and low vertical wind shear favor higher TC genesis over the AS, whereas the joint negative effects of downward motion, mid‐level water vapor decrease, and lower‐level anticyclonic vorticity anomalies result in less TC genesis over the BOBSCS. Numerical experiments show a critical role of the WI SST anomalies in the dipole pattern of vertical motion, which is crucial for the zonal dipole variation of TC genesis.

Distinct Impacts of Two Types of Summer ENSO with Different Temporal Evolutions on the Asian Summer Monsoon: Role of the Tropical Indo-Western Pacific

Abstract ENSO shows great diversity in temporal evolution, exerting different climatic impacts on the global scale. Previous studies suggest that summer ENSOs can be classified into two types with different evolutions: “continuing ENSOs” persist from the previous winter and “emerging ENSOs” newly develop from late spring. In this study, we define two indices for continuing and emerging ENSOs via the linear combination of the two leading modes of the evolutionary anomalies of tropical sea surface temperature (SST) from the prior September to August, which reflects the distinct characteristics of the two types of ENSO well. The two types of ENSO dominate the two leading modes of the SST variability in the tropical Indo–western Pacific (TIWP) and the related Asian summer monsoon anomalies. During continuing ENSOs, the related TIWP SST anomalies show positive anomalies in the tropical Indian Ocean and northwest Pacific and negative ones in the New Guinea region. The warm Kelvin wave forced by the tropical Indian Ocean warming propagates into the tropical western Pacific, exciting the meridional East Asia–Pacific/Pacific–Japan pattern and causing a dipole precipitation pattern in East Asia. During emerging ENSOs, the related TIWP SST anomalies show negative anomalies in the warm pool and positive ones in the western Indian Ocean. The negative SST anomalies in the warm pool induce a westward equatorial Rossby wave conducive to the suppression of the Indian summer monsoon rainfall, whereas the Matsuno–Gill response to the warm pool cooling leads to negative anomalies in East Asian summer rainfall. This mechanism can be simulated well in the CMIP6 and Pacific Ocean–Global Atmosphere experiments. Significance Statement ENSO displays diversity in its amplitude, spatial pattern, temporal evolution, and impact. Although previous studies have systematically investigated the diversity of ENSO spatial locations, there are few studies on the diversity of ENSO evolution and the impacts on the Asian summer monsoon. Here, we define two indices for so-called continuing and emerging ENSOs via the linear combination of the two leading modes of the evolutionary anomalies of tropical sea surface temperature (SST) from the prior September to August. Then, we reveal that the two leading modes of the tropical Indo–western Pacific SST variability are dominated by the two types of ENSO, respectively, and act as a bridge via which the two types of ENSO can exert different impacts on the Asian summer monsoon.

A zonal and segmentation analysis of the relationships between variations in sea-surface temperatures of the western Pacific Ocean and Southeast Asian rainfall

Abstract With the objective of understanding how terrestrial precipitation in Southeast Asia (SEA) responds to both temporal and spatial variations in sea-surface temperature (SST) of the western Pacific Ocean, we first used the North–South and zonal SST variation analyses for the data set named A Group for High-Resolution Sea-Surface Temperature (GHRSST) Level 4 Multiscale Ultrahigh Resolution (MUR). Second, we applied a localized correlation analysis to the SST data set and rainfall data (Integrated Multi-satellitE Retrievals for Global precipitation measurement (IMERG)) in order to determine the boundaries and characteristics of the complex interactions between SST and rainfall variations. The zonal SST variation analysis result suggested that the warm pool in the North of western Pacific zones close to SEA (120 °E–150 °E) was more sustained and less varied than that in the South zones. In terms of the western Pacific SST impact on rainfall in SEA, the correlation analysis confirmed the well-established concept that the SST of the ocean part close to the SEA land, in general, has more influence on SEA rainfall than does the SST of the ocean part far away from SEA. Still, there were some parts of SEA whose rainfall was not governed by the sea–land juxtaposition.

A Secular Shift of the Madden‐Julian Oscillation and Its Relation to Western Pacific Ocean Warming

AbstractIn the context of global warming, understanding changes in the Madden‐Julian Oscillation (MJO) is of great importance, because of its worldwide impact on surface weather and climate. By analyzing the temporal evolution of the phase relationship between Kelvin waves and the MJO, we find a secular phase shift of the MJO after year 2000, at which the MJO exhibits an eastward extension over the Maritime Continent and the Western Pacific Ocean. Further, we examine the influence of sea surface temperature (SST) variations on the anomalous convection and Kelvin waves, and demonstrate that the recent warming trend of the Western Pacific SST is a factor causing the changes of the MJO over this region. The results deepen our understanding of recent modeling and observational studies about the changes of the MJO and confirm the enhanced activity of the MJO over the past few decades.

Statistical prediction of Sri Lankan rainfall during October to December

Sri Lanka receives most rainfall during October to December (OND). Here we construct multiple linear regression models to forecast the OND Sri Lankan rainfall during 1979-2012 for lead times of 1 and 2 months. Correlation analysis was used to examine the relationship between Sri Lankan OND rainfall and global sea surface temperature (SST) anomalies. Three independent predictors were identified through partial least square regression method which includes the southern Atlantic SST tendency, southern Pacific SST tendency and western Pacific and Maritime Continent SST tendency at two different lead times. Three-year-out cross validation concludes that the multiple linear regression models can produce forecast the OND rainfall forecast at correlation coefficient skill of 0.69 and 0.68 for the 1 and 2 month lead times respectively. The physical processes associated with these three predictors show that they contribute to increase in OND rainfall of Sri Lanka.

What Modulates the Intensity of Synoptic-Scale Variability over the Western North Pacific during Boreal Summer and Fall?

AbstractThe present study investigates the factors that affect the year-to-year change in the intensity of synoptic-scale variability (SSV) over the tropical western North Pacific (TWNP) during boreal summer and fall. It is found that the intensity of the TWNP SSV in summer is associated with the equatorial central-eastern Pacific sea surface temperature (SST) anomalies that modulate the background fields through a Rossby wave response both in the source region and along the propagation path of the synoptic-scale disturbances. In fall, the intensity of the TWNP SSV is related to an SST anomaly pattern with opposite anomalies in the equatorial central Pacific and TWNP that modulates the background fields from the equatorial central Pacific to TWNP. However, the equatorial central Pacific SST anomalies alone fail to change the intensity of the TWNP SSV as the induced background field changes are limited to the equatorial central Pacific. It is shown that tropical western Pacific SST anomalies may induce notable changes in the intensity of the TWNP SSV. The relation of the TWNP SSV to the equatorial eastern Pacific SST is weak due to opposite SST anomalies in different types of years. Both seasonal mean and intraseasonal flows provide sources of barotropic energy for the change in the intensity of the TWNP synoptic-scale disturbances in summer. Seasonal mean flow has a main contribution to the barotropic energy conversion for the change in the intensity of the TWNP synoptic-scale disturbances in fall.

The importance of inter‐basin atmospheric teleconnection in the SST footprint of Atlantic multidecadal oscillation over western Pacific

Western Pacific sea surface temperature (SST) multidecadal fluctuations are synchronized to the Atlantic multidecadal oscillation (AMO) phenomenon during the instrumental period. The possible mechanism of the inter-basin synchronization of multidecadal SST variability still remains a matter of discussion regarding the roles of external radiative forcing and internal inter-basin interaction. Here we address this issue using simulations of CMIP5 coupled models and a partially coupled model experiment with prescribed SSTs over the North Atlantic. Observational analysis suggests that in association with the warm AMO phase, prominent SST warming occurs over the western Pacific, accompanied by anomalous low pressures and ascending motion that maintain the warm SST anomalies through positive feedback of local air–sea interaction. The upward motion in the western Pacific corresponds to a significant intensification of Pacific zonal Walker circulation, which is coupled with an increase in the zonal gradient of atmospheric temperature aloft. The CMIP5 model simulated externally forced component of AMO-related changes in western Pacific SST is weak, associated with little change in Pacific zonal circulation showing an absence of anomalous upward motion over the western Pacific, and this is primarily resultant from the negligible changes in the zonal gradient of atmospheric temperature as a direct thermal response to the external radiative forcing. By contrast, the partially coupled model simulation forced by the Atlantic SST variations reasonably reproduces the observed AMO-related changes in western Pacific SST and pan-tropical atmospheric circulation. A surface radiation budget analysis for the western Pacific shows contrasting roles of AMO-related surface incoming solar radiation between the reanalysis/partially coupled model simulation and the forced signal in CMIP5, further confirming the key role of dynamically induced inter-basin atmospheric teleconnection in the multidecadal SST footprint of AMO over the western Pacific.

The role of internal variability in multi-decadal trends of summer rainfall over East Asia–Northwest Pacific

The impacts of internal variability on East Asia–Northwest Pacific (EA–NWP) summer rainfall trends on the multidecadal time scale are invested based on three large ensemble simulations, which have ensemble member of 30, 40 and 100. In all the three simulations, the summer rainfall trends during 1970–2005 are remarkably diverse across the individual ensemble members over the EA–NWP, and the signal-to-noise ratio is lower than 1 over the EA–NWP, suggesting a strong impact of internal variability on EA–NWP summer rainfall trends at this interval. Moreover, we found that the diversity of EA–NWP summer rainfall trends across individual members has a similar leading spatial pattern in all the three ensembles, featuring reverse trends between in Mei-yu region and in the tropical NWP. The leading pattern is likely caused by a gradient between the sea surface temperature (SST) trends in the North Indian Ocean (NIO) and in the tropical western Pacific (WP). When there is a warming trend in the NIO and a cooling trend in the tropical WP, a low-level anomalous anticyclone strengthens over the subtropical NWP, causing a dipole rainfall trend over the EA–NWP. The impact of the east–west SST gradient pattern is confirmed by numerical experiments. Our findings highlight that the internally-generated gradient of NIO–WP SST trends is an important source of the uncertainty in EA–NWP summer rainfall decadal changes in simulations.

Moisture Variability in the East Pearl River Basin since 1894 CE Inferred from Tree Ring Records

Short-term climate change in South China has been extensively studied based on meteorological or hydrological records. However, tree ring-based long-term climate change research is rare, especially in the Pearl River basin, owing to the difficulty in finding old-aged trees. Here, we present a 200-year tree ring width chronology of Pinus kwangtungensis in the east Pearl River basin with reliable coverage from 1894 to 2014. Based on the significant climate-growth relationship between tree growth and annual self-calibrating Palmer drought severity index (scPDSI) from previous May to current April, the pMay-cApr scPDSI was reconstructed for the period 1894–2014. The reconstruction reveals four dry periods during 1899–1924, 1962–1974, 1988–1994, and 2003–2014, and four wet periods during 1894–1898, 1925–1961, 1975–1987, and 1995–2002. Significant spatial correlations between the reconstructed scPDSI and the Climatic Research Unit (CRU) gridded scPDSI indicate that our reconstruction can effectively represent regional moisture variability in the Pearl River basin. Spatial correlations with global sea surface temperatures (SSTs) show that our reconstruction is negatively correlated with northern and western Pacific SSTs while positively correlated with eastern Pacific SSTs, suggesting that SST variability in these domains strongly affects moisture change in the Pearl River basin.

Characteristics of the South China Sea Monsoon from the Onset to Withdrawal before and after 1993/94

The characteristics and possible impact factors of the South China Sea summer monsoon (SCSSM) evolution from onset to withdrawal before and after 1993/94 are investigated using ERA-Interim, CPC rainfall, and OLR data. During the late-onset period of 1979–1993, the SCSSM was characterized by stronger onset intensity and a gradual withdrawal, resulting in a continuous, strong preflood season in Southern China and a slower rain-belt retreat from north to south China in September. In addition, the rain-belt in the Yangtze River basin persisted much longer during summer. However, during the early-onset period in 1994–2016, the SCSSM is associated with a weaker onset intensity and comparatively faster retreat. The advanced preflood season lasted intermittently throughout May and the whole eastern China precipitation lasted until October when it retreated rapidly, making the rain-belt in Southern China persist for an extended duration. Further analysis indicates that a strong modulation of SCS intraseasonal oscillation (ISO) on the SCSSM evolution is observed. There are two active low-frequency oscillations over the SCS in summer during the late-onset period but three during the early-onset period. The wet ISO in the Northwest Pacific propagating northwestward into the SCS and enhanced SCSSM ISO activity may contribute to the early onset and faster withdrawal after 1993/94. The effect of warm western Pacific sea surface temperatures (SST) on the SCSSM evolution is also discussed.

Analysis of Record-High Temperature over Southeast Coastal China in Winter 2018/19: The Combined Effect of Mid- to High-Latitude Circulation Systems and SST Forcing over the North Atlantic and Tropical Western Pacific

AbstractIn winter 2018/19, southeastern coastal China experienced extreme warm temperatures that were due to a weak East Asian winter monsoon. On the basis of observations from 10 meteorological stations and reanalysis data, the large-scale circulation patterns associated with this extreme warm winter and the possible driving mechanism of its related sea surface temperature (SST) anomalies are investigated in this study. During this winter, many places in this region reached their highest winter mean temperature record and had more extreme warm days and fewer extreme cold days relative to climatology. According to the circulation patterns during winter 2018/19, several large-scale circulation conditions associated mainly with the weak East Asian winter monsoon are identified: the eastward shift of the Siberian high and a shallower East Asian trough, which is related to the low blocking frequency over the Aleutian region, are both unfavorable for cold-air intrusion southward. Meanwhile, strong low-level southerly wind anomalies over southeastern China are related mainly to the 2018/19 El Niño event. Furthermore, the possible role of SST anomalies over the North Atlantic and tropical western Pacific Oceans is examined by using an atmospheric general circulation model, suggesting that both the “tripole pattern” of North Atlantic SST and tropical western Pacific SST anomalies in winter 2018/19 played a role in influencing the East Asian trough. The combined effect of all of these factors seems to be responsible for this extreme warm winter over southeastern coastal China.

Quasi-biweekly oscillation of the Asian monsoon rainfall in late summer and autumn: different types of structure and propagation

This study contrasts two types of quasi-biweekly oscillations (QBWOs) over tropical Asia in late-summer and autumn (from August to October). Using a tracking method to calculate the frequency of QBWO events over the Asian monsoon region, two types of QBWOs in monsoon rainfall are revealed. One originates from 110° to 140°E and propagates westward to southern China with a notable impact on the regional rainfall, while the other initiates from 160°E to the dateline and does not affect southern China rainfall significantly. Analysis of the vertical structure of moisture flux shows that the moisture source for type 1 events is dominated by the zonal flux component and that for type 2 the meridional flux component. The nature of the moisture flux determines whether the oscillation can propagate across 120°E and affect rainfall over southern China. Results also show that the strength of the South Asian high and the western Pacific subtropical high differently modulate the generation of the two types of QBWOs. Specifically, mutually stronger (weaker) highs favor the first (second) type of the oscillation. A close relationship also exists between the QBWOs and western Pacific sea surface temperature (SST) anomalies, suggesting that the SST anomalies can potentially trigger the QBWOs.

Data Mining Climate Variability as an Indicator of U.S. Natural Gas.

Anomalously cold winters with extreme storms strain natural gas (NG) markets due to heightened demand for heating and electricity generation. While extended weather forecasting has become an indicator for NG management, seasonal (2–3 month) prediction could mitigate the impact of extreme winters on the NG market for consumers and industry. Interrelated climate patterns of ocean and atmospheric circulation anomalies exhibit characteristics useful for developing effective seasonal outlooks of NG storage and consumption due to their influence on the persistence and intensity of extreme winter weather in North America. This study explores the connection between the Pacific-North American climate systems and the NG market in the U.S., connecting macro-scale oceanic and atmospheric processes to regional NG storage and consumption. Western Pacific sea surface temperatures and atmospheric pressure patterns describe significant variation in seasonal NG storage and consumption. Prediction of these coupled climate processes is useful for estimating NG storage and consumption; this could facilitate economic adaptation toward extreme winter weather conditions. Understanding the implicated impact of climate variability on NG is a crucial step toward economic adaptation to climate change.

Variability of Indian summer monsoon droughts in CMIP5 climate models

Variability of Indian summer monsoon droughts is investigated by computing all-India drought indices namely Percent of Normal Precipitation, Standardized Precipitation Index and percentage area of India under moderate and severe drought conditions. Observations for recent decades, post 1960, exhibit declining trend in monsoon rainfall with frequent occurrence and intensification of droughts along with an increase in percentage of area under moderate and severe drought conditions, in association with variations in sea surface temperature (SST). Historical simulations from CMIP5 models suggest that two models, ACCESS1.0 and INMCM4, could well simulate monsoon rainfall variability, particularly the frequent occurrence of droughts and spatial variability of rainfall during drought years in recent historical period (1961–2005). Future projections of all-India drought indices from these two models indicate frequent droughts during near and mid future (2010–2069) with respect to the recent historical period. Intensification of severe droughts for near and mid future are suggested to be more pronounced over north-central India. The reduction in rainfall in the near and mid future is dynamically consistent with a westward shift in large-scale monsoon circulation, particularly the monsoon trough over South Asia. Interestingly, future projections of monsoon teleconnections indicate a weakening (strengthening) of in-phase (out-of-phase) relationship of all-India drought intensity with the equatorial eastern Pacific and the Indian Ocean (western Pacific) SST. Whereas, a strengthening of in-phase relationship between percentage of area under drought conditions and the equatorial eastern Pacific SST is projected for near and mid future with respect to the recent historical period. These drought features are consistent in both the models.

Pacific sea surface temperature linkages with Tanzania's multi‐season drying trends

During 1999–2014, Tanzania experienced below average precipitation in two important seasons: December to February (DJF) in the south and during March to June (MAMJ) in the northeast. We explore DJF and MAMJ precipitation in the areas with drying trends and examine their relationships with anomalous sea surface temperatures (SSTs) in the Indo‐Pacific and corresponding circulation patterns. At seasonal time scales, precipitation in DJF and MAMJ trend areas appears inversely related to diabatic forcing in the equatorial Pacific. Dominant influence for droughts in DJF is eastern Pacific SST while for droughts in MAMJ it is West Pacific SST. A bivariate regression model with West Pacific and Niño3.4 region SST as predictors is found to recreate multidecadal DJF variability after the 1950s and the extreme drying in MAMJ during the 2000s. The regression model coefficients also indicate differential eastern versus western Pacific forcing for DJF versus MAMJ. Thus, we suggest that recent La Niña‐like conditions, characterized by an enhanced Pacific SST gradient due to cooling in the eastern Pacific and warming in the western Pacific, played a substantial role in Tanzania's recent multi‐season drying trends. SST change scenarios (difference between 2023–2037 and 2000–2014 means) based on CMIP5 projections and observed trends illustrate the uncertainty of future precipitation outcomes, as well as the potential implications of contrasting linkages to eastern versus western Pacific SSTs. These scenarios are mainly optimistic for the DJF southern Tanzania trend area, because it appears dominated by Niño3.4 cooling at both seasonal and decadal time scales. Conversely, our scenarios are quite pessimistic for the MAMJ northeastern Tanzania trend area, because we find a dominant negative influence of warming West Pacific SST.

Recent Acceleration of Arabian Sea Warming Induced by the Atlantic‐Western Pacific Trans‐basin Multidecadal Variability

AbstractArabian Sea (AS) warming has been significantly accelerated since the 1990s, in particular in the spring season. Here we link the AS warming changes to the Atlantic multidecadal oscillation (AMO). A set of Atlantic pacemaker experiments with a slab mixed‐layer ocean model successfully reproduces the AS spring multidecadal variability and its connection with the AMO. An atmospheric teleconnection from the Atlantic to the AS in the preceding winter and associated thermodynamic air‐sea feedback is found to be important. The teleconnection can be reestablished by the atmospheric model when the AMO sea surface temperature (SST) and its trans‐basin footprint over the western Pacific are prescribed simultaneously. The western Pacific SST warming associated with the AMO positive phase induces a Gill‐type Rossby wave over the AS, showing anomalously low pressures and converging southerlies that weaken winter northerlies. Thus, the wind‐evaporation‐SST feedback results in and maintains the AS warm SST anomalies to the subsequent spring.

Variable correspondence between western North Pacific tropical cyclone frequency and East Asian subtropical jet stream during boreal summer: A tropical Pacific sea surface temperature perspective

Fluctuations in the summer relationship between western North Pacific tropical cyclone (TC) frequency (TCF) and East Asian subtropical jet stream (EAJS) are investigated using observations and composite analysis. Here TCF is divided into four categories to assess how different tropical Pacific sea surface temperature (SST) anomalies modulate TCF‐EAJS associations. Results show that high TCF would weaken (strengthen) the south (north) part of EAJS, but with different locally poleward shifts during La Niña‐ and El Niño‐like events. In contrast, during low TCF, a clearly southward shifted, strengthened EAJS occurs only when the western Pacific SST cools, but no significant change is observed when the western Pacific SST warms up. Further analysis shows that the possible reason why TCF has diverse linkages to EAJS is that the TC genesis locations and TC tracks are quite different under different tropical Pacific SST anomalies, even for quasi‐constant TCF. Potential mechanisms responsible for the changing TCF‐EAJS associations can be physically explained by the exceptional configurations of tropospheric meridional temperature gradient anomaly and the deformed‐and‐displaced Pacific–Japan‐like wavetrain, which is mainly due to non‐negligible differences in large‐scale oceanic and atmospheric environments that modulate the TC genesis locations and TC tracks. This study highlights the importance of a tropical SST perspective when analysing TCF‐EAJS associations.

Examining the role of unusually warm Indo‐Pacific sea‐surface temperatures in recent African droughts

Southern Africa (SA) and eastern Africa (EA) experienced a sequence of severe droughts in December–February (SA DJF) 2015–2016, October–December (EA OND) 2016 and March–April–May 2017 (EA MAM). This sequence contributed to severe food insecurity. While climate variability in these regions is very complex, the goal of this study is to analyse the role played by unusually warm Indo–Pacific SSTs, where unusual is defined as a 1‐in‐6 year event. We use observed sea‐surface temperatures (SST) and satellite–gauge rainfall observations, a 20‐member ensemble of Community Atmospheric Model version 5.1 simulations (CAM5), and a 40‐member ensemble of climate change simulations from the Community Earth Systems Model version 1 (CESM1) Large Ensemble Community Project (LENS) to explore climate conditions associated with warm events identified based on eastern and western Pacific SSTs. Our analysis suggests that strong El Niño's may be followed by warm western Pacific SST conditions, which can lead to conditions conducive to successive and potentially predictable droughts in SA DJF, EA OND and EA MAM. We show that different regions of warm SST are related to recent droughts—SA DJF: Niño 3.4; EA OND: western equatorial Pacific (WEP); and EA MAM: western North Pacific (WNP). For DJF and MAM, respectively, the CAM5 model driven with observed SST and the same model driven within a climate change experiment indicate that warmer El Niño's and WNP events produce more intense atmospheric responses, potentially associated with more severe droughts. OND climate seems to be strongly influenced by the Indian Ocean Dipole, which corresponds with some WEP events. Given global warming, we suggest that the extreme Niño 3.4 and west Pacific SST events responsible for 2015–2017 droughts are likely to reoccur, thus humanitarian agencies should prepare to predict and respond to multi‐year drought and substantial food insecurity in SA and EA.

Origins of Biases in CMIP5 Models Simulating Northwest Pacific Summertime Atmospheric Circulation Anomalies during the Decaying Phase of ENSO

Abstract The present study documents the biases of summertime northwest Pacific (NWP) atmospheric circulation anomalies during the decaying phase of ENSO and investigates their plausible reasons in 32 models from phase 5 of the Coupled Model Intercomparison Project. Based on an intermodel empirical orthogonal function (EOF) analysis of El Niño–Southern Oscillation (ENSO)-related 850-hPa wind anomalies, the dominant modes of biases are extracted. The first EOF mode, explaining 21.3% of total intermodel variance, is characterized by a cyclone over the NWP, indicating a weaker NWP anticyclone. The cyclone appears to be a Rossby wave response to unrealistic equatorial western Pacific (WP) sea surface temperature (SST) anomalies related to excessive equatorial Pacific cold tongue in the models. On one hand, the cold SST biases increase the mean zonal SST gradient, which further intensifies warm zonal advection, favoring the development and persistence of equatorial WP SST anomalies. On the other hand, they reduce the anomalous convection caused by ENSO-related warming, and the resultant increase in downward shortwave radiation contributes to the SST anomalies there. The second EOF mode, explaining 18.6% of total intermodel variance, features an anticyclone over the NWP with location shifted northward. The related SST anomalies in the Indo-Pacific sector show a tripole structure, with warming in the tropical Indian Ocean and equatorial central and eastern Pacific and cooling in the NWP. The Indo-Pacific SST anomalies are highly controlled by ENSO amplitude, which is determined by the intensity of subtropical cells via the adjustment of meridional and vertical advection in the models.

Decadal prediction skill of BCC‐CSM1.1 climate model in East Asia

ABSTRACTThe forecast skill of BCC‐CSM1.1 model in summer East Asian climate was evaluated with the decadal prediction experiments launched annually during 1961–2005. Results of the decadal prediction were compared with those of the historical simulation to estimate contribution of the initialization. Improved prediction skill is found for summer surface air temperature (SAT) in central eastern China measured by both root mean square error (RMSE) and anomaly correlation coefficient (ACC) for the forecast years 2–5. The decadal hindcast of precipitation in central eastern China shows increased skill in RMSE but not ACC. Such improvement in East Asia is related to enhanced prediction skill of the western Pacific (WP) sea surface temperature (SST) originated from the oceanic initialization. In the decadal hindcast, the lower tropospheric atmospheric circulation associated with warm WP exhibits an anticyclone over the South China Sea, which resembles the observation. The southwesterly increases over southeast China associated with warm WP, favouring higher SAT in central eastern China. This reveals that the decadal predictions could more realistically reproduce the relationship between summer East Asian atmospheric circulation and WP SST. The 4‐year average forecast within a decade shows that the initialization increases the ACC skill up to forecast years 4–7 for summer SAT in central eastern China.