Summer Asian Pacific Oscillation Research Articles

Recent autumn sea ice loss in the eastern Arctic enhanced by summer Asian-Pacific Oscillation

Recent rapid Arctic sea ice loss was documented as combined results from anthropogenic forcing and climate system internal variability. However, the role of internal variability is not well understood. Here, we propose that the Asian-Pacific Oscillation (APO), an intrinsic atmospheric mode featuring out-of-phase variations in upper-tropospheric temperatures between Asia and the North Pacific, is one driver for autumn sea ice variability in the eastern Arctic. The positive summer APO favors warming of the mid-latitude North Atlantic sea surface temperatures. This warming persists to autumn and in turn triggers strong anticyclonic anomalies over the Barents-Kara-Laptev Seas and weak lower-tropospheric cyclonic anomalies over the East Siberian Sea, enhancing moisture transport into the eastern Arctic. Such changes consequently increase lower-tropospheric humidity, downwelling longwave radiation, and surface air temperature in the eastern Arctic, thereby melting sea ice. Hence, a recent tendency of the summer APO towards the positive phase accelerates autumn sea ice loss in the eastern Arctic.

Assessment of the summer Asian–Pacific Oscillation pattern in CMIP6 models: historical simulations and future projections

Assessment of the summer Asian–Pacific Oscillation pattern in CMIP6 models: historical simulations and future projections

Interdecadal variability of summer extreme rainfall in the Huai River basin of China associated with the Asian‐Pacific Oscillation

AbstractUsing the rainfall data at rain gauge stations in China and the European Center for Medium‐Range Weather Forecast (ECMWF) reanalysis, the daily Asian‐Pacific Oscillation (APO) index, a tropospheric temperature thermal contrast between Asia and the North Pacific, is constructed to investigate an interdecadal relationship between summer (June–July) APO and rainfall in the Huai River basin (HRB) of central‐eastern China. The daily APO index averaged over June 22–July 12 shows an interdecadal shift around the late 1990s. This shift is characterized by a negative phase of the daily APO index during 1987–1996 and a subsequent positive phase during 1997–2006. Accompanying this shift, the occurrence frequency, intensity and amount of extreme rainfall events remarkably increase in the HRB region. The increased rainfall can be reasonably explained by the atmospheric circulation anomalies. Corresponding to the decadal change in the summer daily APO from the negative phase to the positive one, the subtropical high pressure over the western North Pacific moves toward the north and easterly wind anomalies in the middle and lower layers of the troposphere prevail over extratropical East Asia, which strengthens water vapour convergence, the instability of atmospheric stratification and upward motion anomalies in the HRB region.

The Asian–Pacific oscillation over the past millennium in PMIP3 and PMIP4

Atmospheric oscillations in the upper troposphere can exert considerable impacts on regional and global climate change. The Asian–Pacific Oscillation (APO), a recently identified upper-tropospheric teleconnection is not yet well documented during the last millennium. Here we use 13 models from Paleoclimate Modeling Intercomparison Project phase 3 and 4 (PMIP3 and PMIP4, respectively) to first assess their ability in simulating the climatology and interannual variability of the APO in recent decades, and then examine the changes in APO over the last millennium and attribute the associated sources. The evaluation indicates that the models show high fidelity to the observed climatological horizontal and vertical distributions of the boreal summer APO, with the PMIP4 models outperforming the PMIP3 ones. And the majority of the models can reasonably reproduce the out-of-phase teleconnection pattern over the Asia–North Pacific sector and the interannual variability of the APO index (APOI). During the last millennium, the APOI exhibits a remarkable decreasing trend, and the indices averaged in the MCA (LIA) are significantly larger (smaller) referring to the whole millennium. In warm/cold periods, APO changes over East Asia can be attributed to the Tibetan Plateau heating-induced condensation latent heat and the northern part of the East Asian summer monsoon-associated latent heat, whereas the changes over the North Pacific originate from dry adiabatic heat and the superposed weak offsetting latent heat. Additionally, the APOI reveals a notable interannual scale cycle at around five years.

Possible Effect of the Thermal Condition of the Tibetan Plateau on the Interannual Variability of the Summer Asian–Pacific Oscillation

The summer (June–August) Asian–Pacific Oscillation (APO), a large-scale atmospheric teleconnection pattern, is closely associated with climate anomalies over the Northern Hemisphere. Using the NOAA/CIRES twentieth-century reanalysis, the ECMWF twentieth-century atmospheric reanalysis, and the NCEP reanalysis, this study investigates the variability of the summer APO on the interannual time scale and its relationship with the thermal condition over the Tibetan Plateau (TP). The results show that the interannual variability of the APO is steadily related to the summer TP surface air temperature during the last 100 years. Observation and simulation further show that a positive heating anomaly over the TP can increase the upper-tropospheric temperature and upward motion over Asia. This anomalous upward flow moves northward in the upper troposphere, and then turns and moves eastward, before finally descending over the mid- to high latitudes of the central-eastern North Pacific, concurrently accompanied by anomalous upward motion over the lower latitudes of the central-eastern North Pacific. The anomalous downward and upward motions over the central-eastern North Pacific reduce the in situ mid- and upper-tropospheric temperature, mainly through modulating condensation latent heat from precipitation and/or dry adiabatic heat, which ultimately leads to the interannual variability of the summer APO. In this process, the zonal vertical circulation over the extratropical Asian–North Pacific sector plays an important bridging role.

Present and future connection of Asian-Pacific Oscillation to large-scale atmospheric circulations and East Asian rainfall: results of CMIP5

The summer Asian-Pacific oscillation (APO), one of the major modes of climate variability over the Asian-Pacific sector, has a pronounced effect on variations of large-scale atmospheric circulations and climate. This study evaluated the capability of 30 state-of-the-art climate models among the Coupled Model Intercomparison Project Phase 5 (CMIP5) in simulating its association with the atmospheric circulations over the Asian-Pacific region and the precipitation over East Asia. Furthermore, their future connections under the RCP8.5 scenario were examined. The evaluation results show that 5 out of 30 climate models can well capture the observed APO-related features in a comprehensive way, including the strengthened South Asian high (SAH), deepened North Pacific trough (NPT) and northward East Asian jet (EAJ) in the upper troposphere; an intensification of the Asian low and the North Pacific subtropical high (NPSH) as well as a northward shift of the western Pacific subtropical high (WPSH) in the lower troposphere; and a decrease in East Asian summer rainfall (EASR) under the positive APO phase. Based on the five CMIP5 models’ simulations, the dynamic linkages of the APO to the SAH, NPT, AL, and NPSH are projected to maintain during the second half of the twenty-first century. However, its connection with the EASR tends to reduce significantly. Such a reduction might result from the weakening of the linkage of the APO to the meridional displacement of the EAJ and WPSH as a response to the warming scenario.

How the “best” CMIP5 models project relations of Asian–Pacific Oscillation to circulation backgrounds favorable for tropical cyclone genesis over the western North Pacific

Based on the simulations of 32 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), the present study assesses their capacity to simulate the relationship of the summer Asian–Pacific Oscillation (APO) with the vertical zonal wind shear, low-level atmospheric vorticity, mid-level humidity, atmospheric divergence in the lower and upper troposphere, and western Pacific subtropical high (WPSH) that are closely associated with the genesis of tropical cyclones over the western North Pacific. The results indicate that five models can simultaneously reproduce the observed pattern with the positive APO phase accompanied by weak vertical zonal wind shear, strengthened vorticity in the lower troposphere, increased mid-level humidity, intensified low-level convergence and high-level divergence, and a northward-located WPSH over the western North Pacific. These five models are further used to project their potential relationship under the RCP8.5 scenario during 2050–2099. Compared to 1950–1999, the relationship between the APO and the vertical zonal wind shear is projected to weaken by both the multi-model ensemble and the individual models. Its linkage to the low-level vorticity, mid-level humidity, atmospheric divergence in the lower and upper troposphere, and the northward–southward movement of the WPSH would also reduce slightly but still be significant. However, the individual models show relatively large differences in projecting the linkage between the APO and the mid-level humidity and low-level divergence.

Characteristics of the Asian–Pacific oscillation in boreal summer simulated by BCC_CSM with different horizontal resolutions

The summer Asian–Pacific Oscillation (APO) is a major teleconnection pattern that reflects the zonal thermal contrast between East Asia and the North Pacific in the upper troposphere. The performance of Beijing Climate Center Climate System Models (BCC_CSMs) with different horizontal resolutions, i.e., BCC_CSM1.1 and BCC_CSM1.1(m), in reproducing APO interannual variability, APO-related precipitation anomalies, and associated atmospheric circulation anomalies, is evaluated. The results show that BCC_CSM1.1(m) can successfully capture the interannual variability of the summer APO index. It is also more capable in reproducing the APO’s spatial pattern, compared to BCC_CSM1.1, due to its higher horizontal resolution. Associated with a positive APO index, the northward-shifted and intensified South Asian high, strengthened extratropical westerly jet, and tropical easterly jet in the upper troposphere, as well as the southwesterly monsoonal flow over North Africa and the Indian Ocean in the lower troposphere, are realistically represented by BCC_CSM1.1(m), leading to an improvement in reproducing the increased precipitation over tropical North Africa, South Asia, and East Asia, as well as the decreased precipitation over subtropical North Africa, Japan, and North America. In contrast, these features are less consistent with observations when simulated by BCC_CSM1.1. Regression analysis further indicates that surface temperature anomalies over the North Pacific and the southern and western flanks of the Tibetan Plateau are reasonably reproduced by BCC_CSM1.1(m), which contributes to the substantial improvement in the simulation of the characteristics of summer APO compared to that of BCC_CSM1.1.

The Asian–Pacific Oscillation pattern in CMIP5 simulations of historical and future climate

ABSTRACTBased on the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations, this article evaluated the performance of 33 coupled models in simulating the summer Asian–Pacific Oscillation (APO) pattern and projected its change under the RCP4.5 and RCP8.5 scenarios. The results show that most of the individual models have good capacity to simulate the climate mean state of the spatial structure of the summer APO pattern, while some have poor skill in modelling its secular trend and interannual variability. The multimodel ensemble mean (MME) generally outperforms the individual models. It not only well reproduces the spatial distribution of the observed APO pattern, but also reasonably captures the observed negative trend and interannual variability of the APO. The MME projects a decrease in the APO intensity towards the end of 21st century under both the RCP4.5 and the RCP8.5 scenarios. Large differences in projected change under the two forcing scenarios start to emerge about in the 2040s, with larger decreasing under the RCP8.5 than that under the RCP4.5. Over the course of the second half of the 21st century, the APO pattern will shift eastward slightly in addition to the weakening in intensity as compared to the 1950–2005 mean climate. For the individual models, the majority of models agree on the sign of projected decreasing change in the APO intensity and also exhibit a stronger change under the RCP8.5 scenario than under the RCP4.5 scenario.

Preceding Factors of Summer Asian–Pacific Oscillation and the Physical Mechanism for Their Potential Influences

Abstract The authors explore the preceding factors of summertime Asian–Pacific Oscillation (APO) using observations and output from the NCEP Climate Forecast System version 2 (CFSv2). Results show that the winter and spring sea surface temperatures (SSTs) in the tropical central-eastern Pacific (TCEP) and the spring sea level pressure (SLP) over the north Indian Ocean (NIO) are significantly correlated with summer APO. The preceding TCEP SST anomaly tends to exert a delayed impact on summer APO through the following process. The previous winter TCEP SST anomaly persists until spring and results in SLP anomaly over the NIO in spring. The latter induces a vertical motion anomaly over the western Tibetan Plateau, which alters spring rainfall and underlying soil moisture in situ, further modulating local surface air temperature during the following summer and hence the summer APO. The CFSv2 has high skills in predicting the winter and spring TCEP SST and the spring NIO SLP and successfully captures the observed relationships of TCEP SST and NIO SLP with summer APO. This result explains why the CFSv2 is capable of predicting the summer APO teleconnection by several months in advance.

Improving the Prediction of the Summer Asian–Pacific Oscillation Using the Interannual Increment Approach

Abstract The summer Asian–Pacific oscillation (APO) is a dominant teleconnection pattern over the extratropical Northern Hemisphere that links the large-scale atmospheric circulation anomalies over the Asian–North Pacific Ocean sector. In this study, the direct Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) model outputs from 1960 to 2001, which are limited in predicting the interannual variability of the summer Asian upper-tropospheric temperature and the decadal variations, are applied using the interannual increment approach to improve the predictions of the summer APO. By treating the year-to-year increment as the predictand, the interannual increment scheme is shown to significantly improve the predictive ability for the interannual variability of the summer Asian upper-tropospheric temperature and the decadal variations. The improvements for the interannual and interdecadal summer APO variability predictions in the interannual increment scheme relative to the original scheme are clear and significant. Compared with the DEMETER direct outputs, the statistical model with two predictors of APO and sea surface temperature anomaly over the Atlantic shows a significantly improved ability to predict the interannual variability of the summer rainfall over the middle and lower reaches of the Yangtze River valley (SRYR). This study therefore describes a more efficient approach for predicting the APO and the SRYR.

Reconstruction of the March–August PDSI since 1703 AD based on tree rings of Chinese pine (<i>Pinus tabulaeformis</i> Carr.) in the Lingkong Mountain, southeast Chinese loess Plateau

Abstract. We utilised tree-ring cores, collected from three sites at Lingkong Mountain located in the southeast part of the Chinese Loess Plateau (CLP), to develope a regional ring-width chronology. Significant positive correlations between the tree-ring index and the monthly Palmer drought severity index (PDSI) were identified, indicating that the radial growth of trees in this region was moisture-limited. The March–August mean PDSI was quantitatively reconstructed from 1703 to 2008 with an explained variance of 46.4%. Seven dry periods during 1719–1726, 1742–1748, 1771–1778, 1807–1818, 1832–1848, 1867–1932 and 1993–2008 and six wet periods during 1727–1741, 1751–1757, 1779–1787, 1797–1805, 1853–1864 and 1934–1957 were revealed in our reconstruction. Among them, 1867–1932 and 1934–1957 were identified as the longest dry and wet periods, respectively. On the centennial scale, the 19th century was recognised as the driest century. The drying tendency since 1960s was evident. However, recent drought in 1993–2008 was still within the frame of natural climate variability based on the 306 yr PDSI reconstruction. The dry and wet phases of Lingkong Mountain were in accordance with changes in the summer Asian-Pacific oscillation (IAPO) and sunspot numbers, they also showed strong similarity to other tree-ring based moisture indexes in large areas in and around the CLP, indicating the moisture variability in the CLP was almost synchronous and closely related with large-scale land–ocean–atmospheric circulation and solar activity. Spatial correlation analysis suggested that this PDSI reconstruction could represent the moisture variations for most parts of the CLP, and even larger area of northern China and east Mongolia. Multi-taper spectral analysis revealed significant cycles at the inter-annual (2–7 yr), inter-decadal (37.9 yr) and centennial (102 yr) scales. Results of this study are very helpful for us to improve the knowledge of past climate change in the CLP and enable us to prevent and manage future natural disasters.

Asian–Pacific Oscillation signal from a Qomolangma (Mount Everest) ice-core chemical record

AbstractThe Asian–Pacific Oscillation (APO) is a climate pattern related to the thermal differences between the Asian continent and the north Pacific. We present a 1000 year record of the major ions in a 108.83 m ice core from east Rongbuk glacier (28°01′ N, 86°58′ E; 6518ma.s.l.) on the northeast slope of Qomolangma (Mount Everest), and discuss its relationship with a 993 year time series of the summer APO index that has been reconstructed from Beijing’s summer temperature (recorded by a stalagmite) and the Pacific Decadal Oscillation (PDO) index. Empirical orthogonal function (EOF) analysis shows that crustal major ions (Mg2+, Ca2+, SO42–and NO3–) are highly loaded on EOF1, whereas marine major ions (Cl–and Na+) are highly loaded on EOF2. Higher EOF1 is associated with lower upper-troposphere temperature (UTT) over Mongolia, corresponding to a lower APO index and higher surface pressure. Higher EOF2 is associated with higher UTT over Mongolia, corresponding to a higher APO index and lower surface pressure. The higher burden of major ions and higher summer APO index during the period AD 1000–1323 may be due to drought and warm climate of the major-ion source regions. From AD1323 to 1900, EOF1 and EOF2 do not show a consistent correlation with summer APO index, indicating the complex mechanisms of ion transport over the southern Tibetan Plateau (TP) during this period. After AD 1900, the summer APO index is correlated negatively with EOF1 and positively with EOF2, indicating that ion transport over the southern TP during this period is influenced significantly by the APO. Our examinations of the PDO index and major-ion record show that higher PDO corresponds to higher EOF1 and lower EOF2. This suggests that the correlation between the recorded major-ion concentration and the APO index originates in the seesaw relationship in temperature between the Asian continent and the north Pacific.

Is the Interannual Variability of the Summer Asian–Pacific Oscillation Predictable?

Abstract The summer (June–August) Asian–Pacific Oscillation (APO) measures the interannual variability of large-scale atmospheric circulation over the Asian–North Pacific Ocean sector. In this study, the authors assess the predictability of the summer APO index interannual variability and the associated atmospheric circulation anomalies using the 1959–2001 hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF), Centre National de Recherches Météorologiques (CNRM), and the Met Office (UKMO) general circulation models from the Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) project. The results show that these models predict the summer APO index interannual variability well and have higher skill for the North Pacific than for the Asian upper-tropospheric temperature. Meanwhile, the observed APO-related atmospheric circulation anomalies in the South Asian high, the tropical easterly wind jet over the Asian monsoon region in the upper troposphere, the subtropical anticyclone over the North Pacific, and the summer southwest monsoon over Asia in the lower troposphere are reasonably well predicted in their spatial patterns and intensities. Compared with the observations, however, these models display low skill in predicting the long-term varying trends of the upper-tropospheric temperature over the Asian–North Pacific sector or the APO index during 1959–2001.

Simulation and Dynamical Prediction of the Summer Asian–Pacific Oscillation and Associated Climate Anomalies by the NCEP CFSv2

Abstract The Asian–Pacific Oscillation (APO) is a dominant teleconnection pattern linking the climate anomalies over Asia, the North Pacific, and other regions including North America. The National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2) successfully simulates many summer-mean features of the upper-tropospheric temperature, the South Asian high, the westerly and easterly jet streams, and the regional monsoons over Asia and Africa. It also well simulates the interannual variability of the APO and associated anomalies in atmospheric circulation, precipitation, surface air temperature (SAT), and sea surface temperature (SST). Associated with a positive APO are a strengthened South Asian high; a weakened extratropical upper-tropospheric westerly jet stream over North America; strengthened subtropical anticyclones over the Northern Hemisphere oceans; and strengthened monsoons over North Africa, India, and East Asia. Meanwhile, increased precipitation is found over tropical North Africa, South Asia, northern China, and tropical South America; decreased precipitation is seen over subtropical North Africa, the Middle East, central Asia, southern China, Japan, and extratropical North America. Low SAT occurs in North Africa, India, and tropical South America and high SAT appears in extratropical Eurasia and North America. SST increases in the extratropical Pacific and the North Atlantic but decreases in the tropical Pacific. The summer APO and many of the associated climate anomalies can be predicted by the NCEP CFSv2 by up to 5 months in advance. However, the CFSv2 skill of predicting the SAT in the East Asian monsoon region is low.

Modeling variations of summer upper tropospheric temperature and associated climate over the Asian Pacific region during the mid‐Holocene

The summer upper tropospheric temperature change and its association with atmospheric circulations and precipitation over the Asian Pacific region during the mid‐Holocene have been addressed by using outputs from a coupled ocean atmosphere general circulation model performed as part of the second phase of the Paleoclimate Modeling Intercomparison Project. The simulated result shows the summer oscillation pattern similar to the present Asian Pacific oscillation (APO) existed in the mid‐Holocene. When there was a warming (cooling) upper troposphere over East Asia, a cooling (warming) upper troposphere occurred over the midlatitudes of the North Pacific. Compared to the modern climate, however, the simulated mid‐Holocene temperature in the upper troposphere was higher over East Asia and lower over the midlatitudes of the North Pacific, indicating a stronger summer APO in the mid‐Holocene. Corresponding to such a condition, the North Pacific was modeled to be dominated by a high‐level cyclonic circulation difference and a low‐level anticyclonic circulation difference in the mid‐Holocene relative to the present, which favored the subsidence of airflows and thus resulted in less precipitation in this region. Meanwhile, East Asia was simulated to be occupied by an anticyclonic circulation difference in the upper troposphere and a cyclonic circulation difference in the lower troposphere. Accordingly, the ascending motion and the low‐level southerly wind strengthened in East Asia, leading to an increase of local precipitation in the mid‐Holocene. Therefore, the modeled mid‐Holocene climate suggests that the summer rainfall change over the Asian Pacific region may be a result of the strengthened APO in the upper troposphere.

A 150-year reconstructed summer Asian–Pacific Oscillation index and its association with precipitation over eastern China

Reconstructing the long-term series of the East Asian summer monsoon (EASM) indices may help understand long-term variability of EASM and its associations with precipitation. In this study, the summer middle–upper tropospheric temperature over the Asian–North Pacific sector is reconstructed from sea level pressure during the past 150 years, and then an atmospheric thermal contrast between Asia and the North Pacific, called the Asian–Pacific Oscillation (APO) index, is calculated from the reconstructed temperature. The results show that the APO phenomenon may occur in the reconstructed temperature fields, and its index has a significant positive/negative correlation with SST over the extratropical North Pacific/the tropical central–eastern Pacific in the past 150 years. The reconstructed summer APO index shows inter-decadal variability, with a positive phase in the 1870–1890s, the 1920s, and the 1940–1970s, indicating a stronger thermal contrast between Asia and the North Pacific, and with a negative phase in the 1860s, the 1900–1910s, and the 1980–1990s, indicating a weaker thermal contrast. Corresponding to a higher APO index in earlier decades of the twentieth century, there are more rainfall to the south of the Yangtze River and over North China and less rainfall over the Huaihe River valley. In the recent decades, however, more- and less-rain belts shifted southwards when the APO index is higher. During 1850–1900, the reconstructed APO index also showed a significant positive correlation with precipitation in some regions of North China.

Relationship between the Asian-Pacific oscillation and the tropical cyclone frequency in the western North Pacific

The relationship between the Asian-Pacific oscillation (APO) and the tropical cyclone frequency over the western North Pacific (WNP) in summer is preliminarily investigated through an analysis of observed data. The result has shown clearly that APO is significantly and positively correlated to the tropical cyclone frequency in the WNP. If APO is above (below) the normal in summer, more (less) tropical cyclones will tend to appear in the WNP. The present study also addresses the large-scale atmospheric general circulation changes underlying the linkage between APO and the WNP tropical cyclone frequency. It follows that a positive phase of summer APO is concurrent with weakened as well as northward and eastward located western Pacific subtropical high (WPSH), low-level convergence and high-level divergence, and reduced vertical zonal wind shear in the WNP, providing favorable environment for the tropical cyclone genesis, and thus more tropical cyclones will come into being, and vice versa.