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- Author or Editor: Yuan Yang x
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Abstract
Using multiple datasets and a partial correlation method, the authors analyze the different impacts of eastern Pacific (EP) and central Pacific (CP) El Niño on East Asian climate, focusing on the features from El Niño developing summer to El Niño decaying summer. Unlike the positive–negative–positive (+/−/+) anomalous precipitation pattern over East Asia and the equatorial Pacific during EP El Niño, an anomalous −/+/− rainfall pattern appears during CP El Niño. The anomalous dry conditions over southeastern China and the northwestern Pacific during CP El Niño seem to result from the anomalous low-level anticyclone over southern China and the South China Sea, which is located more westward than the Philippine Sea anticyclone during EP El Niño. The continuous anomalous sinking motion over southeastern China, as part of the anomalous Walker circulation associated with CP El Niño, also contributes to these dry conditions.
During the developing summer, the impact of CP El Niño on East Asian climate is more significant than the influence of EP El Niño. During the decaying summer, however, EP El Niño exerts a stronger influence on East Asia, probably due to the long-lasting anomalous warming over the tropical Indian Ocean accompanying EP El Niño.
Temperatures over portions of East Asia and the northwestern Pacific tend to be above normal during EP El Niño but below normal from the developing autumn to the next spring during CP El Niño. A possible reason is the weakened (enhanced) East Asian winter monsoon related to EP (CP) El Niño.
Abstract
Using multiple datasets and a partial correlation method, the authors analyze the different impacts of eastern Pacific (EP) and central Pacific (CP) El Niño on East Asian climate, focusing on the features from El Niño developing summer to El Niño decaying summer. Unlike the positive–negative–positive (+/−/+) anomalous precipitation pattern over East Asia and the equatorial Pacific during EP El Niño, an anomalous −/+/− rainfall pattern appears during CP El Niño. The anomalous dry conditions over southeastern China and the northwestern Pacific during CP El Niño seem to result from the anomalous low-level anticyclone over southern China and the South China Sea, which is located more westward than the Philippine Sea anticyclone during EP El Niño. The continuous anomalous sinking motion over southeastern China, as part of the anomalous Walker circulation associated with CP El Niño, also contributes to these dry conditions.
During the developing summer, the impact of CP El Niño on East Asian climate is more significant than the influence of EP El Niño. During the decaying summer, however, EP El Niño exerts a stronger influence on East Asia, probably due to the long-lasting anomalous warming over the tropical Indian Ocean accompanying EP El Niño.
Temperatures over portions of East Asia and the northwestern Pacific tend to be above normal during EP El Niño but below normal from the developing autumn to the next spring during CP El Niño. A possible reason is the weakened (enhanced) East Asian winter monsoon related to EP (CP) El Niño.
Abstract
Using a Lagrangian trajectory model, contributions of moisture from the Indian Ocean (IO), the South China Sea (SCS), the adjacent land region (LD), and the Pacific Ocean (PO) to interannual summer precipitation variations in southwestern China (SWC) are investigated. Results show that, on average, the IO, SCS, LD, and PO contribute 48.8%, 21.1%, 23.6%, and 3.7% of the total moisture release in SWC, respectively. In summers with the above-normal precipitation, moisture release from the IO and SCS increases significantly by 41.4% and 15.1%, respectively. In summers with below-normal precipitation, moisture release from the IO and SCS decreases significantly by 44.2% and 24.6%, respectively. In addition, the moisture anomalies from the four source regions together explain 86.5% of the total interannual variances of SWC summer precipitation, and the IO and SCS only can explain 75.7%. Variations in moisture transport from the IO, SCS, and LD to SWC are not independent of one another and are commonly influenced by the anomalous anticyclone in the western North Pacific Ocean, which enhances the moisture transport from the IO and SCS by the anomalous southwesterlies over its northwestern quadrant but reduces that from the LD east of SWC by the anomalous westerlies along its northern edge. Anomalous warming in the tropical Atlantic Ocean can modify the Walker circulation, induce anomalous descending motion over the central tropical Pacific, and excite the anomalous anticyclone in the western North Pacific as the classic Matsuno–Gill response. The observed impacts of the tropical Atlantic warming on the anomalous anticyclone and summer precipitation in SWC can be well reproduced in an atmospheric general circulation model.
Abstract
Using a Lagrangian trajectory model, contributions of moisture from the Indian Ocean (IO), the South China Sea (SCS), the adjacent land region (LD), and the Pacific Ocean (PO) to interannual summer precipitation variations in southwestern China (SWC) are investigated. Results show that, on average, the IO, SCS, LD, and PO contribute 48.8%, 21.1%, 23.6%, and 3.7% of the total moisture release in SWC, respectively. In summers with the above-normal precipitation, moisture release from the IO and SCS increases significantly by 41.4% and 15.1%, respectively. In summers with below-normal precipitation, moisture release from the IO and SCS decreases significantly by 44.2% and 24.6%, respectively. In addition, the moisture anomalies from the four source regions together explain 86.5% of the total interannual variances of SWC summer precipitation, and the IO and SCS only can explain 75.7%. Variations in moisture transport from the IO, SCS, and LD to SWC are not independent of one another and are commonly influenced by the anomalous anticyclone in the western North Pacific Ocean, which enhances the moisture transport from the IO and SCS by the anomalous southwesterlies over its northwestern quadrant but reduces that from the LD east of SWC by the anomalous westerlies along its northern edge. Anomalous warming in the tropical Atlantic Ocean can modify the Walker circulation, induce anomalous descending motion over the central tropical Pacific, and excite the anomalous anticyclone in the western North Pacific as the classic Matsuno–Gill response. The observed impacts of the tropical Atlantic warming on the anomalous anticyclone and summer precipitation in SWC can be well reproduced in an atmospheric general circulation model.
Abstract
The authors examine different evolution features of the low-level anticyclone over the tropical northwestern Pacific between eastern Pacific (EP) El Niño events and central Pacific (CP) El Niño events. During EP El Niño, the low-level anticyclone shows an eastward movement from the northern Indian Ocean to the east of the Philippines. During CP El Niño, however, the anticyclone is mostly confined to the west of the Philippines. It is weaker, exhibits a shorter lifetime, and lacks eastward movement compared to the Philippine Sea anticyclone (PSAC) during EP El Niño. Investigation into the possible impact of Indian Ocean (IO) sea surface temperature (SST) on the evolution of the low-level anticyclone during EP and CP El Niño indicates that both SST and low-level atmospheric circulation over the IO are related more strongly with EP El Niño than with CP El Niño. The IO SST tends to exert a more prominent influence on PSAC during EP El Niño than during CP El Niño. During the developing summer and autumn of EP El Niño, the anomalous anticyclone over the northern Indian Ocean excited by positive IO dipole may contribute to an early development of the PSAC. During the winter and decaying spring, the anomalous anticyclone to the east of the Philippines instigated by the IO basin-wide warming mode also favors a larger persistence of the PSAC. During CP El Niño, however, IO SST shows a negligible impact on the evolution of the anticyclone.
Abstract
The authors examine different evolution features of the low-level anticyclone over the tropical northwestern Pacific between eastern Pacific (EP) El Niño events and central Pacific (CP) El Niño events. During EP El Niño, the low-level anticyclone shows an eastward movement from the northern Indian Ocean to the east of the Philippines. During CP El Niño, however, the anticyclone is mostly confined to the west of the Philippines. It is weaker, exhibits a shorter lifetime, and lacks eastward movement compared to the Philippine Sea anticyclone (PSAC) during EP El Niño. Investigation into the possible impact of Indian Ocean (IO) sea surface temperature (SST) on the evolution of the low-level anticyclone during EP and CP El Niño indicates that both SST and low-level atmospheric circulation over the IO are related more strongly with EP El Niño than with CP El Niño. The IO SST tends to exert a more prominent influence on PSAC during EP El Niño than during CP El Niño. During the developing summer and autumn of EP El Niño, the anomalous anticyclone over the northern Indian Ocean excited by positive IO dipole may contribute to an early development of the PSAC. During the winter and decaying spring, the anomalous anticyclone to the east of the Philippines instigated by the IO basin-wide warming mode also favors a larger persistence of the PSAC. During CP El Niño, however, IO SST shows a negligible impact on the evolution of the anticyclone.
Abstract
This study reveals that sea ice in the Barents and Kara Seas plays a crucial role in establishing a new Arctic coupled climate system. The early winter sea ice before 1998 shows double dipole patterns over the Arctic peripheral seas. This pattern, referred to as the early winter quadrupole pattern, exhibits the anticlockwise sequential sea ice anomalies propagation from the Greenland Sea to the Barents–Kara Seas and to the Bering Sea from October to December. This early winter in-phase ice variability contrasts to the out-of-phase relationship in late winter. The mean temperature advection and stationary wave heat flux divergence associated with the atmospheric zonal wave-2 pattern are responsible for the early winter in-phase pattern.
Since the end of the last century, the early winter quadrupole pattern has broken down because of the rapid decline of sea ice extent in the Barents–Kara Seas. This remarkable ice retreat modifies the local ocean–atmosphere heat exchange, forcing an anomalous low air pressure over the Barents–Kara Seas. The subsequent collapse of the atmospheric zonal wave-2 pattern is likely responsible for the breakdown of the early winter sea ice quadrupole pattern after 1998. Therefore, the sea ice anomalies in the Barents–Kara Seas play a key role in establishing new atmosphere–sea ice coupled relationships in the warming Arctic.
Abstract
This study reveals that sea ice in the Barents and Kara Seas plays a crucial role in establishing a new Arctic coupled climate system. The early winter sea ice before 1998 shows double dipole patterns over the Arctic peripheral seas. This pattern, referred to as the early winter quadrupole pattern, exhibits the anticlockwise sequential sea ice anomalies propagation from the Greenland Sea to the Barents–Kara Seas and to the Bering Sea from October to December. This early winter in-phase ice variability contrasts to the out-of-phase relationship in late winter. The mean temperature advection and stationary wave heat flux divergence associated with the atmospheric zonal wave-2 pattern are responsible for the early winter in-phase pattern.
Since the end of the last century, the early winter quadrupole pattern has broken down because of the rapid decline of sea ice extent in the Barents–Kara Seas. This remarkable ice retreat modifies the local ocean–atmosphere heat exchange, forcing an anomalous low air pressure over the Barents–Kara Seas. The subsequent collapse of the atmospheric zonal wave-2 pattern is likely responsible for the breakdown of the early winter sea ice quadrupole pattern after 1998. Therefore, the sea ice anomalies in the Barents–Kara Seas play a key role in establishing new atmosphere–sea ice coupled relationships in the warming Arctic.
Abstract
The recent accelerated Arctic sea ice decline has been proposed as a possible forcing factor for midlatitude circulation changes, which can be projected onto the Arctic Oscillation (AO) and/or North Atlantic Oscillation (NAO) mode. However, the timing and physical mechanisms linking AO responses to the Arctic sea ice forcing are not entirely understood. In this study, the authors suggest a connection between November sea ice extent in the Barents and Kara Seas and the following winter’s atmospheric circulation in terms of the fast sea ice retreat and the subsequent modification of local air–sea heat fluxes. In particular, the dynamical processes that link November sea ice in the Barents and Kara Seas with the development of AO anomalies in February is explored. In response to the lower-tropospheric warming associated with the initial thermal effect of the sea ice loss, the large-scale atmospheric circulation goes through a series of dynamical adjustment processes: The decelerated zonal-mean zonal wind anomalies propagate gradually from the subarctic to midlatitudes in about one month. The equivalent barotropic AO dipole pattern develops in January because of wave–mean flow interaction and firmly establishes itself in February following the weakening and warming of the stratospheric polar vortex. This connection between sea ice loss and the AO mode is robust on time scales ranging from interannual to decadal. Therefore, the recent winter AO weakening and the corresponding midlatitude climate change may be partly associated with the early winter sea ice loss in the Barents and Kara Seas.
Abstract
The recent accelerated Arctic sea ice decline has been proposed as a possible forcing factor for midlatitude circulation changes, which can be projected onto the Arctic Oscillation (AO) and/or North Atlantic Oscillation (NAO) mode. However, the timing and physical mechanisms linking AO responses to the Arctic sea ice forcing are not entirely understood. In this study, the authors suggest a connection between November sea ice extent in the Barents and Kara Seas and the following winter’s atmospheric circulation in terms of the fast sea ice retreat and the subsequent modification of local air–sea heat fluxes. In particular, the dynamical processes that link November sea ice in the Barents and Kara Seas with the development of AO anomalies in February is explored. In response to the lower-tropospheric warming associated with the initial thermal effect of the sea ice loss, the large-scale atmospheric circulation goes through a series of dynamical adjustment processes: The decelerated zonal-mean zonal wind anomalies propagate gradually from the subarctic to midlatitudes in about one month. The equivalent barotropic AO dipole pattern develops in January because of wave–mean flow interaction and firmly establishes itself in February following the weakening and warming of the stratospheric polar vortex. This connection between sea ice loss and the AO mode is robust on time scales ranging from interannual to decadal. Therefore, the recent winter AO weakening and the corresponding midlatitude climate change may be partly associated with the early winter sea ice loss in the Barents and Kara Seas.
Abstract
From a basinwide perspective, the dominant mode of Indian Ocean tropical cyclone genesis (TCG) in September–November (SON) shows an equatorially symmetric east–west zonal dipole pattern, which can explain approximately 13% of the SON TCG variance. This zonal dipole TCG pattern is significantly related to the tripole pattern of the sea surface temperature anomalies (SSTAs) in the tropical Indo-Pacific Ocean (IPT). The IPT, which is a combined interbasin mode and presents a dipole pattern of SSTAs in the tropical Indian Ocean and El Niño–like SSTAs in the tropical Pacific Ocean, can influence the local Walker circulation and zonal dipole TCG pattern over the tropical Indian Ocean. Associated with a positive IPT phase, abnormal ascending (descending) motions are induced and favorable for more (less) water vapor transport to the lower–middle level in the western (eastern) tropical Indian Ocean; significant anticyclonic vorticity anomalies are evoked in the lower level over the eastern tropical Indian Ocean, and weak easterly vertical wind shear appears over the tropical Indian Ocean. Thus, abnormally strong upward motion, abundant water vapor in the lower–middle level, and weak vertical wind shear are favorable for more TCG in the western tropical Indian Ocean, while the combined negative contributions of the vertical motion, lower-level vorticity, and humidity terms result in less TCG in the eastern tropical Indian Ocean.
Abstract
From a basinwide perspective, the dominant mode of Indian Ocean tropical cyclone genesis (TCG) in September–November (SON) shows an equatorially symmetric east–west zonal dipole pattern, which can explain approximately 13% of the SON TCG variance. This zonal dipole TCG pattern is significantly related to the tripole pattern of the sea surface temperature anomalies (SSTAs) in the tropical Indo-Pacific Ocean (IPT). The IPT, which is a combined interbasin mode and presents a dipole pattern of SSTAs in the tropical Indian Ocean and El Niño–like SSTAs in the tropical Pacific Ocean, can influence the local Walker circulation and zonal dipole TCG pattern over the tropical Indian Ocean. Associated with a positive IPT phase, abnormal ascending (descending) motions are induced and favorable for more (less) water vapor transport to the lower–middle level in the western (eastern) tropical Indian Ocean; significant anticyclonic vorticity anomalies are evoked in the lower level over the eastern tropical Indian Ocean, and weak easterly vertical wind shear appears over the tropical Indian Ocean. Thus, abnormally strong upward motion, abundant water vapor in the lower–middle level, and weak vertical wind shear are favorable for more TCG in the western tropical Indian Ocean, while the combined negative contributions of the vertical motion, lower-level vorticity, and humidity terms result in less TCG in the eastern tropical Indian Ocean.
Abstract
This study focuses on statistical analysis of anomalous tropical cyclone (TC) activities and the physical mechanisms behind these anomalies. Different patterns of decaying of the warm sea surface temperature anomaly (SSTA) over the equatorial central-eastern Pacific are categorized into three types: eastern Pacific warming decaying to La Niña (EPWDL), eastern Pacific warming decaying to a neutral phase (EPWDN), and a central Pacific warming decaying year (CPWD). Differences in TC activity over the western North Pacific (WNP) corresponding to the above three types are discussed, and possible mechanisms are proposed. For EPWDL, TC genesis shows a significant positive (negative) anomaly over the northwestern (southeastern) WNP and more TCs move westward and make landfall over the southern East Asian coast. This is attributed primarily to the combined modulation of La Niña and the warm equatorial east Indian Ocean SSTA. For EPWDN, enhanced TC genesis is observed over the northeastern WNP, and suppressed TC activity is located mainly in the zonal region extending from the Philippine Sea to the eastern WNP, close to 160°E. Most of the TCs formed over the eastern WNP experience early recurvature east of 140°E, then move northeastward; hence, fewer TCs move northwestward to make landfall over the East Asian coast. For CPWD, the enhanced TC activity appears over the western WNP. This is due to the weak anomalous cyclonic circulation over the Philippines, primarily caused by the weaker, more westward-shifting warm SSTA compared to that in the previous warming year over the central Pacific.
Abstract
This study focuses on statistical analysis of anomalous tropical cyclone (TC) activities and the physical mechanisms behind these anomalies. Different patterns of decaying of the warm sea surface temperature anomaly (SSTA) over the equatorial central-eastern Pacific are categorized into three types: eastern Pacific warming decaying to La Niña (EPWDL), eastern Pacific warming decaying to a neutral phase (EPWDN), and a central Pacific warming decaying year (CPWD). Differences in TC activity over the western North Pacific (WNP) corresponding to the above three types are discussed, and possible mechanisms are proposed. For EPWDL, TC genesis shows a significant positive (negative) anomaly over the northwestern (southeastern) WNP and more TCs move westward and make landfall over the southern East Asian coast. This is attributed primarily to the combined modulation of La Niña and the warm equatorial east Indian Ocean SSTA. For EPWDN, enhanced TC genesis is observed over the northeastern WNP, and suppressed TC activity is located mainly in the zonal region extending from the Philippine Sea to the eastern WNP, close to 160°E. Most of the TCs formed over the eastern WNP experience early recurvature east of 140°E, then move northeastward; hence, fewer TCs move northwestward to make landfall over the East Asian coast. For CPWD, the enhanced TC activity appears over the western WNP. This is due to the weak anomalous cyclonic circulation over the Philippines, primarily caused by the weaker, more westward-shifting warm SSTA compared to that in the previous warming year over the central Pacific.
Abstract
The meridional geostrophic transport (MGT) in the interior tropical North Pacific Ocean is estimated based on global ocean heat and salt content data. The decadal variations of the zonally and vertically integrated MGT in the tropical North Pacific Ocean are found to precede the Pacific decadal oscillation (PDO) by 1–3 years. The dynamics of the MGT are analyzed based on Sverdrup theory. It is found that the total meridional transport variability (MGT plus Ekman) is dominated by the MGT variability having positive correlations with the PDO index. The Sverdrup transports differ from the total meridional transport significantly and have insignificant correlations with PDO index, suggesting that the MGT variability is not controlled by the Sverdrup dynamics. In comparison, the simulated meridional transport variability in the models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and the Ocean General Circulation Model for the Earth Simulator are dominated by the Sverdrup transports, having insignificant correlations with the simulated PDO indices. The comparison suggests that the non-Sverdrup component in the MGT is important for the predictability of PDO and that significant deficiencies exist in these models in simulating a realistic structure of the tropical ocean gyre variability and predicting the decadal climate variations associated with it.
Abstract
The meridional geostrophic transport (MGT) in the interior tropical North Pacific Ocean is estimated based on global ocean heat and salt content data. The decadal variations of the zonally and vertically integrated MGT in the tropical North Pacific Ocean are found to precede the Pacific decadal oscillation (PDO) by 1–3 years. The dynamics of the MGT are analyzed based on Sverdrup theory. It is found that the total meridional transport variability (MGT plus Ekman) is dominated by the MGT variability having positive correlations with the PDO index. The Sverdrup transports differ from the total meridional transport significantly and have insignificant correlations with PDO index, suggesting that the MGT variability is not controlled by the Sverdrup dynamics. In comparison, the simulated meridional transport variability in the models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and the Ocean General Circulation Model for the Earth Simulator are dominated by the Sverdrup transports, having insignificant correlations with the simulated PDO indices. The comparison suggests that the non-Sverdrup component in the MGT is important for the predictability of PDO and that significant deficiencies exist in these models in simulating a realistic structure of the tropical ocean gyre variability and predicting the decadal climate variations associated with it.
Abstract
The breeding method has been implemented in the NASA Seasonal-to-Interannual Prediction Project (NSIPP) coupled general circulation model (CGCM) with the ultimate goal of improving operational seasonal to interannual climate predictions through ensemble forecasting and data assimilation. This is the first attempt to isolate the evolving ENSO instability and its corresponding global atmospheric response in a fully coupled ocean–atmosphere GCM. The results herein show that the growth rate of the coupled bred vectors (BVs) is sensitive to the ENSO phases of the evolving background flow and peaks about 3 months before an ENSO event. The structure of the dominant growing BV modes also evolves with the background ENSO and exhibits a larger amplitude in the eastern tropical Pacific, reflecting the natural dynamical sensitivity associated with the shallow thermocline at the eastern boundary. The key features of coupled bred vectors of the NSIPP CGCM are reproduced when using the NCEP CGCM, an independently developed coupled general circulation model.
Abstract
The breeding method has been implemented in the NASA Seasonal-to-Interannual Prediction Project (NSIPP) coupled general circulation model (CGCM) with the ultimate goal of improving operational seasonal to interannual climate predictions through ensemble forecasting and data assimilation. This is the first attempt to isolate the evolving ENSO instability and its corresponding global atmospheric response in a fully coupled ocean–atmosphere GCM. The results herein show that the growth rate of the coupled bred vectors (BVs) is sensitive to the ENSO phases of the evolving background flow and peaks about 3 months before an ENSO event. The structure of the dominant growing BV modes also evolves with the background ENSO and exhibits a larger amplitude in the eastern tropical Pacific, reflecting the natural dynamical sensitivity associated with the shallow thermocline at the eastern boundary. The key features of coupled bred vectors of the NSIPP CGCM are reproduced when using the NCEP CGCM, an independently developed coupled general circulation model.
