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- Author or Editor: Shangfeng Chen x
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Abstract
Surface air temperature (SAT) anomalies tend to persist from winter to the following spring over the mid- to high latitudes of Eurasia. The present study compares two distinct cases of Eurasian SAT anomaly evolution and investigates the reasons for the persistence of continental-scale mid- to high-latitude Eurasian SAT anomalies from winter to following spring (termed persistent cases). The persisting SAT anomalies are closely associated with the sustenance of large-scale atmospheric circulation anomaly pattern over the North Atlantic and Eurasia, featuring a combination of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO) and the Scandinavian pattern, from winter to spring. The combined circulation anomalies result in SAT warming over most of mid- to high-latitude Eurasia via anomalous wind-induced temperature advection. The sustenance of atmospheric circulation anomaly pattern is related to the maintenance of the North Atlantic triple sea surface temperature (SST) anomaly pattern due to air–sea interaction processes. The Barents Sea ice anomalies, which form in winter and increase in spring, also partly contribute to the sustenance of atmospheric circulation anomalies via modulating thermal state of the lower troposphere. In the cases that notable SAT warming (cooling) in winter is replaced by pronounced SAT cooling (warming) in the subsequent spring—termed reverse cases—the North Atlantic SST anomalies become small and the Greenland Sea ice change is a response to atmospheric change in spring. Without the support of lower boundary forcing, the atmospheric circulation anomaly pattern experiences a reverse in the spatial distribution from winter to spring likely due to internal atmospheric processes.
Abstract
Surface air temperature (SAT) anomalies tend to persist from winter to the following spring over the mid- to high latitudes of Eurasia. The present study compares two distinct cases of Eurasian SAT anomaly evolution and investigates the reasons for the persistence of continental-scale mid- to high-latitude Eurasian SAT anomalies from winter to following spring (termed persistent cases). The persisting SAT anomalies are closely associated with the sustenance of large-scale atmospheric circulation anomaly pattern over the North Atlantic and Eurasia, featuring a combination of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO) and the Scandinavian pattern, from winter to spring. The combined circulation anomalies result in SAT warming over most of mid- to high-latitude Eurasia via anomalous wind-induced temperature advection. The sustenance of atmospheric circulation anomaly pattern is related to the maintenance of the North Atlantic triple sea surface temperature (SST) anomaly pattern due to air–sea interaction processes. The Barents Sea ice anomalies, which form in winter and increase in spring, also partly contribute to the sustenance of atmospheric circulation anomalies via modulating thermal state of the lower troposphere. In the cases that notable SAT warming (cooling) in winter is replaced by pronounced SAT cooling (warming) in the subsequent spring—termed reverse cases—the North Atlantic SST anomalies become small and the Greenland Sea ice change is a response to atmospheric change in spring. Without the support of lower boundary forcing, the atmospheric circulation anomaly pattern experiences a reverse in the spatial distribution from winter to spring likely due to internal atmospheric processes.
Abstract
This study investigates interdecadal changes in the relationship between interannual variations of boreal spring sea surface temperature (SST) in the North Atlantic and surface air temperature (SAT) over the mid-to-high latitudes of Eurasia during 1948–2014. Analyses show that the connection between the spring North Atlantic tripole SST anomaly pattern and the Eurasian SAT anomalies has experienced marked interdecadal shifts around the early 1970s and mid-1990s. The connection is strong during 1954–72 and 1996–2014 but weak during 1973–91. A diagnosis indicates that interdecadal changes in the connection between the North Atlantic SST and Eurasian SAT variations are associated with changes in atmospheric circulation anomalies over Eurasia induced by the North Atlantic tripole SST anomaly pattern. Further analyses suggest that changes in atmospheric circulation anomalies over Eurasia are related to changes in the position of atmospheric heating anomalies over the North Atlantic, which may be due to the change in mean SST. Marked atmospheric heating anomalies appear over the tropical western North Atlantic during 1954–72 and 1996–2014 but over the subtropical central-eastern North Atlantic during 1973–91. Barotropic model experiments confirm that different background flows may also contribute to changes in anomalous atmospheric circulation over Eurasia.
Abstract
This study investigates interdecadal changes in the relationship between interannual variations of boreal spring sea surface temperature (SST) in the North Atlantic and surface air temperature (SAT) over the mid-to-high latitudes of Eurasia during 1948–2014. Analyses show that the connection between the spring North Atlantic tripole SST anomaly pattern and the Eurasian SAT anomalies has experienced marked interdecadal shifts around the early 1970s and mid-1990s. The connection is strong during 1954–72 and 1996–2014 but weak during 1973–91. A diagnosis indicates that interdecadal changes in the connection between the North Atlantic SST and Eurasian SAT variations are associated with changes in atmospheric circulation anomalies over Eurasia induced by the North Atlantic tripole SST anomaly pattern. Further analyses suggest that changes in atmospheric circulation anomalies over Eurasia are related to changes in the position of atmospheric heating anomalies over the North Atlantic, which may be due to the change in mean SST. Marked atmospheric heating anomalies appear over the tropical western North Atlantic during 1954–72 and 1996–2014 but over the subtropical central-eastern North Atlantic during 1973–91. Barotropic model experiments confirm that different background flows may also contribute to changes in anomalous atmospheric circulation over Eurasia.
Abstract
The western Pacific pattern (WP) is one of the most important atmospheric teleconnections over the Northern Hemisphere (NH) in boreal winter and plays key roles in regulating weather and climate variations over many parts of the NH. This study evaluates the ability of the coupled models participating in CMIP5 and CMIP6 to capture the spatial pattern, dominant frequency, and associated climate anomalies of the winter WP. Ensemble means of the CMIP5 and CMIP6 models well capture spatial structures of the WP, with slightly higher skills for the CMIP6. However, the northern (southern) center of the WP is shifted westward (eastward) relative to the observations, and the strength of the northern center is overestimated in most CMIP5 and CMIP6 models. CMIP6 shows an improvement in simulating the dominant periodicity of the WP. WP-related climatic anomalies in most parts of the NH can be well simulated. However, there exists a large spread across the models in simulating surface air temperature (SAT) anomalies in the Russian Far East and northwest North America, which is attributable to the diversity of the intensity of the WP’s northern lobe. Most CMIP5 and CMIP6 models largely overestimate the WP-related precipitation anomalies over Siberia, which is partly due to the overestimation of mean precipitation there. Furthermore, most models simulate a close relation of the WP and Arctic Oscillation (AO), which does not exist in observation. The CMIP5 and CMIP6 models with weak WP–AO relations have better ability than the models with strong WP–AO relations in capturing the WP-related SAT and precipitation anomalies over the NH, especially over Eurasia.
Abstract
The western Pacific pattern (WP) is one of the most important atmospheric teleconnections over the Northern Hemisphere (NH) in boreal winter and plays key roles in regulating weather and climate variations over many parts of the NH. This study evaluates the ability of the coupled models participating in CMIP5 and CMIP6 to capture the spatial pattern, dominant frequency, and associated climate anomalies of the winter WP. Ensemble means of the CMIP5 and CMIP6 models well capture spatial structures of the WP, with slightly higher skills for the CMIP6. However, the northern (southern) center of the WP is shifted westward (eastward) relative to the observations, and the strength of the northern center is overestimated in most CMIP5 and CMIP6 models. CMIP6 shows an improvement in simulating the dominant periodicity of the WP. WP-related climatic anomalies in most parts of the NH can be well simulated. However, there exists a large spread across the models in simulating surface air temperature (SAT) anomalies in the Russian Far East and northwest North America, which is attributable to the diversity of the intensity of the WP’s northern lobe. Most CMIP5 and CMIP6 models largely overestimate the WP-related precipitation anomalies over Siberia, which is partly due to the overestimation of mean precipitation there. Furthermore, most models simulate a close relation of the WP and Arctic Oscillation (AO), which does not exist in observation. The CMIP5 and CMIP6 models with weak WP–AO relations have better ability than the models with strong WP–AO relations in capturing the WP-related SAT and precipitation anomalies over the NH, especially over Eurasia.
Abstract
This study reveals a marked enhancement in the relationship between the North Atlantic Oscillation (NAO) and North Atlantic tripole (NAT) sea surface temperature (SST) anomaly pattern during boreal spring since the late 1980s. A comparative analysis is conducted for two periods before and after the late 1980s to understand the reasons for the above interdecadal change. During both periods, SST cooling in the northern tropical Atlantic during the positive phase of the NAT SST pattern results in an anomalous anticyclone over the subtropical western North Atlantic via a Rossby wave–type atmospheric response. The westerly wind anomalies along the north flank of the anomalous anticyclone are accompanied by a marked decrease in synoptic-scale eddies over the midlatitudes as well as cyclonic (anticyclonic) vorticity forcings at the north (south) side. As such, an NAO-like dipole atmospheric anomaly is induced over the North Atlantic, which in turn helps to maintain the NAT SST anomaly via modulating surface heat fluxes. The intensity of the synoptic-scale eddy feedback to mean flow is stronger after than before the late 1980s, which is related to interdecadal increase in the intensity of North Atlantic synoptic-scale eddies. This is followed by a stronger NAO-like atmospheric response to the NAT SST anomaly since the late 1980s. Further analysis shows that changes in the spatial structure of the spring NAO may also partly contribute to changes in the spring NAO–NAT SST connection around the late 1980s. In particular, spring NAO-related atmospheric anomalies are weaker and shift northward before the late 1980s, which reduces the contribution of the NAO to a tripole SST anomaly pattern in the North Atlantic.
Abstract
This study reveals a marked enhancement in the relationship between the North Atlantic Oscillation (NAO) and North Atlantic tripole (NAT) sea surface temperature (SST) anomaly pattern during boreal spring since the late 1980s. A comparative analysis is conducted for two periods before and after the late 1980s to understand the reasons for the above interdecadal change. During both periods, SST cooling in the northern tropical Atlantic during the positive phase of the NAT SST pattern results in an anomalous anticyclone over the subtropical western North Atlantic via a Rossby wave–type atmospheric response. The westerly wind anomalies along the north flank of the anomalous anticyclone are accompanied by a marked decrease in synoptic-scale eddies over the midlatitudes as well as cyclonic (anticyclonic) vorticity forcings at the north (south) side. As such, an NAO-like dipole atmospheric anomaly is induced over the North Atlantic, which in turn helps to maintain the NAT SST anomaly via modulating surface heat fluxes. The intensity of the synoptic-scale eddy feedback to mean flow is stronger after than before the late 1980s, which is related to interdecadal increase in the intensity of North Atlantic synoptic-scale eddies. This is followed by a stronger NAO-like atmospheric response to the NAT SST anomaly since the late 1980s. Further analysis shows that changes in the spatial structure of the spring NAO may also partly contribute to changes in the spring NAO–NAT SST connection around the late 1980s. In particular, spring NAO-related atmospheric anomalies are weaker and shift northward before the late 1980s, which reduces the contribution of the NAO to a tripole SST anomaly pattern in the North Atlantic.
Abstract
Previous studies suggested that the boreal spring Arctic Oscillation (AO) exerts a pronounced influence on the following East Asian summer monsoon (EASM) variability. This study reveals that the relationship of spring AO with the following EASM experienced a significant interdecadal change in the early 1970s. The influence of spring AO on the following EASM is weak during the 1950s and 1960s but strong and significant during the mid-1970s through the mid-1990s. The spring AO-related sea surface temperature (SST), atmospheric circulation, and heating anomalies are compared between 1949–71 and 1975–97. Results show that the spring AO-related cyclonic circulation anomaly over the tropical western North Pacific is weaker and located more northward in the former epoch than in the latter epoch. Correspondingly, SST, atmospheric circulation, and heating anomalies over the tropical North Pacific are located more northeastward in the former than latter epoch from spring to summer. In the following summer, the spring AO-related cyclonic circulation anomalies over the tropical North Pacific are located farther away from East Asia in the former epoch. This interdecadal change in the AO–EASM connection may be attributed to a significant change in the intensity of spring North Pacific synoptic-scale eddy activity around the early 1970s from a weak regime to a strong regime, which induces a stronger eddy feedback to the low-frequency mean flow after the early 1970s. This may explain a stronger spring AO-related cyclonic circulation over the tropical western North Pacific and thus a closer relationship between the spring AO and the following EASM in the latter than former epoch.
Abstract
Previous studies suggested that the boreal spring Arctic Oscillation (AO) exerts a pronounced influence on the following East Asian summer monsoon (EASM) variability. This study reveals that the relationship of spring AO with the following EASM experienced a significant interdecadal change in the early 1970s. The influence of spring AO on the following EASM is weak during the 1950s and 1960s but strong and significant during the mid-1970s through the mid-1990s. The spring AO-related sea surface temperature (SST), atmospheric circulation, and heating anomalies are compared between 1949–71 and 1975–97. Results show that the spring AO-related cyclonic circulation anomaly over the tropical western North Pacific is weaker and located more northward in the former epoch than in the latter epoch. Correspondingly, SST, atmospheric circulation, and heating anomalies over the tropical North Pacific are located more northeastward in the former than latter epoch from spring to summer. In the following summer, the spring AO-related cyclonic circulation anomalies over the tropical North Pacific are located farther away from East Asia in the former epoch. This interdecadal change in the AO–EASM connection may be attributed to a significant change in the intensity of spring North Pacific synoptic-scale eddy activity around the early 1970s from a weak regime to a strong regime, which induces a stronger eddy feedback to the low-frequency mean flow after the early 1970s. This may explain a stronger spring AO-related cyclonic circulation over the tropical western North Pacific and thus a closer relationship between the spring AO and the following EASM in the latter than former epoch.
Abstract
The relationship between interannual variations of boreal winter North Atlantic Oscillation (NAO) and northern tropical Atlantic (NTA) sea surface temperature (SST) experienced obvious interdecadal changes during 1870–2012. Similar interdecadal changes are observed in the amplitude of NTA SST anomalies. The mean NTA SST change may be a plausible reason for several changes in the NAO–NTA SST connection. Under a higher mean NTA SST, NTA SST anomalies induce larger wind anomalies over the North Atlantic that produce a tripole SST anomaly pattern and amplify NTA SST anomalies. Comparison of the evolution of anomalies between 1970–86 and 1996–2012 unravels changing roles of El Niño–Southern Oscillation (ENSO) and extratropical atmospheric disturbances in the formation of NTA SST anomalies. During 1970–86, ENSO events play a key role in initiating NTA SST anomalies in the preceding spring through atmospheric circulation changes. With the decay of ENSO, SST anomalies in the midlatitude North Atlantic weaken in the following summer, whereas NTA SST anomalies are maintained up to winter. This leads to a weak NAO–NTA SST connection in winter. During 1996–2012, the preceding spring atmospheric circulation disturbances over the midlatitude North Atlantic play a dominant role in the genesis of a North Atlantic horseshoe (NAH)-like SST anomaly pattern in the following summer and fall. This NAH-like SST anomaly pattern contributes to the development of the NAO in late fall and early winter. The atmospheric circulation anomaly, in turn, is conducive to the maintenance of NTA SST anomalies to winter via changing surface latent heat flux and shortwave radiation. This leads to a close NAO–NTA SST connection in winter.
Abstract
The relationship between interannual variations of boreal winter North Atlantic Oscillation (NAO) and northern tropical Atlantic (NTA) sea surface temperature (SST) experienced obvious interdecadal changes during 1870–2012. Similar interdecadal changes are observed in the amplitude of NTA SST anomalies. The mean NTA SST change may be a plausible reason for several changes in the NAO–NTA SST connection. Under a higher mean NTA SST, NTA SST anomalies induce larger wind anomalies over the North Atlantic that produce a tripole SST anomaly pattern and amplify NTA SST anomalies. Comparison of the evolution of anomalies between 1970–86 and 1996–2012 unravels changing roles of El Niño–Southern Oscillation (ENSO) and extratropical atmospheric disturbances in the formation of NTA SST anomalies. During 1970–86, ENSO events play a key role in initiating NTA SST anomalies in the preceding spring through atmospheric circulation changes. With the decay of ENSO, SST anomalies in the midlatitude North Atlantic weaken in the following summer, whereas NTA SST anomalies are maintained up to winter. This leads to a weak NAO–NTA SST connection in winter. During 1996–2012, the preceding spring atmospheric circulation disturbances over the midlatitude North Atlantic play a dominant role in the genesis of a North Atlantic horseshoe (NAH)-like SST anomaly pattern in the following summer and fall. This NAH-like SST anomaly pattern contributes to the development of the NAO in late fall and early winter. The atmospheric circulation anomaly, in turn, is conducive to the maintenance of NTA SST anomalies to winter via changing surface latent heat flux and shortwave radiation. This leads to a close NAO–NTA SST connection in winter.
Abstract
This study investigates interannual variations of surface air temperature (SAT) over mid- and high latitudes of Eurasia during boreal spring and their association with snow, atmospheric circulation, and sea surface temperature (SST) changes. The leading mode of spring SAT variations is featured by same-sign anomalies over most regions. The second mode features a tripole anomaly pattern with anomalies over the central part opposite to those over the eastern and western parts of Eurasia. A diagnosis of surface heat flux anomalies suggests that snow change contributes partly to SAT anomalies in several regions mainly by modulating surface shortwave radiation but cannot explain SAT changes in other regions. Atmospheric circulation anomalies play an important role in spring SAT variability via wind-induced heat advection and cloud-induced surface radiation changes. Positive SAT anomalies are associated with anomalous westerly winds from the North Atlantic Ocean or with anomalous anticyclone and southerly winds. Negative SAT anomalies occur in regions of anomalous cyclone and northerly winds. Atmospheric circulation anomalies associated with the first mode have a close relationship to spring Arctic Oscillation (AO), indicating the impact of the AO on continental-scale spring SAT variations over the mid- and high latitudes of Eurasia. The atmospheric circulation anomalies associated with the second mode feature a wave pattern over the North Atlantic and Eurasia. Such a wave pattern is related to a tripole SST anomaly pattern in the North Atlantic Ocean, signifying the contribution of the North Atlantic Ocean state to the formation of a tripole SAT anomaly pattern over the mid- and high latitudes of Eurasia.
Abstract
This study investigates interannual variations of surface air temperature (SAT) over mid- and high latitudes of Eurasia during boreal spring and their association with snow, atmospheric circulation, and sea surface temperature (SST) changes. The leading mode of spring SAT variations is featured by same-sign anomalies over most regions. The second mode features a tripole anomaly pattern with anomalies over the central part opposite to those over the eastern and western parts of Eurasia. A diagnosis of surface heat flux anomalies suggests that snow change contributes partly to SAT anomalies in several regions mainly by modulating surface shortwave radiation but cannot explain SAT changes in other regions. Atmospheric circulation anomalies play an important role in spring SAT variability via wind-induced heat advection and cloud-induced surface radiation changes. Positive SAT anomalies are associated with anomalous westerly winds from the North Atlantic Ocean or with anomalous anticyclone and southerly winds. Negative SAT anomalies occur in regions of anomalous cyclone and northerly winds. Atmospheric circulation anomalies associated with the first mode have a close relationship to spring Arctic Oscillation (AO), indicating the impact of the AO on continental-scale spring SAT variations over the mid- and high latitudes of Eurasia. The atmospheric circulation anomalies associated with the second mode feature a wave pattern over the North Atlantic and Eurasia. Such a wave pattern is related to a tripole SST anomaly pattern in the North Atlantic Ocean, signifying the contribution of the North Atlantic Ocean state to the formation of a tripole SAT anomaly pattern over the mid- and high latitudes of Eurasia.
Abstract
Previous studies suggested that spring sea surface temperature anomalies (SSTAs) in the northern tropical Atlantic (NTA) have a marked influence on the succedent winter El Niño–Southern Oscillation (ENSO). In this study, we examine the spring NTA SSTA–winter ENSO connection in a 50-member large-ensemble simulation conducted with the Canadian Centre for Climate Modelling and Analysis second-generation Canadian Earth System Model (CanESM2) and a 100-member ensemble simulation conducted with the Max Planck Institute Earth System Model (MPI-ESM). The observed out-of-phase relation of spring NTA SSTA with winter ENSO can be captured by the multimember ensemble means of the large-ensemble simulations from both models. However, the relation shows a large diversity among different ensemble members attributing to the internal climate variability. The preceding winter North Pacific Oscillation (NPO) is suggested to be an important source of the internal climate variability that modulates the spring NTA SSTA–ENSO connection. The modulation of the winter NPO on the subsequent spring NTA SSTA–winter ENSO relation is seen in both climate modeling and observational datasets. When winter NPO and spring NTA SSTA indices have the same (opposite) sign, the linkage between the spring NTA SSTA and the following winter ENSO tends to be weak (strong). The NPO modulates the spring NTA SSTA–winter ENSO relation mainly via changing the zonal wind anomalies over the tropical western-to-central Pacific induced by the spring NTA SSTA. In addition, our analysis indicates that winter NPO may have a marked effect on the predictability of winter ENSO based on the condition of spring NTA SSTA.
Abstract
Previous studies suggested that spring sea surface temperature anomalies (SSTAs) in the northern tropical Atlantic (NTA) have a marked influence on the succedent winter El Niño–Southern Oscillation (ENSO). In this study, we examine the spring NTA SSTA–winter ENSO connection in a 50-member large-ensemble simulation conducted with the Canadian Centre for Climate Modelling and Analysis second-generation Canadian Earth System Model (CanESM2) and a 100-member ensemble simulation conducted with the Max Planck Institute Earth System Model (MPI-ESM). The observed out-of-phase relation of spring NTA SSTA with winter ENSO can be captured by the multimember ensemble means of the large-ensemble simulations from both models. However, the relation shows a large diversity among different ensemble members attributing to the internal climate variability. The preceding winter North Pacific Oscillation (NPO) is suggested to be an important source of the internal climate variability that modulates the spring NTA SSTA–ENSO connection. The modulation of the winter NPO on the subsequent spring NTA SSTA–winter ENSO relation is seen in both climate modeling and observational datasets. When winter NPO and spring NTA SSTA indices have the same (opposite) sign, the linkage between the spring NTA SSTA and the following winter ENSO tends to be weak (strong). The NPO modulates the spring NTA SSTA–winter ENSO relation mainly via changing the zonal wind anomalies over the tropical western-to-central Pacific induced by the spring NTA SSTA. In addition, our analysis indicates that winter NPO may have a marked effect on the predictability of winter ENSO based on the condition of spring NTA SSTA.
Abstract
This study reveals that the impact of the spring North Pacific meridional mode (PMM) on the following-winter El Niño–Southern Oscillation (ENSO) shows a continuing increase in the past. A comparative analysis is conducted for the high- and low-correlation periods to understand the factors for the strengthened impact of the PMM. The spring PMM-related sea surface temperature (SST) and atmospheric anomalies over the subtropical northeastern Pacific propagate southwestward to the tropical central Pacific via wind–evaporation–SST feedback in the high-correlation period. The tropical SST and atmospheric anomalies further develop to an ENSO-like pattern via positive air–sea interaction. In the low-correlation period, SST and atmospheric anomalies over the subtropical northeastern Pacific related to the PMM cannot extend to the deep tropics. Therefore, the spring PMM has a weak impact on ENSO. The extent to which the PMM-related SST and atmospheric anomalies extend toward the tropics is related to the background flow. The stronger mean trade winds in the high-correlation period lead to an increase in the air–sea coupling strength over the subtropical northeastern Pacific. As such, the spring PMM-related SST and atmospheric anomalies can more efficiently propagate southwestward to the tropical Pacific and exert stronger impacts on the succeeding ENSO. In addition, the southward shifted intertropical convergence zone in the high-correlation period also favors the southward extension of the PMM-related SST anomalies to the tropics and contributes to a stronger PMM–ENSO relation. The variation and its formation mechanism of the spring PMM–winter ENSO relationship appear in both the observations and the long historical simulation of Earth system models.
Significance Statement
The North Pacific meridional mode (PMM) is the leading atmosphere–ocean coupling pattern over the subtropical northeastern Pacific after removing the ENSO variability, with maximum variance during boreal spring. Previous studies indicated that the PMM plays an important role in relaying the impact of the atmosphere–ocean forcings over the extratropics on the tropical ENSO. This study reveals that the impact of the spring PMM on the following winter ENSO shows a continuing increase in the past 70 years. The physical mechanisms for this strengthened impact are further examined. Results obtained in this study have important implications for improving the prediction of the tropical ENSO variability.
Abstract
This study reveals that the impact of the spring North Pacific meridional mode (PMM) on the following-winter El Niño–Southern Oscillation (ENSO) shows a continuing increase in the past. A comparative analysis is conducted for the high- and low-correlation periods to understand the factors for the strengthened impact of the PMM. The spring PMM-related sea surface temperature (SST) and atmospheric anomalies over the subtropical northeastern Pacific propagate southwestward to the tropical central Pacific via wind–evaporation–SST feedback in the high-correlation period. The tropical SST and atmospheric anomalies further develop to an ENSO-like pattern via positive air–sea interaction. In the low-correlation period, SST and atmospheric anomalies over the subtropical northeastern Pacific related to the PMM cannot extend to the deep tropics. Therefore, the spring PMM has a weak impact on ENSO. The extent to which the PMM-related SST and atmospheric anomalies extend toward the tropics is related to the background flow. The stronger mean trade winds in the high-correlation period lead to an increase in the air–sea coupling strength over the subtropical northeastern Pacific. As such, the spring PMM-related SST and atmospheric anomalies can more efficiently propagate southwestward to the tropical Pacific and exert stronger impacts on the succeeding ENSO. In addition, the southward shifted intertropical convergence zone in the high-correlation period also favors the southward extension of the PMM-related SST anomalies to the tropics and contributes to a stronger PMM–ENSO relation. The variation and its formation mechanism of the spring PMM–winter ENSO relationship appear in both the observations and the long historical simulation of Earth system models.
Significance Statement
The North Pacific meridional mode (PMM) is the leading atmosphere–ocean coupling pattern over the subtropical northeastern Pacific after removing the ENSO variability, with maximum variance during boreal spring. Previous studies indicated that the PMM plays an important role in relaying the impact of the atmosphere–ocean forcings over the extratropics on the tropical ENSO. This study reveals that the impact of the spring PMM on the following winter ENSO shows a continuing increase in the past 70 years. The physical mechanisms for this strengthened impact are further examined. Results obtained in this study have important implications for improving the prediction of the tropical ENSO variability.
Abstract
A recent study revealed an impact of the intensity of early spring Aleutian low (AL) on the succeeding winter ENSO. This study examines the ability of 41 climate models that participated in CMIP6 in simulating the early spring AL–winter ENSO connection. It is shown that there exists a large diversity among the models in simulating this AL–ENSO linkage. A number of models capture well the observed AL–ENSO connection and the associated physical processes. However, the AL–ENSO relation in several models is opposite to the observed. Diversity of the AL–ENSO connection is related to the spread in the spatial structure of AL-related atmospheric anomalies over the North Pacific. In the models that capture the observed AL–ENSO connection, weakened AL induces an anomalous anticyclone over the northern middle and high latitudes and an anomalous cyclone over the subtropical North Pacific. The resultant westerly wind anomalies over the tropical western-central Pacific (TWCP) induce an El Niño sea surface temperature (SST) anomaly pattern in the following winter. By contrast, in the models with the AL–ENSO relation opposite to the observations, the AL-associated anomalous anticyclone over the North Pacific extends too southward. As such, the subtropical North Pacific is dominated by northeasterly wind anomalies and SST cooling. The subtropical North Pacific SST cooling induces easterly wind anomalies over the TWCP via wind–evaporation–SST feedback, and leads to a La Niña anomaly pattern in the following winter. The spread in the spatial structure of the AL-associated atmospheric anomalies over the North Pacific is partly due to the diversity in the amplitude of the climatological mean flow.
Significance Statement
A recent study suggested that variation of the AL intensity in early spring could exert a significant impact on the following winter ENSO. It indicated that inclusion of the early spring AL signal could improve the prediction of ENSO and to some extent help reduce the spring predictability barrier of ENSO. To employ the AL as a predictor in the ENSO prediction and forecast, the current climate model should have the ability to simulate realistically the early spring AL variation as well as the physical process linking the early spring AL with the subsequent winter ENSO. Hence, this study examines the performance of the current coupled climate models that participated in the phase 6 of the Coupled Model Intercomparison Project (CMIP6) in simulating the linkage between the early spring AL and the following winter ENSO. We show that there exists a large diversity among the CMIP6 models in simulating the early spring AL–winter ENSO connection. A number of models capture well the observed AL–ENSO connection and the associated physical processes. However, the AL–ENSO relation in several models is opposite to the observed. The factors leading to the spread are further examined. Results of this study would have implications in improving our understanding of the impact of extratropical atmospheric forcing on the ENSO and improving the seasonal forecasting of the ENSO.
Abstract
A recent study revealed an impact of the intensity of early spring Aleutian low (AL) on the succeeding winter ENSO. This study examines the ability of 41 climate models that participated in CMIP6 in simulating the early spring AL–winter ENSO connection. It is shown that there exists a large diversity among the models in simulating this AL–ENSO linkage. A number of models capture well the observed AL–ENSO connection and the associated physical processes. However, the AL–ENSO relation in several models is opposite to the observed. Diversity of the AL–ENSO connection is related to the spread in the spatial structure of AL-related atmospheric anomalies over the North Pacific. In the models that capture the observed AL–ENSO connection, weakened AL induces an anomalous anticyclone over the northern middle and high latitudes and an anomalous cyclone over the subtropical North Pacific. The resultant westerly wind anomalies over the tropical western-central Pacific (TWCP) induce an El Niño sea surface temperature (SST) anomaly pattern in the following winter. By contrast, in the models with the AL–ENSO relation opposite to the observations, the AL-associated anomalous anticyclone over the North Pacific extends too southward. As such, the subtropical North Pacific is dominated by northeasterly wind anomalies and SST cooling. The subtropical North Pacific SST cooling induces easterly wind anomalies over the TWCP via wind–evaporation–SST feedback, and leads to a La Niña anomaly pattern in the following winter. The spread in the spatial structure of the AL-associated atmospheric anomalies over the North Pacific is partly due to the diversity in the amplitude of the climatological mean flow.
Significance Statement
A recent study suggested that variation of the AL intensity in early spring could exert a significant impact on the following winter ENSO. It indicated that inclusion of the early spring AL signal could improve the prediction of ENSO and to some extent help reduce the spring predictability barrier of ENSO. To employ the AL as a predictor in the ENSO prediction and forecast, the current climate model should have the ability to simulate realistically the early spring AL variation as well as the physical process linking the early spring AL with the subsequent winter ENSO. Hence, this study examines the performance of the current coupled climate models that participated in the phase 6 of the Coupled Model Intercomparison Project (CMIP6) in simulating the linkage between the early spring AL and the following winter ENSO. We show that there exists a large diversity among the CMIP6 models in simulating the early spring AL–winter ENSO connection. A number of models capture well the observed AL–ENSO connection and the associated physical processes. However, the AL–ENSO relation in several models is opposite to the observed. The factors leading to the spread are further examined. Results of this study would have implications in improving our understanding of the impact of extratropical atmospheric forcing on the ENSO and improving the seasonal forecasting of the ENSO.
