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- Author or Editor: Sukyoung Lee x
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Abstract
By analyzing El Niño and La Niña composites with 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data, evidence is presented here that the surface air temperature of the Arctic winter (December–February) is anomalously warm during La Niña and cold during El Niño. Surface and top-of-the-atmosphere energy fluxes were used to calculate the composite zonal-mean poleward moist static energy transport. The result shows that the La Niña warming in the Arctic is associated with an increased poleward energy transport in the extratropics. The opposite characteristics are found for El Niño. Because the total tropical convective heating is more localized during La Niña than El Niño, these findings suggest that the Arctic surface air temperature anomalies associated with ENSO may be attributed to the tropically excited Arctic warming mechanism (TEAM). In the tropics, consistent with previous studies, the anomalous poleward energy transport is positive during El Niño and negative during La Niña. Given the debate over whether a warmer world would take on more El Niño–like or La Niña–like characteristics, the findings of this study underscore the need for further investigation of tropical influence on polar climate.
Abstract
By analyzing El Niño and La Niña composites with 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data, evidence is presented here that the surface air temperature of the Arctic winter (December–February) is anomalously warm during La Niña and cold during El Niño. Surface and top-of-the-atmosphere energy fluxes were used to calculate the composite zonal-mean poleward moist static energy transport. The result shows that the La Niña warming in the Arctic is associated with an increased poleward energy transport in the extratropics. The opposite characteristics are found for El Niño. Because the total tropical convective heating is more localized during La Niña than El Niño, these findings suggest that the Arctic surface air temperature anomalies associated with ENSO may be attributed to the tropically excited Arctic warming mechanism (TEAM). In the tropics, consistent with previous studies, the anomalous poleward energy transport is positive during El Niño and negative during La Niña. Given the debate over whether a warmer world would take on more El Niño–like or La Niña–like characteristics, the findings of this study underscore the need for further investigation of tropical influence on polar climate.
Abstract
An increase in the poleward heat or energy transport is often ascribed to a strengthening of the equator-to-pole gradient in temperature or in the top-of-the-atmosphere (TOA) net radiation. While this attribution conforms to the well-established flux–gradient relationship, a counterexample is shown here, demonstrating that a forced atmospheric circulation, triggered by enhanced convection over the western tropical Pacific warm pool and suppressed convection over the eastern tropical Pacific and Indian Oceans, can cause the equator-to-pole gradient in the TOA net radiation to increase.
Abstract
An increase in the poleward heat or energy transport is often ascribed to a strengthening of the equator-to-pole gradient in temperature or in the top-of-the-atmosphere (TOA) net radiation. While this attribution conforms to the well-established flux–gradient relationship, a counterexample is shown here, demonstrating that a forced atmospheric circulation, triggered by enhanced convection over the western tropical Pacific warm pool and suppressed convection over the eastern tropical Pacific and Indian Oceans, can cause the equator-to-pole gradient in the TOA net radiation to increase.
Abstract
Spatial structure of annular modes shows a remarkable resemblance to that of the recent trend in the observed circulation (Thompson et al.). This study performs a series of multilevel primitive equation model simulations to examine the extent to which the annular mode is capable of predicting changes in the zonal-mean flow response to external heat perturbations. Each of these simulations represents a statistically steady state and differs from each other in the values of the imposed tropical heating (
Defining the annular mode as the first empirical orthogonal function (EOF1) of zonal-mean tropospheric zonal wind, it is found that the “climate predictability” is generally high in the small
Abstract
Spatial structure of annular modes shows a remarkable resemblance to that of the recent trend in the observed circulation (Thompson et al.). This study performs a series of multilevel primitive equation model simulations to examine the extent to which the annular mode is capable of predicting changes in the zonal-mean flow response to external heat perturbations. Each of these simulations represents a statistically steady state and differs from each other in the values of the imposed tropical heating (
Defining the annular mode as the first empirical orthogonal function (EOF1) of zonal-mean tropospheric zonal wind, it is found that the “climate predictability” is generally high in the small
Abstract
This study investigates the mechanism behind the recent boreal summer circulation trend pattern and associated high surface temperature anomalies over the Russian Far East. This circulation pattern includes a prominent anticyclone over the Kamchatka Peninsula where heat extremes have been trending upward. Observational analysis and numerical model simulations indicate that latent heating anomalies centered over Yakutia, west of Kamchatka Peninsula, can excite this anticyclone and the downstream circulation trend pattern. However, this anticyclone alone is insufficient for generating the anomalously high temperature over the region. Instead, the high temperature emerges when there is an upstream precursor that resembles the Eurasian circulation trend pattern. Warm advection by this upstream circulation initiates a positive temperature anomaly over the Russian Far East, one week prior to the onset of the anticyclone in this region. As this anticyclone develops, the temperature anomalies further intensify by adiabatic warming and shortwave radiative heating. If upstream circulation anomalies are opposite to those of the Eurasian trend pattern, the initial temperature over the Russian Far East is anomalously negative. As a result, the adiabatic warming and shortwave radiative heating within this anticyclonic region are unable to bring the temperature to an extreme condition. These findings indicate that the temperature extremes over the Russian Far East are contributed by a combination of remote and local circulation forcings and provide insights into subseasonal forecasts of heat waves over this region.
Abstract
This study investigates the mechanism behind the recent boreal summer circulation trend pattern and associated high surface temperature anomalies over the Russian Far East. This circulation pattern includes a prominent anticyclone over the Kamchatka Peninsula where heat extremes have been trending upward. Observational analysis and numerical model simulations indicate that latent heating anomalies centered over Yakutia, west of Kamchatka Peninsula, can excite this anticyclone and the downstream circulation trend pattern. However, this anticyclone alone is insufficient for generating the anomalously high temperature over the region. Instead, the high temperature emerges when there is an upstream precursor that resembles the Eurasian circulation trend pattern. Warm advection by this upstream circulation initiates a positive temperature anomaly over the Russian Far East, one week prior to the onset of the anticyclone in this region. As this anticyclone develops, the temperature anomalies further intensify by adiabatic warming and shortwave radiative heating. If upstream circulation anomalies are opposite to those of the Eurasian trend pattern, the initial temperature over the Russian Far East is anomalously negative. As a result, the adiabatic warming and shortwave radiative heating within this anticyclonic region are unable to bring the temperature to an extreme condition. These findings indicate that the temperature extremes over the Russian Far East are contributed by a combination of remote and local circulation forcings and provide insights into subseasonal forecasts of heat waves over this region.
Abstract
A linear-stochastic model is applied to the 10-day low-pass streamfunction field at 300, 500, and 850 mb for 40 winter seasons of Northern Hemisphere NCEP–NCAR reanalysis data. The linear operator is derived from the observed multilevel covariances, allowing for statistical representation of nonlinear processes. While all empirical normal modes of the system are decaying, increase in the streamfunction variance is possible through nonmodal growth. When the evolution of the streamfunction field following the optimal perturbation is predicted, the Pacific–North American teleconnection pattern (PNA) is found to be the most probable state of the atmosphere. Sixty-eight percent (70%) of positive (negative) PNA events are found to follow high projections onto the leading optimal, suggesting the PNA arises through constructive interference between the decaying modes and may be treated as a linear response to Gaussian white noise stochastic forcing. Implications for PNA timescale and onset mechanisms are also discussed.
Abstract
A linear-stochastic model is applied to the 10-day low-pass streamfunction field at 300, 500, and 850 mb for 40 winter seasons of Northern Hemisphere NCEP–NCAR reanalysis data. The linear operator is derived from the observed multilevel covariances, allowing for statistical representation of nonlinear processes. While all empirical normal modes of the system are decaying, increase in the streamfunction variance is possible through nonmodal growth. When the evolution of the streamfunction field following the optimal perturbation is predicted, the Pacific–North American teleconnection pattern (PNA) is found to be the most probable state of the atmosphere. Sixty-eight percent (70%) of positive (negative) PNA events are found to follow high projections onto the leading optimal, suggesting the PNA arises through constructive interference between the decaying modes and may be treated as a linear response to Gaussian white noise stochastic forcing. Implications for PNA timescale and onset mechanisms are also discussed.
Abstract
This study uses cluster analysis to investigate the interdecadal poleward shift of the subtropical and eddy-driven jets and its relationship to intraseasonal teleconnections. For this purpose, self-organizing map (SOM) analysis is applied to the ECMWF Interim Re-Analysis (ERA-Interim) zonal-mean zonal wind. The resulting SOM patterns have time scales of 4.8–5.7 days and undergo notable interdecadal trends in their frequency of occurrence. The sum of these trends closely resembles the observed interdecadal trend of the subtropical and eddy-driven jets, indicating that much of the interdecadal climate forcing is manifested through changes in the frequency of intraseasonal teleconnection patterns.
Two classes of jet cluster patterns are identified. The first class of SOM pattern is preceded by anomalies in convection over the warm pool followed by changes in the poleward wave activity flux. The second class of patterns is preceded by sea ice and stratospheric polar vortex anomalies; when the Arctic sea ice area is reduced, the subsequent planetary wave anomalies destructively interfere with the climatological stationary waves. This is followed by a decrease in the vertical wave activity flux and a strengthening of the stratospheric polar vortex. An increase in sea ice area leads to the opposite chain of events. Analysis suggests that the positive trend in the Arctic Oscillation (AO) up until the early 1990s might be attributed to increased warm pool tropical convection, while the subsequent reversal in its trend may be due to the influence of tropical convection being overshadowed by the accelerated loss of Arctic sea ice.
Abstract
This study uses cluster analysis to investigate the interdecadal poleward shift of the subtropical and eddy-driven jets and its relationship to intraseasonal teleconnections. For this purpose, self-organizing map (SOM) analysis is applied to the ECMWF Interim Re-Analysis (ERA-Interim) zonal-mean zonal wind. The resulting SOM patterns have time scales of 4.8–5.7 days and undergo notable interdecadal trends in their frequency of occurrence. The sum of these trends closely resembles the observed interdecadal trend of the subtropical and eddy-driven jets, indicating that much of the interdecadal climate forcing is manifested through changes in the frequency of intraseasonal teleconnection patterns.
Two classes of jet cluster patterns are identified. The first class of SOM pattern is preceded by anomalies in convection over the warm pool followed by changes in the poleward wave activity flux. The second class of patterns is preceded by sea ice and stratospheric polar vortex anomalies; when the Arctic sea ice area is reduced, the subsequent planetary wave anomalies destructively interfere with the climatological stationary waves. This is followed by a decrease in the vertical wave activity flux and a strengthening of the stratospheric polar vortex. An increase in sea ice area leads to the opposite chain of events. Analysis suggests that the positive trend in the Arctic Oscillation (AO) up until the early 1990s might be attributed to increased warm pool tropical convection, while the subsequent reversal in its trend may be due to the influence of tropical convection being overshadowed by the accelerated loss of Arctic sea ice.
Abstract
This study examines the role of the latent heating in exciting the upper-level circulation anomaly, which destructively interferes with the climatological stationary wave in the Western Hemisphere during boreal summer. This destructive interference pattern closely resembles the circulation trend that is known to be responsible for surface heat extreme trends. To investigate the mechanism behind this circulation anomaly, daily stationary–transient wave interference and related meteorological variables are analyzed using reanalysis data for the period of 1979–2017. Numerical model simulations forced by reanalysis heating anomalies indicate that the destructive interference pattern is most effectively excited by latent heating anomalies over the North Pacific Ocean and eastern Canada. The North Pacific heating anomaly drives circulation anomalies that not only resemble the destructive interference pattern, but also transport moisture into eastern Canada. The resulting latent heating over eastern Canada drives circulation that further reinforces the destructive interference pattern, which includes a prominent high pressure system over Greenland. Tropical heating also plays a role in driving the destructive interference pattern. On intraseasonal time scales, the destructive interference pattern is preceded by suppressed Indo–western Pacific heating and enhanced North American monsoon heating. On decadal time scales, both heating centers have strengthened, but the trend of the North American monsoon heating was greater than that of the Indo–western Pacific heating. These uneven heating trends help to explain the resemblance between the destructive interference pattern and the circulation trend over the Western Hemisphere.
Abstract
This study examines the role of the latent heating in exciting the upper-level circulation anomaly, which destructively interferes with the climatological stationary wave in the Western Hemisphere during boreal summer. This destructive interference pattern closely resembles the circulation trend that is known to be responsible for surface heat extreme trends. To investigate the mechanism behind this circulation anomaly, daily stationary–transient wave interference and related meteorological variables are analyzed using reanalysis data for the period of 1979–2017. Numerical model simulations forced by reanalysis heating anomalies indicate that the destructive interference pattern is most effectively excited by latent heating anomalies over the North Pacific Ocean and eastern Canada. The North Pacific heating anomaly drives circulation anomalies that not only resemble the destructive interference pattern, but also transport moisture into eastern Canada. The resulting latent heating over eastern Canada drives circulation that further reinforces the destructive interference pattern, which includes a prominent high pressure system over Greenland. Tropical heating also plays a role in driving the destructive interference pattern. On intraseasonal time scales, the destructive interference pattern is preceded by suppressed Indo–western Pacific heating and enhanced North American monsoon heating. On decadal time scales, both heating centers have strengthened, but the trend of the North American monsoon heating was greater than that of the Indo–western Pacific heating. These uneven heating trends help to explain the resemblance between the destructive interference pattern and the circulation trend over the Western Hemisphere.
Abstract
Dynamical mechanisms for the summer Eurasian circulation trend pattern are investigated by analyzing reanalysis data and conducting numerical model simulations. The daily circulations that resemble the Eurasian circulation trend pattern are identified and categorized into two groups based on surface warming signal over central and eastern Europe. In the group with large warm anomaly, the upper-level circulation takes on a wave packet form over Eurasia, and there are enhanced latent heating anomalies centered over the North Sea and suppressed latent heating anomalies over the Caspian Sea. The numerical model calculations indicate that these latent heating anomalies can excite an upper-level circulation response that resembles the Eurasian circulation trend pattern. Additional analysis indicates that trends of these two latent heating centers contribute to the long-term circulation trend. In the weak warm anomaly group, the circulation pattern takes on a circumglobal teleconnection (CGT) pattern, and there is no heating signal that reinforces the circulation. These results indicate that not all CGT-like patterns excite temperature anomalies that are persistent and in phase with the trend pattern, and that quasi-stationary forcings, such as the latent heating anomalies, play an important role in driving the boreal summer circulation anomaly that accompanies the strong and persistent surface temperature signal.
Abstract
Dynamical mechanisms for the summer Eurasian circulation trend pattern are investigated by analyzing reanalysis data and conducting numerical model simulations. The daily circulations that resemble the Eurasian circulation trend pattern are identified and categorized into two groups based on surface warming signal over central and eastern Europe. In the group with large warm anomaly, the upper-level circulation takes on a wave packet form over Eurasia, and there are enhanced latent heating anomalies centered over the North Sea and suppressed latent heating anomalies over the Caspian Sea. The numerical model calculations indicate that these latent heating anomalies can excite an upper-level circulation response that resembles the Eurasian circulation trend pattern. Additional analysis indicates that trends of these two latent heating centers contribute to the long-term circulation trend. In the weak warm anomaly group, the circulation pattern takes on a circumglobal teleconnection (CGT) pattern, and there is no heating signal that reinforces the circulation. These results indicate that not all CGT-like patterns excite temperature anomalies that are persistent and in phase with the trend pattern, and that quasi-stationary forcings, such as the latent heating anomalies, play an important role in driving the boreal summer circulation anomaly that accompanies the strong and persistent surface temperature signal.
