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Abstract
The relationship between latent heating over the Greenland, Barents, and Kara Seas (GBKS hereafter) and Rossby wave propagation between the Arctic and midlatitudes is investigated using global reanalysis data. Latent heating is the focus because it is the most likely source of Rossby wave activity over the Arctic Ocean. Given that the Rossby wave time scale is on the order of several days, the analysis is carried out using a daily latent heating index that resembles the interdecadal latent heating trend during the winter season. The results from regression calculations find a trans-Arctic Rossby wave train that propagates from the subtropics, through the midlatitudes, into the Arctic, and then back into midlatitudes over a period of about 10 days. Upon entering the GBKS, this wave train transports moisture into the region, resulting in anomalous latent heat release. At high latitudes, the overlapping of a negative latent heating anomaly with an anomalous high is consistent with anomalous latent heat release fueling the Rossby wave train before it propagates back into the midlatitudes. This implies that the Rossby wave propagation from the Arctic into the midlatitudes arises from trans-Arctic wave propagation rather than from in situ generation. The method used indicates the variance of the trans-Arctic wave train, but not in situ generation, and implies that the variance of the former is greater than that of latter. Furthermore, GBKS sea ice concentration regression against the latent heating index shows the largest negative value six days afterward, indicating that sea ice loss contributes little to the latent heating.
Abstract
The relationship between latent heating over the Greenland, Barents, and Kara Seas (GBKS hereafter) and Rossby wave propagation between the Arctic and midlatitudes is investigated using global reanalysis data. Latent heating is the focus because it is the most likely source of Rossby wave activity over the Arctic Ocean. Given that the Rossby wave time scale is on the order of several days, the analysis is carried out using a daily latent heating index that resembles the interdecadal latent heating trend during the winter season. The results from regression calculations find a trans-Arctic Rossby wave train that propagates from the subtropics, through the midlatitudes, into the Arctic, and then back into midlatitudes over a period of about 10 days. Upon entering the GBKS, this wave train transports moisture into the region, resulting in anomalous latent heat release. At high latitudes, the overlapping of a negative latent heating anomaly with an anomalous high is consistent with anomalous latent heat release fueling the Rossby wave train before it propagates back into the midlatitudes. This implies that the Rossby wave propagation from the Arctic into the midlatitudes arises from trans-Arctic wave propagation rather than from in situ generation. The method used indicates the variance of the trans-Arctic wave train, but not in situ generation, and implies that the variance of the former is greater than that of latter. Furthermore, GBKS sea ice concentration regression against the latent heating index shows the largest negative value six days afterward, indicating that sea ice loss contributes little to the latent heating.
Abstract
The interference between transient eddies and climatological stationary eddies in the Northern Hemisphere is investigated. The amplitude and sign of the interference is represented by the stationary wave index (SWI), which is calculated by projecting the daily 300-hPa streamfunction anomaly field onto the 300-hPa climatological stationary wave. ERA-Interim data for the years 1979 to 2013 are used. The amplitude of the interference peaks during boreal winter. The evolution of outgoing longwave radiation, Arctic temperature, 300-hPa streamfunction, 10-hPa zonal wind, Arctic sea ice concentration, and the Arctic Oscillation (AO) index are examined for days of large SWI values during the winter.
Constructive interference during winter tends to occur about one week after enhanced warm pool convection and is followed by an increase in Arctic surface air temperature along with a reduction of sea ice in the Barents and Kara Seas. The warming of the Arctic does occur without prior warm pool convection, but it is enhanced and prolonged when constructive interference occurs in concert with enhanced warm pool convection. This is followed two weeks later by a weakening of the stratospheric polar vortex and a decline of the AO. All of these associations are reversed in the case of destructive interference. Potential climate change implications are briefly discussed.
Abstract
The interference between transient eddies and climatological stationary eddies in the Northern Hemisphere is investigated. The amplitude and sign of the interference is represented by the stationary wave index (SWI), which is calculated by projecting the daily 300-hPa streamfunction anomaly field onto the 300-hPa climatological stationary wave. ERA-Interim data for the years 1979 to 2013 are used. The amplitude of the interference peaks during boreal winter. The evolution of outgoing longwave radiation, Arctic temperature, 300-hPa streamfunction, 10-hPa zonal wind, Arctic sea ice concentration, and the Arctic Oscillation (AO) index are examined for days of large SWI values during the winter.
Constructive interference during winter tends to occur about one week after enhanced warm pool convection and is followed by an increase in Arctic surface air temperature along with a reduction of sea ice in the Barents and Kara Seas. The warming of the Arctic does occur without prior warm pool convection, but it is enhanced and prolonged when constructive interference occurs in concert with enhanced warm pool convection. This is followed two weeks later by a weakening of the stratospheric polar vortex and a decline of the AO. All of these associations are reversed in the case of destructive interference. Potential climate change implications are briefly discussed.
Abstract
During the past three decades, the most rapid warming at the surface has occurred during the Arctic winter. By analyzing daily ERA-Interim data, it is found that the majority of the winter warming trend north of 70°N can be explained by the trend in the downward infrared radiation (IR). This downward IR trend can be attributed to an enhanced poleward flux of moisture and sensible heat into the Arctic by poleward-propagating Rossby waves, which increases the total column water and temperature within this region. This enhanced moisture flux is mostly due to changes in the planetary-scale atmospheric circulation rather than an increase in moisture in lower latitudes. The results of this study lead to the question of whether Arctic amplification has mostly arisen through changes in the Rossby wave response to greenhouse gas forcing and its impact on moisture transport into the Arctic.
Abstract
During the past three decades, the most rapid warming at the surface has occurred during the Arctic winter. By analyzing daily ERA-Interim data, it is found that the majority of the winter warming trend north of 70°N can be explained by the trend in the downward infrared radiation (IR). This downward IR trend can be attributed to an enhanced poleward flux of moisture and sensible heat into the Arctic by poleward-propagating Rossby waves, which increases the total column water and temperature within this region. This enhanced moisture flux is mostly due to changes in the planetary-scale atmospheric circulation rather than an increase in moisture in lower latitudes. The results of this study lead to the question of whether Arctic amplification has mostly arisen through changes in the Rossby wave response to greenhouse gas forcing and its impact on moisture transport into the Arctic.
Abstract
Given the recent observational evidence that the positive (negative) phase of the North Atlantic Oscillation (NAO) is the remnant of anticyclonic (cyclonic) wave breaking, this study uses a multilevel primitive equation model to investigate important dynamical attributes of the above wave breaking behavior. For this purpose, a hierarchy of different basic states (two- and three-dimensional) and initial perturbations are used.
With the three-dimensional climatological flow as the basic state, it is found that initial perturbations located equatorward (poleward) and upstream of the climatological Atlantic jet lead to wave breaking similar to that of the positive (negative) NAO phase. Consistently, analysis of observational data indeed shows that the Pacific storm track is displaced equatorward (poleward) prior the onset of the positive (negative) NAO phase. This result suggests that the latitudinal position of the Pacific storm track plays an important role for determining the phase of the NAO. Sensitivity experiments show that individual life cycles resemble each other only within the NAO region, but have large case-to-case variability outside of the NAO region.
Calculations with zonally symmetric basic states fail to produce wave breaking of the correct spatial and temporal scale, underscoring the dynamical significance of the three-dimensional climatological flow.
Abstract
Given the recent observational evidence that the positive (negative) phase of the North Atlantic Oscillation (NAO) is the remnant of anticyclonic (cyclonic) wave breaking, this study uses a multilevel primitive equation model to investigate important dynamical attributes of the above wave breaking behavior. For this purpose, a hierarchy of different basic states (two- and three-dimensional) and initial perturbations are used.
With the three-dimensional climatological flow as the basic state, it is found that initial perturbations located equatorward (poleward) and upstream of the climatological Atlantic jet lead to wave breaking similar to that of the positive (negative) NAO phase. Consistently, analysis of observational data indeed shows that the Pacific storm track is displaced equatorward (poleward) prior the onset of the positive (negative) NAO phase. This result suggests that the latitudinal position of the Pacific storm track plays an important role for determining the phase of the NAO. Sensitivity experiments show that individual life cycles resemble each other only within the NAO region, but have large case-to-case variability outside of the NAO region.
Calculations with zonally symmetric basic states fail to produce wave breaking of the correct spatial and temporal scale, underscoring the dynamical significance of the three-dimensional climatological flow.
Abstract
The teleconnections of the wintertime North Pacific are examined from the continuum perspective with self-organizing map (SOM) analysis. Daily ERA-Interim data for the 1979–2011 period are used. It is found that most of the North Pacific teleconnections can be grouped into several Pacific–North American (PNA)-like, western Pacific (WP)-like, and east Pacific (EP)-like SOM patterns. Each of the SOM patterns has an e-folding time scale of 7–10 days.
The WP-like SOM patterns undergo a decline in their frequency from early to late winter, and vice versa for the EP-like SOM patterns, corresponding to an eastward seasonal shift of the North Pacific teleconnections. This seasonal shift is observed for both phases of the WP and EP patterns, and is only weakly sensitive to the phase of El Niño–Southern Oscillation. It is shown that the interannual variability of the PNA, WP, and EP can be interpreted as arising from interannual changes in the frequency of the corresponding SOM patterns. The WP- and EP-like SOM patterns are found to be associated with statistically significant sea ice cover anomalies over the Sea of Okhotsk and the Bering Sea. The low-level wind and temperature anomalies associated with these patterns are consistent with the changes in sea ice arising from both wind-driven sea ice motion and freezing and/or melting of sea ice due to horizontal temperature advection. Furthermore, widespread precipitation anomalies over the North Pacific are found for all three patterns.
Abstract
The teleconnections of the wintertime North Pacific are examined from the continuum perspective with self-organizing map (SOM) analysis. Daily ERA-Interim data for the 1979–2011 period are used. It is found that most of the North Pacific teleconnections can be grouped into several Pacific–North American (PNA)-like, western Pacific (WP)-like, and east Pacific (EP)-like SOM patterns. Each of the SOM patterns has an e-folding time scale of 7–10 days.
The WP-like SOM patterns undergo a decline in their frequency from early to late winter, and vice versa for the EP-like SOM patterns, corresponding to an eastward seasonal shift of the North Pacific teleconnections. This seasonal shift is observed for both phases of the WP and EP patterns, and is only weakly sensitive to the phase of El Niño–Southern Oscillation. It is shown that the interannual variability of the PNA, WP, and EP can be interpreted as arising from interannual changes in the frequency of the corresponding SOM patterns. The WP- and EP-like SOM patterns are found to be associated with statistically significant sea ice cover anomalies over the Sea of Okhotsk and the Bering Sea. The low-level wind and temperature anomalies associated with these patterns are consistent with the changes in sea ice arising from both wind-driven sea ice motion and freezing and/or melting of sea ice due to horizontal temperature advection. Furthermore, widespread precipitation anomalies over the North Pacific are found for all three patterns.
Abstract
The relationship between local convection, vertically propagating Kelvin waves, and tropical tropopause height variability is examined. This study utilizes both simulations of a global primitive-equation model and global observational datasets. Regression analysis with the data shows that convection over the western tropical Pacific is followed by warming in the upper troposphere (UT) and cooling in lower stratosphere (LS) over most longitudes, which results in a lifting of the tropical tropopause. The model results reveal that these UT–LS temperature anomalies are closely associated with vertically propagating Kelvin waves, indicating that these Kelvin waves drive tropical tropopause undulations at intraseasonal time scales.
The model simulations further show that regardless of the longitudinal position of the imposed heating, the UT–LS Kelvin wave reaches its maximum amplitude over the western Pacific. This result, together with an analysis based on wave action conservation, is used to contend that the Kelvin wave amplification over the western Pacific should be attributed to the zonal variation of background zonal wind field, rather than to the proximity of the heating. The wave action conservation law is also used to offer an explanation as to why the vertically propagating Kelvin waves play the central role in driving tropical tropopause height undulations.
The zonal and vertical modulation of the Kelvin waves by the background flow may help explain the origin of the very cold air over the western tropical Pacific, which is known to cause freeze-drying of tropospheric air en route to the stratosphere.
Abstract
The relationship between local convection, vertically propagating Kelvin waves, and tropical tropopause height variability is examined. This study utilizes both simulations of a global primitive-equation model and global observational datasets. Regression analysis with the data shows that convection over the western tropical Pacific is followed by warming in the upper troposphere (UT) and cooling in lower stratosphere (LS) over most longitudes, which results in a lifting of the tropical tropopause. The model results reveal that these UT–LS temperature anomalies are closely associated with vertically propagating Kelvin waves, indicating that these Kelvin waves drive tropical tropopause undulations at intraseasonal time scales.
The model simulations further show that regardless of the longitudinal position of the imposed heating, the UT–LS Kelvin wave reaches its maximum amplitude over the western Pacific. This result, together with an analysis based on wave action conservation, is used to contend that the Kelvin wave amplification over the western Pacific should be attributed to the zonal variation of background zonal wind field, rather than to the proximity of the heating. The wave action conservation law is also used to offer an explanation as to why the vertically propagating Kelvin waves play the central role in driving tropical tropopause height undulations.
The zonal and vertical modulation of the Kelvin waves by the background flow may help explain the origin of the very cold air over the western tropical Pacific, which is known to cause freeze-drying of tropospheric air en route to the stratosphere.
Abstract
Using daily reanalysis data from 1979 to 2015, this paper examines the impact of winter Ural blocking (UB) on winter Arctic sea ice concentration (SIC) change over the Barents and Kara Seas (BKS). A case study of the sea ice variability in the BKS in the 2015/16 and 2016/17 winters is first presented to establish a link between the BKS sea ice variability and UB events. Then the UB events are classified into quasi-stationary (QUB), westward-shifting (WUB), and eastward-shifting (EUB) UB types. It is found that the frequency of the QUB events increases significantly during 1999–2015, whereas the WUB events show a decreasing frequency trend during 1979–2015.
Moreover, it is shown that the variation of the BKS-SIC is related to downward infrared radiation (IR) and surface sensible and latent heat flux changes due to different zonal movements of the UB. Calculations show that the downward IR is the main driver of the BKS-SIC decline for QUB events, while the downward IR and surface sensible heat flux make comparable contributions to the BKS-SIC variation for WUB and EUB events. The SIC decline peak lags the QUB and EUB peaks by about 3 days, though QUB and EUB require lesser prior SIC. The QUB gives rise to the largest SIC decline likely because of its longer persistence, whereas the BKS-SIC decline is relatively weak for the EUB. The WUB is found to cause a SIC decline during its growth phase and an increase during its decay phase. Thus, the zonal movement of the UB has an important impact on the SIC variability in BKS.
Abstract
Using daily reanalysis data from 1979 to 2015, this paper examines the impact of winter Ural blocking (UB) on winter Arctic sea ice concentration (SIC) change over the Barents and Kara Seas (BKS). A case study of the sea ice variability in the BKS in the 2015/16 and 2016/17 winters is first presented to establish a link between the BKS sea ice variability and UB events. Then the UB events are classified into quasi-stationary (QUB), westward-shifting (WUB), and eastward-shifting (EUB) UB types. It is found that the frequency of the QUB events increases significantly during 1999–2015, whereas the WUB events show a decreasing frequency trend during 1979–2015.
Moreover, it is shown that the variation of the BKS-SIC is related to downward infrared radiation (IR) and surface sensible and latent heat flux changes due to different zonal movements of the UB. Calculations show that the downward IR is the main driver of the BKS-SIC decline for QUB events, while the downward IR and surface sensible heat flux make comparable contributions to the BKS-SIC variation for WUB and EUB events. The SIC decline peak lags the QUB and EUB peaks by about 3 days, though QUB and EUB require lesser prior SIC. The QUB gives rise to the largest SIC decline likely because of its longer persistence, whereas the BKS-SIC decline is relatively weak for the EUB. The WUB is found to cause a SIC decline during its growth phase and an increase during its decay phase. Thus, the zonal movement of the UB has an important impact on the SIC variability in BKS.
Abstract
The projected future behavior of the South Asian summer monsoon (SASM) is analyzed for the 720-ppm stabilization experiments [Special Report on Emissions Scenarios (SRES) A1B] of phase 3 of the Coupled Model Intercomparison Project (CMIP3) simulations. The multimodel ensemble collectively exhibits a clear tendency for weakening SASM circulation and strengthening SASM precipitation during the twenty-first century. These tendencies are consistent, moreover, among multiple realizations for the same model where available.
An annual correlation analysis and thermodynamic energy budget analysis are used to investigate the changes in the monsoon circulation, precipitation (latent heating), and dry static stability across time scales of variation. The strength of the SASM circulation is interpreted in terms of two competing factors: convective latent heating and dry static stability. It is found that on interannual time scales the latent heating wins out over the dry static stability to dominate the interannual fluctuations of the monsoonal circulation. However, the long-term trends in the SASM circulation are governed by the competing effects of the convective latent heating term and the adiabatic term to modify dry static stability in the thermodynamic energy budget.
Abstract
The projected future behavior of the South Asian summer monsoon (SASM) is analyzed for the 720-ppm stabilization experiments [Special Report on Emissions Scenarios (SRES) A1B] of phase 3 of the Coupled Model Intercomparison Project (CMIP3) simulations. The multimodel ensemble collectively exhibits a clear tendency for weakening SASM circulation and strengthening SASM precipitation during the twenty-first century. These tendencies are consistent, moreover, among multiple realizations for the same model where available.
An annual correlation analysis and thermodynamic energy budget analysis are used to investigate the changes in the monsoon circulation, precipitation (latent heating), and dry static stability across time scales of variation. The strength of the SASM circulation is interpreted in terms of two competing factors: convective latent heating and dry static stability. It is found that on interannual time scales the latent heating wins out over the dry static stability to dominate the interannual fluctuations of the monsoonal circulation. However, the long-term trends in the SASM circulation are governed by the competing effects of the convective latent heating term and the adiabatic term to modify dry static stability in the thermodynamic energy budget.
Abstract
This paper reviews the current state of observational, theoretical, and modeling knowledge of the midlatitude storm tracks of the Northern Hemisphere cool season.
Observed storm track structures and variations form the first part of the review. The climatological storm track structure is described, and the seasonal, interannual, and interdecadal storm track variations are discussed. In particular, the observation that the Pacific storm track exhibits a marked minimum during midwinter when the background baroclinicity is strongest, and a new finding that storm tracks exhibit notable variations in their intensity on decadal timescales, are highlighted as challenges that any comprehensive storm track theory or model has to be able to address.
Physical processes important to storm track dynamics make up the second part of the review. The roles played by baroclinic processes, linear instability, downstream development, barotropic modulation, and diabatic heating are discussed. Understanding of these processes forms the core of our current theoretical knowledge of storm track dynamics, and provides a context within which both observational and modeling results can be interpreted. The eddy energy budget is presented to show that all of these processes are important in the maintenance of the storm tracks.
The final part of the review deals with the ability to model storm tracks. The success as well as remaining problems in idealized storm track modeling, which is based on a linearized dynamical system, are discussed. Perhaps on a more pragmatic side, it is pointed out that while the current generation of atmospheric general circulation models faithfully reproduce the climatological storm track structure, and to a certain extent, the seasonal and ENSO-related interannual variations of storm tracks, in-depth comparisons between observed and modeled storm track variations are still lacking.
Abstract
This paper reviews the current state of observational, theoretical, and modeling knowledge of the midlatitude storm tracks of the Northern Hemisphere cool season.
Observed storm track structures and variations form the first part of the review. The climatological storm track structure is described, and the seasonal, interannual, and interdecadal storm track variations are discussed. In particular, the observation that the Pacific storm track exhibits a marked minimum during midwinter when the background baroclinicity is strongest, and a new finding that storm tracks exhibit notable variations in their intensity on decadal timescales, are highlighted as challenges that any comprehensive storm track theory or model has to be able to address.
Physical processes important to storm track dynamics make up the second part of the review. The roles played by baroclinic processes, linear instability, downstream development, barotropic modulation, and diabatic heating are discussed. Understanding of these processes forms the core of our current theoretical knowledge of storm track dynamics, and provides a context within which both observational and modeling results can be interpreted. The eddy energy budget is presented to show that all of these processes are important in the maintenance of the storm tracks.
The final part of the review deals with the ability to model storm tracks. The success as well as remaining problems in idealized storm track modeling, which is based on a linearized dynamical system, are discussed. Perhaps on a more pragmatic side, it is pointed out that while the current generation of atmospheric general circulation models faithfully reproduce the climatological storm track structure, and to a certain extent, the seasonal and ENSO-related interannual variations of storm tracks, in-depth comparisons between observed and modeled storm track variations are still lacking.
Abstract
The behavior in the South Asian summer monsoon (SASM) was analyzed in Coupled Model Intercomparison Project (CMIP3) multimodel historical (20c3m) simulations and in modern observational and reanalysis data. The CMIP3 simulations capture the observed trend of weakening of the SASM circulation over the past half century, but are unable to reproduce the magnitude of the observed weakening trend. While the observations indicate a slight decrease in SASM-related precipitation, the CMIP3 simulations indicate on average a very slight increase, albeit with very large intermodel and intramodel variabilities. The CMIP3 simulations reproduce the observed negative relationship between the SASM and ENSO. The observed weakening trend in this relationship in recent decades, which has been attributed in some studies to anthropogenic forcing, appears to be well within the variability of the CMIP3 multimodel ensemble. For some models, distinct realizations indicate both strengthening and weakening trends that are larger in magnitude than the observed weakening trend.
Abstract
The behavior in the South Asian summer monsoon (SASM) was analyzed in Coupled Model Intercomparison Project (CMIP3) multimodel historical (20c3m) simulations and in modern observational and reanalysis data. The CMIP3 simulations capture the observed trend of weakening of the SASM circulation over the past half century, but are unable to reproduce the magnitude of the observed weakening trend. While the observations indicate a slight decrease in SASM-related precipitation, the CMIP3 simulations indicate on average a very slight increase, albeit with very large intermodel and intramodel variabilities. The CMIP3 simulations reproduce the observed negative relationship between the SASM and ENSO. The observed weakening trend in this relationship in recent decades, which has been attributed in some studies to anthropogenic forcing, appears to be well within the variability of the CMIP3 multimodel ensemble. For some models, distinct realizations indicate both strengthening and weakening trends that are larger in magnitude than the observed weakening trend.
