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Judith Perlwitz and Nili Harnik

Abstract

Recent studies have pointed out the impact of the stratosphere on the troposphere by dynamic coupling. In the present paper, observational evidence for an effect of downward planetary wave reflection in the stratosphere on Northern Hemisphere tropospheric waves is given by combining statistical and dynamical diagnostics. A time-lagged singular value decomposition analysis is applied to daily tropospheric and stratospheric height fields recomposed for a single zonal wavenumber. A wave geometry diagnostic for wave propagation characteristics that separates the index of refraction into vertical and meridional components is used to diagnose the occurrence of reflecting surfaces. For zonal wavenumber 1, this study suggests that there is one characteristic configuration of the stratospheric jet that reflects waves back into the troposphere—when the polar night jet peaks in the high-latitude midstratosphere. This configuration is related to the formation of a reflecting surface for vertical propagation at around 5 hPa as a result of the vertical curvature of the zonal-mean wind and a clear meridional waveguide in the lower to middle stratosphere that channels the reflected wave activity to the high-latitude troposphere.

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Judith Perlwitz and Nili Harnik

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Wave and zonal mean features of the downward dynamic coupling between the stratosphere and troposphere are compared by applying a time-lagged singular value decomposition analysis to Northern Hemisphere height fields decomposed into zonal mean and its deviations. It is found that both zonal and wave components contribute to the downward interaction, with zonal wave 1 (due to reflection) dominating on the short time scale (up to 12 days) and the zonal mean (due to wave–mean-flow interaction) dominating on the longer time scale. It is further shown that the two processes dominate during different years, depending on the state of the stratosphere. Winters characterized by a basic state that is reflective for wave 1 show a strong relationship between stratospheric and tropospheric wave-1 fields when the stratosphere is leading and show no significant correlations in the zonal mean fields. On the other hand, winters characterized by a stratospheric state that does not reflect waves show a strong relationship only between stratospheric and tropospheric zonal mean fields. This study suggests that there are two types of stratospheric winter states, characterized by different downward dynamic interaction. In one state, most of the wave activity gets deposited in the stratosphere, resulting in strong wave–mean-flow interaction, while in the other state, wave activity is reflected back down to the troposphere, primarily affecting the structure of tropospheric planetary waves.

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Tiffany A. Shaw and Judith Perlwitz

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The impact of stratospheric model configuration on modeled planetary-scale waves in Northern Hemisphere winter is examined using the Canadian Middle Atmosphere Model (CMAM). The CMAM configurations include a high-lid (0.001 hPa) and a low-lid (10 hPa) configuration, which were each run with and without conservation of parameterized gravity wave momentum flux. The planetary wave structure, vertical propagation, and the basic state are found to be in good agreement with reanalysis data for the high-lid conservative configuration with the exception of the downward-propagating wave 1 signal. When the lid is lowered and momentum is conserved, the wave characteristics and basic state are not significantly altered, with the exception of the downward-propagating wave 1 signal, which is damped by the act of conservation. When momentum is not conserved, however, the wave amplitude increases significantly near the lid, and there is a large increase in both the upward- and downward-propagating wave 1 signals and a significant increase in the strength of the basic state. The impact of conserving parameterized gravity wave momentum flux is found to be much larger than that of the model lid height. The changes to the planetary waves and basic state significantly impact the stratosphere–troposphere coupling in the different configurations. In the low-lid configuration, there is an increase in wave-reflection-type coupling over zonal-mean-type coupling, a reduction in stratospheric sudden warming events, and an increase in the northern annular mode time scale. Conserving gravity wave momentum flux in the low-lid configuration significantly reduces these biases.

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Tiffany A. Shaw and Judith Perlwitz

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The life cycle of Northern Hemisphere downward wave coupling between the stratosphere and troposphere via wave reflection is analyzed. Downward wave coupling events are defined by extreme negative values of a wave coupling index based on the leading principal component of the daily wave-1 heat flux at 30 hPa. The life cycle occurs over a 28-day period. In the stratosphere there is a transition from positive to negative total wave-1 heat flux and westward to eastward phase tilt with height of the wave-1 geopotential height field. In addition, the zonal-mean zonal wind in the upper stratosphere weakens leading to negative vertical shear.

Following the evolution in the stratosphere there is a shift toward the positive phase of the North Atlantic Oscillation (NAO) in the troposphere. The pattern develops from a large westward-propagating wave-1 anomaly in the high-latitude North Atlantic sector. The subsequent equatorward propagation leads to a positive anomaly in midlatitudes. The near-surface temperature and circulation anomalies are consistent with a positive NAO phase. The results suggest that wave reflection events can directly influence tropospheric weather.

Finally, winter seasons dominated by extreme wave coupling and stratospheric vortex events are compared. The largest impacts in the troposphere occur during the extreme negative seasons for both indices, namely seasons with multiple wave reflection events leading to a positive NAO phase or seasons with major sudden stratospheric warmings (weak vortex) leading to a negative NAO phase. The results reveal that the dynamical coupling between the stratosphere and NAO involves distinct dynamical mechanisms that can only be characterized by separate wave coupling and vortex indices.

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Tiffany A. Shaw and Judith Perlwitz

Abstract

It is well established that interannual variability of eddy (meridional) heat flux near the tropopause controls the variability of Arctic lower-stratospheric temperatures during spring via a modification of the strength of the residual circulation. While most studies focus on the role of anomalous heat flux values, here the impact of total (climatology plus anomaly) negative heat flux events on the Arctic stratosphere is investigated. Utilizing the Interim ECMWF Re-Analysis (ERA-Interim) dataset, it is found that total negative heat flux events coincide with a transient reversal of the residual circulation and cooling of the Arctic lower stratosphere. The negative events weaken the seasonally averaged adiabatic warming.

The analysis provides a new interpretation of the winters of 1997 and 2011, which are known to have the lowest March Arctic lower-stratospheric temperatures in the satellite era. While most winters involve positive and negative heat flux extremes, the winters of 1997 and 2011 are unique in that they only involved extreme negative events. This behavior contributed to the weakest adiabatic downwelling in the satellite era and suggests a dynamical contribution to the extremely low temperatures during those winters that could not be accounted for by diabatic processes alone. While it is well established that dynamical processes contribute to the occurrence of stratospheric sudden warming events via extreme positive heat flux events, the results show that dynamical processes also contribute to cold winters with subsequent impact on Arctic ozone loss. The results highlight the importance of interpreting stratospheric temperatures in the Arctic in the context of the dynamical regime with which they are associated.

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Judith Perlwitz and Hans-F. Graf

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The associated anomaly patterns of the stratospheric geopotential height field and the tropospheric geopetential and temperature height fields of the Northern Hemisphere are determined applying the canonical correlation analysis. With this linear multivariate technique the coupled modes of variability of lime series of two fields are isolated in the space of empirical orthogonal functions. The one dataset is the 50-hPa geopotential height field; the other set consists of different height fields of the tropospheric pressure levels (200, 500, 700, and 850 hPa) and the temperature of the 850-hPa pressure level. For the winter months (December, January, February) two natural coupled modes, a barotropic and a baroclinic one, of linear relationship between stratospheric and tropospheric circulation are found. The baroclinic mode describes a connection between the strength of the stratospheric cyclonic winter vortex and the tropospheric circulation over the North Atlantic. The corresponding temperature pattern for an anomalously strong stratospheric cyclonic vortex is characterized by positive temperature anomalies over higher latitudes of Eurasia. These “Winter Warmings” are observed, for example, after violent volcanic eruptions. The barotropic mode is characterized by a zonal wavenumber 1 in the lower stratosphere and by a PNA-like pattern in the troposphere. It was shown by van Loon and Labitzke that this mode can be enhanced, for example, by El Niños via the intensification of the Aleutian low.

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Nili Harnik, Richard K. Scott, and Judith Perlwitz

Abstract

Observations of the Southern Hemispheric winter conditions indicate that the major warming of September 2002 resulted from a combination of stationary wave-1 and traveling wave-2 forcing events and suggest that wave and mean-flow anomalies present earlier that winter may have also played a role. Quantities such as the location of the zero wind line, the strength and wave geometry of the vortex, and the horizontal and vertical wave fluxes all differed significantly from climatological values throughout much of the 2002 winter. An analysis of the anomalous features suggests the hypothesis that the persistence of a traveling wave 2 may have increased the likelihood of the combination with stationary wave 1, leading to the observed unprecedented increase in upward Eliassen–Palm flux preceding the warming.

The anomalous conditions of the 2002 winter began as early as mid-May of that year and consisted of a large burst of wave flux into the stratosphere and a strong deceleration of the vortex during its early stage of development. The low-latitude easterly anomaly that resulted from this (unprecedented) event appears to have enhanced the poleward focusing of wave activity in the mid- and upper stratosphere during the rest of the winter. The altered wave geometry of the 2002 vortex allowed internal reflection of traveling wave 2, which helps to explain its unusual persistence during the rest of the winter.

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Nili Harnik, Judith Perlwitz, and Tiffany A. Shaw

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Downward wave coupling dominates the intraseasonal dynamical coupling between the stratosphere and troposphere in the Southern Hemisphere. The coupling occurs during late winter and spring when the stratospheric basic state forms a well-defined meridional waveguide, which is bounded above by a reflecting surface. This basic-state configuration is favorable for planetary wave reflection and guides the reflected waves back down to the troposphere, where they impact wave structures. In this study decadal changes in downward wave coupling are analyzed using the Modern Era Retrospective-Analysis for Research and Applications (MERRA) dataset.

A cross-spectral correlation analysis, applied to geopotential height fields, and a wave geometry diagnostic, applied to zonal-mean zonal wind and temperature data, are used to understand decadal changes in planetary wave propagation. It is found that downward wave 1 coupling from September to December has increased over the last three decades, owing to significant increases at the beginning and end of this 4-month period. The increased downward wave coupling is caused by both an earlier onset of the vertically bounded meridional waveguide configuration and a persistence of this configuration into December. The latter is associated with the observed delay in vortex breakup. The results point to an additional dynamical mechanism whereby the stratosphere has influenced the tropospheric climate in the Southern Hemisphere.

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Tiffany A. Shaw, Judith Perlwitz, and Nili Harnik

Abstract

The nature of downward wave coupling between the stratosphere and troposphere in both hemispheres is analyzed using the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) dataset. Downward wave coupling occurs when planetary waves reflected in the stratosphere impact the troposphere, and it is distinct from zonal-mean coupling, which results from wave dissipation and its subsequent impact on the zonal-mean flow. Cross-spectral correlation analysis and wave geometry diagnostics reveal that downward wave-1 coupling occurs in the presence of both a vertical reflecting surface in the mid-to-upper stratosphere and a high-latitude meridional waveguide in the lower stratosphere. In the Southern Hemisphere, downward wave coupling occurs from September to December, whereas in the Northern Hemisphere it occurs from January to March. A vertical reflecting surface is also present in the stratosphere during early winter in both hemispheres; however, it forms at the poleward edge of the meridional waveguide, which is not confined to high latitudes. The absence of a high-latitude waveguide allows meridional wave propagation into the subtropics and decreases the likelihood of downward wave coupling. The results highlight the importance of distinguishing between wave reflection in general, which requires a vertical reflecting surface, and downward wave coupling between the stratosphere and troposphere, which requires both a vertical reflecting surface and a high-latitude meridional waveguide.

The relative roles of downward wave and zonal-mean coupling in the Southern and Northern Hemispheres are subsequently compared. In the Southern Hemisphere, downward wave-1 coupling dominates, whereas in the Northern Hemisphere downward wave-1 coupling and zonal-mean coupling are found to be equally important from winter to early spring. The results suggest that an accurate representation of the seasonal cycle of the wave geometry is necessary for the proper representation of downward wave coupling between the stratosphere and troposphere.

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Judith Perlwitz, Martin Hoerling, and Randall Dole

Abstract

Arctic temperatures have risen dramatically relative to those of lower latitudes in recent decades, with a common supposition being that sea ice declines are primarily responsible for amplified Arctic tropospheric warming. This conjecture is central to a hypothesis in which Arctic sea ice loss forms the beginning link of a causal chain that includes weaker westerlies in midlatitudes, more persistent and amplified midlatitude waves, and more extreme weather. Through model experimentation, the first step in this chain is examined by quantifying contributions of various physical factors to October–December (OND) mean Arctic tropospheric warming since 1979. The results indicate that the main factors responsible for Arctic tropospheric warming are recent decadal fluctuations and long-term changes in sea surface temperatures (SSTs), both located outside the Arctic. Arctic sea ice decline is the largest contributor to near-surface Arctic temperature increases, but it accounts for only about 20% of the magnitude of 1000–500-hPa warming. These findings thus disconfirm the hypothesis that deep tropospheric warming in the Arctic during OND has resulted substantially from sea ice loss. Contributions of the same factors to recent midlatitude climate trends are then examined. It is found that pronounced circulation changes over the North Atlantic and North Pacific result mainly from recent decadal ocean fluctuations and internal atmospheric variability, while the effects of sea ice declines are very small. Therefore, a hypothesized causal chain of hemisphere-wide connections originating from Arctic sea ice loss is not supported.

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