Stratospheric gravity waves excited by a propagating Rossby wave train – A DEEPWAVE Case Study

Stratospheric gravity waves observed during the DEEPWAVE research flight RF25 over the Southern Ocean are analyzed and compared with numerical weather prediction (NWP) model results. The quantitative agreement of the NWP model output and the tropospheric and lower stratospheric observations is remarkable. The high-resolution NWP models are even able to reproduce qualitatively the observed upper stratospheric gravity waves detected by an airborne Rayleigh lidar. The usage of high-resolution ERA5 data – partially capturing the long internal gravity waves – enabled a thorough interpretation of the particular event. Here, the observed and modeled gravity waves are excited by the stratospheric flow past a deep tropopause depression belonging to an eastward propagating Rossby wave train. In the reference frame of the propagating Rossby wave, vertically propagating hydrostatic gravity waves appear stationary; in reality, of course, they are transient and propagate horizontally at the phase speed of the Rossby wave. The subsequent refraction of these transient gravity waves into the polar night jet explains their observed and modeled patchy stratospheric occurrence near 60°S. The combination of both unique airborne observations and high-resolution NWP output provides evidence for the one case investigated in this paper. As the excitation of such gravity waves persists during the quasi-linear propagation phase of the Rossby wave’s life cycle, a hypothesis is formulated that parts of the stratospheric gravity wave belt over the Southern Ocean might be generated by such Rossbywaves trains propagating along the mid-latitude wave guide.

Multidecadal variations in the East Asian winter monsoon and their relationship with the Atlantic Multidecadal Oscillation since 1850

This study investigates the characteristics and physical mechanisms of the multidecadal variations in the East Asian winter (December–January–February) monsoon (EAWM) since 1850 based on multiple observational and reanalysis datasets. The results indicate that the EAWM undergoes multidecadal weakening during the periods of 1869–1919 and 1986–2004 but strengthening during the period of 1920–1985. Similar evolutions can be observed in the time series of the area-averaged winter surface air temperature over East Asia. Associated with the EAWM multidecadal variations, a quasi-barotropic Rossby wave train originating from the subtropical North Atlantic propagating across the Eurasian continent to Northeast Asia also experiences phase shifting at the same time. In its positive phase, the low-level anticyclonic anomaly over the northern Eurasian continent causes a stronger Siberian high; the mid- and high-level cyclonic anomalies over Northeast Asia deepen the East Asian trough and strengthen the East Asian jet stream, respectively. Thus, the positive phase of the wave train is conducive to stronger EAWMs and vice versa. The diagnostic analysis of the Rossby wave source indicates that the upper-tropospheric divergence anomalies over the North Atlantic can favor the excitation of this wave train, and the feedback forcing of high-frequency eddies plays important roles in its maintenance. In addition, the phase shifting of the Atlantic Multidecadal Oscillation (AMO) can induce a similar Rossby wave train across the Eurasian continent, through which it further modulates the multidecadal variations in the EAWM. Warm phases of the AMO are favorable for a stronger EAWM and colder mid-latitude Eurasian continent and vice versa.

Dynamical mechanisms linking Indian monsoon precipitation and the circumglobal teleconnection

The circumglobal teleconnection (CGT) is an important mode of circulation variability, with an influence across many parts of the northern hemisphere. Here, we examine the excitation mechanisms of the CGT in the ECMWF seasonal forecast model, and the relationship between the Indian summer monsoon (ISM), the CGT and the extratropical northern hemisphere circulation. Results from relaxation experiments, in which the model is corrected to reanalysis in specific regions, suggest that errors over northwest Europe are more important in inhibiting the model skill at representing the CGT, in addition to northern hemisphere skill more widely, than west-central Asia and the ISM region, although the link between ISM precipitation and the extratropical circulation is weak in all experiments. Thermal forcing experiments in the ECMWF model, in which a heating is applied over India, suggest that the ISM does force an extratropical Rossby wave train, with upper tropospheric anticyclonic anomalies over east Asia, the North Pacific and North America associated with increased ISM heating. However, this eastward-propagating branch of the wave train does not project into Europe, and the response there occurs largely through westward-propagating Rossby waves. Results from barotropic model experiments show a response that is highly consistent with the seasonal forecast model, with similar eastward- and westward-propagating Rossby waves. This westward-propagating response is shown to be important in the downstream reinforcement of the wave train between Asia and North America.

Multi-model assessment of the late-winter stratospheric response to El Niño and La Niña

<p>The impact of El Niño-Southern Oscillation (ENSO) on the late-winter extra-tropical stratosphere (January-March) is assessed in a multi-model framework. Three state-of-the-art atmospheric models are run with prescribed SST anomalies representative of a strong ENSO event, with symmetric patterns for El Niño and La Niña. The well-known temperature perturbation in the lower stratosphere during El Niño is captured by two models, in which the anomalous warming at polar latitudes is accompanied by a positive geopotential height anomaly that extends over the polar cap. In the third model, which shows a lack of temperature anomalies over the pole, the anomalous anticyclone is confined over Canada and does not expand to the polar cap. This anomalous center of action emerges from the large-scale tropospheric Rossby wave train forced by ENSO, and conservation of potential vorticity around the polar vortex is invoked to link it to the temperature response. No disagreement across models is found in the lower stratosphere for La Niña, whose teleconnection is opposite in sign but weaker. In the middle-upper stratosphere (above 50 hPa) the geopotential height anomalies project on a wavenumber-1 (WN1) pattern for both El Niño and, more weakly, La Niña, and show a westward tilt with height up to the stratopause. It is suggested that this WN1 pattern arises from the high-latitude lower-stratospheric anomalies, and that the ENSO teleconnection to the polar stratosphere can be interpreted in terms of upward propagation of the stationary Rossby wave train and quasi-geostrophic balance, instead of wave breaking. <br>The multi-model assessment, with 50 members for each experiment, contributes to the ERA4CS-funded MEDSCOPE project and includes: EC-EARTH/IFS (L91, 0.01hPa), CNRM/ARPEGE (L91, 0.01hPa), CMCC/CAM (L46, 0.3hPa).</p>

Multi-model assessment of the late-winter stratospheric response to El Niño and La Niña

The impact of El Niño-Southern Oscillation (ENSO) on the late-winter extra-tropical stratosphere (January–March) is assessed in a multi-model framework. Three state-of-the-art atmospheric models are run with prescribed SST anomalies representative of a strong ENSO event, with symmetric patterns for El Niño and La Niña. The well-known temperature perturbation in the lower stratosphere during El Niño is captured by two models, in which the anomalous warming at polar latitudes is accompanied by a positive geopotential height anomaly that extends over the polar cap. In the third model, which shows a lack of temperature anomalies over the pole, the anomalous anticyclone is confined over Canada and does not expand to the polar cap. This anomalous center of action emerges from the large-scale tropospheric Rossby wave train forced by ENSO, and shrinking/stretching around the polar vortex is invoked to link it to the temperature response. No disagreement across models is found in the lower stratosphere for La Niña, whose teleconnection is opposite in sign but weaker. In the middle-upper stratosphere (above 50 hPa) the geopotential height anomalies project on a wavenumber-1 (WN1) pattern for both El Niño and, more weakly, La Niña, and show a westward tilt with height up to the stratopause. It is suggested that this WN1 pattern arises from the high-latitude lower-stratospheric anomalies, and that the ENSO teleconnection to the polar stratosphere can be interpreted in terms of upward propagation of the stationary Rossby wave train and quasi-geostrophic balance, instead of wave breaking.

The Predictability of the Extratropical Flow Response to Recurving Atlantic Tropical Cyclones

Given the ability of recurving Atlantic tropical cyclones (TCs) to disturb the amplitude of Rossby waves on the extratropical jet, this study investigates whether the predictability of the synoptic-scale flow is significantly modified from climatology downstream from and after TC recurvature events. Predictability is evaluated as the standard deviation of isentropic potential vorticity among a 50-member ensemble and is compared to a model climatology. It is shown that forecast uncertainty is dependent upon the relative location of the nearest trough at the time of recurvature and the relative zonal speed between the aforementioned trough and the TC in the 72 hours after recurvature. Predictability is significantly degraded when recurvature occurs downstream of a trough; the elevated uncertainty subsequently propagates downstream along with the trough axis. Furthermore, this study evaluates predictability in spectral space in order to distinguish between uncertainty tied to the exact location of troughs and ridges and uncertainty in Rossby wave amplitude. The wavelet analysis demonstrates that the increase in uncertainty is not solely limited to the trough location, as there is also significantly elevated uncertainty in the Rossby wave amplitude that originates from the upstream trough and spans across downstream troughs and ridges. Uncertainty is also increased near the recurvature longitude in the subset of cases in which the Rossby wave train propagates zonally slower than the TC after recurvature, which is hypothesized to be linked to baroclinic growth processes.

A Northern Hemispheric Wave Train Associated with Interannual Variations in the Bermuda High During Boreal Summer

The processes that lead to the spatial and temporal evolution of the Bermuda High (BH) during July and August (JA) are investigated on the basis of linear regression analysis. The analysis is based on a Bermuda high index (BHI): the difference in standardized, deseasonalized and detrended sea-level pressure (SLP) between northeast of Bermuda (40°N, 60°W) and New Orleans (30°N 90°W). Negative values of BHI indicate a westward expansion of the Bermuda High relative to its climatological-mean location and reduced precipitation in the southeastern US (SEUS), whereas positive values correspond to BH contraction and enhanced precipitation in the SEUS. Linear regression of the 200 hPa geopotential height based on the BHI reveals the existence of a Rossby wave train that extends zonally from the eastern north Pacific to the eastern-north Atlantic. The troughs and ridges associated with this wave train are spatially collocated with the climatological-mean jet stream, indicating that the jet serves as their waveguide. Anomalous troughing in the SEUS associated with this wave train is linked to the contraction of the Bermuda high during JA. The enhanced precipitation is associated with anomalous ascent to the east and south of this trough where anomalous warm advection is observed. Based on these results, it is hypothesized that this Rossby wave train may partially explain the occurrence of suppressed precipitation tied to midsummer drought in the SEUS during July and August. It is found that the BHI has trended from negative to positive in recent decades, suggesting that it may be influenced by low-frequency variability.

The AMV phase-dependence of the connection between February NAO and March surface air temperature over the Tibetan Plateau

<p>The Tibetan Plateau (TP), referred to as the “Asian water tower”, contains one of the largest land ice masses on Earth. The local glacier shrinkage and frozen-water storage are strongly affected by variations in surface air temperature over the TP (TPSAT), especially in springtime. This study reveals a distinct out-of-phase connection between the February North Atlantic Oscillation (NAO) and March TPSAT, which is non-stationary and regulated by the warm phase of the Atlantic Multidecadal Variability (AMV+). The results show that during the AMV+, the negative phase of the NAO persists from February to March, and is accompanied by a quasi-stationary Rossby wave train trapped along a northward-shifted subtropical westerly jet stream across Eurasia, inducing an anomalous adiabatic descent that warms the TP. However, during the cold phase of the AMV, the negative NAO does not persist into March. The Rossby wave train propagates along the well-separated polar and subtropical westerly jets, and the NAO−TPSAT connection is broken. Further investigation suggests that the enhanced synoptic eddy and low-frequency flow (SELF) interaction over the North Atlantic in February and March during the AMV+, caused by the enhanced and southward-shifted storm track, help maintain the NAO anomaly pattern via positive eddy feedback. This study provides a new detailed perspective on the decadal variability of the North Atlantic−TP connections in late winter−early spring.</p>

Accelerated warming in the northern midlatitude summer since the 1990s

<p>The northern midlatitude summer has experienced rapid warming since the 1990s, especially in Europe, Central Siberia-Mongolia, the West Coast of North America as well as several continental Arctic regions. These “hot spots” are connected by a chain of high-pressure ridges from an anomalous wavenumber-5 Rossby wave train in the upper troposphere.  Here by cross-examining reanalysis datasets and a suite of Coupled Model Intercomparison Project Phase 6 (CMIP6) baseline experiments, we demonstrate that the anthropogenically forced response may be intertwined with internal multidecadal variability, making it difficult to partition the 1979-2020 trend with state-of-the-art climate models. Instead, we take a “storyline” approach with a planetary wave model and sensitivity experiments with an Earth system model to explore key underlying driving factors. Our results highlight the importance of multiscale interaction with synoptic eddy via atmosphere-ocean and atmosphere-land coupling in shaping the multidecadal regional warming trend which has enormous socioeconomic implications. </p>

The Atlantic Multidecadal Variability phase-dependence of teleconnection between the North Atlantic Oscillation in February and the Tibetan Plateau in March

The Tibetan Plateau (TP), referred to as the “Asian water tower”, contains one of the largest land ice masses on Earth. The local glacier shrinkage and frozen-water storage are strongly affected by variations in surface air temperature over the TP (TPSAT), especially in springtime. This study reveals that the relationship between the February North Atlantic Oscillation (NAO) and March TPSAT is unstable with time and regulated by the phase of the Atlantic Multidecadal Variability (AMV). The significant out-of-phase connection occurs only during the warm phase of AMV (AMV+). The results show that during the AMV+, the negative phase of the NAO persists from February to March, and is accompanied by a quasi-stationary Rossby wave train trapped along a northward-shifted subtropical westerly jet stream across Eurasia, inducing an anomalous adiabatic descent that warms the TP. However, during the cold phase of the AMV, the negative NAO can not persist into March. The Rossby wave train propagates along the well-separated polar and subtropical westerly jets, and the NAO−TPSAT connection is broken. Further investigation suggests that the enhanced synoptic eddy and low frequency flow (SELF) interaction over the North Atlantic in February and March during the AMV+, caused by the enhanced and southward-shifted storm track, help maintain the NAO anomaly pattern via positive eddy feedback. This study provides a new detailed perspective on the decadal variability of the North Atlantic−TP connection in late winter−early spring.