Scaling of Eliassen-Palm flux vectors

Eliassen-Palm flux is one of the main diagnostics tools for wave propagation and wave-mean flow interaction in atmospheric dynamics and in particular stratosphere-troposphere coupling. Even though the theory has been derived in the 1960s, there is still no consensus about how to display the flux vectors in a plot. This is particularly true where both the troposphere and stratosphere are of importance. Some of the traditional methods are to scale the arrows by either pressure, the exponential of height, the square root of pressure, or even by an arbitrary factor. But the arguments for any of those methods are subjective, and they result in both different amplitudes and direction. Here, we propose an objective way of scaling EP flux vectors, in either linear or logarithmic pressure or height coordinates, which allows for a physically sound representation throughout the entire atmosphere

Ozone-Forced Southern Annular Mode During Antarctic Stratospheric Warming Events

Southern Hemisphere (SH) Stratospheric Warming Events (SWEs) are usually associated with a negative phase of the tropospheric Southern Annular Mode (SAM) during the following summer. In contrast, using ensemble climate model simulations we show that the anomalously high ozone concentrations typically occurring during SWEs can force periods of persistent positive tropospheric SAM in austral spring by increasing lower stratospheric static stability and changing troposphere-to-stratosphere wave propagation. Eventually, the tropospheric SAM switches sign to its negative phase in late spring/early summer, but this ‘downward propagation’ of the stratospheric signal does not occur in simulations without seasonal cycle. We find that the downward propagation is forced both dynamically by adiabatic heating and radiatively by increased shortwave absorption by ozone due to the seasonal cycle. Capturing this ozone forcing mechanism in models requires the inclusion of interactive ozone, which has important implications for the predictive skill of current seasonal forecasting systems

How Frequent Are Antarctic Sudden Stratospheric Warmings in Present and Future Climate?

Southern Hemisphere (SH) stratospheric sudden warmings (SSWs) result in smaller Antarctic ozone holes and are linked to extreme midlatitude weather on subseasonal to seasonal timescales. Therefore, it is of interest how often such events occur and whether we should expect more events in the future. Here, we use a pair of novel multimillennial simulations with a stratosphere-resolving coupled ocean-atmosphere climate model to show that the frequency of SSWs, such as observed 2002 and 2019, is about one in 22 years for 1990 conditions. In addition, we show that we should expect the frequency of SSWs, and that of more moderate vortex weakening events, to strongly decrease by the end of this century

Distribution and biology of the yellow-spotted longicorn beetle Psacothea hilaris hilaris(Pascoe) in Italy

The Asiatic yellow-spotted longicorn beetle, Psacothea hilaris hilaris, was found for the first time in Northern Italy in 2005. As this xylophagous insect is considered one of the most important pests of Morus spp. and of Ficus carica in its countries of origin, a multiyear study was carried out to determine the spread of the pest in Northern Italy, to evaluate its establishment potential and to improve knowledge on its biology in the new habitat. The survey confirmed that P. hilaris hilaris has established in Italy and has colonized an area of about 60 km2. The species overwinters as eggs or larvae. Adults are present from June to October. Damage has been recorded mostly on Ficus carica plants, and very rarely on Morus alba. Both young and older plants, healthy and weakened hosts can be attacked by the pest. Severely attacked plants become weakened and eventually die.Psacothea hilaris hilaris, (capricorne asiatique a taches jaunes) a ete identifie en Italie septentrionale pour la premiere fois en 2005. Puisque cet insecte xylophage est considere comme un des ravageurs les plus importants de Morus spp. et de Ficus carica dans sa zone d\u2019origine, une etude pluriannuelle a ete entreprise pour determiner la dissemination de cet organisme nuisible dans le nord de l\u2019Italie, determiner son potentiel d\u2019etablissement, et ameliorer les connaissances sur sa biologie dans ce nouvel habitat. Les resultats confirment que P. hilaris hilaris s\u2019est etabli en Italie et a colonise une zone d\u2019environ 60 km\ub2. Cette espece passe l\u2019hiver sous forme d\u2019oeuf ou de larve. Les adultes sont presents de juin a octobre. Les degats ont principalement ete observes sur des Ficus carica, et tres rarement sur Morus alba. Le ravageur attaque aussi bien les plantes jeunes qu\u2019agees, saines que malades. Les plantes severement atteintes sont affaiblies, et finissent par mourir

Stratospheric sudden warmings in an idealized GCM

PublishedJournal ArticleAn idealized general circulation model (GCM) with an analytically described Newtonian cooling term is employed to study the occurrence rate of sudden stratospheric warmings (SSWs) over a wide range of parameters. In particular, the sensitivity of the SSW occurrence rates to orographic forcing and both relaxation temperature and damping rate is evaluated. The stronger the orographic forcing and the weaker the radiative forcing (in both temperature and damping rate), the higher the SSW frequency. The separate effects of the damping rates at low and high latitudes are somewhat more complex. Generally, lower damping rates result in higher SSW frequency. However, if the low- and high-latitude damping rates are not the same, SSW frequency tends to be most sensitive to a fractional change in the lower of the two damping rates. In addition, the effect of the damping rates on the stratospheric residual circulation is investigated. It is found that higher high-latitude damping rate results in deeper but narrower circulation, whereas higher low-latitude damping rates cause strengthening of the stream function in the tropical midstratosphere to upper stratosphere. Finally, the relation between easily measured and compared climatological fields and the SSW occurrence rate is determined. The average stratospheric polar zonal mean zonal wind shows a strong anticorrelation with the SSW frequency. In the troposphere, there is a high correlation between the meridional temperature gradient and SSW frequency, suggesting that the strength of synoptic activity in the troposphere may be an important influence on SSW occurrence.National Science FoundationSwiss National Science Foundatio

The impact of SST biases in the tropical east pacific and agulhas current region on atmospheric stationary waves in the southern hemisphere

Climate models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) vary significantly in their ability to simulate the phase and amplitude of atmospheric stationary waves in the midlatitude Southern Hemisphere. These models also suffer from a double intertropical convergence zone (ITCZ), with excessive precipitation in the tropical eastern South Pacific, and many also suffer from a biased simulation of the dynamics of the Agulhas Current around the tip of South Africa. The intermodel spread in the strength and phasing of SH midlatitude stationary waves in the CMIP archive is shown to be significantly correlated with the double-ITCZ bias and biases in the Agulhas Return Current. An idealized general circulation model (GCM) is used to demonstrate the causality of these links by prescribing an oceanic heat flux out of the tropical east Pacific and near the Agulhas Current. A warm bias in tropical east Pacific SSTs associated with an erroneous double ITCZ leads to a biased representation of midlatitude stationary waves in the austral hemisphere, capturing the response evident in CMIP models. Similarly, an overly diffuse sea surface temperature gradient associated with a weak Agulhas Return Current leads to an equatorward shift of the Southern Hemisphere jet by more than 38 and weak stationary wave activity in the austral hemisphere. Hence, rectification of the double-ITCZ bias and a better representation of the Agulhas Current should be expected to lead to an improved model representation of the austral hemisphere

Generation of the Amundsen Sea Low by Antarctic Orography

The Amundsen Sea Low (ASL) is a distinctive feature of the Southern Hemisphere high latitude atmospheric circulation, regulating regional Antarctic climate, meridional heat transport, ocean circulation, and sea-ice in the Amundsen-Bellingshausen Seas. Most previous research on the ASL has focused on its variability with only a few studies attempting to understand why the climatological ASL exists. These studies have proposed different hypotheses to explain the presence of the ASL, however, a clear understanding of the mechanisms responsible for the generation of the ASL remains uncertain. Here we use an atmospheric general circulation model to show that the ASL is a consequence of the interaction between Antarctic topography and the westerly wind jet, with negligible influence from low-latitude teleconnections. A nonrotating fluid flow simulation further suggests that the ASL can be explained by flow separation resulting from the interaction of westerly winds with the topography of Antarctica

Response of Southern Hemisphere Western Boundary Current Regions to Future Zonally Symmetric and Asymmetric Atmospheric Changes

Subtropical western boundary currents (WBCs) are often associated with hotspots of global warming, with certain WBC extension regions warming 3–4 times faster than the global mean. In the Southern Hemisphere, strong warming over the WBC extensions has been observed over the last few decades, with enhanced warming projected into the future. This amplified warming has primarily been linked to poleward intensification of the mid-latitude westerly winds in the Southern Hemisphere. Changes in these winds are often thought of as being zonally symmetric; however, recent studies show that they contain strong zonal asymmetries in certain ocean basins. The importance of these zonal asymmetries for the Southern Ocean has not yet been investigated. In this study, we use an ocean-sea-ice model forced by prescribed atmospheric fields to quantify the contribution of projected zonally asymmetric atmospheric changes in generating future ocean warming and circulation changes in the subtropical WBC regions. We find that the zonally asymmetric component of atmospheric forcing, characterized by a pattern that is consistent across CMIP6 models, can explain more than 30% (>2°C) of the sea surface temperature (SST) warming found in the Tasman Sea and southern Australia region and a sizable fraction of warming in the Agulhas Current region. These changes in SST in both the Indian and Pacific basins are found to be primarily driven by increases in the advection of warm tropical water to the mid-latitudes due to changes in the large-scale subtropical ocean gyres, which in turn can largely be explained by changes in the mid-latitude surface wind stress patterns

Historical and Projected Changes in the Southern Hemisphere Surface Westerlies

The Southern Hemisphere (SH) surface westerlies fundamentally control regional patterns of air temperature, storm tracks, and precipitation while also regulating ocean circulation, heat transport and carbon uptake. Wind-forced ocean perturbation experiments commonly apply idealized poleward wind shifts ranging between 0.5 and 10 degrees of latitude and wind intensification factors of between 10% and 300%. In addition, changes in winds are often prescribed ad hoc as a zonally uniform anomaly that neglects important regional and seasonal differences. Here we quantify historical and projected SH westerly wind changes based on examination of CMIP5, CMIP6, and reanalysis data. We find a significant reduction in the location bias of the CMIP6 ensemble and an associated reduction in the projected poleward shift compared to CMIP5. Under a high emission scenario, we find a projected end of 21st Century ensemble mean wind increase of ∼10% and a poleward shift of ∼0.8° latitude, although there are important seasonal and regional variations

The Impact of Split and Displacement Sudden Stratospheric Warmings on the Troposphere

Although sudden stratospheric warmings (SSWs) can improve subseasonal-to-seasonal forecasts, it is unclear whether the two types of SSW - displacements and splits - have different near-surface effects. To examine the longer-term (i.e., multi-week lead) tropospheric response to displacements and splits, we utilize an intermediate-complexity model and impose wave-1 and wave-2 stratospheric heating perturbations spun-off from a control run. At longer lags, the tropospheric response is found to be insensitive to both the wavenumber and location of the imposed heating, in agreement with freely evolving displacements and splits identified in the control run. At shorter lags, however, large differences are found between displacements and splits in both the control run and the different wavenumber-forced events. In particular, in the control run, the free-running splits have an immediate barotropic response throughout the stratosphere and troposphere whereas displacements take 1–2 weeks before a near-surface response becomes evident. Interestingly, this barotropic response found during CTRL splits is not captured by the barotropically forced wave-2 events, indicating that the zonal-mean tropospheric circulation is somehow coupled with the generation of the wave-2 splits. It is also found that in the control run, displacements yield stronger Polar-Cap temperature anomalies than splits, yet both still yield similar magnitude tropospheric responses. Hence, the strength of the stratospheric warming is not the only governing factor in the surface response. Overall, SSW classification based on vortex morphology may be useful for subseasonal but not seasonal tropospheric prediction