Is the subtropical jet shifting poleward?

The tropics are expanding poleward at about 0.5∘ per decade in observations. This poleward expansion of the circulation is consistently reported using Hadley cell edge metrics and lower-atmospheric tropical edge metrics. However, some upper-atmospheric tropical metrics report smaller trends that are often not significant. One such upper-atmospheric metric is the subtropical jet latitude, which has smaller trends compared to the Hadley cell edge. In this study we investigate the robustness of the weak trends in the subtropical jet position by introducing a new method for locating the subtropical jet, and examining the trends and variability of the subtropical jet latitude. We introduce the tropopause gradient method based on the peak gradient in potential temperature along the dynamic tropopause. Using this method we find the trends in the subtropical jet latitude are indeed much smaller than 0.5∘ per decade, consistent with previous studies. We also find that natural variability within the subtropical jet latitude would not prevent trends from being detected if they were similar to the Hadley cell edge, as trends greater than 0.24∘ per decade could reliably be detected using monthly data or 0.09∘ per decade using daily data. Despite the poleward expansion of the tropics, there is no robust evidence to suggest the subtropical jet is shifting poleward in either hemisphere. Neither the current diagnostic methods nor natural variability can account for the small subtropical jet trends. The most likely explanation, which requires further investigation, is that the subtropical jet position is not tied dynamically to the Hadley cell edge

An adiabatic mechanism for the reduction of jet meander amplitude by potential vorticity filamentation

The amplitude of ridges in large-amplitude Rossby waves have been shown to decrease systematically with lead time during the first 1-5 days of operational global numerical weather forecasts. These models also exhibit a rapid reduction in the isentropic gradient of potential vorticity (PV) at the tropopause during the first 1-2 days of forecasts. This paper identifies a mechanism linking the reduction in large-scale meander amplitude on jet streams to declining PV gradients. The mechanism proposed is that a smoother isentropic transition of PV across the tropopause leads to excessive PV filamentation on the jet flanks and a more lossy waveguide. The approach taken is to analyse Rossby wave dynamics in a single-layer quasi-geostrophic model. Numerical simulations show that the amplitude of a Rossby wave propagating along a narrow but smooth PV front do indeed decay transiently with time. This process is explained in terms of the filamentation of PV from the jet core and associated absorption of wave activity by the critical layers on the jet flanks, and a simple method for quantitatively predicting the magnitude of the amplitude reduction without simulation is presented. Explicitly-diffusive simulations are then used to show that the combined impact of diffusion and the adiabatic rearrangement of PV can result in a decay rate of Rossby waves which is 2-4 times faster than could be expected from diffusion acting alone. This predicted decay rate is sufficient to explain the decay observed in operational weather forecasting models

National Action Plans on Business and Human Rights:an Experimentalist Governance Analysis

Research for this article was partly funded by Danida, “Realising the SDGS: The role of responsible business”.National Action Plans (NAPs) on business and human rights are a growing phenomenon. Since 2011, 42 such plans have been adopted or are in-development worldwide. By comparison, only 39 general human rights action plans were published between 1993 and 2021. In parallel, NAPs have attracted growing scholarly interest. While some studies highlight their potential to advance national compliance with international norms, others criticise NAPs as cosmetic devices that states use to deflect attention from persisting abuses and needed regulation. In response to wider critiques of international human rights norms, and their failure to exact universal state compliance, experimentalist governance theory highlights the dynamic, dialogic and iterative character of human rights implementation as well as the role of stakeholders. In this article, we apply experimentalist governance theory to evaluate the role and character of business and human rights NAPs. Rather than attempting to evaluate NAPs’ ultimate consequences for rights-holders, which appears premature, we focus on NAPs processes. Specifically, we analyse NAPs processes in twenty-five states against five experimentalist governance criteria relating to (i) stakeholder participation; (ii) agreement on a broad problem definition; (iii) local contextualisation; (iv) monitoring and peer review and (v) periodic revision and learning. According to our findings, NAPs on business and human rights in most states demonstrate resemblance to the traits of experimentalist governance. In particular, our analysis points to the emergence of relatively sophisticated and demanding institutional governance mechanisms within NAPs — including the institutionalisation of complex deliberative processes. Nevertheless, our paper also identifies some significant shortcomings in NAPs, related to the lack of inclusion of vulnerable groups and the lack of explicit indicators and targets.Publisher PDFPeer reviewe

Physical factors influencing regional precipitation variability attributed using an airmass trajectory method

A novel Lagrangian framework is developed to attribute monthly precipitation variability to physical processes. Precipitation variability is partitioned into a combination of 5 factors: air mass origin location, origin surface temperature variation, ascent intensity, mass fraction of ascending air and the number of ‘wet’ analysis times per month (> 1 mm/6hrs). Precipitation in a target region is linked to ‘origin’ locations of air masses where the water vapour mixing ratio was last set by boundary layer moistening and is a maximum along back trajectories. Applying the technique to the England and Wales region, the factors together account for 83-89% of the observed summer precipitation variability. The dominant contributor is the number of ‘wet’ analyses, which is shown to be associated with cyclone statistics. The wettest summer months are mainly associated with anomalous cyclone duration rather than the number of cyclones. In addition, surface temperature and saturation humidity at the ‘origin’ locations are found to be below their climatological averages (1979-2013). Therefore the direct thermodynamic effect of anomalous surface temperature on marine boundary layer humidity acts to reduce monthly precipitation anomalies. The decadal precipitation change between phases of the Atlantic Multidecadal Oscillation is approximately 20% of the interannual variability between summer months. Changes in cyclone statistics have an effect six times larger than the direct thermodynamic factor in both monthly and decadal precipitation variability

Diabatic processes and the evolution of two contrasting summer extratropical cyclones

Extratropical cyclones are typically weaker and less frequent in summer as a result of differences in the background state flow and diabatic processes with respect to other seasons. Two extratropical cyclones were observed in summer 2012 with a research aircraft during the DIAMET (DIAbatic influences on Mesoscale structure in ExTratropical storms) field campaign. The first cyclone deepened only down to 995 hPa; the second cyclone deepened down to 978 hPa and formed a potential vorticity (PV) tower, a frequent signature of intense cyclones. The objectives of this article are to quantify the effects of diabatic processes and their parametrizations on cyclone dynamics. The cyclones were analyzed through numerical simulations incorporating tracers for the effects of diabatic processes on potential temperature and PV. The simulations were compared with radar rainfall observations and dropsonde measurements. It was found that the observed maximum vapor flux in the stronger cyclone was twice as strong as in the weaker cyclone; the water vapor mass flow along the warm conveyor belt of the stronger cyclone was over half that typical in winter. The model overestimated water vapor mass flow by approximately a factor of two due to deeper structure in the rearwards flow and humidity in the weaker case. An integral tracer interpretation is introduced, relating the tracers with cross-isentropic mass transport and circulation. It is shown that the circulation around the cyclone increases much more slowly than the amplitude of the diabatically-generated PV tower. This effect is explained using the PV impermeability theorem

Isolating the effects of moisture entrainment on convectively coupled equatorial waves in an aquaplanet GCM

The rate of humidity entrainment in the convective parametrization scheme in a general circulation model affects the simulation of convectively-coupled waves. However, it is unclear whether this is caused directly by the effects of entrainment on waves or indirectly through associated impacts such as on the basic state. Therefore, using an aquaplanet model, we employ a novel framework in which we entrain a weighted average of the resolved humidity field and a prescribed zonally symmetric field, with the weighting controlled by a decoupling parameter. Hence, we can vary the entrainment rate of basic state humidity independently of the entrainment of humidity perturbations, simultaneously minimizing changes in basic state. Thus we isolate the effect of moisture entrainment on the waves. Enhancing entrainment rate increases spectral power over all zonal wavenumbers and frequencies, with an increase in the ratio of eastward-to-westward power. The Kelvin wave speed decreases as entrainment increases, which can be partially accounted for by an associated change in basic state humidity. Increasing the decoupling parameter reduces spectral power in Kelvin waves relative to the background, with only long waves still prominent when entrainment is almost fully decoupled from the resolved moisture field, suggesting the wave structure in humidity is required for convection to organize into short wave structures. For long waves the increase in the ratio of eastward-to-westward power as entrainment rate increases cannot be explained by the changes in the coupling with the wave structure in humidity, but is consistent with the changes in the basic state

Does the representation of flow structure and turbulence at a cold front converge on multi-scale observations with model resolution?

In situ aircraft observations are used to interrogate the ability of a numerical weather prediction model to represent flow structure and turbulence at a narrow cold front. Simulations are performed at a range of nested resolutions with grid spacings of 12 km down to 100 m and the convergence with resolution is investigated. The observations include the novel feature of a low-altitude circuit around the front that is closed in the frame of reference of the front, thus allowing the direct evaluation of area-average vorticity and divergence values from circuit integrals. As such, the observational strategy enables a comparison of flow structures over a broad range of spatial scales, from the size of the circuit itself ($\approx$100 km) to small-scale turbulent fluctuations ($\approx$10 m). It is found that many aspects of the resolved flow converge successfully towards the observations with resolution if sampling uncertainty is accounted for, including the area-average vorticity and divergence measures and the narrowest observed cross-frontal width. In addition, there is a gradual handover from parametrized to resolved turbulent fluxes of moisture and momentum as motions in the convective boundary layer behind the front become partially-resolved in the highest resolution simulations. In contrast, the parametrized turbulent fluxes associated with subgrid-scale shear-driven turbulence ahead of the front do not converge on the observations. The structure of frontal rainbands associated with a shear instability along the front also does not converge with resolution, indicating that the mechanism of the frontal instability may not be well represented in the simulations

Linking African easterly wave activity with equatorial waves and the influence of Rossby waves from the Southern Hemisphere

A connection is found between African easterly waves (AEWs), equatorial westward-moving mixed Rossby-gravity (WMRG) waves and equivalent barotropic Rossby waves (RWs) from the Southern Hemisphere (SH). The amplitude and phase of equatorial waves is calculated by projection of broad-band filtered ERA-Interim data onto a horizontal structure basis obtained from equatorial wave theory. Mechanisms enabling interaction between the wave types are identified. AEWs are dominated by a vorticity wave which tilts eastwards below the African Easterly Jet and westwards above: the tilt necessary for baroclinic wave growth. However, a strong relationship is identified between amplifying vorticity centres within AEWs and equatorial WMRG waves. Although the waves do not phase-lock, positive vorticity centres amplify whenever the cross-equatorial motion of the WMRG wave lies at the same longitude in the upper troposphere (southwards flow) and east of this in the lower troposphere (northwards flow). Two mechanisms could explain the vorticity amplification: vortex stretching below the upper-tropospheric divergence and ascent associated with latent heating in convection in the lower-tropospheric moist northwards flow. In years of strong AEW activity, SH and equatorial upper-tropospheric zonal winds are more easterly. Stronger easterlies have two effects: i) they Doppler shift WMRG waves so that their period varies little with wavenumber (3-4 days) and ii) they enable westward-moving RWs to propagate into the tropical wave guide from the SH. RW phase speeds can match those of WMRG waves, enabling sustained excitation of WMRG. The WMRG waves have an eastwards group velocity with wave activity accumulating over Africa and invigorating AEWs at similar frequencies through the vorticity amplification mechanism