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Stratospheric water vapor affects the Earth's radiative balance and stratospheric chemistry, yet its future changes are uncertain and not fully understood. The influence of deep convection on stratospheric water vapor remains subject to debate. This letter presents a detailed process‐based model study of the impact of convective ice sublimation on stratospheric water vapor in response to CO2 forced climate change. The influence of convective injection is found to be limited by the vertical profile of temperature and saturation vapor pressure in the tropical tropopause layer, not by the frequency of occurrence. Lagrangian trajectory analysis shows the relative contributions to stratospheric water vapor from sublimation and large‐scale transport are approximately unchanged when CO2 is increased. The results indicate the role of convective ice injection for stratospheric water vapor in a warmer climate remains constrained by large‐scale temperatures.

© 2020 The Authors. Quarterly Journal of the Royal Meteorological Society published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society. Flow on a beta-plane driven by a steady localised anticyclonic forcing of potential vorticity (or equivalently a mass source) is considered as a simple model of the Asian monsoon flow in the upper troposphere. Previous authors have noted that the response may be steady, or unsteady, according to the magnitude of the forcing, with the unsteadiness manifested as westward eddy shedding. A detailed study of the transition between steady and eddy-shedding regimes reveals a third regime ('break up'), for intermediate forcing magnitude, where the flow is steady in the neighbourhood of the forcing, but the westward extending plume of low potential vorticity breaks up into isolated anticyclonic vortices some distance away from the forcing region. A related spatio-temporal instability problem for flow on a beta plane is specified and analysed. The flow can be stable, convectively unstable or absolutely unstable. It is argued that these three stability regimes correspond to the steady, break-up and eddy-shedding regimes for the forced flow and good quantitative correspondence between the regimes is demonstrated by explicit solution of the spatio-temporal stability problem.

Concentrations of water vapour entering the tropical lower stratosphere are primarily determined by conditions that air parcels encounter as they are transported through the tropical tropopause layer (TTL). Here we quantify the relative roles of variations in TTL temperatures and transport in determining seasonal and interannual variations of stratospheric water vapour. Following previous studies, we use trajectory calculations with the water vapour concentration set by the Lagrangian dry point (LDP) along trajectories. To assess the separate roles of transport and temperatures, the LDP calculations are modified by replacing either the winds or the temperatures with those from different years to investigate the wind or temperature sensitivity of water vapour to interannual variations and, correspondingly, with those from different months to investigate the wind or temperature sensitivity to seasonal variations. Both ERA-Interim reanalysis data for the 1999–2009 period and data generated by a chemistry–climate model (UM-UKCA) are investigated. Variations in temperatures, rather than transport, dominate interannual variability, typically explaining more than 70 % of variability, including individual events such as the 2000 stratospheric water vapour drop. Similarly seasonal variation of temperatures, rather than transport, is shown to be the dominant driver of the annual cycle in lower stratospheric water vapour concentrations in both the model and reanalysis, but it is also shown that seasonal variation of transport plays an important role in reducing the seasonal cycle maximum (reducing the annual range by about 30 %). The quantitative role in dehydration of sub-seasonal and sub-monthly Eulerian temperature variability is also examined by using time-filtered temperature fields in the trajectory calculations. Sub-monthly temperature variability reduces annual mean water vapour concentrations by 40 % in the reanalysis calculation and 30 % in the model calculation. As with other aspects of dehydration, simple Eulerian measures of variability are not sufficient to quantify the implications for dehydration, and the Lagrangian sampling of the variability must be taken into account. These results indicate that, whilst capturing seasonal and interannual variation of temperature is a major factor in modelling realistic stratospheric water vapour concentrations, simulation of seasonal variation of transport and of sub-seasonal and sub-monthly temperature variability are also important and cannot be ignored.

To explore the mechanisms linking Arctic sea-ice loss to changes in mid-latitude surface temperatures, we conduct idealized modeling experiments using an intermediate general circulation model and with sea-ice loss confined to the Atlantic or Pacific sectors of the Arctic (Barents-Kara or Chukchi-Bering Seas). Extending previous findings, there are opposite effects on the winter stratospheric polar vortex for both large-magnitude (late twenty-first century) and moderate-magnitude sea-ice loss. Accordingly, there are opposite tropospheric Arctic Oscillation (AO) responses for moderate-magnitude sea-ice loss. However, there are similar strength negative AO responses for large-magnitude sea-ice loss, suggesting that tropospheric mechanisms become relatively more important than stratospheric mechanisms as the sea-ice loss magnitude increases. The mid-latitude surface temperature response for each loss region and magnitude can be understood as the combination of an ‘indirect’ part induced by the large-scale circulation (AO) response, and a residual ‘direct’ part that is local to the loss region.

A configuration of an idealized general circulation model has been obtained in which a deep, stratospheric, equatorial, westerly jet is established that is spontaneously and quasi-periodically disrupted by shallow easterly jets. Similar to the disruption of the quasi-biennial oscillation (QBO) observed in early 2016, meridional fluxes of wave activity are found to play a central role. The possible relevance of two feedback mechanisms to these disruptions is considered. The first involves the secondary circulation produced in the shear zones on the upper and lower flanks of the easterly jet. This is found to play a role in maintaining the aspect ratio of the emerging easterly jet. The second involves the organization of the eddy fluxes by the mean flow: the presence of a weak easterly anomaly within a tall, tropical, westerly jet is demonstrated to produce enhanced and highly focused wave activity fluxes that reinforce and strengthen the easterly anomalies. The eddies appear to be organized by the formation of strong potential vorticity gradients on the subtropical flanks of the easterly anomaly. Similar wave activity and potential vorticity structures are found in the ERA-Interim for the observed QBO disruption, indicating this second feedback was active then.

To explore the mechanisms linking Arctic sea-ice loss to changes in mid-latitude surface temperatures, we conduct idealized modeling experiments using an intermediate general circulation model and with sea-ice loss confined to the Atlantic or Pacific sectors of the Arctic (Barents-Kara or Chukchi-Bering Seas). Extending previous findings, there are opposite effects on the winter stratospheric polar vortex for both large-magnitude (late twenty-first century) and moderate-magnitude sea-ice loss. Accordingly, there are opposite tropospheric Arctic Oscillation (AO) responses for moderate-magnitude sea-ice loss. However, there are similar strength negative AO responses for large-magnitude sea-ice loss, suggesting that tropospheric mechanisms become relatively more important than stratospheric mechanisms as the sea-ice loss magnitude increases. The mid-latitude surface temperature response for each loss region and magnitude can be understood as the combination of an ‘indirect’ part induced by the large-scale circulation (AO) response, and a residual ‘direct’ part that is local to the loss region.