[1] described the coincidence of double tropopause (DT) profiles with reduced amounts of ozone in the lower stratosphere (LS) with regions of increased Calcitriol proliferation transport from the tropics to higher latitudes above the subtropical jets. Pan et al. [4] suggested the association of DTs with intrusions of low-latitude air masses with low static stability and low ozone concentrations into the LS at midlatitudes, which they related to Rossby wave breaking events. However, Wang and Polvani [5] have recently suggested using an idealized model that air inside a DT structure comes from high latitudes. Vogel et al. [6] found that, in some cases, the contribution to mixing at levels between 330K and 350K is approximately equivalent to stratospheric and tropospheric intrusion.

However, no clear understanding exists of the origin of MT phenomena, which could be associated with the overlapping of the tropical and extratropical tropopause, or to folding of the tropopause linked to the atmospheric circulation phenomena that characterise these latitudes, such as cut-off low (COL) systems [7], the movement of jet-streams, or baroclinic waves [8, 9]. Recent results show a widening of the Earth’s tropical belt, which could further complicate this picture [10, 11]. In order to address this gap in our understanding, we herein report on the origin of the air immediately above the first and below the second tropopause at the Boulder radiosonde station. We combine the data obtained from the water vapour soundings launched here with the results of reanalysis, based on Lagrangian analysis and fields of potential vorticity (PV).

Our analysis relies on the fact that, if the origin of the MTs is related to excursions of tropical air over the extratropical tropopause, then we might expect there to be a contribution of air rich in moisture to Carfilzomib the extratropical lowermost stratosphere. However, if the origin of the air is associated with the folding of the tropopause and with stratosphere-to-troposphere exchange, we would then expect to find air of stratospheric origin, that is, with a low moisture content and with higher concentrations of ozone.2. Methods We herein consider data for the cold season in the Northern Hemisphere (November to March). The period of study was 1980�C2009 for the computation of vertical profiles from soundings and 1980�C2000 for the Lagrangian analysis, due to the availability of ERA-40 data. We chose the station of Boulder, Colorado (39.9��N, 105.3��W), for our study, mainly because it is a well-studied station and in a location likely to show the first tropopauses at tropical altitudes during summer and at extratropical altitudes during winter [2]. Sounding data were obtained from http://www.esrl.noaa.gov/gmd/ozwv/.