Results
UT/LS and STE
Upper tropospheric and stratospheric ozone measurements of the Jet Propulsion Laboratory lidars located at Mauna Loa Observatory (MLO), Hawaii, and Table Mountain Facility (TMF), California, during summer 2002 were compared to isentropic potential vorticity (IPV) advected on 54 levels from 320 K to 1500 K by the high-resolution model MIMOSA. The correlation between ozone measured by lidar, and the origin of the 10-day backward trajectories of the air parcels sampled was investigated.
Near the tropopause, strong positive correlation between ozone mixing ratio and IPV was observed at both MLO and TMF lidar sites. The largest fluctuations were centered near 350 K and are associated with the meridional displacement of the tropopause by Rossby waves North or South of the observing sites (Figure 1). These large displacements were occasionally accompanied by Rossby wave breaking (RWB), as was identified several times during the summer in the vicinity of the Hawaiian Islands. The evolution with time of the Rossby wave breaking event of July 13 is shown on Figure 2. This event appears to be typical of breaking events previously investigated at midlatitudes, including the southward intrusion of high-PV air originating in the high latitudes lower stratosphere. This time the intrusion was observed to extend deep in the subtropics. Strong positive ozone anomalies were simultaneously measured by the MLO lidar.
Positive correlation between ozone and the equivalent latitude averaged along the parcels’ trajectories was seen up to 475 K in the stratosphere (Figure 1). At and above 750 K, negative correlation was found for both TMF and MLO. For TMF, the altitude dependence of the correlation is similar to that already observed for summer and winter midlatitudes
For MLO, the observed negative correlation was found to be the result of opposite seasonal and interannual tendencies in ozone and equivalent latitude throughout the summer. All other correlations are associated with a higher intra-seasonal variability of both ozone and the parcels’ origin, as compared to their seasonal tendencies. Figure 3 shows an example of parcel back-trajectories ending at MLO at the time of lidar a measurement. The color of each trajectory corresponds to the amount of ozone measured by lidar (rainbow scale from purple (-100%) to red (+100%)).
Figure 1:
(a) shows a height-time cross section of the IPV departure from the seasonal mean (calculated from June 1st to September 30th, 2002, output from MIMOSA every 6-hours above MLO) ) .
(b) is similar to Figure (a) but the average is calculated using the MIMOSA outputs that coincide with the MLO lidar measurements only.
(c) shows the height-time cross section of the ozone mixing ratio departures from the seasonal average as measured by the MLO lidar.
For all 3 figures, the black and purple curves indicate the WMO and dynamical tropopauses respectively, calculated from the coarse grid.
Positive correlation between IPV and ozone is clearly observed throughout the summer. Large synoptic scale variations are observed, with episodes of high IPV and high ozone alternating with episodes of low IPV and low ozone. The core of the summer may be seen as a 3-4 weeks quiet period of low IPV and low ozone mainly during August.
The strongest episode of high IPV occurs around July 13, and is typical of a Rossby wave breaking event. It is described more in detail below.
Figure 2:
Shows IPV maps output from MIMOSA on the 360 K isentropic surface on July 9 (a), 10 (b),11 (c), 12 (d), 13 (e), and 13 (f).
The horizontal (isentropically speaking) wind speed is overplotted with white contours . The black lines on figures (a)-(b) represent the 10-day back trajectory of an air parcel that ended above MLO on July 10. The black lines on figures (c)-(e) represent the 10-day back trajectory of an air parcel that ended above MLO on July 13. The black line on figures (f) represents the 10-day back trajectory of an air parcel that ended above MLO on July 16. For each figure (a)-(d) a star indicates the current position along the trajectory.
The sharp transition from green to red accompanied by strong horizontal winds indicates the position of the “projection” of the tropopause onto the 360-K isentropic surface, and the tropopause jet. The regions of high IPV (red) North of the transition zone indicate a lower stratospheric regime while the regions of low IPV South of it indicate a subtropical upper tropospheric regime.
During the course of these 7 days, the tropopause region located well North of Hawaii is displaced Southward, rolling up clockwise around a high pressure system located just North-West of Hawaii. By July 13, the subtropical jet initially located over or North of Hawaii is pushed South of the Islands, and the tropopause jet takes position above the Islands. By July 16, the tongue of high IPV advected over the Islands separated from the main vortex and gets advected Eastward. The newborn cut-off system did not reconnect with the main vortex after its separation, leading to irreversible transport of lower stratospheric air into the subtropical upper troposphere.
Figure 3: 10-day backward trajectories at 350 K of the 51 air parcels sampled by lidar at MLO between 1 June and 30 September 2002. Each trajectory is color-coded by the deseasonalized ozone mixing ratio as measured by lidar (see text for details). Each parcel is tagged at its starting point by the date of lidar measurement (ending point). A few trajectories that started outside the plotting area (including that ending at MLO on 13-July-2002) have their tag between parentheses just outside the plotting area (not the actual location of their starting points)
References
Leblanc, T., I. S. McDermid, and A. Hauchecorne (2004), A study of ozone variability and its connection with meridional transport in the Northern Pacific lower stratosphere during summer 2002, J. Geophys. Res., DOI:10.1029/2003JD004027.