1. Figure (a-c). Latitude-height distribution of monthly mean ozone flux for the months of (a) January, (b) April and (c) July averaged over years 2000 to 2007. The color contours are the 2-D (up-down, north-south) magnitudes normalized to a fixed grid box (300 km in latitude, 1.5 km in vertical height, ratio of 200:1) and expressed in units of ppb/day for that box. The arrows are unit vectors showing the direction of the contoured flux from each grid box. The thick black dashed line shows the 100 hPa surface; the cyan line, the 2- PVU estimate of the tropopause; the green line, the 380 K surface; the dark blue lines, the 250 ppb O 3 surface (upper) and the 120 ppb O 3 surface (lower); and the black line, the e90 tropopause. These latter quantities are approximate, being derived from the monthly zonal mean values and not from the monthly zonal average of the individual profiles. Fig (d-e). Annual cycles of monthly mean, hemisphere- averaged O 3 fluxes (as Tg/yr) for years 2000-2007 as measured across (d) 100 hPa and (e) the different iso-surfaces of 250 ppb O 3 (dashed red line), 120 ppb O 3 (dash-dot blue) and e90 tropopause (solid black). The true flux across 100 hPa ([w x O 3 ], dashed lines) is accumulated very time step from the O 3 advection; whereas the common approximation is calculated from the monthly mean residual circulation ([w*]) and zonal mean O 3 ([O 3 ]). The tropospheric column (TC) method accumulates the net change in tropospheric column defined by one of the above iso-surfaces every advection step. Fig (f-g). Latitudinal distribution of O 3 fluxes (as Tg/yr) averaged over 3-month seasons (DJF, solid cyan; MAM, dashed green; JJA, dotted red; SON, dash-dot black) for years 2000-2007 across 100 hPa (f) using the residual circulation approximation ([w*]x[O 3 ]) and across the e90 tropopause (g). (h-i) Twelve-month running mean of normalized NH STE O 3 fluxes calculated: with the tropospheric column (TC) method across the e90 tropopause (solid black) and the 250 ppb O 3 iso- surface (solid red); with the lowermost stratosphere (LS) budget method using e90 tropopause (dashed black) and 250 ppb O 3 (dashed red) as the lower boundary; and with the exact flux across 100 hPa (solid green) and the residual circulation at 100 hPa (dashed green). For each method the mean values (as Tg/yr) and correlation coefficients of the interannual variability (lag = 0) with the TC-e90 method are given in the legend. (h) Results are from ECMWF-IFS Cycle 29 show much greater correlations than (i) results from Cycle 36. For a time lag of 3 months, the correlation coefficient of [w x O 3 ] increase to 0.97 (Cycle 29) and to 0.76 (Cycle 36). Is the residual vertical velocity a good proxy for Stratosphere-Troposphere Exchange of ozone? Juno Hsu & Michael Prather, UC Irvine Stratosphere-troposphere exchange (STE) of ozone is key in the budget of tropospheric ozone, in turn affecting climate forcing and global air quality. We investigate the ozone flux pathways in the lowermost stratosphere and compare three commonly used ozone flux diagnostics across the tropopause and nearby iso-surfaces of ozone and pressure by using the UCI chemistry-transport model driven by two different European Centre forecast fields (cycles 29 and 36). STE of ozone calculated based on the ozone mass budget of the lowermost stratosphere and of the tropospheric ozone column yields similar results. However, using the vertical residual velocity as a proxy at best captures the phase of the northern hemisphere (NH) seasonal cycle but fails to capture the interannual variability. The commonly exercised notion that the strength of Brewer-Dobson circulation (BDC) controls STE is not upheld. Climate-change BDC by itself cannot diagnose the ozone STE. Fig (z). The UCI CTM tested this hypothesis with an 8-year set of ECMWF forecast meteorology (years 2000-2007) using two very different versions: merged cycles 29/31 and the more recent cycle 36. The CTM resolution is T42L57 and uses standard UCI chemistry (Linoz v2 for the stratosphere). Cycle 36 has a much more stagnant lower stratosphere as is diagnosed by STE O 3 flux below. The classic diagnostic test of age-of-air vs. N 2 O (Strahan et al., 2011, JGR) is shown here for both cycles with observations approximated (green line).