Total column ozone trends from the NASA Merged Ozone time series 1979 to 2021 showing latitude-dependent ozone recovery dates (1994 to 1998)

Abstract

Monthly averaged total column ozone data (ΩMOD(t,θ)) from the NASA Merged Ozone Data Set (MOD) were examined to show that the latitude-dependent (θ) ozone depletion turnaround dates (TA(θ)) range from 1994 to 1998. TA(θ) is defined as the approximate date when the zonally averaged ozone ceased decreasing. ΩMOD data used in this study were created by combining data from Solar Backscattered Ultraviolet instruments (SBUV/SBUV-2) and the Ozone Mapping and Profiler Suite (OMPS-NP) from 1979 to 2021. The newly calculated systematic latitude-dependent hemispherically asymmetric TA(θ) shape currently does not appear in the suite of chemistry–climate models that are part of the Chemistry–Climate Model Validation Activity (CCMVal), which combines the effects of photochemistry, volcanic eruptions, and dynamics in their estimate of ozone recovery. Trends of zonally averaged total column ozone in percent per decade were computed before and after TA(θ) using two different trend estimate methods that closely agree, Fourier series multivariate linear regression and linear regression on annual averages. During the period 1979 to TA(θ), the most dramatic rates of Southern Hemisphere (SH) ozone loss were PD=-10.9±3 % per decade at 77.5∘ S and -8.0±1.1 % per decade at 65∘ S, which is about double the Northern Hemisphere (NH) rate of loss of PD=-5.6±4 % per decade at 77.5∘ N and 4.4±1 % per decade at 65∘ N for the period 1979 to TA(θ). After TA(θ), there was an increase at 65∘ S of PD=1.6±1.4 % per decade with smaller increases from 55 to 25∘ S and a small decrease at 35∘ N of -0.4±0.3 % per decade. Except for the Antarctic region, there only has been a small recovery in the SH toward 1979 ozone values and almost none in the NH.