Abstract
The total column ozone (TCO) amount varies with latitude, in part due to the difference in tropopause height between the tropics and midlatitudes. This dependency of TCO on latitude has been used to identify the latitudes of the tropical edges and to compute their variations in time. The previously reported poleward movement of the tropical edge latitudes computed from satellite TCO measurements over the past several decades is greater than 3° latitude per decade. This tropical widening rate is significantly larger than a number of independent estimates and if correct suggests a major deficiency in the representation of tropical widening in models. We revisit the previously used TCO tropical edge latitude diagnostic to extend it forward in time with a new data set and to assess its robustness through comparisons with independent tropical edge diagnostics. We find that the previous TCO-based tropical width timeseries contain a spurious jump, likely due to data inhomogeneities. After removal of this jump using an objective statistical breakpoint identification technique, TCO-based tropical widening is reduced to the point that it is not significantly different than other tropical widening estimates. The strong sensitivity of the TCO method to algorithmic choices, its out-of-phase seasonality in the Northern Hemisphere, and its lack of correlation with well-established tropical width metrics on interannual timescales support our conclusion that the TCO-based tropical width diagnostic as previously implemented is not a robust measure of tropical width.
Highlights
A number of different methodologies and characteristics of Earth’s atmosphere have been used to identify the latitudinal boundary between the tropics and extratropics and compute changes in its position with time (e.g., Davis and Rosenlof 2012; Lucas et al 2014; Seidel et al 2008)
We revisit the usage of total column ozone measurements for defining the width of the tropics and quantifying tropical widening rates, with the goal of better understanding the apparent discrepancy between the large TCO-based widening found by Hudson et al 2006 (H06)/ Hudson 2012 (H12) and other studies
The most valid comparison between H12 and our replication should be with the Total ozone mapping spectrometer (TOMS) data sets, because they were used by H12 along with the no longer available TOVS_NEURAL to fill in data gaps in the 1990s between the Nimbus-7 and Earthprobe records
Summary
A number of different methodologies and characteristics of Earth’s atmosphere have been used to identify the latitudinal boundary between the tropics and extratropics and compute changes in its position with time (e.g., Davis and Rosenlof 2012; Lucas et al 2014; Seidel et al 2008). The H03 method is an iterative approach that uses daily gridded TCO data and daily isentropic potential vorticity (PV) from the NCEP-NCAR reanalysis (Kalnay et al 1996) to compute the four ozone regimes (i.e., tropical, midlatitude, polar, and Arctic) and their boundaries.
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