Hemispheric Trends and Interannual Variability of the Tropical Edge Latitudes Inferred from Observations and Climate Models

We present an updated analysis on the trends and variability of the tropical edge latitudes (TELs) inferred from satellite observations, state-of-the-art reanalyses, and recent chemistry-climate model simulations (CCMVAL2).   Previous studies have shown that GPS radio occultation (RO) provides a useful observational dataset for studying the changes in the latitudinal boundaries of the Hadley circulation.  In particular, the study by Ao and Hajj (2013) showed distinct differences between the Northern Hemisphere (NH) and the Southern Hemisphere during the period of 2002–2011, with the NH exhibiting a clear expanding trend while the SH exhibiting no significant trend at all.  Similar methodology is applied here to a longer set of observational data (2002–2015), and the results are compared with the ECMWF Reanalysis Interim, MERRA, MERRA2, and CCMVAL2.   We confirm the hemispheric differences observed after 2002.  The RO trends are consistent with the reanalyses, although the latter show clear differences with each other, especially in SH and after 2005.  Comparison between the reanalyses and CCMVAL2 in 1979–2005 shows that most CCMVAL2 models underestimate the expanding trends, in agreement with previous studies.  In addition, we examine the co-variability between TEL and ENSO and find that most models fail to capture the correct lag correlations, despite being forced by the observed sea surface temperatures.