Regional changes in the annual mean Hadley circulation in recent decades

Abstract

Previous studies suggest that the zonally averaged Hadley circulation (ZAHC) has experienced a robust poleward expansion, and its trend in intensity displays inconsistency among different data sets. This study examines changes in regional HC intensity and poleward edge using six reanalyses, outgoing longwave radiation, and precipitation data sets. HCs in six regions, including Africa (AFHC), the Indian Ocean (IOHC), the western Pacific (WPHC), the eastern Pacific (EPHC), South America (SAHC), and the Atlantic (ATHC), are investigated. Intensity trends in the Northern Hemisphere (NH) WPHC and ATHC and the Southern Hemisphere (SH) EPHC and ATHC are in agreement with each other in the six reanalyses. Furthermore, regional HCs in these domains appear to be intensifying, although not all of the reanalyses show statistically significant trends. For the poleward edge, its trend in the NH AFHC, IOHC, EPHC, SAHC, and ATHC is significantly larger than zero, and the northern HC poleward edge exhibits uniform poleward migrations in these five regions. In the SH, only the trend in the SAHC poleward edge is significantly different from zero. Furthermore, the trend in the SH SAHC poleward edge is significantly larger than those in the SH AFHC, IOHC, and ATHC. The results indicate that the poleward migration of the southern ZAHC poleward edge during recent decades that has been identified by previous studies may be attributed mainly to the poleward migration of the southern SAHC poleward edge. Further analyses suggest that changes in regional HC poleward edges could have a significant impact on regional precipitation anomalies.

Changes in the width of the tropical belt due to simple radiative forcing changes in the GeoMIP simulations

“…However, historical tropical expansion exhibits significant zonal asymmetries (Chen et al, 2014;Lucas and Nguyen, 2015), and some zonally asymmetric dynamics contribute to the longitudinal structure of the meridional overturning circulation (Karnauskas and Ummenhofer, 2014).…”

“…An expansion of the Hadley cells has been detected in reanalyses (Hu and Fu, 2007;Johanson and Fu, 2009;Stachnik and Schumacher, 2011;Davis and Rosenlof, 2012;Davis and Birner, 2013;Nguyen et al, 2013;Chen et al, 2014). Tropical expansion estimates based on reanalyses, however, may suffer from spurious trends and discontinuities in basic meteorological fields (Trenberth et al, 2001;Bengtsson et al, 2004).…”

Regional Widening of Tropical Overturning: Forced Change, Natural Variability, and Recent Trends

“…But outgoing longwave radiation is affected not only by changes in the tropical circulation but also by thermodynamic changes and is thus a poor measure of variability and change in the tropical overturning circulation, at least in the zonal mean (Waugh et al, ). The SLP changes associated with the HC are strongest over oceans (Schmidt & Grise, ), even though the tropical overturning circulation has clear impacts over both land and ocean (Chen et al, ; Freitas et al, ; Huang et al, ; Lucas & Nguyen, ; Zhao & Moore, ). For example, all of the world's hot deserts occur in or near subtropics.…”

The Arctic and Polar cells act on the Arctic sea ice variation

“…The third reanalysis product is the ERA Interim Reanalysis dataset from 1979 to present (Dee et al, 2011) with a horizontal 0.75-degree longitudeÁlatitude resolution and 37 vertical levels, retrieved from the website (www.ecmwf.int/ research/era/do/get/index). The fourth is the Japanese 55-yr Reanalysis (JRA-55) product from 1958 to present (Onogi et al, 2007;Ebita et al, 2011;Chen et al, 2014a) provided by the Japan Meteorological Agency with a horizontal 1.25-degree longitudeÁlatitude resolution and 37 vertical levels, retrieved from the website (www.rda. ucar.edu/datasets/ds628.1/).…”