|Title||The interplay of internal and forced modes of Hadley Cell expansion: lessons from the global warming hiatus|
|Publication Type||Journal Article|
|Year of Publication||2018|
|Authors||Amaya D.J, Siler N., Xie SP, Miller AJ|
|Type of Article||Article|
|Keywords||belt width; black carbon; circulation; climate model; Climate variability; El Nino Southern; global; Global warming hiatus; Hadley Cell expansion; hemisphere tropical expansion; internal variability; Meteorology & Atmospheric Sciences; oscillation; ozone depletion; pacific; Pacific decadal oscillation; precipitation; reanalyses; southern-hemisphere|
The poleward branches of the Hadley Cells and the edge of the tropics show a robust poleward shift during the satellite era, leading to concerns over the possible encroachment of the globe's subtropical dry zones into currently temperate climates. The extent to which this trend is caused by anthropogenic forcing versus internal variability remains the subject of considerable debate. In this study, we use a Joint EOF method to identify two distinct modes of tropical width variability: (1) an anthropogenically-forced mode, which we identify using a 20-member simulation of the historical climate, and (2) an internal mode, which we identify using a 1000-year pre-industrial control simulation. The forced mode is found to be closely related to the top of the atmosphere radiative imbalance and exhibits a long-term trend since 1860, while the internal mode is essentially indistinguishable from the El Nio Southern Oscillation. Together these two modes explain an average of 70% of the interannual variability seen in model "edge indices" over the historical period. Since 1980, the superposition of forced and internal modes has resulted in a period of accelerated Hadley Cell expansion and decelerated global warming (i.e., the "hiatus"). A comparison of the change in these modes since 1980 indicates that by 2013 the signal has emerged above the noise of internal variability in the Southern Hemisphere, but not in the Northern Hemisphere, with the latter also exhibiting strong zonal asymmetry, particularly in the North Atlantic. Our results highlight the important interplay of internal and forced modes of tropical width change and improve our understanding of the interannual variability and long-term trend seen in observations.
|Short Title||Clim. Dyn.|
In this study, we sought to identify the leading modes of internal and forced tropical width variability without any a priori assumptions about what those modes might look like. Using a Joint Empirical Orthogonal Function (EOF) approach, we found that the leading internal mode is closely associated with ENSO variability, the leading forced mode is related to top of the atmosphere (TOA) radiative forcings, and each mode is characterized by symmetric poleward expansion of the climatological Hadley Cells. The forced EOF showed an asymmetry in favor of enhanced Southern Hemisphere (SH) tropical expansion relative to the Northern Hemisphere (NH) . This is possibly due to asymmetric ozone forcing (Kang et al. 2011; Polvani et al. 2011b; Min and Son 2013; Tao et al. 2016), but could also be the result of the minimal observed warming in the Southern Ocean relative to the global average (Li et al. 2013; Armour et al. 2016; Hwang et al. 2017), which may act to strengthen the equator-to-pole temperature gradient, thereby strengthening thermal winds and shifting eddy-momentum flux convergence poleward in the SH (Lu et al. 2008).