Revisiting the Relationship among Metrics of Tropical Expansion

Abstract

There is mounting evidence that the width of the tropics has increased over the last few decades, but there are large differences in reported expansion rates. This is, likely, in part due to the wide variety of metrics that have been used to define the tropical width. Here we perform a systematic investigation into the relationship among nine metrics of the zonal-mean tropical width using preindustrial control and abrupt quadrupling of CO2 simulations from a suite of coupled climate models. It is shown that the latitudes of the edge of the Hadley cell, the midlatitude eddy-driven jet, the edge of the subtropical dry zones, and the Southern Hemisphere subtropical high covary interannually and exhibit similar long-term responses to a quadrupling of CO2. However, metrics based on the outgoing longwave radiation, the position of the subtropical jet, the break in the tropopause, and the Northern Hemisphere subtropical high have very weak covariations with the above metrics and/or respond differently to increases in CO2 and thus are not good indicators of the expansion of the Hadley cell or subtropical dry zone. The differing variability and responses to increases in CO2 among metrics highlights that care is needed when choosing metrics for studies of the width of the tropics and that it is important to make sure the metric used is appropriate for the specific phenomena and impacts being examined.

Disconnect Between Hadley Cell and Subtropical Jet Variability and Response to Increased CO2

“…The lack of correlations between the latitudes of the STJ and EDJ has been shown previously (Davis & Birner, 2017;Solomon et al, 2016;Waugh et al, 2018), and correlations between metrics of the jets are shown in Figure S2 of the supporting information. The lack of correlations between the latitudes of the STJ and EDJ has been shown previously (Davis & Birner, 2017;Solomon et al, 2016;Waugh et al, 2018), and correlations between metrics of the jets are shown in Figure S2 of the supporting information.…”

“…In all models there is a poleward movement of HC and EDJ in response to 4xCO 2 , there is a high correlation in the HC and EDJ response across models, and the slope of the linear fit between change across models is close to the multimodel mean regression coefficient for interannual variations in the piControl runs (see Figure 8 of Waugh et al, 2018). Note that this contrasts the consistencies between the HC edge and EDJ latitude responses.…”

“…For this reason, Figure 2. In Waugh et al (2018), STJ was calculated using the "adjusted_max" method in TropD_Metric_STJ, but here we use a quadratic "fit" method to find the latitude of the maximum value, making it easier to determine the strength of the STJ (see below). Note the color scales are different for each plot.…”

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

“…The climate in the tropics and subtropics is often described as being dominated by the meridional overturning Hadley circulation (HC). The HC is observed to be widening by about 0.2° per decade in each hemisphere since 1979 (S. M. Davis & Rosenlof, ; Fu et al, ; Grise et al, ; Y. Hu & Fu, ; Seidel & Randel, ; Seidel et al, ; Staten et al, ), and both theory and models suggest that this widening will continue during the coming century as a result of continued greenhouse gas (GHG) concentration increases (Chen et al, ; D ' Agostino & Lionello, ; Kim et al, ; Min & Son, ; Nguyen et al, ; Staten et al, ; Waugh et al, ). With over 1 billion people (~17% of Earth's human population) living within ±3° of the present HC edges (Doxsey‐Whitfield et al, ), diagnosing the underlying circulation dynamics associated with recent projections of subtropical aridification (Cook et al, ; Karnauskas et al, , ; Scheff & Frierson, ) is a timely challenge.…”

“…Traditional methods for defining the regional extent of the tropics include outgoing longwave radiation thresholds, sea level pressure (SLP) maxima, and tropopause‐based metrics. 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, ).…”