ITCZ shifts and interhemispheric temperature contrasts

Some paleoproxy papers came out during my graduate studies that showed north-south shifts of the Atlantic ITCZ to be closely correlated to centennial and millenial swings in Greenland temperature. I was fascinated for two reasons. First, it aligned with my dissertation work on the Atlantic ITCZ that showed its north-south position to be acutely senstive to changes. Second, the prevailing thinking of the time was that shifts in tropical convection drove changes to the high latitudes through atmospheric teleconnections, but the paleo evidence suggested otherwise - the high latitudes appeared to be driving tropical convection shifts. My speculation at the time was that high-latitude cooling ‘bled’ into the tropics (motivated by this paper on the temperature effects of the Laurentide ice sheet) , and the resulting cross-equatorial gradient in SST led to a southward ITCZ shift similar to what is observed in the interannual variability.

During my postdoc at the University of Washington, I played around with paleoclimate forcings in an atmospheric general circulation model coupled to a slab ocean to see how they would affect the ITCZ. David Battisti suggested adding sea ice in the North Atlantic (his hypothesis was that sea ice acts as an amplifier for North Atlantic cooling events). To my surprise, it led to a significant southward shift in the Atlantic ITCZ: we reported this result in Chiang et al 2003. In Chiang and Bitz (2005), we examined in detail this high latitude connection to the tropical ITCZ. We proposed a wind-evaporation-SST mechanism to for the extratropical cooling to communicate to the marine tropics, and linked the ITCZ shifts to cross-equatorial energy transports. Tony Broccoli and co-authors independently came to a similar conclusion with regards to the ITCZ shift and energy transports . This result was embraced by the paleo community - see, for example, Chapter 3 of Wally Broecker’s book on glacial cycles.

The climate dynamics community also picked up on this result, and it led to the development of energy flux view of ITCZ shifts starting with these works by Sarah Kang and co-authors. This line of thinking subsequently expanded into a cottage industry within the climate dynamics community. My then-student Andrew Friedman and I wrote a review paper on this development, more from a paleo and applications perspective; Tapio Schneider and colleagues wrote a more theoretical review. Among the more recent developments is the formulation of a 2-D energy flux method, allowing for examining regional tropical convection shifts. This was a notable instance of paleoclimate science influencing the direction of modern-day climate dynamics.

In collaboration with several students and postdocs, we applied this idea to different climate scenarios. Anthropogenic sulfate aerosol forcing, largely concentrated in the northern hemisphere, provided a extratropical cooling effect: Ching Yee Chang and I found that it led to a notable southward shift in the Atlantic ITCZ in coupled model simulations of the 20th century (Chang et al. 2011, Chiang et al. 2013). Abby Swann proposed that northern hemisphere afforestation could lead to a similar effect, in this case a northern hemisphere warming and northward ITCZ shift (Swann et al. 2012). Yuwei Liu focused on an abrupt cooling event in the late 1960’s over the high latitude north Atlantic, showing that it caused weakening of the Eurasian and West African monsoons (Liu et al. 2012, 2014). Ivana Cvijanovic examined the detailed atmospheric energetics associated with a high latitude North Atlantic cooling event, including the role of radiative feedbacks (Cvijanovic and Chiang 2013). Yue Fang examined the effect of an interhemispheric thermal gradient on the tropical Pacific, finding that it modulates ENSO activity (Fang et al. 2008) and also changed the mean east-west gradient and even the cold tongue annual cycle (Chiang et al. 2008). Shih-Yu Lee examined the effect of the Laurentide ice sheet on the tropical Pacific, finding - to our surprise - topographic effects to be quite pronounced, in addition to the thermal effects (Lee et al. 2015).

Encouraged by these applications, Andrew Friedman and I (Friedman et al. 2013) proposed that the interhemispheric thermal contrast - i.e. the surface temperature difference between hemispheres - could provide a useful index of climate change, and one that is distinct from the well-known global mean surface temperature index (see figure 2 from the paper) (of course, energy contrasts would have been a more appropriate metric from a physical standpoint, but one that is hard to quantify). Unlike the latter, the interhemispheric thermal contrast stayed relatively flat over the 20th century, only starting to rise (i.e. relatively warmer north) towards the last 1/4 of it; the reason for this behavior is that anthropogenic aerosols effectively counter the influence of greenhouse gases in the thermal contrast. In the future however, with aerosols projected to decrease and greenhouse gas concentrations projected to increase further, this rising trend will continue unabated. As outlined in a book chapter (Chiang 2016), this may have consequences for tropical rainfall in the coming century. More recently, Friedman et al. (2020) untook a formal detection and attrbution analysis of the interhemspheric thermal contrast over the historical period.

Chang, C.Y., Chiang, J.C.H., Wehner, M.F., Friedman, A.R. and Ruedy, R., 2011. Sulfate aerosol control of tropical Atlantic climate over the twentieth century. Journal of Climate, 24(10), pp.2540-2555.

Chiang, J.C.H. , Biasutti, M. and Battisti, D.S., 2003. Sensitivity of the Atlantic intertropical convergence zone to last glacial maximum boundary conditions. Paleoceanography, 18(4).

Chiang, J.C.H. and Bitz, C.M., 2005. Influence of high latitude ice cover on the marine Intertropical Convergence Zone. Climate Dynamics, 25(5), pp.477-496.

Chiang, J.C., Fang, Y. and Chang, P., 2008. Interhemispheric thermal gradient and tropical Pacific climate. Geophysical Research Letters, 35(14).

Chiang, J.C., Chang, C.Y. and Wehner, M.F., 2013. Long-term behavior of the Atlantic interhemispheric SST gradient in the CMIP5 historical simulations. Journal of Climate, 26(21), pp.8628-8640.

Chiang, J.C., 2016. The Interhemispheric Pattern and Long-Term Variations in the Tropical Climate over the 20th and 21st Centuries. In Climate Change: Multidecadal and Beyond (pp. 255-271).

Cvijanovic, I. and Chiang, J.C., 2013. Global energy budget changes to high latitude North Atlantic cooling and the tropical ITCZ response. Climate dynamics, 40(5), pp.1435-1452.

Fang, Y., Chiang, J.C. and Chang, P., 2008. Variation of mean sea surface temperature and modulation of El Niño–Southern Oscillation variance during the past 150 years. Geophysical research letters, 35(14).

Friedman, A.R., Hwang, Y.T., Chiang, J.C. and Frierson, D.M., 2013. Interhemispheric temperature asymmetry over the twentieth century and in future projections. Journal of Climate, 26(15), pp.5419-5433.

Friedman, A.R., Hegerl, G.C., Schurer, A.P., Lee, S.Y., Kong, W., Cheng, W. and Chiang, J.C., 2020. Forced and unforced decadal behavior of the interhemispheric SST contrast during the instrumental period (1881–2012): Contextualizing the late 1960s–early 1970s shift. Journal of Climate, 33(9), pp.3487-3509.

Liu, Y. and Chiang, J.C.H., 2012. Coordinated abrupt weakening of the Eurasian and North African monsoons in the 1960s and links to extratropical North Atlantic cooling. Journal of Climate, 25(10), pp.3532-3548.

Liu, Y., Chiang, J.C., Chou, C. and Patricola, C.M., 2014. Atmospheric teleconnection mechanisms of extratropical North Atlantic SST influence on Sahel rainfall. Climate dynamics, 43(9), pp.2797-2811.

Lee, S.Y., Chiang, J.C. and Chang, P., 2015. Tropical Pacific response to continental ice sheet topography. Climate Dynamics, 44(9), pp.2429-2446.

Swann, A.L., Fung, I.Y. and Chiang, J.C., 2012. Mid-latitude afforestation shifts general circulation and tropical precipitation. Proceedings of the National Academy of Sciences, 109(3), pp.712-716.