Post tagged Gross moist stability

Understanding changes in the tropical Pacific atmospheric circulation with global warming

The weakening of the Walker circulation in the tropical Pacific is a robust response to global warming in climate models. This can have a global impact on climate, because the convection in the ascending branch of the Walker circulation triggers planetary scale waves that radiate to higher latitudes. In a recent article in the Journal of Atmospheric Sciences (Wills et al. 2017), we study the physical mechanisms responsible for the Walker circulation weakening in an idealized model. Here, we discuss how this work applies to the real-world climate system and how Walker circulation changes are related to tropical Pacific sea-surface temperature changes.

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Understanding the width of the ITCZ

The intertropical convergence zone (ITCZ) is narrow, but why? Was the ITCZ narrower or wider in past climates? How will the width of the ITCZ respond to global warming? These questions challenge our understanding of climate dynamics, and have implications for the impact of climate change in the tropics.

trmm_map_annual_mean_revised
Figure 1. The observed ITCZ: Average precipitation rate (1998-2014) over oceans from the Tropical Rainfall Measuring Mission (colours). The red contour maps where the vertical velocity in the mid-troposphere is zero, i.e. the boundary separating regions of ascending and descending air in the tropics. Vertical velocity data are from the ERA-Interim reanalysis.

As described in the previous blogpost, the ITCZ is a band of intense rainfall that circles the Earth (Fig. 1), moving north and south across the equator over the course of a year following the seasonal cycle of solar insolation. Averaged over a year, the centre of the ITCZ lies just north of the equator. Considerable research has focused on why the ITCZ sits at 6° north on average, and how the ITCZ position varies with climate. What has received comparatively little attention is the width of the ITCZ. Despite being of fundamental importance for controlling tropical climate and sea-surface temperatures (Pierrehumbert 1995), it is not clear what controls the ITCZ width nor how it should respond to changes in climate. Studies with climate models have noted that the ITCZ width depends on interactions between radiation and clouds (Voigt & Shaw 2015) and how the model represents sub-grid scale convection (Kang et al. 2009), but a physical understanding of why the ITCZ width is affected by these processes is lacking. Here we present results from Byrne & Schneider (2016) in which we combine basic theory and idealised climate-model simulations to investigate the physical processes determining the width of the ITCZ and its sensitivity to climate change.

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