The past decade has witnessed increasing scientific focus on detailed aspects of tropical climate at and above the Earth’s surface, specifically on the location of the boundary between the tropical and extratropical zones.  A growing body of literature addresses the possibility of changes in the position and strength of the Hadley cell, jet streams, and zonal-mean atmospheric circulation. These changes could manifest as changes in surface climate, particularly precipitation, and have important societal and ecological consequences. A number of studies have identified poleward movement in the boundary between tropical and extratropical zones; this phenomenon has been referred to as “tropical widening” or “expansion of the tropical belt.”

Tropical widening, at a rate of up to several degrees latitude per decade over the past several decades, has been estimated using a variety of methodologies applied to satellite observations, in situ measurements, and meteorological re-analyses. Although there is a basic understanding of the factors controlling the properties of the Hadley cell and jet streams, there is a large spread among these tropical widening estimates, and there is some evidence that global climate model simulations do not capture the magnitude of observed widening. It is not clear whether the large range of tropical belt width and widening estimates reflects the differing physical properties measured by different definitions of the tropical belt edge location (e.g., definitions based on surface versus upper-air parameters), differences among datasets, or the difference between the hemispheres, seasons, and time periods studied.

Because of potentially adverse societal and ecological impacts, such as shifts in the subtropical dry zones and associated changes in mid-latitude droughts, it is important to understand the causes of tropical width changes, both in the past and in the future. Several different mechanisms of tropical width change have been proposed, including atmospheric composition changes (e.g., greenhouse gases, ozone depletion, aerosols) and internal variability from both the ocean and atmosphere. A better understanding of the relative importance of these proposed drivers is needed to enhance future predictability and help in explaining why observed tropical widening has apparently been greater than expectations from global climate model simulations. Tropical widening also corresponds to shifts in mid-latitude winds that drive ocean circulation, and it has been hypothesized to influence ocean uptake of carbon dioxide and to modulate ocean-cryosphere interactions.

This Chapman Conference will bring together scientists from a wide range of backgrounds to explore and summarize the state of the science and identify future research directions relevant for the problem of tropical widening. A particular emphasis will be on the multidisciplinary nature of the problem with the goal of enabling cross-fertilization among the wide range of climate science subtopics represented by conference participants. To facilitate this, the following four session topics are planned:

1. What determines the width of the tropical belt?

2. How and why has the tropical width changed in the past?

3. How and why might the tropical width change in the future?

4. What are the impacts for the oceans, cryosphere, hydrologic cycle, human society, and ecosystems?