Regionally Varying Assessments of Upper-Level Tropical Width in Reanalyses and CMIP5 Models Using a Tropopause Break Metric

Changes in tropical width can have important societal and economic consequences in a variety of sectors including natural ecosystems, agriculture, and health. Observations suggest the tropics have expanded during the past 30 years due to increased greenhouse gas concentrations in the atmosphere although studies have not agreed on the magnitude of these observed changes. Climate model projections of the future have also indicated a marginal to significant expansion of the tropics and show similar uncertainty in its magnitude. This study utilizes an objective tropopause break method that does not rely on zonal means to define the extent of the tropics. The location of the tropopause break and the associated upper-level boundary between tropical and extratropical air is associated with enhanced stratosphere to troposphere exchange and mixing, with implications to the chemical composition of the stratosphere. Here we show regional variations in the width of the upper-level tropics. The objective methodology compares well with a simple frequency-based methodology applied to radiosonde data, but fails when the tropopause height is changing, as is projected in the future. Modern reanalyses show significant contraction of the tropics over the Eastern Pacific between 1981 and 2015, and slight but significant expansion in other regions. The width of the tropics in historical and future climates simulated by six global climate models, shows similarities and differences with prior work that has only utilized zonal mean width metrics. The models perform well in representing the climatological location of the tropical boundary in the past which gives confidence when using future projections. Significant narrowing trends across the east Pacific lead to zonal mean narrowing in the past contradicting prior work which is attributed to the use of instantaneous data and the ability to create zonally varying locations. In climate model projections of the future, we see a spread in the width projections, especially in the southern hemisphere. A narrowing is projected across the east Pacific and northern hemisphere Americas which could have large impacts on weather and climate in the region. We propose that this objective method that uses instantaneous data rather than monthly-mean and does not require zonal mean data is more appropriate for identifying upper-level tropical width trends. The zonally varying changing tropical width must be connected with local and regional changes to weather, climate, ecosystems, public health and other sectors.