Widening of the tropical belt in a changing climate

A fundamental feature of the Earth’s climate is the distinction between the tropical and extratropical regimes. However, the boundaries of the tropics are not uniquely defined and vary among scientific disciplines. In astronomy and cartography, the edges of the tropical belt are the Tropics of Cancer and Capricorn, at latitudes of ~23.5 degrees north and south, where the sun is directly overhead at solstice. They are determined by the tilt of the Earth’s axis of rotation relative to the planet’s orbital plane, and their location varies slowly, predictably and very slightly — by about 2.5 degrees latitude over 40,000 years. In climatology, on the other hand, there are several indicators of the boundaries of the tropics, and they do not all necessarily yield the same location. Moreover, their positions vary by much larger amounts and much more rapidly and unpredictably than the astronomically defined tropics. Climatologists and geographers have traditionally defined the tropics using classification systems, notably that of Köppen1, based on surface temperature and precipitation patterns. Tropical temperatures are warm, and, except for the major monsoon regions, both seasonal and day-to-day changes are small compared with extratropical climates. Another important feature of the tropics is the prevalence of rain in the moist inner tropical regions near the equator, as distinct from the dry conditions in subtropical regions. This precipitation pattern is largely determined by large-scale atmospheric wind patterns, known as the Hadley circulation, which manifests itself in several ways throughout the atmosphere, as depicted in Fig. 1. The ascending branch of the Hadley circulation, in equatorial regions, carries moisture into the air, promoting rainfall, whereas the descending branches, at the edges of the tropical belt, are notably drier. Within the Hadley circulation cell, atmospheric mass in the lower atmosphere moves towards the equator, whereas outside the cell it moves toward the poles. The latitude at which the net north–south flow is zero can be considered the poleward extent of the Hadley cell and therefore can be used to estimate the width of this tropical circulation. Within the tropical belt, surface winds generally blow from east to west, whereas in midlatitudes they blow from west to east and intensify upward from the surface to form the jet streams. Two jets streams are identifiable in each hemisphere, although they are not always well separated. The subtropical jet, located at the poleward boundaries of the Hadley cell, is the more permanent feature, but the polar front (or eddy-driven) jet, at higher latitudes, is intimately connected with the former through dynamical processes2,3. Thus the distance separating the two subtropical jets on both sides of the equator is another measure of the boundaries of the tropics, and variations in that distance may be linked to aspects of the polar front jets. Within the stratosphere, a major atmospheric flow pattern is the Brewer–Dobson overturning circulation, which consists of largescale upward motion in the tropics and descending motion at higher latitudes. This circulation system carries stratospheric ozone from the tropics poleward, resulting in higher ozone concentrations in the extratropics, allowing for an ozone-based demarcation of the width of the tropics. The tropopause, which separates the troposphere below from the stratosphere above, is higher over the tropics than elsewhere, and it, too, can serve to identify a distinctly tropical region. Thus from various perspectives, climate scientists find clear distinguishing features of the tropics that can be used to estimate the width of this climatic zone. Several recent studies suggest that Widening of the tropical belt in a changing climate