The middleworld serves as a critically important, seasonally variable conduit that governs coupling of the TTL and subtropical jet structure to the middle and high latitudes. In the late winter, this segment of the stratosphere links the tropics above 390 K with high latitudes and couples the tropical upper troposphere through the subtropical jet structure (that dictates isentropic exchange between the troposphere and the stratosphere) at surfaces below 350 K. Key reversals in net meridional flow into and through the middleworld occur in the spring and fall such that concerted flow from the tropics northward in winter give way to regions of equatorward flow in early summer. The equatorward transport in the midlatitude summer middleworld is a result of monsoon circulation.
As evidence from both satellite observations and previous aircraft missions mounts (STRAT, POLARIS, SOLVE, CWVCS, CRYSTAL-FACE, etc.), the importance, seasonal idiosyncrasies, and complexity of middleworld dynamics becomes increasingly evident. So too does the imperative to build a framework of understanding of this region, the lack of which continuously frustrates our ability to predict the behavior of mid-latitude ozone over the coming decades as well as our ability to predict changes in the structure of the TTL and subtropical jet in response to forcing by CO2, CH4, HCFCs, etc. The middleworld that lies between the tropopause and the 380 K θ surface is a convergence zone between tropospheric and stratospheric air (e.g., Holton et al., 1995; Hoskins, 1991). Mixing ratios of ozone and water vapor can span an order of magnitude or more in this region purely due to transport. Therefore, examining transport into the middleworld will offer valuable insight into future changes in ozone at mid- and high latitudes. In particular, a key question remains as to whether the long-term downward trends in mid-latitude column ozone result from chemical loss or whether transport from low latitudes is responsible (WMO, 2003). The issue is critical for properly forecasting future trends in UV dosage levels.