Regional Variations in Gravity Waves, Latent Heating, and the Tropical Circulation

Convective latent heating is an efficient generator of atmospheric gravity waves. Convectively generated gravity waves emanating from rain storms are observed far from their sources and at all levels in the atmosphere: At the surface, in the stratosphere, and even beyond in the mesosphere and thermosphere. They are common in the tropics, but are also observed at mid-latitudes and in all seasons. Convectively generated gravity waves transport momentum and drive remote changes in the circulation where they dissipate, and through this process they link fine-scale precipitation events to mesoscale and global-scale circulation changes.

At tropical latitudes, convectively generated gravity waves occur on a particularly broad range of scales and frequencies. Collectively the full spectrum of gravity waves drives the quasi-biennial oscillation (QBO) in lower stratospheric zonal-mean winds. The key characteristics of the QBO are zonal mean winds that oscillate from easterly to westerly with an average period of 28 months. The circulation is not locked to the annual cycle, but rather the period is inversely related to atmospheric wave momentum transport, or more specifically to the wave momentum forcing. The QBO has known influences on seasonal predictability, but global seasonal prediction models still severely under-resolve gravity waves and their effects on the circulation. Observations show a links between the phase of the El Nino Southern Oscillation (ENSO) and the period and amplitude of the QBO. Additional observational evidence suggests a causal link between the phase of the QBO and modulation in the strength of intraseasonal precipitation variability in the Madden-Julian Oscillation (MJO).

Global characterizations of gravity waves and their momentum transport remain an observational challenge due to their small scales, high frequencies, and intermittent occurrences. Idealized models therefore play a major role in our understanding of tropical gravity waves and their effects on circulation. We present results of idealized model simulations of realistic gravity waves generated by observed precipitation and cloud variability. Regional changes in gravity wave generation and gravity wave drag and their relationships to ENSO and MJO precipitation and circulation patterns are examined. ENSO changes not only the regional pattern of tropical gravity wave sources, but also changes the wave propagation properties and wave effects on the stratospheric circulation. Gravity waves in the model occur in strong, localized, intermittent wave packets that break at lower altitudes than commonly assumed. Gravity wave occurrences above convection near the tropopause are not only tied to regional variations in precipitation, but also show a strong dependence on regional tropopause wind patterns. Our idealized model studies of waves generated by tropical convection show how global-scale waves affect the regional patterns in gravity wave occurrence and affect the net upward flux of zonal momentum into the stratosphere.