On the Causes of the Annual and Semi-Annual Variations of Radiance (or Temperature) from the Tropical Stratosphere

Abstract

Nimbus 3 and 4 satellites carried Satellite Infrared Spectrometers (SIRS-A and SIRS-B, respectively). One of the channels on each spectrometer measured the energy emitted from the stratosphere in a narrow wavelength interval in the middle of the 15-µm CO2 band. The measured radiance changes are indicative of temperature changes in the stratosphere. In the tropics, the annual and semi-annual march of radiance (or temperature) depends on the variations of solar energy absorbed by ozone and on dynamical influences. The radiance changes produced by solar heating of ozone were computed. To do this the concept of ?Newtonian cooling? was also utilized. The computed solar radiances were then compared with the observed radiances; the difference was attributed to dynamical factors. The observed radiances for SIRS-A had a total annual range which did not exceed 6 mW per (m2 ster cm?1). Moreover, the north-south gradient of radiance was very small and there was a strong tendency for the radiances to have a minimum at the equator during much of the year, especially at the solstices. This produces north-south gradients with opposite signs in the Northern and Southern Hemisphere tropics. By contrast, the solar-induced radiances have large annual amplitudes away from the equator, and would produce large radiance (or temperature) gradients. Also, the solar radiance gradients have the same sign on both sides of the equator, especially during the solstices. This would introduce large wind shears in the vertical, and possibly large horizontal wind shears across the equator. The observed radiance changes are often a residual between large solar radiance changes and large dynamical temperature changes. Near the equator the solar radiances agreed approximately with the observed radiances in the period April?July?October. But in November?January?March, dynamic factors dominated. At tropical latitudes away from the equator (e.g., at 2OS) dynamic factors almost completely balance the solar radiances in the annual cycle, leaving a small residual annual component in the observed radiances. The semi-annual amplitude at 20S is larger for the observed radiances than for the solar radiances; thus, the observed values are a combination of solar and dynamically induced temperature changes. The dynamical radiance changes suggest that the air was sinking at 30° latitude in winter and rising there in summer. Also, at the equator, the air was rising in January 1970.