Polar stratospheric optical depth observed between 1978 and 1985

Observations of the stratospheric optical depth at 1.0 μm obtained for high latitudes are presented for a 7‐year period. Weekly averaged data determined from measurements made by the Stratospheric Aerosol Measurement (SAM) II experiment from October 1978 to October 1985 show that the overall yearly values in both polar regions are controlled by volcanic perturbations, with most volcanic effects being experienced in Arctic latitudes. The largest values over this period were observed after the eruption of El Chichon in 1982. Conservatively, peak values found in the Antarctic region were approximately 2.0×10 −2 and in the Arctic region about 5.5×10 −2 . We estimate probable values for these regions to be 2.6×10 −2 and 0.11, respectively. The Arctic values are almost 40 times greater than values observed during the first several months after the instrument was launched. The weekly averaged data also show the seasonal fluctuations due to microphysical and dynamical processes. Comparison of the optical depth record with a weekly averaged 50‐mbar temperature record indicates that polar stratospheric clouds are present in the southern high latitudes each year near this level from early June to early September. A depression observed in the optical depth record each austral spring season is believed to be the result of the downward displacement of particles caused by subsidence and sedimentation during the course of winter. Following the breakup of the vortex, optical depth values increase as aerosol is transported poleward. These features are noted to be present in the Arctic region as well, but on a smaller scale because of the satellite sampling methodology and the averaging scheme employed.