Effect of Gravity Waves From Small Islands in the Southern Ocean on the Southern Hemisphere Atmospheric Circulation

The effect of small islands in the Southern Ocean on the atmospheric circulation in the Southern Hemisphere is considered with a series of simulations using the NASA Goddard Earth Observing System Chemistry-Climate Model in which the gravity wave stress generated by these islands is increased to resemble observed values. The enhanced gravity wave drag leads to a 2K warming of the springtime polar stratosphere, partially ameliorating biases in this region. Resolved wave drag declines in the stratospheric region in which the added orographic gravity waves deposit their momentum, such that changes in gravity waves are partially compensated by changes in resolved waves, though resolved wave drag increases further poleward. The orographic drag from these islands has impacts for surface climate, as biases in tropospheric jet position are also partially ameliorated. These results suggest that these small islands are likely contributing to the missing drag near 60 degrees S in the upper stratosphere evident in many data assimilation products. Plain Language Summary The Southern Ocean contains a number of small islands that cannot be explicitly resolved by contemporary climate models. However, satellite observations indicate that these islands launch gravity waves, and it has been suggested that these gravity waves are important for the Southern Hemisphere atmospheric circulation. We modify the representation of these small islands in the Southern Ocean in the gravity wave scheme of a climate and find that model biases are generally reduced. These results suggest that these small islands are likely contributing to the missing drag near 60 degrees S from 40km to 50km above the surface evident in many data assimilation products.