Is Missing Orographic Gravity Wave Drag near 60°S the Cause of the Stratospheric Zonal Wind Biases in Chemistry–Climate Models?

Abstract

Nearly all chemistry-climate models (CCMs) have a systematic bias of a delayed springtime breakdown of the Southern Hemisphere (SH) stratospheric polar vortex, implying insufficient stratospheric wave drag. In this study the Canadian Middle Atmosphere Model (CMAM) and the CMAM Data Assimilation System (CMAM-DAS) are used to investigate the cause of this bias. Zonal wind analysis increments from CMAM-DAS reveal systematic negative values in the stratosphere near 608S in winter and early spring. These are interpreted as indicating a bias in the model physics, namely, missing gravity wave drag (GWD). The negative analysis increments remain at a nearly constant height during winter and descend as the vortex weakens, much like orographic GWD. This region is also where current orographic GWD parameterizations have a gap in wave drag, which is suggested to be unrealistic because of missing effects in those parameterizations. These findings motivate a pair of free-running CMAM simulations to assess the impact of extra orographic GWD at 608S. The control simulation exhibits the cold-pole bias and delayed vortex breakdown seen in the CCMs. In the simulation with extra GWD, the cold-pole bias is significantly reduced and the vortex breaks down earlier.Changes in resolved wave drag in the stratosphere also occur in response to the extra GWD, which reduce stratospheric SH polar-cap temperature biases in late spring and early summer. Reducing the dynamical biases, however, results in degraded Antarctic column ozone. This suggests that CCMs that obtain realistic column ozone in the presence of an overly strong and persistent vortex may be doing so through compensating errors.

Dynamics of Orographic Gravity Waves Observed in the Mesosphere over the Auckland Islands during the Deep Propagating Gravity Wave Experiment (DEEPWAVE)

“…This small change substantially reduced the model's coldpole bias in austral winter but somewhat degraded its ozone simulations. Alexander and Grimsdell (2013) later extended the AIRS analysis to other subantarctic islands and different austral winters, finding deep gravity waves to be common over many of the islands they studied, and that estimated wave momentum fluxes could explain some, but perhaps not all, of the cold-pole drag deficit inferred from modeling studies such as McLandress et al (2012).…”

“…Alexander et al (2009) inferred significant momentum flux deposition from these waves and, since global models typically treat grid cells containing small islands as ocean rather than land, they speculated that omission of gravity wave drag from small subantarctic islands might explain some or all of the stratospheric ''cold pole'' bias in global models during austral winter (Butchart et al 2011). McLandress et al (2012) tested this idea by inserting artificial subgrid-scale orography into their model's orographic gravity wave drag parameterization near 608S. This small change substantially reduced the model's coldpole bias in austral winter but somewhat degraded its ozone simulations.…”

“…Unfortunately these gravity wave perturbations occur near (and in some cases beyond) the resolution and noise limits of the sensor, so our knowledge of deep gravity wave dynamics over islands remains fragmented. Likewise, global models cannot resolve these waves and presently resort to simple augmentations to existing parameterizations to assess potential effects (McLandress et al 2012). More recently, high-resolution linear and nonlinear models have been used to model orographic gravity waves from South Georgia, revealing deep propagation of waves into the stratosphere and drag effects due to wave breaking on the stratospheric circulation (Jiang et al 2014;Vosper 2015).…”

Stratospheric gravity waves at Southern Hemisphere orographic hotspots: 2003–2014 AIRS/Aqua observations

“…In the winter hemisphere orographic and jet sources play a more important role, and small-scale orographic hotspots may provide a significant contribution to the total gravity wave drag that is currently not well-represented in global climate models McLandress et al, 2012;Alexander and Grimsdell, 2013;Vosper, 2015). More comprehensive observations may help to develop and improve parameterizations to better incorporate the wave drag even for such small sources.…”

Upper atmospheric gravity wave details revealed in nightglow satellite imagery

“…Suomi NPP | VIIRS | Day/Night Band | nocturnal observations | airglow G ravity waves (1) play a central role in the atmospheric circulation (2-4) at space and time scales ranging from regional weather to global climate (5). The momentum imparted by wave breaking modulates the upper atmospheric wind flow, which in turn influences weather and climate patterns through myriad coupling processes (6).…”