The Brewer-Dobson circulation, the meridional overturning of mass in the stratosphere, is important for the distribution of gases in the stratosphere, such as ozone and water vapor, which impact surface climate. I will present a theory that relates the ideal tracer “age of air” to this circulation strength. Age can be determined from certain trace gases, and I have applied the theory to obtain the first data-based estimate of the global meridional overturning circulation of the stratosphere. When I compare this to models and reanalysis data, I find substantial disagreement. I will then show how age of air provides enough information to determine the full circulation by deriving a quantitative relationship between the age distribution and the adiabatic component of the circulation.
Warm-route versus cold-route interbasin exchange in the meridional overturning circulation – Why is the Atlantic saltier than the Pacific?
The interbasin exchange of the meridional overturning circulation (MOC) is studied in an idealized domain with two basins connected by a circumpolar channel in the southernmost region. Gnanadesikan’s (1999) conceptual model for the upper branch of the MOC is extended to include two basins of different widths connected by a re-entrant channel at the southern edge and separated by two continents of different meridional extents. Its analysis illustrates the basic processes of interbasin flow exchange either through the connection at the southern latitude of the long continent (“cold route”) or through the connection at the southern latitude of the short continent (“warm route”). A cold-route exchange occurs when the short continent is poleward of the latitude separating the sub-polar and sub-tropical gyre in the southern hemisphere, otherwise there is warm-route exchange. The predictions of the conceptual model are compared to primitive equation computations in a domain with the same idealized geometry forced by wind-stress, surface temperature relaxation and surface salinity flux. A visualization of the horizontal structure of the upper branch of the MOC illustrates the cold and warm routes of interbasin exchange flows. Diagnostics of the primitive equation computations show that the warm-route exchange flow is responsible for a substantial salinification of the basin where sinking occurs. This salinification is larger when the interbasin exchange is via the warm route, and it is more pronounced when the warm-route exchange flows from the wide to the narrow basin.
Dust in the Wind: Investigating Past and Present Dust Deposition in the Uinta Mountains, Utah
Eolian delivery of mineral dust impacts soil development, contributes to soil fertility, influences surface water chemistry, and alters snowpack albedo in high mountain ecosystems. This study focuses on past and present deposition of mineral dust in the alpine zone of the Uinta Mountains in northeastern Utah. Alpine soil profiles in the Uintas feature a ubiquitous layer of silt ~20 cm thick, indicating that dust deposition has been a significant long-term process in this environment. Four passive dust collectors were deployed in June, 2011, and an additional four were deployed in October, 2015. These collectors document an average dust flux of ~4 g/m2/yr, similar to values measured from snowpack samples in the Wind River (Wyoming) and San Juan (Colorado) Mountains. XRD analysis reveals that the dust is dominated by quartz, potassium feldspar, plagioclase, and illite. Some samples contain amphibole and chlorite. The dust is very well-sorted, with a median size of 8 μm. Geochemical records from lacustrine sediment cores reveal that the flux and properties of dust arriving in the Uinta Mountains have varied over the post-glacial period, likely in response to regional changes in aridity. A specially designed active sampler deployed at an elevation of 3700 m collects separate samples of NNW and SSE provenance. Differences in grain size distribution, mineralogy, and geochemistry of samples from contrasting wind directions indicate the importance of regional dust sources.