Three-Dimensional Transport of the Ertel Potential Vorticity and N2O in the GFDL SKYHI Model

Huijun Yang Department of Geophysical Sciences, University of Chicago, Chicago, Illinois, and Department of Marine Science, University of South Florida, St. Petersburg, Florida

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Abstract

In this study the author investigates the 3D evolution of the Ertel potential vorticity (PV) and N2O in Northern Hemisphere winter from 1 to 10 February of model year 1984 in the GFDL SKYHI model. The diagnosis was done on three isentropic surfaces—800, 450, and 320 K—by using output from the GFDL SKYHI model, which has 1.2° × 1° horizontal resolution, 40 layers in vertical, and 60-s time resolution. The data output was taken twice daily. The high resolution Lagrangian Field Advection Model (FAM), which has no diabatic heating and diffusion, is used to study the evolution of these two fields. The 1° × 1° resolution in FAM gives reasonable results. The following results are found. 1) In the stratosphere there are two kinds of barriers, that is, subtropical barrier and polar barrier for both PV and N2O. The existence of the subtropical barrier is coincident with the subtropical jet. The subtropical barrier is more permeable than the polar barrier. Even though the tropopause acts like a barrier, there is substantial exchange between the troposphere and the stratosphere at 320 K. 2) The inside edge of the polar vortex is marked by a nearby high-PV ring, and the outside edge is indicated by the maximum gradient of N2O at 800 K. 3) On all three isentropic surfaces, both PV and N2O are conserved quite well. 4) Poleward transport from the Tropics to the high latitudes and equaterward transport from the high latitudes to the middle latitudes due to Rossby wave breaking takes place simultaneously and is captured remarkably well in FAM. 5) The polar vortex is kinematically isolated from outside both on the 800- and the 450-K surfaces, with a small amount of outside air entrainment into the edge of the polar vortex on the 450-K surface. The probability distribution function and the finite-time Lyapunov exponents are proven to be useful to characterize the structure and mixing properties of PV and N2O. 6) The transport and mixing channels between the Tropics and the high latitudes have been identified by the positive high Lyapunov exponents. 7) Three clear separate peaks in the PDFs identify with three distinct regions of N2O (tropical reservoir, surf zone, and polar vortex), bounded by the subtropical barrier and the polar barrier between them, suggesting that the mixing may occur separately in each region in the stratosphere. The implications of these findings are discussed briefly.

Abstract

In this study the author investigates the 3D evolution of the Ertel potential vorticity (PV) and N2O in Northern Hemisphere winter from 1 to 10 February of model year 1984 in the GFDL SKYHI model. The diagnosis was done on three isentropic surfaces—800, 450, and 320 K—by using output from the GFDL SKYHI model, which has 1.2° × 1° horizontal resolution, 40 layers in vertical, and 60-s time resolution. The data output was taken twice daily. The high resolution Lagrangian Field Advection Model (FAM), which has no diabatic heating and diffusion, is used to study the evolution of these two fields. The 1° × 1° resolution in FAM gives reasonable results. The following results are found. 1) In the stratosphere there are two kinds of barriers, that is, subtropical barrier and polar barrier for both PV and N2O. The existence of the subtropical barrier is coincident with the subtropical jet. The subtropical barrier is more permeable than the polar barrier. Even though the tropopause acts like a barrier, there is substantial exchange between the troposphere and the stratosphere at 320 K. 2) The inside edge of the polar vortex is marked by a nearby high-PV ring, and the outside edge is indicated by the maximum gradient of N2O at 800 K. 3) On all three isentropic surfaces, both PV and N2O are conserved quite well. 4) Poleward transport from the Tropics to the high latitudes and equaterward transport from the high latitudes to the middle latitudes due to Rossby wave breaking takes place simultaneously and is captured remarkably well in FAM. 5) The polar vortex is kinematically isolated from outside both on the 800- and the 450-K surfaces, with a small amount of outside air entrainment into the edge of the polar vortex on the 450-K surface. The probability distribution function and the finite-time Lyapunov exponents are proven to be useful to characterize the structure and mixing properties of PV and N2O. 6) The transport and mixing channels between the Tropics and the high latitudes have been identified by the positive high Lyapunov exponents. 7) Three clear separate peaks in the PDFs identify with three distinct regions of N2O (tropical reservoir, surf zone, and polar vortex), bounded by the subtropical barrier and the polar barrier between them, suggesting that the mixing may occur separately in each region in the stratosphere. The implications of these findings are discussed briefly.

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