11:30 AM - 11:45 AM
[AAS03-10] Matching the weak temperature gradient and the quasi-geostrophic solutions in a shallow water model on a frictionless f-plane
Keywords:tropics-midlatitude boundary, weak temperature gradient system, quasi-geostrophic system, western boundary currents
The large-scale tropical atmosphere is governed by the weak temperature gradient (WTG) system, in which heating and horizontal divergence are in balance. By contrast, the large-scale midlatitude atmosphere is governed by the quasi-geostrophic (QG) system, in which the Coriolis force and the pressure gradient force are in balance. This difference of atmospheric behaviors are originated from two spatially-variant dimensionless numbers, the Rossby number and the Burger number, both of which depend on the strength of the Earth's rotational effects.
These regions under the WTG and QG systems should be continuously matched at a certain boundary. Therefore, in this study, we aim to theoretically investigate this matching between the WTG and QG solutions at this tropics-midlatitude (TM) boundary. In particular, here we solve a steady-state analytical solution for a smoothly connected QG and WTG systems in a meridional one-dimensional shallow water model on a frictionless f-plane.
In this solution, southerly winds accelerated by heating within the equatorial region are decelerated by radiative cooling in the subtropics. In the midlatitudes, because non-divergent geostrophic winds are dominant, this southerly wind cannot penetrate into the midlatitudes. Therefore, large-scale meridional winds cannot exist at the TM boundary, forming a downward branch of the Hadley circulation. Because the angular momentum originated from the equatorial region cannot be transported into the midlatitudes, a jet stream is generated on the TM boundary. The midlatitude atmospheric state is determined so that the QG solution smoothly matches with the tropical WTG solution, and at the same time, satisfies the boundary condition at the pole. To reproduce the meanderings of the jet stream, the WTG solution near the TM boundary must be further modified by latent heat release from the western boundary currents.
These regions under the WTG and QG systems should be continuously matched at a certain boundary. Therefore, in this study, we aim to theoretically investigate this matching between the WTG and QG solutions at this tropics-midlatitude (TM) boundary. In particular, here we solve a steady-state analytical solution for a smoothly connected QG and WTG systems in a meridional one-dimensional shallow water model on a frictionless f-plane.
In this solution, southerly winds accelerated by heating within the equatorial region are decelerated by radiative cooling in the subtropics. In the midlatitudes, because non-divergent geostrophic winds are dominant, this southerly wind cannot penetrate into the midlatitudes. Therefore, large-scale meridional winds cannot exist at the TM boundary, forming a downward branch of the Hadley circulation. Because the angular momentum originated from the equatorial region cannot be transported into the midlatitudes, a jet stream is generated on the TM boundary. The midlatitude atmospheric state is determined so that the QG solution smoothly matches with the tropical WTG solution, and at the same time, satisfies the boundary condition at the pole. To reproduce the meanderings of the jet stream, the WTG solution near the TM boundary must be further modified by latent heat release from the western boundary currents.