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Dynamical Implications of the Cold Polar Tropopause Bias for Midlatitude Jets: Results from Simple Models, the IPCC Archive and Data Assimilation Experiments

Dynamical Implications of the Cold Polar Tropopause Bias for Midlatitude Jets: Results from Simple Models, the IPCC Archive and Data Assimilation Experiments

Justin Bagley and Eric DeWeaver

The extratropical surface westerlies are a critical component of the climate system, given their influence on local climate, air-sea gas exchange, ocean circulation, and so on. Accurate simulation of the strength, location, and seasonality of the surface westerlies is typically achieved through ad hoc "tuning" of surface and gravity wave drag, tuning which sometimes yields imperfect results. In the current NCAR model the Northern Hemisphere surface westerlies are too strong by a factor of two. Also, the tuning is resolution dependent, as surface westerlies usually shift poleward with increased resolution. Beyond these specific problems, the tuning of the westerlies may obscure fundamental relationships between the strength and location of the westerlies, heat and momentum fluxes due to stormtracks and stationary waves, and other dynamical factors. These relationships may be critical for understanding the response of surface winds to changes in climate forcing.

Using model simulations from the IPCC archive, Lorenz and DeWeaver demonstrated a close association between the radiatively forced rise in tropopause height due to global warming and the concurrent poleward shift of the surface westerlies. This association was reproduced in a simple "dry GCM" in which tropopause height could be externally controlled. The association is of interest for the simulation of surface westerlies because most climate models have a cold bias at the polar tropopause, which is equivalent to a high bias in tropopause height. Thus, we speculate that correcting the cold polar tropopause bias should result in a (possibly undesirable) southward shift of the surface westerlies.

Since its introduction in atmospheric science, the ensemble Kalman filter (EnKF) has typically been employed as a state estimation tool for data assimilation. Here we use the EnKF of the NCAR Data Assimilation Research Testbed (DART) to selectively correct the cold polar tropopause bias in the NCAR Community Atmosphere Model(CAM), for the purpose of identifying, quantifying, and analyzing the impacts of the bias.

We first identify the cold polar tropopause bias through inspection of the zonal mean temperature shift that occurs at each assimilation step. By selectively assimilating only observations poleward of 66N, and between 300 and 150mb, this bias is eliminated. Through evaluation of statistical relationships found in the ensemble, we find that the direct impact of assimilating polar tropopause observations to be localized near the region of the observations. However, the resulting zonal mean circulation throughout the northern hemisphere has also been influenced by the assimilation through secondary effects. The consequences of this influence include equatorward shifts in the extratropical surface westerlies and midlatitude jet, as well as reduced model bias in the zonal mean wind field throughout the hemisphere. These results are broadly consistent with the association between jet location and tropopause height found in the IPCC simulations.