Evaluating blocking indices and investigating eddies’ response to static stability

Abstract

This dissertation addresses two problems in the mid-latitude jet dynamics, namely evaluating indices of blocking anticyclone and investigating eddies' response to static stability. Changes in frequencies of blocking anticyclones are sometimes used to explain changes in surface hot extremes. Several indices have been developed to measure them though there is no consensus. Here, we linearly regress interannual variations of hemispheric continental summer surface hot extreme area on the corresponding variations of blockings in the ERA-Interim reanalysis data and use cross-validation test error to measure the correlation between blockings and extremes. Summing over land and ocean, relative mean-square validation error is at least 0.91 (relative to no knowledge of blocking) for existing indices defined on 500-hPa geopotential height. When excluding blockings over ocean and optimizing parameters/thresholds and partly by using anomaly-based or anomaly-reversal hybrid indices, this can be largely reduced to 0.55. This framework helps to quantify, with an uncertainty estimate, the association of blockings with hot extremes and other extremes as well. Equilibrated eddies' response to static stability has not been investigated in a well-controlled experiment with unchanged jet. With iterative use of linear response function of an idealized dry atmosphere, we control the zonal wind change mostly within 0.2 m/s, when near-surface temperature cools by over 2 K. In such experiment, energy-containing zonal scale decreases by 2~3%. All Rossby radius (+2%), maximum baroclinic growth scale (-1%), Rhines scale (-4 ~ -8%), Rhines scale at the latitude of maximum eddy kinetic energy (-4%) and Kuo scale (0%) do not match accurately. All v'v', u'u', u'v', T'T' and v'T' weaken generally. At 38°S, v'v' and u'v' strengthen in faster eastward-propagating frequencies. Summing over all frequencies, u'v' shows localized strengthening at zonal wavenumber 8, but not v'v'. Eddies at that wavenumber get more banana-shaped in x-y plane and give stronger u'v' per unit of v'v' and u'u'. Eady model, linear baroclinic instability calculations, and inverse energy cascade can explain some features of the spectra, but not why u'v' strengthens while v'v' weakens at that wavenumber. Two-layer quasi-geostrophic model suggests the important role of a prescribed jet location.