Planetary-scale variability in the northern winter and the impact of land–sea thermal contrast

Abstract

This paper investigates the impact of land–sea thermal contrast on large-scale modes of variability during the northern-hemisphere winter, with specific attention to those modes which explain a large proportion of the observed inter-decadal changes in the late 20th century. Two possible mechanisms are considered, where the land–sea contrast plays either a ‘passive’ or an ‘active’ role: namely, the introduction of zonal asymmetries in an (eddy-driven) annular mode, and variations in the thermal balance of a planetary wave pattern with zonal-wavenumber 2. In addition to diagnostics based on re-analysis data, two long perpetual-winter simulations of an intermediate-complexity AGCM are performed, imposing surface boundary conditions with either realistic or artificially reduced land–sea thermal contrast. On the basis of previous studies, indices derived from surface fields are defined to represent the two mechanisms outlined above. Atmospheric patterns co-varying with these indices are first computed from NCEP/NCAR re-analysis, and compared with traditional definitions of dominant modes of variability, such as EOFs and the Cold-Ocean/Warm-Land (COWL) pattern. Subsequently, the indices are applied to fields from the simulations with realistic and reduced land–sea contrast, in order to highlight the impact of thermal asymmetries on the structure of annular and planetary-wave modes. Our results show that the zonal thermal contrast on the western border of the North Atlantic is responsible for the localization of a dipolar structure in sea-level pressure and height fields, which bears a strong resemblance to the North Atlantic Oscillation (NAO). In this region, the response to thermal land–sea contrast is evident on both annular and planetary-wave patterns, with a NAO-like dipole being the dominant regional feature. In the North Pacific, on the other hand, diabatic forcing is balanced primarily by meridional temperature advection, because here (contrarily to the north-west Atlantic region) the surface temperature gradient is stronger in the meridional than in the zonal direction. Therefore, the modification of the annular mode by zonal temperature advection at the Asia-North Pacific border produces a different pattern from that associated with the thermal balance of a COWL-like planetary wave. We conclude that ‘thermally-balanced wave mode’ is a dynamically appropriate description for the pattern previously identified as the second EOF of low-frequency 500-hPa height variability, or empirically defined as the Cold-Ocean/Warm-Land pattern. Since this mode accounts for a large proportion of the upper-air inter-decadal variations in the second half of the 20th century, we suggest that such variations are dynamically consistent over the hemispheric domain; therefore they should be understood in terms of planetary-scale dynamics, rather than by the casual superposition of regional effects.