29 research outputs found

    Wake response to an ocean-feedback mechanism: Madeira Island case study

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    This discussion focused on the numerical study of a wake episode. The Weather Research and Forecasting model was used in a downscale mode. The current literature focuses the discussion on the adiabatic dynamics of atmospheric wakes. Changes in mountain height and consequently on its relation to the atmospheric inversion layer should explain the shift in wake regimes: from a 'strong-wake' to a 'weak-wake' scenario. Nevertheless, changes in SST variability can also induce similar regime shifts. Increase in evaporation, contributes to increase convection and thus to an uplift of the stratified atmospheric layer, above the critical height, with subsequent internal gravity wave activity.Comment: Under review proces

    Why the South Pacific Convergence Zone is diagonal

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    During austral summer, the majority of precipitation over the Pacific Ocean is concentrated in the South Pacific Convergence Zone (SPCZ). The surface boundary conditions required to support the diagonally (northwest-southeast) oriented SPCZ are determined through a series of experiments with an atmospheric general circulation model. Continental configuration and orography do not have a significant influence on SPCZ orientation and strength. The key necessary boundary condition is the zonally asymmetric component of the sea surface temperature (SST) distribution. This leads to a strong subtropical anticyclone over the southeast Pacific that, on its western flank, transports warm moist air from the equator into the SPCZ region. This moisture then intensifies (diagonal) bands of convection that are initiated by regions of ascent and reduced static stability ahead of the cyclonic vorticity in Rossby waves that are refracted toward the westerly duct over the equatorial Pacific. The climatological SPCZ is comprised of the superposition of these diagonal bands of convection. When the zonally asymmetric SST component is reduced or removed, the subtropical anticyclone and its associated moisture source is weakened. Despite the presence of Rossby waves, significant moist convection is no longer triggered; the SPCZ disappears. The diagonal SPCZ is robust to large changes (up to +/-6 degC) in absolute SST (i.e. where the SST asymmetry is preserved). Extreme cooling (change less than -6 degC) results in a weaker and more zonal SPCZ, due to decreasing atmospheric temperature, moisture content and convective available potential energy

    The poleward shift of South Atlantic Convergence Zone in recent decades

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    During austral summer (December–January–February or DJF), intense precipitation over central-eastern Brazil is modulated by the South American Monsoon System and the South Atlantic Convergence Zone (SACZ). Previous studies identified spatial variability in precipitation trends over this region, suggestive of a poleward shift of the SACZ in recent years. To identify underlying mechanisms associated with changes in the precipitation intensity and position of the SACZ, decadal averages of observed precipitation and the mean state of the atmosphere and ocean during three different periods from 1979 to 2014 are compared. Results show evidence of decreasing (increasing) average daily precipitation along the equatorward (poleward) margin of the climatological SACZ, likely related to a poleward shift of the convergence zone. Precipitation reduction along the equatorward margin of the SACZ is associated with weakening of the poleward winds along the eastern Brazilian coast and drying of low-to-mid troposphere (700 hPa) over the tropical Atlantic. These changes in circulation and moisture are likely related to the poleward expansion of the South Atlantic Subtropical High

    The effect of midlatitude transient eddies on monsoonal southerlies over eastern China

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    The strengthening of monsoonal southerlies over East Asia is associated with the westward intensification of the North Pacific subtropical high. Previous work has shown that the seasonal-mean position and strength of subtropical highs are affected by tropical and subtropical diabatic heating. Here it is shown that the synoptic-time-scale strengthening of southerlies over eastern China is dynamically tied to extratropical eddy activity. Composite analysis based on strong southerly wind events highlights an antecedent baroclinic wave train propagating southeastward into eastern China from extratropical central Asia. This wave train generates quasigeostrophic ascent over eastern China that is associated with heavy precipitation. The anomalously cold upper-tropospheric conditions associated with the wave train decrease static stability throughout the lower and middle troposphere in eastern China, while low-level moistening enhances equivalent potential temperature. It is proposed that the resulting reductions in dry andmoist static stability intensify the eddy-induced precipitating ascent. These results illustrate how East Asian monsoon circulation and precipitation can be enhanced by the interaction of midlatitude baroclinic waves with the moist subtropical monsoon region

    Future rise in rain inequality

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