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Modulation of Dynamic Heating in the Winter Extratropics Associated with the Cross-Equatorial Hadley Circulation

Arthur Y. HouData Assimilation Office, Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland

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Andrea MolodData Assimilation Office, Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland

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Abstract

The hypothesis that the cross-equatorial Hadley circulation can modulate the poleward heat transport in the winter extratropics is investigated using the Goddard Earth Observing System (GEOS-1) GCM for 10 northern winter initial conditions. Three-month forecasts are compared with parallel runs from the same initial conditions but with a slightly perturbed radiative forcing in the Tropics. Analysis of the zonal-mean climates shows that, on timescales longer than a month, a persistent intensification in the winter Hadley circulation is positively correlated with increased dynamic cooling in the winter midlatitudes and warming at the high latitudes through-out the troposphere, signaling an increased beat transport toward the winter pole.

While the heating anomaly can undergo significant transient fluctuations in the winter extratropics, the variance of the time-averaged dynamic heating anomaly is dominated by contributions from low zonal wavenumbers (particularly wavenumbers 1 and 2), with minor contributions from wavenumbers 4 and higher, suggesting that the low-frequency planetary-scale waves are the primary vehicle for the increased poleward heat transport, with the synoptic-scale waves assuming a secondary role along the storm tracks.

These results support the earlier idealized GCM study by Hou showing that a stronger winter Hadley circulation induced by a latitudinal shift in tropical convection can lead to enhanced upper-level tropical easterlies and a slightly stronger subtropical winter jet attended by increased poleward heat transport in the winter extratropics. Specific examples were also found in which the zonally averaged response is dominated by regional changes (notably over the North Pacific), indicating these relations may hold locally, as suggested by Bjerknes.

The implication of this work is that the low-frequency variability in the Hadley circulation associated with persistent tropical rainfall anomalies may play an important role in global climate by modulating the subtropical wind shear and the energy available for baroclinic wave growth outside the Tropics.

Abstract

The hypothesis that the cross-equatorial Hadley circulation can modulate the poleward heat transport in the winter extratropics is investigated using the Goddard Earth Observing System (GEOS-1) GCM for 10 northern winter initial conditions. Three-month forecasts are compared with parallel runs from the same initial conditions but with a slightly perturbed radiative forcing in the Tropics. Analysis of the zonal-mean climates shows that, on timescales longer than a month, a persistent intensification in the winter Hadley circulation is positively correlated with increased dynamic cooling in the winter midlatitudes and warming at the high latitudes through-out the troposphere, signaling an increased beat transport toward the winter pole.

While the heating anomaly can undergo significant transient fluctuations in the winter extratropics, the variance of the time-averaged dynamic heating anomaly is dominated by contributions from low zonal wavenumbers (particularly wavenumbers 1 and 2), with minor contributions from wavenumbers 4 and higher, suggesting that the low-frequency planetary-scale waves are the primary vehicle for the increased poleward heat transport, with the synoptic-scale waves assuming a secondary role along the storm tracks.

These results support the earlier idealized GCM study by Hou showing that a stronger winter Hadley circulation induced by a latitudinal shift in tropical convection can lead to enhanced upper-level tropical easterlies and a slightly stronger subtropical winter jet attended by increased poleward heat transport in the winter extratropics. Specific examples were also found in which the zonally averaged response is dominated by regional changes (notably over the North Pacific), indicating these relations may hold locally, as suggested by Bjerknes.

The implication of this work is that the low-frequency variability in the Hadley circulation associated with persistent tropical rainfall anomalies may play an important role in global climate by modulating the subtropical wind shear and the energy available for baroclinic wave growth outside the Tropics.

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