Three-dimensional structure and dynamics of the climatological-mean summertime subtropical highs over the North Pacific and Atlantic (i.e., the Azores high) are investigated. Each of the observed surface highs is accompanied by a meridional vorticity dipole aloft, exhibiting barotropic and baroclinic structures in its northern and southern portions, respectively, in a manner dynamically consistent with the observed midtropospheric subsidence. Each of the highs develops over the relatively cool eastern ocean, where a pronounced near-surface thermal contrast exists with a heated landmass to the east. The authors demonstrate through numerical experiments that those highs can be reproduced in response to a local shallow cooling-heating couplet associated with this thermal contrast, although the upper-level response is somewhat underestimated. The model experiments suggest that the near-surface thermal contrasts associated with those surface subtropical highs over the Pacific and Atlantic can act as sources of the observed planetary waves over the Western Hemisphere. In fact, a wave activity flux for stationary Rossby waves is distinctively upward and diverging toward downstream in the upper troposphere above each of the observed surface highs. The observed wave activity injection is significant into the Azores high but not at all into the Pacific high. Since each of the subtropical highs can be reproduced reasonably well, even for the premonsoon season (i.e., May), in response to a local shallow land-sea heating contrast, it is suggested that the monsoonal convective heating may not necessarily be a significant direct forcing factor for the formation of the summertime subtropical highs. In fact, the model response is quite weak if forced only by mid-and upper-tropospheric convective heating. The present study suggests the presence of a local land-seaatmosphere feedback loop associated with a subtropical high and a continental low to its east, which may be triggered by increasing insolation over land from spring to summer.

ABSTRACT:A diagnostic framework is introduced in which anomalous zonally averaged Rossby wave-activity injection into the stratosphere is decomposed into a contribution solely from zonally confined upward-propagating Rossby wave packets and another from interaction of the wave packets with the climatological planetary waves. To pinpoint the tropospheric sources of the wave packets, a particular form of wave-activity flux is evaluated for the associated circulation anomalies. The framework is applied to reanalysis data for the period prior to a stratospheric sudden warming (SSW) event in January 2006, which was associated with two successive events of above-normal wave-activity injection from the troposphere. In the earlier event, a pair of wave packets that emanated from tropospheric anomalies over the North Pacific and over Europe enhanced the upward wave-activity injection, which was augmented further by their interaction with the climatological planetary wave. In contrast, in the later period a wave packet that emanated from an anticyclonic anomaly over the North Atlantic is found to be the primary contributor to the enhanced planetary wave-activity injection, while its interaction with the climatological planetary wave contributed negatively. The predominant importance of the sole contribution from a single wave packet is also found in a major SSW event observed over Antarctica in September 2002. These results indicate that the diagnostic framework presented in this study is a useful tool for understanding the interaction between anomalies associated with zonally confined wave packets and climatological-mean planetary waves in the study of stratosphere-troposphere dynamical coupling.

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The south Indian Ocean is characterized by enhanced midlatitude storm-track activity around a prominent sea surface temperature (SST) front and unique seasonality of the surface subtropical Mascarene high. The present study investigates the climatological distribution of low-cloud fraction (LCF) and its seasonality by using satellite data, in order to elucidate the role of the storm-track activity and subtropical high. On the equatorward flank of the SST front, summertime LCF is locally maximized despite small estimated inversion strength (EIS) and high SST. This is attributable to locally augmented sensible heat flux (SHF) from the ocean under the enhanced storm-track activity, which gives rise to strong instantaneous wind speed while acting to relax the meridional gradient of surface air temperature. In the subtropics, summertime LCF is maximized off the west coast of Australia, while wintertime LCF is distributed more zonally across the basin unlike in other subtropical ocean basins. Although its zonally extended distribution is correspondent with that of LCF, EIS alone cannot explain the wintertime LCF enhancement, which precedes the EIS maximum under continuous lowering of SST and enhanced SHF in winter. Basinwide cold advection associated with the wintertime westward shift of the subtropical high contributes to the enhancement of SHF, especially around 158-258S, while seasonally enhanced storm-track activity augments SHF around 308S. The analysis highlights the significance of large-scale controls, particularly through SHF, on the seasonality of the climatological LCF distribution over the south Indian Ocean, which reflect the seasonality of the Mascarene high and storm-track activity.