Ozone enhanced layers observed inside the Antarctic ozone hole were investigated using ozonesonde data at Syowa and Neumayer Stations during the dissipation period of the ozone hole in 2003. In 22 ozonesonde observations performed at Syowa and Neumayer Stations during one month after the maximum of ozone hole area, 17 ozone enhanced layers (i.e., up to three layers per one observation) were detected between 14 and 21 km in the lower stratosphere. They always had a thickness thinner than 2.1 km. Maximum ozone mixing ratios in the ozone enhanced layers suggested that most of the layers had their origin in the vortex boundary region. A reverse domain filling analysis indicated that air parcels originating in the vortex boundary region reached Syowa and Neumayer Stations in a form of horizontally-thin filaments. These filamentary structures were generated through chaotic advection due to transient planetary-scale Rossby waves, and became thin also in the vertical because of the vertical shear of horizontal wind. This study suggests that the ozone enhanced layers inside the ozone hole could be a useful measure to identify the poleward Rossby wave breaking and the origin of transported air parcels. A contribution of the ozone enhanced layers to an increase of total ozone was estimated at about 2.1 DU for the analysis period of one month, which is less than one tenth of the increase of vortex average total ozone during the same period. On the other hand, it is corresponding to about a half of the background ozone column in the height region of 14-21 km at the time of the ozone hole area maximum.
The relationship between the boreal spring (or the austral autumn) Antarctic Oscillation (AAO) (March- April) and the West African summer monsoon (WASM) (June-September) is analyzed based on NCEP/NCAR reanalysis data. The results show that the linkage of the boreal spring AAO to the WASM exhibits decadal-scale variations: a strong connection between the two appears over the period 1985-2006 and a weak connection over the period 1970-1984. Further analysis indicates that such an unstable relationship between the two results from the modulation by ENSO events to a large extent. A possible mechanism for the impacts of the boreal spring AAO on the WASM is also discussed. The variability of the boreal spring tropical South Atlantic sea surface temperature (SST) appears to serve as a bridge linking these two systems. The boreal spring AAO produces an anomalous SST over the tropical South Atlantic by exciting an equatorward Rossby wave train over the western Southern Hemisphere (SH). This AAO-related SST anomaly modulates the meridional gradient of moist static energy (MSE) between the Sahel and the Guinea-tropical Atlantic region in the boreal spring. The MSE gradient is of paramount importance for the changes from spring to summer in the West African monsoon because its relaxation along the seasonal cycle is linked to the northward excursion of the WASM system into the African continent. Therefore, an anomalous AAO-related MSE gradient can lead to anomalous Sahel rainfall in the early summer. When this rainfall occurs over the Sahel, the local positive soil moisture-rainfall feedback plays a crucial role in sustaining and prolonging this rainfall anomaly throughout the whole summer.
Changes in precipitation and its extremes in China were analyzed based on daily precipitation series from 1954 to 2006 at 740 stations. Indices of absolute extremes (e.g., heavy rains over 25 mm day1), relative ones (e.g., events exceeding the 95th percentile for local sites and given time in a year), and parameters of ?tted Generalized Extremes Value (GEV) distributions were investigated. Many of the precipitation and index series exhibited climatic jumps during the 1960s, around 1980 and in the 1990s. In particular, the widely-concerned drying process in North China was essentially formed by three drying jumps, indicating a stepwise weakening or southward retreating process of the East Asian summer monsoons during the second half of the 20th century. Changes in the atmospheric circulation in association with the regional drying jump around 1980 were analyzed based on the ECMWF 40 Years Re-Analysis (ERA-40) data available from 1958 to 2002. A signi?cant phase shift was found in summer circulation indices and the northwestern Paci?c Subtropical High during the late 1970s and the early 1980s. The drying jumps also led to abnormal variability in extreme rainfall events in the region, indicating that extreme events with higher intensity could happen in some years during recent decades than earlier, despite a decrease in the long-term mean precipitation.
A 20-member ensemble simulation has been conducted with an atmospheric general circulation model to investigate the time-space characteristics of the leading modes of internal and SST-forced variability in the winter Northern Hemisphere. Each of the 20 integrations is forced by the identical global sea surface temperature (SST) and sea ice history observed for 1959-1998, and differs only in the initial conditions. A variance analysis is performed to quantify the relative importance of the SST-forced and internal variability. In the extratropics, the SST-forced signals are much higher in the North Pacific than in the North Atlantic where month-to-month and interannual variability is dominated by internal chaos. The leading empirical orthogonal function mode resembles the Arctic Oscillation (AO) for both the internal and ensemble-mean variability but with significant differences in correlation between the Pacific and Atlantic centers of action. The Pacific-Atlantic correlation is significantly higher for the ensemble mean than for all but one member integration. The correlation between the Arctic and midlatitude North Atlantic, by contrast, is higher in individual member runs than in the ensemble-mean. These results suggest that SST-forced variability is organized into a hemispheric AO pattern while internal variability is more confined in the North Atlantic sector. Seasonal air-sea interactions in the North Pacific and Atlantic are also discussed.
Formation of dust devils in diurnally-evolving convective mixed layers is studied by means of a large eddy simulation. It is found that a weaker general wind and a stronger surface heat flux for which cellular convection rather than roll convection prevails are favorable for the formation of dust devils. The simulation results show that when the general wind is weak, the maximum vertical vorticity in the convective mixed layer is a monotonically increasing function of w*, where w* is the convective velocity scale for a convective mixed layer. Therefore, dust devils occur most frequently in the early afternoon when the heat flux is large and the convective mixed layer grows to a significant height. The simulated dust devils are found to have a horizontal length scale comparable with observed larger dust devils. They have either one-celled or two-celled structure. Some of them have a one-celled structure initially, but later evolve into a two-celled structure.
Seasonal migration of tropical rainfall is examined in the South American sector and compared with that over Africa, using satellite and reanalysis data. While the African rain band moves continuously back and forth across the equator following the seasonal march of the sun, the South American one displays a peculiar asymmetry between its northward and southward migration. The rain band moves gradually northward from October to July from the Amazon toward the Caribbean Sea, while its return to the Amazon is an abrupt event, with convection developing rapidly in October around 10°S without going through the equator. Over the equatorial Amazon during July-October, equivalent potential temperature (θe) is kept low by the easterly advection of low temperature and humidity air from the equatorial Atlantic, coinciding with the seasonal development of ocean upwelling and a cold tongue in sea surface temperature. The low θe values prevent convection from developing in the equatorial Amazon while warm SST supports convection on the north coast of South America during August-October. Meanwhile solar radiation continues to heat up the land surface to the south, eventually triggering the onset of deep convection there in October. Atmospheric general circulation model experiments were conducted to examine the effect of the Atlantic cold tongue on tropical rain bands. Without the seasonal development of the Atlantic cold tongue, surface θe remains high in September, and rainfall in the equatorial Amazon switches to a pronounced semi-annual cycle with a peak in each equinox. These results illustrate the role of the Atlantic cold tongue in the peculiar meridional migration of the observed South American rain band.