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ABSTRACT This work uses an aerosol-climate coupled system to study the direct climatic effects of dust aerosol on global arid (including hyper-arid) and semi-arid regions. Results show that dust aerosol can cause surface cooling over these regions. The cooling effects of dust aerosol are larger in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH). This asymmetric cooling leads to a severer reduction in evaporation over the low latitudes of the NH compared with their counterpart areas in the SH. In addition, air descent is enhanced (or ascent is weakened) by dust aerosol over the low latitudes of the NH, whereas the reverse happens in the SH. With the anomalous in evaporation and circulation, precipitation is decreased (increased) over the arid and semi-arid areas in the NH (SH) by dust aerosol. Dust aerosol can decrease the potential evapotranspiration in arid and semi-arid regions mainly by decreasing the net radiation flux at the top of the atmosphere, which compensates for the decrease in precipitation caused by dust aerosol. Therefore, as the dominant aerosol in the arid and semi-arid regions, dust aerosol does not exacerbate the aridity over most of these regions. The effects of dust aerosol are indiscernible in the expansion of arid and semi-arid areas on a global scale. [ABSTRACT FROM AUTHOR]

The influence of atmospheric circulation changes reflected in spontaneously occurring sea level pressure (SLP) anomalies upon surface air temperature (SAT) variability and trends is investigated using partial least squares (PLS) regression, a statistical method that seeks to maximally explain covariance between a predictand time series or field and a predictor field. Applying PLS regression in any one of the three variants described in this study (pointwise, PC-wise, and fieldwise), the method yields a dynamical adjustment to the observed NH SAT field that accounts for approximately 50% of the variance in monthly mean, cold season data. It is shown that PLS regression provides a more parsimonious and statistically robust dynamical adjustment than an adjustment method based on the leading principal components of the extratropical SLP field. The usefulness of dynamical adjustment is demonstrated by applying it to the attribution of cold season SAT trends in two reference intervals: 1965-2000 and 1920-2011. The adjustment is shown to reconcile much of the spatial structure and seasonal differences in the observed SAT trends. The dynamically adjusted SAT fields obtained from this analysis provide datasets capable of being analyzed for residual variability and trends associated with thermodynamic and radiative processes. [ABSTRACT FROM AUTHOR]

Seasonally and vertically resolved changes in the strength of the Brewer-Dobson circulation (BDC) were inferred using temperatures measured by the Microwave Sounding Unit (MSU), Stratospheric Sounding Unit (SSU), and radiosondes. Linear trends in an empirically derived 'BDC index' (extratropical minus tropical temperatures), over 1979-2005, were found to be consistent with a significant strengthening of the Northern Hemisphere (NH) branch of the BDC during December throughout the depth of the stratosphere. Trends in the same index suggest a significant strengthening of the Southern Hemisphere branch of the BDC during August through to the midstratosphere, as well as a significant weakening during March in the NH lower stratosphere. Such trends, however, are only significant if it is assumed that interannual variability due to the BDC can be removed by regression of the tropics against the extratropics and vice versa (i.e., exploiting the out-of-phase nature of tropical and extratropical temperatures as demonstrated by previous studies of temperature and the BDC). The possibility that the apparent lower-stratosphere BDC December strengthening and March weakening could point to a change in the seasonal cycle of the circulation is also explored. The differences between a 1979-91 average and 1995-2005 average tropical temperature seasonal cycle in lower-stratospheric MSU data show an apparent shift in the minimum from February to January, consistent with a change in the timing of the maximum wave driving. Additionally, the importance of decadal variability in shaping the overall trends is highlighted, in particular for the suggested March BDC weakening, which may now be strengthening from a minimum in the 1990s. [ABSTRACT FROM AUTHOR]

One pronounced feature in observed latitudinal dependence of lower-stratospheric temperature trends is the enhanced cooling near 30° latitude in both hemispheres. The observed phenomenon has not, to date, been explained in the literature. This study shows that the enhanced cooling is a direct response of the lower-stratospheric temperature to the poleward shift of subtropical jets. Furthermore, this enhanced lower-stratospheric cooling can be used to quantify the poleward shift of subtropical jets. Using the lower-stratospheric temperatures observed by satellite-borne microwave sounding units, it is shown that the subtropical jets have shifted poleward by 0.6° ± 0.1° and 1.0° ± 0.3° latitude in the Southern and Northern Hemispheres, respectively, in last 30 years since 1979, indicating a widening of tropical belt by 1.6° ± 0.4° latitude. [ABSTRACT FROM AUTHOR]