The first global climatology of monsoon low‐pressure systems is presented here, based on the ERA‐Interim reanalysis. Low‐pressure systems are classified into three intensity categories and particular focus is given to systems in the category corresponding to a traditional definition of monsoon depressions. Vortex tracks are identified using an automated algorithm applied to the distributions of 850 hPa relative vorticity, sea‐level pressure and surface wind speed for 1979–2012. Roughly two to three times as many monsoon low‐pressure systems form in the Northern Hemisphere as in the Southern Hemisphere during local summer. The frequency of genesis typically peaks in local summer, but low‐pressure systems form throughout the year in every monsoon region. Interannual variability is weak, with standard deviations of summer counts typically being below 10% of the long‐term summer mean. Regional composites reveal that monsoon depressions in India, the western Pacific and northern Australia share a common structure, consisting of a warm‐over‐cold core and a top‐heavy column of potential vorticity that extends from the surface to the upper troposphere. A separate class of monsoon low‐pressure systems develops over dry regions of West Africa and western Australia, with a shallow composite structure having a warm core in the lower troposphere and cyclonic potential vorticity confined to a thin near‐surface layer. Low‐pressure systems in nearly all monsoon regions are estimated to account for a large fraction, from about 40% to more than 80%, of summer precipitation on the poleward edge of the climatological mean precipitation maxima.

[1] Water vapor in the subtropical troposphere plays an important role in the radiative balance, the distribution of precipitation, and the chemistry of the Earth's atmosphere. Measurements of the water vapor mixing ratio paired with stable isotope ratios provide unique information on transport processes and moisture sources that is not available with mixing ratio data alone. Measurements of the D/H isotope ratio of water vapor from Mauna Loa Observatory over 4 weeks in October-November 2008 were used to identify components of the regional hydrological cycle. A mixing model exploits the isotope information to identify water fluxes from time series data. Mixing is associated with exchange between marine boundary layer air and tropospheric air on diurnal time scales and between different tropospheric air masses with characteristics that evolve on the synoptic time scale. Diurnal variations are associated with upslope flow and the transition from nighttime air above the marine trade inversion to marine boundary layer air during daytime. During easterly trade wind conditions, growth and decay of the boundary layer are largely conservative in a regional context but contribute ∼12% of the nighttime water vapor at Mauna Loa. Tropospheric moisture is associated with convective outflow and exchange with drier air originating from higher latitude or higher altitude. During the passage of a moist filament, boundary layer exchange is enhanced. Isotopic data reflect the combination of processes that control the water balance, which highlights the utility for baseline measurements of water vapor isotopologues in monitoring the response of the hydrological cycle to climate change. Citation: Noone, D., et al. (2011), Properties of air mass mixing and humidity in the subtropics from measurements of the D/H isotope ratio of water vapor at the Mauna Loa Observatory,

A large fraction of the rain received by continental India is produced by cyclonic vortices with outer radii of about 1000 km that are contained within the larger scale South Asian monsoon flow. The more intense occurrences of these vortices are called monsoon depressions; these consist of bottom-heavy columns of relative vorticity that propagate to the northwest in time-mean low-level eastward flow. Previous studies have argued that this apparent upstream propagation is caused by dynamical lifting west of the vortex centre, with the resulting ascent producing vortex stretching that shifts the vortex to the west. Here, analysis of over 100 Indian monsoon depressions is used to show that low-level vortex stretching has a spatial structure inconsistent with the observed propagation and is balanced by other terms in the low-level vorticity budget. Instead, monsoon depressions are shown to consist of potential vorticity maxima that have peak amplitude in the middle troposphere and propagate westward by nonlinear, horizontal adiabatic advection (i.e. beta drift). The precipitating ascent in monsoon depressions makes a more minor contribution to the total storm motion and primarily acts to maintain the upright structure of the vortex. These results suggest a new view of Indian monsoon depressions as potential vorticity columns that propagate primarily by adiabatic dynamics.

[1] The processes controlling the joint distribution of water vapor specific humidity (q) and isotopic ratios (d) at the subtropical relative humidity (RH) minimum are investigated through the development of an advection-condensation model that is applied to an idealized GCM and a GCM nudged by reanalysis data. The first-order characteristics of the joint distribution of d and q values from the subtropical RH minimum can be satisfactorily explained by the model. In an idealized GCM with physics that closely approximates the assumptions in the advection-condensation model, the reconstructed d and q distribution are quite close to that in the model. When compared to an isotopic simulation from a full-physics GCM nudged by reanalysis data, the advection-condensation model reproduces the first-order aspects of the d and q distribution but underestimates the variability. In both cases, the errors can potentially be attributed to insufficient variability in the representation of domains contributing water vapor to the subtropical RH minimum and, in the case of the isotopic reanalysis, to the omission of cloud microphysics. This study suggests that long-term monitoring of water vapor isotopic ratios may provide a way to distinguish between different mechanisms for the projected moistening of the subtropics.Citation: Galewsky, J., and J. V. Hurley (2010), An advection-condensation model for subtropical water vapor isotopic ratios,

O is altered after deposition during austral winter from about À24 to À15‰. More than 70% of the total snow accumulation is tied to convection along the leading edge of cold air incursions of midlatitude air advected equatorward from southern South America. Snowfall amplitude at Quelccaya Ice Cap varies systematically with regional precipitation, atmospheric dynamics, midtroposphere humidity, and water vapor δD. Strongest snowfall gains correspond with positive precipitation anomalies over the western Amazon Basin, increased humidity, and lowered water vapor δD values, consistent with the "amount effect." We discuss ventilation of the monsoon, modulated by midlatitude cold air advection, as potentially diagnostic of the relationship between SASM dynamics and Quelccaya snowfall. Results will serve as a basis for development of a comprehensive isotopic forward model to reconstruct past monsoon dynamics using the ice core δ 18 O record.