Enhanced Moisture Transport Research Articles

Impact of anthropogenic forcing on the Asian summer monsoon as simulated by eight GCMs

The response of the Asian summer monsoon (ASM) to a transient increase in future anthropogenic radiative forcing is investigated by multi‐model global warming experiments. Most models show that, in Asia, the summer monsoon rainfall increases significantly with global warming. On the other hand, the future change in the large‐scale flow indicates a weakening of the ASM circulation. Enhanced moisture transport over the Asian summer monsoon region, associated with the increased moisture source from the warmer Indian Ocean, leads to a larger moisture flux convergence, which is responsible for the intensification of the mean rainfall. Pronounced warming over the tropics in the middle‐to‐upper troposphere causes a reduction in the meridional thermal gradient in the Asian region, which is consistent with the weakened monsoon circulation and eastward shift of the Walker circulation.

Transient Response of ENSO-Monsoon Teleconnection in MRI-CGCM2.2 Climate Change Simulations

This study investigates the impact of anthropogenic climate change on the Indian summer monsoon, and the ENSO-monsoon teleconnection, using the transient climate change simulations of the MRI coupled model (MRI-CGCM2.2). In the present simulations atmospheric greenhouse gas (GHG) concentrations and aerosols are varied to represent observed changes during 1850–2000, and from 2001–2100, at the rate prescribed by the SRES-A2 and B2 scenarios to study the response of the monsoon climate during 2071–2100. All India annual temperature shows warming by 2.35 � C (1.64 � C), while all-India monsoon rainfall (JJAS) indicates an increase by 9% (6%) in SRES-A2 (B2) scenario relative to the present climate (1971–2000). Increase in the monsoon precipitation is evident over parts of south India, parts of the Bay of Bengal and northwest India. The simulations feature stronger warming over South Asia relative to the Indian Ocean, leading to enhanced land-sea temperature contrast. Enhanced moisture in a warmer scenario, coupled with enhanced monsoon circulation, leads to enhanced moisture transport over the Indian region from both the Bay of Bengal and Arabian Sea leading to increased monsoon precipitation. Decomposition of the moisture transport into divergent and non-divergent components shows that the enhanced monsoon rainfall is mainly due to intensification in the non-divergent component of the moisture transport. While strong and continued increase in monsoon rainfall suggests

Moisture Transport and Other Hydrometeorological Features Associated with the Severe 2000/01 Drought over the Western and Central Canadian Prairies

The 54-yr (1948–2001) NCEP–NCAR reanalysis data as well as other information were used to study the moisture transport and associated circulation features for the severe 2000/01 drought over the western and central Canadian Prairies. Most of the moisture for precipitation over the region is from the Pacific Ocean in winter (November–March) and from the Gulf of Mexico in summer (May–August). An analysis shows that the zonal moisture transport from the Pacific Ocean into both the North American continent and the western and central Canadian Prairies during the winter of the 2000/01 agricultural year was the least over the entire study period, and there was no significantly enhanced moisture influx from the Gulf of Mexico into the region to compensate. Very low winter precipitation was produced over the western and central Canadian Prairies as a consequence. During the ensuing summer period, moisture transport from the Gulf of Mexico was significantly less than normal and no significantly enhanced moisture transport from the Pacific Ocean occurred. These conditions collectively resulted in extremely dry surface conditions for the growing season. These moisture transport features were mainly associated with prolonged and extraordinarily strong anomalously high pressures over western North America and their related stronger-than-normal air mass sinking over the western and central prairies and adjacent regions. The anomalous high pressures blocked the moisture from flowing into the western and central prairies and were also associated with the splitting of the jet stream that significantly reduced zonal moisture transport by changing the strength and incoming angle of the airflows from the Pacific Ocean during the winter of 2000/01. Consequences of the stronger-than-normal subsidence were hot and dry surface air during the summer and less precipitation. Collectively, these dynamic factors are favorable for both the formation and the maintenance of droughts.

Carbonate Speleothems in the Dry, Inneralpine Vinschgau Valley, Northernmost Italy: Witnesses of Changes in Climate and Hydrology Since the Last Glacial Maximum

ABSTRACT An interesting association of slope breccia, inactive and active tufa deposits, and speleothems is present in the central Vinschgau Valley, Italy. The occurrence of abundant carbonate cements in fractures and voids of crystalline basement rocks is unexpected considering the fact that this valley is among the driest spots in the entire Alps. Low annual precipitation of 440-530 mm coupled with frequent wind give rise to a semiarid climate and steppe vegetation along the south-facing slopes of the valley. Springs in this area are mostly supersaturated with respect to calcite, and carbonate precipitation occurs locally as tufas and, less well known because of lack of accessibility, as speleothems in the shallow subsurface. The majority of the tufa deposits and speleothems, however, are fossil. Speleothems are composed of low-Mg calcite and calcite-aragonite, respectively. Delicate growth textures including presumable annual lamination caused by pronounced changes in fluorescence intensity are preserved in both calcite and aragonite. Most calcite is a primary precipitate, but small amounts of secondary calcite replacing aragonite are common in most aragonite-bearing samples. The highly radiogenic Sr isotope composition, as well as high concentrations of U, Fe, Sr, and Mg, indicate that the groundwater from which these carbonates precipitated experienced intensive interaction with the host crystalline rocks. The very low tritium concentrations and the lack of a seasonal O isotope variation in modern spring waters, as well as their rather constant hydrochemical composition, also support this suggestion. S isotope data for dissolved sulfate and Ca and Mg sulfate precipitates indicate a sulfide source, i.e., oxidation of sulfide ore minerals in the aquifer, resulting in elevated sulfate and Fe concentrations. Th/U dating of speleothem samples using thermal ionization mass spectrometry yielded ages between 13,710 and 378 yr BP, with most ages falling in the early to middle Holocene. Although no isotopic dates are available for the tufa deposits, field evidence strongly suggests that speleothems, tufa deposits, and carbonate cements in the slope breccia were closely related. We therefore interpret the existence of these terrestrial carbonates as evidence of changes in climate since the middle Holocene. Their presence suggests a higher annual rainfall during the first half of the Holocene, possibly because of enhanced moisture transport from the Mediterranean.

Biases in AMIP model simulations of the east China monsoon system

AMIP model simulations of the east China (5–50°N; 105–122°E) monsoon system are analyzed to study coherent relationships between rainfall and wind annual cycle biases. A comparison with observed interannual variability patterns is carried out to identify the physical processes that explain the biases. The analyses show that poleward displacement of the simulated east Asian jet stream causes the ascending branch of the jet-induced transverse circulation to move north and, as a consequence, produces negative (positive) rainfall biases occur in central (northeast) China. The model simulations show decreased southwesterly flow and ITCZ rainfall over the South China Sea when weaker (versus observations) summer Hadley and Walker circulations are present. This results from diminished model tropical disturbance activity, and highlights the importance of air-sea interactions. In addition, during October–January, intensified model low-level easterlies enhance moisture transport and produce positive local rainfall biases over central and northeast China. Biases in the east China monsoon system are concurrently reflected in the planetary circulation. Enhanced northeast China rainfall results from increased surface pressure over the North Pacific and an amplified zonal pressure gradient along the east China coast. This bias pattern is associated with differences in model representations of topography. On the other hand, the South China Sea experiences an extensive elongated meridional rainfall bias dipole structure that straddles the equator. This is accompanied by a baroclinic vertical pattern over the tropics as well as a barotropic wave train that extends from Australia to the Antarctic, where the teleconnection is likely a direct atmospheric response to tropical convective heating.

Results of global atmospheric circulation experiments on an Earth with a meridional pole-to- pole continent

Four general circulation models, forced by mean annual insolation and having an energy balance ocean, have been run for pole-to-pole continents: (1) low relief, 750 m average elevation; (2) high plateau, 1500 m average elevation; (3) mostly low, 750 m, but with 3 km mountains on the west, and (4) mostly low, 750 m, but with 3 km mountains on the east. The high plateau continent emphasizes the contrast between high and low pressure systems. Marginal mountains on either coast enhance moisture transport from the west to the continental interior, but mountains on the east eliminate the tropical wet belt and produce the most arid scenario. In none of the experiments do zonal climate boundaries trend steeply equatorward from east to west. All of the models produce more arid continental conditions than exist today.