The Impact of Hydrological and Climatic Variations on the Oxygen-18 Content of Atmospheric CO2

The 18O composition of atmospheric CO2 is a potentially valuable tracer of global interactions between the hydrologic and carbon cycles. The observed 18O composition of atmospheric CO2 (hereafter ∂Ca, where d =(R/Rstandard-1) ~ 1000 and R is the molar ratio of heavy to light isotopes) does not show a clear long-term trend, though almost all monitoring stations observed an impressive decrease in ∂Ca from 1992 to 1998. The cause(s) of this and other interannual ∂Ca variations are still relatively unknown, and this work aims to better understand the driving mechanisms that caused the observed interannual ∂Ca variations. Observed interannual ∂Ca anomalies from Mauna Loa were correlated with anomalies of certain meteorological variables that could potentially affect ∂Ca. Negative correlation existed between ∂Ca and both relative humidity and precipitation amount within parts of the tropics. Positive correlations existed between ∂Ca variations and the 18O content of precipitation for the same tropical regions. Rough estimates suggest that about 20% of the decrease in ∂Ca during the 1990s was due to increases in relative humidity and about 80% of the decrease was due to decreases in the d18O value of precipitation (and likely a consequence of increases in the amount of precipitation). A global model was constructed to simulate atmospheric CO2 and CO18O (and thus ∂Ca). This model employed an isotopic land model (ISOLSM) and the Community Atmosphere Model (CAM). The model is used for a series of sensitivity experiments to better understand how both steady-state and interannual varying ∂Ca respond to changes in relative humidity, d18O values of precipitation and water vapor, temperature, and light levels. ∂Ca responded the most to changes in the d18O values of precipitation and water vapor, with moderate responses to relative humidity changes. Model results suggest that the decrease in ∂Ca during the 1990s was due primarily to decreases in the 18O composition of precipitation with a smaller a contribution from increased relative humidity. Thus, observations of ∂Ca may become a powerful integrative tool in the coming decades for monitoring large scale changes in the hydrological cycle should it accelerate under a warming climate, as predicted

Recent contrasting winter temperature changes over North America linked to enhanced positive Pacific‐North American pattern

Recently enhanced contrasts in winter (December‐January‐February) mean temperatures and extremes (cold southeast and warm northwest) across North America have triggered intensive discussion both within and outside of the scientific community, but the mechanisms responsible for these contrasts remain unresolved. Here we use a combination of observations and reanalysis data sets to show that the strengthened contrasts in winter mean temperatures and extremes across North America are closely related to an enhancement of the positive Pacific‐North American (PNA) pattern during the second half of the 20th century. Recent intensification of positive PNA events is associated with amplified planetary waves over North America, driving cold‐air outbreaks into the southeast and warm tropical/subtropical air into the northwest. This not only results in a strengthened winter mean temperature contrast but increases the occurrence of the opposite‐signed extremes in these two regions.Key PointsThe enhanced contrasts in winter mean temperatures and extremes in North America are observedRecent enhancement of positive PNA is a main cause of the contrasting winter temperature changesThe study provides a framework for detection and attribution of climate change in North AmericaPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/115952/1/grl53404_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/115952/2/grl53404.pd

Tracking the Strength of the Walker Circulation With Stable Isotopes in Water Vapor

General circulation models (GCMs) predict that the global hydrological cycle will change in response to anthropogenic warming. However, these predictions remain uncertain, in particular, for precipitation (Intergovernmental Panel on Climate Change, 2013, https://doi .org/10.1017/CB09781107415324.004). Held and Soden (2006, https://doi.org/10.1175/JCLI3990.1) suggest that as lower tropospheric water vapor concentration increases in a warming climate, the atmospheric circulation and convective mass fluxes will weaken. Unfortunately, this process is difficult to constrain, as convective mass fluxes are poorly observed and incompletely simulated in GCMs. Here we demonstrate that stable hydrogen isotope ratios in tropical atmospheric water vapor can trace changes in temperature, atmospheric circulation, and convective mass flux in a warming world. We evaluate changes in temperature, the distribution of water vapor, vertical velocity (omega), advection, and water isotopes in vapor (delta D-v). Using water isotope-enabled GCM experiments for modern versus high-CO2 atmospheres, we identify spatial patterns of circulation change over the tropical Pacific. We find that slowing circulation in the tropical Pacific moistens the lower troposphere and weakens convective mass flux, both of which impact the delta D of water vapor in the midtroposphere. Our findings constitute a critical demonstration of how water isotope ratios in the tropical Pacific respond to changes in radiative forcing and atmospheric warming. Moreover, as changes in delta D-v can be observed by satellites, our results develop new metrics for the detection of global warming impacts to the hydrological cycle and, specifically, the strength of the Walker circulation

Causes and Implications of Extreme Atmospheric Moisture Demand during the Record-Breaking 2011 Wildfire Season in the Southwestern United States

In 2011, exceptionally low atmospheric moisture content combined with moderately high temperatures to produce a record-high vapor pressure deficit (VPD) in the southwestern United States (SW). These conditions combined with record-low cold-season precipitation to cause widespread drought and extreme wildfires. Although interannual VPD variability is generally dominated by temperature, high VPD in 2011 was also driven by a lack of atmospheric moisture. The May–July 2011 dewpoint in the SW was 4.5 standard deviations below the long-term mean. Lack of atmospheric moisture was promoted by already very dry soils and amplified by a strong ocean-to-continent sea level pressure gradient and upper-level convergence that drove dry northerly winds and subsidence upwind of and over the SW. Subsidence drove divergence of rapid and dry surface winds over the SW, suppressing southerly moisture imports and removing moisture from already dry soils. Model projections developed for the fifth phase of the Coupled Model Intercomparison Project (CMIP5) suggest that by the 2050s warming trends will cause mean warm-season VPD to be comparable to the record-high VPD observed in 2011. CMIP5 projections also suggest increased interannual variability of VPD, independent of trends in background mean levels, as a result of increased variability of dewpoint, temperature, vapor pressure, and saturation vapor pressure. Increased variability in VPD translates to increased probability of 2011-type VPD anomalies, which would be superimposed on ever-greater background VPD levels. Although temperature will continue to be the primary driver of interannual VPD variability, 2011 served as an important reminder that atmospheric moisture content can also drive impactful VPD anomalies

Correlations between components of the water balance and burned area reveal new insights for predicting forest fire area in the southwest United States

We related measurements of annual burned area in the southwest United States during 1984–2013 to records of climate variability. Within forests, annual burned area correlated at least as strongly with spring–summer vapour pressure deficit (VPD) as with 14 other drought-related metrics, including more complex metrics that explicitly represent fuel moisture. Particularly strong correlations with VPD arise partly because this term dictates the atmospheric moisture demand. Additionally, VPD responds to moisture supply, which is difficult to measure and model regionally due to complex micrometeorology, land cover and terrain. Thus, VPD appears to be a simple and holistic indicator of regional water balance. Coupled with the well-known positive influence of prior-year cold season precipitation on fuel availability and connectivity, VPD may be utilised for burned area forecasts and also to infer future trends, though these are subject to other complicating factors such as land cover change and management. Assuming an aggressive greenhouse gas emissions scenario, climate models predict mean spring–summer VPD will exceed the highest recorded values in the southwest in nearly 40% of years by the middle of this century. These results forewarn of continued increases in burned forest area in the southwest United States, and likely elsewhere, when fuels are not limiting

Indian Monsoon Onset and the Americas Midsummer Drought: Out-of-Equilibrium Responses to Smooth Seasonal Forcing

Abstract Two dominant high-frequency features of Northern Hemisphere summer climatology are examined in an atmosphere–land general circulation model (AGCM): the sudden onset of rains in south Asia, and the midsummer rainfall minimum in the tropical Americas. A control simulation succeeds in capturing these observed features fairly well. A slowed-calendar experiment is performed, to see whether these features are close to equilibrium with seasonally evolving forcings (orbital geometry and SST). The results indicate that some lag (disequilbrium) within the AGCM delays south Asian onset by about a month, from May in the experiment when seasonal forcing evolves extremely slowly to June in the normal, full-speed seasonal cycle. Disequilibrium also acts to delay and limit the amplitude of the Americas midsummer drought, and the associated intrusion of the Atlantic subtropical high into the Intra-Americas Seas’ region. It is hypothesized that early summer (centered on the solstice) temperature over mid- and high-latitude continents, which differs greatly between experiment and control, drives the low-latitude rainfall differences. A more mysterious pole-to-pole, annual-mean, zonal wave-1 difference is also found in the slowed-calendar experiment

Effects of global irrigation on the near-surface climate

Irrigation delivers about 2,600 km3 of water to the land surface each year, or about 2% of annual precipitation over land. We investigated how this redistribution of water affects the global climate, focusing on its effects on near-surface temperatures. Using the Community Atmosphere Model (CAM) coupled to the Community Land Model (CLM), we compared global simulations with and without irrigation. To approximate actual irrigation amounts and locations as closely as possible, we used national-level census data of agricultural water withdrawals, disaggregated with maps of croplands, areas equipped for irrigation, and climatic water deficits. We further investigated the sensitivity of our results to the timing and spatial extent of irrigation. We found that irrigation alters climate significantly in some regions, but has a negligible effect on global-average near-surface temperatures. Irrigation cooled the northern mid-latitudes; the central and southeast United States, portions of southeast China and portions of southern and southeast Asia cooled by ~0.5 K averaged over the year. Much of northern Canada, on the other hand, warmed by ~1 K. The cooling effect of irrigation seemed to be dominated by indirect effects like an increase in cloud cover, rather than by direct evaporative cooling. The regional effects of irrigation were as large as those seen in previous studies of land cover change, showing that changes in land management can be as important as changes in land cover in terms of their climatic effects. Our results were sensitive to the area of irrigation, but were insensitive to the details of irrigation timing and delivery