Abstract

The influence of surface thermohaline forcing on the variability of the Atlantic meridional overturning circulation (MOC) at mid–high latitudes is investigated using output from three Intergovernmental Panel on Climate Change (IPCC) coupled climate models. The method employed is an extension of the surface-forced streamfunction approach, based on water mass transformation theory, used in an earlier study by . The maximum value of the MOC at 48°N is found to have a significant lagged relationship with the maximum surface-forced streamfunction in the region north of 48°N with a surface density greater than σ ₀ = 27.5 kg m⁻³. This correlation peaks when the index of the surface-forced streamfunction leads the MOC by 2–4 yr, depending on the coupled model considered. A method for estimating the MOC variability solely from the surface forcing fields is developed and found to be in good agreement with the actual model MOC variability in all three of the models considered when a past averaging window of 10 yr is employed. This method is then applied with NCEP–NCAR reanalysis surface flux fields for the period 1949–2007 to reconstruct MOC strength over 1958–2007. The reconstructed MOC shows considerable multidecadal variability but no discernible trend over the modern observational era.

Abstract

A global isopycnal coordinate GCM is used to investigate the processes that drive the meridional circulation, transformation, and interocean exchange of water masses in the Southern Ocean. The noneddy-resolving model (mesh size 1.25°) includes an active mixed layer, parameterized bolus transport, and seasonally varying surface fluxes. The model gives a plausible picture of the formation and circulation of subantarctic mode water (SAMW) and Antarctic Intermediate Water (AAIW). Progressively denser versions of SAMW and AAIW form in the Indian and Pacific Oceans as the Antarctic Circumpolar Current drifts south and loses buoyancy.

SAMW forms predominantly in the Indian Ocean, at a rate of 20 Sv (Sv ≡ 10⁶ m³ s⁻¹), while AAIW forms mainly in the Pacific sector, at a rate of 8.5 Sv. Throughout the circumpolar zone 25°–42.5°S, there is a net formation of 11 Sv of SAMW, largely by surface cooling. This SAMW is exported northward across 25°S into the subtropical gyres. The properties, distribution, and recirculation of SAMW and AAIW compare well with observations. The authors differentiate the effects of surface fluxes and mixing in transforming water masses in two distinct circumpolar zones. South of 42.5°S, surface buoyancy gain (due to a slight dominance of freshening over cooling) and diapycnal mixing are shown to play a roughly equal role in lightening water (at a peak diapycnal flux of 9 Sv across σ = 27.3), and in forming AAIW.

The meridional overturning is computed as a function of density and decomposed. The parameterized bolus transport opposes the northward surface Ekman drift and southward deep geostrophic flow. Denser waters are not in steady state and the meridional overturning streamfunction gives a misleading impression of dense water transformation in the Southern Ocean. A “transformation streamfunction” is introduced that gives the correct (model) transformation rates; this is believed to be a powerful tool in diagnosing models that drift.

The implications for model North Atlantic Deep Water (NADW) are considerable. In the model, NADW is transported southward across 25°S in the Atlantic sector at a rate of 15.7 Sv. South of 25°S, NADW and Circumpolar Deep Water (CDW) are consumed by interior diapycnal mixing at a rate of 5.7 Sv. NADW and CDW are exported northward across 25°S in the Indo-Pacific sector at a rate of 19.5 Sv. The 9.5 Sv imbalance amounts to a steady loss of NADW and CDW from the Southern Ocean, highlighting the unsteadiness of dense water masses in the model.

Abstract

The response of the Northern Hemisphere winter stratosphere to the Pacific decadal oscillation (PDO) is examined using the Whole Atmosphere Community Climate Model. A 200-yr preindustrial control simulation that includes fully interactive chemistry, ocean and sea ice, constant solar forcing, and greenhouse gases fixed to 1850 levels is analyzed. Based on principal component analysis, the PDO spatial pattern, frequency, and amplitude agree well with the observed PDO over the period 1900–2014. Consistent with previous studies, the positive phase of the PDO is marked by a strengthened Aleutian low and a wave train of geopotential height anomalies reminiscent of the Pacific–North American pattern in the troposphere. In addition to a tropospheric signal, a zonal-mean warming of about 2 K in the northern polar stratosphere and a zonal-mean zonal wind decrease of about 4 m s⁻¹ in the PDO positive phase are found. When compositing PDO positive or negative winters during neutral El Niño years, the magnitude is reduced and depicts an early winter forcing of the stratosphere compared to a late winter response from El Niño. Contamination between PDO and ENSO signals is also discussed. Stratospheric sudden warmings occur 63% of the time in the PDO positive phase compared to 40% in the negative phase. Although this sudden warming frequency is not statistically significant, it is quantitatively consistent with NCEP–NCAR reanalysis data and recent observational evidence linking the PDO positive phase to weak stratospheric vortex events.

Abstract

Estimates of the recent mean and time varying water mass transformation rates associated with North Atlantic surface-forced overturning are presented. The estimates are derived from heat and freshwater surface fluxes and sea surface temperature fields from six atmospheric reanalyses—the Japanese 25-yr Reanalysis (JRA), the NCEP–NCAR reanalysis (NCEP1), the NCEP–U.S. Department of Energy (DOE) reanalysis (NCEP2), the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-I), the Climate Forecast System Reanalysis (CFSR), and the Modern-Era Reanalysis for Research and Applications (MERRA)—together with sea surface salinity fields from two globally gridded datasets (World Ocean Atlas and Met Office EN3 datasets). The resulting 12 estimates of the 1979–2007 mean surface-forced streamfunction all depict a subpolar cell, with maxima north of 45°N, near σ = 27.5 kg m⁻³, and a subtropical cell between 20° and 40°N, near σ = 26.1 kg m⁻³. The mean magnitude of the subpolar cell varies between 12 and 18 Sv (1 Sv ≡ 10⁶ m³ s⁻¹), consistent with estimates of the overturning circulation from subsurface observations. Analysis of the thermal and haline components of the surface density fluxes indicates that large differences in the inferred low-latitude circulation are largely a result of the biases in reanalysis net heat flux fields, which range in the global mean from −13 to 19 W m⁻². The different estimates of temporal variability in the subpolar cell are well correlated with each other. This suggests that the uncertainty associated with the choice of reanalysis product does not critically limit the ability of the method to infer the variability in the subpolar overturning. In contrast, the different estimates of subtropical variability are poorly correlated with each other, and only a subset of them captures a significant fraction of the variability in independently estimated North Atlantic Subtropical Mode Water volume.

Abstract

This paper introduces the three-dimensional Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), which treats atmospheric dynamics, radiation, and chemistry interactively for the height range from the earth’s surface to the thermosphere (approximately 250 km). It is based on the latest version of the ECHAM atmospheric general circulation model of the Max Planck Institute for Meteorology in Hamburg, Germany, which is extended to include important radiative and dynamical processes of the upper atmosphere and is coupled to a chemistry module containing 48 compounds. The model is applied to study the effects of natural and anthropogenic climate forcing on the atmosphere, represented, on the one hand, by the 11-yr solar cycle and, on the other hand, by a doubling of the present-day concentration of carbon dioxide. The numerical experiments are analyzed with the focus on the effects on temperature and chemical composition in the mesopause region. Results include a temperature response to the solar cycle by 2 to 10 K in the mesopause region with the largest values occurring slightly above the summer mesopause. Ozone in the secondary maximum increases by up to 20% for solar maximum conditions. Changes in winds are in general small. In the case of a doubling of carbon dioxide the simulation indicates a cooling of the atmosphere everywhere above the tropopause but by the smallest values around the mesopause. It is shown that the temperature response up to the mesopause is strongly influenced by changes in dynamics. During Northern Hemisphere summer, dynamical processes alone would lead to an almost global warming of up to 3 K in the uppermost mesosphere.

The Mackenzie River is the largest North American source of freshwater for the Arctic Ocean. This basin is subjected to wide fluctuations in its climate and it is currently experiencing a pronounced warming trend. As a major Canadian contribution to the Global Energy and Water Cycle Experiment (GEWEX), the Mackenzie GEWEX Study (MAGS) is focusing on understanding and modeling the fluxes and reservoirs governing the flow of water and energy into and through the climate system of the Mackenzie River Basin. MAGS necessarily involves research into many atmospheric, land surface, and hydrological issues associated with cold climate systems. The overall objectives and scope of MAGS will be presented in this article.

Abstract

MetPy is an open-source, Python-based package for meteorology, providing domain-specific functionality built extensively on top of the robust scientific Python software stack, which includes libraries like NumPy, SciPy, Matplotlib, and xarray. The goal of the project is to bring the weather analysis capabilities of GEMPAK (and similar software tools) into a modern computing paradigm. MetPy strives to employ best practices in its development, including software tests, continuous integration, and automated publishing of web-based documentation. As such, MetPy represents a sustainable, long-term project that fills a need for the meteorological community. MetPy’s development is substantially driven by its user community, both through feedback on a variety of open, public forums like Stack Overflow, and through code contributions facilitated by the GitHub collaborative software development platform. MetPy has recently seen the release of version 1.0, with robust functionality for analyzing and visualizing meteorological datasets. While previous versions of MetPy have already seen extensive use, the 1.0 release represents a significant milestone in terms of completeness and a commitment to long-term support for the programming interfaces. This article provides an overview of MetPy’s suite of capabilities, including its use of labeled arrays and physical unit information as its core data model, unit-aware calculations, cross sections, skew T and GEMPAK-like plotting, station model plots, and support for parsing a variety of meteorological data formats. The general road map for future planned development for MetPy is also discussed.

The Community Earth System Model (CESM) is a flexible and extensible community tool used to investigate a diverse set of Earth system interactions across multiple time and space scales. This global coupled model significantly extends its predecessor, the Community Climate System Model, by incorporating new Earth system simulation capabilities. These comprise the ability to simulate biogeochemical cycles, including those of carbon and nitrogen, a variety of atmospheric chemistry options, the Greenland Ice Sheet, and an atmosphere that extends to the lower thermosphere. These and other new model capabilities are enabling investigations into a wide range of pressing scientific questions, providing new foresight into possible future climates and increasing our collective knowledge about the behavior and interactions of the Earth system. Simulations with numerous configurations of the CESM have been provided to phase 5 of the Coupled Model Intercomparison Project (CMIP5) and are being analyzed by the broad community of scientists. Additionally, the model source code and associated documentation are freely available to the scientific community to use for Earth system studies, making it a true community tool. This article describes this Earth system model and its various possible configurations, and highlights a number of its scientific capabilities.

The MacKenzie Global Energy and Water Cycle Experiment (GEWEX) Study, Phase 1, seeks to improve understanding of energy and water cycling in the Mackenzie River basin (MRB) and to initiate and test atmospheric, hydrologic, and coupled models that will project the sensitivity of these cycles to climate change and to human activities. Major findings from the study are outlined in this paper. Absorbed solar radiation is a primary driving force of energy and water, and shows dramatic temporal and spatial variability. Cloud amounts feature large diurnal, seasonal, and interannual fluctuations. Seasonality in moisture inputs and outputs is pronounced. Winter in the northern MRB features deep thermal inversions. Snow hydrological processes are very significant in this high-latitude environment and are being successfully modeled for various landscapes. Runoff processes are distinctive in the major terrain units, which is important to overall water cycling. Lakes and wetlands compose much of MRB and are prominent as hydrologic storage systems that must be incorporated into models. Additionally, they are very efficient and variable evaporating systems that are highly sensitive to climate variability. Mountainous high-latitude sub-basins comprise a mosaic of land surfaces with distinct hydrological attributes that act as variable source areas for runoff generation. They also promote leeward cyclonic storm generation. The hard rock terrain of the Canadian Shield exhibits a distinctive energy flux regimen and hydrologic regime. The MRB has been warming dramatically recently, and ice breakup and spring outflow into the Polar Sea has been occurring progressively earlier. This paper presents initial results from coupled atmospheric-hydrologic modeling and delineates distinctive cold region inputs needed for developments in regional and global climate modeling.

Abstract

Understanding observed changes to the global water cycle is key to predicting future climate changes and their impacts. While many datasets document crucial variables such as precipitation, ocean salinity, runoff, and humidity, most are uncertain for determining long-term changes. In situ networks provide long time series over land, but are sparse in many regions, particularly the tropics. Satellite and reanalysis datasets provide global coverage, but their long-term stability is lacking. However, comparisons of changes among related variables can give insights into the robustness of observed changes. For example, ocean salinity, interpreted with an understanding of ocean processes, can help cross-validate precipitation. Observational evidence for human influences on the water cycle is emerging, but uncertainties resulting from internal variability and observational errors are too large to determine whether the observed and simulated changes are consistent. Improvements to the in situ and satellite observing networks that monitor the changing water cycle are required, yet continued data coverage is threatened by funding reductions. Uncertainty both in the role of anthropogenic aerosols and because of the large climate variability presently limits confidence in attribution of observed changes.