Features Of General Circulation Research Articles

Recreating Observed Convection‐Generated Gravity Waves From Weather Radar Observations via a Neural Network and a Dynamical Atmospheric Model

AbstractConvection‐generated gravity waves (CGWs) transport momentum and energy, and this momentum is a dominant driver of global features of Earth's atmosphere's general circulation (e.g., the quasi‐biennial oscillation, the pole‐to‐pole mesospheric circulation). As CGWs are not generally resolved by global weather and climate models, their effects on the circulation need to be parameterized. However, quality observations of GWs are spatiotemporally sparse, limiting understanding and preventing constraints on parameterizations. Convection‐permitting or ‐resolving simulations do generate CGWs, but validation is not possible as these simulations cannot reproduce the CGW‐forcing convection at correct times, locations, and intensities. Here, realistic convective diabatic heating, learned from full‐physics convection‐permitting Weather Research and Forecasting simulations, is predicted from weather radar observations using neural networks and a previously developed look‐up table. These heating rates are then used to force an idealized GW‐resolving dynamical model. Simulated CGWs forced in this way closely resembled those observed by the Atmospheric InfraRed Sounder in the upper stratosphere. CGW drag in these validated simulations extends 100s of kilometers away from the convective sources, highlighting errors in current gravity wave drag parameterizations due to the use of the ubiquitous single‐column approximation. Such validatable simulations have significant potential to be used to further basic understanding of CGWs, improve their parameterizations physically, and provide more restrictive constraints on tuning with confidence.

Making the Case for High-Resolution Regional Ocean Reanalyses: An Example with the Red Sea

Abstract Coastal oceans host 40% of the world population and amount to $1.5 trillion of the global economy. Studying, managing, and developing the coastal regions require decades-long information about their environment. Long-term ocean measurements are, however, lacking for most coastal regions and often global reanalyses are used instead. These are, however, coarse in nature and tuned for the global circulations. The Red Sea (RS) is a narrow basin connected to the Indian Ocean through the Bab-al-Mandab Strait. Despite being the busiest commercial crossroad and hosting the world’s third largest coral reef system, the RS lacks long-term observations. A recent increase in population and an unprecedented acceleration in governmental and industrial developments further emphasized the need for long-term datasets to support its development and the sustainability of its habitats, and to understand its response to a changing climate. Toward this end, we have generated a 20-yr high-resolution reanalysis for the RS (RSRA) using a state-of-the-art ensemble data assimilation system incorporating available observations. Compared to global reanalyses, RSRA provides a markedly better description of the RS general and mesoscale circulation features, their variability, and trends. In particular, RSRA accurately captures the three-layer summer transport through the Bab-al-Mandab, simulated as two-layer transport by some global reanalyses. It further reproduces the seasonal anomalies, whereas global reanalyses misidentify some seasons as anomalous. Global reanalyses further overestimate the interannual variations in salinity, misrepresent the trend in temperature, and underestimate the trend in sea level. Our study clearly emphasizes the importance of generating dedicated high-resolution regional ocean reanalyses.

Blocking and General Circulation in GFDL Comprehensive Climate Models

Abstract This study examines the climatology and dynamics of atmospheric blocking, and the general circulation features that influence blocks in GFDL’s atmosphere-only (AM4) and coupled atmosphere–ocean (CM4) comprehensive models. We compare AM4 and CM4 with reanalysis, focusing on winter in the Northern Hemisphere. Both models generate the correct blocking climatology and planetary-scale signatures of the stationary wave. However, at regional scales some biases exist. In the eastern Pacific and over western North America, both models generate excessive blocking frequency and too strong of a stationary wave. In the Atlantic, the models generate too little blocking and a weakened stationary wave. A block-centered compositing analysis of block-onset dynamics reveals that the models 1) produce realistic patterns of high-frequency (1–6-day) eddy forcing and 2) capture the notable differences in the 500-hPa geopotential height field between Pacific and Atlantic blocking. However, the models fail to reproduce stronger wave activity flux convergence in the Atlantic compared to the Pacific. Overall, biases in the blocking climatology in terms of location, frequency, duration, and area are quite similar between AM4 and CM4 despite the models having large differences in sea surface temperatures and climatological zonal circulation. This could suggest that other factors could be more dominant in generating blocking biases for these GCMs. Significance Statement Atmospheric blocks are persistent high pressure systems that can lead to hazardous weather. Historically, climate models have had trouble capturing blocks, but recent changes in the models might lead to improvements. As such, the work herein investigates the spatial distribution, prevalence, duration, size, and dynamics of wintertime blocking in recent NOAA climate models. Overall, these models capture the long-term-average spatial pattern of blocking, and properly reproduce key dynamical features. However, the models produce too much blocking in the western United States, and too little over the northern Atlantic Ocean and Europe. These blocking biases are consistent with atmospheric stationary waves biases, but not jet stream bias. This downplays the role of jet biases in the models being responsible for blocking biases.

High-Resolution Operational Ocean Forecast and Reanalysis System for the Indian Ocean

AbstractA good understanding of the general circulation features of the oceans, particularly of the coastal waters, and ability to predict the key oceanographic parameters with good accuracy and sufficient lead time are necessary for the safe conduct of maritime activities such as fishing, shipping, and offshore industries. Considering these requirements and buoyed by the advancements in the field of ocean modeling, data assimilation, and ocean observation networks along with the availability of the high-performance computational facility in India, Indian National Centre for Ocean Information Services has set up a “High-Resolution Operational Ocean Forecast and Reanalysis System” (HOOFS) with an aim to provide accurate ocean analysis and forecasts for the public, researchers, and other types of users like navigators and the Indian Coast Guard. Major components of HOOFS are (i) a suite of numerical ocean models configured for the Indian Ocean and the coastal waters using the Regional Ocean Modeling System (ROMS) for forecasting physical and biogeochemical state of the ocean and (ii) the data assimilation based on local ensemble transform Kalman filter that assimilates in situ and satellite observations in ROMS. Apart from the routine forecasts of key oceanographic parameters, a few important applications such as (i) Potential Fishing Zone forecasting system and (ii) Search and Rescue Aid Tool are also developed as part of the HOOFS project. The architecture of HOOFS, an account of the quality of ocean analysis and forecasts produced by it and important applications developed based on HOOFS are briefly discussed in this article.

Atmospheric blocking in an aquaplanet and the impact of orography

Abstract. Many fundamental questions remain about the roles and effects of stationary forcing on atmospheric blocking. As such, this work utilizes an idealized moist general circulation model (GCM) to investigate atmospheric blocking in terms of dynamics, geographical location, and duration. The model is first configured as an aquaplanet, then orography is added in separate integrations. Block-centered composites of wave activity fluxes and height show that blocks in the aquaplanet undergo a realistic dynamical evolution when compared to reanalysis. Blocks in the aquaplanet are also found to have similar life cycles to blocks in model integrations with orography. These results affirm the usefulness of both zonally symmetric and asymmetric idealized model configurations for studying blocking. Adding orography to the model leads to an increase in blocking. This mirrors what is observed when comparing the Northern Hemisphere (NH) and Southern Hemisphere (SH), where the NH contains more orography and thus more blocking. As the prescribed mountain height increases, so do the magnitude and size of climatological stationary waves, resulting in more blocking overall. Increases in blocking, however, are not spatially uniform. Orography is found to induce regions of enhanced block frequency just upstream of mountains, near high pressure anomalies in the stationary waves, which is poleward of climatological minima in upper-level zonal wind, while block frequency minima and jet maxima occur eastward of the wave trough. This result matches what is observed near the Rocky Mountains. Finally, an analysis of block duration suggests blocks generated near stationary wave maxima last slightly longer than blocks that form far from or without orography. Overall, the results of this work help to explain some of the observed similarities and differences in blocking between the NH and SH and emphasize the importance of general circulation features in setting where blocks most frequently occur.

Performance of One-Way and Two-Way Nesting Techniques Using the Shelf Circulation Modelling System for the Eastern Canadian Shelf

The performance of one-way and two-way nesting techniques is assessed in this study using model results produced by a regional ocean circulation modelling system for the eastern Canadian shelf. The assessment is made in terms of dynamical consistency between the parent model (PM) and child model (CM), representation of general circulation features, and reduction of numerical noise generated during the interaction of the PM and CM. It is demonstrated that the feedback from the CM to the PM in numerical experiments using two-way nesting ensures that the large-scale circulation produced by the PM and CM be dynamically consistent over the region where the two model domains overlap. In comparison with one-way nesting, two-way nesting leads to a better representation of coastal currents over the Gulf of St. Lawrence and the Scotian Shelf and improves the large-scale circulation in the results produced by the PM. This study also examines an alternative two-way nesting technique based on the semi-prognostic method in which differences between the PM and CM densities are used to adjust the horizontal momentum balance in the PM. Model results demonstrate the advantage of the semi-prognostic method in eliminating numerical noise during the feedback from the CM to PM while ensuring dynamical consistency between the two model components.

The thermosphere general circulation modeling with the parametrization of radiative processes

The paper presents a new version of the global three-dimensional general circulation model of the Earth’s thermosphere (90-500 km) with high spatial resolution (2 × 2.5 × 80), including consistent calculation of radiative processes. Based on a detailed analysis of the reproduction of the various components of radiation transfer a good agreement of radiation balance with empirical data is shown in the new model. Analytical estimations and model results proved that the thermosphere global state formation is essentially determined by the ratio between the radiation heating and heat sink due to molecular parameters, as well as by the lower boundary conditions. On the base of the preliminary model identification with empirical data a satisfactory reproduction of the thermal balance and the thermospheric general circulation features is shown.

The Hydrometeorology of Niger River Basin

This paper examines the mean annual cycle of rainfall and general circulation features over the Niger River Basin consisting of mainly West Africa and Central Africa regions. Rainfall is examined using a 1400-station archive compiled by earlier workers. Other circulation features are examined using the NCEPNCAR reanalysis dataset. Important features of the reanalysis zonal wind field are shown to compare well with the seasonal evolution described by the radiosonde observations. In addition to the well-known African easterly jet (AEJ) of the Northern Hemisphere, the seasonal evolution of its Southern Hemisphere counterpart is also described. Thermal wind calculations show that although the southern jet is weaker, its existence is also due to a local reversal of the surface temperature gradient. In the upper troposphere, a strong semiannual cycle is shown in the 200-mb easterlies and a feature like the tropical easterly jet (TEJ) is evident south of the equator in January and February. The paper describes the movement of the rainbelt between central and West Africa. An asymmetry in the northward and southward migration of the rainbelt is evident. The paper discusses the influence that the jets may have on rainfall and possible feedback effects of rainfall on the jets. Evidence suggests that the midtropospheric jets influence the development of the rainy season, but also that the rainfall affects the surface temperature gradient and in turn the jets. In the Northern Hemisphere, east of 200E, the axis of the TEJ is located so that it may promote convection by increasing upper-level divergence. However, west of 100E and in the Southern Hemisphere, the location of the TEJ is consistent with the suggestion that it is the equatorward outflow of convection that produces the TEJ. The paper notes that rainfall and river flow is largely influenced by groundwater base flow, and a return to sustained river flow requires replenishment of the aquifers, which is possible only with cumulative raining years. The paper confirms that there is correlation between the decrease in rainfall and low river flows.

Evaluation of the Circulation Patterns in the Black Sea Using Remotely Sensed and <i>in Situ</i> Measurements

The objective of the present work is to provide an overview of the general circulation features in the Black Sea basin. In order to achieve this, 18 years (1993-2010) of satellite data coming from the Aviso website were analyzed. A description of the general circulation patterns in the Black Sea is first presented. This is followed by statistical analyses of the satellite data in 20 points covering the entire area of the sea. The reference points were chosen as follows: 12 points along the Rim cyclonic current, 3 points inside the Rim cyclonic current, 4 points on the edge of two of the biggest anticyclonic gyres outside the Rim current and one point in the northwestern shelf area of the basin. Rose graphics were drawn for the reference points for winter and summer time. Finally, 9 years of in situ data obtained from the Gloria drilling platform were analyzed and compared with the satellite data. The present study shows that most of the reference points are sensitive to seasonal changes. The current velocities depend mostly on the points location: the points located on the Rim current and on the nearshore anticyclonic eddies present higher values than the ones located in or outside the general circulation features.

A Note on Pacific Decadal Oscillation, El Nino Southern Oscillation, Atlantic Multidecadal Oscillation and the Intertropical Front in Sahel, Africa

The advance and retreat of the Sahara Desert are repetitive in decadal time scale. This climate variability, unique in the world, results from the irregular motion of the Intertropical Front (ITF), a discontinuity originated from the convergence of two air masses over the African continent. It results from the convergence of a dry air mass flowing off the North African anticyclone and a southwestern warm and humid air mass that originates off the South Atlantic anticyclone. As it crosses the equator, this air stream is shifted rightwards, becoming southwesterly winds and transporting moisture into the continent. The equatorial thermal low associated with ITF over the continent is deeper and more intense than the Intertropical Convergence Zone (ITCZ) one and its thermodynamic properties differ remarkably, moisture concentration in particular. The ITF annual latitudinal displacement is larger than that of the Atlantic ITCZ, reaching 20?N commonly during the North Hemisphere summer. In this manuscript one discuss about the possibility that Pacific Decadal Oscillation (PDO) is not a regular physical oscillation and its phases duration may not be predictable and be dependent of large magnitude seismic events, which are rare and random. An important point, however, is that the Pacific sea surface temperature (SST) pattern, once established and recognized, may remain for a relative long (decades) period. Its SST pattern, due to its large area extension, modulates the global climate forcing the large scale general circulation features, ENSO in particular, to produce distinct regional impacts, depending on the signal and strength of the SST anomalies. This happened with the ITF in the Sahel and may possibly have occurred with other large scale atmospheric structures, such as the Asian Monsoon, convergence zones and subtropical anticyclones centers positioning.

Low-latitude mesospheric vertical winds observed using VHF radar

[1] Using long-term data (January 1998 to December 2009) collected from Mesosphere-Stratosphere-Troposphere (MST) radar located at a tropical station, Gadanki (13.5°N, 79.2°E), India, variability of low-latitude mesospheric vertical wind is investigated for the first time. The emphasis is on sub-daily, monthly, seasonal and annual variabilities in vertical wind in the altitude region 65–85 km. Possible sources of errors in the vertical wind measurement at mesospheric altitudes from the MST radar are discussed. The observed mesospheric vertical wind is generally upward in all the seasons. During winter and summer, significant sub-daily variations are noticed followed by spring and fall equinoxes. The vertical wind can reach occasionally values as high as 5 ms−1 but most of the time (95%) it is, in general, less than ∼2.63 ms−1. The present observations are consistent with the general circulation features in recent models for low latitude locations with northward and upward wind prevailing throughout the year representing part of meridional circulation.

Season division and its temporal and spatial variation features of General atmospheric circulation in East Asia

Using the NCEP-DOE Reanalysis 2 data of air temperature, geopotential height, relative humidity and wind field of 17 pressure levels from 1979 to 2009, and the Multi-factor Fisher optimal dissection method, the seasons of the general atmospheric circulation in East Asia are diuided. The main results are as follows. (1)The seasonal establishment of atmospheric general circulation is initiated from subtropic of the stratosphere in East Asia. In the troposphere the spring and the summer first start from the tropical regions, and then extend into middle latitudes. The autumn and the winter first start from middle latitudes, and then extend into low latitudes. (2) In the troposphere the seasonal establishment of atmospheric general circulation is more consistent, and it is very different from that in the stratosphere. And the seasonal establishments of the middle and the low level of the stratosphere are not very consistent. (3)In the past 30 years, the four seasons have started much earlier in about 6 years, in which years some stronger El Nio phenomena just happened. That implies that the El Nio phenomenon causes abnormal seasonal variation.

The blocking anticyclones: the state of studies and forecasting

In connection with analysis of anomalous weather in summer of 2010, the state-of-the-art is reviewed of climatology of the blocking anticyclones, of mechanism for their onset and persistence, as well as possibilities of their forecasting. The main results are quoted on occurrence frequency, intensity, and duration of blockings and on their connection with features of general circulation of the atmosphere in the Northern and Southern hemispheres. As the primary cause of the blocking formation, nonlinear instability of the Rossby waves is considered with intense energy exchange with planetary waves and with synoptic-scale eddies. Mathematical means have been developed to investigate the energy exchange on the real data of objective analysis (reanalysis). This approach can be applied to analyze causes of anomalous duration of the summer blocking. From the point of view of forecasting, blockings do not differ much from other atmospheric pressure systems. Namely, within the framework of medium-range forecasting, the blocking onset, under condition of its absence in the initial fields, can be predicted with a projection of several days; also, the ensemble forecasts are generally more successful than those obtained with any single model. The blocking duration and decay can be predicted successfully within the medium-range forecasting.

Assessment of regional seasonal predictability using the PRECIS regional climate modeling system over South America

The purpose of this study was to evaluate the accuracy and skill of the UK Met Office Hadley Center Regional Climate Model (HadRM3P) in describing the seasonal variability of the main climatological features over South America and adjacent oceans, in long-term simulations (30 years, 1961–1990). The analysis was performed using seasonal averages from observed and simulated precipitation, temperature, and lower- and upper-level circulation. Precipitation and temperature patterns as well as the main general circulation features, including details captured by the model at finer scales than those resolved by the global model, were simulated by the model. However, in the regional model, there are still systematic errors which might be related to the physics of the model (convective schemes, topography, and land-surface processes) and the lateral boundary conditions and possible biases inherited from the global model.

Hydrodynamic modelling of mesoscale eddies in the Black Sea

A three dimensional structure of mesoscale circulation in the Black Sea is simulated using the Proudman Oceanographic Laboratory Coastal Ocean Modelling System. A number of sensitivity tests reveal the response of the model to changes in the horizontal resolution, time steps, and diffusion coefficients. Three numerical grids are examined with x-fine (3.2 km), fine (6.7 km) and coarse (25 km) resolution. It is found that the coarse grid significantly overestimates the energy of the currents and is not adequate even for the study of basin-scale circulation. The x-fine grid, on the other hand, does not give significant advantages compared to the fine grid, and the latter is used for the bulk of simulations. The most adequate parameters are chosen from the sensitivity study and used to model both the basin-scale circulation and day-to-day variability of mesoscale currents for the months of May and June of 2000. The model is forced with actual wind data every 6 h and monthly climatic data for evaporation, precipitation, heat fluxes and river run-off. The results of the fine grid model are compared favourably against the satellite imagery. The model adequately reproduces the general circulation and many mesoscale features including cyclonic and anticyclonic eddies, jets and filaments in different parts of the Black Sea. The model gives a realistic geographical distribution and parameters of mesoscale currents, such as size, shape and evolution of the eddies.

Simulating the upper ocean circulation on the belize shelf: An application of a triply nested-grid ocean circulation model

We present a three-level nested-grid ocean circulation modeling system for the Belize shelf of the western Caribbean Sea. The nested-grid system has three subcomponents: a coarse-resolution outer model of the western Caribbean Sea; an intermediate-resolution middle model of the southern Meso-American Barrier Reef System; and a fine-resolution inner model of the Belize shelf. The two-way nesting technique based on the semi-prognostic method is used to exchange information between the three subcomponents. We discuss two applications of the nested-grid system in this study. In the first application we simulate the seasonal mean circulation in the region, with the nested system forced by monthly mean surface fluxes and boundary forcing. The model results reproduce the general circulation features on the western Caribbean Sea and meso-scale circulation features on the Belize shelf. In the second application, we simulate the storm-induced circulation during Hurricane Mitch in 1998, with the nested-grid system forced by the combination of monthly mean forcing and idealized wind stress associated with the storm. The model results demonstrate that the storm-induced currents transport a large amount of estuarine waters from coastal regions of Honduras and Guatemala to offshore reef atolls.

Deep water mass characteristics and interannual variability in the North and Central Aegean Sea

The characteristics and interannual variability of the deep water masses in the North and Central Aegean Sea are being investigated through the data sets of the Hellenic Navy Hydrographic Service (HNHS) and the MEDATLAS 1997 project. In the period between 1987 and 1993, the densest deep water in the Mediterranean has been produced in the Aegean Sea (with σ θ densities reaching up to 29.6 kg/m 3), contributing to what has been called the Eastern Mediterranean Transient. The examination of time series of mean integrated values of θ, S and σ θ below the depth of 500 dbar reveals the significant deep water density increase after 1987 in all of the deep basins in the area. Data suggest that the density increase of 1987–1988 is mainly attributed to a temperature drop, while in 1993, an even more intense density increase is observed, characterized this time by an abrupt salinity increase. We assume that the increased salinity necessary to produce deep water masses with the observed characteristics was not locally produced but rather advected from the Levantine through the South Aegean. After 1993, no new deep water formation episodes have been observed. A series of Θ– S diagrams derived from HNHS CTD casts covering the period between 1993 and 2000, depict the different characteristics of the deep water masses in the area. As 1993 marks the end of the formation period, observed differences between basins in that year must be attributed to different deep water formation sites. Thereafter, the stagnating deep water in the North and Central Aegean basins has been slowly gaining buoyancy by losing salt and gaining heat. The rate at which this phenomenon takes place varies between different deep basins. It is suggested that these variations are linked to the different volumes of each basin as well as to the general circulation features of the Aegean Sea.

A Global-Scale Examination of Monsoon-Related Precipitation

Abstract A pentad version of the Global Precipitation Climatology Project global precipitation dataset is used to document the annual and interannual variations in precipitation over monsoon regions around the globe. An algorithm is described that determines objectively wet season onset and withdrawal for individual years, and this tool is used to examine the behavior of various characteristics of the major monsoon systems. The definition of onset and withdrawal are determined by examining the ramp-up and diminution of rainfall within the context of the climatological rainfall at each location. Also examined are interannual variations in onset and withdrawal and their relationship to rainy season precipitation accumulations. Changes in the distribution of “heavy” and “light” precipitation events are examined for years in which “abundant” and “poor” wet seasons are observed, and associations with variations in large-scale atmospheric general circulation features are also examined. In particular, some regio...

The Seasonal Evolution of the Atmospheric Circulation over West Africa and Equatorial Africa

Abstract This paper examines the mean annual cycle of rainfall and general circulation features over West Africa and central Africa for 1958–97. Rainfall is examined using a 1400-station archive compiled by the first author. Other circulation features are examined using the NCEP–NCAR reanalysis dataset. Important features of the reanalysis zonal wind field are shown to compare well with the seasonal evolution described by the radiosonde observations. In addition to the well-known African easterly jet (AEJ) of the Northern Hemisphere, the seasonal evolution of its Southern Hemisphere counterpart is also described. Thermal wind calculations show that although the southern jet is weaker, its existence is also due to a local reversal of the surface temperature gradient. In the upper troposphere, a strong semiannual cycle is shown in the 200-mb easterlies and a feature like the tropical easterly jet (TEJ) is evident south of the equator in January and February. The paper describes the movement of the rainbelt ...

Modelling the Danube-influenced North-western Continental Shelf of the Black Sea. I: Hydrodynamical Processes Simulated by 3-D and Box Models

A hydrodynamical modelling of the Black Sea is presented using the GHER 3-D primitive equation model. Results of a very high resolution model with 5 km grid size are analysed and the dominant features of general circulation in the Black Sea are highlighted. Compared with a coarse resolution model, forced with the same climatic monthly averaged atmospheric data, the high resolution model exhibits stronger variability, including frontal structures and coastal upwellings induced by baroclinic instabilities, in particular along the Turkish coast. A comparison of the shelf-open sea exchanges, with particular focus on the cold intermediate waters, shows that both models lead to a replenishment of the CIL in the basin interior by cold waters formed on the shelf. Due to the better representation of frontal structures, the high resolution model is an appropriate candidate for coupling with a biological model. Problems of calibration, interpretation and data availability typically arise from this kind of coupled 3-D model. In order to overcome such difficulties, the results of the 3-D hydrodynamical model are used to guide the development of an integrated 0-D box model capable of achieving the objectives of projects like EROS21, where the effect of changes in the Danube inflows on the shelf ecosystem is investigated. The 0-D model is thus designed to cover this region and is obtained through integration over an appropriate variable volume. The integration procedure shows where the weaknesses of an 0-D approach might lie. Diagnoses in the 3-D model of the integral quantities show the range of uncertainty one can expect in the exchange laws of the 0-D model.