scholarly journals Interannual Variations of Wind Regimes off the Subtropical Western Australia Coast during Austral Winter and Spring

2012 ◽  
Vol 25 (16) ◽  
pp. 5587-5599 ◽  
Author(s):  
Evan Weller ◽  
Ming Feng ◽  
Harry Hendon ◽  
Jian Ma ◽  
Shang-Ping Xie ◽  
...  
Keyword(s):  
Western Australia ◽  
Geopotential Height ◽  
General Circulation ◽  
Circulation Model ◽  
Interannual Variations ◽  
Storm Events ◽  
Austral Winter ◽  
Easterly Wind ◽  
Upper Troposphere ◽  
Australian Continent

Abstract Off the Western Australia coast, interannual variations of wind regime during the austral winter and spring are significantly correlated with the Indian Ocean dipole (IOD) and the southern annular mode (SAM) variability. Atmospheric general circulation model experiments forced by an idealized IOD sea surface temperature anomaly field suggest that the IOD-generated deep atmospheric convection anomalies trigger a Rossby wave train in the upper troposphere that propagates into the southern extratropics and induces positive geopotential height anomalies over southern Australia, independent of the SAM. The positive geopotential height anomalies extended from the upper troposphere to the surface, south of the Australian continent, resulting in easterly wind anomalies off the Western Australia coast and a reduction of the high-frequency synoptic storm events that deliver the majority of southwest Australia rainfall during austral winter and spring. In the marine environment, the wind anomalies and reduction of storm events may hamper the western rock lobster recruitment process.

The Simulation of Stationary and Transient Geopotential-Height Eddies in January and July with a Spectral General Circulation Model

1984 ◽  
Vol 41 (8) ◽  
pp. 1394-1419 ◽  
Author(s):  
Robert C. Malone ◽  
Eric J. Pitcher ◽  
Maurice L. Blackmon ◽  
Kamal Puri ◽  
William Bourke
Keyword(s):  
General Circulation Model ◽  
Geopotential Height ◽  
General Circulation ◽  
Circulation Model

scholarly journals Interannual Variations of Summer Monsoons: Sensitivity to Cloud Radiative Forcing

1998 ◽  
Vol 11 (8) ◽  
pp. 1883-1905 ◽  
Author(s):  
O. P. Sharma ◽  
H. Le Treut ◽  
G. Sèze ◽  
L. Fairhead ◽  
R. Sadourny
Keyword(s):  
Optical Properties ◽  
Radiative Forcing ◽  
General Circulation ◽  
Initial Conditions ◽  
Model Simulation ◽  
Circulation Model ◽  
Interannual Variations ◽  
Radiative Effects ◽  
High Concentration ◽  
Sea Temperature

Abstract The sensitivity of the interannual variations of the summer monsoons to imposed cloudiness has been studied with a general circulation model using the initial conditions prepared from the European Centre for Medium-Range Forecasts analyses of 1 May 1987 and 1988. The cloud optical properties in this global model are calculated from prognostically computed cloud liquid water. The model successfully simulates the contrasting behavior of these two successive monsoons. However, when the optical properties of the observed clouds are specified in the model runs, the simulations show some degradation over India and its vicinity. The main cause of this degradation is the reduced land–sea temperature contrast resulting from the radiative effects of the observed clouds imposed in such simulations. It is argued that the high concentration of condensed water content of clouds over the Indian land areas will serve to limit heating of the land, thereby reducing the thermal contrast that gives rise to a weak Somali jet. A countermonsoon circulation is, therefore, simulated in the vector difference field of 850-hPa winds from the model runs with externally specified clouds. This countermonsoon circulation is associated with an equatorial heat source that is the response of the model to the radiative effects of the imposed clouds. Indeed, there are at least two clear points that can be made: 1) the cloud–SST patterns, together, affect the interannual variability; and 2) with both clouds and SST imposed, the model simulation is less sensitive to initial conditions. Additionally, the study emphasizes the importance of dynamically consistent clouds developing in response to the dynamical, thermal, and moist state of the atmosphere during model integrations.

Comparison of the Semtner and Chervin eddy-resolving global ocean model with LUCIE and satellite observations in the Leeuwin Current region

1996 ◽  
Vol 47 (3) ◽  
pp. 509 ◽  
Author(s):  
CJC Reason ◽  
AF Pearce
Keyword(s):  
Western Australia ◽  
Satellite Data ◽  
General Circulation ◽  
Maximum Velocity ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Global Ocean ◽  
Western Coast ◽  
Model Output ◽  
Leeuwin Current

Output from the Semtner and Chervin eddy-resolving global ocean general circulation model is compared with observations from the Leeuwin Current Interdisciplinary Experiment (LUCIE) and satellite data for the coastal waters of Western Australia. The model output is a snapshot over the domain 9-43�S, 90-120�E for a day in mid July 1987, which is during the season that the Leeuwin Current is expected to be well established along the western and southern coasts of Western Australia. Maximum Leeuwin Current velocities in the model are of the order of 60 cm s-1 and are found in the southern part of the current on the western coast and around into the Great Australian Bight. At depths below about 200 m, and centred near 400 m, there is an equatorward-flowing undercurrent with maximum velocity of order 25 cm s-1. Comparison of temperature and salinity cross-sections with LUCIE observations reveals that the model output for this day exhibits many realistic features. In particular, the model fields display a number of prominent meanders and eddies on the Leeuwin Current as well as further offshore. Consistent with observations, mesoscale features associated with the Leeuwin Current are concentrated between 25�S and the Cape Mentelle region; the flow in the northern part of the Leeuwin Current and the North West Shelf may be too weak to induce eddy-generating instabilities. Prominent in the model output are two large meanders on the Leeuwin Current between 25�S and 29�S and two anticyclonic eddies further downstream; features similar to these are evident in satellite data during winter 1987.

Tropical Connections to Climatic Change in the Extratropical Southern Hemisphere: The Role of Atlantic SST Trends

2014 ◽  
Vol 27 (13) ◽  
pp. 4923-4936 ◽  
Author(s):  
Graham R. Simpkins ◽  
Shayne McGregor ◽  
Andréa S. Taschetto ◽  
Laura M. Ciasto ◽  
Matthew H. England
Keyword(s):  
Climatic Change ◽  
Atmospheric Circulation ◽  
Southern Hemisphere ◽  
Geopotential Height ◽  
General Circulation ◽  
Rossby Waves ◽  
Circulation Model ◽  
Hadley Cell ◽  
Austral Spring ◽  
The Impact

The austral spring relationships between sea surface temperature (SST) trends and the Southern Hemisphere (SH) extratropical atmospheric circulation are investigated using an atmospheric general circulation model (AGCM). A suite of simulations are analyzed wherein the AGCM is forced by underlying SST conditions in which recent trends are constrained to individual ocean basins (Pacific, Indian, and Atlantic), allowing the impact of each region to be assessed in isolation. When forced with observed global SST, the model broadly replicates the spatial pattern of extratropical SH geopotential height trends seen in reanalyses. However, when forcing by each ocean basin separately, similar structures arise only when Atlantic SST trends are included. It is further shown that teleconnections from the Atlantic are associated with perturbations to the zonal Walker circulation and the corresponding intensification of the local Hadley cell, the impact of which results in the development of atmospheric Rossby waves. Thus, increased Rossby waves, forced by positive Atlantic SST trends, may have played a role in driving geopotential height trends in the SH extratropics. Furthermore, these atmospheric circulation changes promote warming throughout the Antarctic Peninsula and much of West Antarctica, with a pattern that closely matches recent observational records. This suggests that Atlantic SST trends, via a teleconnection to the SH extratropics, may have contributed to springtime climatic change in the SH extratropics over the past three decades.

scholarly journals First outcomes from the CNR-ISAC monthly forecasting system

2012 ◽  
Vol 8 (1) ◽  
pp. 77-82 ◽  
Author(s):  
D. Mastrangelo ◽  
P. Malguzzi ◽  
C. Rendina ◽  
O. Drofa ◽  
A. Buzzi
Keyword(s):  
Geopotential Height ◽  
General Circulation ◽  
Circulation Model ◽  
Forecast Errors ◽  
Preliminary Evaluation ◽  
National Council ◽  
Probabilistic Forecasting ◽  
Monthly Basis ◽  
Standard Pressure ◽  
Forecasting System

Abstract. A monthly probabilistic forecasting system is experimentally operated at the ISAC institute of the National Council of Research of Italy. The forecasting system is based on GLOBO, an atmospheric general circulation model developed at the same institute. The model is presently run on a monthly basis to produce an ensemble of 32 forecasts initialized with GFS-NCEP perturbed analyses. Reforecasts, initialized with ECMWF ERA-Interim reanalyses of the 1989–2009 period, are also produced to determine modelled climatology of the month to forecast. The modelled monthly climatology is then used to calibrate the ensemble forecast of daily precipitation, geopotential height and temperature on standard pressure levels. In this work, we present the forecasting system and a preliminary evaluation of the model systematic and forecast errors in terms of non-probabilistic scores of the 500-hPa geopotential height. Results show that the proposed forecasting system outperforms the climatology in the first two weeks of integrations. The adopted calibration based on weighted bias correction is found to reduce the systematic and the forecast errors.

scholarly journals Forcing of the Upper-Tropospheric Monsoon Anticyclones

2019 ◽  
Vol 76 (7) ◽  
pp. 1937-1954 ◽  
Author(s):  
Leong Wai Siu ◽  
Kenneth P. Bowman
Keyword(s):  
General Circulation ◽  
Asian Monsoon ◽  
Lower Stratosphere ◽  
Warm Season ◽  
Circulation Model ◽  
Western Hemisphere ◽  
North American Monsoon ◽  
Upper Troposphere ◽  
Forcing Mechanisms ◽  
Good Agreement

Abstract During the boreal warm season (May–September), the circulation in the upper troposphere and lower stratosphere is dominated by two large anticyclones: the Asian monsoon anticyclone (AMA) and North American monsoon anticyclone (NAMA). The existence of the AMA has long been linked to Asian monsoon precipitation using the Matsuno–Gill framework, but the origin of the NAMA has not been clearly understood. Here the forcing mechanisms of the NAMA are investigated using a simplified dry general circulation model. The simulated anticyclones are in good agreement with observations when the model is forced by a zonally symmetric meridional temperature gradient plus a realistic geographical distribution of heating based on observed tropical and subtropical precipitation in the Northern Hemisphere. Model experiments show that the AMA and NAMA are largely independent of one another, and the NAMA is not a downstream response to the Asian monsoon. The primary forcing of the NAMA is precipitation in the longitude sector between 60° and 120°W, with the largest contribution coming from the subtropical latitudes within that sector. Experiments with idealized regional heating distributions reveal that the extratropical response to tropical and subtropical precipitation depends approximately linearly on the magnitude of the forcing but nonlinearly on its latitude. The AMA is stronger than the NAMA, primarily because precipitation in the subtropics over Asia is much heavier than at similar latitudes in the Western Hemisphere.

scholarly journals ENSO Amplitude Modulation Associated with the Mean SST Changes in the Tropical Central Pacific Induced by Atlantic Multidecadal Oscillation

2014 ◽  
Vol 27 (20) ◽  
pp. 7911-7920 ◽  
Author(s):  
In-Sik Kang ◽  
Hyun-ho No ◽  
Fred Kucharski
Keyword(s):  
Observational Data ◽  
Wind Stress ◽  
General Circulation ◽  
Southern Oscillation ◽  
Coupled Model ◽  
Circulation Model ◽  
Atlantic Multidecadal Oscillation ◽  
Positive Phase ◽  
Central Pacific ◽  
Easterly Wind

Abstract The mechanism associated with the modulation of the El Niño–Southern Oscillation (ENSO) amplitude caused by the Atlantic multidecadal oscillation (AMO) is investigated by using long-term historical observational data and various types of models. The observational data for the period 1900–2013 show that the ENSO variability weakened during the positive phase of the AMO and strengthened in the negative phase. Such a relationship between the AMO and ENSO amplitude has been reported by a number of previous studies. In the present study the authors demonstrate that the weakening of the ENSO amplitude during the positive phase of the AMO is related to changes of the SST cooling in the eastern and central Pacific accompanied by the easterly wind stress anomalies in the equatorial central Pacific, which were reproduced reasonably well by coupled general circulation model (CGCM) simulations performed with the Atlantic Ocean SST nudged perpetually with the observed SST representing the positive phase of the AMO and the free integration in the other ocean basins. Using a hybrid coupled model, it was determined that the mechanism associated with the weakening of the ENSO amplitude is related to the westward shift and weakening of the ENSO zonal wind stress anomalies accompanied by the westward shift of precipitation anomalies associated with the relatively cold background mean SST over the central Pacific.

scholarly journals Abrupt Circulation Responses to Tropical Upper-Tropospheric Warming in a Relatively Simple Stratosphere-Resolving AGCM

2012 ◽  
Vol 25 (12) ◽  
pp. 4097-4115 ◽  
Author(s):  
Shuguang Wang ◽  
Edwin P. Gerber ◽  
Lorenzo M. Polvani
Keyword(s):  
Climate Change ◽  
General Circulation ◽  
Climate Models ◽  
Climate Model ◽  
Polar Vortex ◽  
Circulation Model ◽  
Hadley Cell ◽  
Critical Threshold ◽  
Intergovernmental Panel ◽  
Upper Troposphere

Abstract The circulation response of the atmosphere to climate change–like thermal forcing is explored with a relatively simple, stratosphere-resolving general circulation model. The model is forced with highly idealized physics, but integrates the primitive equations at resolution comparable to comprehensive climate models. An imposed forcing mimics the warming induced by greenhouse gasses in the low-latitude upper troposphere. The forcing amplitude is progressively increased over a range comparable in magnitude to the warming projected by Intergovernmental Panel on Climate Change coupled climate model scenarios. For weak to moderate warming, the circulation response is remarkably similar to that found in comprehensive models: the Hadley cell widens and weakens, the tropospheric midlatitude jets shift poleward, and the Brewer–Dobson circulation (BDC) increases. However, when the warming of the tropical upper troposphere exceeds a critical threshold, ~5 K, an abrupt change of the atmospheric circulation is observed. In the troposphere the extratropical eddy-driven jet jumps poleward nearly 10°. In the stratosphere the polar vortex intensifies and the BDC weakens as the intraseasonal coupling between the troposphere and the stratosphere shuts down. The key result of this study is that an abrupt climate transition can be effected by changes in atmospheric dynamics alone, without need for the strong nonlinearities typically associated with physical parameterizations. It is verified that the abrupt climate shift reported here is not an artifact of the model’s resolution or numerics.

scholarly journals A Simple Kinematic Source of Skewness in Atmospheric Flow Fields

2012 ◽  
Vol 69 (2) ◽  
pp. 578-590 ◽  
Author(s):  
Fay Luxford ◽  
Tim Woollings
Keyword(s):  
Geopotential Height ◽  
General Circulation ◽  
Dynamical Behavior ◽  
Kinematic Model ◽  
Circulation Model ◽  
Jet Stream ◽  
Jet Streams ◽  
Close Link ◽  
Nonlinear Dynamical ◽  
Kinematic Effect

Abstract Geopotential height fields exhibit a well-known pattern of skewness, with distributions that are positively skewed on the poleward side of the midlatitude jets/storm tracks and negatively skewed on the equatorward side. This pattern has often been interpreted as a signature of nonlinear dynamical features, such as blocking highs and cutoff lows, and there is renewed interest in the higher moments of flow variables as indicators of the nature of the underlying dynamics. However, this paper suggests that skewness can arise as a simple kinematic consequence of the presence of jet streams and so may not be a reliable indicator of nonlinear dynamical behavior. In support of this, reanalysis data are analyzed to demonstrate a close link between the jet streams and the skewness patterns. Further evidence is provided by a simple stochastic kinematic model of a jet stream as a Gaussian wind profile. The parameters of this model are fitted to data from the reanalysis and also from an aquaplanet general circulation model. The skewness of the model’s geopotential height and zonal wind fields are then compared to those of the original data. This shows that a fluctuating jet stream can produce patterns of skewness that are qualitatively similar to those observed, although the magnitude of the skewness is significantly overestimated by the kinematic model. These results suggest that this simple kinematic effect does contribute to the observed patterns of skewness but that other processes (such as nonlinear dynamics) likely also play a role.

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