Circulation In Middle Latitudes Research Articles

Analysis of decadal characteristics of leading teleconnections based on complex networks theory

We construct a complex network of the middle latitude circulation system of north hemisphere (NCNH). The decadal characteristic of leading teleconnection and its probable correlation with abrupt climate change are analyzed base on the circulation system network. Results show that (1) action centres of NAO, EUPA and WP have all moved during different periods in the recent 40 years and there has been a common abrupt change process as the end of the 1970s and beginning of the 1990s separately; (2) the new index which denotes the importance of teleconnection is defined using the node degree of NCNH. The temporal change of new index shows that the leading modes were PNA, EUPA and WP before the 1980s; index of NAO notably increased and all the five teleconnections were comparatively robust after 1980s; the index of AO notably increased and other modes comparatively decreased after 1987. So, center and index changes of decadal leading teleconnections may be another important phenomenon of abrupt climate change.

The east asian monsoon simulation with IAP AGCMs-a composite study

Preliminary analyses on simulation of East Asian monsoon (EAM) by IAP 9-level and 2-level Atmosphereic General Circulation Models of AMIP run were made. The analyses include the seasonal and interannual variations of EAM circulation, the monsoon index defined with 850 hPa wind, the circulation differences of composite strong and weak monsoon years and the regional precipitation over China monsoon region. Two models can give reasonable simulation on the seasonal variation of the subtropical high over Western Pacific and the middle latitude circulation index, the observed global circulation differences for strong and weak EAM. However, the abilities of the models on reproducing the cross-equatorial wind of EAM system, the interannual variation of EAM and the regional precipitation variability are unsatisfactory. Improvements on EAM simulation should contain both the better representation on the regional surface details (orography, surface albedo and surface water and energy flux) and the improvement of the tropical global circulation modelling.

The statistical properties of general atmospheric circulation: Observational evidence and a minimal theory of bimodality

AbstractIn this paper we address the problem of constructing a minimal model of atmospheric circulation able to describe the bulk of the statistical properties of the general circulation in middle latitudes. After a brief reexamination of the Charney‐DeVore theory of multiple equilibria, we compare the statistical properties that can be deduced from it with those resulting from analysis of observations of the northern hemisphere middle‐latitude circulation at 500 mb. While the prediction concerning the amplitude of the waves seems to be confirmed by the presence of a clear bimodality of the statistical distribution, we find no trace of bimodality in the zonal wind. We show how the theory can be brought into agreement with observations by introducing nonlinearity into the wave equation in such a way that the Charney‐DeVore resonance curve is bent, producing different states corresponding to the same value of the zonal wind. We finally address ourselves to the question of what physical processes determine the observed unimodal statistics of zonal wind, a subject of future work.

The statistical properties of general atmospheric circulation: observational evidence and a minimal theory of bimodality

In this paper we address the problem of constructing a minimal model of atmospheric circulation able to describe the bulk of the statistical properties of the general circulation in middle latitudes. After a brief reexamination of the Charney-DeVore theory of multiple equilibria, we compare the statistical properties that can be deduced from it with those resulting from analysis of observations of the northern hemisphere middle-latitude circulation at 500 mb. While the prediction concerning the amplitude of the waves seems to be confirmed by the presence of a clear bimodality of the statistical distribution, we find no trace of bimodality in the zonal wind. We show how the theory can be brought into agreement with observations by introducing nonlinearity into the wave equation in such a way that the Charney-DeVore resonance curve is bent, producing different states corresponding to the same value of the zonal wind. We finally address ourselves to the question of what physical processes determine the observed unimodal statistics of zonal wind, a subject of future work.

Possible influences of solar radiation variations on the atmospheric circulation in the northern hemisphere of the earth

The strength of the circumpolar zonal circulation of the atmosphere in the northern hemisphere, expressed by an index calculated from the air pressure gradient between 35° N and 65°N, shows a remarkable ‘secular change’ in this century in the years 1935 to 1938, and probably again in the years after 1970. A contemporary change and parallel course with a high positive correlation can be found in a solar radiation index calculated by means of sunspot numbers and solar faculae areas. A good correlation was also found between this solar radiation index and a series of UV measurements on Mt. Wilson. The possible mechanism of action in the atmosphere will be discussed, and it is shown that this solar radiation index is also correlated with circulation parameters in the troposphere and stratosphere over Europe. There are indications of a decrease of the action from the upper to the lower atmosphere. The consequences of this secular change of circulation for the climatic history in the 20th century in Europe are presented and from that a hypothetical retrospective view of the European climatic history before the 20th century, based on the total series of sunspot numbers, is briefly discussed. In a general view with smoothed data it can be concluded that the sun seems to determine the basic structure of the north hemispheric circulation in middle latitudes, if a strong influence of sunspots and solar faculae on the solar radiation is assumed.

The role of interaction with middle latitude circulation in the behaviour of the Southwest Monsoon of 1972 and 1979 : Raman, C. R. V., Y. P. Rao and S. M. A. Alvi, 1980. Curr. Sci., 49(4): 123–129

The role of interaction with middle latitude circulation in the behaviour of the Southwest Monsoon of 1972 and 1979 : Raman, C. R. V., Y. P. Rao and S. M. A. Alvi, 1980. Curr. Sci., 49(4): 123–129

On the role of the Asian monsoon in the angular momentum and kinetic energy balances of the tropics

The balance conditions of angular momentum and time-mean kinetic energy and their annual variations are studied for the northern hemisphere tropical belt. The belt is divided into two roughly equal-sized parts, the monsoon and the extramonsoon regions. The data used consist of all available daily rawinsonde reports from the world aerological network for the two 5-year periods 1958-63 and 1968-73.
 
 In winter, the trade winds in the monsoon and extramonsoon regions are both sources of relative westerly angular momentum for the middle latitude circulation. However, it is found that the angular momentum gained in the extramonsoon region of the tropics (26 Hadley) is mostly destroyed by a net southward flow of mass in that region (on the order of 30 cm sec-l), and becomes regenerated in the monsoon region by a net northward flow of mass there. This excess of angular momentum together with the angular momentum picked up locally in the monsoon region (17 Hadley) is almost all exported across its northern boundary (35 Hadley). It is further found that in winter the tropics are also an important source of mean kinetic energy for middle latitudes. Again almost all export of kinetic energy was found to take place across the northern boundary of the monsoon sector. Most of this energy must be generated through the pressure gradient term inside the monsoon region itself (about 4 watt m-2), the transformation from transient eddy kinetic energy being very small. The proper evaluation of the zonal pressure gradient appears to be the main stumbling block in the present study, preventing us from estimating the generation and thereby, as a residual, the frictional dissipation in the two regions.
 
 In summer, the extramonsoon region remains a source of angular momentum (20 Hadley), but the monsoon region with its surface westerlies acts as a sink (-18 Hadley), leading to a sharp reduction (and even a midsummer reversal) of the export into middle latitudes. Also the export of mean kinetic energy almost vanishes in summer, except for a small southward transfer across the equator. Probably most of the summer generation and dissipation of mean kinetic energy in the tropics (about I watt m-2) occur in the monsoon sector itself without appreciable exchange with other regions. The calculations for two 5-year periods give very similar estimates and thereby show the reliability of the results.

On the advective and diffusive heat balance in the interior of a subtropical ocean

An exponential solution to the thermoeline equations is scaled and calibrated for the intermediate circulation in middle latitudes of the open ocean in accordance with the scaling and results of the more complete numerical models of Bryan & Cox. Evaluation of the average potential vorticity for a number of stations is a main feature of the calibration method. The results of tests on the calibrated model in which the wind stress parameter E is varied show good qualitative agreement with the published numerical results for the region in which the analytical model is valid. The effects of varying the vertical thermal diffusion parameter K are investigated. A significant feature of the results is the lack of appreciable difference in temperature distributions for the cases when E = K = 1 (the reference case) and E = 1, K = 0 (the purely advective case). The relative sizes of terms in the heat-balance equation are investigated. It is shown that the dominant balance near the surface (but beneath the frictional layer) is between horizontal and vertical advection if the ratio E / KC >1 where C is the dimensionless thermocline scale parameter evaluated from the potential vorticity equation. The value of the above ratio is approximately 5 in the reference case. For all values of the parameters it is shown that the ratio of horizontal advection to vertical diffusion is not less than one just beneath the surface layer in middle latitude regions of surface downwelling. DOI: 10.1111/j.2153-3490.1971.tb00584.x

On the advective and diffusive heat balance in the interior of a subtropical ocean

An exponential solution to the thermoeline equations is scaled and calibrated for the intermediate circulation in middle latitudes of the open ocean in accordance with the scaling and results of the more complete numerical models of Bryan & Cox. Evaluation of the average potential vorticity for a number of stations is a main feature of the calibration method. The results of tests on the calibrated model in which the wind stress parameter E is varied show good qualitative agreement with the published numerical results for the region in which the analytical model is valid. The effects of varying the vertical thermal diffusion parameter K are investigated. A significant feature of the results is the lack of appreciable difference in temperature distributions for the cases when E = K = 1 (the reference case) and E = 1, K = 0 (the purely advective case). The relative sizes of terms in the heat-balance equation are investigated. It is shown that the dominant balance near the surface (but beneath the frictional layer) is between horizontal and vertical advection if the ratio E / KC >1 where C is the dimensionless thermocline scale parameter evaluated from the potential vorticity equation. The value of the above ratio is approximately 5 in the reference case. For all values of the parameters it is shown that the ratio of horizontal advection to vertical diffusion is not less than one just beneath the surface layer in middle latitude regions of surface downwelling. DOI: 10.1111/j.2153-3490.1971.tb00584.x

THE EASTERN PACIFIC HURRICANE SEASON OF 1968

A statistical resume of the season is presented and the lack of a valid climatology for comparison is pointed out. Background for the report is provided through discussion of limited basic data and description of the unique hurricane environment. Contributions of 1968 hurricanes to general knowledge of storms in the area include new ideas relating to inflow of cool air off cold water to the north and a demonstration that vertical shear has diagnostic value for revealing stoppage of the hurricane convective chimney. A few generalities as to the relationship between 1968 hurricane and storm behavior and middle latitude circulation are considered. A chronicle of individual storms and hurricanes gives particular attention to important data, mainly satellite pictures, and includes an example of a small intense hurricane with radius of tropical storm intensity winds of approximately 40 mi.

Breaks in the Indian summer monsoon as a phenomenon of interaction between the easterly and the sub-tropical westerly jet streams

The present paper contains the results of the study of a synoptic situation over India during a period of 8 days in which normal monsoon conditions were followed by a break and were further followed by a return to normal monsoon. This study has shown that during breaks in the monsoon, a trough in the middle latitude westerlies with a jet embedded in it increases considerably in its amplitude and gets retarded as it moves into the Tibetan Plateau on account of its entering a region of weaker basic current. The large-amplitude trough weakens or destroys completely the Tibetan high at the 500 mb level and appreciably weakens the easterlies at the higher levels to the south of the Himalayas. Protruding into India and Pakistan, the large amplitude trough contributes to the development of heavy rainfall along and near the foot of the Himalayas which is characteristic of the break conditions. The southward protrusion of the trough and its subsequent movement eastward, is also indirectly responsible for a corresponding shift of the high over Iran and Arabia, which, in its turn, seems to lead to the formation of a secondary jet core in the easterlies west of 80° E between 20° N and 25° N. Consequent on the above developments, the westerly jet which had retreated to the north of the Himalayas at the time of the onset of the monsoon reenters the Indo-Pakistan sub-continent during the breaks. Thus we see during break conditions, the remarkable spectacle of two jets of entirely different types-the easterly and the sub-tropical westerly—within a short latitudinal distance of each other and dynamically interacting with each other.
 The above detailed study, supplemented by a general examination of the monthly mean 700 mb charts for the northern hemisphere for a period of 10 years, suggests that active monsoon over India is closely associated with high index circulation in middle latitudes over Asia and neighbourhood, while weak monsoon is closely associated with low index circulation over the same region.

Mean meridional circulations in the atmosphere

AbstractThe mean meridional circulation in winter and summer is derived as a function of latitude and height from an analysis of observed winds in the area 160°W‐0‐40°E, 75°N‐15°N. The results confirm the classical idea of a direct circulation in low latitudes, but the indirect circulation in middle latitudes is stronger than expected. An investigation of the energy released by the meridional circulation implies that it is of comparable importance to large‐scale disturbances in the energy balance of the general circulation.

Low Latitude Vorticity Injections and the Development of Large Scale Anomalous Circulation Patterns

Some general characteristics of synoptic developments in the 500 mb vorticity field are discussed with particular attention devoted to sudden injections of vorticity and to changes in circulation patterns downwind therefrom. Attention is then focused on vorticity injections generated at low latitudes, but not too low to interact in an important way with the middle latitude circulation. Composite maps based upon numerous cases are shown which reveal the development of large scale anomalous patterns in the westerlies during the 3 days subsequent to low latitude vorticity injection. The forecasting implications are discussed.

Recent variations in the general circulation as related to glacier retreat in northern Scandinavia

A review is given of the investigations on variations in the general circulation in middle latitudes made byWillett andPetterssen. According toWillett a «high-index» type of circulation pattern with a strong zonal flow has probably caused the recent climatic improvement in the northern latitudes while the «low-index» type with meridional flow would create a cooling off in the same areas. According toPetterssen an other type of «low-index» pattern with a weak zonal flow and strong meridional exchange of air has been the essential cause of the warming up of the northern latitudes in Europe. This implies that the definition of the «low-index» type is not clear and gives rise to misunderstanding. Making use of our experience on glacier retreat in northern Scandinavia it is concluded that thePetterssen type of circulation more probably than the «high-index» type has caused the warming-up in the North Atlantic area from which the retreat of the glaciers has followed.