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T. N. Krishnamurti

Abstract

Daily upper-wind analysis at 200 mb was carried out between the equator and 45N, around the globe, for December 1955 to February 1956 to map the subtropical jet stream. It turned out that this current formed a pattern of three standing long waves which maintained nearly a steady state. This made possible an analysis of the meteorological variables in a coordinate system fixed with respect to the jet stream axis at 200 mb.

Monthly mean maps and vertical cross sections of wind, temperature, moisture and mass distribution were computed in this coordinate system. The jet-stream core is situated close to the 200-mb surface and has a maximum speed close to 70 m per sec in the mean integrated around the world; this is mainly due to very high speeds over Africa and Asia. Mean velocities close to or above 160 kn are found in the ridges of the meandering jet axis, and lower speeds are observed in the troughs. Computations of mass circulation across the jet-stream axis revealed (for all three months of winter) a thermally direct system of the order of a few knots in the sense of the classical Hadley circulation. The center of the mass-circulation cell was about 15 deg lat equatorward of the jet axis.

Mean structure of the wave is obtained by analysis of fields of temperature, wind and height of constant pressure surfaces.

Some simple calculations are made to evaluate the strength of the computed mass circulation in the heat balance and kinetic-energy balance of the atmospheric general circulation. Rossby's hypothesis of constant vorticity transport is examined.

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T. N. Krishnamurti

Abstract

Calculations of budgets of heat, angular momentum and kinetic energy for the tropical latitudes are obtained in a coordinate system following the subtropical jet stream axis at the 200-mb surface.

Comparisons of the fluxes of various quantities with those required at certain latitudes show that the vertical mass circulation has a very important role in the tropics and that the daily eddies are important on the poleward side of the jet stream.

Large fluxes of various atmospheric properties are computed for 3 different months. The month of January, 1956, shows very large export of kinetic energy and angular momentum from tropics into middle latitudes.

Statistics of the middle-latitude zonal and meridional motion are studied. It is shown that the month of January, 1956, was one of very low zonal index, blocking, and very slow eastward wave motion. Possible connection between lower and higher latitude circulations is suggested.

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T. N. Krishnamurti

Abstract

Observational evidence of a major east-west circulation during the northern summer is presented in this paper. The geometry of this circulation is portrayed in the form of the streamlines of the divergent part of the wind for the seasonal mean motion field. It is, furthermore, shown that 1) the intensity of this circulation is comparable to that of the Hadley type circulation, 2) the circulation is thermally direct, 3) there is a generation of kinetic energy by these east-west over-turnings of mass, and 4) this circulation is distinctly different from the so called Walker circulation, the latter being a southern extension of the more vigorous east-west circulation.

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T. N. Krishnamurti
and
H. N. Bhalme

Abstract

In this paper the elements of a monsoon system are defined, and its oscillations are determined from spectral analysis of long observational records. The elements of the monsoon system include pressure of the monsoon trough, pressure of the Mascarene high, cross-equatorial low-level jet, Tibetan high, tropical easterly jet, monsoon cloud cover, monsoon rainfall, dry static stability of the lower troposphere, and moist static stability of the lower troposphere. The summer monsoon months over India during normal monsoon rainfall years are considered as guidelines in the selection of data for the period of this study. The salient result of this study is that there seems to exist a quasi-biweekly oscillation in almost all of the elements of the monsoon system. For some of these elements, such as the surface pressure field, monsoon rainfall, low-level cross-equatorial jet and monsoon cloudiness, the amplitude of this oscillation in quasi-biweekly range is very pronounced. For the spectral representation of the time series, the product of the spectral density times frequency is used as the ordinate and the log of the frequency as the abscissa. Dominant modes are also found in the shorter time scales (<6 days). A sequential ordering of elements of the monsoon systems for the quasi-biweekly oscillation is carried out in terms of their respective phase angle. The principal result here is that soon after the maximum dry and moist static instabilities are realized in the stabilizing phase, there occur in sequence an intensification of the monsoon trough, satellite brightness, Mascarene high, Tibetan high and the tropical easterly jet. Soon after that the rainfall maximum over central India, arising primarily from monsoon depressions, is found to be a maximum.

In the second part of this paper we offer some plausible mechanisms for these quasi-biweekly oscillations.

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T. N. Krishnamurti
and
Y. Ramanathan

Abstract

This paper examines some observational aspects of the evolution of energy exchanges and differential heating during the GARP Monsoon Experiment MONEX. The main findings are that:

  1. 1) A large increase in the kinetic energy of the total flow field and the nondivergent component of the flows occurs over the Arabian Sea about a week prior to the onset of monsoon rains over central India.

  2. 2) The field of differential heating moves during the onset period to a favorable position for the generation of eddy available potential energy and its release to eddy kinetic energy.

  3. 3) The release of the eddy available potential energy goes to enhance the kinetic energy of the divergent circulations.

  4. 4) The kinetic energy of the divergent circulation does not increase much with time. This energy is shown to he transferred rapidly to the nondivergent motion via a number of interaction functions. The orientation of the divergent flows is shown to be of prime importance in these transfers during the onset, active, break and revival periods of the monsoons.

Based on the above observational findings a series of numerical prediction experiments ire conducted to examine the sensitivity of the monsoon onset to initially imposed fields of differential heating. The results of some 96 h integrations seem to confirm the large sensitivity in the evolution to the field of heating. The results of time integrations also show that the kinetic energy of the monsoon circulations increases via the rapid increase of interactions among the irrotational and nondivergent modes.

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Da-Sheng Yang
and
T. N. Krishnamurti

Abstract

A study of the potential vorticity budget for the low-level flows over the Arabian Sea and Indian Ocean is presented here. This study covers a 17-day period between 11 and 27 June 1979 during the GARP Monsoon Experiment (MONEX). Data sets for this study include the special observing systems for the Global Experiment, i.e., cloud winds from the geostationary satellites, surface and upper air data from the First GARP Global Experiment, FGGE and MONEX, data from low-level constant-level balloons, upper air data from dropwindsonde aircraft, and the World Weather Watch. The selection of the 17-day period is important logically for both phenomena and from the availability of the maximum data sets, over oceans during the global experiment. As many as 50 soundings per day were available from this observing system. The framework for the calculations are three equations that describe the changes in potential vorticity, absolute vorticity and the dry static stability. These equations include as relevant forcing the effects of shortwave and longwave radiative processes, shallow and deep cumulus convection, surface friction, and the air-sea transfers of momentum, heat and moisture.

The period of this study coincides with that of the onset and establishment of deep moist westerlies of the monsoons. The Somali jet established itself around 10°N during this period and the onset of monsoon rain had begun. A large increase of positive potential vorticity on the cyclonic shear side of the low-level jet is attributed to an increase of both the absolute vorticity and of the dry static stability. The mechanisms responsible for this increase are explored via detailed budget calculations. The salient mechanisms arise from the covariance of potential vorticity and differential radiative heating along the vertical, and vertical convergence of potential vorticity by the cumulus mass flux, which contributes to a generation of potential vorticity. On the other hand, the role of horizontal advection and friction seems to be opposite.

The meridional flow is countergradient, such that horizontal advection brings low values of dry static stability, absolute vorticity and potential vorticity into the region of their large positive values north of the equator. The wind-stress curl in this region is positive; it contributes to a diminution of the absolute as well as potential vorticity.

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T. N. Krishnamurti
and
D. Subrahmanyam

Abstract

The analyzed wind field at 850 mb during the summer monsoon experiment (MONEX) is subjected to a time series analysis to confirm the existence of a peak in the time range of 30–50 days. Having established that, this study presents a mapping of the motion field for this time scale. In this short paper we illustrate the steady meridional propagation of a train of troughs and ridges that seem to form near the equator and dissipate near the Himalayas. The meridional scale of this mode is around 300 km, and its meridional speed of propagation is ∼0.75° latitude per day. The amplitude of the wind for this mode is around 3–6 m s−1. The salient contributions here are the mapping and the demonstration of a very regular behavior of this mode; its existence is here noted from a period well before the onset of monsoon, i.e., from early May to late July. Three major storms during MONEX were noted to form within the trough line of this system, and the period of major cessation of rains over the Indian subcontinent was noted to occur around the period of arrival near 20°N of a ridge line of this mode. A more detailed analysis of this strongly divergent mode will be published at a later date.

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Z. Zhang
and
T. N. Krishnamurti

Abstract

In this note, the local solution of Gill for the symmetric and the antisymmetric heat sources and sinks is extended to the entire global tropical belt by the authors. The symmetric and antisymmetric heating forced solutions of the motion field derived from the linearized shallow-water equations describe many aspects of the winter and the summer monsoon circulations. The extension, provided here, describes the response of tropical circulation to the global tropical heat sources and sinks retrieved from a satellite-based field of the outgoing longwave radiation through regression algorithms. Such a prescribed field is expanded into a low-order set of trigonometric functions in the zonal direction and parabolic cylinder functions along the meridional coordinate. Upon sequential substitution of these modes into the linearized shallow-water equations on a beta plane, it is possible to solve for the heating forced solutions in a closed analytical form for the entire Tropics. The solutions exhibit most of the climatological features of the wintertime and the summertime circulations of the lower troposphere, such as the trades of the two hemispheres and the Asian monsoon with its elements; for example, monsoon trough, cross-equatorial flows, southwest monsoon current, Mascarene high, and the subtropical anti-cyclones. Furthermore, the solutions also describe features such as the heat lows over Arabia, the Sahara, and Mexico. Overall, these solutions confirm the importance of diabatic heat sources and sinks in the shaping of the tropical climate.

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T. N. Krishnamurti
and
D. R. Chakraborty

Abstract

Many low-frequency phenomena such as the Madden–Julian oscillation (MJO) or the El Niño–Southern Oscillation (ENSO) exhibit rapid growth where they appear to be undergoing a phase locking with other time scales such as the annual cycle. The purpose of this paper is to illustrate an example of phase locking of two different time scales. In this instance it is shown that during such epochs of phase locking a large increase in nonlinear energy exchange occurs from one time scale to the other. This paper utilizes the ECMWF Re-Analysis (ERA-40) datasets for the year 2001 to examine this problem. This study is a sequel to a recent modeling study where the maintenance of the MJO time scale was examined from scale interactions, especially with synoptic-scale waves with ∼2–7 day periods. It was shown that a pair of waves on the synoptic time scale can satisfy certain selection rules and undergo triad interactions (kinetic energy to kinetic energy exchanges) and transfer energy. This present study illustrates the fact that during epochs of phase locking such nonlinear interactions can become very large, thus portraying the importance of phase locking. These explosive exchanges are shown from two perspectives: an approach based on kinetic energy exchanges in the frequency domain and another that invokes the boundary layer dynamics in the frequency domain.

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T. N. Krishnamurti
and
Vince Wong

Abstract

In this paper we investigate a baroclinic boundary layer of cross-equatorial flow over the Arabian Sea with a fine-mesh (200 m) vertical resolution numerical model. The dynamical model utilizes a prescribed vertically varying pressure field from observations. The vertical variation of eddy diffusivity is parameterized. Long-term steady state on a meridional-vertical plane is obtained via integrations of the numerical model starting from an initial Ekman balance. The principal results of the study include a realistic simulation of the vertical structure of the cross-equatorial flow. The balance of forces and boundary-layer transitions between strong cross-equatorial flow, the low-level jet and an ITCZ over the northern Arabian Sea are analyzed in some detail. We note that an advective boundary layer is dominant across the equator, and the core of the low-level jet is located toward the poleward edge of the advective boundary layer. North of this region a gradual increase of upward motion is noted and this region is identified as the Intertropical Convergence Zone (ITCZ). The ITCZ occurs in a transition zone between an advective and a quasi-Ekman boundary layer.

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