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  • Author or Editor: A. D. Vernekar x
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J. D. Opsteegh
and
A. D. Vernekar

Abstract

A steady-state, linear, two-level primitive equation model is used to simulate the January standing wave pattern as a response to mountain, diabatic and transient eddy effects. The model equations are linearized around an observed zonal mean state which is a function of latitude and pressure. The mountain effect is the vertical velocity field resulting from zonal mean wind over the surface topography. The diabatic heating is calculated using parameterized forms of the heating processes. The transient-eddy effects, i.e., the flux convergence of momentum and heat by transient eddies, are computed from observations. Separate responses of the model are computed for each of the three forcing functions.

The amplitude of the response to diabatic heating is small compared to observed values. The vertical structure is highly baroclinic. At the upper level, the phase of the waves is approximately in agreement with the observations. The amplitude of the response to mountain forcing is comparable with observations. The wavelength of the response in the Pacific sector is shorter than observed. The vertical structure is equivalent barotropic. The combined response to diabatic heating and mountain forcing is dominated by the contribution from the mountains. The phase shows some agreement with the observations, but the Aleutian low is located too far to the west and an unrealistic high appears to the west of the dateline.

The amplitude of the response to transient eddy effects is comparable to the observations in middle and low latitudes. At high latitudes the amplitudes are much too large. The assumption of linearity is not valid for strong forcing at high latitudes where the zonal wind is very weak. The vertical structure of the response is almost equivalent barotropic.

A comparison of the responses to mountain and transient eddy effects shows an interesting phase relationship. The troughs produced by the transient forcing are found in the lee of the troughs produced by the mountains (very close to the ridge) indicating that transient forcing is organized by the mountain effects.

The combined model response to all three forcing functions shows good agreement with observations except at very high latitudes.

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A. WIIN-NIELSEN
and
A. D. VERNEKAR

Abstract

The mean meridional circulation calculated earlier from observed values of transports of momentum and sensible heat by solving the zonally averaged form of the quasi-geostrophic omega equation is used to investigate the influence of this secondary flow on the zonally averaged values of the wind and temperature in the atmosphere. The contributions from the horizontal transport processes and the mean meridional circulations are computed separately in order to estimate their relative importance. It is found that the mean meridional circulation counterbalances the horizontal transport of momentum in the upper troposphere, while the two effects work in the same direction in the lower part of the atmosphere. With respect to changes in the zonally averaged temperature field, it is found that the effect of the mean meridional circulation opposes the effect of the horizontal transport of sensible heat almost everywhere.

The recent results of calculations of the mean meridional circulation are also used to discuss the role of zonal heating and friction in quasi-geostrophic models.

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A. D. Vernekar
,
J. Zhou
, and
J. Shukla

Abstract

The authors successfully model and simulate the observed evidence that anomalously high winter/spring Eurasian snow cover is linked to weak rainfall in the following summer Indian monsoon. It is shown that excessive snow cover in February reduces June to September precipitation over India. The excessive snow cover is associated with a weak monsoon characterized by higher sea level pressure over India, a weaker Somali jet, weaker lower tropospheric westerlies, and weaker upper tropospheric easterlies. The weak monsoon is also associated with weaker secondary circulations. The remote response to excessive Eurasian snow cover is to reduce the strength of trade winds in the eastern equatorial Pacific Ocean. Energy used in melting excessive snow reduces the surface temperature over a broad region centered around the Tibetan Plateau. Reduced surface sensible heat flux reduces the midtropospheric temperature over the Tibetan Plateau. The result is to reduce the midtropospheric meridional temperature gradient over the Indian peninsula, which weakens the monsoon circulation.

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M.J. Fennessy
,
J.L. Kinter III
,
B. Kirtman
,
L. Marx
,
S. Nigam
,
E. Schneider
,
J. Shukla
,
D. Straus
,
A. Vernekar
,
Y. Xue
, and
J. Zhou

Abstract

A series of sensitivity experiments are conducted in an attempt to understand and correct deficiencies in the simulation of the seasonal mean Indian monsoon with a global atmospheric general circulation model. The seasonal mean precipitation is less than half that observed. This poor simulation in seasonal integrations is independent of the choice of initial conditions and global sea surface temperature data used. Experiments are performed to test the sensitivity of the Indian monsoon simulation to changes in orography, vegetation, soil wetness, and cloudiness.

The authors find that the deficiency of the model precipitation simulation may be attributed to the use of an enhanced orography in the integrations. Replacement of this orography with a mean orography results in a much more realistic simulation of Indian monsoon circulation and rainfall. Experiments with a linear primitive equation model on the sphere suggest that this striking improvement is due to modulations of the orographically forced waves in the lower troposphere. This improvement in the monsoon simulation is due to the kinematic and dynamical effects of changing the topography, rather than the thermal effects, which were minimal.

The magnitude of the impact on the Indian monsoon of the other sensitivity experiments varied considerably, but was consistently less than the impact of using the mean orography. However, results from the soil moisture sensitivity experiments suggest a possibly important role for soil moisture in simulating tropical precipitation, including that associated with the Indian monsoon.

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P. N. Vinayachandran
,
Adrian J. Matthews
,
K. Vijay Kumar
,
Alejandra Sanchez-Franks
,
V. Thushara
,
Jenson George
,
V. Vijith
,
Benjamin G. M. Webber
,
Bastien Y. Queste
,
Rajdeep Roy
,
Amit Sarkar
,
Dariusz B. Baranowski
,
G. S. Bhat
,
Nicholas P. Klingaman
,
Simon C. Peatman
,
C. Parida
,
Karen J. Heywood
,
Robert Hall
,
Brian King
,
Elizabeth C. Kent
,
Anoop A. Nayak
,
C. P. Neema
,
P. Amol
,
A. Lotliker
,
A. Kankonkar
,
D. G. Gracias
,
S. Vernekar
,
A. C. D’Souza
,
G. Valluvan
,
Shrikant M. Pargaonkar
,
K. Dinesh
,
Jack Giddings
, and
Manoj Joshi

Abstract

The Bay of Bengal (BoB) plays a fundamental role in controlling the weather systems that make up the South Asian summer monsoon system. In particular, the southern BoB has cooler sea surface temperatures (SST) that influence ocean–atmosphere interaction and impact the monsoon. Compared to the southeastern BoB, the southwestern BoB is cooler, more saline, receives much less rain, and is influenced by the summer monsoon current (SMC). To examine the impact of these features on the monsoon, the BoB Boundary Layer Experiment (BoBBLE) was jointly undertaken by India and the United Kingdom during June–July 2016. Physical and biogeochemical observations were made using a conductivity–temperature–depth (CTD) profiler, five ocean gliders, an Oceanscience Underway CTD (uCTD), a vertical microstructure profiler (VMP), two acoustic Doppler current profilers (ADCPs), Argo floats, drifting buoys, meteorological sensors, and upper-air radiosonde balloons. The observations were made along a zonal section at 8°N between 85.3° and 89°E with a 10-day time series at 8°N, 89°E. This paper presents the new observed features of the southern BoB from the BoBBLE field program, supported by satellite data. Key results from the BoBBLE field campaign show the Sri Lanka dome and the SMC in different stages of their seasonal evolution and two freshening events during which salinity decreased in the upper layer, leading to the formation of thick barrier layers. BoBBLE observations were taken during a suppressed phase of the intraseasonal oscillation; they captured in detail the warming of the ocean mixed layer and the preconditioning of the atmosphere to convection.

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