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Richard S. Lindzen and Sumant Nigam


We examine the importance of pressure gradients due to surface temperature gradients to low-level (p ≥ 700 mb) flow and convergence in the tropics over time scales ≳ 1 month. The latter plays a crucial role in determining the distribution of cumulonimbus convection and rainfall.

Our approach is to consider a simple one-layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically—consistent with ECMWF analyzed data. The top of the layer is taken at 700 mb. The influence from higher levels is intentionally suppressed by setting horizontal pressure gradients and frictional stresses to zero at the top of the layer. Horizontal convergence within the layer is taken up by cumulonimbus mass flux. However, the development of the cumulonimbus mass flux is associated with a short relaxation time [O(½ hr)] (roughly the development time for such convection). During this short time, horizontal convergence acts to redistribute mass so as to reduce horizontal pressure gradients. This effect proves important in the immediate neighborhood of the equator.

Our results show that flows forced directly by surface temperature are often comparable to observed low-level flows in both magnitude and distribution.

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Sumant Nigam and Richard S. Lindzen


A linear, primitive equation stationary wave model having high vertical and meridional resolution is used to examine the sensitivity of orographically forced (primarily by Himalayas) stationary waves at middle and high latitudes to variations in the basic state zonal wind distribution. We find relatively little sensitivity to the winds in high latitude but remarkable sensitivity to small variations in the subtropical jet. Fluctuations well within the range of observed variability in the jet can lead to large variations in the stationary waves of the high latitude stratosphere, and to large changes even in tropospheric stationery waves. Implications for both sudden warmings and large-scale weather are discussed.

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M. Barlow, S. Nigam, and E. H. Berbery


The relationship between the three primary modes of Pacific sea surface temperature (SST) variability—the El Niño–Southern Oscillation (ENSO), the Pacific decadal oscillation, and the North Pacific mode—and U.S. warm season hydroclimate is examined. In addition to precipitation, drought and stream flow data are analyzed to provide a comprehensive picture of the lower-frequency components of hydrologic variability.

ENSO and the two decadal modes are extracted from a single unfiltered analysis, allowing a direct intercomparison of the modal structures and continental linkages. Both decadal modes have signals in the North Pacific, but the North Pacific mode captures most of the local variability. A summertime U.S. hydroclimatic signal is associated with all three SST modes, with the linkages of the two decadal modes comparable in strength to that of ENSO.

The three SST variability modes also appear to play a significant role in long-term U.S. drought events. In particular, the northeastern drought of the 1960s is shown to be closely linked to the North Pacific mode. Concurrent with the drought were large positive SST anomalies in the North Pacific, quite similar in structure to the North Pacific mode, and an example of a physical realization of the mode. Correspondingly, the 1962–66 drought pattern had considerable similarity to the drought regression associated with the North Pacific mode. Analysis of upper-level stationary wave activity during the drought period shows a flux emanating from the North Pacific and propagating over the United States. The near-equivalent-barotropic circulation anomalies originating in the North Pacific culminate in a cyclonic circulation over the East Coast that, at low levels, opposes the climatological inflow of moisture in an arc over the continent from the Gulf Coast to the Northeast, consistent with the observed drought.

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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


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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