The tenth IMSC, Beijing, China, 2007 - International Meetings on ...

for without negatively impacting other parts of the field. This would appear to be especially the
case in reconstructing precipitation and drought fields that have a lot of relatively small-scale
spatial variability in them. To avoid this problem in reconstructing gridded Palmer Drought
Severity Indices (PDSI) over North America from tree rings, and to use only those tree-ring
chronologies that are likely to have true drought signals in them, a method of point-by-point
regression (PPR) was developed and extensively tested. PPR reconstructs each grid point of
PDSI separately from all others using principle components regression analysis. As
constructed, PPR has total control over which tree-ring predictors are used at each grid point
through two steps that determine the selection of candidate predictors: a search radius for
locating likely predictors of PDSI and a screening probability that identifies those predictors
within the search radius that are likely to contain statistically useful information about past
drought. This predictor selection process is applied at each grid point in the reconstruction of
the PDSI field such that not all tree-ring predictors are used to reconstruct drought at all
locations of the grid. Thus, the PPR method operates locally within the possibly overlapping
limits of the search radius used. Even though PPR does not theoretically guarantee the
preservation of the joint space-time variability of the PDSI field because of its point-wise
method of estimation, tests reveal that the inherent spatial patterns of drought variability over
North America are faithfully preserved in the PPR-reconstructed field.
High resolution modeling of the monsoon circulation in the Indian Ocean
N. A. Diansky
E-mail:dinar@inm.ras.ru
V. B. Zalesny
S. N. Moshonkin
A. S. Rusakov
Institute of Numerical Mathematics, Russian Academy of Sciences, 119333, Gubkina
Results on the monsoon circulation in the Indian Ocean simulated with a σ-coordinate
ocean model developed at the Institute of Numerical Mathematics, the Russian Academy of
Sciences is presented. <strong>The</strong> model has a horizontal resolution of 1/8°×1/12° and contains 21
σ-layers of uneven thickness. Realistic bottom topography and land geometry are used. <strong>The</strong>
numerical experiments were carried out for 15 years starting from the Levitus climatology for
January and monthly mean climatic atmospheric forcing from the NCEP reanalysis data. <strong>The</strong>
annual cycle of the surface and subsurface currents and temperature and salinity fields were
analyzed. <strong>The</strong> model reproduces well the Summer Monsoon and the Winter Monsoon currents
and their time evolution and spatial structures. <strong>The</strong> Somali Current is adequately modeled.
During the Summer Monsoon, the velocities of the current exceed 2 m/s, while the total mass
transport is approximately 70 Sv. <strong>The</strong> model results show that a reversal of the Somali Current
from the northern direction in the summer to the southern direction in the winter is
accompanied by the generation of anticyclonic eddies, which drift westward owing to the
β-effect and dissipate either near the Somali shore or in the Gulf of Aden. <strong>The</strong> monsoon
variability of the equatorial surface current and equatorial subsurface countercurrent system
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