RESEARCHERS Frontier Newsletter No.13 Jan.2001


Dr.Takahashi photograph of face

Dr. Jun Takahashi

International Arctic Research Center (IARC)

I have been participating in the FRSGC/IARC since March 1999 .I am interested in the transportation mechanisms of the dense water in the polar regions with using numerical models.

In the Arctic coastal region,the dense water is performed by the surface cooling, and carried to the basin by transportation mechanisms.It seems that the dense water is the source of the intermediate and deep water in the Arctic ocean.The transportation mechanisms of the dense water affects the intermediate and deep circulation in the Arctic and cold halocline,and therefore it is important to clarify the mechanisms.

I have paid attention to an effect of mesoscale eddies on the continental shelf/slope,and have carried out numerical experiments with using a simple shallow water model on a bottom layer.An simulat- ed eddy moves to a basin apart from coast, because of nonlineality of eddy current and meridional variation of ambient potential vorticity due to change of layer height.This implies that the mesoscale eddies can con- tribute to the transportation of the dense water with instructing eddy generating mechanisms.

The bottom Ekman transportation is important as the transportation mechanism. It carries an alongshore current to a basin by the bottom friction.I plan to examine the effect with using OGCM suppose the Arctic.I would like to contribute the para- meterization of the BBL on the global model and clarification of the mechanisms.

Dr.Tomita photograph of face

Dr. Hirofumi Tomita

Integrated Modeling Research Program

I have joined the Integrated Modeling Research Program in April 1999 .At the university,while studying the thermo- fluid dynamics of a relatively simple system such as Benard convection,I became interested in the geophysical fluid dynamics.

I,a member of the Next Generation Model Research Group,am now engaged in the development of the dynamical core,which describes the dynamics of general circula- tion in numerical models.Most of today's atmospheric general circulation models employ the spectral method to represent meteorological fields.Although this method has great advantage of high numerical accu- racy over the other methods,it is pointed out that the numerical efficiency becomes worse at simulations with very higher reso- lution.

We can then reconsider the use of grid method as an alternative of spectral method. If we use the simple longitude- latitude grid, however,the similar problem of computa- tional inefficiency occurs because the longi- tudinal lines become dense at the polar regions.To overcome this problem,we should use a grid system having nearly homogeneous spacing..One of those grid systems is the icosahedral one.

I'm constructing the dynamical core for this grid system.Until now,the icosahedral grid model has not demonstrated yet its fea- sibility in the long term simulation of cli- mate.Without doubt,we have to overcome a number of problems before we are satis- fied with obtained results.I am convinced that my work is steadily progressing to that goal.

Dr.Nonaka photograph of face

Dr. Masami Nonaka

Masami Nonaka

International Pacific Research Center (IPRC)

Part of water subducted into the subsur- face layer in the subtropical gyre is trans- ported to the tropics and finally to the equa- torial region and upwells there.The exis- tence of this ocean circulation connecting the subtropics and the tropics is suggested by the tritium and salinity distributions in the tropical subsurface layer.Mean fields of temperature and salinity in the tropics sug- gest that they are strongly affected by the water subducted in the subtropics and trans- ported into the tropics.

As well as the mean fields,anomalies in the tropics may be caused by the water.Analysis of historical observed temperature data suggested that temperature anomalies in the North Pacific associated with the decadal/interdecadal variations there propagate to the equatorial region and cause decadal/interdecadal varia- tions of sea surface temperature (SST)in the equatorial region.As shown in the El Ni n o phenomena,equatorial SST anomalies may cause anomalies in the atmospheric cir- culation and anomalous global climate vari- ations.

These suggest an important role of the ocean circulation connecting the subtropics and the tropics in the climate variations. Then,I have studied the pathway of the water exchange between the subtropics and the tropics and equatorward propagation of the North Pacific decadal temperature anomalies via the ocean circulation,using ocean general circulation model numerical- ly simulating realistic Pacific Ocean circula- tions.And I have been studying decadal variations of strength of the circulation and their effect on the equatorial SST variations. These studies would show the importance of ocean in the climate variations on decadal time scale,especially the impor- tance of temperature anomaly propagations in the ocean to force the atmosphere remotely.

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