Chiaki Kobayashi

Brewer-Dobson circulation diagnosed from JRA-55

We examine the Brewer-Dobson circulation (BDC) in the lower stratosphere diagnosed from the Japanese 55u2009year Reanalysis (JRA-55) data set and compare it with the BDCs diagnosed from the other reanalyses (i.e., ERA-Interim and JRA-25), and JRA-55-related data sets (i.e., JRA-55C, created by assimilating only in situ observations, and JRA-55AMIP, a simulation by a prediction model). The climatological mean seasonal change of the BDC in JRA-55 is similar to that in ERA-Interim but considerably different from that in JRA-25. Dynamical and thermodynamical consistencies among the variables are greatly improved in the JRA-55 data set. The interannual variations of the annual mean tropical upwellings in JRA-55 are almost coincident with those in JRA-55C. It suggests that the weakly increasing trend of the BDC found in JRA-55 does not have anything to do with changes in the satellite observing system. The climatological mean tropical upwelling diagnosed from JRA-55 is stronger than that from JRA-55AMIP. This difference is presumed to partly link to the models inability to simulate the quasi-biennial oscillation (QBO). The JRA-55AMIP data set, a simple simulation performed without data assimilation, does not simulate the QBO, whereas the JRA-55 data set represents the QBO with the aid of data assimilation. The climatological mean zonal mean states of JRA-55AMIP considerably deviate from those of JRA-55 in the lower stratosphere. The deviation of JRA-55AMIP is similar to the modulation pattern by the QBO in specific phase regions in which the BDC is rather weak. The simulated QBO might enhance the BDC and reduce the model biases in the tropical lower stratosphere.

A study of dynamical seasonal prediction of potential water resources based on an atmospheric GCM experiment with prescribed sea-surface temperature

Abstract The inter-annual variability and potential predictability of seasonal mean potentially available water resources (precipitation, P, minus evaporation, E), are investigated based on SST-forced ensemble dynamical seasonal atmospheric prediction using the Japan Meteorological Agency global model. High temporal correlations between the observed P—E and the model-ensemble average prediction of P—E are found in the central part of the USA for boreal winter and spring, and in eastern Brazil for boreal spring and summer, while a negative correlation is found in India for all seasons. The correlation in Siberia is low in all seasons, and that in Japan is high only in boreal winter. The P—E potential predictability is highest in the tropics and lower in the extra-tropics. High variance ratios exceeding 0.4 are confined to limited land areas. In addition to river basins where seasonal discharge predictions using statistical models were possible in previous works, the Congo, Mekong and Columbia rivers, were also found to be promising with high-potential predictability.

Phase shift of the seasonal cycle in the Hadley Circulation in recent decades

[1] A trend of phase shift in the seasonal cycle of the Hadley circulation is detected from the three reanalyses data. There is a strengthening trend of the North cell of Hadley circulation in March and April, and a strengthening trend of the South cell of Hadley circulation in September and October. The result indicates the properties of the boreal winter circulation tend to remain into boreal spring, and the boreal summer circulation into boreal autumn. Three major reanalyses show qualitatively the same tendency. The seasonal cycle in zonal averaged tropical precipitation and OLR, which are independent from the reanalyses, also shows a phase shift consistent with the Hadley circulation especially in September.