The Persistence Of The Soil Moisture In Middle-High Latitude Of The Northern Hemisphere And Its Relationship With Atmospheric Circulation Variability

Soil moisture plays a very important role on climate change.There are many researchs about the timescales of soil moisture memory for the single monitoring site.They derived the timescales of several months for soil moisture anomaly especially in the deep soil layer.But,because of the lack of observed soil moisture data in mang areas,there is little research about the soil moisture memory in massive scale.In this paper,we investigated the spatial distribution and seasonal variation of the global soil moisture memory with the daily soil moisture data sets in the widely-used European Centre for Medium Range Weather Forecasting(ECMWF)40-year reanalysis(ERA40).In the first soil layer(7cm)we derived timescales of above 6 months for the northern part of north africa,the near-equatorial part of africa and the northern part of sorth America in all the year.In the antumn and winter,we derived timescales of 3-6 months or more for most eastern regions of Eurasian and the northern part of north America.In different soil layers,the spatial distribution and seasonal variation of soil moister memory were consistent with the first soil layer.For a single grid point,the deeper the soil depth,the greater the timescales.Recent modeling and observational studies had highlighted the importance of soil moisture for regional climate and had investigated the feedback of soil moisture on the temperature、precipitation and regional atmospheric circulation.However,they had seldom examined the impact of soil moisture on hemispheric circulation.After removing the influence of trend and the signal of ENSO on the monthly soil moisture and 500hPa height field data sets of ERA40,We applied a lagged maximum covariance analysis to investigate linear covariability between the soil moisture in middle-high latitude of the northern hemisphere and the 500hPa height field in north Atlantic area.We found that in the winter(JFM)and summer(JJA),the NAO-like atmospheric circulation had a significant effect on the soil moisture in the lagged month.In addition,soil moisture in autumn to early winter(spring to early summer)could significantly effect the lagged winter(summer)NAO-like atmospheric circulation.The soil moisture anomaly in Siberia areas、the Tibetan and Mongolian plateau areas and the northern part of north america areas(the northern part of north america areas and the Tibetan plateau areas)had significant effect on the lagged winter(summer)atmospheric circulation.Thses effections were connected with these areas having long timescales of soil moisture anomaly in autumn-eraly winter or spring-eraly summer.The leading time of soil moisture anomaly in minddle-high latitude indicates an implication for skillful predictability of atmospheric variability.The CCM3 model has performed identical idealized experiments to address the role of soil moisture.We used the experiment(PnAn)and the fixed soil moisture experiment(PnAn_SM)to verify the results of ERA40 data sets.Both two experiments are Neutral_Pacific and Neutral_Atlantic removing the influence of SST.We found that because of the soil moisture,the squared-covariance fraction of NAO increased from 40.3%to 52.7%in the winter,from 45.2%to 50.9%in the summer.Compared with the ERA40 data sets,the PnAn experiment could simulate the interaction between the soil moisture and the NAO.