Strengthening relationship between ENSO and western Russian summer surface temperature

Western Russia (WR) experienced an extremely hot summer in 2010 that caused tremendous social and economic losses. The WR summer surface temperature (WRST) in the observational record is characterized by substantial interannual variability superimposed on the secular warming trend. Analysis of the 130 year observational record reveals that a strong and significant inverse relationship between WRST interannual variability and the tropical El Nino-Southern Oscillation (ENSO) has emerged during the past three decades. The ENSO influence on the summer extratropical atmospheric circulation was weak before 1980 but became strong and significant afterward, showing a structure similar to the East Atlantic/WR teleconnection pattern. This pattern is associated with rising/falling upper level geopotential height over WR, which leads to the warming/cooling of surface and tropospheric air temperatures. Numerical simulations from a theoretical linear baroclinic model and Atmospheric Model Intercomparison Project models further suggest that the enhancement of the ENSO teleconnection to WR may be attributable to a change in the ENSO-related tropical thermal forcing. A tripole-type rainfall anomaly pattern over tropical Pacific and Atlantic is found to be associated with ENSO in the past three decades. The tripole heating pattern can excite a Rossby wave that extends northwestward reaching WR and is necessary for the strong influence of ENSO on WR summer climate.