Hovmöller diagrams of climate anomalies in NCEP/NCAR reanalysis from 1948 to 2009

Abstract

Atmospheric and oceanic reanalysis data of the past 60 years are visualized by time–latitude cross sections which inform about temporal and latitudinal variability, changes, and meridional circulation of the climate system on time scales from years to decades. The diagrams ease the understanding of climate dynamics in a similar way as the Hovmöller diagram has clarified the zonal propagation of synoptic-scale waves in a latitude band. Thus the time–latitude cross sections are referred to as Hovmöller diagrams of climate anomalies. Various parameters are selected at the Earth’s surface level: air temperature, pressure, sea surface temperature, column-integrated atmospheric water vapour (IWV), surface relative humidity, and surface wind. Contour lines of one parameter are overlayed on the color image of the other parameter so that relations and coincidences of trends, oscillations, and sudden changes of the climate are revealed. Temporal variations of IWV and surface relative humidity appear to be related to the strength of the Southern polar front jet. In case of ENSO 1997/1998, the equatorial IWV enhancement is accompanied by acceleration of the meridional circulation cells of the Southern Hemisphere. Meridional surface wind data suggest the existence of double polar cells in both hemispheres. The Southern polar cells switched to a higher circulation speed in 1997, and surface relative humidity increased over Antarctica. A sudden and persistent decrease of surface pressure occurred in Antarctica around 1980. Since 2000, a rapid increase of surface air temperature, sea surface temperature, and IWV occurs in the Northern Hemisphere. These results of NCEP/NCAR reanalysis have to be cross-validated with other reanalyses, climate models, and pure observations which will be a major endeavour.