A Preliminary Application Of Spherical Harmonic Analysis In Temperature Field Reconstruction

Reconstruction of temperature variations from proxy data has become a primary means to understand the ancient climate changes. There are many methods now be used to reconstruct the temperature fields. However, all of them have deficiencies. For example, all of the methods are based on the assumption that the spatial patterns of the temperature for nearly a thousand years are the same as the past century; some of these methods require all of the proxy data series are the same length, therefore, the shorter proxy data are either discarded which causes the existing proxy series cannot be efficiently utilized or extended which will inevitably introduces a new error. Aim at the disadvantages, the spherical harmonic analysis method was applied to the reconstruction of the global temperature field. Firstly, based on instrumental data of the stations which were distributed on land, we firstly reconstructed the spatial distribution of global temperature field from1951to2010. The comparison with that of global instrumental data proved that the reconstructed temperature field is reliable on land whenever the instrumental data stations are intensive or sparse. Only for oceanic province where no instrumental data station exists, the results of reconstructed temperature field are not good. Further studies show that at least40-50stations were required to reconstruct a creditable global temperature field by spherical harmonics analysis; and the reconstruction can only reflect the broad-scale spatial temperature changes. When more stations were used in temperature reconstruction, clear regional changes of temperature field were reproduced. The method of spherical harmonics analysis was then applied to the reconstruction of the ancient temperature field, and the results show that spatial patterns of the global temperature field in the last millennium has been changing; before1350, the average global temperature fluctuations is small, and cold period and warm period alternated. But since1350, the global average temperature fluctuated obviously, the apparent cold periods emerged and the warm period divided these cold periods; half of the20th century is the warmest period in the past five hundred years; and through analysis of the spatial distribution of the temperature field, it was found that the warming of the global average temperature is not due to the temperature rising of some regions, but by the warming of cold regions. And in the20th century, the spatial distribution of the temperature field is more uniform compared to other time periods.