SSM/I can receive radiation information of surface and near-surface penetrating through clouds. Compared with crossing scanning sensors

such as AMSU-A

SSM/I that utilizes conical scanning can avoid limb effects on AMSU-A measurements. Thus

this SSM/I measurement can be applied to analyze spatial and temporal variations in polar climate. Spectrum

wavelet

and Empirical Orthogonal Function（EOF） analyses were used to analyze the SSM/I measurements from 1998 to 2008 in a polar area. Results of spectrum and wavelet analyses show significant Four-Month Oscillations（FMO） in SSM/I channel 19V/H and 37V/H measurements. The FMO of the brightness temperature peaked at the beginning of March

July

and November in the Arctic and at the middle of April

August

and December in the Antarctic. The intensity of FMO varied inter-annually. The intensity was stronger in 1999

2002

and 2005 than that in the other years in the Arctic and in 1998

2001

2005

and 2008 in the Antarctic. FMO was also detected in reanalysis of ERA-Interim surface skin temperatures and sea ice area. The mean brightness temperature of the surface varied with increasing surface skin temperature in the Arctic from December to May of next year

when sea ice area almost remains the same. From June to October

surface skin temperature was nearly invariable

and the mean brightness temperature varied with increasing sea ice area in the Arctic. When the FMO of the mean surface temperature and sea ice area were relatively high

the FMO of the mean brightness temperature peaked. Conversely

when both FMO of the mean surface skin temperature and sea ice area were relatively low

the FMO of the brightness temperature went to the valley value. Compared with the surface skin temperature

melting and freezing of sea ice exhibited greater impacts on the brightness temperature. The EOF results showed the spatial characteristics of FMO in the Arctic and Antarctic. Most areas in the Arctic presented the same FMO

but the phase differed in Weddell Sea（20°W–60°W

60°S–75°S） and Lazarev Sea（20°W–30°E

60°S–70°S） in the Antarctic. The phase of 60°S–70°S latitude band were opposite from that of 70°S–80°S latitude band within 90°W–180° W around the Antarctic area. Significant FMO signals exist in the brightness temperature of SSM/I channel 19 V/H and 37 V/H. The FMO of surface climate variables in the Arctic and Antarctic was confirmed by combining FMO with ERA reanalysis temperature. The intensity of the FMO varied inter-annually. The FMO of the brightness temperature reflected the combined effect of variation in the surface skin temperature and the melting and freezing process of sea ice. For the spatial characteristics of the brightness temperature

FMO presented synchronous variations in the Arctic and varied from region to region in the Antarctic.