SSM/I仪器采用圆锥扫描方式,避免了AMSU-A等跨轨迹横扫仪器观测资料中存在的临边效应,其观测资料更加适用于空间特征分析。因此,利用1998年-2008年SSM/I仪器极地观测亮温资料,经谱分析、小波分析以及经验正交函数(EOF)分析,验证了南、北极地区的4个月振荡(后简称FMO)现象,揭示了南、北极地区FMO现象的时空分布特征。结果显示:(1)SSM/I 19V/H,37V/H通道在南、北极的亮温观测中有显著的FMO现象,北极亮温值FMO峰值出现在3月、7月和11月上旬,南极FMO峰值出现在4月、8月和12月中旬;(2)北极的FMO增强年为1999年、2002年和2006年;南极为1998年、2001年、2005年和2008年;(3)从12月到次年5月,当海冰面积基本不变时,亮温值随表面温度变化而变化;6到10月,表面温度基本不变时,亮温值随海冰面积变化而变化。当海冰FMO和表面温度FMO均位于相对较大值时,亮温的FMO达到峰值;当海冰FMO和温度FMO都相对较小时,则亮温的FMO到达谷值,其中海冰的影响较大;(4)北极大部分地区呈现同位相的FMO现象;南极不同区域间有显著差异:威德尔海(Weddell Sea, 20°W-60°W, 60°S-75°S)与拉扎列夫海域(Lazarev sea,20°W-30°E, 60°S-70°S)FMO位相相反;90°W到180°W的绕极海域内60°S到70°S纬度带与70°S到80°S纬度带的FMO位相相反。综上,SSM/I对极地的观测亮温中有显著的FMO现象,其振荡强度存在年际变化。地表亮温的FMO主要受到表面温度变化和海冰凝结融化过程的共同作用。在空间特征上,北极大部分地区呈现同位相的FMO现象;南极不同区域间有显著差异。

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.