孟加拉湾海平面异常的年际变化及其对ENSO的响应

摘要

The low frequency change of sea level anomalies (SLA) averaged in the Bay of Bengal was investigated on the basis of altimeter data given by CNES. The rising rate of mean sea level was 1.9 mm/a, Detailed analysis showed that the sea level variability was closely related to ENSO. When an E1 Nifio event took place in 1997, the regional sea level of the Bay of Bengal showed a negative anomaly. When a La Nifia event happened in 1998, a reverse situation took place. Sea level variability in the Bay of Bengal had a strong response to ENSO. Sea level variability in the Bay of Bengal and Nino3(SST) ran almost synchronously in opposite phase. ENSO can influence sea level change of the Bay of Bengal by influencing wind field over the bay and the equatorial area. During E1 Nifio years, there was an anticyclone over the Bay of Bengal, causing an Ek- man transport to pile up water in the centre and at the same time reducing the water around the bay. Wind anomaly over the equator area can excite Kelvin waves and influence water temperature over thermocline in the Bay of Bengal, changing the steric sea level and also the sea level. Steric sea level anomaly was the main reason for the low frequency change of the SLA. Through wavelet coherence analysis, it was found that the steric sea level anomaly had a good coherence with Nino3 on 3.5-5.0 year periods. The opposite phase indicated that when E1 Nifio took place, SST over East Tropical Pacific increased and the mean steric sea level in Bay of Bengal decreased. Being part of the West Pacific-East Indian Ocean warm pool, the sea water temperature in Bay of Bengal lowered when an E1 Nifio took place, causing the steric sea level to decrease.%利用卫星高度计数据分析了1993年1月至2009年12月孟加拉湾海平面变化特征。分析表明，孟加拉湾海平面的平均上升速率为1．9mm／a。孟加拉湾SLA（海平面异常）明显地受ENSO（厄尔尼诺与南方涛动）循环的调制，前者对东太平洋海表面温度异常Nin03指数呈现显著的反位相相关。厄尔尼诺事件发生时（如1997年），孟加拉湾平均SLA呈现负值；拉尼娜事件发生时（如1998年），孟加拉湾平均SLA呈现正值。ENSO循环通过孟加拉湾海域风场异常和赤道处的西风异常来影响孟加拉湾sLA。厄尔尼诺事件发生前，首先在西太／东印度洋出现赤道西风异常，之后孟加拉湾上空出现反气旋式风场异常，通过Ekman抽吸使得海湾四周的海平面异常下降，海湾中心的海平面异常上升：拉尼娜事件发生时则正好相反。比容海平面异常对孟加拉湾平均海平面异常的年际变化也有贡献。经小波相干分析发现，在3．5—5．0年周期的范围，孟加拉湾的比容海平面异常和Nino3指数存在位相相反的相干；由于孟加拉湾是西太．东印度洋暖池的一部分，当厄尔尼诺事件发生时，暖池温度下降．孟加拉湾海水温度也下降．故热胀冷缩引起比容海平面降低。