2006 Volume 84 Issue 4 Pages 567-579
Intraseasonal variations (ISV) of climate from the central Indian Ocean to the western Pacific are investigated at high frequencies (20-50 day period) and low frequencies (50-100 day period) in association with El Niños, mainly using 7 years of zonal wind, 8 years of precipitation, and 31 years of outgoing long-wave radiation (OLR) data observed from satellites. Zonal wind, validated against Tropical Atmosphere Ocean (TAO) buoy measurements at 147°E, 0° and 156°E, 0°, indicated a larger portion of ISV was contained in high frequencies during the 1999-2000, 2000-0l, 2002-03, and 2003-04 seasons, as compared to the 1996-97 and 2001-02 seasons, with the latter seasons occurring a year before El Niño events. For 1996-97 and 2001-02, a particular frequency of 3 cycles per season (64-day period) explained a significant portion of the zonal wind ISV in a coherent band from the Indian Ocean to the western Pacific. Furthermore, it was found that the westerly wind signal in this region propagated eastward. Satellite-based precipitation was likewise examined, and showed a similar pattern in time and space.
Consistently, OLR data indicated that the ratio of low-frequency ISV to high-frequency ISV was significantly correlated with the Nino 3.4 index at a one-year lead time. Low-frequency variance alone also showed a significant, albeit weaker, correlation, and high-frequency variance indicated no correlation. We suggest that high-frequency variance acts as noise, reducing the relationship between low-frequency ISV and El Niño/Southern Oscillation. Thus, a strong low-frequency variance in combination with a weak high-frequency variance (high ratio value) is important for understanding the initiation of El Niños. Another interesting feature observed from OLR was the area with the largest ratios of low-frequency to high-frequency ISV had shifted westward during the 30-year record. The westward shift suggests that the eastern Indian Ocean is playing a more important role in ISV-El Niño relations in recent years.