Lisan Yu's Homepage > Projects

Subject I. Scale interactions between westerly wind bursts (WWB), Madden-Julian Oscillation (MJO), and ENSO

Project Description Subject I. Scale interactions between westerly wind bursts (WWB), Madden-Julian Oscillation (MJO), and ENSO The MJO, also referred to as the "30-60-day oscillation" or the "intraseasonal oscillation", is the dominant mode of variability on intraseasonal timescales. It significantly affects wind, SST, and rainfall patterns throughout the global Tropics and also subtropics. It plays an important role in the timing of active and break phases of the Indian and Australian Monsoon. The MJO is characterized by an eastward propagation of both enhanced and suppressed tropical rainfall and is most evident over the warm pool region from the Indian Ocean to the western Pacific Ocean. Because most tropical rainfall is convective, and convective cloud tops are very cold and emit little longwave radiation, detection of MJO can be made using outgoing longwave radiation (OLR) derived from NOAA's polar- orbiting and geostationary satellites.

The relationship between WWB, MJO, and ENSO remains as an open question. The mechanisms through which the interactions between synoptic-intraseasonal atmospheric variability and the interannual ENSO can occur are yet to be identified. MJO and WWB are substantially different. The MJO has a much larger spatial and temporal scales than those of WWB, while the occurrence frequency of the MJO is much less than that of WWB. The MJO is also featured by its eastward propagation and coupling between dynamic and convective fields, which are not usually observed with WWB. Nevertheless, most, although not all, WWB are associated with the MJO. The strong WWB during onsets of recent ENSO warm events appear to be all accompanied by strong MJO activities.

High resolution and high accuracy satellite observations provide the best means to study the scale interactions. This project is to derive possible interaction pattern between WWB, MJO, and ENSO using satellite observations. Results from this analysis contribute to the understanding of the role of synoptic-intraseasonal atmospheric variability in ENSO irregularity.

Subject II. Interannual variability of the Indian Ocean and its relationship with ENSO

SST in the Indian Ocean has distinct interannual variability despite the existence of strong annual cycle associated with the monsoon. Some but not all of the variability is related to ENSO. There appears to have climate fluctuations , such as the tropospheric biennial oscillation (TBO) and an east-west oriented equatorial Indian Ocean dipole (IOD) pattern, that occur independent of ENSO entrema. TBO is observed to possess eastward phase propagation from the Indian Ocean toward the Pacific Ocean, and its role in linking monsoon variability with low frequency processes in the Pacific Ocean is suggested. On the other hand, the distinction between the IOD and ENSO is less clear. The correlation between ENSO and IOD SST anomalies is statistically significant, and especially so if the correlation is performed on a season-by-season basis. This leads to the question as to whether the IOD is an integral part of ENSO or a coupled ocean-atmosphere system akin to ENSO physics but independent of ENSO occurrence.

The objective of this project is to investigate the upper ocean processes in the Indian Ocean and their variability on interannual timescales and their relationship with variability on ENSO timescales in the Pacific Ocean utilizing satellite observations of surface vector wind, SST, sea surface height (SSH), cloud, precipitation fields as well as in situ observations from ships and buoys.