Zhaoyong Guan

Impact of the Indian Ocean dipole on the relationship between the Indian monsoon rainfall and ENSO

The influence of the recently discovered Indian Ocean Dipole (IOD) on the interannual variability of the Indian summer monsoon rainfall (ISMR) has been investigated for the period 1958-1997. The IOD and the El Nino/Southern Oscillation (ENSO) have complementarily affected the ISMR during the last four decades. Whenever the ENSO-ISMR correlation is low (high), the IOD-ISMR correlation is high (low). The IOD plays an important role as a modulator of the Indian monsoon rainfall, and influences the correlation between the ISMR and ENSO. We have discovered that the ENSO-induced anomalous circulation over the Indian region is either countered or supported by the IOD-induced anomalous meridional circulation cell, depending upon the phase and amplitude of the two major tropical phenomena in the Indo-Pacific sector.

Individual and Combined Influences of ENSO and the Indian Ocean Dipole on the Indian Summer Monsoon

The relative influences of the ENSO and Indian Ocean Dipole (IOD) events on the Indian summer rainfall were studied using observational data and an atmospheric general circulation model (AGCM). The composite analysis of rainfall anomalies demonstrates that the IOD, while significantly influencing the Indian summer monsoon rainfall, also significantly reduces the impact of ENSO on the Indian summer rainfall whenever these events with the same phase co-occur. The AGCM experiments have shown that during an El Nino event, the Walker circulation over the tropical Indo-Pacific region is modulated; a low-level anomalous divergence center over the western Pacific and an anomalous convergence zone over the equatorial Indian Ocean are induced. Furthermore, an anomalous zone of convergence over the Myanmar and south China regions is induced during an El Nino event. These zones of anomalous convergence are complemented by anomalous divergence over the Indian region, causing anomalous subsidence and weakened rainfall. When a strong positive IOD event simultaneously occurs with El Nino, the latters influence on the Indian monsoon is reduced by both poles of the IOD through the following mechanism: an anomalous divergence center, as compared to the summers when an El Nino alone occurs, is introduced in the eastern tropical Indian Ocean. From this center, the anomalous divergent flow crosses the equator, and this air, while weakening the El Nino-induced divergence over the western Pacific, also leads to convergence over the Indian monsoon region. This results in the reduction of the ENSO-induced subsidence and the related rainfall deficit over the eastern flank of the Indian monsoon trough region and adjoining regions to the east. On the other hand, over the western part of the tropical Indian Ocean sector, part of the anomalous ascending motion from the warm pole of the positive IOD event subsides just to the north of the equator, moves northward, ascends, and causes surplus rainfall. This reduces the ENSO-induced rainfall deficit over western India, the western part of the monsoon trough, and parts of Pakistan. The AGCM experiments also demonstrate that positive IOD events amplify the ENSO-induced dryness over the Indonesian region.

Influence of the Indian Ocean Dipole on the Australian winter rainfall

[1]xa0Using an atmospheric general circulation model and observed datasets of sea surface temperature and rainfall, we studied the influence of the Indian Ocean Dipole (IOD) on the Australian winter rainfall. The IOD has significant negative partial correlations with rainfall over the western and southern regions of Australia. These negative partial correlations extend south-eastward from Indonesia all the way to south east Australia. Our atmospheric general circulation model sensitivity experiments indicate that cold sea surface temperature anomalies prevailing west of the Indonesian archipelago during the positive IOD events introduce an anomalous anticyclonic circulation at lower levels over the eastern tropical and subtropical Indian Ocean, and over much of the Australian continent. It is also apparent that the response of the atmosphere to the IOD in this region is baroclinic, causing anomalous subsidence and anomalous reduction in the rainfall over the affected regions of Australia.

The unusual summer of 1994 in East Asia: IOD teleconnections

[1]xa0An extremely hot and dry summer of 1994 was reported in East Asian countries. Using observational data, we have demonstrated that the Indian Ocean Dipole (IOD) is at least one possible cause of the abnormal East Asian summer climate. An anomalous cyclonic circulation over the western Pacific and the southern China weakened the monsoonal northward flow in the lower troposphere. An anomalous anticyclonic circulation with the equivalent barotropic structure around Japan, Korea and the northeastern part of China caused the hot and dry summer of 1994. This accumulation of the lower potential vorticity in the Far East is related to the wave activity from the Mediterranean/Sahara region. The monsoon-desert mechanism connects a Rossby wave source with the IOD-induced diabatic heating around the Bay of Bengal. Another Rossby wave-train pattern was generated in the upper troposphere and propagates northeastward from the southern China. Both the Rossby wave patterns influenced the circulation changes over East Asia.

Interhemispheric oscillations in the surface air pressure field

The low frequency variability in the surface air pressure (SAP) field is investigated using the NCEP/NCAR 40-year reanalysis data. The EOF1 (EOF2), which accounts for 36% (21%) of total variance of the zonal mean anomalous SAP, corresponds to the air mass oscillation in the middle and high latitudes of the Southern (Northern) Hemisphere. While the total air mass is conserved globally, EOF3 (14.5%) represents the air mass redistribution between the two hemispheres on time-scales ranging from several months to a decade. This is referred to as the interhemispheric oscillation (IHO). An index is defined to describe the IHO variability. The EOF3 explains 65% of the total variance of the IHO index. Moreover 14.5% of the total variance of the zonal mean SAP anomalies represents the variability of the IHO.