Abstract
AbstractThis study examines the respective influences of Pacific and Indian Ocean couplings on tropical intraseasonal oscillation (ISO) in boreal winter (November–April). Three basin‐coupling experiments are performed with a coupled atmosphere‐ocean general circulation model (CGCM), in which air‐sea coupling is limited respectively to the Indian Ocean, the Pacific Ocean, and both the oceans. The modelling results show that zonal ISO propagations can be found in the Indo‐Pacific region with or without the ocean coupling; however, the propagation signals are enhanced by the coupling. In this particular model the Pacific Ocean coupling has a stronger influence on the ISO propagation than the Indian Ocean coupling. Without the Pacific coupling, the ISO propagation signal is weakened significantly when it enters the Pacific Ocean sector. Without the Indian Ocean coupling, the simulated ISO intensity is weakened less in the Indain Ocean, and significant zonal propagations of ISO can still be simulated in the sector. The relative importance of the ocean coupling is likely related to the smaller ISO‐related sea surface temperature (SST) anomalies in the Indian Ocean than in the Pacific Ocean. In this particular CGCM, the ocean coupling affects the ISO propagation mainly through a wind‐evaporation‐SST feedback rather than a cloud‐radiation‐SST feedback, while in observations both processes are equally important. To further confirm the importance of the ocean coupling, forced atmospheric GCM experiments are performed with SSTs prescribed from the climatologies produced in the basin‐coupling experiments. It is found that the eastward propagating feature seen in the CGCM experiments is weakened and dominated by the strong standing feature in the forced AGCM runs. The difference demonstrates the contribution of the ocean coupling to the ISO propagation, and also confirm that biases in the mean SST and low‐level wind caused by the ocean coupling are responsible for the spurious standing ISO oscillation feature in the central Pacific and Indian Oceans. Copyright © 2009 Royal Meteorological Society
Highlights
Since the pioneering work of Madden and Julian (1971, 1972), tropical intraseasonal oscillation (ISO) has been increasingly recognized as a phenomenon important to both climate and weather
The observed (Figure 6(a)) rainfall anomalies show a familiar four-stage development process: (1) initiation over equatorial Africa and/or western Indian Ocean (2) rapid intensification when passing through the Indian Ocean (3) mature evolution characterized by a weakening in the Maritime Continent and redevelopment over the western Pacific and (4) dissipation when approaching the dateline
To understand how the ocean coupling affects the ISO propagation, we examine in Figure 7 the evolutions of the sea surface temperature anomalies (SSTA) and wind stress anomalies associated with the composite ISO lifecycle in the observations and the three coupled atmosphere-ocean general circulation model (CGCM) runs
Summary
Since the pioneering work of Madden and Julian (1971, 1972), tropical intraseasonal oscillation (ISO) has been increasingly recognized as a phenomenon important to both climate and weather. Wang and Xie (1998) coupled a simple atmospheric model to a mixed-layer ocean and showed that intraseasonal SST variations can affect surface pressure and enhance or reduce ISO convective activity. Watterson (2002) used the CSIRO Mark coupled atmosphere-ocean general circulation model (CGCM) to examine the sensitivity of ISO to model ocean configurations Fu and Wang (2004) further demonstrated the importance of SST feedback to the ISO evolutions They coupled the ECHAM4 AGCM to an intermediate ocean model and showed that this hybrid-coupled model can produce realistic propagating ISO.
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