Sort Options:

• Sort By Last Name
• Sort By Title

Other:

• Print All

Sort by time-scale:

• Short
• Intermediate
• Long
• SHOW ALL

The Effect of High-Frequency, Observed SST Forcing on AGCM Simulations of Indian Monsoon Intraseasonal Variability

The Effect of High-Frequency, Observed SST Forcing on AGCM Simulations of Indian Monsoon Intraseasonal Variability

Nicholas P. Klingaman (1), Peter M. Inness (1), Julia M. Slingo (1), Hilary Weller (1)

(1) Climate Directorate of the National Centre for Atmospheric Science and Department of Meteorology, University of Reading

The Indian summer monsoon provides much of the subcontinent's annual precipitation and is an essential moisture source for agricultural production. While the monsoon exhibits substantial interannual variations, its intraseasonal variability is of greater magnitude and hence of critical importance for predictability. This intraseasonal variability is concentrated in the 30-50 day band and comprises a northwards-propagating oscillation(NPISO) between active and break phases of enhanced and reduced precipitation over India, while the opposing phase dominates over the equatorial Indian Ocean.

Recent investigations using general circulation models(GCMs) have suggested that the NPISO is an intrinsically atmospheric mode, but that atmosphere-ocean coupling is required to achieve realistic intensity and propagation speed. Specifically, coupled GCMs (CGCMs) outperform atmosphere-only GCMs (AGCMs) due to the CGCMs ability to represent the near-quadrature phase relationship between sea-surface temperatures (SSTs) and atmospheric deep convection. Without atmosphere-ocean feedbacks, AGCMs too quickly initiate convection over warm SST anomalies. These studies have forced their AGCM simulations with SSTs from either a previous CGCM simulation or the NCEP (Reynolds) satellite SST product, however, both of which substantially underestimate intraseasonal SST variability across the tropical Indian Ocean.

In this study, we have forced the Hadley Centre Atmospheric Model (HadAM3) with a new, high-frequency, observed SST dataset from the UK Met Office. These SSTs show significantly more variability in the intraseasonal band than the existing NCEP dataset. Two thirty-member ensembles have been conducted at 1 degree spatial resolution: one forced by daily SSTs and the other forced by monthly mean SSTs. These ensembles are compared to determine whether high-frequency, realistic SSTs can improve AGCM simulations of the intraseasonal variability of the Indian monsoon. Composite active and break events are constructed from each ensemble and compared to similar composites taken from reanalysis to examine the degree to which each ensemble properly represents the physical mechanisms underlying the NPISO.