Indian Ocean and Indian summer monsoon: relationships without ENSO in ocean–atmosphere coupled simulations

• Published in: Climate dynamics 2017-08, Vol.49 (4), p.1429-1448
• Main Authors: Crétat, Julien, Terray, Pascal, Masson, Sébastien, Sooraj, K. P., Roxy, Mathew Koll
• Format: Article
• Language: English
• Subjects: Atmosphere; Cells; Circulation; Climatology; Computer simulation; Convection; Convection heating; Coupled modes; Diabatic heating; Earth and Environmental Science; Earth Sciences; El Nino; El Nino phenomena; El Nino-Southern Oscillation event; Environmental aspects; Equatorial regions; Forces (mechanics); Frameworks; Geophysics/Geodesy; Hadley cells; Heating; Meridional circulation; Moisture; Monsoon circulation; Monsoons; Observations; Ocean currents; Ocean temperature; Ocean, Atmosphere; Ocean-atmosphere interaction; Ocean-atmosphere system; Oceanography; Oceans; Quadrupoles; Rain; Rainfall; Rainfall variability; Sciences of the Universe; Sea surface; Sea surface temperature; Southern Oscillation; Spring; Summer; Summer monsoon; Surface temperature; Temperature effects; Tropical climate; Variability; Walker circulation; Wind
• ISSN: 0930-7575; 1432-0894
• DOI: 10.1007/s00382-016-3387-x

The relationship between the Indian Ocean and the Indian summer monsoon (ISM) and their respective influence over the Indo-Western North Pacific (WNP) region are examined in the absence of El Niño Southern Oscillation (ENSO) in two partially decoupled global experiments. ENSO is removed by nudging the tropical Pacific simulated sea surface temperature (SST) toward SST climatology from either observations or a fully coupled control run. The control reasonably captures the observed relationships between ENSO, ISM and the Indian Ocean Dipole (IOD). Despite weaker amplitude, IODs do exist in the absence of ENSO and are triggered by a boreal spring ocean–atmosphere coupled mode over the South-East Indian Ocean similar to that found in the presence of ENSO. These pure IODs significantly affect the tropical Indian Ocean throughout boreal summer, inducing a significant modulation of both the local Walker and Hadley cells. This meridional circulation is masked in the presence of ENSO. However, these pure IODs do not significantly influence the Indian subcontinent rainfall despite overestimated SST variability in the eastern equatorial Indian Ocean compared to observations. On the other hand, they promote a late summer cross-equatorial quadrupole rainfall pattern linking the tropical Indian Ocean with the WNP, inducing important zonal shifts of the Walker circulation despite the absence of ENSO. Surprisingly, the interannual ISM rainfall variability is barely modified and the Indian Ocean does not force the monsoon circulation when ENSO is removed. On the contrary, the monsoon circulation significantly forces the Arabian Sea and Bay of Bengal SSTs, while its connection with the western tropical Indian Ocean is clearly driven by ENSO in our numerical framework. Convection and diabatic heating associated with above-normal ISM induce a strong response over the WNP, even in the absence of ENSO, favoring moisture convergence over India.