Abstract Regional ocean–atmospheric interactions in the summer tropical Indo–northwest Pacific region are investigated using a tropical Pacific Ocean–global atmosphere pacemaker experiment with a coupled ocean–atmospheric model (cPOGA) and a parallel atmosphere model simulation (aPOGA) forced with sea surface temperature (SST) variations from cPOGA. Whereas the ensemble mean features pronounced influences of El Niño–Southern Oscillation (ENSO), the ensemble spread represents internal variability unrelated to ENSO. By comparing the aPOGA and cPOGA, this study examines the effect of the ocean–atmosphere coupling on the ENSO-unrelated variability. In boreal summer, ocean–atmosphere coupling induces local positive feedback to enhance the variance and persistence of the sea level pressure and rainfall variability over the northwest Pacific and likewise induces local negative feedback to suppress the variance and persistence of the sea level pressure and rainfall variability over the north Indian Ocean. Anomalous surface heat fluxes induced by internal atmosphere variability cause SST to change, and SST anomalies feed back onto the atmosphere through atmospheric convection. The local feedback is sensitive to the background winds: positive under the mean easterlies and negative under the mean westerlies. In addition, north Indian Ocean SST anomalies reinforce the low-level anomalous circulation over the northwest Pacific through atmospheric Kelvin waves. This interbasin interaction, along with the local feedback, strengthens both the variance and persistence of atmospheric variability over the northwest Pacific. The response of the regional Indo–northwest Pacific mode to ENSO and influences on the Asian summer monsoon are discussed.
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The North Pacific Pacemaker Effect on Historical ENSO and Its Mechanisms
Studies have indicated that North Pacific sea surface temperature (SST) variability can significantly modulate El Niño–Southern Oscillation (ENSO), but there has been little effort to put extratropical–tropical interactions into the context of historical events. To quantify the role of the North Pacific in pacing the timing and magnitude of observed ENSO, we use a fully coupled climate model to produce an ensemble of North Pacific Ocean–Global Atmosphere (nPOGA) SST pacemaker simulations. In nPOGA, SST anomalies are restored back to observations in the North Pacific (>15°N) but are free to evolve throughout the rest of the globe. We find that the North Pacific SST has significantly influenced observed ENSO variability, accounting for approximately 15% of the total variance in boreal fall and winter. The connection between the North and tropical Pacific arises from two physical pathways: 1) a wind–evaporation–SST (WES) propagating mechanism, and 2) a Gill-like atmospheric response associated with anomalous deep convection in boreal summer and fall, which we refer to as the summer deep convection (SDC) response. The SDC response accounts for 25% of the observed zonal wind variability around the equatorial date line. On an event-by-event basis, nPOGA most closely reproduces the 2014/15 and the 2015/16 El Niños. In particular, we show that the 2015 Pacific meridional mode event increased wind forcing along the equator by 20%, potentially contributing to the extreme nature of the 2015/16 El Niño. Our results illustrate the significant role of extratropical noise in pacing the initiation and magnitude of ENSO events and may improve the predictability of ENSO on seasonal time scales.
more » « less- Award ID(s):
- 1637450
- NSF-PAR ID:
- 10121056
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 32
- Issue:
- 22
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- p. 7643-7661
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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