The Eugene Rasmusson Symposium


Bio-Climate Feedbacks and ENSO Phase-locking

Raghu Murtugudde, ESSIC/Univ. of Maryland, College Park, MD; and J. Ballabrera-Poy, R. H. Zhang, and A. J. Busalacchi

Rasmusson and Carpentar (1982) was a seminal paper that laid the foundation for much of TOGA and the modern-day ENSO research. As a fresh graduate, I arrived to the DC area to be mesmerized by Gene's enthusiastic acceptance of new ideas even as far-fetched as the ones that I presented claiming that marine biology could be as important to the annual cycle in the eastern tropical Pacific as any other ocean-atmosphere process considered thus far. His encouragement was one of the reasons to continue down that path which led to a demonstration that the mixed layer-thermocline interactions are indeed crucially controlled by the conversion of light to heat by the phytoplankton. It is now confirmed in most coupled climate models that bio-climate feedback is quite an important contributor to the annual cycle in the tropical oceans. The coarse horizontal and vertical resolutions of the coupled climate models still make it impossible for the full exploitation of this coupled feedback. Hence, a high resolution ocean GCM coupled to a statistical atmosphere and a simple wind-SST equation model is employed to demonstrate the role of the bio-climate feedbacks on ENSO phase-locking, amplitude, and frequency. It is shown that the better resolution of the annual cycle due to accurate representation of the biological feedbacks results in greatly improved Bjerknes feedback and hence the amplitude and frequency of ENSO simulations in addition to its phase-locking behavior. The basic feedbacks are that the enhanced heating just below the mixed layer during boreal spring months weaken the stratification and deepen the mixed layer and reduce the Ekman divergence in the surface layer. The subtle differences in the amount of radiation that escapes the bottom of the mixed layer leads to better simulation of the thermocline and hence the mixed layer and thus the coupling between the ocean and the atmosphere as seen by the correlation between NINO3 SST and zonal wind-stresses in the western-central Pacific warm pool during the growing phase of ENSO. It would be a joy to present the new results in front of the elderly statesman of the field, Gene Rasmusson.

Poster Session 1, Poster Session
Thursday, 18 January 2007, 9:45 AM-11:00 AM, Exhibit Hall C

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