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Miller, A. J., M. A. Alexander, G. J. Boer, F. Chai, K. Denman, D. J. Erickson III, R. Frouin, A. J. Gabric, E. A. Laws, M. R. Lewis, Z. Liu, R. Murtugudde, S. Nakamoto, D. J. Neilson, J. R. Norris, J. C. Ohlmann, R. I. Perry, N. Schneider, K. M. Shell, and A. Timmermann, 2003: Potential feedbacks between Pacific Ocean ecosystems and interdecadal climate variations. Bull. Amer. Met. Soc., 84, 617-633.


INTRODUCTION

Oceanic ecosystems influence climate on many time and space scales. But this influence is not well understood. It is clear that interdecadal1 physical climate variations occur in the Pacific sector. Interdecadal sea surface temperature (SST) anomalies show a "canonical" structure (e.g., Tanimoto et al. 1993; Zhang et al. 1997), with central North Pacific SSTs near the subtropical front bracketed to the east, north, and south by oppositely signed SSTs. A second SST pattern is centered around the subpolar front in the Kuroshio-Oyashio Extension region (Deser and Blackmon 1995; Nakamura et al. 1997) and tends to lag the subpolar front SST anomalies by a few years on interdecadal timescales (Miller and Schneider 2000). The North Pacific thermocline observations show interdecadal structures linked to wind stress curl forcing (Miller et al. 1998; Tourre et al. 1999) and to anomalous subduction from buoyancy forcing (Deser et al. 1996; Schneider et al. 1999). These observed patterns may indicate preferred natural modes of variability in the climate system or may be simply a passive response to the long-term atmospheric forcings.

The mechanism responsible for interdecadal climate variability in the Pacific, however, are not very clear. A number of possible physical feedback loops (i.e., coupled oscillations) in the ocean-atmosphere system have been proposed to explain aspects of the observed and modeled climate variations (e.g., Latif 1998; Miller and Schneider 2000). However, interdecadal feedback mechanisms cannot be unambiguously identified in existing observations and they are not robust in the coupled general circulation models (CGCMs) whose results have been analyzed. Moreover, stochastic driving of the ocean by the atmosphere can potentially account for a considerable portion of observed and modeled interdecadal variability (Barsugli and Battisti 1998); Frankignoul et al. 1997) with or without feedbacks.

While physical-biological feedbacks may only account for a small fraction of the observed interdecadal climate variability, it is nevertheless of interest to ask if these exist and are of potential importance. We therefore attempt to answer a sequence of questions:

  • What mechanisms might allow Pacific Ocean biological responses to influence variations of the physical climate system on interdecadal timescales?
  • Do these oceanic biological feedback mechanisms increase or decrease the Pacific Ocean-atmosphere sensitivity to physical climate interactions?
  • In what key regions of the Pacific Ocean might these biological influences be active?
  • How can these possible biological feedbacks be tested with models and observations?

The goal of this paper is to motivate a coordinated modeling and observational effort to study coupled physical-biological mechanisms of interdecadal climate variability in the Pacific. (This discussion applies as well to climate processes with interannual and centennial timescales.)
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1 We use the term "interdecadal" to loosely refer to timescales that are longer than interannual (ENSO) and shorter than centennial (greenhouse gas forcing).