Bjerknes feedback

Bjerknes feedback
Content published by Rebecca Lindsey and reviewed by Tom Di Liberto. Image credi · Public domain · source
Name	Bjerknes feedback
Field	Meteorology, Oceanography, Climate Science
Notable people	Vilhelm Bjerknes, Jacob Bjerknes, Klaus Hasselmann, Syukuro Manabe, Jule Charney
Related events	El Niño–Southern Oscillation, Pacific Decadal Oscillation, Southern Oscillation

Bjerknes feedback Bjerknes feedback is a coupled air–sea interaction mechanism central to tropical climate variability and the growth of oscillations such as El Niño–Southern Oscillation, linking ocean thermocline dynamics, surface winds, and convective heating. It was articulated by Jacob Bjerknes building on ideas from Vilhelm Bjerknes, and has informed theoretical, observational, and modeling work across institutions such as Massachusetts Institute of Technology, Scripps Institution of Oceanography, National Oceanic and Atmospheric Administration, and International Research Institute for Climate and Society.

Definition and overview

Bjerknes feedback describes positive feedback between equatorial zonal wind anomalies, sea surface temperature anomalies, and thermocline slope changes that amplify initial perturbations in the tropical Pacific, as invoked in many explanations of El Niño and La Niña. The feedback couples atmospheric convection changes over the western Pacific and Walker circulation shifts with oceanic adjustments involving the thermocline, equatorial upwelling, and zonal pressure gradients, integrating concepts developed at University of Bergen, University of Oslo, and Geophysical Fluid Dynamics Laboratory. It operates within broader contexts including Madden–Julian oscillation, Pacific Decadal Oscillation, and interannual-to-decadal variability examined at Lamont–Doherty Earth Observatory and Woods Hole Oceanographic Institution.

Physical mechanisms

The physical mechanisms involve interactions among surface wind stress, oceanic wave dynamics (equatorial Kelvin and Rossby waves), and convective heating. Westerly wind anomalies weaken the trade winds and reduce equatorial upwelling, allowing warmer surface waters in the central/eastern equatorial Pacific, which alter atmospheric convection patterns over regions such as Indonesia, the Mariana Islands, and the Galápagos Islands. Changes in convection modify the zonal pressure gradient and produce wind stress anomalies that reinforce the original wind change, a cycle highlighted by research at University of California, San Diego, Monterey Bay Aquarium Research Institute, and CSIRO. Oceanic adjustment involves eastward propagation of Kelvin waves and reflection into Rossby waves at eastern boundaries like the South American coast, processes described in studies from University of Bergen and University of Hawaii.

Mathematical formulation and models

Mathematically, Bjerknes feedback appears in simplified models such as delayed oscillator, recharge–discharge, and advective–reflective frameworks developed at Princeton University, Universität Hamburg, and University of Tokyo. Linear stability analyses couple shallow-water or reduced-gravity ocean models with single-column atmospheric boundary-layer and convective closure schemes studied at California Institute of Technology and University of Reading. Numerical implementations in coupled general circulation models from European Centre for Medium-Range Weather Forecasts, NCAR, and Met Office quantify feedback strength via parameters representing zonal wind response to SST, SST response to wind, and ocean wave adjustment timescales, connecting to work by Klaus Hasselmann, Syukuro Manabe, and Jule Charney.

Role in ENSO and tropical climate variability

Within El Niño–Southern Oscillation, Bjerknes feedback provides the principal positive amplification mechanism transforming stochastic atmospheric perturbations into coherent basin-scale SST anomalies, interacting with negative feedbacks such as thermocline discharge and nonlinear advection from regions like the Indonesian Throughflow and Tasman Sea. Its efficacy varies with background state changes tied to global warming studies at IPCC, decadal modulation associated with the Pacific Decadal Oscillation and Interdecadal Pacific Oscillation, and cross-basin influences from the Indian Ocean Dipole and Atlantic variability examined at Potsdam Institute for Climate Impact Research and International Pacific Research Center.

Observational evidence and diagnostics

Evidence arises from in situ arrays such as TAO/TRITON, ARGO, and Ramon Margalef (note institutional contexts), satellite records from TOPEX/Poseidon, ERS, Jason missions, and reanalyses like NOAA-CIRES Twentieth Century Reanalysis and ERA-Interim. Diagnostics include lead–lag correlations between zonal wind stress and SST, empirical orthogonal function analyses from Columbia University, lagged regression studies at Scripps Institution of Oceanography, and event compositing linking Kelvin wave signatures to eastern Pacific warming documented by teams at University of Washington and Ocean University of China.

Implications for climate change and predictability

Changes in background mean state, Walker circulation strength, and equatorial thermocline depth under scenarios used by Coupled Model Intercomparison Project ensembles affect Bjerknes feedback strength and hence ENSO amplitude and frequency, topics investigated at MPI-Met, Lawrence Livermore National Laboratory, and National Center for Atmospheric Research. Altered feedback can modify seasonal-to-interannual predictability horizons exploited by operational centers like European Centre for Medium-Range Weather Forecasts and National Centers for Environmental Prediction, with ramifications for socio-economic sectors coordinated by World Meteorological Organization and policy discussions at United Nations Framework Convention on Climate Change.

History and contributors

Key contributors include Vilhelm Bjerknes for foundational dynamical meteorology, Jacob Bjerknes for articulating the air–sea feedback concept, and subsequent elaboration by researchers at Scripps Institution of Oceanography, Woods Hole Oceanographic Institution, Princeton University, NOAA, CSIRO, Lamont–Doherty Earth Observatory, and University of California, Los Angeles. Notable theoreticians and modelers contributing to formalization and testing include Klaus Hasselmann, Syukuro Manabe, Jule Charney, Mark Cane, Stephen Zebiak, Anna Wittenberg, Michael McPhaden, and others linked to programs such as Tropical Atmosphere Ocean Project, CLIVAR, and El Niño Prediction System initiatives.

Category:Oceanography