Intrinsic interannual oceanic variability and its impact on decadal predictability
Christopher Wolfe, Stony Brook University, SoMAS, Stony Brook, NY, United States, Paola Cessi, University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States and Bruce D Cornuelle, University of California San Diego, La Jolla, CA, United States
Abstract:
In the classical paradigm for the ocean’s influence on climate variability, the ocean acts primarily as a heat reservoir, providing memory to the climate system due to its large heat capacity. The spectrum of climate variability could therefore be thought of as atmospheric white noise “reddened” by the ocean’s memory. As climate models increased in complexity, the dynamical response of the ocean to atmospheric forcing became appreciated. Many forms of low-frequency (interannual and longer) variability are now thought to be due to weakly damped oceanic oscillations excited by atmospheric noise (e.g., the Atlantic multidecadal oscillation) or fully-coupled, self-sustaining oscillations of the atmosphere/ocean system (e.g., El Niño/Southern Oscillation). While many ocean-only (i.e., uncoupled to an atmosphere) models exhibit
spontaneous low-frequency variability once dissipation rates are sufficiently reduced, the role of this intrinsic oceanic variability in the climate system remains poorly understood.
A series of simulations using a realistic, global ocean/ice model similar to that used in the ECCOv4 product finds variability on interannual to decadal timescales, even though the model is forced with annually-repeating atmospheric fields. The variability takes the form of temperature and salinity anomalies (from the seasonal cycle) with horizontal scales of hundreds to thousands kilometers, vertical scales of up to a kilometer, and sufficient amplitude to create surface heat flux anomalies exceeding 30 W m-2. The anomalies are organized on basin scales—occupying the majority of the North Atlantic Ocean and the entirety of the South Atlantic/Indian Ocean “supergyre”—and resemble the basin modes found in idealized ocean models. This kind of intrinsic variability has implications for decadal climate predictability since the response to atmospheric perturbations which project onto these internal modes becomes dominated by the intrinsic variability after about a decade. In this presentation, we discuss the dynamics of this intrinsic variability, show how it is modulated by external forcing perturbations, and quantify its impact on the decadal predictability of ocean model.
