A delayed oscillator model for the quasi-periodic multidecadal variability of the NAO

Cheng Sun,Fei-Fei Jin,Jianping Li

doi:10.1007/s00382-014-2459-z

Abstract

Abstract Wavelet analysis of the annual North Atlantic Oscillation (NAO) index back to 1659 reveals a significant frequency band at about 60 years. Recent NAO decadal variations, including the increasing trend during 1960–1990 and decreasing trend since the mid-1990s, can be well explained by the approximate 60-year cycle. This quasi 60-year oscillation of the NAO is realistically reproduced in a long-term control simulation with version 4 of the Community Climate System Model, and the possible mechanisms are further investigated. The positive NAO forces the strengthening of the Atlantic meridional overturning circulation (AMOC) and induces a basin-wide uniform sea surface temperature (SST) warming that corresponds to the Atlantic multidecadal oscillation (AMO). The SST field exhibits a delayed response to the preceding enhanced AMOC, and shows a pattern similar to the North Atlantic tripole (NAT), with SST warming in the northern North Atlantic and cooling in the southern part. This SST pattern (negative NAT phase) may lead to an atmospheric response that resembles the negative NAO phase, and subsequently the oscillation proceeds, but in the opposite sense. Based on these mechanisms, a simple delayed oscillator model is established to explain the quasi-periodic multidecadal variability of the NAO. The magnitude of the NAO forcing of the AMOC/AMO and the time delay of the AMOC/AMO feedback are two key parameters of the delayed oscillator. For a given set of parameters, the quasi 60-year cycle of the NAO can be well predicted. This delayed oscillator model is useful for understanding of the oscillatory mechanism of the NAO, which has significant potential for decadal predictions as well as the interpretation of proxy data records.

Highlights

We focus on the possible influence of the Atlantic multidecadal oscillation (AMO) and North Atlantic tripole (NAT) patterns on North Atlantic Oscillation (NAO) quasi-periodic multidecadal variability, which has not been well addressed in the literature
This study investigated the quasi-periodicity of the NAO over multidecadal timescales
Principal Oscillation Pattern (POP) analysis revealed two dominant sea surface temperature (SST) patterns over the North Atlantic in association with the 50–70 year cycle, and this cycle is manifested by the alternating appearance of these two SST patterns

Summary

Introduction

Many studies have been conducted on the cause of the NAO decadal variability, but there is still little consensus Several factors, such as greenhouse gas emissions and warming in tropical oceans, have been suggested to account for the NAO interdecadal variations. We further show that there is a two-way interaction between the NAO and AMO; that is, the NAO has a positive forcing effect on the AMO, consistent with the previous findings, while the AMO in turn provides a delayed negative feedback on the NAO by inducing a meridional SST gradient pattern These positive and negative feedback mechanisms act successively, leading to the quasi-periodic multidecadal variability of the NAO.

Data and methodology

Statistical method

Quasi‐periodic multidecadal variability of the NAO and possible mechanisms

Possible mechanisms involved

Findings

Summary and discussion