Emerging negative Atlantic Multidecadal Oscillation index in spite of warm subtropics

Eleanor Frajka-Williams,Claudie Beaulieu,Aurelie Duchez

doi:10.1038/s41598-017-11046-x

Eleanor Frajka-Williams, Claudie Beaulieu + Show 1 more

Open Access

https://doi.org/10.1038/s41598-017-11046-x

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Abstract

Sea surface temperatures in the northern North Atlantic have shown a marked decrease over the past several years. The sea surface in the subpolar gyre is now as cold as it was during the last cold phase of the Atlantic Multidecadal Oscillation index in the 1990s. This climate index is associated with shifts in hurricane activity, rainfall patterns and intensity, and changes in fish populations. However, unlike the last cold period in the Atlantic, the spatial pattern of sea surface temperature anomalies in the Atlantic is not uniformly cool, but instead has anomalously cold temperatures in the subpolar gyre, warm temperatures in the subtropics and cool anomalies over the tropics. The tripole pattern of anomalies has increased the subpolar to subtropical meridional gradient in SSTs, which are not represented by the AMO index value, but which may lead to increased atmospheric baroclinicity and storminess. Here we show that the recent Atlantic cooling is likely to persist, as predicted by a statistical forecast of subsurface ocean temperatures and consistent with the irreversible nature of watermass changes involved in the recent cooling of the subpolar gyre.

Highlights

The Atlantic Multidecadal Oscillation (AMO, Fig. 1a) is an index of decadal variability in the Atlantic based on sea surface temperatures (SSTs)
The AMO is typically constructed by averaging North Atlantic SSTs and subtracting a background time series to remove the anthropogenic changes
In contrast with SSTs, subsurface temperature anomalies can persist for months or years, leaving a longer-term impact on the overlying ocean surface, and so here we focus on upper ocean heat content (OHC)

Summary

Introduction

The Atlantic Multidecadal Oscillation (AMO, Fig. 1a) is an index of decadal variability in the Atlantic based on sea surface temperatures (SSTs). Over the recent few years, striking changes in Atlantic SSTs have occurred in the subpolar gyre, where the cold anomaly that developed from 2013–2015 was termed the ‘cold blob’ in the press[13]. This cold anomaly resulted from extremely harsh winters of 2013–2015, characterised by strong surface heat loss[14] which resulted in persistent cooling of the upper ocean[15] and drove deep ocean convection[16, 17]. We investigate whether the cold subpolar anomaly is likely to persist and consider how the present cold state of the AMO may evolve

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