A reversal of climatic trends in the North Atlantic since 2005

Journal name:
Nature Geoscience
Volume:
9,
Pages:
513–517
Year published:
DOI:
doi:10.1038/ngeo2727
Received
Accepted
Published online

In the mid-1990s the North Atlantic subpolar gyre warmed rapidly1, which had important climate impacts such as increased hurricane numbers2 and changes to rainfall over Africa, Europe and North America3, 4. Evidence suggests that the warming was largely due to a strengthening of the ocean circulation, particularly the Atlantic Meridional Overturning Circulation1, 5, 6, 7. Since the mid-1990s direct and indirect measurements have suggested a decline in the strength of the ocean circulation8, 9, which is expected to lead to a reduction in northward heat transport10, 11. Here we show that since 2005 a large volume of the upper North Atlantic Ocean has cooled significantly by approximately 0.45°C or 1.5 × 1022J, reversing the previous warming trend. By analysing observations and a state-of-the-art climate model, we show that this cooling is consistent with a reduction in the strength of the ocean circulation and heat transport, linked to record low densities in the deep Labrador Sea9. The low density in the deep Labrador Sea is primarily due to deep ocean warming since 1995, but a long-term freshening also played a role. The observed upper ocean cooling since 2005 is not consistent with the hypothesis that anthropogenic aerosols directly drive Atlantic temperatures12.

At a glance

Figures

  1. Recent upper ocean trends in the North Atlantic.
    Figure 1: Recent upper ocean trends in the North Atlantic.

    a, Linear trend in SST calculated over 1990–2004 from HadISST. The stippling shows where the fitted trend is larger than 2 standard errors of the residuals (see Methods). b,c, As a, but for 0–700m temperature (T700, b) and salinity (S700, c) as calculated from the EN4 data set. df, Same as ac, but for the 2005–2014 period. g, Time series of T700 and S700 averaged over the eastern North Atlantic (50°–10°W, 35°–65°N, boxed area shown on panel e) and the deep Labrador Sea density (DLS density), which is the 1,000–2,500m average density (σ2) in the Labrador Sea (60°–35°W, 50°–65°N, boxed area shown on panel a). Anomalies in g are made relative to 1961–1990.

  2. The role of the atmosphere in recent changes in the North Atlantic.
    Figure 2: The role of the atmosphere in recent changes in the North Atlantic.

    a, Linear trend in SLP calculated over 1990–2004 from NCEP reanalysis. The stippling shows where the fitted trend is larger than 2 standard errors of the residuals (see Methods). b,c, Same as a, but for wind-stress curl (WSC, b) and annual-mean net surface fluxes (SHF, c), as calculated from the NCEP reanalysis data set. Note that positive wind-stress curl anomalies in b represent increased Ekman upwelling, and positive SHF anomalies in c represent a warming of the ocean. df, Same as ac, but for the 2005–2014 period.

  3. Simulated ocean trends following a reduction in deep Labrador Sea density.
    Figure 3: Simulated ocean trends following a reduction in deep Labrador Sea density.

    a, Composite of 15-year linear trends in SST following the nine strongest trends in Labrador Sea density, where SST trends are offset by five years (that is, the first year used to compute the SST trend lags the first year used to calculate the deep Labrador Sea density index by five years). Stippling shows where trends are significant at p ≤ 0.1, see Methods for details. b,c, Same as a, but for 0–700m average temperature anomaly (T700) and 0–700m average salinity anomaly (S700). d, Standardized time series of deep Labrador Sea density (DLS density) and the eastern SPG (~38°–10°W, 50°–62.5°N; boxed area shown on panel b) 0–700m temperature (ESPG T700) anomaly for a portion of the simulation. e, Lead/lag relationship between rolling 15-year trends in deep Labrador Sea (DLS) density, and the 15-year trends in AMOC at 40°N (with Ekman component removed—see Methods, magenta), NAO index, Labrador Sea 0–700m temperature (LS T700), and the eastern SPG (ESPG) for 0–700m temperature (T700, solid) and 0–700m salinity (S700, dash). Positive lags show where the deep Labrador Sea density is leading the other variables. Note that for e the Labrador Sea density anomalies are multiplied by −1 to show how the metrics evolve before and after a negative trend in deep Labrador Sea density.

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Affiliations

  1. NCAS-Climate, Department of Meteorology, University of Reading, Reading RG6 6BB, UK

    • Jon Robson,
    • Pablo Ortega &
    • Rowan Sutton

Contributions

J.R. and R.S. jointly conceived the study. J.R. and P.O. analysed the observational and model data. J.R. led the writing of the manuscript with contributions and input from all authors.

Competing financial interests

The authors declare no competing financial interests.

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