Does External Forcing Interfere with the AMOC's Influence on North Atlantic Sea Surface Temperature and Arctic Climate?

Transcription

1 Does External Forcing Interfere with the AMOC's Influence on North Atlantic Sea Surface Temperature and Arctic Climate? Neil Tandon and Paul Kushner Department of Physics, U of Toronto Tandon and Kushner, J. Climate in press, and in prep. Support: NSERC CanSISE (Canada) Fondation BNP-Paribas PRECLIDE (France)

2 AMOC-NASST Relationship Liu et al., Science (2009) adapted from Delworth et al., J. Clim. (1993) (AMOC = Atlantic meridional overturning circulation NASST = North Atlantic sea surface temperature)

3 Models are highly inconsistent! historical simulations ( ) from CMIP5, Linearly detrended Zhang and Wang, 2013

4 Models are highly inconsistent! historical simulations ( ) from CMIP5, Linearly detrended Zhang and Wang, 2013

5 Key question 1: Why do models seem so inconsistent in how they represent the AMOC-NASST relationship?

6 Internal variability explains some... Historical simulations ( ), linearly detrended Thick: first realization

7 ... but external forcing explains more! PI control simulations (499 years), linearly detrended, 156 year chunks

8 CESM Large Ensemble Reveals External Forcing lag [years] CESM Realizations CESM Ens Mean CMIP5 Mean ERSSTv3

9 CESM1 Large Ensemble Key point: Removing external forcing is required to obtain robust AMOC-NASST correlation.

10 Implications for Predictability Removing external forcing reduces NASST predictability and to a lesser extent AMOC predictability.

11 Key question 1: Why do models seem so inconsistent in how they represent the AMOC-NASST relationship? Simple key point: Removing external forcing is required to obtain robust AMOC-NASST correlation. This affects inferred predictability. (Tandon and Kushner, in press) Key question 2: So what does this imply for Arctic climate?

12 Is the AMOC Key for Arctic Climate? Not Clear AMOC correlations with A-S-O Sea Ice Concentration CMIP5 PI Control 500 years CMIP5 historical 1 realization (10 year low pass filter)

13 Conclusion The empirical AMOC-NASST relationship is strongly affected by external forcing, which can affect its timing, sign, and inferred predictability. Cleaner relationships like positive AMOC- NASST correlations at zero lag are produced when external forcing is removed, either in pictl simulations or in the CESM LENS. Linear detrending is not suitable: timescales of forced and unforced variations are not wellseparated enough. Internal AMOC itself appears to exert only a weak influence on Arctic climate.

14 What Is the Role of External Forcing? stronger AMOC external forcing weaker ocean stratification * cooler NASST weaker poleward heat transport weaker AMOC stronger poleward heat transport warmer NASST increased ocean stratification * 14 external forcing * or some combination of feedbacks

15 AMOC-Arctic Sea Ice Relationship Correlations with AMOC Index PIControl with 10 year low pass filtering Mahajan et al. 2011, J. Climate

16 Internal AMOC/NASST Variability stronger AMOC weaker ocean stratification * cooler NASST stronger poleward heat transport warmer NASST weaker poleward heat transport weaker AMOC increased ocean stratification * * or some combination of feedbacks 16

17 How about Regional Ocean Circulation? Perhaps Subpolar Gyre Index correlations with A-S-O Sea Ice Thickness PI Control CMIP5 historical (non-filtered)

18 How about Regional Ocean Circulation? Perhaps Subpolar Gyre Index correlations with A-S-O Sea Ice Concentration PI Control CMIP5 historical (non-filtered)

19 CESM1 Large Ensemble Key point: Removing external forcing is required to obtain robust AMOC-NASST correlation.

20 Implications for Predictability

21 Spectra 21

22 22

23

24

25

26

27 Conclusion Recent large initial condition ensemble calculations provide new insights into the division between forced and internal variability. They provided a test of plausibility of extracting forced climate signals in observed ocean temperatures, which can then be in turn used for attribution of regional hydroclimate variability. They allowed us to disentangle AMOC-NASST relationship in different climate simulations, revealing a time evolving forcing being commingled with modes of internal climate variability. Several other LE projects are ongoing, including those with applications to climate event attribution. For many applications, fewer realizations might be sufficient to infer the forced signal or temperature trends (Thompson et al. 2014, Ting et al. 2009). But the communicative power and simplicity of this brute force approach carries considerable advantages.

28 Initial Tests of the Method h(x) using g L (t) Observed, Prescribed SST for Simulations Impact on h(x) of using g Th (t) Observed and Predicted Land Temperatures Spatial patterns obtained from different methods are similar. Bichet et al The gain factor affects the timing of the hydroclimate response to the SST forcing.

29 JFM Precipitation CRU AMIP FORCED Southwestern wintertime precip reduction and northwestern increase attributable to internal PDO variability. Bichet et al. in prep

30 AMOC Effect on Temperature Delworth et al. 30 (1993)

31