Home > models, Natural Variability, Research Papers > Knight 2005: Persistent Natural Thermohaline Circulation Cycles

Knight 2005: Persistent Natural Thermohaline Circulation Cycles

2011 February 11

While the Decadal (Climate) Prediction System not only allows for internal natural variability, it feeds short term predictions (10 yrs or so) into slightly tweaked HadCMv3 model. Nevertheless, the near term decadal warming in the DePreSys doesn’t seem significantly slower than iconic 0.2C/decade IPCC AR4 warming. Nowhere near as slow as the ‘line+sine’ or ‘exp+sine’ “models.” Is there a natural “forcing” strong enough to justify the 60 year sine with ~ 0.1C amplitude (in the GSAT)?

I don’t know. But following a pointer in Smith 2007 (DePreSys), leads to the following paper:

A Signature of Persistent Natural Thermohaline Circulation Cycles in Observed Climate
Jeff R. Knight, Robert J. Allan, Chris K. Folland, Michael Vellinga, and Michael E. Mann
GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L20708, doi:10.1029/2005GL024233, 2005

Abstract: Analyses of global climate from measurements dating back to the nineteenth century show an ‘Atlantic Multidecadal Oscillation’ (AMO) as a leading large-scale pattern of multidecadal variability in surface temperature. Yet it is not possible to determine whether these fluctuations are genuinely oscillatory from the relatively short observational record alone. Using a 1400 year climate model calculation, we are able to simulate the observed pattern and amplitude of the AMO. The results imply the AMO is a genuine quasi-periodic cycle of internal climate variability persisting for many centuries, and is related to variability in the oceanic thermohaline circulation (THC). This relationship suggests we can attempt to reconstruct past THC changes, and we infer an increase in THC strength over the last 25 years. Potential predictability associated with the mode implies natural THC and AMO decreases over the next few decades independent of anthropogenic climate change


I’ve redacted the references to make it a little easier to read.

Introduction:
The AMO is a coherent pattern of multidecadal variability in surface temperature centred on the North Atlantic Ocean. It has been linked with the occurrence of Sahel drought, variability in Northeast Brazilian rainfall, North American climate and river flows, and the frequency of Atlantic hurricanes. Marked fluctuations of this pattern occurred in the 20th century, with intervals between successive peaks and troughs of roughly 65 years. A link with variability in the THC has been suggested, as the mean THC transports sufficient heat northward (1.2 PW at 30N) to warm the Northern Hemisphere by several C. Assessments of whether the AMO is a long-lived phenomenon, and whether it is related to the THC, are hindered by relatively short global climate records and insufficient subsurface ocean data. Palaeoclimate proxies suggest AMO variability extending back over several centuries, but these have uncertainties. As such, long simulations from numerical climate models are needed to investigate multidecadal variability. Spatially coherent modes have been found in some models, although with limited success in capturing the pattern of the AMO. Here we look for the AMO and THC links in a 1400 year simulation with the HadCM3 climate model, isolating internal variability by using constant levels of external climate forcing.

Hey! That’s the HadCM3 model again. Guess it get around. So the AMO is notable in actual climate variability aside from the temperature record. It can be seen in the paleoclimate record, but not strong enough to overcome uncertainty. AMO-like coherent phenomena can be seen in some models but without reproducing the AMO pattern itself. And in the next section (not quoted), the AMO may be associated with temperatures in western North America, Europe and Africa, and southern Asia. A wide spread hemispheric phenomena.

THC strengths for the next 35 years (Figure 4c). All the members of this ensemble show a downturn in the strength of the THC within a decade of the present day, suggesting that the THC is currently at or near a peak and likely to diminish thereafter. Further, each analogue segment becomes negative in the next 3 decades, reaching an average minimum of 0.70 Sv, similar to reconstructed levels for the minima of the 1910s and 1970s.

That’s something I hadn’t seen before. AMO/THC variability are modeled and indicate a drop in SST anomalies over the next few decades. Does this show up in HadCM3 global forecasts? The CMIP3 model means? Need to start digging into model data.

Cruising through the several conclusions….

Our 1400 year model simulation exhibits multidecadal climate variability with a similar pattern and amplitude to that of the AMO in observations. Together with the similarity of the simulated 70 – 120 year period to the observed 65 year period, and the range of periods derived from palaeodata (40 –130 years), this suggests the model simulates a realistic AMO. …

The simulated temperature changes associated with THC variability cannot fully explain the 0.6C of 20th century warming seen in both global and Northern Hemisphere mean temperature, but are large enough to modify estimates of the rate of anthropogenic climate change …. (someone should tell Dr Spencer)

The modelled AMO-THC link suggests we can make an SST-based reconstruction of past THC changes. …

The quasi-periodic nature of the model’s AMO suggests that in the absence of external forcings at least, there is some predictability of the THC, AMO and global and Northern Hemisphere mean temperatures for several decades into the future. We utilise this to forecast decreasing THC strength in the next few decades. This natural reduction would accelerate anticipated anthropogenic THC weakening, and the associated AMO change would partially offset expected Northern Hemisphere warming. This effect needs to be taken into account in producing more realistic predictions of future climate change.

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