October 29, 2008

A Further Look into the AMO (and Atlantic Hurricanes)

Filed under: Climate Extremes, Hurricanes

There is a degree of disagreement among climate scientists as to whether or not a phenomenon known as the Atlantic Multidecadal Oscillation (AMO) is a true physical mechanism operating in the Atlantic Ocean (e.g., Delworth and Mann, 2000; Knight et al., 2005; Zhang, 2007), or whether it is largely a manifestation of the pattern of the anthropogenic influence on the earth’s climate (Mann and Emanuel, 2006). The subject is of considerable interest in that many researchers have identified other climate phenomenon that seem to be related to the patterns of the AMO—primary among which are the patterns of Atlantic hurricane activity (e.g. Goldenberg et al., 2001). Thus, the source of the AMO likely sheds light on the source of Atlantic hurricane frequency and intensity fluctuations—are they primarily natural in origin, or are they primarily caused by human emissions of greenhouse gases and aerosols?

The AMO is an oscillating pattern of sea surface temperatures (SSTs) in the Atlantic Ocean. In its “negative” phase, SSTs are cool in the North Atlantic and relatively warm in the South Atlantic, in the AMO’s “positive” phase, the situation is reversed, and SSTs are warmer in the North Atlantic and relatively cooler in the South Atlantic (Figure 1 bottom). Accompanying the SST swings are changes in a host of other atmospheric variables.

The long-term history of the AMO appears to show a that couple of phase changes have occurred in the past century or so—a negative phase from about 1900 to the 1920s, followed by a positive phase from the 1930s to the 1960s, followed by another negative phase from the 1970s to the mid-1990s, and a return to the positive phase from about 1995 through the present (Figure 1, top).

Figure 1. Timeseries of the AMO index (top) and the AMO SST pattern (bottom). When the AMO is in its positive phase (red regions in the top chart), the SST is above normal in the red regions in the bottom chart, including in the main hurricane development region in the tropical North Atlantic. The situation is reversed for negative AMO phases (blue regions in the top chart) (source: Knight et al., 2005).

Many hurricane researchers have pointed out a similar pattern in Atlantic hurricane activity, with low levels of activity in the early few decades of the 20th century and in the 1970s and 1980s, and higher levels of activity in the 1930s through the 1960s, and again in the years since 1995.

Figure 2. Tropical cyclone counts in the North Atlantic basin, 1900-2007 (data source: National Hurricane Center).

However, other researchers have pointed out the similarity in the global temperatures and AMO patterns (thinking that it is more likely that global temperature patterns drove Atlantic SST patterns rather than vice versa) which led them to several major conclusions (from Mann and Emanuel, 2006):

1) “In short, there is no evidence that a natural climate oscillation such as the AMO contributes to long-term tropical North Atlantic SST variations.”

2) “There is a strong historical relationship between tropical Atlantic SST and tropical cyclone activity extending back through the late nineteenth century. There is no apparent role of the AMO.”


3) “The underlying factors appear to be the influence of (primarily anthropogenic) forced large-scale warming, and an offsetting regional cooling overprint due to late twentieth century anthropogenic tropospheric aerosol forcing.”

In other words, the AMO plays no role in Atlantic SSTs in the area that influences Atlantic hurricane patterns, and further, the real reason behind the hurricane patterns, including the recent increase in activity, is anthropogenic climate change.

As further proof that the AMO is not important in the tropical Atlantic, and thus that it couldn’t be related to Atlantic hurricane activity, Mann and Emanuel (2006) present this evidence:

Moreover, if the AMO were indeed responsible for anomalous recent tropical North Atlantic warmth, the THC [thermohaline circulation] should have exhibited a trend toward anomalous strength in recent decades, since model simulations indicate that tropical North Atlantic surface warmth associated with the AMO is in phase with the strength of the North Atlantic THC [see, e.g., Figure 3 in Knight et al., 2005]. Yet the only direct oceanic measurements available suggest a decrease, not an increase, in the THC between the late 1950s and the past decade (Bryden et al., 2005). [emphasis added]

The thermohaline circulation, or THC, that Mann and Emanuel refer to is the general circulation of waters through the global ocean that is driven by latitudinal temperature and salinity differences (see here for more details) and has ties in to the Gulf Stream and patterns of Atlantic SSTs.

Now, as we have covered previously, research published subsequent to “Bryden et al., 2005” has shown that the results reported by Bryden et al. to great fanfare—that the THC was slowing down (just as climate models suggest that it should be as the greenhouse effect strengthens), were in fact, in error.

More just-published research furthers this point and in fact, directly takes on the conclusions of Mann and Emanuel (2006). Rong Zhang of Princeton’s Geophysical Fluid Dynamics Laboratory (GFDL) has examined the historic behavior of the Atlantic Ocean’s portion of the THC, known as the Atlantic meridional overturning circulation (AMOC) using models and observations, and found that,

1) the AMOC has been strengthening in recent years,

2) the AMOC is strongly related to the patterns of Atlantic SST (i.e. the AMO),

3) multidecadal oscillation in the AMOC and the AMO are manifest in unforced climate model simulations (i.e., their existence is not dependent on anthropogenic influences),


4) “This result suggests that AMOC variations might have played an important role in the multidecadal variations of the Atlantic hurricane activities…”

Basically, these conclusions are diametrically opposed to those of Mann and Emanuel (2006). A fact commented on by Zhang himself:

The relationships identified here and the fact that they are reproduced in a millennium control simulation using a state-of-art coupled model bring significant new evidence that the observed AMO is linked to AMOC variations (Delworth and Mann, 2000; Knight et al., 2005; Zhang, 2007) rather than merely a 20th century artifact of changes in radiative forcing (Mann and Emanuel, 2006). [emphasis added]

Zhang’s findings join a ever-growing list of papers that suggest that natural variability (e.g., Knutson et al., 2008), rather than anthropogenic global warming, has been largely responsible for the observed patterns (including the recent increase) in hurricane activity in the Atlantic ocean.


Delworth, T. L., and M. E. Mann, 2000, Observed and simulated multidecadal
variability in the Northern Hemisphere. Climate Dynamics, 16, 661–676.

Goldenberg, S. B., et al., 2001. The recent increase in Atlantic hurricane activity: Causes and implications. Science, 293, 474– 479.

Knight, J. R., et al., 2005. A signature of persistent natural thermohaline circulation cycles in observed climate. Geophysical Research Letters, 32, L20708, doi:10.1029/

Knutson, T., et al., 2008. Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geosciences, 1, 359 – 364.

Mann, M.E., and K. A. Emanuel, 2006. Atlantic hurricane trends linked to climate change. Eos: Transactions of the American Geophysical Union, 87, 233-244.

Zhang, R., 2007. Anticorrelated multidecadal variations between surface
and subsurface tropical North Atlantic, Geophysical Research Letters, 34, L12713,

Zhang, R., 2008. Coherent surface-subsurface fingerprint of the Atlantic meridional
overturning circulation. Geophysical Research Letters, 35, L20705, doi:10.1029/2008GL035463.

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