Early twentieth-century warming linked to tropical Pacific wind strength

From the ENSO-can-contribute-to-global-warming department, we have a new paper from the National Center for Atmospheric Research (NCAR) and the University of Arizona (UA). It’s Thompson et al. (2014) Early twentieth-century warming linked to tropical Pacific wind strength.  The abstract reads:

Of the rise in global atmospheric temperature over the past century, nearly 30% occurred between 1910 and 1940 when anthropogenic forcings were relatively weak¹. This early warming has been attributed to internal factors, such as natural climate variability in the Atlantic region, and external factors, such as solar variability and greenhouse gas emissions. However, the warming is too large to be explained by external factors alone and it precedes Atlantic warming by over a decade. For the late twentieth century, observations and climate model simulations suggest that Pacific trade winds can modulate global temperatures^{2, 3, 4, 5, 6, 7}, but instrumental data are scarce in the early twentieth century. Here we present a westerly wind reconstruction (1894–1982) from seasonally resolved measurements of Mn/Ca ratios in a western Pacific coral that tracks interannual to multidecadal Pacific climate variability. We then reconstruct central Pacific temperatures using Sr/Ca ratios in a coral from Jarvis Island, and find that weak trade winds and warm temperatures coincide with rapid global warming from 1910 to 1940. In contrast, winds are stronger and temperatures cooler between 1940 and 1970, when global temperature rise slowed down. We suggest that variations in Pacific wind strength at decadal timescales significantly influence the rate of surface air temperature change.

Thompson et al. (2014) failed to mention a number of things in the abstract. (1) The multidecadal variations in trade wind strength are a result of the multidecadal variations in the dominance of El Niños versus La Niñas in the tropical Pacific (ENSO).  The trade winds are weaker during multidecadal periods when El Niños dominate and they’re stronger during periods of La Niña dominance.  We can see these decadal and multidecadal variations in El Niño and La Niña dominance by smoothing a sea surface temperature-based ENSO index (NINO3.4 region SST anomalies based on HADISST data) with a 121-month running-average filter.

Figure 3-129 is from my upcoming book.

(2) The trade winds were once again weaker during the late 20^th Century warming period, a period when El Niño events were stronger, lasted longer, and occurred more frequently than La Niña events. So, if weaker trade winds contributed to global warming during the early 20^th century warming period, they also contributed to the warming since the mid-1970s.

(3) Because climate models used to hindcast surface temperatures during the 20^th Century do not properly simulate the coupled ocean-atmosphere processes associated ENSO (El Niño and La Niña), and because those models do not properly simulate the “skewness” of ENSO, those climate models cannot be used for global warming attribution studies. And if they can’t be used for attribution, they have no value when projecting climate into the future.

(4) ENSO is fueled by sunlight.

The NCAR press release for Thompson et al. (2014) is Coral Reveals Long-Term Link Between Pacific Winds, Global Climate.

In the Introduction to my book Who Turned on the Heat? – The Unsuspected Global Warming Culprit, El Niño-Southern Oscillation, I wrote:

Climate models used by the Intergovernmental Panel on Climate Change (IPCC) cannot match the sea surface temperature records that show how often and how strongly ENSO events have occurred since 1900. Climate models can’t even simulate the ENSO events since the start of the recent warming period in the mid-1970s. However, the models need to be able to mimic the historical instrument-based ENSO records.  In fact it’s critical that they do, and it’s easy to understand why.  The strength of ENSO phases, along with how often they happen and how long they persist, determine how much heat is released by the tropical Pacific into the atmosphere and how much warm water is transported by ocean currents from the tropics toward the poles.  During a multidecadal period when El Niño events dominate (a period when El Niño events are stronger, when they occur more often and when they last longer than La Niña events), more heat than normal is released from the tropical Pacific and more warm water than normal is transported by ocean currents toward the poles—with that warm water releasing heat to the atmosphere along the way.   As a result, global sea surface and land surface temperatures warm during multidecadal periods when El Niño events dominate.  They have to.  There’s no way they cannot warm.  Conversely, global temperatures cool during multidecadal periods when La Niña events are stronger, last longer and occur more often than El Niño events.  That makes sense too because the tropical Pacific is releasing less heat and redistributing less warm water than normal then.

Nice to see the climate science community supporting that.

The Introduction to Who Turned on the Heat? is included in the free preview here, and the book is on sale only in pdf format here for U.S.$5.00.

[Thanks to blogger Andrew for the heads-up at WUWT here.]