Article: England, M. H., McGregor, S., Spence, P., Meehl, G. A., Timmermann, A., Cai, W., Gupta, A. S., McPhaden, M. J., Purich, A., Santoso, A. Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus. Nature Clim. Change Advanced Online Publication, 1-6 (2014). DOI: 10.1038/NCLIMATE2106
It should not be surprising that the global surface air temperature trend has been warming over the twentieth century. This is consistent with post industrial increase in atmospheric carbon dioxide, a consequence of humankind’s increased burning of fossil fuels. What may be a little puzzling are the periods of weaker warming, or even cooling, sometimes lasting for decades. These interruptions in the general warming trend of surface air temperatures are sometimes referred to as hiatuses; this terminology may be used to perhaps ward off the idea of a global warming “reversal”, which has been claimed by climate deniers.
The concept of warming hiatuses is consistent with a long term warming trend; there is inherent complexity in the interactions of the Earth’s climate system and the continuous increase of greenhouse gas concentrations. Currently, we have been stuck in a persistent warming hiatus since 2001. This is not a new phenomenon, the period of time from 1940s to the 1970s is also considered to be a warming hiatus. Warming resumed between 1975 and 2000, with the highest rates of warming occurring between 1975 and 1985 (Figure 1).
Climate scientists have been proposing possible causes to warming hiatuses, with explanations including higher heat uptake by the ocean, a period of reduced warmth from the sun, and changes in atmospheric concentrations of water vapor (a greenhouse gas) and aerosols. There is a growing consensus that the subsurface ocean is experiencing enhanced heat uptake. Consider the following: If the Earth remains in a state of global warming, but the surface air temperature trend has stalled or reversed, then some other component of the Earth must be receiving more heat. This concept is supported by observations of Pacific Ocean temperature.
To better understand the current warming hiatus, it would be beneficial to compare the current scenario to the most recent warming hiatus, with the expectation that there must be similar mechanisms in operation. During the 1940s – 1970s and the current warming hiatuses, a noticeable characteristic has been cooler surface ocean temperatures in the East Pacific. Expanding on this observation, both hiatuses have been characterized by a negative phase of the Interdecadal Pacific Oscillation (IPO). Quite simply, the IPO is a pattern of climate variability, not that different in concept from the El Niño Southern Oscillation. Just as El Niño has a positive phase and a negative phase (referred to as La Niña) with differing intensities in trade winds, and surface ocean temperatures, the IPO has a series of climate conditions during its positive and negative phase. The negative phase, which is consistent with the two most recent warming hiatuses, is characterized by a cool tropical Pacific and strengthened trade winds. During both warming hiatuses, trade wind strength increased, to levels higher than typically associated with just a negative phase of IPO. The increased wind strength results in changes in ocean circulation.
Upwelling (the movement of deeper, cooler water to the surface) is enhanced at the equator due to increases in a process called Ekman transport. The general idea here is that the net movement of water is perpendicular to the direction that the wind is blowing due to the combination of wind friction on the surface of the water and the Coriolis force, which is a force due to the rotation of the Earth, towards the right in the Northern Hemisphere and the left in the Southern Hemisphere. The trade winds at the equator and the opposing Coriolis Forces in the two hemispheres causes a divergence of water away from the equator. As water is pushed away from the equator, cooler water rises (upwells) to replace the displaced water, resulting in a cooling of surface ocean temperatures, especially seen in the Eastern Equatorial Pacific. Pacific ocean currents along the equator and along the continental margins also tend to increase in speed, increasing upwelling. As the surface equatorial Pacific cools, there is a net heat gain experienced at depth, attributed to enhanced convergence of the pycnocline (the portion of the water column where density increases rapidly with depth) (Figure 2).
To better understand how the observed strengthening of winds during the warming hiatuses impacts global climate, researchers performed numerous experiments using a global ocean model. The use of simulated models allows researchers to modify various parameters such as global wind patterns and solar radiation as if you were a higher power tweaking these parameters on Earth. This allows researchers to run the clock forward and make observations of how the Earth responds to these initial conditions and forcings. For these experiments, researchers observed the model response to climatological monthly winds and to observations of Pacific wind trends from 1992-2011. The model responded consistently with cooling in the eastern tropical Pacific, warming in the western Pacific and Indian Oceans, increased Ekman divergence at the equator, and increased ocean circulation, all consistent with the trends that have been observed.
The model also responded with cooling in the surface tropical Pacific, with enhanced heat uptake at depth, which are driven by the increase of wind strength. The combined effects of stronger winds and resulting changes in ocean circulation contribute to the lower global average surface air temperatures, with the temperature changes only becoming detectable 5 years following the increase in wind speed. The model experiments also predict that 80% of the cooling of surface air temperature occurs after the year 2000. One aspect not entirely accounted for in any of the model experiments was the intensity of the wind acceleration in the current warming hiatus, with model extremes less than half of the observed acceleration.
Running the model into the future is dependent on how tropical wind patterns change over the next decade. With current trade winds remaining unchanged, the model predicts a persistent warming hiatus lasting until 2020, similar to the duration of the last warming hiatus. In the event that strong trade winds begin to diminish, the model predicts a short and abrupt end to the warming hiatus, with a return to warming more typical of post industrial conditions (Figure 3).
Despite overwhelming scientific evidence and nearly unanimous acceptance by the scientific community, climate change is largely divided in the eyes of the media and the general public. A phrase such as warming hiatus, or the casual interpretation of a surface air temperature plot indicating a cooling trend since 2001 could be easily transformed into an argument that global warming is not being caused by burning of fossil fuels. It is important to understand that these warming hiatuses have been observed in the past, and have always been relatively short-lived anomalies from an increasing temperature trend following the increase in atmospheric carbon dioxide concentrations. Essentially these warming hiatuses are natural climate variations temporarily reducing the impact of global warming and certainly do not invalidate the fact that human induced climate change will continue unless significant measures are taken to reduce carbon emissions.