The theory and estimation of the role of cloud in changing Earth’s dynamic energy balance is an area of fundamental weakness in climate science. Low level stratiform cloud forms over cool ocean water and dissipates over warm. The Pacific Ocean is where sea surface temperature (SST) varies most. SST changes dramatically across the Pacific Ocean as a result of a shifting balance between cold, turbulent, nutrient rich and acidic water rising in the eastern Pacific and the suppression of upwelling of sub-surface currents by a warm surface layer. A thermally enhanced satellite image as of the 7th of February 2011 can be found at this NOAA site. It shows the ‘V’ shaped wedge of cold water typical of the 20 to 40 year cool La Niña dominant mode of the Pacific multi-decadal pattern. It covers a good part of the planet.
The Pacific multi-decadal pattern involves modulation of the frequency and intensity of the ENSO. This can most easily be seen in the multivariate ENSO index (MEI) of Klaus Wolter in Figure 1. A bias is seen towards La Niña (blue) conditions prior to 1976/77, a shift thereafter to an El Niño (red) bias and a subsequent shift after 1998 back to a cooler bias. The MEI is based on six observed variables over the tropical Pacific. These six variables are: sea-level pressure, zonal and meridional components of the surface wind, sea surface temperature, surface air temperature, and total cloudiness fraction of the sky.
Figure 1: Multivariate ENSO Index (Source: NOAA; click on link)
The risk in focusing on a specific index is that the woods will be obscured by the trees. A 2007 study by Anastasios Tsonis – ‘A new dynamical mechanism for major climate shifts’ – shows that ENSO is part of a global and chaotic system. There are tremendous energies cascading through powerful systems. However, given the dramatic changes in upwelling of cold water on interannular to decadal and millennial timescales in the eastern Pacific – this is what drives most of the changes in global average surface temperature, hydrology and marine biology. Tsonis (2009) observed in a sediment record a ‘chaotic bifurcation’ from La Niña to El Niño dominated conditions 5000 years ago. This resulted in the drying of the Sahel and changed the path of human cultural development. Professor Jonathon Nott of James Cook University was interviewed by the ‘The Australian’ newspaper in the tense day before cyclone Yasi hit. Cyclones in Australia are much bigger and much more frequent in La Niña years than otherwise. He said that ‘what the record shows is we go through extended periods, hundreds of years, of high activity and extended periods of little activity.’ God help us – the past 150 years have been a period of little activity. There is little to suggest that we have more than skimmed the surface of Pacific Ocean variability.
In the past 60 years, observation from ships show cloud change over the same periods as the well known modes of Pacific Ocean decadal SST variability. A change to less cloud in the shift to a warm El Niño dominated Pacific decadal pattern in the late 1970’s and a change to more cloud following the shift to a La Niña dominated cool mode since 1998. Satellite measurements have since quantified decadal changes in outgoing shortwave and longwave radiative flux associated with cloud changes in the Pacific.
Burgman et al (2008) use a variety of data sources to examine decadal variability of surface winds, water vapour (WV), outgoing longwave radiation (OLR) and clouds. They conclude that the ‘most recent climate shift, which occurred in the 1990s during a period of continuous satellite coverage, is characterized by a ‘La Niña’ SST pattern with significant signals in the central equatorial Pacific and also in the north-eastern subtropics. There is a clear westward shift in convection on the equator, and an apparent strengthening of the Walker circulation. In the north-eastern subtropics, SST cooling coinciding with atmospheric drying appears to be induced by changes in atmospheric circulation. There is no indication in the wind speed that the changes in SST or WV are a result of changes in the surface heat flux. There is also an increase in OLR which is consistent with the drying. Finally, there is evidence for an increase in cloud fraction in the stratus regions for the 1990s transition as seen in earlier studies.’
In a study that was widely interpreted as a demonstration of a positive global warming cloud feedback, Amy Clement and colleagues (2009) presented observational evidence of decadal change in cloud cover in surface observation of clouds from the Comprehensive Ocean Atmosphere Data Set (COADS). ‘Both COADS and adjusted ISCCP data sets show a shift toward more total cloud cover in the late 1990s, and the shift is dominated by low- level cloud cover in the adjusted ISCCP data. The longer COADS total cloud time series indicates that a similar magnitude shift toward reduced cloud cover occurred in the mid-1970s, and this earlier shift was also dominated by marine stratiform clouds. . . Our observational analysis indicates that increased SST and weaker subtropical highs will act to reduce NE Pacific cloud cover.’ As was clearly stated in the paper, the evidence was for a decadal cloud feedback. The feedbacks correspond exactly to changes in the Pacific multi-decadal pattern.
A number of studies have demonstrated the connection of ENSO to radiative flux and therefore to cloud. In an analysis of global warming cloud feedbacks, Dessler (2010) used short term variations in surface temperature and CERES data to determine that cloud cover was negatively correlated with temperature. Dessler also plotted ENSO against surface temperature leaving no doubt that ENSO was the primary cause of the short term temperature variations. Leaving aside anthropogenic global warming – the finding of a positive feedback here is in the first instance an ENSO feedback. As was reported, ‘the climate variations being analysed here are primarily driven by ENSO, and there has been no suggestion that ENSO is caused by cloud variations.’ The study takes a statistical approach that may gloss over the difference in processes in play in ENSO and from global warming.
Zhu et al (2007) found that cloud formation for ENSO and for global warming have different characteristics and are the result of different physical mechanisms. The change in low cloud cover in the 1997-1998 El Niño came mainly as a decrease in optically thick stratocumulus and stratus cloud. The decrease is negatively correlated to local SST anomalies, especially in the eastern tropical Pacific, and is associated with a change in convective activity. ‘During the 1997–1998 El Niño, observations indicate that the SST increase in the eastern tropical Pacific enhances the atmospheric convection, which shifts the upward motion to further south and breaks down low stratiform clouds, leading to a decrease in low cloud amount in this region. Taking into account the obscuring effects of high cloud, it was found that thick low clouds decreased by more than 20% in the eastern tropical Pacific… In contrast, most increase in low cloud amount due to doubled CO2 simulated by the NCAR and GFDL models occurs in the subtropical subsidence regimes associated with a strong atmospheric stability.’
The surface observed decadal atmospheric changes have quantified support in satellite measurements of top of atmosphere radiative flux. This is what NASA/GISS says about the International Satellite Cloud Climatology Project data. The ‘slow increase of global upwelling LW (infrared or heat) flux at TOA from the 1980’s to the 1990’s, which is found mostly in lower latitudes, is confirmed by the ERBS records.’ ‘The overall slow decrease of upwelling SW (visible light) flux from the mid-1980’s until the end of the 1990’s and subsequent increase from 2000 onwards appears to be caused, primarily, by changes in global cloud cover (although there is a small increase of cloud optical thickness after 2000) and is confirmed by the ERBS measurements.’
Wong et al (2006) find that ‘comparison of decadal changes in ERB with existing satellite-based decadal radiation datasets shows very good agreement among ERBS Nonscanner WFOV Edition3_Rev1, HIRS Pathfinder OLR, and ISCCP FD datasets.’ They estimate the 15 year stability uncertainty of the radiative flux anomaly data (for all three datasets) at 0.3W/m2 to 0.4W/m2.
All global warming in the past 50 years, the period in which the IPCC say most warming occurred because of anthropogenic greenhouse gas emissions, happened between 1977 and 1998. This is exactly the same period as the last warm El Niño dominated Pacific decadal mode. In the instrumental record, the trajectory of global surface temperature mirrors the Pacific Ocean states. Cool to the late 1970’s, warm to 1998 and cool since. Sea surface temperature is negatively correlated to marine stratiform cloud. Multiple satellite data sources show that over most of the period of warming there was planetary cooling in the infrared band where greenhouse gases were expected to result in warming – and strong planetary warming as a result of less cloud reflecting less sunlight back into space. As a testable hypothesis, the current cool La Niña mode of the Pacific decadal pattern will lead to increased cloud cover and global cooling over another decade or three. After that, in a chaotic climate, it is anyone’s guess.
Biosketch. Robert styles himself in the blogosphere as a Chief Hydrologist. ‘Cecil Terwilliger (brother to Sideshow Bob) was Springfield’s Chief Hydrological and Hydrodynamical Engineer. He opined that this was a sacred vocation in some cultures. The more I thought about this the more it resonated with me. I am an hydrologist by training, profession and (much more) through a deep fascination with water in all its power and beauty. Given the importance of water to us practically and symbolically, there is more than an element of the sacred.’ –http://www.earthandocean.robertellison.com.au/
Moderation note: this is a technical thread, moderated for relevance.