The Intergovernmental Panel on Climate Change (IPCC) has reported that most of the warming of the past 50 years resulted from an increase in concentrations of greenhouse gases in the atmosphere (IPCC, 2007).  The recent history of global surface temperatures includes a period of cooling between 1946 and 1975, warming at about 0.2^OC/decade between 1976 and 1998 and no warming from 1999 to 2010.

That the El Niño Southern Oscillation (ENSO) has an impact on global surface temperature is well known – ENSO causes heating or cooling of the atmosphere.  One physical interpretation of atmospheric heating and cooling is as a result of heat transfer between the Pacific Ocean and the atmosphere – to a cool ocean surface in a La Niña and from a warm ocean surface in an El Niño.  All other things being equal - this cannot have much impact on ocean and atmospheric heat content beyond a decade at most.   Less well known is the relationship between the Pacific Decadal Oscillation (PDO) and ENSO.    A cool mode PDO, over 20 to 30 years, sees cooler than average sea surface temperature (SST) in the northern Pacific and more frequent and intense La Niña.  A warm mode PDO is defined as warmer than average SST in the north eastern Pacific over 20 to 30 years and is associated with more frequent and intense El Niño. ENSO and the PDO involve observable changes in frigid and nutrient rich upwelling of turbulent sub-surface currents influencing SST and biological productivity across the Pacific.

Mechanisms for a decadal surface temperature impact include cloud formation dynamics involving SST and release to the atmosphere of dimethyl sulphide from oceanic phytoplankton.  As well, small changes involving ‘top down’ solar forcing appear to cause modulation of complex and dynamic ENSO atmosphere and ocean couplings, and thus may initiate chaotic bifurcation in Pacific Ocean climate states.  Understanding the physical evidence of Pacific Ocean climate shifts provides a clearer picture of the reasons behind 20^th century decadal cycles of atmospheric warming and cooling than has generally been the case.  The implications of chaotic climate shifts in the Pacific Ocean include a potential lack of global warming for another decade or so at least.

Deep oceanic currents are driven by thermohaline circulation and by the rotation of the planet.  The deep currents interact with a sun warmed surface layer that is a hundred or more metres deep.  Deep ocean currents occasionally push through the warm surface layer in the south eastern Pacific in one of the major areas for upwelling on the planet.  Upwelling subsurface water is both frigid and rich in nutrients leading to booms and busts in biological activity affecting fisheries, mammals and birds off the Pacific coast of South America.  This area is designated as Large Marine Ecosystem (LME) No. 13, is amongst the most productive environments in the world and is known as the Humboldt Current.  A good introduction is provided by National Ocean and Atmospheric Administration (NOAA) at their LME/Humboldt Current web page.

The thermal evolution of the Humboldt Current is best understood in terms of ENSO.  ENSO is an oscillation between El Niño and La Niña states over a 2 to 7 year cycle.  An El Niño is defined as sustained SST anomalies greater than 0.5^O C (in the Nino 3 region) over the central pacific.  Conversely, a La Niña is defined as sustained SST anomalies less than -0.5^O C.   The oscillations are driven by complex interactions of cloud, wind, sea surface pressure and temperature, planetary rotation and surface and subsurface currents.   The short explanation is that the Pacific trade winds set up conditions for a La Niña.  Trade winds, south-easterly in the Southern Hemisphere and north-easterly in the Northern Hemisphere, pile up warm surface water against Australia and Indonesia.  Water vapour rises in the western Pacific creating low pressure cells that strengthen the trade winds piling yet more warm water up in the western Pacific.  Cool, subsurface water rises in the eastern Pacific and spreads westward.  At some point the trade winds falter and warm water spreads out westward across the Pacific.  Figure 1 shows thermally enhanced satellite images of SST of the Pacific Ocean during the warm ocean surface conditions of the 1997/98 El Niño and thecool conditions of the 2000 La Niña.

Figure 1: The 1998 El Niño and2000 La Niña(Source: NOAA)

SST anomalies for the Nino 3 and Nino 3.4 regions of the central Pacific are shown below in Figure 2 for the period 1950 to 2000.   El Niño are shown as red and La Niña blue.

Figure 2:Nino3 and Nino 3.4 Central Pacific Sea Surface

Temperature Anomalies (Source: UCAR)

One physical link between SST in the Pacific Ocean and atmospheric temperature is straight forward.  It is simply a matter of heating and cooling as a result of heat transfer between the Pacific Ocean and the atmosphere – from a warm ocean surface in an El Niño and to a cool surface in a La Niña.  Figure 3 shows a satellite derived monthly temperature record of the lower atmosphere.  Note the record surface temperature early in 1998 and the rapid cooling to 2000.  ENSO is the largest cause by far of interannual surface temperature variability.

Figure 3: Average Monthly Lower Atmosphere Temperature Anomalies

(Source: Dr Roy Spencer)

McLeanet el (2009) examined the Southern Oscillation Index (SOI), a leading indicator for ENSO, against global tropospheric temperature anomalies (GTTA).  A graph of the raw data is shown in Figure 4.  By considering a 7 month lagged time series derivative of GTTA, McLean and colleagues were able to show that 81% of variance in tropical tropospheric temperatures was related to changes in ENSO.

Figure 4: SOI vs. GTTA (after McLean et al 2009)

‘Overall the results suggest that the Southern Oscillation exercises a consistently dominant influence on mean global temperature, with a maximum effect in the tropics, except for periods when equatorial volcanism causes ad hoc cooling. That mean global tropospheric temperature has for the last 50 years fallen and risen in close accord with the SOI of 5–7 months earlier shows the potential of natural forcing mechanisms to account for most of the temperature variation.’ (McLean et al 2009)

Without wishing to labour the obvious – the peaks and troughs of the atmospheric temperature mirror the ENSO state.

Table 1: ENSOStatesand Atmospheric Temperature Trends

The other observation that is commonly taken from Pacific SST anomalies is the shift in 1976/77 to more frequent and intense El Niño.  This is sometimes referred to as the ‘Great Pacific Climate Shift’ and much effort has gone into examining ENSO proxy data to establish if the shift was unusual in the climate record.  Mann et al (2000) found that the intensity of the 1998 and 1983 El Niños, and the length of the early 1990’s El Niño, may be unusual.  Gergis and Fowler (2006) found that both El Niño and La Niña increased in frequency in the 20^th century.  Verdon and Franks (2006) found no change in ENSO frequency.      Braganza et al (2008) also found no change in ENSO frequency – although their proxy record is limited to Pacific Basin records and stops in 1982 sacrificing relevance for accuracy.

The 1976/77 Pacific Climate Shift is more easily seen in the multivariate ENSO index (MEI) of Klaus Wolter in Figure 5.  A bias is seen towards La Niña (blue) conditions prior to the late 1970’s, 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 5: Multivariate ENSO Index (Source: NOAA)

A link between ENSO and the PDO has been shown (eg Hamlet and Lettenmeier 1999, Yu and Zwiers 2001, Hildalgo and Dracup 2002, Chan and Zhou 2005, Verdon and Franks, 2006, Kunkel and Pearce 2006, Wang et al 2008).  Excellent recent reviews of the PDO and climate impacts are provided by Mestas-Nuñez and Miller 2006 and Hartmann et al 2005.

Verdon and Franks (2006) used ‘proxy climate records derived from paleoclimate data to investigate the long-term behaviour of the PDO and ENSO. During the past 400 years, climate shifts associated with changes in the PDO are shown to have occurred with a similar frequency to those documented in the 20th Century. Importantly, phase changes in the PDO have a propensity to coincide with changes in the relative frequency of ENSO events, where the positive phase of the PDO is associated with an enhanced frequency of El Niño events, while the negative phase is shown to be more favourable for the development of La Niña events.’

The PDO index shown in Figure 6 shows the state of north eastern Pacific SST – a cool mode to the late 1970’s followed by a warm mode.  The 20 to 30 year cycles are strongly evident in the latter half of the 20^th century – when records are more extensive.

‘The PDO was named by Mantua et al (1997), who demonstrated a connection between salmon abundance and SST in the northern Pacific.   SST varied over 20 to 30 year cycles in phase with changes in salmon abundance.  SST were cooler than average for 20 to 30 years – a cool mode of the PDO, and then warmer than average over 20 to 30 years, a warm mode.  A warm mode PDO is associated with reduced abundance of coho and chinook salmon in the Pacific Northwest, while a cool mode PDO is linked to above average abundance of these fish.  The biology responds to cold but nutrient rich sub surface water upwelling in the north eastern Pacific.  The abundance of salmon was greatest in the period between the mid 1940’s and mid 1970’s, least in the period 1976 to 1998 and has, in recent years, rebounded to values not seen since the 1970s.’ (JISOA- Climate Impacts Group)

Figure 6:  PDO Index (Source: JISOA)

The question naturally arises as to whether this is a recurring cycle or an artefact of 20^th century anthropogenic climate change.  Because ENSO and the PDO are the source of most variability in global hydrology – the persistence of the cycle can be examined in tree ring and other proxies.  Figure 7 shows one such reconstruction.

Figure 7: PDO Reconstruction from a Californian Tree Ring Proxy (after Biondi

et al 2001)

The best that may be said of all the reconstructions is that there does appear to be cool and warm PDO modes – decadal changes – over the past few hundred years.  ENSO and the PDO vary naturally and substantially in ways that are not simple oscillations about a stationary mean.  Attribution, prediction and quantification remain problematic, however, the background variability of Pacific climate states should engender caution in attribution of most of recent global warming to greenhouse gases.  These vary with ENSO on interannual scales and with the PDO on decadal scales.  There is also evidence of millennial variation in a shift 5,000 years ago from La Niña dominant to El Niño dominant conditions that is defined by Tsonis (2009) as a chaotic bifurcation.

It seems evident that not all of the surface temperature increase between 1976 and 1998 was caused directly by infrared radiation trapping by greenhouse gases.   At least some of the temperature increase was the result of energy transfer from the ocean to the atmosphere in a warm Pacific mode amplification of El Niño.  Cloud cover in the Pacific changes as a result of changes in SST and this seems also be a factor in modulating the global energy balance.  ‘Tropical and subtropical low-level marine clouds consist of optically thick stratocumulus clouds, which usually form over the regions associated with relatively cold sea surface temperatures (SST) and atmospheric subsidence, and optically thin shallow cumuli in the tradewind regime. These low-level clouds play a pivotal role in the global climate system not only by affecting radiative budgets but also by promoting heat and moisture exchange between the sea-surface, the boundary layer, and the overlying troposphere.’ (Zhu Ping et al 2007)

There is as well a biologically mediated interaction of Pacific Ocean variability with cloud formation.  Dimethyl sulphate (DMS) is released to the atmosphere from phytoplankton in the Earth’s oceans.  DMS is oxidized in the atmosphere to sulfur-containing compounds.  The sulphur compounds both reflect sunlight and have the potential to create new aerosols which act as cloud condensation nuclei. The massive production of sulphur compounds over the oceans may have a significant impact on the Earth's climate - see for instance Thomas et al (2010).  Remembering the biological effects of changing patterns of frigid and nutrient rich upwelling in the eastern Pacific – a cool Pacific phase will see phytoplankton blooms leading to increased sulpur compounds in the atmosphere and increased cloud cover.  Less upwelling, as seen in the period 1976 to 1998 - sees much less biological production in the surface waters of the Pacific.

Observational evidence of decadal change in cloud cover is seen in a 2009 study by Amy Clements and colleagues using 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.’ (Clement et al 2009)

Shifts in biology, cloud cover, surface temperature and SST are all part of a picture of climate shift on decadal timescales.  The climate shift since 1998 shows the ongoing influence of cycles of natural climate variability.

The influence of clouds on climate can be seen in the terms of the global energy storage formula.

E_IN/S = E_OUT/S + d(GES)/dt.

By the law of conservation of energy - the average energy in, E_IN/S, at the top of atmosphere (TOA) in a period is equal to the average energy out, E_OUT/S, plus the rate of change in global energy storage, d(GES)/dt.  Energy in (in the visible and infrared spectrum) varies marginally in the 11 year Schwabe solar cycle, perhaps a little more over the longer term due to solar variation and, due to orbital changes, over an Ice Age.  On a decadal timescale – energy in (at TOA) is more or less constant.

Most change in the global energy balance occurs in energy out resulting in an equal and opposite change to global energy storage – planetary heating and cooling.  Energy out is measured by satellite in the short wave and infrared spectrums.  Changing cloud cover influences the global energy balance by reflecting more or less visible light back into space - and thereby reducing or increasing the amount of energy coming into Earth’s climate system.  Energy is stored in the Earth’s climate system - mostly as heat in the oceans.  If the trend in energy out in a period is negative then the rate of change of global energy storage is positive and the planet warms and vice versa.

The ISCCP-FD data in Figure 8 shows a negative trend in reflected short wave radiation between the 1980’s and 1990’s.  More energy in the short wave band was entering the climate system in the late 1990’s than in the mid 1980’s – a warming trend in the short wave band as a result of changes in cloud cover.

While the early satellite data is not conclusive by any means, the results are consistent with observed decadal changes in Pacific Ocean SST and with theoretical cloud formation mechanisms.  There is a decreasing trend in reflected shortwave energy (along with sudden increases as a result of volcanic sulphur emissions – note the cooling in 1992 in Figure 3) from the start of records to the end of the 1990’s followed by an increase thereafter.  Short term changes in short wave energy flux is a result primarily of changing global cloud cover - so we have decreasing cloud cover (especially low level cloud in the subtropical and tropical Pacific) to the late 1990’s and thereafter an increase – consistent with surface observations.

Figure 8: ISCCP-FD Upwelling Short Wave Radiation Flux at Top of

Atmosphere (Source ISCCP-FD)

If the quantum of change in this (and in ERBE) data is to be believed – the changes in reflected shortwave are climatologically much more significant than any other factor in Earth’s recent climate.

Figure 9: ISCCP-FD Upwelling Long Wave Radiation Flux at Top of

Atmosphere (Source ISCCP-FD)

It is notable that the long wave radiation flux trend is to increased emissions – or planetary cooling – in both ISCCP and ERBE data.  This also reveals changes in low level cloud – reduced low level cloud cover results in reduced infrared trapping by cloud.  Infrared trapping by clouds is the reason why overcast nights are warmer than a night with clear skies.  The data shows that clearing skies between the 1980’s and 1990’s reversed the anticipated trend of greenhouse gas warming in the infrared.

The IPCC in AR4 (S 3.4.3) notes the trends in both ISCCP and ERBE data but says that this is inconsistent with surface observations of clouds. Later compilations of, especially, Pacific cloud observations provides new evidence to the contrary.  The net outgoing radiation flux (short wave plus long wave) trend is negative and the planet warmed in the period.

Figure 10 shows a thermally enhanced satellite image as of November 2010.  It shows both the cool SST associated with the current La Niña and the cool pool of water in the north east Pacific typical of the PD0.

Figure 10: Global SST Anomalies 4^th November 2010 (Source: NOAA)

One potential cause of Pacific Ocean variability is shown by Lockwood et al (2010).  ‘During the descent into the recent exceptionally low solar minimum, observations have revealed a larger change in solar UV emissions than seen at the same phase of previous solar cycles. This is particularly true at wavelengths responsible for stratospheric ozone production and heating. This implies that ‘top-down’ solar modulation could be a larger factor in long-term tropospheric change than previously believed, many climate models allowing only for the ‘bottom-up’ effect of the less-variable visible and infrared solar emissions. We present evidence for long-term drift in solar UV irradiance, which is not found in its commonly used proxies.’

Judith Lean (2008) commented that ‘ongoing studies are beginning to decipher the empirical Sun-climate connections as a combination of responses to direct solar heating of the surface and lower atmosphere, and indirect heating via solar UV irradiance impacts on the ozone layer and middle atmospheric, with subsequent communication to the surface and climate. The associated physical pathways appear to involve the modulation of existing dynamical and circulation atmosphere-ocean couplings, including the ENSO and the Quasi-Biennial Oscillation. Comparisons of the empirical results with model simulations suggest that models are deficient in accounting for these pathways.’

One theory sees solar mediated changes in the thermosphere and stratosphere influencing the troposphere in the polar regions.  The southern hemisphere has seen a trend to more positive (by convention low pressure cells above the polar region) values of the Southern Annular Mode (SAM) index to the late 1990’s.  The SAM Index below is an index of sea level pressure (SLP) in the Antarctic and sub-Antarctic.  Modellers have suggested that the positive trend is, in part, a result of greenhouse gases and loss of ozone.  There are, however, too many unknowns for models to be definitive.

Figure 11: Southern Annual Mode Index (Source IPCC 4AR)

Higher SLP (negative SAM) in the polar region results in storm tracks pushing further into lower latitudes.  More cold Southern Ocean surface water accumulates off the South American coast.  Here it dilutes the warm and buoyant surface layer allowing more upwelling of turbulent subsurface currents.  The cool water upwelling in the Humboldt Current drifts westward across the Pacific driven first of all by equatorial winds and currents that are an artefact of planetary spin.  As the cool water moves westward, ocean and atmosphere coupling reinforce Walker and Hadley circulation strengthening tradewinds and the evolving La Niña pattern.

With a positive SAM, westerlies in the Antarctic Vortex increase in velocity and storm tracks are constrained to higher latitudes.  As a result, surface flows in the Antarctic Circumpolar Current tend to stay further to the south.  Flow increases through the Drake Passage between the Antarctic Peninsula and the southern tip of South America.  An El Niño pattern evolves from water warming in the south eastern Pacific, in the area of the Humboldt Current, and impeding subsurface upwelling.

The smoothed curve above is useful for visualising change over time.  But we should remember that the Sun itself is a complex and dynamic system – far more complex than the classic Poincaréthree body orbital problem that preceded chaos theory by 75 years – and look for abrupt rather than smooth transitions, climate shifts, in all climate indices including surface temperature.

An alternate approach to understanding the PDO and the ENSO as behaving like a complex and dynamic system in chaos theory emerged from a 2007 study by Tsonis et al.  They constructed a numerical network model from 4 observed ocean and climate indices - including the ENSO, the PDO, the North Atlantic Oscillation (NAO) and the Pacific North America Pattern (PNA), capturing major modes of climate variability in the period 1900–2000.  The network model synchronized around 1909, the mid 1940’s and 1976/77 – after which the climate state shifted.  A later study (Swanson and Tsonis 2009) found a similar shift in 1998/2001. They found that where a ‘synchronous state was followed by a steady increase in the coupling strength between the indices, the synchronous state was destroyed, after which a new climate state emerged. These shifts are associated with significant changes in global temperature trend and in ENSO variability.’

Identifying shifts in Pacific Ocean climate states as chaotic does little to engender confidence in predictability.  However, given the persistence of alternating states over a couple of decades in the instrumental and observational evidence, a broad prediction might be possible.  Currently, a decade old cool mode PDO continues to intensify in the north eastern Pacific – as shown in the thermally enhanced satellite image (Figure 10) above.  The current cool Pacific mode explains the lack of global warming over the past decade and suggests that the surface temperature hiatus seems likely to persist for another decade or two.

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