In our last post Fritz Vahrenholt and Frank Bosse looked at solar activity and the impacts on the North Atlantic Oscillation. Today they present a post here on more papers on the NAO.
By Dr. Sebastian Lüning and Prof. Fritz Vahrenholt
(Translated/ edited by P Gosselin)
In our book “The Neglected Sun” we describe how climate is driven significantly by ocean and solar cycles. The climate science establishment prefers to ignore all of this. But lately things have turned around, as a number of scientists have been researching the subject and are publishing on this almost on a weekly basis. It is indeed becoming increasingly clear that ocean cycles are in fact coupled to a certain extent with solar activity. An important paper on this was published in the Environmental Research Letters in May, 2015. A team of scientists led by M. B. Andrews of the Hadley Centre of the British Met Office examined the relationship between the North Atlantic Oscillation (NAO) and solar activity cycles. It has long been known that the NAO swings to the positive mode when solar activity is strong. Conversely negative NAO values often fall when solar activity is weak. For more information of the NAO see Wikipedia here.
The North Atlantic Oscillation (NAO) is a climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level between the Icelandic low and the Azores high. Through fluctuations in the strength of the Icelandic low and the Azores high, it controls the strength and direction of westerly winds and storm tracks across the North Atlantic. It is part of the Arctic oscillation, and varies over time with no particular periodicity. Unlike the El Niño-Southern Oscillation phenomenon in the Pacific Ocean, the NAO is a largely atmospheric mode. It is one of the most important manifestations of climate fluctuations in the North Atlantic and surrounding humid climates.
The North Atlantic Oscillation is closely related to the Arctic oscillation (AO) or Northern Annular Mode (NAM), but should not be confused with the Atlantic Multidecadal Oscillation (AMO).
Westerly winds blowing across the Atlantic bring moist air into Europe. In years when westerlies are strong, summers are cool, winters are mild and rain is frequent. If westerlies are suppressed, the temperature is more extreme in summer and winter leading to heat waves, deep freezes and reduced rainfall. A permanent low-pressure system over Iceland (the Icelandic Low) and a permanent high-pressure system over the Azores (the Azores High) control the direction and strength of westerly winds into Europe. The relative strengths and positions of these systems vary from year to year and this variation is known as the NAO. A large difference in the pressure at the two stations (a high index year, denoted NAO+) leads to increased westerlies and, consequently, cool summers and mild and wet winters in Central Europe and its Atlantic facade. In contrast, if the index is low (NAO-), westerlies are suppressed, northern European areas suffer cold dry winters and storms track southwards toward the Mediterranean Sea. This brings increased storm activity and rainfall to southern Europe and North Africa.”
Andrews and his colleagues tried to determine if there is an empirical NA/solar relation in a simulation model. In earlier attempts the models were unable to get a handle on magnitude of the effect. But this time around the scientists were more successful. They recognized that they had indeed overseen a 3-year time lag with which the NAO follows the sun. Andrews et al also see an indication that other processes play a role – other than the pure atmospheric warming, and the dynamic effects thereof, taken into account up to now. The paper’s abstract follows:
A simulated lagged response of the North Atlantic Oscillation to the solar cycle over the period 1960–2009
Numerous studies have suggested an impact of the 11 year solar cycle on the winter North Atlantic Oscillation (NAO), with an increased tendency for positive (negative) NAO signals to occur at maxima (minima) of the solar cycle. Climate models have successfully reproduced this solar cycle modulation of the NAO, although the magnitude of the effect is often considerably weaker than implied by observations. A leading candidate for the mechanism of solar influence is via the impact of ultraviolet radiation variability on heating rates in the tropical upper stratosphere, and consequently on the meridional temperature gradient and zonal winds. Model simulations show a zonal mean wind anomaly that migrates polewards and downwards through wave–mean flow interaction. On reaching the troposphere this produces a response similar to the winter NAO. Recent analyses of observations have shown that solar cycle–NAO link becomes clearer approximately three years after solar maximum and minimum. Previous modelling studies have been unable to reproduce a lagged response of the observed magnitude. In this study, the impact of solar cycle on the NAO is investigated using an atmosphere–ocean coupled climate model. Simulations that include climate forcings are performed over the period 1960–2009 for two solar forcing scenarios: constant solar irradiance, and time-varying solar irradiance. We show that the model produces significant NAO responses peaking several years after extrema of the solar cycle, persisting even when the solar forcing becomes neutral. This confirms suggestions of a further component to the solar influence on the NAO beyond direct atmospheric heating and its dynamical response. Analysis of simulated upper ocean temperature anomalies confirms that the North Atlantic Ocean provides the memory of the solar forcing required to produce the lagged NAO response. These results have implications for improving skill in decadal predictions of the European and North American winter climate.”
Other modellers also took a closer look at the sun’s control over the North Atlantic Oscillation. In June, 2014, Lin et al. presented a situation of a Atlantic ocean cycle (AMOC) slowdown for the 1915-1935 period in the Climate of the Past Discussion. The authors saw the rise in solar activity after 1914 as a trigger and were able to show the effect in their simulation. The abstract:
An abrupt slowdown of Atlantic Meridional Overturning Circulation during 1915–1935 induced by solar forcing in a coupled GCM
In this study, we explore an abrupt change of Atlantic Meridional Overturning Circulation (AMOC) apparent in the historical run simulated by the second version of the Flexible Global Ocean–Atmosphere–Land System model – Spectral Version 2 (FGOALS-s2). The abrupt change is noted during the period from 1915 to 1935, in which the maximal AMOC value is weakened beyond 6 Sv (1 Sv = 106 m3 s−1). The abrupt signal first occurs at high latitudes (north of 46° N), then shifts gradually to middle latitudes (∼35° N) three to seven years later. The weakened AMOC can be explained in the following. The weak total solar irradiance (TIS) during early twentieth century decreases pole-to-equator temperature gradient in the upper stratosphere. The North polar vortex is weakened, which forces a negative North Atlantic Oscillation (NAO) phase during 1905–1914. The negative phase of NAO induces anomalous easterly winds in 50–70° N belts, which decrease the release of heat fluxes from ocean to atmosphere and induce surface warming over these regions. Through the surface ice–albedo feedback, the warming may lead to continuously melting sea ice in Baffin Bay and Davis Strait, which results in freshwater accumulation. This can lead to salinity and density reductions and then an abrupt slowdown of AMOC. Moreover, due to increased TIS after 1914, the enhanced Atlantic northward ocean heat transport from low to high latitudes induces an abrupt warming of sea surface temperature or upper ocean temperature in mid–high latitudes, which can also weaken the AMOC. The abrupt change of AMOC also appears in the PiControl run, which is associated with the lasting negative NAO phases due to natural variability.
Another paper on the solar impact on North Atlantic cycles – especially the NAO – appeared in the journal Annales Geophysicae. Sfîcă and colleagues reported there in February 2015:
The influence of solar activity on action centres of atmospheric circulation in North Atlantic
We analyse the response of sea level pressure and mid-tropospheric (500 hPa) geopotential heights to variations in solar activity. We concentrate on the Northern Hemisphere and North Atlantic in the period 1948–2012. Composite and correlation analyses point to a strengthening of the North Atlantic Oscillation and weakening (i.e. becoming more zonal) of the Pacific/North American pattern. The locations of points with lowest and highest sea level pressure in the North Atlantic change their positions between low and high solar activity.
Let’s now move to the Pacific Ocean. Here find the “El Niño–Southern Oscillation” (ENSO) phenomenon. Daniel Howard, Nir Shaviv and Henrik Svensmark are now able to show that the global sea level fluctuations (excluding the long-term trend) is at least 70% controlled by the ENSO and solar activity fluctuations. This team of scientists published their results in May 2015 in the Journal of Geophysical Research:
The solar and Southern Oscillation components in the satellite altimetry data
With satellite altimetry data accumulating over the past two decades, the mean sea level (MSL) can now be measured to unprecedented accuracy. We search for physical processes which can explain the sea level variations and find that at least 70% of the variance in the annually smoothed detrended altimetry data can be explained as the combined effect of both the solar forcing and the El Niño–Southern Oscillation (ENSO). The phase of the solar component can be used to derive the different steric and eustatic contributions. We find that the peak to peak radiative forcing associated with the solar cycle is 1.33 ± 0.34 W/m2, contributing a 4.4 ± 0.8 mm variation. The slow eustatic component (describing, for example, the cryosphere and large bodies of surface water) has a somewhat smaller peak to peak amplitude of 2.4 ± 0.6 mm. Its phase implies that warming the oceans increases the ocean water loss rate. Additional much smaller terms include a steric feedback term and a fast eustatic term. The ENSO contributes a peak to peak variation of 5.5 ± 0.8 mm, predominantly through a direct effect on the MSL and significantly less so indirectly through variations in the radiative forcing.”
Also see the works by Joseph Fletcher on the subject of the sun/ENSO.