With regard to longer‐term trends and multidecadal variability, van Loon et al . [ 2012 ] showed for the Atlantic, where the Climate Research Unit's North Atlantic Oscillation (NAO) index goes back nearly 200 years, that a Gleissberg minimum causes the baroclinity to increase with stronger westerlies and an associated tendency toward positive NAO, whereas in the Gleisberg maximum, it slackens with weaker westerlies and a greater tendency for negative NAO. In the following, we shall examine how decadal phenomena, the peaks in the 11 year sunspot cycle and the Pacific Decadal Oscillation, as well as the North Atlantic Oscillation (NAO) and the sunspot peaks, interact with each other. The values of the NAO are from the Climate Research Unit at the University of East Anglia: Gibraltar minus Southwest Iceland. The sunspot data are in the ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SUNSPOT_NUMBERS/INTERNATIONAL/maxmin/MAXMIN, and the time series of PDO are in the NOAA/ERL/PSD websites, as are the sea level pressures and sea surface temperatures.

Two of the major features of decadal time scale climate variability in the Pacific region have been identified, one from external forcing from variations in total solar irradiance associated with the 11 year peaks in the sunspot cycle [ van Loon et al ., 2007 ; van Loon and Meehl , 2008 ; van Loon , 2012 ] and another from internally generated climate variability associated with the Pacific Decadal Oscillation (PDO) [ Mantua et al ., 1997 ]. The PDO is defined based on North Pacific sea level pressure and sea surface temperature (SST) variability and is similar in pattern to the Interdecadal Pacific Oscillation based on Pacific basinwide SST variability [ Power et al ., 1999 ]. In the North Atlantic, there have been connections made for certain time periods under certain conditions between the internally generated North Atlantic Oscillation (NAO) and the solar cycle [e.g., Ineson et al ., 2011 ; Hood et al ., 2013 ; Scaife et al ., 2013 ].

2 The Pacific Ocean Region

The correlation pattern of the PDO with Hadley2 sea level pressure (SLP) is shown in Figure 1a for the southern summer/northern winter months of November‐December‐January when the relationships are strongest. The period used for the correlations is 1949–2012, available from the NOAA/ERL/PSD websites noted above. There are negative correlation values in the North Pacific near the Aleutians for the positive phase of the PDO and a weakened SLP gradient across the equatorial Pacific (positive values to the west, negative values to the east) that gives westerly anomaly winds (weaker upwelling and warmer water) in the equatorial region. Meanwhile for peak years in the 11 year sunspot cycle, Figure 1b shows composite mean sea level pressure anomalies for November to January in the 15 sunspot peaks since the one in 1860 [see van Loon et al., 2007; Meehl et al., 2008; van Loon and Meehl, 2008; van Loon, 2012]. As noted in van Loon et al. [2007], anomalies in the Gulf of Alaska greater than about 2 hPa are significant at the 95% level. The pattern is similar to that in Figure 1a but with opposite sign, with positive SLP anomalies in the Gulf of Alaska and a strengthened equatorial Pacific SLP gradient with small‐amplitude positive anomalies in the east and negative anomalies in the west. Clearly, the data earlier in the record are more sparse and possibly less reliable when it comes to anomaly patterns. However, van Loon et al. [2007] show that composite SLP patterns for peak solar years that include mainly nineteenth and early twentieth century years have nearly the same pattern as composites for later in the twentieth century, suggesting that the SLP reconstructions for early in the record are capturing the essential elements of Pacific region SLP.

Figure 1 Open in figure viewer PowerPoint (a) Correlations between sea level pressure and an index of the Pacific Decadal Oscillation, November–January; (b) composite sea level pressure anomalies (hPa) in 15 sunspot peaks, November–January; (c) sea level pressure anomalies (hPa) when the PDO is in phase with the sunspot peaks, November–January; (d) sea level pressure anomalies (hPa) when the sunspot peaks are out of phase with the PDO, November–January; (e) sea surface temperature anomalies (°C) when the sunspot peaks are in phase with the PDO, November–January; and (f) sea surface temperatures anomalies (°C) when the sunspot peaks are out of phase with the PDO, November–January.

To identify how the two decadal oscillations interact, we divide the sea level pressure anomalies in the sunspot peaks into those out of phase with the PDO (i.e., peak solar years as in Figure 1b and negative PDO years with a pattern opposite to that in Figure 1a; November to January 1907, 1917, 1968, 1989, 2000, and 2012) and those that were in phase with the PDO (November to January 1928, 1937, 1947, and 1979). The mean from which the anomalies are made is 1900–2000. The sample is admittedly small, and the size of the actual values may thus not be representative of a longer series. But the overall distribution of positive and negative anomalies is likely indicative of the sense of the interaction between the two decadal oscillations, and the patterns are physically consistent with current understanding of processes and mechanisms as will be discussed below.

The SLP anomalies in the years when the PDO is in phase with the sunspot peaks are shown in Figure 1c and the anomalies when they are out of phase are in Figure 1d. When they are in phase (Figure 1c), the SLP is above normal with larger‐amplitude values farther north in the Gulf of Alaska closer to the Aleutians compared to the total peak solar pattern in Figure 1b, with negative anomalies in the western Bering Sea and eastern Russia. The SLP anomalies are small and below normal over the Pacific tropics with little anomalous zonal SLP gradient. When they are out of phase in Figure 1d, the positive SLP anomalies in the Gulf of Alaska extend well across the Bering Sea and into eastern Russia. In the eastern tropical Pacific, the anomalies are positive, with negative anomalies to the west, thus producing a strengthened mean zonal SLP gradient across the equatorial Pacific.

The differences between the two phases are also evident in SST. When the sunspot maxima are in phase with the PDO (Figure 1e), there is a mixture of weak positive and negative equatorial eastern Pacific sea surface temperature (SST) anomalies. But when the sunspots are out of phase with the PDO (Figure 1f), there are larger and more westward extensive negative SST anomalies in the eastern equatorial Pacific than in the total composite of peak solar years in van Loon et al. [2007].

To a first order, comparing Figures 1a–1d, the SLP anomalies from the PDO and peak solar composites are roughly linearly additive. This has been noted earlier for interactions between externally forced and internally generated decadal variability associated with the mid‐1970s climate shift [Meehl et al., 2009b]. There appears to be a similar additive process going on for the SST anomalies, with the positive and negative equatorial Pacific SST aniomalies for “in phase” PDO and solar roughly canceling out in Figure 1e. When they act in the same direction for “out of phase” in Figure 1f, there are relatively larger negative SST anomalies than in peak solar years alone.

As noted above, for the larger number of years in the peak solar composite in Figure 1b, it was noted by van Loon et al. [2007] and Meehl et al. [2008] that North Pacific SLP anomalies greater than about 3 hPa are significant at the 95% level. But this significance level drops for the lower number of samples from the PDO/solar composites where SLP interannual standard deviations in the North Pacific are roughly 3 to 5 hPa, about the size of most of the anomalies in that region in Figures 1c and 1d. SST anomalies in the equatorial Pacific greater than about 0.6°C for peak solar years in van Loon et al. [2007] and Meehl et al. [2008] are shown to be significant at the 95% level. The additive nature of the SST anomalies for out‐of‐phase years in Figure 1f is roughly twice those values, so even for the smaller number of samples, these anomalies are still significant at the 95% level.

Examined individually, the four in‐phase years all show similar elements of the SLP anomaly pattern in the composite in Figure 1c, while all but one of the composite out‐of‐phase years in Figure 1d show a similar pattern in the North Pacific.

Nevertheless, there is the possibility that the small sample size here is not representing what is actually taking place in the climate system. However, there is evidence that these responses are physically consistent with several mechanisms that are likely producing these SLP and SST patterns in the Pacific. For the PDO, it has been postulated that internally generated climate variability associated with a kind of slow motion, delayed action oscillator [e.g., White et al., 2003] could produce El Niño–Southern Oscillation (ENSO)‐like variability on decadal time scales in the Pacific basin that depends crucially on tropical‐midlatitude interaction [Meehl and Hu, 2006]. For the response to solar forcing, two mechanisms have been identified that can produce ENSO‐like patterns in the Pacific [Gray et al., 2010]. The first is a top‐down mechanism [Haigh, 1996] whereby for peak solar years, there is greater ultraviolet radiation absorbed by stratospheric ozone that heats the tropical stratosphere causing an increase in meridional temperature gradient. There is a consequent strengthening of the circumpolar vortex as the signal propagates downward into the lower stratosphere and upper troposphere, which affects wave propagation from the troposphere to the stratosphere. This produces stronger subsidence in the subtropics and an increase in upward vertical motion in the deep tropics with an increase in convection in the western tropical Pacific regions of climatological rainfall [Balachandran et al., 1999]. The associated increases of convective heating in the troposphere from the PDO and solar mechanisms drive anomalous atmospheric Rossby waves that contribute to the anomalous SLP in the North Pacific as noted in Figure 1a and shown for Figure 1b by Meehl et al. [2008]. The second is a bottom‐up mechanism whereby greater total solar irradiance in peak solar years drives differential regional heating of the ocean surface across the relatively cloud‐free subtropics compared to the deep tropics. Through a chain of processes, this ends up producing stronger climatological precipitation in the western tropical Pacific, a stronger Walker circulation, greater‐magnitude trade winds that produce greater upwelling in the equatorial eastern Pacific, and cooler SSTs there [Meehl et al., 2008]. Both of these mechanisms combine in the same sense to produce the anomaly patterns shown in Figure 1b and have been reproduced in climate model simulations [Meehl et al., 2008, 2009a].

Another process involved with the top‐down mechanism involves the polar vortex. As it strengthens in peak solar years, the westerlies in the troposphere strengthen, and this contributes to a positive phase of the NAO [e.g., Ineson et al., 2011]. This process would also contribute to stronger westerlies in the North Pacific and positive SLP anomalies there for peak solar years (and negative SLP anomalies for solar minimum years) [Ineson et al., 2011]. There is some evidence of this pattern in Figure 2a for years when the NAO and the peak solar years are in phase, which makes it tempting to say that the North Pacific and North Atlantic responses are coordinated. However, since there is more than one process affecting the North Pacific response as noted above, there are years when the North Pacific and North Atlantic are not consistent (e.g., Figure 2b), and this is discussed further below. In any case, though there is a small sample for the PDO and solar years in Figure 1, the responses are physically consistent with existing processes and mechanisms that have been identified in the climate system.