Effect of solar activity

Uda (1962) noticed that most of the years of cold (warm) oceanic temperatures in the area east of Japan were correlated with the periods of minimum (maximum) sunspot activity. Rule 65 confirms this relationship for the period 1961-2006. In 2007, however, the temperature sharply increased and remained above normal through 2008, despite the low level of solar activity. Solar cycle 24 is expected to peak in late 2011 or mid-2012, and based on rule 65, the temperature east of Japan should follow the cycle.
Pokudov (1978) examined a relationship between solar activity and meandering of the Kuroshio Current.  He found that when the solar activity is high, the Kuroshio axis tends to be farther south, with the higher probability to form a meander.

Regime shifts

Fig. 1. Regime shifts in (a) winter (DJF), (b) summer (JJA) and (c) annual Pacific Decadal Oscillation indices, as obtained using the regime shift detector with the following parameters: Target significance level p* = 0.1, regime length l = 20 years, and Huber parameter h = 1.

Climate regime shifts in the Pacific-North American region during the 20th century are described in detail in Rodionov (2002).  A major regime shift in the Pacific climate, from a cold to warm PDO phase, occurred in the late 1970s. There have been a lot of discussions lately that the PDO is switching back to the cold phase again (e.g, click here and here), after scientists from NASA's Jet Propulsion Laboratory announced that it was more than just a La Niña event that occurred in 2008 (see here). A similar announcement was made during the peak of a prolonged La Niña event that started in the fall of 1998 and lasted for nearly two years (see here). Soon after, however, the PDO returned to the positive territory and its winter 2003 value was the highest since 1941 (Fig. 1a). The regime shift detector applied to the winter PDO time series showed no indication of a regime shift in the late 1990s. It may be that the negative PDO value in 2008 is again just a result of the concurrent La Niña event and not a regime shift.

Fig. 2. Changes in mean winter (DJF) SST from 1978-1998 to 1999-2008.

The situation, however, is more complicated that it seems. Some argue that there was a regime shift in the late 1990s, but not of the kind that is reflected in the PDO (Bond et al., 2003). As noted by Minobe (2002), a difference between the climate shifts in the 1990s and 1970s was that the 1999 shift involved prominent warming of SSTs in the western North Pacific, whereas the cooling in this region in the 1970s was quite small.

Unlike the winter PDO index, the summer and annual PDO indices exhibit a statistically significant (p < 0.03) shift in 1999 (Fig. 1 b,c). But even in winter, the difference in SSTs between 1999-2008 and 1978-1998 closely resembles the negative PDO pattern (Fig. 2). Furthermore, the regional manifestations of the regime shift in the late 1990s are quite convincing. For example, the magnitude of the 1999 regime shift in the central North Pacific was greater that that of the 1977 shift (Fig. 3a). A highly statistically significant (p < 0.004) shift in 1999 is also evident in the waters off Baja California Peninsula (Fig. 3b).

Fig. 3. As in Fig.1, except for winter (DJF) SST in (a) central North Pacific (20-35N, 170E-160W), and (b) off Baja California Peninsula (20-25N, 125-110W).

All this indicates that 1999 marks the beginning of a new climate regime similar in many respects to a negative PDO phase (but not necessarily its classical spatial pattern). The 1999 regime shift was also found in biological records (Schwing and Moore, 2000; Batten and Welch, 2004). It is important to underscore that during a series of weak-to-moderate El Niño events in 2002/03, 2004/05, and 2006/07, SST anomalies in the central North Pacific remained positive (Fig. 3a), as during the negative PDO. The fact that the North Pacific responded stronger to La Niña than El Niño events suggests that it was a true climate regime shift in 1999.

Relationship with the AMO

Fig. 4. Cross-correlation functions between the PDO and AMO indices for unsmoothed values, 11-yr running averages, and residuals after removing the running averages.

The time series of SST averaged over the North Atlantic and linearly detrended to remove warming signal exhibits long-term changes, with cold and warm phases lasting 20-40 years. The alteration between these phases is known as the Atlantic Multidecadal Oscillation (AMO). For the annual unfiltered values, the correlation between the PDO and AMO indices reaches r = -0.53 (significant at p < 0.05) when the AMO is leading the PDO by 18 years (Fig. 4). If both indices are smoothed by 11-yr running averages, the correlation coefficient reaches r = -0.86, when the AMO is leading by 16 years (Fig. 4). The cross-correlation function in Fig. 4 does not necessarily indicate a causal relationship between the PDO and AMO; it just emphasizes that both indices have strong 70-yr components in their fluctuations, which are roughly in quadrature with respect to each other. Zhang and Delworth (2007) proposed a mechanism for the influence of the AMO on the North Pacific multidecadal variability. Their modeling results suggest that the AMO can contribute to the component of the PDO that is linearly independent of ENSO. Dima and Lohmann (2007) proposed a different conceptual model, in which the multidecadal signal is transferred from the Atlantic to the Pacific through the Tropics.

Fig. 5. Annual AMO and PDO indices smoothed by 11-yr running averages, with the AMO leading PDO by 16 years. Years on the abscissa are those of the PDO.

Whatever the mechanism may be (or lack thereof), the lagged AMO-PDO statistical relationship can be used to estimate further progression of the PDO. As shown in Fig. 5, the PDO will probably be decreasing until about 2017, although this may be just a manifestation of the 70-yr cycle in the PDO itself. Numerical experiments suggest that the 70-yr cycle is not generated by external forcing and is likely to be an internal cycle mediated by ocean-atmosphere interactions (Andronova and Schlesinger, 2000).

The 70-yr cycle is not the only cycle common for the North Pacific and North Atlantic. When the running 11-yr averages are removed from the PDO and AMO series, the cross-correlation function between the residuals reveals a distinct quasi-decadal cycle (Fig. 4). The magnitude of the correlation between the residuals reaches -0.39 (p < 0.05, data 1905-2002), when the AMO is leading the PDO by 11 years.

Fig. 6. The residuals in the annual PDO and AMO indices after removing the 11-yr running averages and smoothing by 3-yr running averages.

When the residuals are smoothed by running 3-yr averages, the correlation between the two time series at the 11-yr lead time jumps to -0.75 for the period 1952-2001. Figure 6 shows the time series of the residuals (with the AMO leading by 11 years), along with the PDO forecast values based on the regression between the AMO. As one can see from this figure, the decadal component of the PDO index will likely to decline by 2009 and then increase by 2012. This is generally consistent with rule 351, which suggests that the North Pacific index is expected to increase until 2010. Also shown in Fig. 6 are annual (unfiltered) values of the PDO for 2002-2007 and forecast PDO values based on the regression with the AMO.

Relationship with North American climate

Fig. 7. Winter (DJF) SAT in Detroit and PDO indices smoothed by 11-yr running averages, with the SAT leading PDO by 22 years. Years on the abscissa are those of the PDO.

An oscillation with the period of 65-70 years can also be found in North American climate (Schlesinger and Ramankutty, 1994). A good example of the presence of this cycle in surface temperature is a time series of winter SAT in Detroit. The correlation between this time series and the winter PDO index (both smoothed by 11-yr running averages) reaches -0.8 when the SAT is leading by 22 years. As shown in Fig. 7, not only the phases of the 70-yr cycle in the SAT, but even smaller-scale variations, such as cooling in the 1960s, coincide with similar variations in the PDO. A strong warming trend in the Midwest in the past 30 years suggests that the PDO will continue to decline through 2025, possibly reaching record low values. The mechanism of this relationship, however, is unknown. 

Conclusion

A major climate regime shift occurred in the North Pacific in 1999, in conjunction with the contemporaneous shift in the equatorial Pacific. The regime shift of 1999 was not as persuasive as the 1977 shift, and the climate has not simply returned to its pre-1977 state. The SST anomaly pattern established since 1999 was not close enough to the classical negative PDO phase (particularly in the western North Pacific), so that the shift could not be detected in the winter PDO index. However, the key elements of the negative PDO pattern, such as positive SST anomalies in the central North Pacific and negative anomalies along the west North American coast, are present in the new pattern. The PDO index is expected to decline until about 2017, with some slowdown in 2009 (due to a possible El Niño event) and in 2010-2012 (due to quasi-decadal variability).

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