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Thursday, June 11, 2009

The Reemergence Mechanism

I’ve moved to WordPress.  This post can now be found at The Reemergence Mechanism

I provided a number of quotes from Newman et al (2003) “ENSO-Forced Variability of the Pacific Decadal Oscillation” in my post Misunderstandings about the PDO – REVISED that ran in parallel at WattsUpWithThat. WUWT link:

The first quote received numerous comments at WUWT. I was surprised, however, that portions of the second one garnered little response. The second quote was, “Anomalous tropical convection induced by ENSO influences global atmospheric circulation and hence alters surface fluxes over the North Pacific, forcing SST anomalies that peak a few months after the ENSO maximum in tropical east Pacific SSTs (Trenberth and Hurrell 1994; Alexander et al. 2002). This ‘atmospheric bridge’ explains as much as half of the variance of January–March seasonal mean anomalies of SST in the central North Pacific (Alexander et al. 2002). Furthermore, North Pacific SSTs have a multiyear memory during the cold season. Deep oceanic mixed layer temperature anomalies from one winter become decoupled from the surface during summer and then ‘reemerge’ through entrainment into the mixed layer as it deepens the following winter (Alexander et al. 1999). Thus, over the course of years, at least during winter and spring, the North Pacific integrates the effects of ENSO." [Emphasis added]

I found the thoughts of “multiyear memory”, of “reemergence”, and of global oceans, not just the Pacific, integrating the effects of ENSO to be interesting ones, and they prompted the question, have I already illustrated these effects?


The following quote is from the abstract of Alexander and Dreser (1995) in “A Mechanism for the Recurrence of Wintertime Midlatitude SST Anomaliies” [Journal of Physical Oceanography, Article: pp. 122–137, Volume 25, Issue 1 (January 1995).

“In the early 1970s, Namias and Born speculated that ocean temperature anomalies created over the deep mixed layer in winter could be preserved in the summer thermocline and reappear at the surface in the following fall or winter.” The abstract concludes, “These results suggest that vertical mixing processes allow ocean temperature anomalies created over a deep mixed layer in winter to be preserved below the surface in summer and reappear at the surface in the following fall, confirming the Namias–Born hypothesis.”

Link to Alexander and Dreser paper:

In other words, wintertime SST anomalies in certain portions of the global oceans can and do repeat in subsequent falls and winters. Alexander and Dreser provided a graphic, Figure 1, that illustrates and discusses the process of reemergence.

Figure 1


In the paper “‘Reemergence’ Areas of Winter Sea Surface Temperature Anomalies in the World’s Oceans,” Hanawa and Sugimoto (2004) illustrated the areas of the global oceans, Figure 2 (Hanawa and Sugimoto Figure 1), where they were able to isolate reemergence using multiple datasets. Their abstract reads in part, “Using datasets of sea surface temperature (SST), surface heat flux, upper ocean thermal data, and climatological temperature and salinity profiles, we try to detect ‘reemergence’ areas of winter SST anomalies in the world’s oceans, and describe characteristics of these areas in terms of mixed layer depth (MLD), annual mean heat flux and properties of waters formed in winter mixed layer. Eventually, seven reemergence areas are found: four in the Northern Hemisphere and three in the southern Hemisphere. All areas have a large seasonal variation of MLD, and are the regions where annual mean heat fluxes are relatively small except for two regions in the Northern Hemisphere…”
Figure 2

Unfortunately, I have not found a link to a free version of Hanawa and Sugimoto (2004), but here’s a link to the abstract:

So, in short, reemergence occurs in all oceans, and Hanawa and Sugimoto were able, using the chosen methods, to detect it in the identified areas of the global oceans. Let me reword that. Hanawa and Sugimoto are not saying that reemergence does not occur in the remainder of the global oceans. The areas they identified are dependent on the criterion they established to detect reemergence.

Confirmation: The final paragraph in their Summary and Remarks (Yes, I paid the $9.00) reads “Previous authors [e.g., Alexander and Timlin, 1999] have reported much wider reemergence areas in the North Pacific: latitudinal belt along 40_N (see their Figure 2). Further Coe¨tlogen and Frankignoul [2003] have pointed out that the reemergence could also occur in the remote area due to the advection by the strong current such as the Gulf Stream. In the present study, as mentioned in earlier section, we set rather severe criterions to firmly and robustly detect the reemergence areas. Therefore, the areas detected in the present study are very conservative.”


The IPCC mentions the phenomenon of reemergence in Chapter 3 “Observations: Surface and Atmospheric Climate Change”. Refer to page 289 of AR4, subheading “3.6.3 Pacific Decadal Variability”. They write, “The inter-decadal time scale of tropical Indo-Pacific SST variability is likely due to oceanic processes. Extratropical ocean influences are also likely to play a role as changes in the ocean gyre evolve and heat anomalies are subducted and re-emerge (Deser et al., 1996, 1999, 2003; Gu and Philander,1997)." [Boldface added]

The word reemerge (or re-emerge) does not appear in any form in Chapter 5 “Observations: Oceanic Climate Change and Sea Level” of the IPCC’s AR4, as one would expect since it is an accepted oceanic process. And reemergence makes no other appearance in AR4.


As noted in Newman et al (2003), “Furthermore, North Pacific SSTs have a multiyear memory during the cold season. Deep oceanic mixed layer temperature anomalies from one winter become decoupled from the surface during summer and then ‘reemerge’ through entrainment into the mixed layer as it deepens the following winter. Thus, over the course of years, at least during winter and spring, the North Pacific integrates the effects of ENSO." Since reemergence is the phenomenon that causes the integration, and since reemergence has been found in all oceans except the Arctic and Southern Oceans, it therefore seems logical that at least portions of oceans where reemerging SST anomalies have been found would integrate the effects of ENSO.

Let’s consider that for a moment. An El Nino event releases significant amounts of heat from the eastern tropical Pacific. In response, lower troposphere temperature (TLT) anomalies increase globally. Changes in atmospheric circulation increase SST in all oceans. Also in response, global ocean currents and other oceanic processes redistribute the ENSO heat around the global oceans. Each winter in the years following an El Nino event, the SST anomaly reemerges, extending the lifetime of the El Nino impacts on global SST anomalies. La Nina events typically follow El Nino events. If, big if, all El Nino events were followed by La Nina events of equal and opposite magnitude, the events would tend to counteract one another, regardless of whether we’re discussing the immediate effects or the extended effects due to reemergence.

- BUT -

El Nino events are not followed by La Nina events of equal and opposite magnitude. In fact, there are epochs when the frequency and magnitude of El Nino events greatly outweigh those of La Nina events. During those periods, the global oceans would integrate the effects of a predominance of El Nino events. This should appear as a gradual rise in global SST anomalies. The opposite would hold true for periods when the frequency and magnitude of La Nina events outweigh those of El Nino events. The result of the global oceans integrating the effects of ENSO during a period of when La Nina events dominate should appear as a gradual decrease in global SST anomalies.

By smoothing NINO3.4 SST anomalies with a 121-month filter, Figure 3, the periods when El Nino and La Nina events dominated become apparent. In general, these epochs correspond to periods when global SST anomalies and global land plus sea surface temperatures rose or fell. That is, when the frequency and magnitude of El Nino events dominated, global temperatures rose and when the frequency and magnitude of La Nina events dominated, global temperatures decreased.
Figure 3


A running total is a simple integral, and in previous posts, Reproducing Global Temperature Anomalies With Natural Forcings for example, I’ve used a running-total of HADISST and HADSST2 NINO3.4 SST anomalies to illustrate the cumulative effects of ENSO on global surface temperature.

Note: The calculation of a running total is explained and illustrated well in Figure A of the following link:http://blogs.techrepublic.com.com/howdoi/?p=188

I provided a likely reason for the apparent cumulative effect of ENSO in those earlier posts. The effect appeared to result from the step changes in global surface temperature caused by El Nino and La Nina events. Refer to my posts:
Can El Nino Events Explain All of the Global Warming Since 1976? – Part 1
Can El Nino Events Explain All of the Global Warming Since 1976? – Part 2

While those step changes exist, it seems likely that reemergence plays a major role by helping to maintain the temperatures after the initial ENSO-induced shift.

And to illustrate how well a running total of NINO3.4 SST anomalies simulates the underlying Global SST anomaly curve, Figure 4 is a comparative graph of HADISST Global SST anomalies and a running total of HADISST NINO3.4 SST anomalies, where the NINO3.4 SST anomalies were scaled by a factor of 0.0045. (It doesn’t take much of the ENSO signal to create the effect.) Note that the Base Years for the NINO3.4 SST anomalies are 1950 to 1979. The scaled running total of SST anomalies simulates global SST anomalies equal to or better than most GCMs.
Figure 4


HADISST data is available through the KNMI Climate Explorer website:

1 comment:

David said...

Weird, but on that picture of the regions, they look like shadows of the continents. Like the continents are releasing the heat into the ocean. Maybe, maybe not, but it certainly looks cool.


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