I don’t like to start a post with notes, but someone is bound to comment on the difference between the linear trend of the data used in this post and the trends of more current depictions of Sea Level. I wanted to clear that up before proceeding.
This post uses sea level data from 1993 to 2003 that is the product of CLS : Collecte Localisation Satellites. It is the sea level dataset presented by the AVISO website:
But the CLS Sea Level data in this post stops in 2003.
The CLS presentation is similar to the graph from the University of Colorado site:
The CLS data used in this post is available through the KNMI Climate Explorer. If and when KNMI updates the CLS sea level data or adds another sea level dataset that runs to present times, I will revise this post. Also, note that the linear trend of the Global Sea Level dataset presented in this post is less than the linear trends of the current versions available from the University of Colorado and AVISO. Specifically, the linear trend of the CLS data from 1993 to 2003 is 2.6mm/year, but from 1993 to 2009, the Sea Level linear trend presented by the University of Colorado in their 2009 Release 3 data is 3.1 to 3.2 mm/year (depending on the application of inverse barometer). However, Figure 1 illustrates an earlier version of the University of Colorado data, their 2004 Release 3. The trend presented in it is 2.8mm/year. The data in this post better reflects the trend of the earlier University of Colorado data.
One last note: Keep in mind that the rise in sea level is the product of thermal expansion and of the increase in mass from glacial runoff and the like.
And with that out of the way, here’s the body of the post.
IS THE RISE IN SEA LEVEL REALLY AS MONOTONOUS AS SHOWN?
It’s hard to imagine that the relatively steady rise in Sea Level has the El Nino-Southern Oscillation (ENSO) as one of its major components, but the Sea Level anomaly data available through the KNMI Climate Explorer shows that fact quite well.
Figure 2 shows a curve of Global Sea Surface Height anomalies from January 1993 to December 2003. The graph is typical of many depictions of Sea Level. The period includes a few years before and after the El Nino of 1997/98. Note what appears to be a slight rise and fall caused by that El Nino on top of a constant rise in Sea Level. Appearances are deceiving. That monotonous rise in sea level is not so monotonous. There are major variations taking place that are the results of ENSO events.
The Global Sea Level anomaly data in Figure 2 appears in many of the graphs in this post. It is used to highlight the greater variability of other datasets.
The AVISO website linked in the opening notes also provides a map of Sea Level trends from 1993 to present. Refer to Figure 3. The greatest rise in Sea Level occurs over the Western tropical Pacific, but over the Eastern Pacific, Sea Levels have declined. Also note the pattern in the Pacific: Cool colors in the eastern Pacific, up along the West Coast of North America, wrapping around an area of warm colors in the central North Pacific. This pattern is similar to the depictions of the Pacific Decadal Oscillation (PDO).
Since the PDO is an aftereffect of ENSO, the pattern implies an ENSO relationship. To confirm this, I segmented the Sea Level data, Figure 4, to capture the areas with the greatest rises. The color-coding of the areas on the map agrees with the colors used in the graphs of the respective sea level anomaly data. I also selected an area in the eastern equatorial Pacific, the area used for the Cold Tongue Index (6S-6N, 180-90W), to serve as a reference of the timing of the ENSO events. And as you’ll see, the Pacific Warm Pool data can also be used in the same capacity since it appears to mirror the Cold Tongue Index data.
SEA LEVEL VARIATIONS OF THE AREAS USED AS ENSO REFERENCES
Figure 5 is a comparative graph of Cold Tongue Index (CTI) and global Sea Level anomalies. The variations in Cold Tongue Index sea level dwarf those of the global dataset. The 1994/95, 1997/98, and 2002/03 El Nino events stand out, as does the El Nino conditions that occurred in 1993. Again, the CTI data is being provided for timing purposes.
Like the Cold Tongue Index data, the ENSO-induced variations in Sea Level anomalies for the Pacific Warm Pool, Figure 6, are significantly larger than the global data. Note that the Pacific Warm Pool Sea Level anomaly trend (0.63 cm/year) is more than double the global trend (0.26 cm/year).
The Pacific Warm Pool Sea Level anomalies appear to mirror the Cold Tongue Index data. Refer to Figure 7. I’ll use the Pacific Warm Pool Sea Level Anomalies as an ENSO reference graph also.
Keep in mind that a significant El Nino event, such as the 1997/98 El Nino, not only releases heat into the atmosphere, it causes warm waters to be transported via ocean currents from the tropics to the mid-latitudes of the Pacific. Atmospheric circulation patterns, rainfall, cloud cover, surface winds, etc., also change during and after El Nino events. The sea level variations illustrated in the following are products of the changes in atmospheric circulation, or the transport of warm water away from the tropical Pacific, or both.
THE SEA LEVEL VARIATIONS FOR THE AREAS SHOWN WITH ELEVATED TRENDS
The step change in the Central North Pacific Sea Level data is obvious in Figure 8. Note that the linear trend of this dataset is 0.585 cm/year, more than twice that of the Global trend (0.26 cm/year).
The sea level data provides an excellent way to illustrate the lag between the North Pacific and ENSO. Figure 9 compares the sea level anomaly data for the Central North Pacific with scaled Pacific Warm Pool data. Note how the rise in the Central North Pacific data (an area that is prevalent in illustrations of the PDO) lags the Pacific Warm Pool data by approximately 6 months.
The Sea Level anomalies for the Southwest Pacific dataset are shown in Figure 10. Its linear trend (0.51 cm/year) is almost twice that of Global Sea Level, 0.265 cm/year.
Figure 11 shows two step changes in the Sea Levels of the Southwest Pacific, with the first the result of the 1994/95 El Nino and the second caused by the 1997/98 El Nino. Note, however, that the Southwest Pacific does not respond to the 2002/03 El Nino with an upward step.
The Sea Level anomalies for the portion of the Antarctic Circumpolar Current (ACC) that stretches from the east coast of South America to the south-central Indian Ocean are illustrated in Figure 12. The linear trend (0.265 cm/year) is slightly higher than the Global trend of 0.26 cm/year. It must have risen more in recent years, because that trend is not exceptional. Regardless, note the difference in the magnitude of the annual variations before and after 1997/98. The amplitude of the month-to-month variations are much greater after the 1997/98 El Nino than they were before it.
Smoothing the Atlantic-Indian Ocean ACC Sea Level data with a 13-month running-average filter, Figure 13, reveals the upward step change that occurred as a result of the 1997/98 El Nino.
THE UNUSUAL DATASET
The data for the portion of the Antarctic Circumpolar Current (ACC) from the southeast portion of the South Indian Ocean to the southwest portion of the Pacific Ocean are illustrated in Figure 14. The data hugs the Antarctic Coast; therefore, much of the area is covered with sea ice for part of the year. It’s the noisiest dataset illustrated in this post. Its linear trend is slightly higher than the global trend. So what’s so unusual about it?
There are step changes in the sea level anomalies for that section of the ACC, Figure 15, but they are out of synch with ENSO events. I’m sure I could track down some explanation, but for now we’ll leave it as a curiosity, since it really doesn’t have a major impact the global trend.
WHAT ABOUT THE REST OF THE OCEANS?
To illustrate the impact of ENSO on larger areas of the global oceans, I’ve divided the global sea level data into two segments. Refer to Figure 16 for the coordinates.
Figure 17 is a comparison of those datasets with global sea level anomalies. The curve of the East Pacific and Atlantic Ocean data resembles the Cold Tongue Index curve, and the curve of the Indian Ocean through the Western and Central Pacific resembles the Pacific Warm Pool curve. How close are the resemblances?
A note regarding the next two graphs. I do realize I’ve scaled the Cold Tongue Index and Pacific Warm Pool data so they capture most on the rises in the sea level of the other datasets during the 1997/98 El Nino. I am not implying that the contribution to the mass of the oceans caused by glacier runoff and the like stopped during that period. The scaling was provided to illustrate the correlation and the potential impacts of significant El Nino events on sea level.
Figure 18 compares Scaled Sea Level Anomalies for the Cold Tongue Index and the Sea Level Anomalies of the East Pacific and Atlantic Oceans (plus the portions of the Arctic and Southern Oceans captured by the coordinates of 90S-90N, 150W-30E). Note how the East Pacific and Atlantic Ocean data correlates well with the Cold Tongue Index data before the 1997/98 El Nino but diverges from the Cold Tongue Index data after that El Nino. For this area of the global oceans, does this imply that that a significant portion of the thermal expansion-caused rise in sea level from 1993 to late 1997 was caused primarily by the 1997/98 El Nino itself? And that the additional rise in sea level from 1997 to 2003 (beyond the rise caused by the lingering effects of the changes in atmospheric circulation and the transport of warm water from the tropical Pacific to the mid latitudes of the Pacific) was an aftereffect of the El Nino caused by the release of heat into the troposphere? Refer to my post about “RSS MSU TLT Time-Latitude Plots…” for illustrations of the step changes in TLT anomalies resulting from significant El Nino events. The El Nino-induced rise in TLT anomalies should add to the sea level rise caused by the mass contribution from glacier melt, etc.
And in Figure 19, I’ve compared Scaled Sea Level Anomalies for the Pacific Warm Pool and the Sea Level Anomalies of the Indian Ocean and the West and Central portions of the Pacific (plus the portions of the Arctic and Southern Oceans captured by the coordinates of 90S-90N, 30E-150W). And again, the greatest divergence between the datasets occurs after the 1997/98 El Nino. The same questions would apply to this area of the global oceans. That is, for this area of the global oceans, do the correlation before 1997 and the divergence after imply that a significant portion of the thermosteric rise in sea level from 1993 to late 1997 was primarily caused by the 1997/98 El Nino? And that the additional rise in sea level from 1997 to 2003 (beyond the rise caused by the lingering effects of the changes in atmospheric circulation and the transport of warm water from the tropical Pacific to the mid latitudes of the Pacific) was an aftereffect of the El Nino caused by the release of heat into the troposphere? And I’ll add the qualifier again. That is, the El Nino-induced rise in TLT anomalies would add to the sea level rise caused by the mass contribution from glacier melt, etc, already taking place.
I originally titled this post “ENSO Is The Primary Driver of Sea Level Anomalies”, but then toyed with a few others before deciding on the present title. Based on the comparisons in this post it appears that ENSO is in fact the primary driver of yearly and multiyear variations in Sea Level anomalies. Significant ENSO events also appear to have major impacts on decadal trends. It’s unfortunate that the dataset doesn’t begin in the early 1980s, because it would be interesting to see the impacts of the significant 1986/87/88 and 1997/98 El Nino events in sequence. And again, if and when KNMI updates the sea level data on their Climate Explorer website, I will update this post. It would also be interesting to try to determine why the rise in sea level flattened from 2006 to 2007 then rebounded in 2008.
Thanks to Carl Wolk of the website “Climate Change by Erl Happ and Carl Wolk” for his post “Sea Level Data Exposes El Nino’s Secret.” It gave me the idea for this post, which exposed a few secrets of the monotonous rise in the sea level data.
The CLS Sea Level data used in this post is available through the KNMI Climate Explorer website: