Guest Post by Bob Tisdale
The region east of the Philippines stands out on satellite-based sea level trend maps. See Figure 1, which is from the Map of Sea Level Trends webpage from the University of Colorado Sea Level Research Group. I’ve also shown the coordinates of the region that border it.
THE UNUSUAL SEA LEVEL RISE EAST OF THE PHILIPPINES APPEARS TO BE ENSO RELATED
That small pocket of extraordinary sea level rise looks like the remnants of a slow moving off-equatorial Rossby wave, or a series of them.
During an El Niño, a huge volume of warm water travels from the western equatorial Pacific to the east. See the animation here. That animation shows the evolution of the 1997/98 El Niño and its impacts on sea level residuals, starting with the two initial Kelvin waves carrying warm water east.
The El Niño does not consume all of the warm water that had traveled east. That is, not all of the heat available is released to the atmosphere, because much of the warm water that had traveled east is still below the surface of the eastern equatorial Pacific. So all of that leftover warm water has to go somewhere at the end of the El Niño. It doesn’t just disappear. The leftover warm water is returned to western tropical Pacific, at about 5N-15N latitude, and sometimes at 15S-5S, as part of phenomena called off-equatorial Rossby waves. The Rossby wave north of the equator during the decay of the 1997/98 El Niño can be seen here. It’s a continuation of the sea level residual animation above. (Note how there appears to be a secondary El Niño event taking place in the western tropical North Pacific while the La Niña evolves along the equator. That’s why ENSO indices can’t be used when trying to explain the rise in global temperatures. The ENSO indices can’t account for ENSO residuals.)
So that small pocket of high sea level trends is exactly what we would expect from a group of off-equatorial Rossby waves returning El Niño leftovers back to the western tropical Pacific.
TIME-SERIES GRAPH OF THE SEA LEVEL ANOMALIES FOR THE REGION EAST OF THE PHILIPPINES
For months, I’ve wanted to plot the data for that region, so that I could get a rough idea of its contribution to the global rate of sea level rise. Unfortunately, the Interactive Sea Level Time Series Wizard at the CU website had been taken down for service. It’s operational again.
As the title of this post suggests, the contribution of the region east of the Philippines to the global sea level rise isn’t a lot. It looks bad on the map, but due to its small size (only about 1.4% of the surface of the sea-ice-free global oceans) it doesn’t add much to the global rate of sea level rise, less than 5%.
Figure 2 presents the time-series graph of the sea level anomalies for the region east of the Philippines. I divided the region bordered by the coordinates of 5N-15N, 125E-165E into sixteen 5-deg latitude by 5-deg longitude grids. Using the CU sea level wizard, I downloaded the time-series data for the center of each of those 5×5 grids. I then averaged the data for the 2 separate latitude bands (5N-10N and 10N-15N), and then took a weighted average of the sea level data for those 2 latitude bands to account for the very slight differences in area. (The area weighting actually makes little difference for two latitude bands that close to the equator.)
There is a very strong ENSO component to the sea level data for the region of unusual sea level rise, east of the Philippines. The 1997/98 and 2009/10 El Niños are the causes of the strong dips in sea level anomalies there. The 3-year La Niña that trailed the 1997/98 El Niño is visible, as are the double-dip La Niñas that preceded and followed the 2009/10 El Niño. Note the sharp drop-off this year in response to the (attempted) early evolution of the 2014/15 El Niño.
NOTE: For most of the global oceans, longitudes must be entered into CU sea level wizard as negative numbers. For example, the longitude of 127.5E is entered as -232.5. (Both ways you’re 52.5 degrees west of the dateline.) The sea level wizard rounds it to the nearest single digit.
GLOBAL SEA LEVEL DATA COMPARED TO THE REGION EAST OF THE PHILIPPINES
Figure 3 compares the global sea level data from the University of Colorado (here) to the sea level anomalies for the region east of the Philippines. I didn’t bother to reference them to a common time period. The rate of sea level rise for the region east of the Philippines is about 3 times faster than the global rate. That’s about what we would have expected based on the color coding of the trend map.
HOW MUCH OF AN IMPACT DOES THE RISE IN SEA LEVEL EAST OF THE PHILIPPINES HAVE ON THE GLOBAL TREND?
Using the NOAA Latitude/Longitude Distance Calculator we can get an approximate estimate of the surface area of the region bordered by the coordinates of 5N-15N, 125E-165E. It’s roughly 4.9 million km^2. The surface area of the global oceans is about 391 million km^2, and, of that, sea ice has on average covers about 18.1 million km^2 annually. The region of the unusual sea level rise east of the Philippines, therefore, represents less than 1.5% of the surface of the global oceans not covered in sea ice. We can then area-weight the data for the coordinates of 5N-15N, 125E-165E by that percentage and subtract it from the global data. The remainder is compared to the global sea level data in Figure 4.
The difference in the rates of sea level rise is very small, only about 4.5%.
THE OLD UNIVERSITY OF COLORADO SEA LEVEL TREND ERROR MAP
Yes, I’m aware of the old sea level trend error map that’s available from the WayBack Machine. That map used to be posted on the University of Colorado sea level website. See Figure 5. The archived 2007 source of the map is here. The estimated errors in that region east of the Philippines were comparable to the trend.
That error map is no longer posted by the University of Colorado. Why? You’d have to ask them.
But the volatility and the excessive trend of the sea level in that region should be caused by ENSO residuals. And, bottom line, the sea level rise for that region has little impact on the global rate.
Will the sea levels in that region continue at that pace into the future? Much of it depends on ENSO, and that’s something climate models still can’t simulate. Extending that excessive trend into the future would be foolish, especially when we have no understanding of what ENSO will do in the decades to come.