>UPDATE October 22, 2009
On October 15, 2009, the NODC corrected errors in the Ocean Heat Content data for the period of April through June 2009. This post has been updated with that corrected data.
The ENSO-induced step changes in the Ocean Heat Content (OHC) of the Tropical Pacific, Tropical Atlantic, South Pacific, South Indian, and South Atlantic datasets were illustrated in ENSO Dominates NODC Ocean Heat Content (0-700 Meters) Data. In this post, we’ll take a look at the natural variables that impact the North Atlantic OHC.
The rise in the North Atlantic OHC anomalies since 1955, as reconstructed by Levitus et al (2009), is strikingly high when compared to Global OHC. As shown in Figure 1, the North Atlantic OHC linear trend is almost three times that of Global OHC.
Discussions of North Atlantic Sea Surface Temperature (SST) normally include the Atlantic Multidecadal Oscillation (AMO). The AMO is a naturally occurring cycle in SST with a frequency that varies from 50 to 80 years. NOAA ESRL represents it as detrended North Atlantic SST anomalies. The AMO is typically illustrated over a period that begins much earlier than 1955 (the start of the OHC dataset), so for reference purposes, Figure 2 shows the NOAA ESRL AMO from 1955 to present. By 1955, the AMO had already dropped from its peak in 1952. It declined until 1975, then rose through present times. It may (or may not) have peaked in 2005.
While the variability of North Atlantic SST has an impact on North Atlantic OHC, detrending North Atlantic OHC anomaly data, Figure 3, shows that it is influenced by other factors, too. The El Nino-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) turn out to be the other major natural influences.
LOW LATITUDE (TROPICAL) NORTH ATLANTIC OHC (0-25N)
Figure 4 compares Low Latitude (0-25N) North Atlantic OHC anomalies to the OHC anomalies of the North Atlantic from 0 to 65N. The tropical OHC anomaly trend is slightly lower than the trend for the basin.
On the other hand, the linear trend of the Tropical North Atlantic OHC anomalies is significantly greater than (almost twice) the linear trend of the Tropical South Atlantic OHC anomalies. Refer to Figure 5.
NOTE: This post discusses North Atlantic OHC, but the Tropical South Atlantic OHC data, when compared to the Tropical North Atlantic OHC data, helps illustrate the El Nino-induced step changes and a curious lag. Refer to Figure 6. There are very clear upward steps in the Tropical South Atlantic OHC data. The first takes place in the late 1970s, and the second starts in 2001. Curiously, these rises lag similar upward steps in the Tropical North Atlantic OHC. I have no explanation for the 1- to 2-year lag in the Tropical South Atlantic data–wouldn’t want to speculate about it.
Figure 7 is a comparison of Tropical North Atlantic OHC anomalies and scaled NINO 3.4 SST anomalies. The upward step in the Tropical North Atlantic OHC anomaly data in the late 1970s coincides with the rise in NINO3.4 SST anomalies from the multiyear La Nina of 1973/74/75/76 to the 1976/77 El Nino. And the substantial rise early in this decade occurs during the transition from the multiyear La Nina of 1998/99/00 to the 2002/03 El Nino. I have searched for but have not found any papers that discuss these upward step responses in OHC (in any basin) to multiyear La Nina events; then again, the Levitus (2009) OHC data is a new dataset.
HIGH LATITUDE NORTH ATLANTIC OHC (45N-65N)
Figure 8 is a comparison of linear trends of North Atlantic and High-Latitude North Atlantic OHC anomalies. The High Latitude OHC anomalies are dominated by the long-term decline in OHC until the rapid rise in mid-1990s.
After the 1970s, there is no apparent correlation between NINO3.4 SST anomalies and High-Latitude North Atlantic OHC anomalies. Refer to Figure 9.
Lozier et al (2008) “The Spatial Pattern and Mechanisms of Heat-Content Change in the North Atlantic” identifies the driver of decadal North Atlantic OHC variability. Link:
Lozier et al (2008) found that “…the large-scale, decadal changes in wind and buoyancy forcing associated with the NAO is primarily responsible for the ocean heat-content changes in the North Atlantic over the past 50 years.” They write in the abstract, “The total heat gained by the North Atlantic Ocean over the past 50 years is equivalent to a basinwide increase in the flux of heat across the ocean surface of 0.4 ± 0.05 watts per square meter. We show, however, that this basin has not warmed uniformly: Although the tropics and subtropics have warmed, the subpolar ocean has cooled. These regional differences require local surface heat flux changes (±4 watts per square meter) much larger than the basinwide average. Model investigations show that these regional differences can be explained by large-scale, decadal variability in wind and buoyancy forcing as measured by the North Atlantic Oscillation index. Whether the overall heat gain is due to anthropogenic warming is difficult to confirm because strong natural variability in this ocean basin is potentially masking such input at the present time.”
Here’s the lead-in to the initial quote above. I wouldn’t want anyone to think I edited out something that contradicts the quote. “A comparison of the zonally integrated heat-content changes as a function of latitude (Fig. 4B) confirms that the NAO difference can largely account for the observed gyre specific heat-content changes over the past 50 years, although there are some notable differences in the latitudinal band from 35° to 45°N. Thus, we suggest that the large-scale, decadal changes in wind and buoyancy forcing associated with the NAO is primarily responsible for the ocean heat-content changes in the North Atlantic over the past 50 years.”
The decadal variations in the NAO (inverted and scaled) do appear to agree with the High-Latitude North Atlantic OHC anomalies, Figure 10, until the aftermath of the 1997/98 El Nino.
MID-LATITUDE NORTH ATLANTIC OHC (25N-45N)
The linear trend of the Mid Latitude North Atlantic OHC anomaly data, Figure 8, is also higher than the linear trend of the North Atlantic basin OHC anomalies.
The Mid-Latitude North Atlantic OHC anomaly data is compared to NINO3.4 SST Anomalies in Figure 12. There are a number of times when the Mid-Latitude North Atlantic OHC anomalies appear to follow or be impacted by some (but not all) ENSO events. Let’s divide the Mid-Latitude North Atlantic OHC anomalies into two more datasets, Northern (35N-45N) and Southern (25N-35N) Mid Latitudes, to see what that reveals.
NORTHERN MID-LATITUDE NORTH ATLANTIC (35N-45N)
Figure 13 is a map of Levitus et al (2005) OHC linear trends cropped from Figure 5.2 of the IPCC’s AR4. The text for Figure 5.2 reads, “Linear trends (1955–2003) of change in ocean heat content per unit surface area (W m–2) for the 0 to 700 m layer, based on the work of Levitus et al. (2005a). The linear trend is computed at each grid point using a least squares fit to the time series at each grid point. The contour interval is 0.25 W m–2. Red shading indicates values equal to or greater than 0.25 W m–2 and blue shading indicates values equal to or less than –0.25 W m–2.”
I do realize there are significant differences between Levitus et al 2005 and 2009 datasets, but as illustrated in Figure 13, I’m only using it to locate a North Atlantic “Hotspot”. It is unfortunate that Levitus et al have elected to provide very limited color coding in their maps of OHC and OHC trend data, but the exceptional number of gradients within the area I’ve blocked off indicate a major rise in OHC. The coordinates I’ve used for that area are 35N-45N, 78W-50W. I’ve identified it as the Northwest Mid-Latitude North Atlantic in Figures 14 through 16.
Link to IPCC AR4 Chapter 5:
The OHC anomalies for the North Atlantic and the Northwest Mid-Latitude North Atlantic are illustrated in Figure 14. The linear trend for the Northwest Mid-Latitude North Atlantic OHC anomalies is more than double the North Atlantic (basin-wide) trend, and more than 7 times the global OHC trend. Refer back to Figure 1 for the global trend.
As noted on the comparison of the Northwest Mid-Latitude North Atlantic OHC anomalies and NINO3.4 SST anomalies, Figure 15, there is no apparent correlation between those two datasets.
This brings us to the comparison of the NAO, as suggested by Lozier et al (2008), and the Northwest Mid-Latitude North Atlantic OHC anomalies, Figure 16. There does appear to be agreement between the decadal variations of the two datasets.
The variability of the Western Mid-Latitude North Atlantic OHC dominates that Northern latitude band of the Mid-Latitude North Atlantic, as shown in Figure 17.
SOUTHERN MID-LATITUDE NORTH ATLANTIC (25N-35N)
All of the latitude bands discussed so far appear to be impacted by a natural variable. The Southern portion (25N-35N) of the Mid-Latitude North Atlantic OHC anomalies, however, does not. The variability of the Southern Mid-Latitude North Atlantic OHC anomalies bears little resemblance to that of the Northern Mid-Latitude North Atlantic OHC anomalies, Figure 18.
The variability of the Southern Mid-Latitude North Atlantic OHC anomalies bears little resemblance to that of the Tropical North Atlantic OHC anomalies, Figure 19.
Significant ENSO events do not induce upward step changes in the Southern Mid-Latitude North Atlantic OHC anomalies, Figure 20. In fact, there are no step changes, just that unusual rise in the 1970s, followed by the decrease from the late 1970s to the early 1980s, and a relatively monotonous rise from the early 1980s to present.
And the decadal variability of the NAO does not appear to agree with the Southern Mid-Latitude North Atlantic OHC anomalies, Figure 21.
Please do not jump to conclusions with my next note. I am simply making an observation.
The rise in the early 1970s and fall in the late 1970s-early 1980s does appear to agree with the bias discovered in the older Levitus et al (2005) dataset. Refer to Figure 22, which is Figure 1 from Levitus et al (2009). Link to Levitus et al (2009) “Global ocean heat content 1955–2008 in light of recently revealed instrumentation problems”:
I have no way to confirm if the “hump” in the Southern Mid-Latitude North Atlantic OHC anomalies is the result of residual biases. There may be an explanation for the disagreement between that latitude band and the others.
The CDC NAO data, the NODC OHC data, and the HADISST data used for NINO3.4 SST anomalies are available through the KNMI Climate Explorer:
UPDATE October 8, 2009:
KNMI corrected a problem in its NODC Ocean Heat Content data on October 1, 2009. The error grew in effect with the distance from the equator. It changed the scale of the variations and the trends but did not drastically change the overall shape of the curves. The problem, therefore, had no real impact on this post, which illustrated the timing of ENSO-induced step changed in the tropical North Atlantic OHC and the effects of the NAO on high-latitude North Atlantic OHC. But to prevent a disagreement between the data presented in this post and future ones, I have redone the graphs in the following and revised the text where appropriate.
This post also updates the data through June 2009.