As noted in the July 2013 Sea Surface Temperature (SST) Anomaly Update, the extratropical North Pacific sea surface temperature anomalies (24N-65N, 100E-100W) made an unexpected surge in July 2013. NOAA may make a fuss about this in their State of the Climate Report this month, so I figured I’d better address it.
Before we begin, I first want to note that the Pacific sea surface temperature anomalies, as a whole, still show little to no warming over the past 20 years. Refer to Figure 1. But the models prepared for the IPCC’s upcoming 5th Assessment Report have simulated that the Pacific sea surface temperatures should have warmed about 0.4 deg C in the last 2 decades.
People are going to try to blame the recent warming event in the extratropical North Pacific on human-induced global warming, but before they do that, they’ll need to explain why the Pacific as a whole hasn’t warmed in 2 decades.
THE EXTRATROPICAL NORTH PACFIC WAS WARM IN JULY 2013
There’s no doubt that the sea surface temperature anomalies for the extratropical North Pacific, based on the Reynolds OI.v2 data, are at record levels. See Figure 2.
Note: The seasonal cycle in the sea surface temperatures there peaks in August, so that’s not the warmest the sea surface temperatures (not anomalies) have been in extratropical North Pacific, but, unless there’s a drop in the anomalies this month, expect the alarmists at NOAA to be chiming in on that a month from now.
Notice, however, how relatively flat the sea surface temperatures had been in the extratropical North Pacific since the early 1990s—prior to the surge. In fact, as shown in Figure 3, the sea surface temperatures of the extratropical North Pacific have warmed at a very slow pace (0.029 deg C/decade) even with the July 2013 reading, while the models indicate it should have warmed at a rate that is almost 7.5 times faster.
Looking at the weekly data since January 2001, Figure 4, the sea surface temperature anomalies for the extratropical North Pacific during the week of July 31, 2013 were also at their warmest—though not too much higher than the value reached in August 2004. And for those interested, the seasonal cycle in the anomalies there also typically peaks in August.
THE CARRY OVER TO THE NORTH PACIFIC AND THE PACIFIC AS A WHOLE
With a spike that large, the sea surface temperature anomalies of the entire North Pacific are also at record levels (Figure 5).
The Pacific as a whole (Figure 6) also showed a surge.
The tropical Pacific is experiencing ENSO-neutral conditions (not an El Niño and not a La Niña), but the recent rise in Pacific sea surface temperature anomalies (Figure 6) looks like the start of an El Niño event. (And for those wondering, sea surface temperature anomalies in the South Pacific cooled very slightly in July 2013.)
A REMINDER OF THE SOUTH ATLANTIC
When I saw the surge in the North Pacific this month, I was reminded of the curious surge in the South Atlantic sea surface temperature anomalies a few years ago. Refer to Figure 7. Other than the dip and rebound in the early 90s and second smaller dip in the late 1990s, sea surface temperature anomalies in the South Atlantic remained relatively flat from the late 1980s to 2008. Then in early 2009 there was a sudden upward shift. Sea surface temperature anomalies remained elevated for a few years in the South Atlantic and then dropped back to previous levels.
I have yet to find a paper that explains that sudden surge in South Atlantic sea surface temperatures, but I also stopped looking for an explanation a few years ago.
SURGE IN NORTH PACIFIC ALSO APPEARS IN NOAA’s ERSST.v3b DATA
Figure 8 compares the sea surface temperature anomalies for the extratropical North Pacific using the NOAA’s Reynolds OI.v2 data and their ERSST.v3b reconstruction. The ERSST.v3b data is used in the NOAA/NCDC combined global land air plus surface temperature anomaly product. While the Reynolds OI.v2 data is more volatile (a function of the satellite data it also uses), the sudden surge in the extratropical Pacific sea surface temperatures also appears in the ERSST.v3b data, which is based only on measurements from ship inlets and buoys (fixed and floating).
In Figure 9, the extratropical North Pacific data from the ERSST.v3b reconstruction is presented since the start of the dataset in 1854. The data is very volatile there, and it’s prone to sudden shifts and spikes. In addition to the recent spike, the one in the late 1960s also stands out.
There are two very pronounced multidecadal cooling periods in the sea surface temperature anomalies of the extratropical North Pacific. Of course, as shown in Figure 10, the climate models prepared for the IPCC’s upcoming 5th Assessment Report cannot simulate those cooling periods. As a result, they fail to properly simulate the warming that took place, when it took place. And they definitely failed to capture the upward shift in the late 1980s, which was caused by a shift in wind patterns. That well-known shift was then followed by a relatively flat temperature period (Refer back to Figure 3), until the recent upward spike.
MONTHLY CHANGES OF THAT MAGNITUDE ARE NOT UNUSUAL
The new spike gives us a record high sea surface temperature anomaly in the extratropical North Pacific, so it’s unusual in that regard. But the magnitude of the monthly change from June to July is not unusual. Figure 11 presents monthly change in the sea surface temperature anomalies for the extratropical North Pacific, where the data represents the value of the month being plotted minus the value of the previous month.
However, when we look at the satellite era, starting the data in 1980 for example, Figure 12, the sudden spike is unusual.
You’ll note that the change from May to June 2013 was even greater than the change from June to July. So for those interested in the 2-month change, refer to Figure 13. A 2-month warming of that magnitude is more unusual, but they have occurred in the past.
I’m sure, if you were to evaluate changes for periods of different lengths–3 months, 4 months, etc.–you will find a period when the recent change appears unusual, but then you’d have to consider the fact that the data have been infilled in earlier years and that we do not know the true extent of the monthly variations in decades past. And there’s something else you need to consider, which we’ll get to in a few moments.
WHAT CAUSED THE SUDDEN SURGE?
It will probably be a year or so before someone publishes a paper about it, but I suspect the sudden upward spike in extratropical North Pacific sea surface temperatures was caused by a shift in wind patterns, related to a change in sea level pressure. In other words, it’s likely weather related.
For example: The North Pacific Index represents the sea level pressure of the central extratropical North Pacific (30N-65N, 160E-140W). Trenberth and Hurrel created the North Pacific Index (aka NPI) for their 1995 paper Decadal atmosphere-ocean variations in the Pacific. So the interrelationships between sea level pressure, wind patterns and sea surface temperatures have been known for decades. Trenberth and Hurrel (1995) used the North Pacific Index to explain the unusual variability in the sea surface temperatures of the North Pacific.
Continuing our example, the North Pacific Index data at the UCAR website lags by a few months, so I used the ICOADS-base sea level pressure data from those coordinates from the KNMI Climate Explorer. Figure 14 presents the July “North Pacific Index” sea level pressures for Julys starting in 1981. As shown, the last time the sea level pressures in the extratropical North Pacific were that high was in 1999, during the La Niña of 1998-01.
I created 3 animations while I prepared this post and downloaded one from the Unisys website. You may need to click start them. Animation 1 is the most recent animation of sea surface temperature anomalies from Unisys.
Animation 2 presents the July sea surface temperature anomaly maps from 2007 to 2013.
Animation 3 includes the weekly sea surface temperature anomaly maps from the week centered on June 5, 2013 through July 31, 2013.
Animations 2 and 3 are based on maps created at the NOAA NOMADS website.
And Animation 4 is of sea level residuals since the first of the year from the JPL website here. The unusual warming of the extratropical North Pacific does not appear to have impacted the sea level residuals there—yet.
It’s unfortunate that the JPL sea level residual data is not available in a user-friendly format.
WHAT DOES THE SOURCE DATASET SHOW?
Always save the best for last.
ICOADS is the source dataset used by NOAA and the UKMO for their sea surface temperature products. It was updated in 2009. See the ICOADS webpage here.
The ICOADS data for the extratropical North Pacific, Figure 15, confirms the spike in July 2013—BUT—it shows sea surface temperature anomalies there were comparable in July 2004.
And that means, based on the new and improved source data, the July 2013 sea surface temperatures in the extratropical North Pacific were the same as they were in July 2004. See Figure 16.
Before you jump to conclusions, I believe the NOAA ERSST.v3b data is based on the older ICOADS source data. And keep in mind that the Reynolds OI.v2 data is based on satellite data and ICOADS in situ data from ships and buoys, also likely the older version. The only sea surface temperature reconstruction based on the new ICOADS data that I’m aware of is HADSST3.
WHAT DOES THE FUTURE BRING?
Is the sudden surge in the sea surface temperatures of the extratropical North Pacific an indication of an upward shift like the one in the late 1980s? Or is it simply a short-term spike like the one in late 1960s (and in 2004 based on the source data)? We’ll just have to watch and see what happens. And we’ll have to see what the UKMO has to say with their HADSST3 and HADISST datasets, but they lag the NOAA data by a month (HADSST3) or two (HADISST).