There’s been speculation that Anthropogenic Global Warming had an impact on the Sea Surface Temperatures that fueled Hurricane Irene. Let’s put that to rest.
I downloaded the Sea Surface Temperature anomalies for the 5-degree (longitude and latitude) grids, Figure 1, that captured the track of Hurricane Irene, and created a weighted average of the Sea Surface Temperature anomalies for those grids. The weighting accounted for latitude and percentage of ocean surface per grid, since portions of some of the grids were covered by land mass.
Figure 1
The Unisys Hurricane Irene Tracking map is available here:
http://weather.unisys.com/hurricane/atlantic/2011/IRENE/track.gif
SATELLITE-ERA SEA SURFACE TEMPERATURE ANOMALIES
Figure 2 is a time-series graph of the weekly satellite-based Reynolds OI.v2 Sea Surface Temperature anomalies for the track of Hurricane Irene. (The more recent weekly data for that dataset starts the week centered on January 3, 1990.) Also highlighted in red is the SST anomaly (+0.714 deg C) for the week centered on August 24, 2011. The most recent Sea Surface Temperature anomalies were elevated but nowhere near record levels. The linear trend of +0.045 deg C per decade for the hurricane track is also not exceptional. It’s about 56% of the linear trend for Global Sea Surface Temperature anomalies (+0.08 deg C per decade) for the same period.
Figure 2
Figure 3 provides a longer-term graph of the Sea Surface Temperature anomalies for the track of Hurricane Irene, using monthly Reynolds OI.v2 SST data. Again, Sea Surface Temperature anomalies were elevated but not unusually high in August 2011. Note also how the trend for the data is relatively flat at +0.016 deg C per decade. It is about 18% of the trend for global SST anomalies, which are approximately +0.09 deg C per decade.
Figure 3
A LOOK AT A LONGER-TERM DATASET IS REVEALING
The Hadley Centre’s HADISST Sea Surface Temperature dataset starts in 1870. Unfortunately, there is a lag in its update, so the HADISST data currently ends in June 2011. Luckily, another two months of data would not change the following results appreciably.
Figure 4 illustrates the Sea Surface Temperature anomalies and linear trend for the track of Hurricane Irene from January 1950 to June 2011. The trend is negative, meaning the Sea Surface Temperatures for the track of Hurricane Irene have cooled over that period. Since it’s a noisy dataset, I’ve also included the curve of the data smoothed with a 13-month running-average filter.
Figure 4
Figure 5 illustrates the Sea Surface Temperature anomalies for the Hurricane Irene track since the start of the dataset in 1870. Note how the Sea Surface Temperatures rose during the early 20thCentury warming period, but then cooled since 1950.
Figure 5
This becomes clearer if we smoothed the data with a 121-month running-average filter, Figure 6.
Figure 6
THE SST ANOMALIES OF A PORTION OF IRENE’S TRACK HAVE COOLED SIGNIFICANTLY SINCE 1950
Figure 7 presents a map of the change in the Sea Surface Temperature anomalies of the Western North Atlantic from 1950 to 2010 based on local linear trends. The map was cropped from a global map created at the GISS Surface Temperature Analysis Global Map Making Webpage.I’ve highlighted the portion of Irene’s track used in this post where Sea Surface Temperature anomalies have cooled since 1950.
Figure 7
The Sea Surface Temperature anomalies from 1870 for that portion of Irene’s track (25N-35N, 80W-75W) are shown in Figure 8. The green curve shows the data smoothed with a 13-month filter. Also illustrated in red are the Sea Surface temperature anomalies since January 1950, along with the linear trend (- 0.06 deg C per decade). Based on the linear trend, Sea Surface Temperature anomalies for that portion of Irene’s track dropped approximately 0.37 deg C since 1950.
Figure 8
THE NORTHERN AND SOUTHERN PORTIONS OF IRENE’S TRACK
I had originally planned to end the post at that point, but someone was bound to ask for time-series graphs of the Northern and Southern portions of the track of Hurricane Irene. Figure 9 illustrates the grids included for those subsets.
Figure 9
As illustrated in Figure 10, the Sea Surface Temperature anomalies of the Northern Portion peaked in 1949, similar to the overall dataset, but dropped significantly until the late 1960s, when they rose again. On the other hand, the Sea Surface Temperature anomalies of the Southern Portion did not peak until 1960 and made a minor dip and rebound since then. In both cases, Sea Surface Temperature anomalies were warmer during the 1950s and 1960s than they are today.
Figure 10
A NOTE ABOUT THE RISE IN GLOBAL SEA SURFACE TEMPERATURES
Earlier I noted that Global Sea Surface Temperature anomalies have a linear trend of 0.09 deg C per decade during the satellite era; that is, since November 1981. Some readers new to discussions of Sea Surface Temperature during that period are bound to assume the rise in Sea Surface Temperature has an anthropogenic cause. I have illustrated and discussed in numerous posts over the past few years how and why most if not all of the rise in Sea Surface Temperature anomalies result from the a combination of two natural processes: the El Niño-Southern Oscillation or ENSO and the Atlantic Multidecadal Oscillation or AMO. My most recent posts that discussed this at an introductory level were ENSO Indices Do Not Represent The Process Of ENSO Or Its Impact On Global Temperature and Supplement To “ENSO Indices Do Not Represent The Process Of ENSO Or Its Impact On Global Temperature”.
SOURCES
The Reynolds OI.v2 SST anomaly data is available through the NOAA NOMADS webpage:
http://nomad3.ncep.noaa.gov/cgi-bin/pdisp_sst.sh?lite=
The HADISST SST anomaly data is available through the KNMI Climate Explorer:
http://climexp.knmi.nl/selectfield_obs.cgi?someone@somewhere
Bob Tisdale - Climate Observations 
