What Causes Sea Surface Temperature (SST) To Rise?

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THE SOURCE OF ENERGY NECESSARY TO RAISE SSTs

I was recently asked by a blogger at another website, “What is the source of the energy necessary to raise SSTs?”

The ultimate source of energy necessary to raise SSTs would be an increase in solar irradiance, regardless of whether the increase in solar irradiance resulted from variations in the solar cycle, or from changes in cloud cover, or from a reduction in stratospheric volcanic aerosols. The impact of shortwave radiation (visible light) on SST depends on factors such as the turbidity of the water and sea surface albedo, which in turn depends on other variables including wind speed and chlorophyll concentration. Additionally, an increase in downward longwave (infrared) radiation would warm the top few centimeters of the oceans, and through mixing caused by waves and wind stress turbulence, would warm the mixed layer of the ocean. This in turn would affect the temperature gradient between the mixed layer and skin, dampening the outward flow of heat from the ocean to the atmosphere.

Since Total Solar Irradiance does not vary significantly, it is represented that the increase in anthropogenic greenhouse gases is, therefore, responsible for the rise in SST since the mid-1970s.

HOWEVER

There are other natural processes that can raise and lower SSTs, including Thermohaline Circulation/Meridional Overturning Circulation, upwelling, and…

ENSO EVENTS

My posts at WattsUpWithThat and here at my blog…
Can El Nino Events Explain All of the Global Warming Since 1976? – Part 1
Can El Nino Events Explain All of the Global Warming Since 1976? – Part 2
…that were supplemented by…
Supplement To “Can El Nino Events Explain All Of The Warming Since 1976?”
Supplement 2 To “Can El Nino Events Explain All Of The Warming Since 1976?”
… illustrated the step changes (increases) in the East Indian and West Pacific Ocean SST anomalies (60S-65N, 80W-180, or approximately 25 to 30% of the global ocean between 60S and 65N) that resulted from the 1986/87/88 and 1997/98 El Nino events. The step increases in the East Indian-West Pacific SST anomalies (black curve) in response to the 1986/87/88 and 1997/98 El Nino events (purple curve) are very obvious in Figure 1.

http://s5.tinypic.com/16h0tuq.jpg
Figure 1

The El-Nino Induced step changes did not occur after the 1982/83 and 1991/92 El Nino events because the El Nino heat distribution processes were suppressed by the El Chichon and Mount Pinatubo eruptions. This is why Sato Index data (green curve) appears in Figure 1 and in many of the following graphs. It’s there as a reminder of the timing of the volcanic eruptions.

WHAT CAUSES THE EAST INDIAN AND WEST PACIFIC OCEAN SST ANOMALIES TO RISE IN STEPS?

During an El Nino event, water that was below the surface of the Pacific Warm Pool is transported east. Much of it is “deposited” on the surface of the Eastern Equatorial Pacific. Then, during the subsequent La Nina, surface currents and trade winds drive the warm water west to the surface of the East Indian and West Pacific Oceans. In other words, before the El Nino, the warm water was below the surface and not included in SST measurement; then after the El Nino, the warm water was on the surface and included in the SST measurement. It’s that simple. This was illustrated and discussed in detail in the posts “Can El Nino Events Explain All of the Global Warming Since 1976?” linked above.

DO THE STEP CHANGES APPEAR IN ANY OTHER OCEAN SST ANOMALY DATASETS?

The step changes DO appear in the “global” SST anomaly dataset between 60S and 65N as shown in Figure 2:
http://i41.tinypic.com/71mbd3.jpg
Figure 2

The steps DO NOT appear in Northeast Pacific SST anomaly data, Figure 3. The Northeast Pacific SST anomalies (brown curve) vary in synch with NINO3.4 SST anomalies (purple curve):
http://i39.tinypic.com/2z5kkmh.jpg
Figure 3

They DO NOT appear in the combined Southeast Pacific and South Atlantic SST anomaly data (gray curve), Figure 4. The Southeast Pacific and South Atlantic SST anomaly data also “track” NINO3.4 SST anomalies (purple curve):
http://i39.tinypic.com/2ivehp3.jpg
Figure 4

They DO NOT appear in the Western Indian Ocean SST (bronze curve) anomaly data, though there are some lags in the responses to the La Ninas, Figure 5:
http://i40.tinypic.com/rwmers.jpg
Figure 5

However, they DO appear in the East Indian-West Pacific SST anomaly data as noted above and illustrated in Figure 1, and they DO appear in the North Atlantic SST anomaly data (red curve), Figure 6:
http://i43.tinypic.com/2ezsk92.jpg
Figure 6

In fact, the step changes in the East Indian-West Pacific SST anomaly data (green curve) are very similar in scale to those in the North Atlantic (red curve), Figure 7, even though the North Atlantic is influenced by AMOC:
http://i39.tinypic.com/15cocop.jpg
Figure 7

There are divergences between the two datasets in later years, indicating that there is an additional factor that influences North Atlantic SST anomalies--Atlantic Meridional Overturning Circulation. The divergence could also imply that the 2002/03, 2004/05, and 2006/07 El Nino events had a greater influence on the North Atlantic than they did on the East Indian and West Pacific Oceans.

The North Atlantic step changes were discussed in my post:
There Are Also El Nino-Induced Step Changes In The North Atlantic.

WHERE DO EL NINO EVENTS GET THEIR ENERGY?

The next question would logically be, Where do El Nino events get the energy necessary to raise SSTs in steps in the East Indian and West Pacific Oceans and in the North Atlantic?

As mentioned above, the source of warm water for El Nino events is the Pacific Warm Pool. William S. Kessler of the NOAA Pacific Marine Environmental Laboratory, in his webpage titled “Frequently-(well, at least once)-asked-questions about El Niño”, provides a basic description of El Nino events. Refer to:

http://faculty.washington.edu/kessler/occasionally-asked-questions.html#q1
In it, he writes, “During El Niño events, this entire system relaxes. The trade winds weaken, particularly west of the Dateline, and the piled-up water in the west sloshes back east, carrying the warm pool with it.”

AND WHERE DOES THE PACIFIC WARM POOL GET ITS SUPPLY OF WARM WATER?

The Pacific Warm Pool receives part of its supply of warm water from prior El Nino events. The warm water that was “sloshed” eastward during the el Nino is returned to the West Pacific by the Pacific Equatorial Currents. This was discussed and illustrated with a video in Recharging The Pacific Warm Pool.

The results include a step change in the Western Equatorial Pacific Warm Water Volume. It is quite visible after the 1997/98 El Nino, the blue curve in Figure 8.

http://i37.tinypic.com/28sxc8m.jpg
Figure 8

Step changes after the 1986/87/88 and 1997/98 El Nino events are also visible in The Indo-Pacific Warm Pool Index (brown curve) shown in Figure 9.
http://i41.tinypic.com/biukd5.jpg
Figure 9

And part of the recharging of the Pacific Warm Pool is simply the return to “normal” equatorial Pacific conditions following an El Nino event. This can be seen in the video included in “Cross-Sectional Views of Three Significant El Nino Events – Part 1.”

ADDITIONALLY, SHARP DECREASES IN CLOUD COVER OVER THE PACIFIC WARM POOL DURING EL NINO EVENTS ALLOW THE SUN TO WARM SSTs THERE

In “ENSO Surface Shortwave Radiation Forcing over the Tropical Pacific” (2008), Pavlakis et al illustrated the correlation between NINO3.4 SST anomalies and Downward Shortwave Radiation (visible light) anomaly (DSR-A) at the surface for the Western Pacific (10S–5N, 120–140E).
http://www.atmos-chem-phys-discuss.net/8/6697/2008/acpd-8-6697-2008-print.pdf

They further explain that the changes in Downward Shortwave Radiation are caused mostly by changes in Cloud Amount. As illustrated in Figure 10, during the 1997/98 El Nino, Downward Shortwave Radiation (black curve) rose almost 25 Watts/meter^2 as a result. During the El Nino events with lower NINO3.4 SST anomalies, the increases in Downward Shortwave Radiation were proportionately lower.
http://i41.tinypic.com/2435kbb.jpg
Figure 10

These ENSO-caused changes in cloud amount were further discussed in Recharging The Pacific Warm Pool Part 2.

UPDATE November 20, 2009: I’m in the process of writing yet another post on the multiyear aftereffects of major El Nino events, and I felt a discussion in it would add much to this post, so I’ve included it here. I've also changed the Figure numbers so that the run in sequence here.

CLOUD AMOUNT ASPECT OF ENSO RECHARGE PHASE IS DISCUSSED IN PAVLAKUS ET AL (2008)

In “ENSO Surface Shortwave Radiation Forcing over the Tropical Pacific” (2008), Pavlakis et al illustrated inverse relationship between NINO3.4 SST anomalies and Downward Shortwave Radiation (visible light) anomaly (DSR-A) at the surface for the Central and Eastern Tropical Pacific:
http://www.atmos-chem-phys-discuss.net/8/6697/2008/acpd-8-6697-2008-print.pdf

The Pavlakis et al Figure 6 is shown here as Figure 11. It compares NINO3.4 SST anomalies and Downward Shortware Radiation (DSR), which is visible light, for two areas of the Central and Eastern Equatorial Pacific. Note how when NINO3.4 SST anomalies are negative, indicating a La Nina, DSR anomalies are positive, indicating that more sunlight is reaching and entering the Central and Eastern Equatorial Pacific Ocean, warming it. The opposite happens during an El Nino: NINO3.4 SST anomalies are positive and DSR anomalies are negative, indicating that less visible light is warming the Central and Eastern Equatorial Pacific Ocean. Keep in mind that during the El Nino phase, there may be less sunlight entering the Central and Eastern Pacific Ocean, but this is happening during the discharge phase of ENSO. Then, when the El Nino fades, cloud amount drops and DSR increases, recharging the heat released during the El Nino.
http://i47.tinypic.com/eqs7d3.png
Figure 11

And some might find the Pavlakis et al Figure 1 informative. It illustrates, “The distribution of downward shortwave radiation at the surface (DSR), over the tropical and subtropical Pacific for the three month period November, December, January (NDJ); (a) eleven neutral years average, (b) average for five El Nino years, (c) average for five La Nina years.” I’ve animated the individual cells of their Figure 1 in my Figure 12. Pavlakis et al detail the source of Figure 12 (Their Figure 6) on page 4, under the heading of “Long-term surface shortwave radiation.”

http://i48.tinypic.com/2a9cxfq.gif
Figure 12
Cell a:
http://i49.tinypic.com/27ytmc8.png
Cell b:
http://i47.tinypic.com/vsmkd4.png
Cell c:
http://i49.tinypic.com/1gg07s.png

IS THERE EVIDENCE OF AN IMPACT OF ANTHROPOGENIC GREENHOUSE GASES ON THE RECHARGE MODE OF ENSO?

The next logical point to address would be how much of the ocean heat recharge during the La Nina events could be attributable to the constantly increasing infrared radiation from Anthropogenic Greenhouse Gases and how much could be attributable to the rise in visible light from the decrease in cloud amount. This unfortunately raises the debate about the impacts of infrared radiation and visible light on Ocean Heat Content. Downward Shortwave Radiation (DSR), which is visible light, penetrates and warms the ocean for 100+ meters, while infrared radiation or Downward Longwave Radiation (DLR) only penetrates the top few centimeters. So the order of magnitude of the temporary increase in DSR (visible light) is many times greater than the long-term increase in DLR (infrared radiation) from greenhouse gases. But the argument has been presented that DLR (infrared radiation), through mixing caused by waves and wind stress turbulence, would warm the mixed layer of the ocean. This in turn would impact the temperature gradient between the mixed layer and skin, dampening the outward flow of heat from the ocean to the atmosphere. The end result according to the argument: OHC would rise due to an increase in DLR (infrared radiation) caused by increases in greenhouse gas emissions.

However, refer to the Tropical Pacific OHC data, Figure 13. While there is no doubt that there is a positive trend in the Tropical Pacific OHC data, the graph shows decadal and multidecadal periods of decreasing OHC, not gradually rising OHC as one would expect if greenhouse gases had an effect on the tropical Pacific. The heat lost during these long-term decreases is replaced and additional heat is added during two multiyear periods that coincide with the multiyear La Nina events of 1973/74/75/76 and 1998/99/00. Specifically, for the decade from 1963 to 1973, OHC anomalies drop gradually from ~0.04 GJ/m^2 to ~-0.3 GJ/m^2, and for the two decades from 1977 to 1997 (1999), OHC anomalies drop gradually from ~0.16 GJ/m^2 to ~-0.12 GJ/m^2 (~-0.16 GJ.m^2). During the multiyear (4-year) period between them, from 1973 to 1977, OHC anomalies rose from ~-0.3 GJ/m^2 to ~0.16 GJ/m^2; this appears to be a recharge caused by the multiyear 1973/74/75/76 La Nina. The 1995/96 upsurge in tropical Pacific OHC was explained in McPhaden (1999) “Genesis and Evolution of the 1997-98 El Nino,” as a result of “stronger than normal trade winds associated with a weak La Nina in 1995–96.”
http://www.pmel.noaa.gov/pubs/outstand/mcph2029/text.shtml

http://i36.tinypic.com/eqwdvl.png

Figure 13

With the anomalous rise in Tropical Pacific OHC in 1995/96 shown in Figure 13, it could be argued that the rise in tropical Pacific OHC of ~-0.08 GJ/m^2 to ~0.24 GJ/m^2 from 1998 to late 2001 was a rebound to the values established by the 1995/96 La Nina, or it could be argued that the rise in tropical Pacific OHC was caused by the multiyear 1998/99/00 La Nina, similarly to the 1973/74/75/76 La Nina.

And as discussed above, La Nina events have been established by Trenberth et al as the periods of OHC recharge. The decadal and multidecadal declines in tropical Pacific OHC with the short-term recharges do not appear to be consistent with what should be expected if the constantly increasing infrared radiation from greenhouse gases had a measureable effect on OHC.

END OF UPDATE

SOURCES

ERSST.v2 data was used for the post “Can El Nino Events Explain All of the Global Warming Since 1976?” Parts 1 and 2 and the supplements. NOAA NOMADS was the source:
http://nomads.ncdc.noaa.gov/#climatencdc

As of this writing, NOAA is updating the ERSST.v2 data on NOMADS and long-term data is not available. However, the KNMI Climate Explorer website also has ERSST.v2 data. HADISST data could be used to verify the graphs, too; it makes little difference. The step changes are there regardless of the dataset.
http://climexp.knmi.nl/selectfield_obs.cgi?someone@somewhere

In my post “There Are Also El Nino-Induced Step Changes In The North Atlantic,” OI.v2 SST data was used primarily. It’s available through NOMADS here:
http://nomad3.ncep.noaa.gov/cgi-bin/pdisp_sst.sh?lite

The Sato Index Data is available from GISS at:
http://data.giss.nasa.gov/modelforce/strataer/
Specifically:
http://data.giss.nasa.gov/modelforce/strataer/tau_line.txt