On Measuring Ocean Heat Flux

Following Dr. Bernaerts’ discussion that Oceans Make Climate, and that naval activity has an effect, this post overviews issues concerning the heat flux at the boundary between sea surface and atmosphere.

The Basics

mmedson

The graph displays measures of heat flux in the sub-tropics during a 21-day period in November. Shortwave solar energy shown above in green labeled radiative is stored in the upper 200 meters of the ocean. The upper panel shows the rise in SST (Sea Surface Temperature) due to net incoming energy. The yellow shows latent heat cooling the ocean, (lowering SST) and transferring heat upward, driving convection.

From
An Investigation of Turbulent Heat Exchange in the Subtropics
James B. Edson

“One can think of the ocean as a capacitor for the MJO (Madden-Julian Oscillation), where the energy is being accumulated when there is a net heat flux into the ocean (here occurring to approximately November 24) after which it is released to the atmosphere during the active phase of the MJO under high winds and large latent heat exchange.”
http://www.onr.navy.mil/reports/FY13/mmedson.pdf

Turbulence Changes Both Parts of the Heat Flux

As mentioned above, this flux is not in equilibrium or steady state, but constantly subject to turbulence, both natural and man-made. Therein lies the difficulty in measuring it accurately and documenting changes over time. The study above, while not addressing ships, shows that latent heat varies considerably with turbulence.

“Turbulence in the surface layer of the ocean contributes to the transfer of heat, gas and momentum across the air-sea boundary. As such, study of turbulence in the ocean surface layer is becoming increasingly important for understanding its effects on climate change.”

“Moving surface vessels such as ships typically produce wakes which are highly visible in ocean SAR images, where the region behind the vessel displays a region of wake turbulence and surface currents which produce a visible backscattering response.”

http://www.ocean-sci-discuss.net/9/2851/2012/osd-9-2851-2012-print.pdf

Turbulence Changes the Ocean Albedo

Ship Wake

Schematic of a typical turbulent ship wake as viewed by SAR.
Measurement of turbulence in the oceanic mixed layer using Synthetic Aperture Radar (SAR)
S. G. George and A. R. L. Tatnall 2012

The incoming solar energy is reduced by the “bright water” resulting from air bubbles and foam in the wake.

“The albedo change over land caused by land‐use and land‐cover modifications is well documented [Forster et al., 2007]. However, modification of the ocean albedo by human activities is unknown, even though the oceans cover 70% of Earth’s surface and absorbs approximately 93% of incident solar radiation.”

“This study provides new insights into ship‐generated disturbances on the ocean surface, which have received little attention in climate studies, but is potentially significant for the ocean‐ atmosphere energy balance and could affect climate.”

“The strong enhancement of ocean reflectance in the ship wake is unambiguous, and >100% in most cases in the spectral range from the ultraviolet to the near‐infrared (0.340 mm ≤l≤ 2.205 mm), and clearly seen in the ocean BRDF measurements. These results are derived from angular and spectral measurements of the intensity of reflected solar radiation from an airborne instrument over several regions of the ocean disturbed by the ship wakes. The implication for the global radiation budget at the top of the atmosphere has been demonstrated in this study.”
Gatebe et al 2011
http://onlinelibrary.wiley.com/doi/10.1029/2011GL048819/pdf

However authors of this study do not estimate albedo effect from shipping to be significant at this time.

“Changes in surface albedo represent one of the main forcing agents that can counteract, to some extent, the positive forcing from increasing greenhouse gas concentrations. Here, we report on enhanced ocean reflectance from ship wakes over the Pacific Ocean near the California coast, where we determined, based on airborne radiation measurements that ship wakes can increase reflected sunlight by more than 100%. We assessed the importance of this increase to climate forcing, where we estimated the global radiative forcing of ship wakes to be -0.00014 plus or minus 53% Watts per square meter assuming a global distribution of 32331 ships of size of greater than or equal to 100000 gross tonnage. The forcing is smaller than the forcing of aircraft contrails (-0.007 to +0.02 Watts per square meter), but considering that the global shipping fleet has rapidly grown in the last five decades and this trend is likely to continue because of the need of more inter-continental transportation as a result of economic globalization, we argue that the radiative forcing of wakes is expected to be increasingly important especially in harbors and coastal regions.”

There are some efforts to measure the infrared signature of ship wakes, including emitted energy.

“The sea surface turbulent trailing wake of a ship, which can be rather easily observed in the infrared by airborne surveillance systems, is a consequence of the difference in roughness and temperature between the wake and the sea background. We have developed a phenomenological model for the infrared radiance of the turbulent wake by assuming that the sea surface roughness is dependent upon the turbulent intensity near the sea surface. . .Given the incident solar, atmospheric, and sky infrared radiances, we calculate the reflected and emitted sea surface radiance from both the wake and the background. We compare the infrared contrast of the wake with infrared image data obtained in an airborne trial.”

Modeling the turbulent trailing ship wake in the infrared
Vivian Issa and Zahir A. Daya 2014
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA555507

Conclusion

Ocean turbulence is being studied but not as extensively as atmospheric turbulence. In both domains, drawing climate conclusions is challenging. There is an albedo effect of a ship’s wake that reflects solar SW, but one study considers it a small effect. The release of latent heat varies significantly with wind changes, but the effect from shipping is not known. Other ocean effects from shipping are not discussed here, such as additional release of CO2 and ice-breaking in the Arctic .

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8 comments

  1. ArndB · April 11, 2015

    My more practical view concerning ship wakes. The possible impact of ‚changes in surface albedo‘ (reflected sunlight) by ship wakes measured by SAR is presumably a very minor aspect. A more decisive factor is the change within the water column over the depths affected. A water column of six meters, has a heat capacity double as high as the entire air column above (7-10’000 meters). A change in temperature and salinity structure throughout a wake-water column is most likely not ‘neutralized’ within a short time period, but could last for days or longer. Temperature and salinity is the driver of the sea interior, which includes the changes caused by ship wakes (and wind).

    Like

    • Ron Clutz · April 11, 2015

      Thanks for clarifying that for me. I see on reading more carefully you wrote that the change in the thermal structure is the important factor.

      I have also found interesting 2 of your recent articles and have added them to the earlier post.

      Offshore Wind-parks and mild Winters.
      Contribution from Ships, Fishery, Wind-parks etc.
      25th February, 2015
      http://climate-ocean.com/2015/K/k-.pdf

      After a moderate March now a cold April? April 4, 2015
      http://climate-ocean.com/2015/K-m2.html

      Like

      • ArndB · April 13, 2015

        Thanks for recognizing the current topic on offshore wind-farms by adding a link to the recent post.

        Like

  2. steverichards1984 · April 12, 2015

    I beg to differ!

    If a vessel disturbs a 6 metre water column, the temperature of the sea water, post passing, would quickly equalise with that of the surrounding water. Differences in salinity would cause ‘micro’ currents that would quickly equalise salinity.

    A quick test would be to construct a 1x1x1 metre tank full of water, salt or fresh. Place a 1kW water heater at the bottom of the tank central between the 4 sides.

    Place pt100 temperature sensors in various locations with in the water.

    Power the heater for 5 minutes and monitor the sensors.

    I am confident that the water within the tank will rise in temperature, the temperature nearest and above the heater will rise first and fastest, the sensors furthest away will record a slower rise.

    Depending upon the amount of insulation attached to the surface of the tank, the final temperature should reach its theoretical value dependant upon energy input and insulation.

    Of course there will be a thermal gradient as the warmer water will always rise above the cooler water.

    Moving to the ships wake: the sea surface and air will be at different temperatures ensuring a regular movement of energy. Evaporation will be taking place. Due to this, the sea water column will be in turmoil, with energy flowing into the water or out of the water. Along comes our ships wake, creating massive disturbance. Post ship, the original ‘regulating activity’ will reassert itself, reproducing close to the original conditions, I would imagine quite quickly.

    Like

    • Ron Clutz · April 12, 2015

      Do you think the process you describe results in additional release of heat away from the ocean into the air? As you say, afterwards thermal equilibrium returns since any loss is small relative to the ocean’s heat capacity.

      Like

  3. Ron Clutz · April 13, 2015

    It was impressive in the Edson study referenced at the top of this post that light winds produced net heat loss of 100 W/m2, and the winds were able to lower the SST in that patch of ocean by 1C in about a week.

    Like

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