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 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.
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.”
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.”
Turbulence Changes the Ocean Albedo
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
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
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 .