Abstract
Ship-based measurements of the air-sea temperature difference (air-sea ΔT) are analyzed and presented with a focus on the temperature difference probability distribution and causes of its stability and variability. By taking the measurements with a hyperspectral infrared radiometer, temperatures of both the lower atmosphere and the skin layer of the ocean are acquired by a single, well-calibrated instrument. Consider that for each radiometric cruise deployment, when the air-sea ΔT measurements are plotted in a histogram, a mean air-sea ΔT peak at about -1 K becomes apparent regardless of measurement location or time of year. The large amount of air-sea ΔT data centered around -1 K suggests that there is a ‘feedback’ loop involving oceanic and atmospheric variables that stabilizes the air-sea ΔT around -1 K. Within a system, “feedback” is defined as the process in which a part of the output of the system is returned to its input to influence or regulate its further output. A feedback loop may be characterized as positive or negative. Following an initial disturbance, a positive feedback loop will lead to a growth in the initial perturbation, and could push a previously stable system past a critical condition and into another state. A negative feedback loop will act to lessen the initial disturbance, keeping the system within a stable state. When describing climate systems, there are a number of variables that allow for the formation of both positive and negative feedback loops. Data from the multiple cruises point to the negative ‘tail’ of the air-skin ΔT being caused by a combination of low wind speeds (less than 3 m/s) and intense solar radiation. Wind speeds from 3 m/s to 9 m/s cause an increase in sensible and latent heat fluxes that serve as negative feedbacks that stabilize the air-sea ΔT around its -1 K mean. At wind speeds above 10 m/s and low humidity levels, the latent heat flux acts as a positive feedback, and depending on the strength of other oceanic and atmospheric heat fluxes, may cause the air-skin ΔT to become more negative. A flow diagram of initial disturbances and the resulting feedbacks is presented, demonstrating the necessary conditions and consequences of the negative and positive feedback loops with regard to the air-skin ΔT.