Air-Sea Interface [chapter]

2013 Ocean Biogeochemical Dynamics  
34 The sea-surface microlayer (SML) at the air-sea interface is < 1 mm deep but it is 35 physically, chemically and biologically distinct from the underlying water and the atmosphere 36 above. Wind-driven turbulence and solar radiation are important drivers of SML physical and 37 biogeochemical properties. Given that the SML is involved in all ocean-atmosphere 38 exchanges of mass and energy, its response to solar radiation, especially in relation to how it 39 regulates the air-sea exchange of
more » ... ir-sea exchange of climate-relevant gases and aerosols, is surprisingly poorly 40 characterised. 41 MILAN (sea-surface MIcroLAyer at Night) was an international, multidisciplinary 42 campaign designed to specifically address this issue. In spring 2017, we deployed diverse 43 sampling platforms (research vessels, radio-controlled catamaran, free-drifting buoy) to study 44 full diel cycles in the coastal North Sea SML and in underlying water, and installed a land-45 based aerosol sampler. We also carried out concurrent ex situ experiments using several 46 microsensors, a laboratory gas exchange tank, a solar simulator, and a sea spray simulation 47 chamber. 48 In this paper we outline the diversity of approaches employed and some initial results 49 obtained during MILAN. Our observations of diel SML variability, e.g. the influence of 50 changing solar radiation on the quantity and quality of organic material, and diel changes in 51 wind intensity primarily forcing air-sea CO2 exchange, underline the value and the need of 52 multidisciplinary campaigns for integrating SML complexity into the context of air-sea 53 interaction. 54 55 CAPSULE 56 MILAN was a multidisciplinary, international study examining how the diel 57 variability of sea-surface microlayer biogeochemical properties potentially impacts ocean-58 atmosphere interaction, in order to improve our understanding of this globally important 59 process. 60 61 BACKGROUND 62 The sea-surface microlayer (SML) occupies the uppermost tens to hundreds of µm of 63 the ocean surface and is ubiquitous (Wurl et al. 2011). Consequently, it is in direct contact 64 with the atmosphere and covers around 70% of Earth's surface. Compared to the underlying 65 bulk water, the SML is characterised by distinct biological and physicochemical properties 66 (Cunliffe et al. 2013), which is important given that all ocean-atmosphere exchanges of mass 67 and energy must necessarily cross it. An improved understanding of the SML is thus essential 68 for studying air-sea exchange processes that have important implications for global 69 biogeochemical cycles, climate regulation, and air quality (Wurl et al. 2017). 70 The SML experiences instantaneous meteorological forcing by e.g., solar radiation, 71 wind and precipitation. Solar radiation directly influences the thermal and saline boundary 72 layer with variable thicknesses in the order of 1000 μm (Saunders 1967) and 200 μm 73 (Katsaros 1980), respectively. Evaporation and precipitation have a strong influence on the 74 thermal and saline properties of the SML (Schlüssel et al. 1997). Furthermore, recent 75 measurements show that large enrichment of organic material in the SML reduces evaporation 76 from the sea-surface (Wurl et al. 2018). The SML also experiences higher exposure to UV 77 radiation than does the underlying water column, because light attenuation by optically active 78 components and water itself reduce light levels exponentially with increasing depth. Whereas 79 light levels in the SML always exceed 98% of surface irradiance, in coastal environments 80 with large amounts of suspended and dissolved organic matter only around 10% of surface 81 UV-B irradiance may reach 0.2 to 5 m depth (Smyth 2011; Tedetti and Sempéré 2006). 82 Wind induced formation of small capillary waves, and microscale breaking causes 83 SML disruption (Ocampo-Torres et al. 1994). After disruption, re-establishment of the 84 thermal skin layer occurs (Jessup et al. 2009) and depends on wind speed. Re-establishment 85 occurs on a timescale of between several seconds and around one minute, the latter being 86
doi:10.2307/j.ctt3fgxqx.6 fatcat:qdvnddtbdfcwdcnt5r5y2nfafm