Monitoring the Air-Sea Interface

There are few places on earth as dynamic as the boundary between ocean and atmosphere. This is where carbon dioxide is exchanged between ocean and air, where the air deposits plastics, sand, and atmospheric pollutants into the ocean, and where the ocean outgasses nitrous oxide and oxygen into the atmosphere, to name a few. Such air-sea fluxes influence multiple aspects of our climate, cycles such as the water and carbon cycle, trade winds, and even the ozone layer itself.

Gauging air-sea fluxes is a challenging affair, especially as many fluxes cannot be measured directly. Instead, a suite of “essential ocean and climate variables” is used to estimate fluxes. Estimating heat fluxes, for example, requires high measurements of surface winds, surface humidity, air temperature, surface albedo (upward solar radiation), downward longwave radiation, longwave surface emissivity, and sea surface skin temperature. Such measurements aren’t only vital for air-sea interface studies, but for configuring, calibrating, and improving ocean, climate, and weather models that we use to predict, plan for, and mitigate against changing conditions.

In recent years, numerous scientists and organizations have highlighted the need for more measurements of essential ocean and climate variables. In 2020, the Scientific Committee on Oceanic Research launched a new working group dedicated to improving our knowledge and monitoring of air-sea interface. In 2021, their program—Observing Air-Sea Interactions Strategy (OASIS)—was adopted as a Program within the UN Decade of Ocean Science for Sustainable Development for matching several Decade outcomes: “a predicted ocean”; “a safe ocean”; “a healthy and resilient ocean”; and “a sustainable and productive ocean.”

Eyes in the Sky

Satellites provide near-global and increasingly finer-scale data on a host of essential ocean and climate variables. Sitting some 1,300 kilometres above us, the recently-launched Sentinel-6 Michael Freilich satellite, fitted with Thales Alenia Space’s Poseidon-4 altimeter, will provide altimetry data at an accuracy of up to 2.9 centimetres. Altimeter readings can be used to calculate the transfer of gases such as dimethyl sulphide and carbon dioxide.

Satellites have revolutionised data collection of a host of ocean and climatic variables, but there are some limitations. Some variables, such as sea level pressure, cannot be measured with satellites. In other cases, variables can be measured, but because of the orbit of the satellites, they aren’t collected from polar regions. One example is the Global Precipitation Measurement mission, which observes rain and snow between 65°N and 65°S. Scale is also a consideration. Sub-mesoscale features important to air-sea fluxes may be too small for satellites to detect.

Satellite technology and coverage are improving, but in situ platforms are, and likely will be for some time, major players in data collection.

Ship-Shape Observations

Arguably the oldest and longest-running ocean-climate data collection program is the World Meteorological Organization’s Voluntary Observing Ship (VOS) scheme. Since 1853, passenger, cargo, and other ships have recorded and reported a range of at-sea oceanographic and meteorological measurements such as wind speed and direction, humidity, sea state, and sea temperature using either specially fitted or the ships’ own instruments. While historically measurements were submitted using snail-mail, today, the WMO Global Telecommunication System makes it possible to deliver data in near-real-time. Some 7,700 ships contributed data via the VOS in the mid-1980s, but today this number has almost halved.

global drifter array 1

Location of the surface drifters in the Global Drifter Array program. (As of Jan 24, 2022). Obtained from https://www.aoml.noaa.gov/phod/gdp/. (Image credit: NOAA)

Other ship-based measurements come from dedicated research vessels. Unsurprisingly the instruments onboard these vessels can be more sophisticated than may be found onboard VOS participants. For example, the R/V Pourquoi Pas, operated by French oceanographic institute Ifremer, is equipped with multiple instruments, including a Sea-Bird Scientific Thermosalinograph that can measure temperature and conductivity and a weather station that includes a Seatronics barometer to measure pressure. In 2017, instruments onboard the R/V Pourquoi Pas enabled scientists to study the impact of Saharan dust in the air-sea interface during a deposition event in the Mediterranean Sea.

Like any data collection method, ship-based observations do have their drawbacks. VOS data tends to come from in and around shipping lanes and coastal areas, particularly those relating to larger ports in the Northern hemisphere, and tend to have a bias for fairer weather (ships avoid the worse weather as much as possible). Although research vessels may stray off the beaten shipping lane and collect more specialised data, the number of vessels is invariably far fewer than the VOS.

Staying in One Place

Moored buoys networks can provide long-term in situ measurements on a host of ocean and climate variables needed to calculate air-sea fluxes, including surface currents, humidity, wind, and short and longwave radiation. The Indian moored buoy network, developed by India’s National Institute of Ocean Technology (NIOT), has provided real-time measurements for almost 25 years across coastal and deepwater locations in the Arabian Sea and Bay of Bengal. Data is transmitted every three hours, and additional high-frequency data can be stored on the moorings for retrieval later.

wave glider 24 day mission

Setup of the Wave Gliders with instruments in the 25-day mission between San Clemente and San Nicholas Islands. (Image credit: Grare et al, 2021)

The network has provided vital data for a host of studies on the air-sea interface. One recent example comes from Samar Ghose (Indian Institute of Technology Bhubaneswar) and colleagues who studied how heat flux varied in and out of the monsoon seasons in the two basins. He found latent heat flux to be higher during the monsoon season, with pre-monsoon wind speeds and relative humidity regulating latent heat flux and sea surface and air temperature in sensible heat flux. However, there were some variations. The role of relative humidity, for example, was much stronger during the southwest monsoon season in the Arabian Sea than in the Bay of Bengal.

Going with the Flow

While moored buoy networks provide vital long time series of data from fixed locations, enabling studies of the essential ocean and climate variables that control air-sea fluxes over time, the Global Drifter Array program employs some 1,300 surface drifters to gather data as they float on the ocean surface.

Manufactured by specialist companies such as Marlin-Yug, longevity of the drifters varies, but drifter #26028 currently holds the record for transmitting data for over ten years. Transmitting data every few hours, all drifters measure sea surface temperature, near-surface current velocity, and location. Since the mid-1990s, drifters have been equipped with pressure sensors, providing more accurate reading than those taken from ships due to proximity to the sea surface. A smaller number of drifters are also equipped with sensors to measure wind, salinity, and waves.

Alongside research, surface drifters play a pivotal role in calibrating and validating measurements obtained from satellites. These include the Sentinel-3 satellite, which has contributed to climate products such as the European Centre for Medium-Range Weather Forecasts’ ERA-Interim global atmospheric reanalysis.

Without the need for onboard people-power, surface drifters—and equally moored buoys—aren’t just cheaper to deploy than ships, they can be used in areas where in situ observations aren’t typically gathered. This includes places or times where conditions are hazardous, where ship traffic is low, and particularly remote ocean areas.

Of course, drifters face their own set of challenges. By their very nature, surface drifter movements are dictated by surface flow, potentially taking drifters away from areas of interest. In equatorial regions, for example, maintaining drifter presence is difficult as divergent surface flows tend to drag the drifters into the sub-tropics.

Other challenging locations include the Polar regions, particularly when sea ice poses a threat to manned and unmanned vehicles, and powerful storms can take drifters off course. In fact, when it comes to air-sea fluxes and essential ocean and climate variables, the remote and vast Southern Ocean is one of the most data-deficient Oceans on Earth. However, a new breed of instruments is helping unlock its secrets

The Future is Autonomous

Autonomous (or unmanned) underwater and surface vehicles were first developed in the late 1950s, but it is only in recent years that they are becoming more prominent in research. Unlike drifters, these vehicles have some propulsion, enabling researchers to send them to specific locations for data collection. For example, in 2017, a Liquid Robotics Wave Glider vehicle equipped with a host of sensors such as laser wave gauges and ADCPs set out on a 25-day data-collection mission between San Clemente and San Nicholas Islands to understand how fluxes operate across and into the ocean mixed layer. In the winter of 2018/2019 a Wave Glider, paired with a Teledyne Webb Slocum G2 glider set out on a 56-day mission south of the Antarctic Polar front to gather data on carbon dioxide outgassing during winter storms.

The Wave Glider and Slocum G2 glider aren’t the only autonomous vehicles contributing to air-sea interface research in the Southern Ocean. In 2019 autonomous surface vehicle Saildrone successfully circumnavigated Antarctica in 196 days. Equipped with an anemometer, a CTD to measure sea surface temperature and salinity, and a system to collect a surface and marine boundary layer atmospheric carbon dioxide and sea-level atmospheric pressure, the platform delivered hourly carbon dioxide flux estimates. If Southern Ocean storms weren’t challenging enough, in September 2021, a specially modified version of the vehicle became engulfed by Hurricane Sam, a Category 4 event. Not only did the Saildrone survive, but it came out with the world’s first video footage from inside a hurricane.

Autonomous vehicles like the Saildrone, Wave Glider, and Slocum G2 glider won’t completely replace other in situ instruments. However, their ability to tolerate harsh conditions and collect data for long periods with an array of different instruments, combined with ongoing improvements in A.I., automation, and robotics, means autonomous vehicles are likely to become ever more critical in our quest to understand, measure, and predict the air-sea interface.

This feature appeared in Environment, Coastal & Offshore (ECO) Magazine's 2022 Spring edition, to read more access the magazine here.

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