How we forecast the behaviour of our oceans

For those working at sea or living near the coast, forecasts of wave height, ocean currents or storm surges are just as vital as forecasts of the weather.

We routinely run a number of ocean forecast computer models that help organisations, such as ferry operators and oil companies, to plan their operations at sea, and allow accurate warnings of coastal flooding to be issued. These include:

• ocean circulation analysis and forecast models for the deep ocean
• ocean circulation forecast models for the seas of the continental shelf that surrounds the UK (shelf seas)
• wave forecast models
• a high-resolution sea-surface temperature analysis
• coupled ecosystem forecast models that describe changes in the chemical and biological make-up of both deep ocean and UK shelf-seas

To further develop these capabilities we have established a National Centre for Ocean Forecasting (NCOF) in association with Proudman Oceanographic Laboratory, Plymouth Marine Laboratory, National Oceanography Centre, Southampton and the NERC Environmental Systems Science Centre. The aim of NCOF is to establish ocean forecasting as part of the national infrastructure, based on world-class research and development.

For more details and sample data please visit the NCOF web site

Deep ocean

Although far removed from our coastal communities, the deep ocean exerts an important influence on the weather we all experience. As industry explores the deep ocean to meet our energy and other resource needs, predicting temperature and current through the entire water column will ensure safe and effective operations. Understanding the processes of change of the deep ocean is vital to improving our understanding of climate change.

The Met Office has run the global Forecasting Ocean Assimilation Model (FOAM) daily since 1997, forecasting three-dimensional ocean currents, temperature, salinity and sea-ice concentration, thickness and velocity.

FOAM incorporates observational data into ocean and sea-ice models and produces an accurate representation of the present ocean state. In order to forecast how ocean temperatures, currents and the sea-ice field will evolve, atmospheric forecasts of surface pressure, wind and solar heating are used to drive the model.

Each daily run uses observations from the previous 10 days, including:

• temperature and salinity profiles including data from Argo floats and moored instruments
• sea-surface height data measured from earth orbiting satellites
• ship, buoy and satellite sea-surface temperature observations
• sea-ice concentration fields received from the Canadian Meteorological Centre

Mobile circulation patterns, such as ocean eddies, fronts and jets, are the ocean equivalent of the storm systems we recognise from weather charts. Model resolution is the key to ensuring that these features are properly captured. In order to do this, a number of high-resolution configurations, are nested within the global FOAM model.

Shelf seas

Tides and storm surges have a major influence on the character of the regional seas around the UK, and predicting these correctly is of critical importance to mariners.

Following the catastrophic storm surge that flooded part of the south-east of England in 1953, the Met Office established a storm tides forecast service and now make daily runs of a storm surge model developed for the Environment Agency by the Proudman Oceanographic Laboratory (POL).

The storm surge model is specifically designed to calculate changes in water level due to tide and surge. To forecast other key characteristics of our coastal seas (e.g. currents at the surface and through the water column, sea temperature, water quality) a more detailed model — the Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) — is used.

Both models are driven by tides (gravitational effects of the sun and moon) and forecasts of atmospheric parameters, including surface pressure, wind and solar heating. Close to the coastline the sea-bed becomes more complex, so a number of high-resolution configurations are used to represent the sea’s response to this complexity as accurately as possible.

Waves

On stormy days around our coasts breaking waves provide us with a constant reminder of the power of the oceans. High seas are dangerous for mariners and bring the risk of coastal flooding.

Over the past two decades, we have run and maintained wave models to provide predictions of wave conditions, globally and around the UK.

Waves develop as a result of the wind blowing over the sea surface, leading to high 'wind-waves' developing during the strongest storms. Away from storm centres, wind-wave energy becomes more uniform and moves without further external influence — at this stage the waves are termed as swell. Wave energy is lost from the ocean when the waves break (e.g. white-capping in windy conditions) and due to friction as waves move through shallow water.

The Met Office model represents all these processes, including shallow-water physics and is driven by forecasts of surface winds.

Outputs from the models are used for a variety purposes, from predicting offshore vessel motion characteristics to forecasts of coastal waves and overtopping.

Sea-surface temperature

The Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) system produces a high-resolution (1/20°, approx. 5 km) daily analysis of the current sea-surface temperature (SST) for the global ocean. OSTIA uses satellite data provided by the Global High Resolution Sea Surface Temperature Pilot Project (GHRSST-PP), together with in-situ observations, to determine the sea-surface temperature.

More details and sample data can be found on the OSTIA pages on the NCOF web site

Ocean ecosystems

Marine ecosystem modelling is of fundamental importance in understanding issues of water quality, environmental health, and maintaining sustainable ecologies. The science of forecasting marine ecosystems has many challenges, but the Met Office has committed to developing this field by becoming one of the first centres to run an operational forecast model.

Our models simulate the carbon-cycle and predict the biological and chemical make-up of both the open ocean and shelf seas. For example, in the shelf-seas around the UK we use models which separate the ecosystem into functional groups, including:

• one bacteria, four phytoplankton and three zooplankton functional groups
• a fully resolved diurnal cycle
• variable carbon to chlorophyll ratios
• independent nutrient pools for carbon, nitrogen, phosphorous and silicate

Open-ocean ecosystem forecasts are currently not operational, but are planned to become part of the operational deep ocean modelling system (from 2009) through the inclusion of the Hadley Centre Ocean Carbon Cycle (HadOCC) model in the Forecasting Ocean Assimilation Model (FOAM). HadOCC is a Nutrient, Phytoplankton, Zooplankton, Detritus (NPZD) ecosystem model with the addition of two components representing carbon in the ocean: dissolved inorganic carbon and alkalinity.