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# Ocean and Sea Ice Forecasting

In many situations it is important to be able to make ocean forecasts. One application is to be able to issue warnings about risk of flooding in coastal areas. Other applications include forecasts of the spreading of oil spill, ice formation, ice compression, and wave height.

Ocean forecasts are done in a similar manner as weather forecasts are done. The variables of greatest interest are

• significant wave height
• sea level
• ice extent (ice concentration, thickness,and ridges)
• currents
• sea-surface temperature

The first variable, significant wave height, is calculated at SMHI using a wave model called SWAN (Simulating Waves Nearshore). All other variables are calculated using the coupled ice-ocean model HIROMB (High-Resolution Operational Model for the Baltic).

In general, to be able to make forecasts, two things are needed:

• a good initial condition, i.e. a description of the state of the ocean right now, and
• a method to, by using the laws of physics, calculate how the state (of the ocean in our case) develops over time.

The first item above, about the initial condition, is fulfilled by starting with a first guess of the ocean state, which is typically a six-hour forecast valid at the starting time of the forecast. This first guess is then modified slightly using observations of e.g. sea-surface temperature, valid near the starting time of the forecast. The merging of the first guess and the observations is done in an optimal way to minimize errors, using statistics of forecast errors as well as observation errors. This is usually referred to as "data assimilation", and is a science of its own.

The reason why data assimilation is needed to make forecasts is that numerical forecast models contain errors which grow over time. This makes the ocean state drift away from the true state which sooner or later affects the forecast quality in a negative way. Assimilation of observations is usually done before each forecast, i.e. with an interval of 6-24 hours depending on the type of forecasts.

To satisfy the second demand above, numerical circulation models are usually employed to make forecasts from 48 hours up to 4 weeks ahead. To make shorter, more detailed forecasts, the atmospheric model HIRLAM (High-Resolution Limited-Area Model) is currently employed, which is being run and developed at SMHI. The longer ocean forecasts are forced using the correspondingly longer foreasts from ECMWF (European Centre for Medium-Range Weather Forecasts). It is an international organization in which Sweden is a member through SMHI.

Figure 1. Example of a sea-level forecast for Kalix. Red: observations, blue: HIROMB 1 nautical mile resolution, cyan: HIROMB 3 nautical miles resolution. In this case the forecast length is 48 hours.
Enlarge picture
Figure 2. Forecast of ice thickness and ice drift in the Bothnian Bay. Example of a 4-day forecast.
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Last updated 31 August 2009
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