Marine weather forecasting is the process by which mariners and meteorological organizations attempt to forecast future weather conditions over the Earth's oceans. Mariners have had rules of thumb regarding navigation around tropical cyclones for many years, dividing a storm into halves and sailing through the normally weaker and more navigable half of their circulation. Marine weather forecasts by various weather organizations can be traced back to the sinking of the Royal Charter in 1859 and the RMS Titanic in 1912. The wind is the driving force of weather at sea, as wind generates local wind waves, long ocean swells, and its flow around the subtropical ridge helps maintain warm water currents such as the Gulf Stream. The importance of weather over the ocean during World War II led to delayed or secret weather reports, in order to maintain a competitive advantage. Weather ships were established by various nations during World War II for forecasting purposes, and were maintained through 1985 to help with transoceanic plane navigation. Voluntary observations from ships, weather buoys, weather satellites, and numerical weather prediction have been used to diagnose and help forecast weather over the Earth's ocean areas. Since the 1960s, numerical weather prediction's role over the Earth's seas has taken a greater role in the forecast process. Weather elements such as sea state, surface winds, tide levels, and sea surface temperature are tackled by organizations tasked with forecasting weather over open oceans and seas. Currently, the Japan Meteorological Agency, the United States National Weather Service, and the United Kingdom Met Office create marine weather forecasts for the Northern Hemisphere.
History
There are various origins for government-issued marine weather forecasts, generally following maritime disasters.
United Kingdom In October 1859, the steam clipper Royal Charter was wrecked in a strong storm off Anglesey. 450 people lost their lives. Due to this loss, Vice-Admiral Robert FitzRoy introduced a warning service for shipping in February 1861, using telegraph communications. This remained the United Kingdom Met Office's primary responsibility for some time afterwards. In 1911, the Met Office had begun issuing marine weather forecasts which included gale and storm warnings via radio transmission for areas around the British Isles. This service was discontinued during and following World War I, between 1914 and June 1921, and again during World War II between 1939 and 1945.
United States
The first attempt as a marine weather program within the United States was initiated in New Orleans, Louisiana by the United States Army Signal Corps. A January 23, 1873 memo directed the New Orleans Signal Observer to transcribe meteorological data from the ship logs of those arriving in port. Marine forecasting responsibility transferred from the United States Navy to the Weather Bureau in 1904, which enabled the receipt of timely observations from ships at sea. The sinking of RMS Titanic in 1912 played a pivotal role in marine weather forecasting globally. In response to that tragedy, an international commission was formed to determine requirements for safer ocean voyages. In 1914, the commission's work resulted in the International Convention for the Safety of Life at Sea. In 1957, in order to help address marine issues, the United States Weather Bureau started to publish the Mariners Weather Log bi-monthly publication to report past weather conditions primarily over Northern Hemisphere oceans, information regarding the globe's tropical cyclone seasons, to publish monthly climatologies for use of those at sea, and to encourage voluntary ship observations from vessels at sea. Within the United States National Weather Service (NWS), forecast weather maps began to be published by offices in New York City, San Francisco, and Honolulu for public use. North Atlantic forecasts were shifted from a closed United States Navy endeavor to a National Weather Service product suite via radiofacsimile in 1971, while northeast Pacific forecasts became publicly available by the same method in 1972. Between 1986 and 1989, the portion of the National Meteorological Center (NMC) known as the Ocean Products Center (OPC) was responsible for marine weather forecasting within the NWS. Between August 1989 and 1995, the unit named the Marine Forecast Branch also was involved in providing objective analysis and forecast products for marine and oceanographic variables. The Marine Prediction Center, later renamed the Ocean Prediction Center, assumed the U.S. obligation to issue warnings and forecasts for portions of the North Atlantic and North Pacific oceans once it was created in 1995.
Importance of the wind
Development of warm ocean currents The trade winds blow westward in the tropics, and the westerlies blow eastward at mid-latitudes. This wind pattern applies a stress to the subtropical ocean surface with negative curl across the north Atlantic Ocean. The resulting Sverdrup transport is equatorward. Because of conservation of potential vorticity caused by the poleward-moving winds on the subtropical ridge's western periphery and the increased relative vorticity of northward moving water, transport is balanced by a narrow, accelerating poleward current, which flows along the western boundary of the ocean basin, outweighing the effects of friction with the western boundary current known as the Labrador current. The conservation of potential vorticity also causes bends along the Gulf Stream, which occasionally break off due to a shift in the Gulf Stream's position, forming separate warm and cold eddies. This overall process, known as western intensification, causes currents on the western boundary of an ocean basin, such as the Gulf Stream, to be stronger than those on the eastern boundary.
Swell dispersion and wave groups
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