An oceanic climate, also known as a marine climate or maritime climate, is the temperate climate sub-type in Köppen classification represented as Cfb or represented as Do under the Trewartha climate classification, typical of west coasts in higher middle latitudes of continents, generally featuring warm summers and cool to mild winters (for their latitude), with a relatively narrow annual temperature range and few extremes of temperature under Köppen's system or mediocre growing seasons with mild winters under Trewartha's. Oceanic climates can be found in both hemispheres generally between 40 and 60 degrees latitude, with subpolar versions extending to 70 degrees latitude in some coastal areas and some hot-summer oceanic climates extending to just north of 35 degrees North latitude in North America and Honshu and just south of 35 degrees North latitude in Mainland China under the Trewartha classification. Other varieties of climates usually classified together with these under the Köppen system include subtropical highland climates, represented as Cwb or Cfb, and subpolar oceanic or cold subtropical highland climates, represented as Cfc or Cwc. Subtropical highland climates occur in some mountainous parts of the subtropics or tropics, some of which have monsoon influence, while their cold variants and subpolar oceanic climates occur near polar or tundra regions.
Cause
Physical mechanisms
Oceanic climates are primarily influenced by the thermal properties of the ocean and by the prevailing atmospheric circulation of the mid-latitudes. Water has a higher heat capacity than land and can store large amounts of solar energy during the warmer months, releasing this energy gradually during the colder months. This reduces seasonal temperature contrasts in coastal areas compared with locations at similar latitudes farther inland. In the mid-latitudes, prevailing westerly winds frequently transport maritime air masses from the ocean toward the continents. The passage of extratropical cyclones and associated frontal systems contributes to frequent changes in weather and to precipitation throughout much of the year. As a result, oceanic climates generally have relatively mild summers, cool to mild winters, and a comparatively small annual temperature range. Ocean currents can further modify these characteristics by transporting heat between different parts of the ocean. The effect of ocean circulation is particularly important in western Europe, where the North Atlantic Ocean and its associated circulation contribute to relatively mild conditions compared with other regions at similar latitudes. However, regional climate is determined by the combined effects of oceanic heat transport, atmospheric circulation, latitude, elevation, and topography rather than by any single ocean current.
Oceanic climates are not necessarily found in coastal locations on the aforementioned parallels; however, in most cases oceanic climates parallel higher middle latitude oceans. The polar jet stream, which moves in a west to east direction across the middle latitudes, advances low pressure systems, storms, and fronts. In coastal areas of the higher middle latitudes (45–60° latitude), the prevailing onshore flow creates the basic structure of most oceanic climates. Oceanic climates are a product and reflection of the cool ocean adjacent to them. In the autumn, winter, and early spring, when the polar jet stream is most active, the frequent passing of marine weather systems creates the frequent fog, cloudy skies, and light drizzle often associated with oceanic climates. They are typically found poleward of Mediterranean climates, except in Australia where they are poleward of both such climates and humid subtropical climates due to the shape of the continent. Only in Europe do they penetrate far inland, where they eventually transition into warm-summer humid continental climates; in other continents, they are blocked by a large mountain range or limited by nearby oceans.
The North Atlantic Gulf Stream, a tropical oceanic current that passes north of the Caribbean and up the East Coast of the United States to North Carolina, then heads east-northeast to the Grand Banks of Newfoundland, is thought to greatly modify the climate of northwest Europe. As a result of the North Atlantic Current, west coast areas located in high latitudes like Ireland, the UK, and Norway have much milder winters (for their latitude) than would otherwise be the case. The lowland attributes of western Europe also help drive marine air masses into continental areas, enabling cities such as Dresden, Prague, and Vienna to have maritime climates in spite of being located well inland from the ocean.
Variability Although oceanic climates are characterized by relatively moderate temperatures, they are not free from substantial interannual variability or extreme weather events. Variations in atmospheric circulation can influence temperature, precipitation, storm tracks, and the frequency of dry or wet periods.
In the North Atlantic region, the North Atlantic Oscillation (NAO) is an important source of climate variability. Changes in the pressure difference between the Azores High and the Icelandic Low influence the strength and position of the westerly winds and the tracks of extratropical cyclones. Other regions with oceanic climates can be influenced by large-scale climate patterns such as the El Niño–Southern Oscillation, the Pacific Decadal Oscillation, and the Southern Annular Mode.
The term "oceanic climate" therefore describes a long-term climatic regime rather than a particular type of daily weather. Individual years can include heat waves, cold spells, droughts and heavy precipitation.
Precipitation and topography Locations with oceanic climates tend to feature frequent cloudy conditions with precipitation, low-hanging clouds, and frequent fronts and storms. Thunderstorms are normally few, since strong daytime heating and hot and cold air masses meet infrequently in the regions, but are more common in subtropical highland climates where these air masses meet more frequently due to the influence of hotter weather in the subtropics or tropics, especially in monsoon-influenced climates. In most areas with an oceanic climate, precipitation comes in the form of rain for the majority of the year. Most oceanic climate zones, however, experience at least one snowfall per year. Snowfall is more frequent and commonplace in the subpolar oceanic climates due to the colder weather in those locations.
Topography Topography can produce substantial variations within regions classified as having an oceanic climate. Moist maritime air rising over mountain ranges is cooled adiabatically, often producing increased precipitation on windward slopes. Areas located on the leeward side of mountain ranges may receive substantially less precipitation because of the rain-shadow effect. Consequently, locations with oceanic climates can have markedly different precipitation totals over relatively short distances. Elevation also reduces air temperature and can cause areas at higher elevations to experience cooler conditions than nearby lowland locations, including more frequent snowfall during winter.
Temperature Under the Köppen climate classification system Overall temperature characteristics of the oceanic climates feature cool temperatures and infrequent extremes of temperature. In the Köppen climate classification, oceanic climates have a mean temperature of or higher (or or higher) in the coldest month, compared to continental climates where the coldest month has a mean temperature of below (or ). Summers are warm but not hot, with the warmest month having a mean temperature below .
Poleward of the latter is a subtype of it, the subpolar oceanic climate (Köppen Cfc), with long but relatively mild (for their latitude) winters, and cool and short summers with average temperatures of at least for one to three months. Examples in the Northern Hemisphere include parts of coastal Iceland, the mountains of Scotland, parts of Shetland, and the British Columbian coast in Canada. In the Southern Hemisphere, examples include extreme southern Chile and Argentina (such as Punta Arenas and Ushuaia), the Falkland Islands, parts of southeastern Australia, and much of New Zealand.
Under the Trewartha climate classification system Oceanic climates are defined very differently under the Trewartha climate classification system.
In temperate areas, oceanic climates exist where four to seven months average at least regardless of summer heat or lack thereof, so long as the coldest month averages at least . This allows some places with hot summers, such as New York City, the District of Columbia, and Louisville, Kentucky, to have oceanic climates under the Trewartha system, although these climates are considered humid subtropical under the Köppen system due to summer heat instead of the length of the growing season being the trigger for Köppen.
In boreal/subpolar areas, Trewartha defines areas like Anchorage that have winters averaging over as oceanic climates even if they are insufficiently mild to be classified as oceanic under Köppen too like Reykjavík is, as opposed to the freezing isotherm used in temperate climates. Nonetheless, Köppen and Trewartha concur on the summer temperature of a subpolar climate, with one to three months averaging over being the trigger for both.
Locations
Europe
Oceanic climates in Europe occupy a large stretch of land, from Norway, the British Isles and southeast to some parts of northern Turkey.
Western Europe is almost exclusively oceanic between 45°N to 54.913°N; including most of France (away from the Mediterranean), nearly all of Belgium, the Netherlands, Austria, most of Luxembourg, most of Denmark, western Germany, northwestern Switzerland, extreme southern Sweden, and Poland's immediate coastal cities, such as Gdynia.
While most of Southern Europe is climatically Mediterranean, some parts of Southern Europe (specifically in the northwestern part of the Iberian Peninsula) also have oceanic climates — which is the case of parts of Galicia, an autonomous community in Spain, including the province of A Coruña and its capital, the city of A Coruña.
However, these instances of the climate are highly variable, and often somewhat anomalous. For example, the western Azores off the coast of Portugal are too wet in summer to be Mediterranean, and too mild in summer to be humid subtropical, though they often have winter means above , unusual for European oceanic climates.
Another anomalous case can be found in northwestern Turkey, including northern Istanbul. These places are, in a strict air-mass sense, not oceanic: they are affected by southerlies directly from the Mediterranean, and polar intrusions from Siberia. Yet their position near the Black Sea makes them too wet in summer to be Mediterranean, too mild during winter to be humid continental, and not hot enough in summer to be humid subtropical; therefore Köppen classifies them as oceanic. Despite their anomalous position, however, their temperatures, around in winter and in summer, are not wholly atypical for European oceanic climates.
Some Eastern European regions such as the north of Croatia and Serbia and some parts of the Czech Republic, also have oceanic climates; these are generally near the boundary for being humid continental.