In recent years, one of the most commonly used phrases in weather and climate discussions has been along the lines of, “Since we are in an El Niño pattern...”, followed by different analyses or opinions on current or forecasted weather. In fact, there is a periodic on-going cyclic weather pattern variation or oscillation called ENSO or El Niño–Southern Oscillation. This is a cyclic change in the wind and sea surface temperature over parts of the tropical eastern Pacific Ocean. This oscillation has two phases, one warm phase - El Niño, and one cold phase - La Niña.
The observed atmospheric component of this is the Southern Oscillation, which is most clearly observed through prevailing high and low surface air pressure over the western Pacific. El Niño makes conditions favorable for a high pressure situation, while La Niña caters to a low pressure situation.
The global effects of ENSO are seen by the atmospheric component, i.e., the air pressure and the circulation pattern set up by the prevailing pressure centers. And, increasingly common, the impact of any weather is more rainfall and general water supply effect, rather than actual temperature or wind. Since the wind pattern oscillation is shifted to fold around the high/low pressure, it causes the upper air winds pattern (a.k.a the jet streams), for example over the US west coast, to shift south.
The location of these jet streams is a major factor in producing winter weather patterns at mid-latitudes globally. For example, instead of coming ashore in the US Pacific Northwest as usual, the southern jet stream hits California carrying moisture and storms. Californians experienced this from January through March of this year with higher than normal rainfall – a rather welcome effect for drought stricken California, yet far from enough to normalize the water levels in dams and reservoirs.
In Europe, the El Niño effect is less obvious although the winters are typically colder and drier in Northern Europe, and milder and more wet in Southern Europe. The European summer weather pattern in an El Niño year is often debated, but there seems to be a correlation of wetter and cooler summers in Northern Europe during strong events of El Niño.
The El Niño effects, lasting a season or so, are fairly well known among the public but what is less commonly known is that the cooling phase of ENSO, La Niña, often lasts longer - up to several years.
Strong El Niño anomaly years are often followed by 2 or 3 years of La Niña type anomaly. Hence, La Niña, arguably, has more significant consequences for the most critical prevailing effect of weather, the water supply for public consumption and agriculture irrigation.
As we transition from an El Niño phase to a La Niña phase of ENSO, the world’s leading tropical weather forecast centers are forecasting an about average tropical weather outlook for the North Atlantic and the North Pacific hurricane seasons. This is based on long term forecast analysis using coupled ocean/atmosphere/land/sea-ice models where interaction between the most significant contributors of global weather pattern creation is simulated.
Until next time, stay weather aware and weather prepared!
Karl Eggestad, Global Sales Director for Metacast, ChyronHego