La Nina Stays Intact While Cooling North Atlantic Tropics is a Surprise

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A steady resilient La Nina continues in the equatorial East Pacific as defined by the Nino34 SSTA index. Interestingly, the tropical North Atlantic has also turned cooler recently including a negative phase tropical North Atlantic index which is rare in the past 20 years. These SSTA trends are an important diagnostic to hurricane season forecasts issued later this month/early April.

Multivariate ENSO IndexIndexRank
JAN/FEB 2018-0.73115 of 69 where 1 = Strongest La Nina and 69 = Strongest El Nino

 Discussion: The initial set of seasonal tropical cyclone outlooks for the North Atlantic and East Pacific in 2018 will be released later in March and early April by various providers. Governing the activity forecasts is the Pacific ENSO and upper ocean heat in the Atlantic.

According to the multivariate ENSO index (MEI) for JAN/FEB 2018 the 2017-18 La Nina is peaking in strength albeit a weak La Nina compared to other episodes since 1950. The Nino34 SSTA has stubbornly remained moderately supportive of La Nina for several months (despite warming of the upper oceans of the East Pacific since January).

La Nina is forecast to fade this spring but for the moment is hanging on and there are some models maintaining La Nina well into the tropical cyclone season. The weak La Nina potential scenario continuing supports a very active hurricane season in the North Atlantic.

Meanwhile sudden cooling occurred in the equatorial Atlantic the last 1-2 months and the tropical North Atlantic index dipped to -0.2C during February. The negative TNA value is rare during the warm character of the Atlantic the last 20 years. The Atlantic multi-decadal oscillation (AMO) representing a basin-wide index of North Atlantic SSTA has also cooled into neutral phase. A cooler tropical North Atlantic would limit fuel for hurricane development.

Forecast models indicate the North Atlantic will re-warm for tropical cyclone season. However, the recent cool trend was not forecast by most global SSTA models.

Based on current diagnostics the upper air patterns are projected supportive of above normal hurricanes in the North Atlantic during tropical cyclone season. However, the tropical North Atlantic has cooled and will need to re-warm to allow the ENSO hurricane support to flourish.

Climate Impact Company issues the initial 2018 North Atlantic tropical season outlook by March 23rd.

Fig. 1: The NINO34 SSTA identifies a steady cool La Nina in the equatorial East Pacific while North Atlantic SSTA indices are also cooling especially the tropical North Atlantic index which measures SSTA in the main development region for hurricanes in the North Atlantic tropics in-between the Caribbean and northwest coast of Africa.

Fig. 2: Global SSTA observations for yesterday identify a lingering La Nina and sudden cooling in the equatorial Atlantic.