Discussion: Two of three TOP providers (Colorado State University Tropical Meteorology Project and Tropical Storm Risk U.K.) of seasonal tropical cyclone activity for the North Atlantic issued their initial forecasts last week (Table 1). NOAA/NHC issues their forecast in May. Colorado State University updates their forecast in early June while Tropical Storm Risk U.K. updates their forecast monthly. Climate Impact Company issued an analog forecast last week also updated in early June. All providers update their seasonal forecasts around August 1st when the most active part of the season begins. The official start of the North Atlantic tropical cyclone season is June 1st.
|Tropical Storms||Hurricanes||Intense Hurricanes||Accumulated cyclone energy|
Table 1: Seasonal forecasts of North Atlantic tropical cyclone activity for 2018 by CSU, TSR and CIC compared the 30-year normal and last year.
The initial forecasts for 2018 each agree that 2018 will not be as active as 2018. CSU is most aggressive, TSR least aggressive and CIC in-the-middle with forecasts near the 30-year normal. Of particular interest, and not commonly discussed is the accumulated cyclone energy (ACE) forecast which measures storm intensity at below normal levels for 2018 by TSR and CIC while CSU is above normal. Ultimately, the ACE index is the best single parameter to describe the intensity of a given season. Last year was 226 which is 3rd highest on record.
Of course it takes just 1 hurricane to hit a high population area to cause havoc which ca occur somewhat independent of the seasonal activity. However, the (likely) greatly diminished ACE index forecast for 2018 compared to last year is a good indicator that the risk of a Harvey-like disaster is not eliminated but lowered considerably.
The lead climate diagnostic for the less active than 2017 forecast is the cool nature of the North Atlantic tropics as mid-spring approaches (Fig. 1). Tropical waters were much warmer last year at this time (Fig. 2) and a leading indicator of seasonal tropical cyclone activity, the Atlantic multi-decadal oscillation (AMO) and tropical North Atlantic (TNA) Index which were collectively near record warmth last year have cooled considerably during quarter 1 of 2018. The early year cooling is typical but there are questions on how much recovery occurs by summertime.
Fig. 1: Last week of March global sea surface temperature anomalies. The North Atlantic tropics are near to cooler than normal.
Fig. 2: April 2017 global SSTA as provided by ECMWF indicate a general warming of the North Atlantic basin.
Important is the consideration of the AMO and TNA index in seasonal forecasts of tropical cyclone activity in the North Atlantic. ENSO is part of the forecast process, but not the only part.
AMO and TNA monthly index…
Leading global sea surface temperature anomaly (SSTA) forecasts warm the North Atlantic for summer, possibly too warm BUT leave the tropical North Atlantic comparatively cooler (Fig. 3). The tropics are where most tropical cyclones initiate and implied is that upper ocean and surface heat is near normal and not nearly as warm as last year.
Fig. 3: Global SSTA forecast for JUL/AUG/SEP 2018 by NMME model. Note lack of any warmth in the North Atlantic tropics.
Another indicator of a diminished 2018 tropical cyclone year outside of ENSO and AMO/TNA is the Bureau of Meteorology/Australia forecast of the negative phase of the Indian Ocean Dipole (-IOD). During this condition the western tropical Indian Ocean is cooler than normal. Many tropical waves entering the tropical North Atlantic after crossing tropical Africa acquire their initial intensity based on how warm the surface water is in the source region (western IO). If the western IO waters are cooler than normal Africa tropical waves are likely diminished in intensity and less likely to become hurricanes in the North Atlantic.