Sharp warming of the tropical North Atlantic in August

Strong Positive Pacific Meridional Mode Causes Central Tropical Pacific Warming
08/24/2018, 7:06 am EDT
European Model: 3 Tropical Events North Atlantic Next Wednesday
08/29/2018, 1:36 pm EDT
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The tropical/subtropical central and east North Atlantic basin has warmed dramatically in August. The Madden Julian oscillation convective phase is forecast by NCEP to engage this warmer surface water producing increased thunderstorm activity that can lead to tropical waves and tropical cyclones early-to-middle September. As the climatological peak (Sep. 10) of the season approaches the North Atlantic tropics will become active.

Discussion: Entering the tropical cyclone season the outer North Atlantic tropics were unusually cool forcing forecasters to lower seasonal hurricane activity forecasts due to limited upper ocean heat to fuel stronger storms. Interestingly, the past 30 days has brought dramatic warming to the tropical and subtropical North Atlantic central and east basin. The basin temperature average is now +0.56C (Fig. 1) with nearly all of the warming (to above normal) occurring during the past 30 days (Fig. 2).

In the Main Development Region for North Atlantic hurricanes represented by the region east of the Caribbean Sea to the northwest coast of Africa where the tropical North Atlantic (TNA) index is measured the basin average is now a marginally warm +0.36C (Fig. 3) after a 30-day change of a whopping +1.32C warm-up (Fig. 4).

In early September the convection phase of the Madden Julian oscillation (MJO) is forecast to shift to the North Atlantic/Africa tropics (Fig. 5) enhancing vertical currents causing an expanse of the intra-tropical convergence zone (ITCZ). The evolution of this pattern will inspire North Atlantic tropical cyclone activity as the middle third of September approaches.

Fig. 1:  North Atlantic basin sea surface temperature anomaly observations.

Fig. 2:  North Atlantic basin SSTA 30-day change observations.

Fig. 3:  SSTA across the Main Development Region for North Atlantic hurricanes is now marginally warmer than normal.

Fig. 4:  The 30-day change of SSTA in the MDR is SUBSTANTIALLY warmer.

Fig. 5:  NCEP 14-day MJO forecast indicates MJO shifts to the tropical North Atlantic to Africa in early-to-middle September likely enhancing North Atlantic tropical cyclone risk.