Northern Hemisphere Tropical Cyclone ACE Index 75% Of Normal

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The 2021 northern hemisphere accumulated cyclone energy index (ACE) is about 75% of normal so far in 2021. Only the North Atlantic basin ACE index is above normal at 79.2 (29% above normal). Interestingly (and rare), both the northeast and northwest Pacific basins have observed below normal ACE index for the tropical cyclone season so far in 2021.

Fig. 1: Accumulated cyclone energy index for the season for the North Atlantic, Northeast Pacific, Northwest Pacific and northern hemisphere according to Colorado State University data.

Discussion: The 2021 northern hemisphere accumulated cyclone energy index (ACE) is about 75% of normal so far in 2021. Only the North Atlantic basin ACE index is above normal at 79.2 (29% above normal). Interestingly (and rare), both the northeast and northwest Pacific basins have observed below normal ACE index for the tropical cyclone season so far in 2021. Most striking is only 4 hurricanes and 1 major hurricane in the northwest Pacific where 9.8 and 4.7 are normal (respectively). There are likely many contributors to the under-performing intensity of the northern hemisphere tropical cyclone activity in 2021 so far. One unusual possibility as a contributor is the negative phase of both the Pacific and Atlantic Meridional Mode (-PMM and -AMM) which indicate cross-equatorial flow of trade winds from the northern to the southern hemisphere lowering the potential upper ocean heat of the deep tropics of North Atlantic and especially the Pacific basin (although this pattern may be reversing in September).