June 30th marked the end of a record-calm year for tropical cyclone activity in the Southern Hemisphere. A cyclone “year†extends from July 1 to June 30 south of the equator (this is because cyclones peak in the warm season, which straddles adjacent calendar years in the Southern Hemisphere). For the 2016-17 year, this entire half of the world racked up just 45% of the accumulated cyclone energy(ACE) that it sees in a typical year, based on the climatological period 1981-2010. ACE incorporates a tropical cyclone’s lifespan as well as its peak winds.

“It’s pretty remarkable that an entire hemisphere can have less than half of their normal ACE in a season,†noted Phil Klotzbach (Colorado State University) in an email. The previous record-low Southern Hemisphere ACE for a July-to-June year was 54% of average, in 2008-09.

The large-scale rising motion needed for sustained tropical cyclone action was often in the wrong place during the southern summer of 2016-17. The strongest areas of rising motion across the southern tropics and subtropics were located over land, across southern Africa and western Australia. Meanwhile, downward motion predominated at these latitudes across the South Indian and Southwest Pacific Oceans, where most of the Southern Hemisphere’s tropical cyclones develop. “A good way to kill a tropical cyclone season is to have upward motion over land!†noted Klotzbach.

Below are the Southern Hemisphere stats for 2016-17, together with the numbers recorded in a typical year. The data comes from Klotzbach’s real-time roundup page for global tropical cyclone activity.

Index	2016-17	July-June avg (2016-17)	% of long-term average recorded in 2016-17
Named storms	19	27	70%
Named storm days	63	125	50%
Hurricanes	9	14	64%
Hurricane days	21	45	47%
Major hurricanes	4	7	57%
Major hurricane days	4	14	29%
ACE	96	211	45%

Tranquil north and south—except for the Atlantic

While the Southern Hemisphere has been exceptionally quiet when it comes to tropical cyclones, the Northern Hemisphere hasn’t exactly been raging with activity thus far in 2017. Together, this has given the planet a paltry amount of ACE in recent months. Typically, the Northern Hemisphere sees 59.8 units of ACE in the first six months of a calendar year. But the January-to-June total for 2017 is just 14.2 units—a mere 24% of average. The typical Northern Hemisphere tropical cyclone lasts five days, but this year’s longest-lived has been Hurricane Dora in the East Pacific, which only lasted 2.75 days as a named storm.

The only one of the six basins tracked by Klotzbach that’s been having a busier-than-average season is the North Atlantic. Here, ACE is running at a peppy 215% of average. But we’re talking tiny numbers: The North Atlantic has seen a grand total of 2.8 units of ACE, compared to an average to date of 1.3. Although there have already been three named storms in the North Atlantic, none of them became hurricanes. For that matter, there has yet to be a single typhoon (a hurricane-strength cyclone) in the Northwest Pacific, whereas a typical year has already seen 8 “typhoon days†by this point.

For the climatological period 1981-2010, here is the average annual global ACE for each basin, as calculated from Klotzbach’s datasets. The Southern Hemisphere statistics apply to the July-to-June span preceding each calendar year.

Annual global average:  771.6
Northwest Pacific:  305.1  (40% of global average)
South Indian:  138.4  (18%)
Central/East Pacific:  131.8  (17%)
North Atlantic:  105.6  (14%)
South Pacific:  71.6  (9%)
North Indian:  19.1  (2%)

Figure 1. Visible MODIS image of Cyclone Enawo taken at 1024Z (5:24 am EST) on March 7, 2017. At the time, Enawo had just made landfall over northeast Madagascar as a Category 4 storm with 145 mph winds. Enawo was one of the strongest tropical cyclones in the Southern Hemisphere during 2016-17, along with Cyclone Donna, which affected parts of New Caledonia and Vanuatu in the Southwest Pacific in early May 2017. Image credit: NASA.

Why the quietude?

On average, the planet produces about 75-100 named storms and about 40-60 hurricane-strength cyclones per year. Those ranges are more durable than you might expect, in part because of the “balancing†effects of the El Niño-Southern Oscillation (ENSO). A strong El Niño event will tend to favor hurricanes in the Central and Northeast Pacific while inhibiting them in the Atlantic; the opposite is true for a strong La Niña event.

ENSO has little year-to-year effect on the total number of named storms in the Northwest Pacific—but it does have an influence on ACE in this key region. El Niño years tend to produce stronger, longer-lived typhoons, which in turn helps boost global ACE for those years. Since 1970, some years have seen less than 500 units of global ACE, while others have seen more than 1000 units. (These annual ACE statistics, compiled by Ryan Maue of WeatherBell, combine each July-to-June SH year with the following January-to-December NH year.)

Global ACE peaked at near-record levels in 2015, in sync with the record-strong 2014-16 El Niño event. At the same time, the amount of heat held in the planet’s oceans dipped in 2015. This was a departure from the oceanic heat accumulation that’s predominated since 1970, as human-produced greenhouse gases trap more and more heat. It’s also a sign that large amounts of heat and moisture escaped the oceans in 2015, some of it helping to fuel that year’s crop of intense tropical cyclones. “It is not unexpected that there is a quiet period subsequently, as a recovery and recharge of ocean heat content takes place,†said global water and energy cycle expert Kevin Trenberth (National Center for Atmospheric Research) in an email.

How the heat and moisture get transported

On a yearly average, the tropics accumulate far more heat than the poles, and tropical cyclones are one of the mechanisms of heat transfer from lower to higher latitudes. But there are other ways this energy can get transferred, Trenberth pointed out. An active midlatitude jet stream can pull enormous amounts of tropical heat and moisture poleward, often in the form of atmospheric rivers. The U.S. West Coast was hammered by soggy storms in the past winter, leading to California rainfall that averaged 124% to 178% of normal by region for the water year ending June 30, as summarized by Jan Null (Golden Gate Weather Services).

Heat and moisture can also escape tropical oceans via monsoonal lows, which don’t qualify as tropical cyclones but which can still spread extreme, prolonged rains into subtropical regions.  When such lows move over Australia, they are sometimes dubbed “landphoons,†and they were unusually prevalent in 2016-17, closing Uluṟu-Kata Tjuṯa National Park on Christmas Day and giving the Northern Territory 148% of its average wet-season rainfall. Extreme rainfall also inundated parts of Southern Africa in January and large sections of Southeast Asia, including Thailand and the Philippines, in January and February. Such events remind us that while ACE is a valuable tool for assessing tropical cyclones, there is much more to the grand cycles that shuttle heat and water around our planet.

For those in the U.S. who are starting a long holiday weekend, Happy Fourth! We’ll be back with a new post on Monday 7/3.

Figure 2. A family navigates floodwaters after heavy rains trigger flooding on January 2, 2017, in the Sungai Kolok district of southern Thailand’s Narathiwat province. Image credit: Madaree Tohlala/AFP/Getty Images.