|Above: A satellite image from late Saturday, Feb. 18, shows a well-structured Cyclone Kelvin just inland across northwest Australia (left) and a still-impressive Tropical Cyclone Gita (right) churning across the Southwest Pacific en route to New Zealand. Image credit: UK Met Office, @metofficestorms.|
New Zealand is in line for very stormy conditions on Tuesday night local time, as Tropical Cyclone Gita arcs its way toward the South Island of NZ. The most recent forecast from the New Zealand Met Service (3Z Monday) called for Gita to become post-tropical before it reaches the north end of the South Island. In its forecast issued at 12Z Monday, the Joint Typhoon Warning Center also predicted Gita would be post-tropical by landfall, with top sustained winds of 45 knots (52 mph). At 15Z Monday (10:00 am EST), Gita was located roughly 500 miles northwest of Wellington, NZ, heading southeast at 23 mph.
Even as a post-tropical cyclone, Gita will slam New Zealand with a variety of impacts. Waves may top 18 feet in some areas, according to the NZ Met Service, and wind gusts could top 90 mph at higher elevations of the northern South Island. The city known as “Windy Wellington” will live up to its moniker, with gusts possibly reaching 80 mph on Tuesday night. Rainfall intensities could approach 2”/hour, modest by U.S. standards but quite high for New Zealand. On Monday local time, Gita generated swells of 14-16 feet along the Gold Coast of east central Australia, closing all of the region’s famed beaches.
Tall order for a tropical cyclone
Most of New Zealand lies poleward of 35°S latitude (roughly the same distance from the equator as North Carolina), so it’s difficult for a tropical cyclone to maintain its identity as it approaches the island nation. Gita has been remarkably resilient so far—still a symmetric warm-core cyclone in phase space diagrams produced by Florida State University at 6Z Monday.
As of 12Z Monday, Gita was traversing sea surface temperatures (SSTs) around 25°C (77°F)—normally too cool to sustain tropical development—and was experiencing fierce wind shear of greater than 40 knots. A strong outflow channel at upper levels has kept Gita going as a tropical cyclone despite these negative factors. Even cooler waters and stronger shear lie ahead, though, heralding a rapid transition to post-tropical status over the next 24 hours.
The website Force Thirteen posted this map of New Zealand landfalls related to tropical cyclones.
|Figure 1. Radar images of Tropical Cyclone Kelvin taken before landfall (left) and after landfall (right) showed that intensification of the storm took place after landfall. Image credit: Australian Bureau of Meteorology (BOM).|
Tropical Cyclone Kelvin hits Australia, then intensifies over land
An unusual meteorological event played out in northwest Australia over the weekend, when Category 1 Tropical Cyclone Kelvin made landfall then intensified after landfall. Visible and infrared satellite loops showed that the eye grew more distinct for a few hours after landfall, and the eyewall clouds surrounding the eye expanded in areal coverage. Radar loops also showed an increased in the storm’s organization after landfall, with the low-level spiral bands becoming more organized. According to the Joint Typhoon Warning Center, Kelvin reached its peak intensity of 90 mph when the storm was centered about ten miles inland at 0Z Sunday, February 18. The storm then began to rapidly weaken about three hours later, as it pushed farther inland. Kelvin was rapidly intensifying as it approached landfall, but nevertheless, we normally expect tropical cyclones to quickly weaken once their center moves over land. This occurs because the storm is then cut off from the source of heat and moisture that powers it—the warm ocean waters.
Actually, it turns out that Kelvin’s behavior is not that unusual over northern Australia, and there have been a number of tropical cyclones that have intensified over land in this part of the world. A 2008 paper published in Monthly Weather Review by hurricane scientists Kerry Emanuel, Jeff Callaghan, and Peter Otto, A Hypothesis for the Redevelopment of Warm-Core Cyclones over Northern Australia, studied the issue using a simple tropical cyclone model coupled to a one-dimensional soil model, and came up with a plausible theory for these storms, which they dubbed “agukabams”. Northern Australia has a deep layer of very hot, sandy soil, and when this soil is wetted by the first rains of an approaching cyclone, the land provides a ready source of heat and moisture to the landfalling storm, allowing it to intensify for a few hours. The computer simulations showed that once the storms get sufficiently isolated from their oceanic source of moisture, the rainfall they produce is insufficient to keep the soil wet enough to transfer significant quantities of heat, and the storms then decay rapidly.
We asked Clark Evans, Associate Professor of the Atmospheric Science Program at the University of Wisconsin-Milwaukee, to comment on Kelvin’s case, and he had this to say:
Kelvin is an interesting case. It doesn't seem to fit perfectly into the class of "agukabams" that Kerry Emanuel, Jeff Callaghan, and Peter Otto wrote about in 2008 in Monthly Weather Review. In general, those storms intensify further inland, once rainfall and the associated upper soil cooling it fosters have abated slightly. Here, Kelvin intensified during and after landfall. Despite this, however, I suspect that the underlying physical mechanism supporting Kelvin's intensification is similar to that posed by Kerry and his coauthors. Unfortunately, I don't have access to observed soil temperature and moisture data for Australia just prior to Kelvin's landfall to dig deeper into this suspicion, though.
Having unfettered inflow of warm, moist air from the Indian Ocean was likely important as well, similar to my 2011 work on the overland intensification of the remnants of Tropical Storm Erin over the U.S. southern Great Plains with Russ Schumacher and Tom Galarneau. Kerry and his coauthors thought about this in their 2008 paper as well, but the simplified modeling framework they used in their study did not account for it. Thus, the two processes would seem to both be necessary but not sufficient conditions, but a full-physics modeling study would be needed to parse out the underlying physics.
A garden-variety season gets more active
Thus far, 2017-18 hasn’t been an especially noteworthy season for tropical cyclones in the Southern Hemisphere. Real-time statistics on accumulated cyclone energy (ACE) maintained by Phil Klotzbach (Colorado State University) show that the South Indian Ocean basin west of 135°E longitude, which includes cyclones affecting Western Australia, was running close to average for seasonal ACE, while the South Pacific east of 135°E has seen less than 80% of its average ACE.
La Niña conditions, which have been in place during the current Southern Hemisphere tropical cyclone season, tend to produce warmer SSTs and reduced wind shear in the waters along the west side of Australia, both of which favor tropical cyclone activity. About twice as many cyclones make landfall in Australia during La Niña than during El Niño, according to the nation’s Bureau of Meteorology. Significant flooding in Australia is also more likely when La Niña holds sway. The effects of El Niño and La Niña are more complex across New Zealand, though La Niña years tend to be warmer than average.
Bob Henson co-wrote this post.