Through the later part of August, the Northern
Hemisphere has had much more tropical cyclone (TC) activity than any
other season on record (reasonably reliable global records date back to
the early 1970s). We often evaluate TC activity through the use of the
Accumulated Cyclone Energy (ACE) metric. This metric effectively
combines frequency and intensity into one number to give you an idea of
how active a season has been. Through August 24, we have seen much more
Northern Hemisphere ACE than any other year since 1971. The primary
driver of this record setting ACE has been the Northwest Pacific which
has seen nearly three times as much ACE to date as normal!
Figure 1. Northern Hemisphere TC Activity Through August 24.
Figure 2: NW Pacific ACE through August 24.
Northern Hemisphere activity is usually divided into four basins: the North Atlantic, the Northeast Pacific, the Northwest Pacific and the North Indian Ocean. The breakdown between the Northeast and Northwest Pacific Oceans is usually done at the International Date Line (180°). Very few storms cross between basins, although crossover storms do occur, especially between the Northeast and Northwest Pacific (like Halola did earlier this year). A typical breakdown of the globe into its six TC basins is provided below:
Figure 3. Global Tropical Cyclone Basins.
One of the big questions is: Why have we seen so much TC activity this year around the globe? While we have discussed in an earlier blog about the detrimental impacts of El Niño on Atlantic hurricane activity, it actually enhances TC activity in both the Northeast and Northwest Pacific basins, primarily through alterations in vertical wind shear (change in wind direction with height) patterns. Too much shearing in the vertical tears apart TCs that are attempting to form. As a reminder, El Niño is characterized by anomalously warm sea surface temperatures (SSTs) in the tropical eastern and central Pacific. The strong El Niño in 2015 is clearly seen by looking at a current SST anomaly map.
Figure 4. Current SST anomaly map across the tropical and subtropical Pacific.
The remainder of this blog briefly looks at a couple of additional Northern Hemisphere records that have been set so far, while the next part will look specifically at the record setting activity that has occurred in the Northwest Pacific. An additional blog will focus on the North Central Pacific (the portion of the Northeast Pacific basin from 180-140°W). A final blog will then look at some of the physical drivers of this variability.
One of the most impressive statistics of the 2015 Northern Hemisphere TC season has been the intensity of the tropical cyclones that have formed. Already, we have had eight typhoons reach Cat. 4-5 intensity in the Northwest Pacific (>130 mph winds), and four hurricanes have reached this same threshold in the Northeast Pacific. The combined 12 Cat.4-5s through August 24 is four more than any other season since 1971.
Figure 5. Northern Hemisphere Cat. 4-5 Hurricanes Forming by August 24.
There are certainly data issues early in the record, especially prior to the mid-1980s when satellite intensity estimates of tropical cyclones became standardized across the globe, but regardless, the number has been quite impressive. In addition, the percentage of hurricanes reaching Cat. 4-5 intensity has also been quite impressive. 63% of all hurricanes have reached that threshold this year, which puts it behind only 2007 (64%) for TCs reaching that threshold by this date.
Figure 6. Percentage of Northern Hemisphere Hurricanes Reaching Cat. 4-5 Intensity by August 24.
Chris Landsea, a forecaster and researcher at the National Hurricane Center, and I have recently had a paper accepted for publication in Journal of Climate where we show a stabilization in the number of Cat. 4-5 hurricanes globally since 1990 (when satellite data is likely most reliable). It will be interesting to see if the large increase in 2015 is just a blip on the radar or the start of an increasing trend.
The next part of this blog will look in more detail at Northwest Pacific TC activity. I hope to post this blog later this week.
Figure 1. Northern Hemisphere TC Activity Through August 24.
Figure 2: NW Pacific ACE through August 24.
Northern Hemisphere activity is usually divided into four basins: the North Atlantic, the Northeast Pacific, the Northwest Pacific and the North Indian Ocean. The breakdown between the Northeast and Northwest Pacific Oceans is usually done at the International Date Line (180°). Very few storms cross between basins, although crossover storms do occur, especially between the Northeast and Northwest Pacific (like Halola did earlier this year). A typical breakdown of the globe into its six TC basins is provided below:
Figure 3. Global Tropical Cyclone Basins.
One of the big questions is: Why have we seen so much TC activity this year around the globe? While we have discussed in an earlier blog about the detrimental impacts of El Niño on Atlantic hurricane activity, it actually enhances TC activity in both the Northeast and Northwest Pacific basins, primarily through alterations in vertical wind shear (change in wind direction with height) patterns. Too much shearing in the vertical tears apart TCs that are attempting to form. As a reminder, El Niño is characterized by anomalously warm sea surface temperatures (SSTs) in the tropical eastern and central Pacific. The strong El Niño in 2015 is clearly seen by looking at a current SST anomaly map.
Figure 4. Current SST anomaly map across the tropical and subtropical Pacific.
The remainder of this blog briefly looks at a couple of additional Northern Hemisphere records that have been set so far, while the next part will look specifically at the record setting activity that has occurred in the Northwest Pacific. An additional blog will focus on the North Central Pacific (the portion of the Northeast Pacific basin from 180-140°W). A final blog will then look at some of the physical drivers of this variability.
One of the most impressive statistics of the 2015 Northern Hemisphere TC season has been the intensity of the tropical cyclones that have formed. Already, we have had eight typhoons reach Cat. 4-5 intensity in the Northwest Pacific (>130 mph winds), and four hurricanes have reached this same threshold in the Northeast Pacific. The combined 12 Cat.4-5s through August 24 is four more than any other season since 1971.
Figure 5. Northern Hemisphere Cat. 4-5 Hurricanes Forming by August 24.
There are certainly data issues early in the record, especially prior to the mid-1980s when satellite intensity estimates of tropical cyclones became standardized across the globe, but regardless, the number has been quite impressive. In addition, the percentage of hurricanes reaching Cat. 4-5 intensity has also been quite impressive. 63% of all hurricanes have reached that threshold this year, which puts it behind only 2007 (64%) for TCs reaching that threshold by this date.
Figure 6. Percentage of Northern Hemisphere Hurricanes Reaching Cat. 4-5 Intensity by August 24.
Chris Landsea, a forecaster and researcher at the National Hurricane Center, and I have recently had a paper accepted for publication in Journal of Climate where we show a stabilization in the number of Cat. 4-5 hurricanes globally since 1990 (when satellite data is likely most reliable). It will be interesting to see if the large increase in 2015 is just a blip on the radar or the start of an increasing trend.
The next part of this blog will look in more detail at Northwest Pacific TC activity. I hope to post this blog later this week.
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