Mid August is fast approaching and so is the middle part of the Pacific Tropical Cyclone Season. Contrary to what was originally forecasted, some surprising developments have taken place for the Eastern and Western Pacific with regards to their respective tropical forecasts and levels of activity. This blog will cover some of the current sea surface temperatures (SSTs) patterns and their effect on tropical cyclone activity, forecast teleconnections, and what has been observed across the entire basin thus far.
The sea surface temperature (SST) anomalies in the equatorial Pacific this season are having a big impact on the frequency of Tropical Cyclones, most notably in the Eastern Pacific. El Nino Southern Oscillation (ENSO) depicts a Central Pacific based La Nina. The SST anomaly presentation, as of August 6th, shows that warmer than normal SSTs persist from near 120W Longitude, eastward to Central America and NW South America. Near the date line (180W) and points to the west show much cooler than normal SSTs. This placement of a mid-ocean cold pool sandwiched between two areas of anomalously warmer waters are commonplace for second year La Nina’s like the one occurring this Summer. Keeping the Central Pacific based La Nina in mind, the Central Pacific Hurricane Season has been very quiet, with only Hurricane Darby crossing West of 140W longitude into the basin this year.
FIG 1: NOAA Coral Reef 5km SST Anomaly map of Pacific basin, as of August 6th (Source: NOAA office of satellite and product operations)
The North West Pacific basin has yielded a total of 13 Tropical Depressions, 5 Tropical Storms, and 2 Typhoons, through August 16th. As of August 16th, 2022 Accumulated Cyclone Energy still remains well below the climatological average for the year so far. Accumulated Cyclone Energy or (ACE) is calculated for each individual tropical cyclone based on the system’s duration and overall intensity. Adding up the ACE values for each named system that develops in a given basin gives a good idea of the overall level of activity for that basin’s entire tropical cyclone season. Weather models continue to depict drier conditions (less tropical development) in the Central South China Sea and east of the Philippines, with wetter conditions, (increase in tropical development) expected in the Northern South China Sea, East China Sea, and ENE of Luzon, over the next several weeks.
FIG 2: North West Pacific Basin observed ACE based on only JTWC named systems through August 16th, 2022 (Source: Colorado State University Department of Atmospheric Science | Tropical Meteorology Project)
FIG 3: Seasonal Forecast Tropical Storm Density Anomaly : June – November 2022 (Source: ECMWF Long Range Forecast – SEAS5)
The Eastern Pacific’s active Hurricane season this year has defied some climatological variables that would otherwise indicate a below average tropical season, resulting in some surprising observations. Through August 16th, 10 Tropical Depressions, 10 Tropical Storms, 7 Hurricanes, and 2 Major Hurricanes have formed. The Pacific Decadal Oscillation or (PDO) has consistently remained negative all year. A negative PDO is defined as warmer than normal sea surface temperatures prevailing across the NW portion of the basin, while cooler than normal sea surface temperatures prevail across the eastern portion of the basin and near the equatorial regions. Such a SST signature, typically found in La Nina years just like this year across the basin, would historically result in less tropical development for the region, however this is not the case in 2022 due to several factors. Number one, despite the presence of the negative PDO, waters have been anomalously warm across the entirety of the basin, outside of the La Nina itself. Very warm SSTs persist across the Western half of the basin while the Eastern half is slightly cooler.
FIG 4: PDO value by month from Jan 2022 through July 2022.(Source: National Centers for Environmental Information | via NOAA)
Number two, warmer than normal SSTs persist from near 120W longitude eastward to the Mexican and South American coasts. The anomalous warmth in sea surface temperatures acts as fuel for showers and thunderstorms to flourish and additionally enhances rainfall and thunderstorm activity along the Intertropical Convergence Zone (ITCZ) located in this general area during the summer. Number three, the area associated with the pocket of warmer water near Mexico and South America has helped to weaken some of the easterly trade winds that are normally experienced during a La Nina and negative PDO season. Number four, this weakening of the easterly winds has allowed the ITCZ to move more poleward (further north) and has allowed for thunderstorm complexes to become more organised and develop into tropical cyclones with increased frequency. These factors have resulted in an ACE total for the Eastern Pacific of 80.8 units compared to the climatological average of 60.4 units, or 133% above average. Moral of the story, it’s been an above average season.
FIG 5: North East Pacific Basin observed ACE and total named systems from NHC through August 16th, 2022 (Source: Colorado State University Department of Atmospheric Science | Tropical Meteorology Project)
In conclusion, tropical development has remained below average for the Western Pacific with more of a westward bias in development across the basin for the South China Sea and East China Sea while areas east of the Philippines remain less active. The East has been characterised by more frequent development of tropical cyclones and higher ACE. This increase has ultimately been attributed to the very warm SSTs near and just off Mexico, Central America, and Northwest South America; leading to a decrease in windshear, a more active ITZC, and thus more thunderstorm activity fueling Tropical Cyclone formation. The North West Pacific has met the expectations of the pre-season outlook, while the Eastern Pacific has been unexpectedly active compared to the preseason forecast, despite an ongoing ENSO La Nina pattern. The 2022 season has held some surprises so far and more are likely to come.
Stay connected and safe.