NOAA's 2015 Eastern Pacific Hurricane Season Outlook indicates that an above-normal season is most likely, with a 70% chance of an above-normal season, a 25% chance of a near-normal season, and only a 5% chance of a below normal season.
The main climate factor expected to enhance this season is El Niño, which is now present and is predicted to last throughout the hurricane season.
Predictions from NOAA's Climate Forecast System (CFS), NOAA Geophysical Fluid Dynamics Lab (GFDL) model FLOR-FA, the European Centre for Medium Range Weather Forecasting (ECMWF), and the EUROpean Seasonal to Inter-annual Prediction (EUROSIP) ensemble are all suggesting above-normal tropical cyclone activity in the eastern Pacific this season, though their forecast skill for the region is limited at this lead time.
An important measure of the total seasonal activity is NOAA's Accumulated Cyclone Energy (ACE) index, which accounts for the combined strength and duration of tropical storms and hurricanes during the season.
NOAA estimates a 70% chance that the 2015 seasonal ACE index will be in the range of 110% – 190% of the median.
El Niño decreases the vertical wind shear over the eastern tropical Pacific, which favors more and stronger tropical storms and hurricanes and this is already affecting the wind and rainfall patterns across the equatorial and subtropical Pacific Ocean.
Also, the sea surface temperatures patterns that have been associated with decreased hurricane activity in the eastern Pacific since 1995 are not expected during the 2015 season, and are therefore not expected to compete with El Niño's conducive wind patterns.
For the 2015 hurricane season, NOAA estimates a 70% chance of occurrence for each of the following ranges of activity:
- 15-22 named storms,
- 7-12 hurricanes,
- 5-8 major hurricanes,
- An ACE range 110%-190% of the median.
Bars depict number of named systems (open/yellow), hurricanes (hatched/green), and category 3 or greater (solid/red), 1886-2004. Image credit: NOAA.
Science behind the outlook
NOAA's 2015 seasonal hurricane outlook reflects the persistence and possible strengthening of the current El Niño as the season progresses.
At present, SSTs are above average across the eastern half of the equatorial Pacific, and the largest departures exceed +1.0oC. SST anomalies averaged across the Niño 3.4 region (which spans the east-central equatorial Pacific between 120oW-170oW) are currently +1.0oC. This value is at CPC's lower threshold for a moderate-strength El Niño. The Niño 3.4 region is a main region used by the CPC to help monitor and assess the strength of El Niño.
The current sub-surface thermal structure also reflects El Niño, with above-average temperatures present from the surface to 150m depth between the date line and the west coast of South America. This anomalous warmth was initially linked to a downwelling equatorial oceanic Kelvin wave that was triggered in February. Unlike previous downwelling Kelvin waves over the past twelve months, this most recent wave was associated with a strong ocean-atmosphere coupling that led to El Niño.
Additionally, anomalous westerly trade winds (i.e. lighter easterly trade winds) have been present since March across the central and east-central equatorial Pacific Ocean, contributing to the warmer SSTs and to the enhanced convection.
Additional periods with exceptionally weak trade winds could further strengthen El Niño this hurricane season. One such period is currently occurring in association with a westerly wind burst over the western equatorial Pacific.
The average forecast (orange line) of the dynamical models (closed markers) in the suite of IRI/ CPC Niño 3.4 SST forecasts suggests that El Niño could become a strong event (Niño 3.4 SST anomalies ≥ 1.5oC) as the hurricane season progresses.
Some models (such as the CFS Hi-Res model and the ECMWF) are predicting an exceptionally strong El Niño while others are predicting a moderate-strength event (Niño 3.4 SST anomalies between 1.0-1.5oC). The statistical model forecasts (open markers) are generally cooler than the dynamical model predictions, and on average (green line) show a borderline moderate-strength El Niño during the hurricane season. These differing forecasts, combined with overall lower predictive skill during March-May, produce uncertainty as to exactly how strong El Niño will become.
Global sea surface temperatures: differences from the previous multi-decadal signal
The current and predicted global SST patterns, along with the dominant multi-decadal signals since 1995, are shown.
The observed March-April 2015 SST anomaly pattern (top left) reflects 1) a projection of the Atlantic SST anomalies onto the cold phase of the AMO, and 2) a horseshoe-shaped pattern of below-average SSTs (blue arc) in the western Pacific. NOAA's CFS high-resolution model (Top Right) predicts these two patterns to persist through the hurricane season. These conditions differ from the global patterns of SST anomalies observed during much of the preceding 1995-2014 period of reduced activity in the eastern Pacific hurricane basin (Bottom).
This year, NOAA does not expect the low-activity era SST anomaly patterns to develop, which means that they do not expect the recent multi-decadal signal to offset El Niño's conducive wind patterns. Instead, the predicted SST anomaly patterns this season are more typical of the 1981-1994 high-activity era for eastern Pacific hurricanes. During that period, 62% of the hurricane seasons were above normal, 23% were near normal, and only 15% were below normal. Seasons during the period averaged about 18 named storms, 10.5 hurricanes, and 5 major hurricanes, with an ACE value of 150% of the median. This average value falls within NOAA's definition for an above-normal season, elevated activity is comparable to the predicted 2015 ranges.
In contrast, eastern Pacific hurricane seasons have been less active since 1995.
All North Atlantic and Eastern North Pacific major hurricanes (at least Category 3 on the Saffir-Simpson Hurricane Scale).
During 1995-2014, 40% of the seasons were below normal, 35% were near normal, and only 25% were above normal. Seasons during this period averaged about 13 named storms, 7 hurricanes, and 3 major hurricanes, with an ACE value of 88% of the median.
Despite this recent low-activity era, the eastern Pacific did have an above-normal hurricane season last year.
If the 2015 season is also above normal, it would mark the first time since 1997-98 that two consecutive seasons were above normal.
Tropical Storm "Andres" on May 28, 2015. Image credit: NASA Terra/MODIS.
The first named storm of the 2015 season – Tropical Storm "Andres" – formed on May 28, 2015 about 1 100 km (690 miles) southwest of Manzanillo, Mexico. Andres is expected to intensify and become a hurricane on May 29, far off the coast of Mexico.
The season officially started on May 15, and will end on November 30.
Data and analysis source: NOAA 2015 Eastern Pacific Hurricane Season Outlook. Issued May 27, 2015.
Featured image credit: NASA/NOAA GOES Project.
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