1. Predicting El Niño and La Niña (also called ENSO) impacts is an ongoing scientific challenge facing climate scientists today. Such forecasts made during the spring generally have limited skill.
2. Many combinations of named storms and hurricanes can occur for the same general set of climate conditions. For example, one cannot know with certainty whether a given climate signal will be associated with several short-lived storms or fewer longer-lived storms with greater intensity.
3. Weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months, possibly affecting seasonal hurricane activity.

2013 Eastern Pacific Hurricane Outlook Summary

NOAA’s 2013 eastern Pacific Hurricane Season Outlook indicates a 55% chance of a below-normal season, a 35% chance of a near-normal season, and a 10% chance of an above normal season. See NOAA definitions of above, near-, and below- normal seasons. The eastern Pacific hurricane region covers the eastern North Pacific Ocean east of 140^oW north of the equator.

This outlook is based on the analysis and prediction of three climate signals:

• The ongoing climate conditions that have been associated with reduced eastern Pacific hurricane activity since 1995,
• Expected ENSO-neutral conditions, meaning El Niño is not expected to develop and strengthen the seasonal activity, and
• Expected near-average or below-average sea-surface temperatures in the eastern equatorial Pacific Ocean.

Historically, these conditions tend to suppress eastern Pacific hurricane activity. We estimate a 70% chance of occurrence for each of the following ranges of activity this season:

• 11-16 named storms,
• 5-8 hurricanes,
• 1-4 major hurricanes,
• An ACE range 60%-105% of the median.

The seasonal activity is expected to fall within these ranges in 7 out of 10 seasons with similar climate conditions and uncertainties to those expected this year. They do not represent the total possible ranges of activity seen in past similar years.

Note that the predicted ranges are centered below the official NHC 1981-2010 seasonal averages of 15 named storms, 8 hurricanes, and 4 major hurricanes.

There will be no further updates to this outlook.

DISCUSSION

1. Expected 2013 Activity

This Outlook is a general guide to the expected overall activity for the 2013 eastern Pacific hurricane season. It is not a seasonal hurricane landfall forecast, and it does not imply levels of activity for any particular location.

Known climate signals and evolving oceanic and atmospheric conditions, combined with dynamical model forecasts, indicate that a below-normal 2013 eastern Pacific hurricane season is most likely. This outlook calls for a 55% chance of a below-normal season, a 35% chance of a near-normal season, and a 10% chance of an above-normal season.

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. We estimate a 70% chance that the 2013 seasonal ACE index will be in the range of 60%-105% of the median. According to NOAA’s definitions of season strength, an ACE value below 80% of the 1981-2010 median indicates a below normal season, and a value of 80%-115% of the median indicates a near-normal season.

Consistent with the expected ACE range, other likely (70% chance) ranges of activity for 2013 are 11-16 named storms, 5-8 hurricanes, and 1-4 major hurricanes.

This outlook is consistent with climate model forecasts. Predictions from NOAA’s Climate Forecast System (CFS), the European Centre for Medium Range Weather Forecasting (ECMWF), and the EUROpean Seasonal to Inter-annual Prediction (EUROSIP) ensemble are all suggesting below-normal or near-normal tropical cyclone activity in the eastern Pacific this season, though their forecast skill for the region is limited at this lead time.

2. Science behind the outlook

This outlook is based on the analysis and prediction of three climate signals. The first is a continuation of conditions that have been suppressing eastern Pacific hurricane activity since 1995. The second is a likely continuation of ENSO-neutral conditions through the season. The third is an expectation for near- or below-average sea-surface temperatures in the Niño-3 region of the eastern tropical Pacific Ocean. SSTs in this region are an additional predictor for eastern Pacific hurricane activity, especially in the presence of ENSO-neutral conditions.

a. Expected continuation of low-activity hurricane era in the eastern Pacific

East Pacific hurricane seasons have been less active since 1995, while the Atlantic Basin has experienced increased activity. During 1995-2012, 39% of eastern Pacific hurricane seasons were below normal, 39% were near normal, and only 22% 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.

Associated with the climate conditions that have contributed to this reduction in eastern Pacific hurricane activity, the Pacific Inter-Tropical Convergence Zone (ITCZ) has been shifted farther north, allowing for extensive southwesterly flow into the eastern tropical North Pacific. Also, the upper-level ridge over Mexico has been stronger than average, resulting in enhanced upper-level easterly winds. This combination of factors leads to increased vertical wind shear, which typically limits the number, intensity, and duration of tropical storms and hurricanes. Also, these storms tend to form closer to Mexico and closer to cooler ocean temperatures, both of which limit their duration.

The levels of hurricane activity since 1995 in the eastern Pacific (and the Atlantic basin) contrast sharply with those of the preceding period 1982-1994. The eastern Pacific was much more active during this earlier period, while the Atlantic basin was more suppressed. For the eastern Pacific, 62% of hurricane seasons during 1982-1994 were above normal, 23% were near normal, and only 15% were below normal. Seasons during this 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.

b. ENSO-neutral conditions

The ENSO (El Niño/ Southern Oscillation) is an important climate predictor for eastern Pacific hurricane activity. The three phases of ENSO are El Niño, La Niña, and Neutral. El Niño acts to reduce the vertical wind shear, and is therefore more conducive to eastern Pacific hurricane activity. Conversely, La Niña increases the wind shear and usually suppresses the activity. These typical impacts can be strongly modulated by conditions associated with the tropical multi-decadal signal.

SSTs in the eastern equatorial Pacific (i.e. the Niño-3 region) are also used to predict seasonal eastern Pacific hurricane activity, especially in the presence of ENSO-neutral conditions.

ENSO-neutral conditions have been present since last summer. Currently, equatorial Pacific SSTs are near average and the Niño 3.4 index is slightly below zero. The equatorial Pacific sub-surface temperatures and oceanic heat content are also near average.

Most models contained in the suite of IRI/ CPC Niño 3.4 SST forecasts predict ENSO-neutral conditions to continue through ASO, with the statistical model forecasts being generally cooler than the dynamical model forecasts. The CFS T-382 high-resolution model predicts La Niña for ASO, which is consistent with its forecast for increased vertical wind shear across the eastern tropical Pacific.

The observations, ENSO model forecasts, the official CPC/IRI ENSO forecast issued in early May, all suggest that ENSO-Neutral conditions are likely to continue through the summer and fall (55% chance). El Niño is not expected to develop this summer and enhanced the seasonal activity.

NOAA FORECASTERS

Climate Prediction Center
Dr. Gerald Bell, Meteorologist, Gerry.Bell@noaa.gov
Dr. Jae Schemm, Meteorologist, Jae.Schemm@noaa.gov