FORECAST OF ATLANTIC HURRICANE ACTIVITY FOR OCTOBER-NOVEMBER 2006 AND SEASONAL UPDATE THROUGH SEPTEMBER

 

We have experienced average hurricane activity through September.  August was inactive, but September had above-average activity.  We expect October to have below-average activity largely due to developing El Niño conditions in the central and eastern Pacific.  November activity in El Niño years is very rare.

 

(as of 3 October 2006)

 

 

 

By Philip J. Klotzbach[1] and William M. Gray[2]

 

with special assistance from William Thorson[3]

 

 

 

This forecast as well as past forecasts and verifications are available via the World Wide Web at http://hurricane.atmos.colostate.edu/Forecasts


Emily Wilmsen, Colorado State University Media Representative, (970-491-6432) is available to answer various questions about this forecast

 

 

 

Department of Atmospheric Science

Colorado State University

Fort Collins, CO 80523

Email: amie@atmos.colostate.edu


[1] Research Associate

[2] Professor Emeritus of Atmospheric Science

[3] Research Associate

 

ATLANTIC BASIN SEASONAL HURRICANE FORECAST FOR 2006

 

 

Full Season Tropical Cyclone Parameters and their 1950-2000 Climatology (in parentheses)

 

Full Season Adjusted 3 Aug. 2006 Forecast

Full Season Adjusted 1 Sep. 2006 Forecast

 

Observed Activity Through September

 

Updated Oct.-Nov. Forecast

 

Full Season Adjusted 3 Oct. 2006 Forecast

Named Storms (NS) (9.6)

15

13

9

2

11

Named Storm Days (NSD) (49.1)

75

50

48

10

58

Hurricanes (H) (5.9)

7

5

5

1

6

Hurricane Days (HD) (24.5)

35

13

18.50

4

23

Intense Hurricanes (IH) (2.3)

3

2

2

0

2

Intense Hurricane Days (IHD) (5.0)

8

4

3

0

3

Net Tropical Cyclone Activity (NTC) (100%)

140

90

83

12

95

 

 

 

 

Notice of Author Changes

 

By William Gray

The order of the authorship of these forecasts has been reversed from Gray and Klotzbach to Klotzbach and Gray.  After 22 years (since 1984) of making these forecasts, it is appropriate that I step back and have Phil Klotzbach assume the primary responsibility for our project’s seasonal, monthly and landfall probability forecasts.  Phil has been a member of my research project for the last six years and has been second author on these forecasts for the last five years.  I have greatly profited and enjoyed our close personal and working relationships.

 

Phil is now devoting more time to the improvement of these forecasts than I am.  I am now giving more of my efforts to the global warming issue and in synthesizing my projects’ many years of hurricane and typhoon studies.

 

Phil Klotzbach is an outstanding young scientist with a superb academic record.  I have been amazed at how far he has come in his knowledge of hurricane prediction since joining my project six years ago.  I foresee an outstanding future for him in the hurricane field.  I expect he will make many new forecast innovations and skill improvements in the coming years.  I plan to continue to be closely involved in the issuing of these forecasts for the next few years. 



ABSTRACT

 

Information obtained through 30 September 2006 shows that we have so far experienced an average Atlantic basin hurricane season.  August had substantially below-average activity (only 45% of average) while September had above-average activity (about 140% of average).  US landfall has been well below average.  No hurricanes have made landfall along the US coastline this year.  Eighty-three percent of the average full season Net Tropical Cyclone (NTC) activity has occurred so far this year.  In an average year, 80 percent of the seasonal average NTC of 100 occurs by the end of September.

Our October-only forecast calls for 2 named storms, 1 hurricane, 0 major hurricanes and NTC activity of 12 which is below the October-only average value of 18.  We forecast no tropical cyclone activity in November. Our below-average prediction for October-November activity is largely due to the rapid emergence of an El Niño event during the latter part of this summer.

Atlantic basin tropical cyclone (TC) activity for the 2006 season will be considerably less than the seasonal activity we anticipated in our earlier forecasts issued in early December, early April, late May and early August.  We judge this reduced seasonal activity to be due to mid-level dryness in the tropical Atlantic (with large amounts of African dust) which greatly reduced August activity and to the rapid late summer development of an El Niño event which we and nearly all ENSO forecasts did not anticipate.   


VERIFICATION OF SEPTEMBER MONTHLY FORECAST

 

CSU forecast and verification of September-only hurricane activity issued on 2 September.

 

 

 

Tropical Cyclone Parameters and 1950-2000 September Average (in parentheses)

 

September 2006

Statistical Forecast

 

Adjusted September 2006 Forecast

 

September 2006 Verification

Named Storms (NS) (3.4)

3.4

5

4

Named Storm Days (NSD) (21.7)

17.2

20

30.50

Hurricanes (H) (2.4)

3.2

3

4

Hurricane Days (HD) (12.3)

5.5

10

18.25

Intense Hurricanes (IH) (1.3)

1.7

2

2

Intense Hurricane Days (IHD) (3.0)

2.5

4

3

Net Tropical Cyclone Activity (NTC) (48.0)

45.4

59

66

 

Our September 2006 forecast verified quite well.  Even though conditions in August were not favorable for Atlantic basin tropical cyclone activity, we predicted that they would likely become more favorable for hurricane development in September, and this prediction verified quite well.  Dry air continued to predominate across the tropical Atlantic in September; however, vertical wind shear was below average for most of the month.  Four named storms (Florence, Gordon, Helene and Isaac) formed during September, and all four of these storms became hurricanes.  Gordon and Helene became major hurricanes.  A more in-depth discussion of our September-only forecast is in section 5.



1        Introduction

 

Our Colorado State University research project has shown that a sizable portion of the year-to-year variability of Atlantic tropical cyclone (TC) activity can be hindcast with skill significantly exceeding climatology. These forecasts are based on a statistical methodology derived from 55 years of past global reanalysis data and a separate study of prior analog years which have had similar global atmosphere and ocean precursor circulation features. Qualitative adjustments are added to accommodate additional processes which may not be explicitly represented by our statistical analyses.

 

2        2006 Atlantic Basin Activity through September

 

As of the end of September, the 2006 hurricane season has had 83 percent of the NTC activity of the average hurricane season. June-July 2006 had approximately average activity while August had greatly reduced (approximately 45%) TC activity compared with the typical August.  September activity was somewhat above the long-term average with 4 named storms, 4 hurricanes and 2 major hurricanes forming during the month.  As of 1 October, a total of 9 named storms, 5 hurricanes and 2 major (Cat. 3-4-5) hurricanes have developed this season.  Through September, the climatological (1950-2000) average number of named storms, hurricanes and major hurricanes is 7.5, 4.5, and 2.1, respectively. Through September 2006, the Atlantic basin has thus witnessed 120, 111, and 95 percent of average named storm, hurricane, and major hurricane activity, respectively.  Table 1 shows observed Atlantic basin tropical cyclone activity by storm, and Figure 1 displays Atlantic basin tracks for the 2006 season. 

 

Table 1: Observed 2006 Atlantic basin tropical cyclone activity through September.

 

 

Highest

Category

 

 

Name

 

 

Dates

 

Peak Sustained Winds (kts)/lowest SLP (mb)

 

 

NSD

 

 

HD

 

 

IHD

 

 

NTC

TS

Alberto

June 11-14

60 kt/995 mb

2.75

 

 

2.7

TS

Beryl

July 19-21

50 kt/1001 mb

2.75

 

 

2.7

TS

Chris

Aug. 1-4

55 kt/1001 mb

3.25

 

 

2.8

TS

Debby

Aug. 23-26

45 kt/1000 mb

3.25

 

 

2.8

H-1

Ernesto

Aug. 25–Sep. 1

65 kt/988 mb

6.00

0.25

 

6.8

H-1

Florence

Sep. 5-12

80 kt/972 mb

7.50

2.75

 

9.0

IH-3

Gordon

Sep. 8-20

105 kt/955 mb

9.25

7.50

1.25

24.2

IH-3

Helene

Sep. 14-24

110 kt/954 mb

10.75

7.50

1.75

26.4

H-1

Isaac

Sep. 28-Present

70 kt/989 mb

2.50

0.50

 

5.7

Totals

9

 

 

48.00

18.50

3.00

83.1

 

 

 

Figure 1:  2006 Atlantic basin hurricane tracks through September.  Figure courtesy of Weather Underground (http://www.weatherunderground.com/).

 

 

3        Predictions of Individual Monthly Atlantic TC Activity

A new aspect of our climate research is the development of TC activity predictions for individual months. There are often monthly periods within active and inactive Atlantic basin hurricane seasons which do not conform to the overall season. For example, 1961 was an active hurricane season (NTC of 222), but there was no TC activity during August; 1995 had 19 named storms, but only one named storm developed during a 30-day period during the peak of the hurricane season between 29 August and 27 September. By contrast, the inactive season of 1941 had only six named storms (average 9.3), but four of them developed during September. During the inactive 1968 hurricane season, three of the eight named storms formed in June (June average is 0.5).

We have conducted new research to see how well various sub-season or individual monthly trends of TC activity can be forecast. This effort has recently been documented in papers by Blake and Gray (2004) for August and Klotzbach and Gray (2003) for September. These reports show that it is possible to develop skillful prediction schemes for August-only and September-only Atlantic basin tropical cyclone activity. We have also developed a separate October forecast scheme. On average, August, September, and October have about 26%, 48%, and 17% or 91% of the Atlantic basin's NTC activity. Initial August-only forecasts have now been made by Blake for the last seven years (2000-2006), and the verification of these forecasts is promising, despite this year’s significant over-forecast. The verification of the September-only and October-only forecasts also appears to show skill.

3.1       Independent October-only Forecast

 

We had previously developed a statistical forecast of October-only activity; however, we did not have an explicit ENSO predictor in this scheme, and therefore we have discontinued its use for this year.  Our research over the past couple of years indicates that ENSO is of critical importance in determining October and November activity. We intend to develop a new statistical forecast by next summer for October activity that takes into account El Niño conditions.  This forecast will likely be developed using a similar methodology to what was used for our new August seasonal statistical forecast that uses predictors obtained from surface data.  Using only surface data, we can develop the scheme on data extending back to 1900.  Our new October forecast scheme will be developed on 1949-2005 and then tested on 1900-1948.  A forecast scheme that shows skill over 106 years is likely to be quite robust. 

 It is expected that the currently rapidly developing El Niño in the central and eastern Pacific will likely bring an early end to the Atlantic basin hurricane season.  El Niño conditions typically cause a greater suppression of the latter part of the hurricane season than they do in the earlier portion.  Table 2 displays average (1950-2000) October TC activity compared with years when the August-September Nino 1+2 and Nino 3 indices average greater than 0.5ºC above average.  For our October forecast calculation, this will be our definition of an El Niño.  Based on correlation data from 1949-2005, October Atlantic basin hurricane activity is more affected by SSTs in the eastern Pacific than SSTs in the central Pacific.

Table 2 shows that there is typically a marked reduction in October Atlantic hurricane activity in El Niño years.  Table 3 gives probabilities of percentage reductions in October TC activity in El Niño years.  The probabilities for one or more intense hurricanes, two or more hurricanes and an October NTC value of 20 or greater drops considerably when SSTs in the east-central Pacific (Nino 3 temperatures) are greater than 0.5ºC above normal.  These probabilities drop even more when SSTs are above normal in the eastern Pacific (Nino 1+2 temperatures greater than 0.5ºC above normal).  We estimate that the Nino 1+2 and Nino 3 indices for August-September will average around 0.7ºC above average.  Note the large reduction in October activity, especially for intense hurricane and intense hurricane days in El Niño years.  No intense (or major) hurricanes have been observed to form after 1 October in El Niño years since 1950.

 

Table 2: Average October Atlantic basin tropical cyclone activity compared with October Atlantic basin tropical cyclone activity in El Niño years (defined as the August-September Nino 1+2 index and the August-September Nino 3 index averaging greater than 0.5ºC above normal).

 

TC

Parameter

Average October (1950-2000)

El Niño

October

 

Ratio (El Niño/Average)

NS

1.6

1.3

81%

NSD

8.9

6.5

73%

H

1.1

0.7

64%

HD

4.4

2.8

64%

IH

0.4

0.0

0%

IHD

0.9

0.5

56%

NTC

17.5

9.9

57%

 

Table 3: Probabilities of one or more intense hurricanes, two or more hurricanes and an NTC of over 20 for various October ENSO conditions. 

 

 

All Years

Nino 3 > 0.5ºC

Nino 1+2 > 0.5ºC and Nino 3 > 0.5ºC

Prob. 1 or more IH

30% (17 out of 56)

14% (2 out of 14)

0% (0 out of 12)

Prob. 2 or more H

27% (15 out of 56)

14% (2 out of 14)

8% (1 out of 12)

Prob. NTC > 20

38% (21 out of 56)

21% (3 out of 14)

17% (2 out of 12)

 

 

We have basically kept our October forecast the same as we forecast in early September.  The number of hurricane days has been increased from 3 to 4 due to Hurricane Isaac which will likely accrue 1-2 hurricane days in early October before dissipating.  We continue to call for below-average October activity.  We foresee an NTC of about 70 percent of the typical October value.  In round numbers, we are forecasting 2 named storms, 1 hurricane, 0 intense hurricanes and an NTC of 12 for October. 

 

Table 4 displays a summary of this year’s hurricane activity through September and our projection for the rest of the season.  We expect October activity to be below the October climatological average.  We predict no activity in November. 

 

Table 4:  Summary of hurricane activity through September 2006 and projected hurricane activity for the remainder of the year. 

 

Tropical Cyclone Parameters and 1950-2000 Full Season Climatology (in parentheses)

Observed TC Activity Through September

Updated October-November Forecast

 

Updated Full Season Forecast

Named Storms (NS) (9.6)

9

2

11

Named Storm Days (NSD) (49.1)

48

10

58

Hurricanes (H) (5.9)

5

1

6

Hurricane Days (HD) (24.5)

18.50

4

23

Intense Hurricanes (IH) (2.3)

2

0

2

Intense Hurricane Days (IHD)

3

0

3

Net Tropical Cyclone Activity (NTC) (100%)

83

12

95

 

3.2       U.S. Landfall Probability Forecast for October-November

 

We have recently developed a methodology for calculating the probability of hurricane landfall along the entire U.S. coastline for the month of October.  Based on a 1950-2001 dataset, we have calculated the probability of various intensity classes of tropical cyclones making landfall in October.  These forecast probabilities are based on our October NTC forecast values.  Since we are not predicting any activity for November, the probability for October-November will be the same as it is for October.  For our remainder of the season prediction of an NTC value of 12, our United States landfall probabilities are given in Table 5.  Landfall probabilities for the U.S. are below average for this October based on a below-average forecast of Atlantic basin NTC.

 

Table 5:  Estimated probability (expressed in percent) of one or more U.S. landfalling tropical storms (TS), category 1-2 hurricanes (HUR) and category 3-4-5 hurricanes (IH) making landfall along the entire U.S. coastline for October-November 2006 based on an October-November NTC forecast of 12.  The long-term mean October probability of one or more landfalling systems during the last 52 years is given in parentheses. 

 

Storm Category

October 2006 Probability

Named Storm (29%)

22%

Hurricane (15%)

14%

Intense Hurricane (6%)

4%

 

3.4       November 2006 Activity

 

            Tropical cyclone activity in November is not very frequent.  Since 1950, 41% of years have witnessed the development of one or more named storms in November, while only 27% of years have witnessed the development of one or more hurricanes.  Only four years since 1950 (7%) have had a major hurricane develop in November.  The probability of named storm, hurricane and major hurricane development becomes even less likely in El Niño years.  For our calculations, we define an El Niño year to be a year where August-September Nino 3 temperatures are 0.5ºC or greater above the long-period average, years where August-September Nino 3 temperatures are 0.5ºC or less below the long-period average are La Niña years, and all other years are classified as neutral.   Using this classification, 14 years since 1950 are defined as El Niño years, 22 years are defined as La Niña years, and the other 20 years are classified as neutral.  Table 6 displays the probability of development of a named storm, hurricane and major hurricane for all years, La Niña years, neutral years and El Niño years, respectively.  Note the large decrease in probability for November tropical cyclone formation with increasing August-September Nino 3 anomalies.  We estimate that the August-September average Nino 3 value for 2006 will be approximately 0.7ºC above average which fits our definition of an El Niño year, and therefore we think that development of tropical cyclones in November is unlikely.  It should also be noted that none of the ten years with Nino 3 anomalies of 0.7ºC or greater witnessed named storm formation in November. 

 

Table 6: Probability of November development of a named storm, hurricane and major hurricane for all years, La Niña years, neutral years, and El Niño years, respectively.

 

 

All Years

La Niña Years

Neutral Years

El Niño Years

Prob. 1 or more NS

41% (23 out of 56)

57% (12 out of 21)

45% (9 out of 20)

14% (2 out of 14)

Prob. 1 or more H

27% (15 out of 56)

38% (8 out of 21)

30% (6 out of 20)

7% (1 out of 14)

Prob. 1 or more IH

7% (4 out of 56)

14% (3 out of 21)

5% (1 out of 20)

0% (0 out of 14)

 

4          Seasonal and Monthly Prediction Summary

      Table 7 displays a summary of this year’s hurricane activity through September and our projection for the rest of the season.  We expect activity in October-November to be below average. 

Table 7: Summary of hurricane activity through September 2006 and projected hurricane activity for the remainder of the year

 

 

 

Tropical Cyclone Parameters and 1950-2000 Full Season Climatology (in parentheses)

 

Observed TC Activity through September

 

Updated

 October-November

 Forecast

 

Updated

 Full Season

 Forecast

Named Storms (NS) (9.6)

9

2

11

Named Storm Days (NSD) (49.1)

48

10

58

Hurricanes (H) (5.9)

5

1

6

Hurricane Days (HD) (24.5)

18.50

4

23

Intense Hurricanes (IH) (2.3)

2

0

2

Intense Hurricane Days (IHD) (5.0)

3

0

3

Net Tropical Cyclone Activity (NTC) (100)

83

12

95

 

 

5        Discussion

 

5.1       Early Season (June-August) Discussion

 

June-July 2006 had about average activity with two named storms forming during the two-month period (Alberto and Beryl).  The June-July long-period (1950-2000) average is approximately 1.5 named storm formations and 0.6 hurricane formations.  Unlike 2005 when we witnessed two major hurricanes (Dennis and Emily) develop and intensify in the tropical Atlantic and Caribbean, we did not see any activity in the deep tropics during June and July 2006.  August 2006 had about average named storm activity, but the amount of hurricane and intense hurricane activity was well below average.  Only one hurricane formed during August (Ernesto), and it lasted less than one day due to interaction with Caribbean basin landmasses.  Unexpected subsidence, dry air and mid-level dust across the tropical Atlantic during the month of August are believed to be the primary contributors to the surprisingly inactive August.  This subsidence is believed to be a result of the development of weak El Niño conditions in the eastern and central equatorial Pacific.  See our 1 September forecast discussion for a more in-depth discussion of August 2006. 

 

5.2              September Discussion

 

September had above-average activity when evaluated by the NTC metric.  This represents the ninth consecutive September that has had above normal NTC activity.  September 2006 accrued 66 NTC units, which is somewhat more than the 1950-2000 average of 48.  Although El Niño conditions continued to develop in the central and eastern Pacific, vertical wind shear in the tropical Atlantic was actually somewhat below normal.  Figure 2 shows vertical wind shear across the tropical Atlantic (0-20ºN, 20-60ºW) for the 2006 Atlantic basin hurricane season.  Note that values in September 2006 were generally below the long-term average.   Atlantic basin sea surface temperatures also remained above average throughout the month.  We think that the likely inhibiting factor that kept September from being a very active month was the continued predominance of dry air in the tropical Atlantic.  Figure 3 displays water vapor brightness temperatures across the tropical Atlantic (0-20ºN, 20-60ºW).  Note that brightness temperatures remained above average (i.e. less moisture) throughout most of the month of September, as they had in August.  The continued dominance of subsidence across the tropical Atlantic may in part be due to a shift in the Walker Circulation associated with the developing El Niño.

We consider our September monthly forecast to be a success.  We predicted that despite an inactive early season, we would see above-average activity in September, and this is what occurred.  Our forecast predicted that three hurricanes and two major hurricanes would develop during September, and four hurricanes and two major hurricanes formed.

 

 

Figure 2:  Vertical wind shear (850-200 mb) across the tropical Atlantic (0-20ºN, 20-60ºW) from June-September.  Note that vertical wind shear values in September have generally been below the long-period average.  Figure courtesy of the Cooperative Institute for Research in the Atmosphere (CIRA) from the Tropical Cyclone Formation Probability Product (DeMaria et al. 2001). 

 

 

Figure 3:  Water vapor brightness temperature across the tropical Atlantic (0-20ºN, 20-60ºW) from January-September.  Note that brightness temperatures remained above average for most of September.  Brightness temperatures are a proxy for mid-level moisture, with cooler temperatures indicating more moisture.  Figure courtesy of the Cooperative Institute for Research in the Atmosphere (CIRA) from the Tropical Cyclone Formation Probability Product (DeMaria et al. 2001). 

 

 

5.3              ENSO Discussion

 

One of the extraordinary features of the 2006 Atlantic basin hurricane season has been the rapid onset of El Niño conditions in the tropical Pacific.  The warming of the eastern and central Pacific during the summer of 2006 has been truly remarkable.   Only 1997 witnessed a larger increase in Nino 3 anomalies from June-July to August-September than has the 2006 season.  But 1997 June-July Nino 3 anomalies were already well above average while 2006 June-July anomalies were not. This is by far the largest warming of SST anomalies in the tropical Pacific for a year that was not already in a substantial El Niño or La Niña event.  August-September 2006 sea surface temperatures in Nino 3 have warmed by approximately 0.6ºC from their June-July values.  The increase was 0.9ºC during 1997; however, 1997 was already a strong El Niño event by the beginning of the summer.  The largest increase in SST anomalies in Nino 3 during this same time period for neutral ENSO conditions at the start of June was 1979 which witnessed an increase of 0.4ºC during the summer.  Table 8 displays June-July and August-September Nino 3 values for the ten years with the largest warming between these two sets of months. 

 

Table 8: June-July Nino 3 temperatures, August-September Nino 3 temperatures and the change in temperature from June-July to August-September for the ten years with the largest warming between these two periods. 

 

 

Year

 

June-July Nino 3 (ºC)

August-September Nino 3 (ºC)

August-September – June-July Nino 3 (ΔT) (ºC)

1997

2.1

3.0

0.9

1988

-1.9

-1.4

0.5

1972

1.2

1.7

0.5

1982

1.0

1.4

0.4

1979

0.1

0.5

0.4

1952

-0.6

-0.2

0.4

2004

-0.1

0.2

0.3

1984

-0.8

-0.5

0.3

1976

0.6

0.9

0.3

 

 

 

 

2006

0.1

0.7

0.6

 

 

The rapid warming of SSTs in the eastern and central Pacific was not forecast well by the statistical and dynamical ENSO models.  Using ENSO prediction information obtained from the monthly technical ENSO updates provided by the International Research Institute for Climate and Society (IRI) shows that an average of the 12 dynamical and 8 statistical models did not predict this year’s rapid warming between June-July and August-September.  These models predict sea surface temperatures in Nino 3.4 for three month increments (e.g. July-August-September).  We estimate that Nino 3.4 for July-August-September (JAS) of 2006 will be approximately 0.6ºC.  Table 8 displays the statistical model consensus, the dynamical model consensus, and the combined model consensus for predicted Nino 3.4 temperatures for July-August-September 2006 for various lead times from January-July.  Note that the rapid warming was not predicted by the model consensus.  The inability to predict this fast-developing ENSO event certainly made our forecast of the 2006 Atlantic basin hurricane season more difficult. 

 

Table 8: Predicted July-August-September 2006 Nino 3.4 values for the consensus of dynamical models, statistical models and all models from various lead times.  Model output information was obtained from IRI.

 

 

 

 

Date

 

 

 

Dynamical Model JAS Nino 3.4 Prediction (ºC)

 

 

 

Statistical Model JAS Nino 3.4 Prediction (ºC)

 

 

All Model Nino 3.4 JAS Prediction (ºC)

19 January

0.2

0.1

0.1

15 February

0.1

-0.1

0.0

15 March

0.1

0.0

0.1

18 April

0.1

0.1

0.1

17 May

0.1

0.1

0.1

13 June

0.2

0.1

0.2

19 July

0.4

0.2

0.3

 

 

 

 

Observed

 

 

0.6

 

By 1 August, one can usually make a good extrapolated El Niño forecast for the August-October period.  However, this was not the case this year. 

The typical onset of east Pacific warm SST anomalies associated with El Niño conditions occurs between spring and early summer.  The usual El Niño can be detected by June-July.  However, this year’s July Nino 3 SST anomaly was only 0.22ºC and showed little increase from May and June.  But, there was an unexpected and surprisingly strong Nino 3 warming during August-September of approximately 0.6ºC.  There is now a weak to moderate El Niño event.  This year’s late El Niño event is similar to the late onset of 1986, but this year’s warming from July to September is considerably stronger than 1986.  Looking back at the historical records of El Niño onset events for the 20th century, this season appears to be about the strongest two-month warming from July to September.

 It is difficult to attribute this sudden warming to one particular cause, but we believe that intense, long-lived Hurricane Ioke which developed in the central Pacific on August 20 and tracked slowly westward across the central and eastern Pacific was an important contributor to the dramatic transition from neutral to El Niño conditions.  Ioke caused strong westerly anomalies to develop at low latitudes near the dateline.  These westerlies drove the warm anomalies in the western and central Pacific eastward. 

 

5.4              El Nino’s Influence on Tropical Atlantic Predictors

 

During the period of August-September 2006, the tropical Atlantic basin manifested some conditions typical of an El Niño year.  El Niño years typically have the following tropical Atlantic conditions: 

 

1)      stronger than normal 200 mb (~12 km) zonal winds (positive U)

2)      dryer middle tropospheric moisture conditions (negative q – specific humidity)

3)      somewhat higher than average sea level pressure anomalies (positive SLPA)

4)      somewhat higher than average sea surface temperature anomalies (positive SSTA)

 

Table 9 shows the standard deviation (SD) of the average Atlantic basin U, q, SLPA and SSTA values for the 10 highest August-September Nino 3 SSTAs versus the 10 lowest August-September Nino 3 SSTAs of the last 56 years and compares these with 2006 August and September values.

 

Table 9: Difference of August-September tropical Atlantic basin parameter standard deviation (SD) values for the 10 warmest minus the 10 coldest Nino 3 SSTA values between 1950-2005.  Data from August-September 2006 is given for comparison

 

 

Tropical Atlantic Parameter

SD of the Average of the 10 Warmest – 10 Coldest August-September Pacific Nino 3 SSTA

 

2006

 

 

August

September

August

September

ΔU (5-20ºN, 10-90ºW)

+1.3

+1.5

+0.5

-0.8

Δ 600 mb q (5ºS-25ºN, 10-90ºW)

-0.6

-0.3

-0.4

-0.5

ΔSLPA (5-25ºN, 10-70ºW)

+1.0

+0.6

-0.5

-0.3

ΔSSTA (5-25ºN, 10-70ºW)

+0.2

+0.4

+1.6

+1.8

 

 

 

 

 

ENSO Parameter Difference:

-ΔU + Δq – ΔSLPA + ΔSSTA

-3.1

-2.8

-2.0

-1.2

 

There are some El Niño or La Niña months or seasons when the 200 mb zonal wind is the dominant impact, while at other times it is the mid-level moisture content or surface pressure which appears to be more dominant. For example, in August-September of 2006, zonal wind anomalies and sea level pressure anomalies were near their long-term averages.  It appears that subsidence and dry middle levels in the tropical Atlantic were the likely causes of diminished activity in August 2006. 

There also are periods when the tropical Atlantic does not respond as strongly to the Nino 3 sea surface temperature anomalies as it does in other periods.  It is well-known that the global response to Pacific equatorial SSTA changes can vary between different years and different global locations. 

 

6        Forthcoming 2006 Verification and Initial Forecast of 2007 Hurricane Activity

 

A verification and discussion of all 2006 forecasts will be issued on Friday 17 November 2006.  Our first seasonal hurricane forecast for the 2007 hurricane season will be issued in early December 2006.  All of these forecasts will be made available on the web at: http://hurricane.atmos.colostate.edu/Forecasts.