Christopher W. Landsea
NOAA Climate and Global Change Fellowship,
NOAA/AOML/Hurricane Research
Division, Miami, Florida
Gerald D. Bell
NOAA/NWS/NCEP/Climate Prediction Center, Washington, D.C.
William M. Gray
Department of Atmospheric Science, Colorado State University,
Fort Collins, Colorado
Stanley B. Goldenberg
NOAA/AOML/Hurricane Research Division, Miami, Florida
(Manuscript received 3 September 1996, in final form 18 March 1997)
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The 1995 Atlantic hurricane season was a year of near-record hurricane
activity with a total of 19 named storms (average is 9.3 for the base period
1950-1990) and 11 hurricanes (average is 5.8), which persisted for a total of 121 named storm days
(average is 46.6) and 60 hurricane days (average is 23.9), respectively.
There were 5 intense (or major) Saffir-Simpson category 3, 4 or 5
hurricanes (average is 2.3 intense hurricanes) with 11.75 intense hurricane
days (average is 4.7). The net tropical cyclone activity, based upon the combined values of named storms, hurricanes, intense hurricanes and their days present, was 229 percent of the
average.
Additionally, 1995 saw the return of hurricane activity to the deep
tropical latitudes:
7 hurricanes developed south of 25°
N (excluding all of the Gulf
of Mexico) compared with just 1 during all of 1991-1994.
Interestingly, all seven storms that formed south of 20øN in
August and September recurved to the northeast without making
landfall in the United States.
The sharply increased hurricane activity during 1995 is attributed to
the
juxtaposition of virtually all of the large scale features over the
tropical North Atlantic that
favor tropical cyclogenesis and development.
These include extremely low vertical wind shear, below normal
sea level pressure, abnormally warm ocean waters, higher than
average amounts of total precipitable water and a strong
west phase of the stratospheric quasi-biennial oscillation.
These various environmental factors were in strong contrast to those
of the very unfavorable conditions that accompanied the extremely quiet
1994 hurricane season.
The favorable conditions for the 1995 hurricane season
began to develop as far back
as late in the previous Winter. Their onset well ahead of the start of
the
hurricane season indicates that they are a cause of the increased
hurricane
activity, and not an effect. The extreme duration of the atmospheric
circulation anomalies over the tropical North Atlantic is
partly attributed
to a transition in the equatorial Pacific from warm episode conditions
(El Niño) to cold episode
conditions (La Niño) prior to the onset of the hurricane season.
Though the season as a whole was extremely active, 1995's Atlantic
tropical cyclogenesis showed a strong intraseasonal variability with
above normal storm frequency during August and October and below normal
for September.
This variability is likely attributed to changes in the
upper tropospheric circulation across the tropical North Atlantic,
which resulted in a return to near normal vertical shear during
September. Another contributing factor to the reduction
in tropical cyclogenesis during September
may have been a temporary return to near normal SSTs across
the tropical and subtropical North Atlantic, caused by
the enhanced tropical cyclone activity during August.
Seasonal hurricane forecasts for 1995 issued at Colorado State
University on 30 November 1994, 5 June 1995 and 4 August 1995
correctly anticipated an above-average season, but
underforecast the extent of the extreme hurricane activity.
During the 1991-1994 period, tropical storm and hurricane
activity
over the Atlantic basin (i.e., the North Atlantic, the Caribbean, and
the
Gulf of Mexico) averaged the lowest on record since the beginning of
reliable
archives in the mid-1940s (Landsea et al. 1996). During these years,
there
was a
average of 7.2 named storms (i.e., tropical storms and hurricanes -
those
tropical cyclones having maximum sustained one minute surface winds of
at least
18 m s-1), 3.8 hurricanes (winds of at least 33 m
s-1), and 1.0
intense (or
major) hurricanes (winds of at least 50 m s-1, and indicated
by categories
3-5 on the Saffir-Simpson scale (Simpson 1974)). Additionally, the
Caribbean
Islands experienced no hurricanes during this period - and none since
Hurricane Hugo in 1989 -
the longest hurricane-free interval for that region in this century.
In contrast, the June-November
1995 Atlantic hurricane season was extremely active in
nearly every aspect Table 1,
Fig. 1). During this season, there were
19
named storms, 11 of which became hurricanes and 5 of which reached
intense
hurricane status. Additionally, 5 named storms (2 being hurricanes)
struck
the mainland United States and 3 hurricanes affected the countries
surrounding
the Caribbean Sea.
It was apparent by late 1994 that environmental conditions were
becoming
increasingly favorable for an active 1995 Atlantic hurricane season,
as indicated by the first seasonal hurricane forecast issued in late
November
1994 (Gray 1994). The update of this forecast in early June (Gray et
al. 1995a)
continued the prediction for an active hurricane season, while the
forecast
in early August (Gray et al. 1995b) indicated the likelihood of an even
more
active season than was
previously expected. Collectively, these forecasts predicted more
named
storms, hurricanes and intense hurricanes than any other presented in
the
twelve year history of real-time forecasting at Colorado State
University.
Active hurricane seasons in the Atlantic basin
are generally associated with a reduction
of tropospheric vertical wind
shear (typically measured between 850 and 200 mb)
within the critical 10 to 20 °
N latitude belt stretching from
North Africa to Central America (Gray et al. 1993). [Hereafter,
following Goldenberg and Shapiro (1996), this region will be referred
to
as the "main development region" (MDR).]
In contrast, quiet years are generally associated with above normal
vertical wind shear in this region, with values larger than the 7.5-10
m s-1 which appears to be the threshold beyond which
unfavorable
conditions occur (Zehr 1992, DeMaria et al. 1993).
There are several local and remote factors that control the
interannual variability of Atlantic
basin hurricane activity, oftentimes through their influence on
tropospheric vertical wind shear:
Gray et al. (1993) have suggested that abnormally low SLP in
the MDR reflects a poleward shift and/or a strengthening of the
Intertropical Convergence Zone (ITCZ). Both of these situations
contribute to
less subsidence and drying in the MDR through which
easterly waves move.
An enhanced ITCZ also provides more large-scale low level cyclonic
vorticity to incipient tropical cyclones, thereby creating an
environment
that is more favorable for tropical
cyclogenesis (Gray 1968).
Knaff (1996) also indicates that abnormally low SLP is
accompanied by a deeper moist boundary layer and a weakened
tradewind inversion.
In contrast, above
normal SLP tends to be associated with opposite conditions which are
unfavorable for tropical cyclogenesis.
The Atlantic basin tropical cyclone "best track" data are provided by
the U. S. Tropical Prediction Center/National Hurricane Center
in the form of six hourly positions and intensities of all tropical
cyclones reaching named storm status. Documentation of this database
is found in Jarvinen et al. (1984), with discussion of its
strengths and weaknesses in Neumann et al. (1993) and
Landsea (1993).
Sea surface temperature data are obtained from the high-resolution
data set of Reynolds and Smith (1995). The data are derived from an
optimal
interpolation of in situ ship and buoy data supplemented by
satellite
SST retrievals on a 2° grid spacing. Deviations from
long-term means are computed with respect to a 1950-1979 base period.
Station rainfall, sea level pressure and wind data are those
available in real-time from the Global Telecommunications System,
and are typically compared
against long-term (1950-1990) average values as described in
Gray et al. (1994) and Landsea and Gray (1992).
Additionally, gridded (2.5°
interval) fields of
sea level pressure, wind and height are utilized from the
National Center for Environmental Prediction/National Center for
Atmospheric Research reanalysis project (Kalnay et al. 1996).
Tropospheric vertical shear is calculated as the vector difference
of the 200 minus the 850 mb flow field vectors at each grid point.
Anomalies are calculated with respect to the 1979-1995 base period
monthly
means.
Finally, monthly estimates of total precipitable water were obtained
from the
National Environmental Satellite, Data and Information Service
(NESDIS)/Office
of Research and Applications (Ferraro et al. 1996).
These estimates are derived from passive microwave measurements of
the 22 GHz channel from the Special Sensor Microwave Imager.
Anomalies are calculated with respect to the 1987-1995 base period
monthly means.
The 1995 Atlantic hurricane season began with the
development of Hurricane Allison on 3 June and ended
with the dissipation of Hurricane Tanya on 1 November.
The total number of named
storms (NS) was 19, yielding 121 named storm days (NSD) (Table 1, Fig.1).
Eleven of these storms reached hurricane (H) strength, yielding
60 hurricane days (HD) during the season.
Five of these hurricanes reached intense
hurricane (IH) status, yielding 11.75 intense hurricane days (IHD).
For the season as a whole,
all but one of the designated tropical cyclone activity
parameters were more than twice the long-period (1950-1990)
means
( Table 2 ).
Additional details on individual tropical cyclones during 1995 can be
found in Lawrence et al. (1997).
Though extremely active, the 1995 hurricane season
is not unprecedented. Ten other hurricane
seasons in the last 110 years have had comparable activity [i.e. nearly
equivalent or greater
activity in at least one of the tropical cyclone parameters listed in
Table 2]
(Table 3) , with a very active
season occurring
approximately once every 10-15 years.
The record number of named storms observed is 21 during the 1933
season1.
The 1995 and 18872
seasons each had 19 named storms, followed by
the 1969 and 1936 seasons, in which 17 named storms were recorded.
The 11 hurricanes recorded during 1995 were second only to the 12
observed in 1969, and was tied with the 1950 and 1916 seasons.
The total of five intense hurricanes during 1995 was
exceeded in 1950 (7) and 1961, 1926 and 1916 with 6 each.
Hurricane Destruction Potential (HDP) - a combined measure of hurricane
intensity and duration - reached a total of 172, almost two and a half
times the long-term average.
The parameter "Net Tropical Cyclone" (NTC)
activity is useful for characterizing how
active the season has been overall
in terms of the total cyclone frequency, intensity, and
duration (Gray et al. 1994). NTC combines six
measures of tropical cyclone activity (NS, NSD, H, HD, IH, IHD)
into an average percentage
value compared with the 1950 to 1990 climatology. The NTC value for
1995 was 229%, and was the largest value recorded since 1950 (243%).
NTC during 1995 was also exceeded in 1926 (237%) and 1893 (250%). The
active years of 1961 and 1933 recorded NTC values of 222% and 215%,
respectively.
The upsurge in hurricane activity during 1995 was most dramatic
in the latitudes equatorward of 25 °
N excluding
the Gulf of Mexico, where seven hurricanes developed
compared to only one during the entire 1991-1994 period
(Hurricane Chris in 1993).
The Caribbean countries were struck by
three of these hurricanes (two intense - Luis and Roxanne), after
experiencing a record five years with no hurricanes at all.
Hurricane Luis was of particular interest during 1995 because of its
long duration at IH status and its destructiveness
in the Caribbean. Typically, intense hurricanes
remain so for an average of
just 2-3 days. Luis maintained that status for 8.25 days.
This is the third longest duration of an intense hurricane observed
in the period of reliable records (i.e. since
1944), exceeded only by Hurricanes Donna in 1960
(9.0 IHD) and Esther in 1961 (8.5 IHD).
During August-October, vertical wind shear the Atlantic basin's MDR
normally exhibits a strong westerly component and exceeds
the critical 7.5-10 m s-1 threshold for tropical cyclone
formation
(Fig. 2a).
This large shear results from a combination of upper level (200 mb)
westerly
winds averaging 3-6 m s-1 (Fig. 2b), in association with a
mean tropical
upper tropospheric trough (or TUTT; Sadler 1976; Fitzpatrick et al.
1995),
and low level (850 mb) easterly tradewinds averaging 6-7 m
s-1
(Fig. 2c). Thus,
very active hurricane seasons require that the vertical wind shear in
this
region be substantially reduced from the climatological mean.
During August-October 1995, the magnitude of the vertical shear over
the North
Atlantic decreased to 2-6 m s-1 throughout the MDR
(Fig. 3a), approximately 3-5 m
s-1 below the climatological mean
(Fig. 4a). This
decrease reflected a nearly complete elimination of the normal pattern
of
westerly shear over much of the region, and resulted from a combination
of
(1) reduced upper level westerlies ( 4c, Table 4),
with an actual reversal to upper level easterlies throughout
the Caribbean Sea eastward to approximately 60
°W
(Figs. 3b and 5),
and (2) weaker than normal low level easterlies across the
tropical North Atlantic. Farther north, the westerly vertical
shear observed in the region poleward of approximately
25°N
averaged near normal to slightly above normal during August-October.
Similar regional
variations in vertical shear have also been noted during other active
hurricane seasons (Goldenberg and Shapiro 1996).
Together, this pattern of vertical shear strongly delineated
the observed sites of tropical cyclogenesis during 1995, with sixteen
tropical
storms forming south of 25°
N
including all of the Gulf of Mexico (Fig. 1)
compared to the 1950-1990 average of 6.9 storms, and three tropical
storms
forming north of 25°
N
compared to an average of 2.4 storms.
The large scale pattern of upper tropospheric easterly anomalies and
lower
tropospheric westerly anomalies that contributed to reduced vertical
shear
over the tropical North Atlantic during 1995 also extended well
westward
to the tropical Northeast Pacific. However, in this latter region,
these
anomalies produced a sharp increase in vertical shear to 9-20 m
s-1
(Fig. 4a), which is 6-9 m
s-1 larger than the typically low values
observed in association with the climatological mean 200 mb subtropical
ridge (Fig. 3a, 3b). This increase
in vertical shear contributed to a
marked decrease in Northeast Pacific cyclone activity during 1995
(Rappaport
1997),
contrasting with the quite numerous tropical cyclones normally
are observed in that region.
It is interesting that the same anomalous forcing in the Atlantic and
Northeast Pacific tropical cyclone basins produces such a dichotomous
response because of the differences in background climatological flow.
The circulation anomaly patterns first appeared over the MDR
during February and March 1995 (Fig.
6b), and reflected a reversal from
the anomalous cyclonic circulation and anomalous westerly shear
patterns
observed over the region during December 1994-January 1995
(Fig. 6a).
By April-May, easterly shear anomalies dominated the entire tropical
North Atlantic(Fig. 6c), as the
anomalous anticyclonic circulation center
moved poleward to approximately 28° N,
These conditions then continued during
June and July (Fig. 6d) and
subsequently persisted through the remainder of
the hurricane season. The large spatial scale and extreme persistence
of
these anomalies since early 1995 strongly
suggests that they were a cause, rather than
an effect, of the active 1995 hurricane season.
A second contributing factor to the extremely active 1995
hurricane
season was a persistent pattern of below normal sea level pressure
throughout
the tropical North Atlantic (Fig.
7c, Table 4).
During August-October, a large scale pattern of negative SLP
anomalies dominated the North Atlantic, the United States and
Central America, with SLPs averaging
up to 2 mb below normal throughout the northern portion of the MDR,
and more than 2.5-3 mb below normal throughout the
Gulf of Mexico.
These conditions are consistent with the pattern of low level wind
anomalies and vertical shear described previously
(Fig. 4d).
Below normal SLPs also
began to cover the tropical North Atlantic during April-May
1995 (Fig. 7a). By June-July the
entire tropical North Atlantic was
dominated by below normal SLP, which subsequently persisted
through the remainder of the hurricane season (Fig. 7c). Again, the
large spatial scale and extreme persistence of these negative SLP
anomalies since
early 1995 suggests that they were also a contributing factor to,
rather than an effect of, the
active 1995 hurricane season.
A third contributing factor to the active 1995 hurricane season
was a persistent, large scale pattern of above normal SSTs
(0.5°
C warmer than average)
across the MDR during August-October (Figs. 8c).
Above normal SSTs first developed over large portions of the
tropical North Atlantic during April-May
(Fig. 8a), with anomalies exceeding +0.5°
C.
This pattern expanded during June-July (Fig. 8b), as
anomalies increased to more than +1.0°
C
throughout the region.
This persistent pattern of above normal SSTs is consistent
with the large scale pattern of negative SLP anomalies during the
period,
and
also supports the interpretation that favorable environmental
conditions were in place well prior to the onset of the active
August-October
period.
An additional likely contributor to the development and growth of the
Atlantic tropical cyclones during August-October was above
normal total precipitable water across the
tropical North Atlantic, the Caribbean Sea, and the Gulf of Mexico
(Fig. 9c). These conditions are
consistent with the overall tendency
for above normal SSTs and below normal SLPs throughout the region.
The more humid than normal TPW values
became established during June-July throughout the MDR
(Fig. 9b), following
a period of abnormally dry TPW values during April-May
(Fig. 9a).
During the 1995 Atlantic hurricane season, the stratospheric zonal
winds at both 30 mb and 50 mb were easterly with an amplitude of
2.5-5 m s-1 (Fig. 10a),
indicating only weak vertical wind
shear between these two levels. At both levels, these easterlies
were much weaker than normal, consistent with the westerly phase
of the stratospheric quasi-biennial oscillation
(Fig. 10b). As
discussed in section 1, the westerly phase of the stratospheric
QBO tends to promote active hurricane seasons (Gray 1984a, Shapiro
1989). Thus these stratospheric conditions also favored an active
hurricane season during 1995.
Following January 1995, SSTs over the tropical Pacific began a
rapid
decline toward normal. By April-May,
the area of positive SST anomalies had disappeared from the tropical
Pacific east of the date line (Fig. 11b)
and below normal SSTs had
developed over much of the eastern equatorial Pacific.
During August-October 1995,
below normal SSTs spread westward to cover the entire tropical Pacific
east of the dateline, with the largest negative anomalies observed over
the east central
equatorial Pacific (Fig. 11c).
Accompanying this transition to cold
episode (La Niño) conditions, the pattern of anomalous tropical
convection
also reversed, with suppressed convective activity near the date
line and enhanced convective activity over Indonesia (Halpert et al.
1996). Collectively, these conditions reflected a reversal from the
atmospheric and oceanic anomaly patterns that had dominated the
tropical
Pacific since late 1990. This evolution matches the persistent
pattern (since February) of easterly shear anomalies over the
tropical North Atlantic. It also corresponds well with the dramatic
decrease in the magnitude of the vertical shear during the peak of the
1995
Atlantic hurricane season (Fig.
4a, 5), compared to the
relatively high shear
conditions that prevailed during the past four hurricane seasons
(Landsea
et al. 1996).
Rainfall totals during June-September 1995
were near normal to moderately dry over much of the African Sahel,
defined
as the region from Senegal to Sudan roughly between 10-20 °
N
(Fig. 12a). Overall the Western
Sahel, the area with the strongest
concurrent association with Atlantic intense hurricane activity
(Landsea
and Gray 1992), had
near average rainfall during the June-September 1995 rainy season, with
an area-averaged precipitation amount of -0.20
standardized deviations below the long-term mean Fig. 12b)
3.
This value is within the
middle quintile of rainfall years since 1950 (i.e. within the one-fifth
of
all years closest to the long-term mean) and is significantly wetter
than most the previous 25 years, which averaged -0.57 standardized
deviations. Overall, these observations do
suggest that the Western Sahel rainfall was not a major contributor to
the
active 1995 Atlantic hurricane season.
There could not be two hurricane seasons more different than
1994 and 1995. Nine hurricanes were observed during
August-October 1995, compared to only one hurricane during the
same three months of 1994 (Table 5),
values well above and well below the climatological mean
of five hurricanes expected during these months, respectively.
All other tropical cyclone parameters
currently used to summarize a given hurricane
season also highlight the dramatic differences between the
extremely active 1995 season and the inactive 1994 season
during the traditional August-October period peak of activity (Table
5).
While the juxtaposition of conducive environmental conditions led to
a busy 1995 hurricane season, August-October 1994 hurricane activity
was suppressed by a combination of inhibiting conditions:
abnormally strong upper-level westerlies and
enhanced vertical wind shear, abnormally high SLP and abnormally
cool SSTs over the MDR along with a moderate El Niño
and a strong easterly phase of the QBO (Table 5).
Notably, the Western Sahel rainfall was near normal
during both years (Fig. 12b).
The season began when Hurricane Allison formed
over the eastern Gulf of Mexico in early June, followed by two
tropical storms (Barry and
Chantal) during the first half of July (Fig. 13).
Subsequently, tropical cyclone formation occurred primarily during two
month-long periods, 30 July-29 August and 27 September-27 October
separated by the inactive period of 30 August to 26 September.
This clustering of tropical
cyclone activity over 20-30 day periods interspersed with
20-30 day quiet periods is typical of other active Atlantic
hurricane seasons as well, as was
observed during the 1950, 1955, 1985 and 1990 seasons.
In fact, Gray (1979) noted a clustering of tropical cyclogenesis on
the
time scales of a few weeks
in both the Northern and Southern hemispheres. In the Indian
and western Pacific, this clustering is often forced by the
Madden-Julian
Oscillation (MJO), with more than twice as many cyclones forming in the
"wet" MJO phase than in the "dry" MJO phase (Liebmann et al. 1994).
Shapiro and Goldenberg (1993) has shown,
however, that the MJO is negligible in amplitude during the summer
months over the Atlantic MDR.
This suggests that the MJO may not be responsible for the intraseasonal
variations in Atlantic tropical cyclones, though it does not discount
the possibility that other intraseasonal variations in the
atmospheric circulation may be tied to the "clustering" of Atlantic
tropical cyclogenesis.
During 1995, nine named storms (Dean, Erin,
Felix, Gabrielle, Humberto, Iris, Jerry, Karen and Luis) formed
during the 31 days between 30 July and 29 August. This is
equivalent to the formation of a new
named storm just over every three days. Of these nine
storms, five became hurricanes (Erin, Felix,
Humberto, Iris and Luis) and two became intense hurricanes
(Felix and Luis).
The first three weeks of September define the climatological peak
of the hurricane season (Neumann et al. 1993). However, between
29 August when Luis was named
and 27 September when Noel was named, only one new system formed -
Intense Hurricane Marilyn.
On average, 4.1 named storms form in
the Atlantic basin during this 28 day period.
A second burst of 6 new named storms formed between 27 September
and 27 October. Of these new storms, four became hurricanes
and two became intense hurricanes (Noel - H, Opal - IH,
Pablo - TS, Roxanne - IH, Sebastien - TS and Tanya - H).
The large scale circulation changes associated with the active and
inactive periods of hurricane activity during 1995 are shown in
Fig. 14.
During August and October, there was a large scale pattern of
anomalous cyclonic streamfunction over Western Canada and anomalous
anticyclonic streamfunction throughout eastern North America.
This latter feature
is consistent with reduced westerly flow (and actual easterly flow
in some cases) at upper levels throughout the MDR, and thus with
reduced vertical wind shear.
In contrast, an opposite pattern of streamfunction anomalies is
evident during September, with near normal geostrophic winds observed
across
the tropical North Atlantic, the Caribbean and the Gulf of Mexico.
These conditions are then consistent with the observed return to near
normal values of vertical shear across the MDR during the month.
Thus a large scale change in the atmospheric circulation across
North America appears to have been a primary contributor to the
reduced hurricane activity observed during September.
Another contributor to the decreased tropical cyclogenesis during
September
was a weakening of the widescale warm SST anomalies and a creation of
some
cool anomalies in the MDR
and in the North Atlantic subtropics (Fig. 15b) compared to that observed
during August (Fig. 15a) and
October (Fig. 15c).
These cooler SSTs during September appeared directly under
the late July-late August tropical cyclone tracks (Fig. 15b).
Additionally, they were accompanied by a temporary return to near
normal SLPs during the month (Table 4). Thus
both atmospheric and oceanic conditions favored increased hurricane
activity during August and October and relatively fewer hurricanes in
September.
Tropical cyclones are known
to force cooling of the SST by a combination of vertical turbulent
mixing
and upwelling of cooler sub-surface water (e.g. Shay et al. 1989).
The timescale that the oceanic mixed layer takes to recover to original
pre-tropical cyclone conditions can vary from a few days to a few
weeks depending on the location in the basin,
depth of the thermocline and the time of year (Black 1983).
It is suggested here that the tropical cyclones themselves can induce
a negative feedback that operates on a monthly timescale:
during a several week period of multiple tropical cyclones, the SSTs
are
cooled significantly, reducing the potential
for tropical cyclones to develop. After a few weeks, the SSTs
return to their original warm state and multiple tropical cyclones
are again possible, dependent upon other genesis factors being
favorable. The scenario appears to be what occurred during 1995.
Most of the tropical storms and hurricanes during 1995
originated over the central
tropical North Atlantic, moved toward the west-northwest, and then
recurved back to the northeast before affecting North America or
Central
America (Fig. 1). In fact, of the
seven named storms that developed during
August and September in the MDR (Felix, Humberto, Iris,
Karen, Luis, Marilyn and Noel), only Felix crossed the 70°
W
longitude and threatened the continental United States. Even this
system
eventually recurved to the northeast without making landfall.
Since 1944, three-fourths of the August-September
named storms forming
equatorward of 20°
N and east of 55°
W
have recurved or dissipated over the open ocean,
while 18% have made landfall along the continental
United States and 7% have struck Central America or Mexico. Thus the
frequency of recurving systems during 1995 is much larger than is
expected to occur climatologically.
This repetition of recurving tropical cyclone tracks during 1995
is attributed to an anomalous 500 mb circulation over the western
North Atlantic, the
approximate steering level for hurricanes (e.g. Elsberry 1995).
In particular,
there was an eastward shift of the climatological mean subtropical
ridge normally centered east of
Florida near 65°
W (Fig. 16a), to
approximately 55°
W
during August-September 1995 (Fig.
16b), along with an amplification of the mean trough over the
southeastern
United States(Figs. 16b, c).
This anomaly pattern was associated with
enhanced southerly flow over the Western Atlantic near 30°
N,
65°
W during August-September.
This flow pattern directed the hurricanes northward
into the mean westerly current, where they subsequently recurved to the
northeast without making landfall.
However, such anomalous steering flow variations were not present in
the preceeding months, thus making anticipating such track
variations difficult.
The seasonal hurricane forecasts for 1995, issued by
Gray (1994) and Gray et al. (1995a,b) on 30 November 1994, 7 June 1995
and 4 August 1995, respectively, are given in Table 6 4.
The verification of the 4 August forecasts of upcoming August-November
tropical cyclones is shown in Table 7.
All of these forecasts correctly called for an above average season,
with all
measures of activity above the long term mean.
However, these forecasts did not anticipate
the extreme nature of the 1995 hurricane season.
Nonetheless, the forecasts successfully indicated a
marked upswing in tropical cyclone activity during
1995, following the extremely inactive 1991-94
seasons. For example, in late November 1994, it was stated that "the
1995 season should be much more active than the four recent
1991 through 1994 hurricane seasons, and especially in the tropical
regions at latitudes south of 25 °
N" (Gray 1994).
The primary factor leading to this forecast was "the anticipated
dissipation of the
long running equatorial Pacific warm water event which [had then]
persisted for over four consecutive years."
By June 1995 it was evident that
"the El Niño, stratospheric QBO, West African rainfall, and
Atlantic sea surface temperature anomalies [were] all coming together
to
promote the large-scale wind and thermal-moisture conditions which
are associated with an active season" (Gray et al. 1995a).
Additionally, it was suggested at that time
that the probability of hurricane activity
within the Gulf of Mexico and the Caribbean would be higher than at
any time since 1989. Finally, by August 1995,
most of the global and regional meteorological features known to be
associated with active Atlantic hurricane seasons
were evident. Gray et al. (1995b) indicated that
there was "a very high statistical
probability that 1995 will experience a very active hurricane
season."
The 1995 Atlantic hurricane season featured 19 named storms (the
average is 9.3), with 11 of these systems reaching hurricane status
(the
average is 5.8). This is the second largest number of named storms
observed in any hurricane season (June-November) since 1871, and the
second largest number of hurricanes observed in any season since 1886.
Of these 11 hurricanes, 5 reached intense hurricane status (the average
is 2.3) - the most observed in the Atlantic basin since 1964. This
active hurricane season followed four consecutive years (1991-1994) of
extremely low Atlantic tropical cyclone activity.
During the 1995 Atlantic hurricane season an abnormally large fraction
of the named storms (89%, or 17 of 19) developed from African easterly
waves.
As with most seasons, the majority of the storms developed during the
August-October period, although there was a relative lull in formation
from
late August through late September in 1995, during the climatological
peak of the season.
Seven of the hurricanes formed south of
25°
N
excluding the Gulf of Mexico, compared to only one
hurricane which formed in this region during the entire 1991-1994
period.
Additionally, all named storms that formed in the tropical North
Atlantic
during 1995 exhibited strongly recurving tracks, which
prevented their landfall on the east coast of the United States. This
extreme repetition of recurving hurricane tracks resulted from a
systematic
eastward displacement of the mid-tropospheric subtropical
ridge position to near 55°
W,
and the development of southerly flow to the west of the mean ridge
axis
over the western North Atlantic.
Perhaps the primary factor for the
increased hurricane activity during 1995 can be attributed to a
favorable,
large scale pattern of extremely low vertical wind shear throughout the
MDR. This reduced vertical shear actually reflected a
large scale pattern of anomalous easterly (westerly) winds at upper
(lower)
tropospheric
levels extending from Africa to approximately 140°
W.
These same anomalies extended across the Northeast Pacific tropical
cyclone basin and - due to a different climatological mean flow in that
region - resulted in increased shear and substantially reduced tropical
cyclone activity during the 1995 season. These large scale
conditions first appeared in February and March 1995, subsequently
persisted throughout the hurricane season. This extreme
persistence and large spatial scale of these anomalous wind and shear
patterns indicates that they were a cause, rather than an
effect, of the active 1995 hurricane season.
In addition to changes in the large-scale flow fields, the enhanced
Atlantic hurricane activity has also been linked to below normal sea
level pressure, abnormally warm ocean waters and very humid values of
total precipitable water. These favorable conditions all developed by
the start of the hurricane season in June.
Our results suggest a positive interplay between the anomalously warm
SSTs, low SLPs and humid TPW values over the tropical North Atlantic
during 1995, which ultimately favored tropical
cyclogenesis and intensification. Qualitatively (Fig. 17a), the abnormally
warm SSTs over the MDR help to lower surface pressures
hydrostatically by directly warming the lower troposphere5.
The reduced meridional pressure gradient of the modified SLP
acts to reduce the low level tradewind
easterlies, thereby contributing to a further warming of the ocean
temperatures via reduced oceanic upwelling (Enfield and Mayer 1996).
Finally, the TPW is increased due to less subsidence drying and
increased moisture flux from the ocean surface associated with
the lowered SLP and increased SST, respectively. In contrast, opposite
conditions of abnormally cool SSTs, high SLPs and dry TPW
values (Fig. 17b)
are more typical of inactive hurricane seasons.
An additional local environmental factor that likely contributed
to
the active 1995 hurricane season was the westerly phase of the
stratospheric quasi-biennial oscillation (QBO). This westerly phase of
the QBO
enhances Atlantic basin
hurricane activity, while the easterly phase is typically associated
with
suppressed hurricane activity.
Remote climate factors can significantly affect the
interannual variability of Atlantic basin hurricane activity, primarily
through their low frequency modulation of the distribution of vertical
shear.
These same factors provide the main long range forecast signal for
seasonal hurricane activity up to 11 months in advance. During 1995,
the
most important of these climate factors appears to have been a dramatic
transition from the prolonged late 1991-early
1995 warm episode (El Niño) to cold episode (La Niño)
conditions during February-August. This transition
contributed to the extreme duration of the atmospheric
circulation anomalies over the North Atlantic during
1995, and to the dramatic reversal in these anomaly patterns from
those which dominated during the last four hurricane seasons. This
strongly reduced vertical shear in the MDR (and slightly above
normal shear north of 25°
N) during 1995 is consistent with
conditions observed during
previous La Niño events (Goldenberg and Shapiro 1996).
A second remote climate factor previously
identified as a contributor to increased
Atlantic basin hurricane activity is enhanced western Sahel
precipitation. However, we find that near normal rainfall
was observed throughout the Western Sahel during 1995.
Thus this effect was likely a
neutral contributor to the observed increase in Atlantic
hurricane activity this year.
This near normal value of western Sahel rainfall was a surprise,
however, considering the strong and very stable correlations shown in
other studies (e.g. Landsea and Gray 1992, Landsea et al. 1992,
Goldenberg and Shapiro 1996) between June-September rainfall and
Atlantic tropical cyclone activity.
Despite the lack of positive contribution of Sahelian rainfall toward
the 1995 Atlantic hurricane season, the combined conducive effects of
low vertical wind shear, low SLPs, warm SSTs, humid TPW, a westerly
phase
of the stratospheric QBO moderate and a La Niño event
allowed for the 1995 hurricane season to be extremely active. In
strong contrast to 1995, the 1994 hurricane season had very quiet
conditions especially during the traditional peak in activity during
August-October. The same factors that promoted increased activity
in 1995 were shown to be in an opposite, inhibiting state during
1994.
The correct anticipation of two factors: the end of the long
running
Pacific warm episode and the onset of the westerly phase of the QBO led
to
successful forecasts of an active hurricane season as early as late
November
1994. These long range forecasts are also significant, in that they
indicated
a reversal from the suppressed hurricane activity observed during the
previous
four years, and thus were a marked departure from both "persistence"
and
"climatology".
Finally, there was
substantial inter-monthly variability in tropical cyclone activity
during
the 1995 season, with named storms forming primarily during two one
month
periods: 30 July-29 August and 27 September-27 October.
The downturn in activity
during September likely resulted from a return to
near normal vertical shear over the Gulf of Mexico and
western Caribbean, caused by
a series of large amplitude troughs propagating across the
eastern United States and the northern Gulf of Mexico. In contrast the
Caribbean and Gulf of Mexico regions were dominated by
very low vertical shear values during August and October, caused by
a persistent upper level ridge that extended southward from the eastern
United States to the Yucatan Peninsula. These observations highlight
the
sometimes important contribution to the short-term variability of
Atlantic basin tropical cyclone activity from slowly-evolving,
mid-latitude
weather systems. A second effect that likely also contributed to the
reduction of September tropical cyclones was cooler SSTs throughout
the MDR that were produced by the tropical
cyclones that passed through the area the previous month. By October,
the SSTs
had recovered to their originally favorable warm anomaly state.
Some have asked (e.g. Begley 1996) whether the increase in hurricanes
during 1995 is related to the global surface temperature increases
that have been observed over the last century, some contribution of
which is often ascribed to increases in anthropogenic "greenhouse"
gases
(Houghton et al. 1996). We conclude that such an
interpretation is not warranted, particularly in light of the large
scale patterns of oceanic and atmospheric conditions that can
be linked coherently and tangibly to the observed interannual
variability of hurricane activity. Additionally, Atlantic hurricane
activity has actually decreased significantly in both frequency of
intense hurricanes and mean intensity of all named storms over
the past few decades (Landsea
et al. 1996). This holds true even with the inclusion of 1995's
Atlantic hurricane season. It is likely that this
multi-decadal variability (Landsea et al. 1992, Gray et al. 1996)
is primarily of natural origin, and is partly related to the sometimes
large interdecadal variability of the
Sahel rainfall (Nicholson 1989), Southern Oscillation (Quinn et al.
1987)
and to known atmospheric
circulation patterns that control the distribution of vertical shear
over the tropical North Atlantic.
Acknowledgements
The authors are indebted to a number of
individuals who have furnished us with the data
or who have given us valuable assessments of the
current state of global atmospheric and oceanic conditions:
Lixion Avila, Ken Berry, Pete Black, Dave Enfield, Pat Fitzpatrick,
John Knaff,
Robert Kohler, James Kossin, Vern Kousky, Vadlamani Kumar, Richard
Larson,
Miles Lawrence, Douglas LeCompte, Dennis Mayer, Dave Misonis,
Max Mayfield, Colin McAdie,
Paul Mielke, Rodrigo Ortiz, Richard Pasch, Edward Rappaport, Tom Ross,
John Sheaffer, Richard Taft, Wassila Thiao, William Thorson and Ray
Zehr.
Ralph Ferraro and Sheldon Kusselson analyzed the SSM/I total
precipitable
water and suggested that this aspect of the environmental conditions
be compared with the Atlantic basin tropical cyclone activity.
Lloyd Shapiro, Hugh Willoughby and two anonymous reviewers
provided very helpful reviews of the manuscript.
This research analysis and forecast has been supported by research
grants from the National Science Foundation (NSF) and National
Atmospheric and Oceanic Administration (NOAA) National Weather
Service and Climate Prediction Center. The lead author was
funded for this work through the 1995-1996 NOAA Postdoctoral
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ABSTRACT
1 Introduction
The primary purpose of this paper is to describe and diagnose the
large scale local environmental conditions which contributed to the
active
1995 Atlantic hurricane season, and to assess qualitatively the
influence
of known remote factors such as ENSO and Western
Sahel precipitation on these conditions.
During 1995, all of the above factors with the exception of
Western Sahel rainfall favored enhanced
Atlantic basin tropical cyclone formation. In contrast, nearly all of
these factors were unfavorable during the 1991-1994 period.
The data sets are briefly described in section 2.
A summary of Atlantic basin tropical cyclone activity is then
presented in section 3.
The precursor and concurrent environmental conditions during
the 1995 hurricane season are described in sections 4 and 5,
respectively.
Contrasts of the quite different 1994 and 1995 hurricane seasons
are presented in section 6.
The intraseasonal variability of tropical cyclogenesis during 1995 is
then described in section 7.
In section 8, the
preponderance of recurving tropical cyclones is described and reasons
for its occurrence discussed.
A verification of the 1995 seasonal forecasts is then presented
(section
9), followed by a summary and discussion of results (section 10).
2 Data
3 Summary of 1995 Atlantic tropical cyclone
activity
4 Local environmental conditions associated with the
active
1995 hurricane season
Several persistent local factors combined to produce the
extremely
active 1995 hurricane season. These were (1) reduced vertical wind
shear;
(2) below normal sea level pressure; (3) abnormally warm ocean waters
with correspondingly large amounts of lower tropospheric water vapor;
and (4) a strong west phase of the stratospheric QBO.
These favorable conditions allowed for an abnormally large number (89%,
or
17 of 19) of tropical cyclones to develop from African easterly waves
(Pasch et. al. 1998).
Typically, an average of only
61% of the tropical cyclones form from easterly waves in a given
season.
In fact, during three weeks in August, six of the eight
easterly waves that moved from North Africa through the MDR developed
into
a tropical cyclone.
It is also possible that the active hurricane season was due in part
to stronger than normal easterly waves originating from Africa as
suggested in section 1.
However, high quality data at a dense enough network to capture
interannual variations in the over-ocean easterly
wave structure differences is not available at this time. Thus
we are unable at this time to further test this hypothesis.
a. Vertical Wind Shear
b. Sea level pressure (SLP)
c. Atlantic sea surface temperature (SST)
d. Atlantic total precipitable water (TPW)
e. The stratospheric Quasi-Biennial Oscillation (QBO)
5 Remote environmental conditions contributing to the
active 1995 hurricane season
The local environmental conditions that favor tropical cyclone
development
over the North Atlantic exhibit substantial interannual variability,
which
is influenced by at least two remote climate phenomena: the ENSO cycle,
and western Sahel rainfall. These
phenomena also contribute to a surprisingly strong long range
predictive
signal for Atlantic basin seasonal tropical cyclone activity up to 11
months
in advance (Gray et al. 1992a, 1993, 1994).
a. The 1995 transition from warm episode (El Niño) to cold
episode (La Niño) conditions
The late 1990-early 1995 period was dominated by warm
episode (El Niño)
conditions in the tropical Pacific (Trenberth and Hoar 1996).
During this period, tropical cyclone activity over the North
Atlantic averaged the lowest on record since the beginning of reliable
archives in the mid-1940s (Landsea et al. 1996). These warm episode
conditions culminated during
the 1994/95 winter season (Fig. 11a)
with return to mature phase of warm ENSO conditions
for the third time in four years (Halpert et al. 1996).
b) Western Sahel rainfall
6 Contrast of the 1994 and 1995 hurricane seasons
7 Intraseasonal variations of 1995 hurricane
activity
8 Recurving tropical cyclone tracks
9 Verification of forecasts for the 1995 Hurricane
Season
10 Summary and Discussion
11 References
1 The Atlantic tropical cyclone record before 1944 is likely incomplete since storms may have been missed or their intensity misclassified due to lack of satellite and aircraft monitoring (Neumann et al. 1993, Landsea 1993).
2 Recent reanalysis by Fern ndez-Partag s and Diaz (1996) has identified that 1887 had at least 19 named storms - making it as active as 1995's busy season in terms of named storms. Only the named storm count for 1887 in Table 3 has been updated; the remaining values are the original "best track" data.
3 This rainfall index is based upon a slightly modified region with a reduced number of stations from that presented in Landsea and Gray (1992). This new index is designed to better take into account the area's identified spatial variability (see Nicholson and Palao 1993 and Moron 1994) and to utilize stations that have reliable, long-term records that are also readily accessible in real-time. See Landsea et al. (1997) for details.
4 The third author made a qualitative adjustment to the 30 November 1994 forecast at the National Hurricane Conference in Atlantic City on April 14, 1995 (Gray 1995). This was based on a then faulty assessment of March ENSO and Atlantic sea surface temperature conditions.
5 This negative relationship of SLP and SST anomalies in the tropical North Atlantic is not uniform around the tropics. Weisberg and Wang (1997) show that many regions of the tropics show no consistent SLP-SST relationship or even an in-phase association.