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SCIENCE
REPORTS
5.1
Seasonal Forecasting at ECMWF (D. L. T. Anderson, ECMWF)
ECMWF has
set up an operational coupled model system to produce global seasonal
forecasts. One of the main contributors to predictability on these timescales
is ENSO. The 97/98 El Niņo, in particular its onset, was well predicted.
Because the coupled model is global it can also represent the impact of
ENSO outside the tropical Pacific, for example, on the high latitudes
and the Asian Monsoon.
Errors in the forecasts
can arise from uncertainties in the atmosphere model, ocean model and
initial
conditions (ocean,
atmosphere, land). Some deficiencies in the coupled model forecasts of
equatorial Pacific SSTs over the 90s are discussed. Errors in the forecasts
due to uncertainties in the ocean initial conditions are examined using
coupled forecasts starting from different initialization methods (with
and without ocean data assimilation and different assimilation set ups).
Results show the ocean data assimilation can have a large positive impact
on the coupled model forecasts of ENSO. Links are found between these
forecasts and uncertainties in the ocean initial state in the central
and western equatorial Pacific, as well as the north western tropical
Pacific, outside the TOGA-TAO array region. The lack of salinity observations
means that errors in the salinity field cannot be corrected. Assimilating
temperature but not salinity can lead to dynamical imbalances along the
equator.
5.2
Basin-Wide Adjustment of Mass and Warm Pool Displacement During the 1997-98
El Niņo-La Niņa (J. Picaut, IRD/ORSTOM and NASA/Goddard Space Flight Center)
An intriguing
feature of the successive 1997-98 El Niņo-La Niņa events is their appearance
at depth months before their signature in sea surface temperature. The
subsurface TAO data showed that the anomalous downward movement of the
thermocline in the central-eastern Equatorial Pacific was underway several
months before the appearance of the 1997 El Niņo warming in sea surface
temperature. Similar feature, associated with an upward movement of the
thermocline, was also evident prior to the sea surface cooling associated
with the onset of the 1998 La Niņa. In situ and TOPEX/Poseidon derived
currents indicate that strong mass transport within the equatorial wave
guide was already pushing the warm pool toward the east (west) as early
as February 1997 (January 1998). Possible mechanisms for these original
features are explored through the readjustment of water masses in the
equatorial wave guide associated with the displacements of the eastern
edge of the warm pool. The role of horizontal and vertical advection,
of equatorial waves and their reflection on both ocean boundaries in this
readjustment of mass and of thermocline depth all along the equatorial
band is investigated. This is done through the use of TOPEX/Poseidon and
TAO data, the Gent and Cane oceanic general circulation model forced by
observed winds and a simplified coupled ocean-atmosphere model. In particular
it was found that the strong westerly winds bursts end of 1996 - early
1997 (much probably originating from the Indian Ocean and the North/Pacific)
acting in phase with the basic ENSO oscillation, were the main reason
for the 1997 El Niņo to be so powerful. As for the basic ENSO oscillation,
it appears to be due to equatorial wave reflections on both ocean boundaries,
in accord with the delayed action oscillator and the revised advective/reflective
mechanisms. Similarly, the sudden shift into La Ni¤a appeared to be due
to equatorial wave reflection on both ocean boundaries together with the
remote effect of weak easterly winds in the far western Pacific.
5.3
Impact of TAO vs. ERS Wind Stresses On Simulations of the Tropical Pacific
Ocean During the 1993-1998 Simulated by the OPA OGCM (C. Menkes, LODYC
and IRD/ORSTOM)
A simulation
of the 1993-1998 period is performed with the OPA OGCM forced by weekly
ERS 1-2 wind stress data from IFREMER/CERSAT extending the work of Grima
et al (1998) for the 1992-1995 period in which the quality of the ERS-1
forcing fields proved to give high quality OPA simulations of the tropical
Pacific. Heat and fresh water fluxes used in this simulation are climatological,
computed from the ECMWF 1979-1993 reanalysis. First, SSTs are restored
to Reynold's weekly SSTs with a restoring term of 40 W m °K. The implied
heat flux by this restoring term is stored during the simulation. Second,
this heat flux is added to the forcing fluxes and the model is rerun using
this fluxes without any restoring terms. This results in simulations in
which SST is now a free parameter (Vialard et al., 1998).
The simulation is
evaluated relative to different data sets. First, a comparison to TOPEX/POSEIDON
sea level anomalies shows a mean correlation of 0.84 in the 5°N-5°S band
and a mean rms difference of 4.3 cm. The mean rms of the model and data
sea level anomalies are respectively 5.3 cm and 7.9 cm. That suggests
that the ERS wind stresses may be too weak in the equatorial Pacific band.
Second, comparisons with TAO equatorial currents are performed. At the
surface (-15 m depth) , correlations and rms differences between model
and TAO currents at 0°, 156°E, 0°, 170°W, 0°, 140°W and 0°, 110°W average
to 0.72 and 22.4 cm. However, simulated zonal current display a well-marked
bias at 140°W and 110°W in the vertical. This bias is characterized by
too weak and too shallow an equatorial undercurrent, and variability of
the current structure that is too weak in the vertical. Comparisons between
ERS and TAO winds at 10 m reveal that the ERS wind zonal gradient along
the equator is weaker than observed in the in-situ data. Secondly, the
ERS stress variability is weaker than observed variability at TAO moorings
by 10-20%. These results are consistent with the findings of Graber et
al. (1996) who found such systematic biases when comparing 10 m neutral
winds from different ERS products to TAO buoys. However, among those ERS
products, the IFREMER/CERSAT product was the least biased. In any case,
such biases in the ERS products are also consistent with the lack of variability
observed in the simulation.
We aim at improving
the model simulation by including the information contained in the TAO
wind data. To do so stresses are computed at each TAO mooring using stability-dependent
formulae based on Liu et al. (1979) and Smith (1988). A combined data
set ERS+TAO is constructed by optimally inserting weekly TAO stresses
into the ERS baseline forcing. A new simulation is performed using the
ERS+TAO stresses. Significant improvements are observed. First, in terms
of simulated sea level amplitudes, the mean rms of the model sea level
anomalies are now 7 cm while mean correlation and rms difference are unchanged.
Second, correlation with zonal currents are improved above the thermocline.
For example, simulated surface current correlations and rms differences
with TAO zonal currents amount to 0.79 and 22.6 cm. In the east, the EUC
core is now well located, and the variability of the current structure
in the vertical is much improved at 140°W and 110°W. Among the model parameters,
SST is quite dramatically affected by such changes. In the ERS+TAO run,
there can be changes greater then 2°C in the eastern Pacific compared
to the ERS run, during the 1997-1998 El Niņo. Such changes are achieved
and dominated by enhanced vertical processes such as enhanced entrainment
of cold water at the mixed layer base and enhanced vertical diffusion
processes.
To this day, ERS 1-2
IFREMER wind products are possibly among the best observed winds available
on a long time period. However, these results tend to show that there
are still unresolved biased when comparing to TAO buoys that need to be
understood. In particular, Graber et al. (1996) show that other comparisons
with buoy networks outside the equatorial band seem less biased. This
point is being investigated. However, if such biases in the equatorial
band are proven to be true, as these simulations suggest, this could have
several implications for ocean atmosphere climate predictions in models
that assimilate such remotely sensed data. Finally, these results point
out how crucial it is to have synoptic observations of in situ data such
as those provided by the TAO array and the developing PIRATA array.
References:
Liu, W. T., K. B. Katsaros, and J. A. Businger (1979), Bulk Parameterization
of Air-sea Exchanges of Heat and Water Vapor Including the Molecular Constraints
at the Surface, J. Phys. Oceanogr., 10, 1722-1735.
Graber, H. C, N. Ebuchi, R. Vakkayil (1996), Evaluations of ERS1 scatterometer
winds with wind and wave ocean buoy observations, Technical Report,
RSMAS 96-003
Grima, N., A. Bentamy, P. Delecluse, K. Katsaros, C. Levy, and Y. Quilfen
(1998), Sensitivity study of an OGCM to satellite wind-stress forcing,
Journal of Geophysical Research, in press.
Smith, S. D. (1988), Coefficients for sea surface wind stress, heat flux
and wind profiles as a function of wind speed and temperature,Journal
of Physical Oceanography,93, 15,467-15,472.
Vialard, J. (1998), The role of salinity in an OGCM during the 1997-1998
El Niņo, in preparation.
5.4
Genesis and Evolution of the 1997-98 El Niņo (M. J. McPhaden, NOAA/Pacific
Marine Environmental Laboratory)
This presentation
describes the onset, development, and sudden end of the 1997-98 El Niņo,
using data from the Tropical Atmosphere Ocean (TAO) Array of moored buoys
and other data sets of the ENSO observing system. Warm sea surface temperature
(SST) anomalies erupted in the tropical eastern Pacific during April-June
1997, and by July 1997 SST anomalies were the highest observed in the
past hundred years. SST anomalies in the eastern Pacific subsequently
exceeded 5°C, making this the strongest El Niņos on record. The onset
of the El Niņo was characterized by surface winds along the equator that
were punctuated by a series of westerly events of increasing intensity
and eastward fetch. These westerly episodes, associated with enhanced
Madden and Julian Oscillations in the atmosphere, locally drove warm water
eastward near the equator. These westerly events also excited downwelling
equatorial Kelvin waves that propagated into the eastern Pacific, depressing
the thermocline by over 90 m in late 1997. At the same time, the thermocline
in the western Pacific shoaled by 20-40 m in response to anomalous upwelling
Rossby wave generation. Changes in SST due to ocean dynamical effects
and anomalous air-sea heat exchanges led to changes in the location and
intensity of atmospheric deep convection and precipitation in the tropics.
Resultant anomalous heating of the atmosphere altered the position of
the sub-tropical jet streams and storm tracks, affecting weather patterns
worldwide. The event ended abruptly with the return of normal trade winds
in May-June 1998, which upwelled thermocline water to cool the surface
in the eastern and central equatorial Pacific. These observations will
be used to test the delayed oscillator theory for ENSO, and the implications
for the predictability of ENSO will be discussed.
5.5
Rectification of the MJO into the ENSO Cycle (W.S. Kessler, NOAA/PMEL
and R. Kleeman, BMRC)
An ocean general circulation model, forced with idealized,
purely oscillating winds over the western equatorial Pacific similar to
those observed during the Madden-Julian Oscillation, developed rectified
low-frequency anomalies in SST and zonal currents, compared to a run in
which the forcing was climatological. The rectification in SST resulted
from increased evaporation under stronger-than-normal winds of either
sign, from changes in the vertical temperature gradient that were correlated
with oscillations of upwelling speed at the intraseasonal frequency, and
from zonal advection due to nonlinearly-generated equatorial currents.
The net rectified signature produced by the MJO-like winds was SST cooling
(about 1°C) in the west Pacific, and warming (about 0.3°C) in the central
Pacific, thereby flattening or reversing the background zonal SST gradient.
It is hypothesized that, in a coupled system, such a pattern of SST anomalies
would tend to spawn additional westerly wind anomalies as a result of
SST-induced changes in the low-level pressure gradient. This was tested
in an intermediate coupled model initialized to 1 January 1997, preceding
the 1997-98 El Niņo. On its own, the model hindcast a (relatively weak)
warm event, but when the effect of the rectified SST pattern was added,
the hindcast El Niņo became about 30% stronger due to the hypothesized
additional westerlies. The results suggest that the MJO can interact constructively
with the ENSO cycle, and therefore is more than just "weather noise" that
introduces irregularity to the cycle. This implies that developing the
capacity to predict, if not individual MJO events, then the conditions
that affect their amplitude, would enhance predictability of the strength
of oncoming El Niņos over present abilities as demonstrated in 1996-97.
5.6
Yoshida Jet Detected in JAMSTEC Subsurface Current Moorings (K. Kutsuwada,
Tokai University)
Long-term measurements of oceanic currents in the subsurface
layers by upward-looking acoustic Doppler current profilers (ADCP) at
three stations (142°E, 147°E and 165°E) on the equator have been performed
as a part of the Tropical Ocean Climate Study (TOCS) by the Japan Marine
Science and Technology Center (JAMSTEC). Time series of the current data
are analyzed to examine variations which are related to the onset of El
Ni¤o events. Some episodes in which strong eastward flow covers the upper
layer above about 100 m are recognized during the period from December
1996 to November 1997, corresponding to the onset and mature phases of
the 1997-98 El Niņo event. In these episodes of surface eastward jet,
the vertically-averaged (0-100m) eastward flow was larger than 0.5 m s
and attained to 1.3 m s at the westernmost station (142°E) during the
strongest episode occurring in December 1996. The surface eastward jet
in the onset phase of the El Ni&#ntilde;o event (December 1996 and
March 1997) occurred in the western Pacific at all the three stations,
while in the mature phase (from April to November 1997) it was recognized
only at the easternmost station (165°E). To investigate variations of
the wind stress during these episodes, we construct data sets of daily
surface wind-stress vectors on a 1° x 1° grid in the tropical Pacific
using satellite scatterometer data (ADEOS/NSCAT and ERS-2 supplied by
JPL and IFREMER, respectively). In all the episodes, westerly winds (bursts)
stronger than 5 m s are predominant over the western equatorial Pacific
covering our mooring stations. Dominance of the westerly wind bursts (WWB)
is confined to the west of 160°E in the onset phase of the El Niņo event,
while the WWBs migrate eastward to an area east of the dateline in the
mature phase after May 1997. We attempt to interpret the occurrence of
surface eastward jets as Yoshida jets in response to local wind forcing.
Results reveal that in some episodes such as the strongest one in December
1996, the enhancement of the surface eastward current estimated from NSCAT
wind data is almost identical to that observed in the mooring data. This
means that the onset of this episode is interpretable in terms of Yoshida
jet dynamics. On the other hand, in the termination as well as in the
onset of other episodes, significant discrepancies are found between the
estimated and observed enhancements of the surface eastward currents.
These results suggest that the effect of remote wind forcing plays an
important role during these periods, indicating the necessity for information
on the zonal pressure gradient along the equator.
5.7
Remotely-Forced Effects on Currents and Water Properties in the Coastal
Upwelling System Off Northern and Central Chile (S. Hormazabal Fritz,
Regional Program for Physical Oceanography and Climate, U. Concepcion)
A recent
seven-year (1991-1998) continuous record of currents over the continental
slope at 30°S off Chile, at 220 m, 485 m, 750 m depths in 850 m of water,
is used to show interannual modulation of intraseasonal coastal trapped
waves in the eastern South Pacific ocean. The first six years of the continuous
records correspond to the physical oceanography component of the bilateral
research program "Marine Natural Resources - SAREC contribution to the
international JGOFS Eastern Boundary Current Study off Chile" (1991-1997).
This study is jointly sponsored by SAREC/SIDA (Sweden) and CONICYT (Chile).
The last nine months of data are from the continuation of the observations
within the Chilean research program FONDAP-Humboldt (1997-2000), which
is a study of the circulation and physical-biological interactions in
the Humboldt current system, and their impacts on regional bio-geochemical
cycling.
The analysis of these
records of currents confirms earlier results (Shaffer et al, 1997) that
show a remarkably efficient transmission through the ocean of strong 40-70
day waves, driven by equatorial wind events. These waves travel first
as equatorial Kelvin waves, along to equator and later as coastal-trapped
waves along the coasts of South America. Wavelet analysis of along-shore
currents shows that intraseasonal time scale variability associated with
coastal trapped waves is seasonally strongest during the austral summer.
It is also strongest during El Nino events (1991-1992, 1994 and 1997-1998)
and weakest during La Nina events (1995-1996). Currents observations at
750 m depth show clearly an enhanced equatorward flow near the bottom
during warm ENSO events. The analysis of records of sea level, sea surface
temperature and coastal wind collected during five years (1991-1995),
and sea surface temperatures obtained through remote sensing in the austral
summers (1991 and 1992), show that sea surface temperature is controlled
by local wind through coastal upwelling and strongly modulated by coastal
trapped waves in the coastal area of northern Chile (18°-33°S). In the
intraseasonal band, the sea surface temperature signal does not propagate
alongshore, and is not related to the wind stress, particularly off Iquique
(20°S), but sea level perturbations are followed by sea surface temperature
anomalies with an 11-12 day lag-time. These relations are explained by
changes in the depth of the thermocline due to coastal trapped waves that
generate differences in water temperature entering the upper mixed surface
layer due to coastal upwelling, modifying its heat budget and the sea
surface temperature. In this way, the sea surface temperature is being
strongly modulated by coastal trapped waves.
Reference:
Shaffer, G., O. Pizarro, L. Djurfeldt, S. Salinas, and J. Rutlant (1997):
Circulation and low-frequency variability near the Chilean coast: Remotely
forced fluctuations during the 1991-1992 El Niņo, J. Phys. Oceanogr.,27,
217-235.
5.8
TRITON Salinity Measurements (K. Ando and Y. Kuroda, JAMSTEC)
The surface layer
in the western equatorial Pacific is characterized by a warm and fresh
water pool of surface water. The zonal displacement of this pool is strongly
related to the ENSO phenomena. The surface fresh water is formed by the
heavy rainfall over the area, and to this area high salinity surface water
from the east is transported and subducted below the surface fresh water
by the westward South Equatorial Current. The contribution of salinity
to density change in this warm fresh pool cannot be neglected. For example,
lack of salinity data will cause errors in estimation of dynamic height
and buoyancy in the mixed layer. The measurement of salinity is therefore
crucial to understanding the western tropical Pacific. Twelve conductivity/temperature
(CT) sensors are attached to each TRITON buoy, and more than 140 CT sensors
are planned in the upper (0-750 m) western tropical Pacific as part of
the TRITON project. A necessary first step is to verify the quality of
our salinity data for meeting TRITON scientific goals.
The current status
of the TRITON salinity measurement project is the validation of salinity
data from four buoys (48 sensors) in March-June 1998 along 156°E, as well
as comparing laboratory pre- and post-calibrations of each sensor. The
temperature calibration by JAMSTEC shows good agreement (within 2 mK difference
for all sensors) with that by manufacturer (SeaBird Electronics). The
conductivity calibration by JAMSTEC shows a 0.8 mS m difference on average
(equivalent to about 0.004-0.008 psu in salinity). The drift of the temperature
sensors is very small (within 2 mK per year) on average, and the drift
of conductivity sensor is 0.002 psu per month (0.024 psu per year) on
average. Salinity data from the buoys are sometimes missing because of
hardware or electrical problems, and there are occasional unrealistic
values. However, data return was about 80% in real-time. In the mixed
layer, temperature data shows strong diurnal cycles, and salinity data
seem to show realistic variability related to local rainfall events.
5.9
SSS Changes in the Western Tropical Pacific During the 1996 La Niņa and
1997 El Niņo period: The present ORSTOM-TSG network in the Pacific. (T.
Delcroix, ORSTOM/Noumea)
Sea-surface salinity
(SSS) changes during the 1996 La Niņa and 1997 El Niņo events are analysed
along the Fiji-Japan shipping track, based on 20 thermosalinograph sections.
In the equatorial band, above-average SSS (35.2 - 35.4 psu instead of
35 psu) were observed in 1996, consistent with a well-marked south equatorial
current, an unusually-strong equatorial upwelling, and below-average precipitation.
From January to August 1997, the SSS decreased sharply from 35.2 - 33.8
psu (lowest recorded monthly value over the last 20 years), compatible
with a reversal of zonal current, the occurrence of equatorial downwelling,
and above-average precipitation. From September to November 1997, the
SSS remained almost constant (34.2 psu), consistent with the opposite
effects of eastward current, likely bringing low saline water from the
Pacific warm pool, and of evaporative cooling, vertical mixing and below-average
precipitation which all tend to increase SSS. The impacts of the SSS changes
on sea level are discussed. The present status of the ORSTOM thermosalinograph
network in the Tropical Pacific is also presented together with scheduled
studies for assimilating SSS data in numerical models.
5.10
Observing Tropical SSS in the Future by Integrating TAO and Satellite
Data (G. Lagerloef, Earth and Space Research, Seattle)
The ability to measure surface ocean salinity by passive
microwave (L-band) remote sensing has been known for several decades and
demonstrated with airborne instruments. Satellite mission designs in the
U.S. and in Europe are presently being considered by cognizant space agencies.
Because requirements for salinity are very similar to requirements for
measuring soil moisture, these missions will measure both parameters,
although the radiometric signal over the ocean is much smaller than over
land.
This presentation
first outlined the basic considerations for tropical dynamics with the
following points:
- Interannual SSS
variations of ņ1 psu are observed in the tropics, caused mainly by variations
in rainfall and advection.
- SSS fields are
needed to balance the upper ocean freshwater budget, initialize coupled
climate models and validate freshwater flux parameterizations.
- It is possible
to improve the initialization of tropical ocean climate models with
SSS data by adjusting T-S curves with SSS and altimeter sea level data
(e.g. recent work by Reynolds, Ji, Vossepol).
- Tropical SSS retrievals
likely will be more precise than those in temperate and polar latitudes
because of brightness temperature sensitivity to salinity increases
with SST.
- TAO/PIRATA SSS
data will be essential to improving and validating satellite SSS measurements.
Satellite SSS error covariances can be estimated.
- Optimal interpolation
of satellite, TAO/PIRATA and other in situ SSS (similar to Reynolds
SST) will provide the best estimate of SSS fields in the tropics. This
will yield high-resolution SSS fields with known errors which are essential
for coupled model development.
The physical principles
governing salinity remote sensing then were presented, along with the
various errors and corrections that must be applied. The satellite mission
concepts were then be described, along with their approval status within
agency programs. These include the HYDROSTAR mission proposed to NASA
earlier in 1998 with selections to be announced before the end of the
year, and the European MIRAS/RAMSES/SMOS sensor which will be proposed
to ESA on 30 November 1998 with selections to be announced mid-1999. If
approved, these satellites may be launched in the 2002-2003 time frame,
and could be planned in sequence to extend the potential time series to
more than five years. The relative salinity retrieval aspects were addressed,
and it is apparent that large-scale climatological variability in the
tropics will be resolved. Accuracies may be <0.5 psu at 1° x 1° resolution
and ~1 week time scales, and perhaps ~0.2 psu on monthly time scales.
Accuracies ~0.1 psu appear possible in the future with more precise radiometer
systems under study. For additional reference, the draft of the First
Workshop Report, Salinity Sea Ice Working Group (SSIWG), held at La Jolla,
CA, USA, 7-8 February 1998, can be found at http://www/esr/org/lagerloef/ssisg/ssiwgrep1.v2.html
.
5.11
Estimating Rainfall in the Tropics Using the Fractional Time Raining and
New Efforts at Obtaining Minute Resolution Rainfall Data in the Pacific
(J. Ensworth, University of Oklahoma)
The relationship
between the fractional time raining and tropical rainfall amount is investigated
using rain gauge data and a point process model of tropical rainfall,
building upon the work originally published by Morressey et al (1994).
Both the strength and the nature of the relationship are dependent upon
the resolution of the data used to estimate the fractional time raining.
It is found that highly accurate estimates of rainfall amounts over periods
of one month or greater can be obtained from the fractional time raining
so long as high-time resolution (minute) data are used. It is demonstrated
that the relationship between the fractional time raining and monthly
atoll rainfall is quasi-homogeneous within the monsoon trough region of
the equatorial western Pacific. Optical rain gauges also reflect this
homogeneity but with an overall offset/overcatch indicated. Programs have
been implemented to make these minute resolution rainfall measurements
through the University of Oklahoma's Environmental Verification and Analysis
Center (EVAC). SPaRCE (South Pacific Rainfall Climate Experiment), GLOBE
and the DOE/ARM projects involve school participation in rainfall collection
using automated weather stations.
Reference:
Morrissey, M., W.F. Krajewski, and M.J. McPhaden (1994): Estimating rainfall
in the tropics using the fractional time raining. J. Appl. Met.,
33, 387-393.
5.12
Easterly Waves and Convective Organization in the Eastern Equatorial Pacific
ITCZ (Y. Serra, University of Washington)
Atmospheric
sounding data are used to identify easterly wave activity in the East
Pacific Intertropical Convergence Zone (ITCZ) during the Tropical East
Pacific Process Study (TEPPS). TEPPS collected surface meteorological
data, Doppler radar volumes, atmospheric soundings and rainfall data from
the NOAA ship RONALD H. BROWN while on station at 7.8°N, 125°W from 08-23
August 1997. The main objective of the experiment was to understand the
discrepancies between satellite microwave and infrared radiometer estimates
of rainfall in this data poor region of the Pacific. In addition to results
related to this objective, easterly wave activity was discovered as a
prominent time scale of variability in the meridional wind and humidity
data. The meridional wind shows variability on the order of 4 days, with
a amplitudes of -5 to 12 m s-1 from the surface to 2 or 4 km. Maximum
amplitudes in moisture variance are >ņ1 g kg-1 from the surface to
4 or 6 km. There were three cycles of the wave observed in the data. The
maxima in the moisture occurred about a day before the maxima in the southerlies
for two of these three events. An in situ convective indicator, calculated
from the radar data, confirms a connection between convection over the
ship and the meridional wind variance, with the convection occurring in
the enhanced southerlies. As the ship provides just a point measurement
of the winds, the location of the convection with respect to the surface
convergence related to any wave passage cannot be determined. Satellite
infrared radiometer (IR) data were also examined for 4-day activity. However,
comparisons between IR cold cloud tops (<235°K) and the radar convective
activity index indicate that IR data are not a reliable indicator of convection
in the east Pacific (S. Yuter, personal communication). Only cloud systems
>100 km and longer than 24 hrs in duration were consistently detected
as IR cold cloudiness. Nevertheless, these data reveal non-propagating
variability in IR temperatures on the scale of 4-5 days. The lack of propagation
may be related to the failure of IR to detect all stages of the cloud
systems and the use of daily averaged data, currently the highest time
resolution available for the preliminary data set. Continued work in this
area will include a closer look at the IR temperature data, using warmer
cloud top temperatures and higher time resolution, as well as analysis
of surface mooring data and NCEP and ECMWF model analyses.
5.13
Relationships between the Atlantic and Indo-Pacific El Niņo: Predictability
of African Climate (M. Jury, University of Zululand)
Links between
the oceans and the atmospheric circulation over Africa prior to and during
historical El Niņo events were revealed through empirical studies using
NCEP reanalysis composites and numerical studies using GCM simulations.
Anti-phase relationships of tropical SST on either side of Africa were
outlined. Warming of the tropical eastern Pacific increases upper level
westerly winds downstream over tropical Africa. Cooling occurs in the
eastern Atlantic as surface easterlies drive equatorial upwelling prior
to an Indo-Pacific El Ni¤o event. The sub-tropical jet decelerates over
southern Africa, creating a high pressure cell which blocks the supply
of moisture during austral summer. Warmer sea temperatures in the western
Indian Ocean attract convection away from southern Africa.
It is concluded that
the atmospheric circulation generated by warm El Niņo conditions in the
Indo-Pacific Oceans opposes that which develops over the Atlantic Ocean
during a warm event there. Coupling of the ENSO circulation over Africa
is dependent on the response of the Atlantic Walker Cell and underlying
ocean. The PIRATA array is ideally placed to assist our understanding
of the uptake of ENSO signals at seasonal lead times. PIRATA has already
proved its worth in long-range predictions for Africa. During a December
1997 forecast meeting in Windhoek it was pointed out that the El Niņo
situation was unusual for Southern Africa with warming in the tropical
eastern Atlantic Ocean reducing the potential for drought. In hindsight,
this was indeed the case. Further dynamical analysis of ocean-atmosphere
interactions is proposed as part of the CLIVAR Africa project, and this
could ensure continued involvement by African scientists in TIP activities.
5.14
Upper Layer Hydrology and Circulation Variability in the Western Equatorial
Atlantic (B. Bourles, IRD/ORSTOM, K. Pailler, University of Brest, and
Y. Gouriou, IRD/ORSTOM)
The western
Tropical Atlantic is an area of crucial interest with regard to the thermohaline
circulation and associated mass, heat, and salt transports. There, the
North Brazil Current (NBC) and the North Brazil Undercurrent (NBUC) transport
warm upper ocean waters toward the northern hemisphere, contributing to
balance the southward flow of cold North Atlantic Deep Water, and closing
the meridional overturning cell of the global thermohaline circulation.
The NBC and NBUC feed surface and subsurface eastward flows at different
depths and different latitudes. Namely, from south to north, the NBUC
feeds the South Equatorial Undercurrent, whereas the NBC feeds the Equatorial
Undercurrent, the North Equatorial Undercurrent and the North Equatorial
Countercurrent (NECC). The quantification of mean and seasonal transports
and water mass contents of these different currents is made very difficult
due to a large spatial and time variability encountered in this region,
and to eddies that detach from the NBC retroflection from boreal summer
to winter. Four surveys, carried out in the western Tropical Atlantic,
in January-March 1993, January-March 1994, September-October 1995 and
April-May 1996 as part of the World Ocean Circulation Experiment Hydrographic
Program, allowed to improve our knowledge of some main aspects of the
circulation. Off the continental shelf, while the NBC retroflection is
total from June to January, the continuity of the NBC along the American
continent toward the Caribbean Sea is confirmed in boreal spring in the
upper layer. The southeastward flowing current, observed around 3øN, 45øW
and supplied by the NBC retroflection (but also by a North Equatorial
Current recirculation) feeds, in the upper layer, the EUC and the NECC,
and in the subthermocline layer, the EUC and the Northern Subsurface Countercurrent.
Over the continental shelf, estimates of the transport off French Guiana
indicate an annual mean coastal flow as high as 3.9ņ1.2 Sv from the coastline
to the 200 m isobath (located 200 km offshore). There is no clear-cut
seasonal variability of this coastal flow, which is strongly modulated
by the offshore mesoscale eddies shed from the NBC retroflection loop.
The western Tropical
Atlantic is also a region where different water masses are present. The
waters formed within the subtropical region of both hemispheres exhibit
salinity values up to 37 psu just above the thermocline. At the surface,
the Amazon discharge is responsible for the presence of large fresh (S<34
psu) water lenses of about 30 m thickness, extending eastward as far as
30°W in boreal fall, when entrained by the NECC. Such different water
masses superimposition implies significant vertical salinity gradients,
largely superior to those observed in the western Pacific Ocean, that
induce strong haloclines, and thus marked pycnoclines, yielding to the
presence of "barrier layers." It is noticeable that fresh surface waters
are generally associated to positive sea surface temperature anomalies,
that may be the consequence of barrier layer effects. Furthermore, the
determination of the mixed layer depth, by applying the commonly used
criteria, indicates values of 3-5 m within the Amazon water lenses.
5.15
The Barrier Layer in the Atlantic Ocean (S. Masson, LODYC/University of
Paris)
Many papers
highlight the importance of the barrier layer in the vertical structure
of the western Pacific ocean. The barrier layer is defined as a salinity
stratified layer separating the warm surface layer from the thermocline.
More recently, a barrier layer has been observed in the west Atlantic.
However, this barrier layer does not seem to be created by a maximum of
precipitation like in Pacific ocean. An ocean general circulation model
forced by the 15-year ECMWF reanalysis fluxes (1979-1993) is therefore
used to investigate the barrier layer formation mechanisms and to evaluate
its impact on the vertical structure of Atlantic Ocean. Each year, this
simulation generates a 40 m thick barrier layer between July and September
offshore of the mouth of the Amazon. The seasonal variation of the barrier
layer is mainly caused by the current dynamics. In surface layer, the
North Brazilian Current (NBC) and the North Equatorial Counter Current
system brings offshore fresh water of Amazon, while in subsurface, salty
water from the south Atlantic is advected by the North Brazilian Undercurrent
and the North Equatorial Undercurrent system. Our results do not show
that input of fresh water by precipitation or by river runoff affect significantly
the seasonal variation of the barrier layer.
In this study, interannual
variations of the barrier layer are also investigated. The results of
the simulation show a minimum barrier layer thickness in 1983 and 1987.
In these two years, a strong NBC seems to advect the Amazon water along
the coast, limiting the decrease of sea surface salinity offshore. Presence
of a robust barrier layer in the simulations encourages us to investigate
the effect of the hyaline stratification on vertical mixing. By running
an experiment in which the mixed layer is computed only as a function
of temperature, we establish the increase of the NBC retroflection by
trapping the current in the shallow pycnoclines that result from barrier
layer formation.
5.16
Formation and Spreading of Arabian Sea High-Salinity Water Mass (S. Prasanna
Kumar, NIO)
The formation
and seasonal spreading of Arabian Sea high-salinity water mass (ASHSW)
were studied based on the monthly mean climatology of temperature and
salinity in the Arabian Sea, north of the equator and west of 80°E, on
a 2° x 2° grid. The ASHSW forms in the northern Arabian Sea during winter
and spreads southwards along 24 sigma-t surface against the prevailing
weak zonal currents. The eastern extent of the core is limited by the
strong northward coastal current flowing along the west coast of India.
During the southwest monsoon, the northern part of the core shoals under
the influence of the Findlater jet, while the southern extent deepens.
Throughout the year, the southward extent of the ASHSW is inhibited by
the equatorial currents. The atmospheric forcing that leads to the formation
of ASHSW was delineated using the monthly mean climatology of heat and
fresh water fluxes. Monsoon winds dominate all the flux fields during
summer (June-September) while latent heat release during the relative
calm of the winter (November-February) monsoon, driven by cool, dry continental
air from the north, results in an increased density of the surface layer.
Thus, excess evaporation over precipitation, and turbulent heat loss exceeding
the radiative heat gain, cool the surface waters of the northern Arabian
Sea during winter and drive convective formation of ASHSW.
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