Learning to Predict Climate Variations Associated with El Nino and the Southern Oscillation Accomplishments and Legacies of the TOGA Program (1996) / Chapter Skim
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3. COMPONENTS OF THE U.S. TOGA PROGRAM
3. COMPONENTS OF THE U.S. TOGA PROGRAM
Pages 22-80
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From page 22... ...
Process studies, especially TOGA COARE, were performed to fill gaps in our knowledge of crucial dynamics on smaller spatial and temporal scales. Models, especially coupled models, and prediction systems were developed to address many aspects of ENSO.
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From page 23... ...
The TOGA Program benefited from activities related to predicting seasonal-to-interannual climate variations that were not considered part of $45.0 S40.0 CO o $35-0 ._ - $30.0 .= $25.0 $20.0 ~:5 ~ $1 5.0 _ $1 0.0 o $5.0 s0.0 $4.2 NSF OCE 80.2 NOAA NOS $2.6 - \ NOAA EPOCS / FY 1994 ($ in Millions) 32.2 31 -I 1 1 1 1 1 1 1 1 39.0 ~1 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 Fiscal Year 85.5 AtC'= ATA, $5.9 NASA MTPE ~9.
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From page 24... ...
As predictions of the phases of ENSO began to show skill, priorities shifted to measuring those quantities most useful for initializing and evaluating coupled models. We now have the ability to measure the quantities of highest priority sea surface temperature, surface winds, and subsurface thermal structure-plus some other quantities of interest.
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From page 25... ...
The problem for observing ENSO was to measure enough of the tropical Pacific, both at the surface and at depth, and to do it frequently enough to resolve the evolving atmospheric winds and ocean thermal structure. At the beginning of TOGA (see Figure 5, top)
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From page 26... ...
26 LEARNING TO PREDICT CLIME TE VARIA T1ONS TOGA in Situ Ocean Observing System Pacific Basin ~ ° .~: \ \ it/ ~! ~ ; Moo / _' ~ January 1985 By _:r _r , _~ July 1990 ~ ~ .
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From page 27... ...
Island and coastal tide gauges that reported to the TOGA Sea Level Center are shown with circles. Drifting buoys are indicated schematically with short arrows; each arrow represents ten actual drifters.
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From page 28... ...
(3) subsurface temperature and salinity 15 Tropical ocean surface 2° latitude by 30 days 0.03 PSU salinity 10° longitude 16 Tropical ocean surface 2° latitude by 30 days 0.1 m/see circulation 10° longitude 17 Sub-surface equatorial 30° longitude as available 0.1 m/see currents (at 5 levels)
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From page 29... ...
A particular measurement platform usually contributed to measurements of more than one quantity of interest. For example, the TOGA TAO moored array provides data on surface winds, surface-air temperature, sea surface temperature, surface atmospheric pressure, and upper-ocean thermal structure, with additional data on upper ocean currents at some of the array elements.
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From page 30... ...
They are an effective means of obtaining broad, basin-wide coverage of sea surface temperature, sea-level pressure, and near-surface currents. In the late 1970s, Doppler-ranging from the French navigational system ARGOS became operational on NOAA TIROS (Television Infrared Observation Satellite)
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From page 31... ...
Its technical objectives were, over a three-year period, to learn to use VOS to maintain a group of 160 drifters and to select the most robust elements from the competing drifter designs. Its scientific objectives were to obtain a basin-wide field of surface currents and sea surface temperature of the tropical Pacific for the purpose of studying processes that determine the evolution of sea surface temperature.
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From page 32... ...
Of primary interest to TOGA were sea surface temperature data from drifters. These data are critical because they provide well-distributed benchmarks for adjusting and correcting the inaccuracies and biases in maps of sea surface temperature derived from other sensors (such as those on satellites or VOS)
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From page 33... ...
TAO Array The establishment of the Tropical Atmosphere/Ocean (TAO) moored array of instruments in the tropical Pacific (see Figure 5, bottom)
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From page 34... ...
However, the basin-wide TAO array was not completed until very late in the TOGA program. It was conceived because all other existing means for real-time monitoring of the Pacific, whether in situ or remote, had proved inadequate to fulfilling the goals of TOGA.
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From page 35... ...
However, wind measurements at three or four sites in the equatorial Pacific were not sufficient to describe the surface wind field in the region extending from approximately 5°S to 5°N, where knowledge of surface winds is critically important for forecasting variations of the equatorial upperocean thermal and flow fields (McCreary 1976~. Because the launch of a
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From page 36... ...
However, better information on the variability of surface winds in the tropics was needed to determine the appropriate scales and sampling strategies. A study of 30 years of data from tropical Pacific islands collected by the New Zealand Meteorological Service was undertaken to determine the relevant spatial and temporal scales of variations in the surface winds.
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From page 37... ...
The planned array would span the equatorial wave guide, from 8°S to 8°N, and extend from the eastern Pacific all the way to the western Pacific. Although the instruments also measured surface temperature and subsurface thermal structure, the spacing and sampling design was based on the wind variability because so little was known about the spatial scales of temperature variability.
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From page 38... ...
is NOAA Satel/iite / ~\ At, ~ / LEARNING TO PREDICT CLIMATE VARIA TIONS ,4~, T' ~' \ \ \ \ / I Buoy Status | Quality Controls \ \` / / ~ , ~ Sensor _ Calibration Data Managemen / Calibration Data Base ( System :) 3.8m above Wind Sensy/o satellite water surface ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Y: Air Temperaure/ ,~ ~ .
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From page 39... ...
The TAO array is generally agreed to be the observational element that is most critical to sustain into the post-TOGA era, for two lcey reasons. First, it measures the high-priority fields (sea surface temperature, surface winds, and upper-ocean thermal structure)
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From page 40... ...
. The lack of comprehensive satellite coverage of the equatorial Pacific surface-wind field played a major part in the justification and deployment of the TAO array.
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From page 41... ...
During the course of TOGA, remote measurements of a number of quantities e.g., sea surface temperature, sea level, water vapor, and cloud fractionwere obtained from both research and operational satellites. Unfortunately, the TOGA Program ended prior to rigorous assessment of the ultimate value to ENSO monitoring and prediction of many of the remotely sensed quantities.
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From page 42... ...
The large-scale monitoring of sea surface temperature using infrared satellite measurements from the operational AVHRR provided a critical contribution throughout the entire TOGA Program. These measurements were the principal source of information going into the sea surface temperature indices that were used to validate coupled atmosphere-ocean prediction models.
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From page 43... ...
The initialization of models for making ENSO predictions requires specification of the subsurface thermal structure because the ocean's evolution on seasonal-to-interannual time scales is controlled, in part, by the planetary waves evident in the thermal structure. The VOS network and the TAO array supply the only regular observations of subsurface temperature.
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From page 44... ...
Climatology of the near-surface equatorial ocean at 110°W. Shown as a function of depth and month are (top panel)
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From page 45... ...
Tide gauges that are part of the TOGA Sea Level Network provide most of the measurements. It is difficult to obtain a picture of the evolving thermal structure of the Pacific from the Sea Level Network alone, because this network is limited to coastal stations and islands, and the islands are poorly distributed throughout the interior of the Pacific.
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From page 46... ...
19919. The Geosat data have been used to track the space-time progression of a number of eastward-propagating Kelvin-wave pulses across the equatorial Pacific basin in response to westerly winds west of the dateline during the 1986-1987 ENSO warm phase (Miller et al.
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From page 47... ...
Heat and moisture fluxes between the ocean surface and the atmosphere are a major influence on sea surface temperature. Throughout most of the tropics, the sensible heat flux from the ocean into the atmosphere is relatively small (on the order of 10 Wm-2)
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From page 48... ...
It can also be applied retrospectively as long as the sea surface temperature is sufficiently well known. Reanalysis of atmospheric data sets using atmospheric general-circulation models offers the possibility of preparing long time series of surface fluxes.
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From page 49... ...
Over the next decade, a number of national and international earth-observing satellite missions are expected to provide unprecedented concurrent estimates of wind velocity at the ocean surface wind speed at the ocean surface, columnar water vapor, sea surface height, tropical rain rates, sea surface temperature, albedo, cloud fraction, surface irradiance, and ocean color. However experience has shown that the promise and potential of timely remotely sensed observations can go unfulfilled for reasons of cost, schedule, complexity, or lack of strong advocacy.
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From page 50... ...
A number of limited-duration experiments were conducted to determine the relative importance of several processes and to improve the representation of those processes in models. These experiments aimed to reduce the uncertainties associated with numerical simulations and predictions of month-to-month fluctuations of sea surface temperature, especially in the eastern tropical Pacific.
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From page 51... ...
The TOGA Coupled Ocean-Atmosphere Response Experiment (COARE) was conducted to study the strong air-sea interactions in the western equatorial Pacific Ocean, where sea surface temperatures are warmer than 29°C and where deep convection and heavy precipitation occur (Webster and Lukas 1992~.
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From page 52... ...
TOGA COARE was motivated by: · the difficulty that coupled ocean-atmosphere models have in adequately simulating the mean state and variability of the thermal and flow fields in the lower atmosphere and upper ocean, because of large uncertainties in the components of the net air-sea flux, onset of atmospheric convection, and ocean mixing parameterization; a dearth of high-quality measurements of air-sea fluxes of heat, moisture, and momentum in the western tropical Pacific; uncertainty about the importance of the influence of rainfall upon buoyancy and mixing in the upper ocean; concerns about the interpretation of space-based observations of sea surface temperature and surface wind; and multiple-scale interactions that extend the oceanic and atmospheric influence of the western Pacific warm-pool area to other regions and vice versa. Because of the size and complexity of TOGA COARE, a TOGA COARE international Project Office (TCIPO)
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From page 53... ...
Leading up to the IOP, conditions in the tropical Pacific had relaxed from warm to near normal, with slightly cold surface waters in the eastern equatorial Pacific (see Figure 9~. However, a cold event did not materialize and there was a return to warmer-than-normal surface waters in the central equatorial Pacific.
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From page 54... ...
Monthly-mean anomalies of sea surface temperature averaged over 5°S to 5°N during the TOGA years. Anomalies are based on the adjusted optimal-interpolation climatology of Reynolds and Smith ( 1994~.
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From page 55... ...
Also, the burst cooled the western Pacific through air-sea heat fluxes and upper-ocean mixing The net effect was to displace the warm pool eastward. Observations of rainfall over the ocean were made using Doppler weather radars on ships and on research aircraft.
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From page 56... ...
Sampling frequency for satellite-derived sea surface temperature by infrared instruments is reduced by the abundant cloudiness of the warm-pool region, leading to potential biases. Short-wave radiation penetrates to depths below the mixed layer, contributing significantly to heat content variations there, which could influence sea surface temperature and create temperature inversions.
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From page 57... ...
S TOGA PROGRAM 57 rates of change in sea surface temperature during warming and cooling periods in the warm pool (Smyth et al.
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From page 58... ...
1995~. Evaporation, computed from moored-buoy measurements, was anticorrelated with sea surface temperature in the western Pacific because of the light winds (Zhang and McPhaden 1995~.
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From page 59... ...
Their actual existence and their hypothesized role in the development of anomalies on interannual time scales of sea surface temperature were questioned. Oceanic synoptic-scale (2080 day periods)
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From page 60... ...
Of particular interest, for example, is whether or not there is a downstream rectification of sea-surface-temperature variations associated with synoptic disturbances traveling from the western equatorial Pacific to the central and eastern equatorial Pacific, causing seasonal and longer variations in the central and eastern region. Tropic Heat The Tropic Heat program examined the processes that contribute to the maintenance and evolution of the cold-water tongue in the eastern and central equatorial Pacific.
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From page 61... ...
taken anywhere in the upper ocean, let alone at the equator. Detailed shipboard measurements of air-sea fluxes were compared to satellite observations, providing early guidance for the TOGA Heat Flux project, as well as for TIWE (Tropical Instability Wave Experiment)
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From page 62... ...
Special attention was devoted to the cusp-like wave structures characteristic of the instability waves evident in fields of sea surface temperature. Conditional sampling of the thermohaline and velocity structures associated with fronts in the upper ocean was assisted by AVHRR images received directly on board one of the research vessels (Flament et al.
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From page 63... ...
Variations of sea surface temperature with a 30day period were greatest near 2°N, where there is a sharp meridional temperature front. However, below the surface, variations in temperature with a 30-day period were greatest around 5°N, associated with vertical and meridional displacements of the sharp thermocline (McPhaden 1996~.
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From page 64... ...
Subduction of warm salty water from the central equatorial Pacific below the less saline water of the warm pool, along with vertical mixing, provides the time-averaged balance to the freshwater flux (Shinoda 1993, Shinoda and Lukas 19959. Questions raised by WEPOCS observations of the upper ocean and measurements of air-sea fluxes were instrumental in the development of TOGA COARE.
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From page 65... ...
The North Equatorial Countercurrent and the North Equatorial Undercurrent also transport water from the North Brazil Current, which was retroflected from the boundary. Northern Hemisphere and Southern Hemisphere water masses are mixed along the western boundary between the North Brazil Current and the North Equatorial Current.
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From page 66... ...
MODEL/NO ENS O The major modeling advance of the TOGA period was the successful simulation of the ENSO cycle using coupled models of the atmosphere and ocean for the region of the tropical Pacific. Great strides were made in understanding the importance of wind stress from the atmosphere for determining sea surface temperature, and also in understanding the importance of sea surface temperature in forcing atmospheric conditions.
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From page 67... ...
Ocean Moclels Two major advances in tropical ocean modeling during the TOGA decade were the simulation of sea surface temperature in a variety of ocean models, and the development of an operational hindcast -analysis system for the tropical Pacific Ocean. The pre-TOGA years saw the maturation of the linear theory of equatorial ocean waves their excitation by wind stresses, their propagation eastward as Kelvin modes and westward as Rossby modes, and their effect on vertical displacements of the thermocline.
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From page 68... ...
Heat fluxes at the ocean surface play a central role in controlling the variability of sea surface temperature, especially variability related to the annual cycle. Models with simple parameterizations (see, e.g., Seager et al.
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From page 69... ...
TOGA PROGRAM] 69 Stratus-topped layers, which exist mostly over the cooler waters, intercept the incoming solar flux, thereby helping to maintain the relatively cold sea surface temperatures.
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From page 70... ...
In the cold-tongue regions, stratus clouds influence the far-eastern Pacific (say within 1000 km of the coast) , where the variations of the heat fluxes through the ocean surface are in phase with the variations of sea surface temperature.
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From page 71... ...
Prediction of sea surface temperature, a boundary condition for the atmosphere, allows the forecast of concomitant atmospheric statistics. Predictions of eastern tropical Pacific sea surface temperature can be made a season to a year or more in advance (e.g., Latif et al.
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From page 72... ...
monthly statistical forecasts were being made by the Climate Analysis Center for 3-month-mean sea surface temperature in several regions of the tropical Pacific and Indian Oceans (Graham et al. 1987a, b; Barnston and Ropelewslci 1992~.
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From page 73... ...
Initial conditions of the atmosphere were derived from the ocean-model simulation of sea surface temperature. Thus, no observations of sea surface temperature or subsurface temperatures were utilized.
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From page 74... ...
-- .,,/ :`Pe~e ~ Landis\ \ I 1 1 1 I T r ~ 0 3 6 9 12 15 18 21 24 Lead Time in Months Figure 10. Correlations for predictions of anomalies of equatorial sea surface temperature using fully coupled atmosphere~cean models.
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From page 75... ...
The NMC system provided forecasts of sea surface temperature for the tropical Pacific, as well as forecasts of rainfall and surface temperature around the globe. The skill level of this model for about the first two seasons was higher than that of other systems because an observed ocean thermal state is used for initialization rather than a proxy based on the history of the winds (similar results were obtained by Rosati et al.
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From page 76... ...
As of the end of TOGA, routine predictions of sea surface temperature based on ENSO dynamics were being made using the Zebiak and Cane model, various statistical models, and the NMC coupled model. These predictions were published in the NOAA Climate Diagnostics Bulletin and the NOAA Experimental Long Lead Forecast Bulletin.
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From page 77... ...
S TOGA PROGRAM 77 climate prediction.
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From page 78... ...
for the program that became TOGA concentrated solely on ENSO in the tropical Pacific. The international TOGA plan (WCRP 1985)
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From page 79... ...
While the resulting TOGA Observing System, and its array of moored buoys in the tropical Pacific, was one of the great achievements of the program, much of the system was not available until nearly the end of the program. The usefulness of the system for making forecasts of seasonal-to-interannual climate variations has still not been fully evaluated.
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From page 80... ...
federal agencies are uniform and available for all years of the program, that financial data from other countries are limited, and that some issues remained unresolved through the preparation of this report on the relationship of the advisory and review mechanism (e.g., the TOGA Panel) to the program and the several sponsoring agencies all point to the difficulties of coordinating the large international TOGA Program.
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