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T. P. Barnett

Abstract

This is the second of three papers describing the interaction between the Monsoon System and the Pacific Trade Wind fields. The current study concentrates on the tropical band within ±30° of the equator, an earlier study (Part I) concentrated on the region ±10° of the equator.

The results of the current study show that the two wind systems are strongly coupled across the tropical latitudes at interannual time scales with coherent variations apparent in the surface wind field from Africa to South America. It appears that the equatorial regions are coupled most strongly to the Southern Hemisphere. The couplings and interaction between the two systems are dependent on the phase of the annual cycle. The apparent temporal bimodality observed in Part I in the near-equatorial band is no longer seen when the full tropical band is analyzed. There is only a slight preference in the wind system for anomalous convergence over Indonesia. The eastward propagation of anomalous zonal wind in the equatorial region is still evident in this analysis.

The results suggest that the atmosphere changes its state in a way that is only broadly related to changes in the sea surface temperature (SST) in the central Pacific, Thus it appears that mechanisms other than those associated with the Pacific SST may be required to explain much of the variability described in this paper. It also appears that the climatic signal being described here is but part of an even larger mode of climatic variability.

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T. P. Barnett

Abstract

Long-range Naval aircraft using AXBT's obtained meridional temperature sections from the central Pacific Ocean along 158 and 170°W between 30 and 50°N at approximately monthly intervals between November 1974 and April 1977 (29 months). Analyses of these sections show that the seasonal cycle in the central ocean is confined largely to the upper 100 m. The phase of the seasonal cycle increases with depth so that at 100 m it lags the surface by three months. Exceptions to the above statements occur in two narrow bands centered on latitudes 42 and 36°N where the seasonal cycle apparently penetrates in phase to the limit of observations (300 m) except in the interval 100–150 m which lags the surface by 1–2 months. Approximately 90–95% of the variance in the seasonal change of heat storage in the study region can be accounted for by air/sea heat exchange and some type of vertical mixing. Horizontal and vertical advection were of limited and little use, respectively, in reproducing the seasonal cycle variance.

The spatially coherent features of the non-seasonal, or anomaly, field were confined largely to the upper 100 m, with small spatial scales dominating the variance field below 100 m. This implies that the often studied sea surface temperature anomalies are associated with a thermal structure largely confined to the mixed layer. It was not possible to explain quantitatively a significant portion of the variance of the heat storage anomalies in terms of currently available estimates of air/sea heat exchange and advective processes. This result is partially due to noise introduced into the heat budget calculations by sampling variability. However, the major reason for the result appears to be poor estimates of the heat budget source terms. Estimates of these source terms must be substantially improved if quantitative understanding of interannual ocean variability is to be achieved.

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T. P. Barnett

Abstract

Analysis of a near-global sea level pressure (SLP) data set for the period 1951-80 was carried out in terms of complex empirical orthogonal function

A large-scale, propagating SLP signal was discovered that seems to include variability previously associated with the Southern Oscillation, monsoons and tropical midlatitude teleconnection patterns In this sense, the new feature offers an empirical framework for many of the well-known features of the global climate system. Extensive comparisons show the feature to be observable in the raw data field, and thus not a figment of the analysis technique. The large SLP signal may have its origins in a SLP/snow feedback loop that acts over the region from Siberia to India. It is suggested that the maintenance of the SLP signal involves SLP/precipation feedbacks and, more importantly, the excitation of a major natural mode of climate variation. The SLP signal is only excited at time scales larger than about 18 mouths, i.e., time scales characteristic of oceanic processes. The GCM results of Lau also suggest the importance of ocean-atmosphere interactions in maintaining the SLP signal. However, forcing solely by SST does not appear capable of explaining the entire SLP signal.

The North Atlantic 0scillation (NAO) was found to be another natural mode of global climate variation. The spatial response of this mode. which closely resembled a simple standing wave, may be excited over a wide range of frequencies. The appearance of this mode in both a Geophysics Fluid Dynamic Laboratory (GFDL) model simulation and NCAR Community Climate Model (CCM) simulation with limited interannual forcing suggests the NAO to be a singular expression of internal atmospheric dynamics The NAO unrelated in both space and time to the large SLP signal discussed above.

Both the NAO and the large-scale SLP modes exhibit dichotomous behavior in time. On one hand they can appear as quasi-regular, continuous elements of the global climate system. Alternatively, they exhibit behavior characteristic of a multiequilibrium system.

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T. P. Barnett

Abstract

Results of a recent study show eastward propagation of information in the low-frequency variations of the tropical sea-level pressure (SLP) field. The current work extends that analysis to investigate the vertical structure of this signal. It is found that the propagating signal exists to a height of at least 850 mb. At 500 mb the signal is not so clear, while at 200 mb there is little evidence of propagation of information. Instead, the low-frequency variations in 200 rnb height appear coherent for the tropical belt around the entire globe. The analysis suggests that the anomalies discussed here appear first at the surface and later at higher levels in the atmosphere.

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T. P. Barnett

Abstract

Numerous hypotheses have been proposed to explain interannual changes in equatorial water temperatures. It is shown that many of these hypotheses can be tested by expressing them in terms of a statistical-dynamical model based on the heat balance equation. The ability of the resulting model to account for variance in a 20-year record of observed water temperature provides a hitherto unavailable, quantitative measure of the hypothesis consistency.

Field data were used in conjunction with the models to show that the following general conclusions are consistent with available observations: 1) The advective terms (both horizontal and vertical) in the heat balance equation account for 30–50% of the variance in records of interannual changes in near-equatorial SST. 2) The advective changes are closely related to significant changes in the trade wind field, particularly those occurring near the equator and just west of the dateline, as well as major changes in sea level across the entire Pacific Basin.

Specific hypotheses about interannual changes in water temperature were tested. The following conclusions were found to be consistent with the available data: 1) Eastward advection of heat by the North Equatorial Countercurrent is far more important (29% of the variance) to the heat balance of the eastern tropical Pacific than local heating or consequences of long-term variations in the Northeast Trades. 2) At Talara, Peru, 48% of the SST variance was predictable one month in advance using basin-wide fluctuations in sea level as predictors. This suggests the importance to the heat balance off Peru of eastward advection of heat by currents or wave phenomena. Of less importance (14%) was trans-equatorial flow across the Galapagos front. Upwelling induced by local changes in the wind stress was not important, on the interannual time scale, in the estimate of SST. 3) Temperature changes in the central equatorial Pacific (Christmas Island) were consistent with the mechanisms of local upwelling at the equator (25%) and advection from the east (33%).

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T. P. Barnett

Abstract

A statistical study suggests that sea surface temperatures (SST) anomalies in the Pacific can be used to forecast subsequent changes in surface air temperature anomaly over North America. The SST's generally produce higher hindcast skill than do forecasts made from sea level pressure (SLP) or from persistence. The skill associated with the hindcasts was generally low and dependent on both location and season. Over a large region of the central United States significant predictions could not be made for any season and combination of SST and SLP predictors. The results of the study were found to be insensitive to changes in model structure, treatment of the predictands and methods of skill scoring. Independent tests of the predictive relations gave results in excellent agreement with those discussed above from the dependent data set.

The major predictive ability comes from water temperature and sea level pressure variations in the equatorial and tropical Pacific Ocean. Fluctuations of SST and SLP in the central North Pacific were of comparatively small value in predicting subsequent air temperature anomalies. The spatial distribution of predictors suggests the zonal SST gradients contribute most to the predictive skill while it is a coherent, near Pacific-wide fluctuation in the subtropical ridge that leads to the SLP skill. A physical hypothesis involving the interaction of the tropical ocean, subtropical and polar jet was constructed to explain these results.

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T. P. Barnett

Abstract

Advanced statistical techniques have been used to conduct a study of the relationships between ocean and atmosphere variables in the tropical Pacific Ocean. The results of the study show that the ocean variables can hindcast features of the trade wind field (TWF) variability several months into the future. The results are compatible with the notion that the Hadley and Walker cells are associated with east-west and north-south sea surface temperature (SST) gradients as suggested by Bjerknes. However, the level of skill in even specifying the strength of these cells is small, suggesting mechanisms other than those associated with SST are responsible for much of the observed variability in those systems.

TWF predictors can hindcast themselves and ocean variables at both short lead times and lead times near one year. The main components of the wind field responsible for this long-lead-time skill are associated with the southeast trades and a near-equatorial band in the western Pacific. The results also suggest that large El Niño events are predictable a year in advance. This conclusion was supported by an independent test which successfully forecast SST anomalies off Peru one year in advance for the period 1976–79.

The results of the study have been used to test specific ideas and scenarios regarding the physical mechanisms responsible for large-scale air-sea interactions in the tropical Pacific. The results add new ideas and’ additional depth to previous work plus help formulate a more cohesive description of large-scale events. During regimes of cold equator SST the northeast and southeast trade wind field merge and extend into the western Pacific. At these times there is no clear minimum in zonal component of the wind system over the western and central ocean between 15°N and 15°S. During warm equatorial SST situations the two TWF's are separated as evidenced by a strong minimum or even reversal of the zonal component in the region noted above. The associated changes in zonal stress and wind stress curl between warm and cold events is very large. During the transition from cold to warm equator the equatorial SST anomalies move westward from South America to near the dateline. The wind anomalies in the equatorial region move simultaneously from the western Pacific to the central ocean. Both types of anomaly meet in the central ocean during the summer/fall season.

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T. P. Barnett

Abstract

Studies of surface wind fields, sea surface temperature (SST), precipitation and sea level pressure in the tropical band extending ±30° of the equator from Africa to South America led to the following conclusions. The Southern Oscillation, El Niflo, and climatic variations in the Monsoon System are all part of one global scale phenomenon. The new information is that this phenomenon appears in the sea level pressure field to have a strong propagating component that appears first in the northern Indian Ocean and moves eastwardinto the eastern Pacific. Similar propagation of information was found in the surface wind field and equatorial precipitation regimes. These same conditions were amply demonstrated during the 1982-83 event and so it may be concluded that the evolution of that event bears many similarities to those in the historical record studies referred to in this paper.

In the surface wind field of the equatorial wave guide, the large-scale signal appears to take the form of a forced Kelvin wave. The mechanism that drives this wave appears to be latent heat release associated with precipitation anomalies that are phase-locked to and propagate with the surface wind anomalies. The long time scales associated with the atmospheric anomalies may be associated either with the slow interaction between the Walker and Hadley cells or with ocean-atmosphere coupling.

Variations in the SST in the central equatorial Pacific appear to be due almost exclusively to advective processes and not to local air-sea heat exchange, a result in accord with that of other studies. All SST anomalies in the equatorial region seen to be closely related to earlier variations in the zonal wind over the maritime continent, e.g., perturbations in the Indonesian Low which appear as the eastward propagating node of the Pacific Walker cell. It appears to be this feature of the wind field that eventually forces the observed ocean response.

Extremely limited data show a remarkable coincidence between an empirically-derived histogram of recurrence intervals between El Niflo events derived from wind field considerations alone and similar histograms derived by Quinn et al. from completely different data sets. The former distribution function assumes that tropical climate variations can be characterized as frequency-modulated processes.

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T. P. Barnett

Abstract

The common variance between 100-yr-long control runs from 11 coupled global climate models (CGCMs) has been studied by use of common empirical orthogonal functions (EOFs). The results suggest that there is a considerable disparity between the CGCMs estimates of internal variability. About one-half of this difference can be attributed to model drift or other low-frequency variations in several of the models. However, even after accounting for this effect, it was found that the models can easily differ by a factor of 2 or more for the energy levels in different EOF mode (wave) numbers. Comparison with observations showed that no one model consistently reproduced the observed partial eigenvalue spectrum. Again, differences between observed and model energy levels were commonly a factor of 2 or more. It is speculated that at least some of the disagreement is due to the relative coarse resolution of the models used in this study.

Separate analysis of a 1000-yr control run of the Geophysical Fluid Dynamics Laboratory model suggested that intramodel variability is much smaller than intermodel variability. It was also found that an estimate of the anthropogenic signal due to greenhouse gases and aerosols from the Max Planck Institute had strong spatial similarities to the leading modes of the models’ common EOFs. This fact complicates the detection/attribution problem.

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T. P. Barnett

Abstract

The time history of the Monsoon System over the Indian Ocean has been developed from ship observations and merged with the Wyrtki-Meyers Pacific Trade Wind field. The interaction of these two massive wind systems has been studied by a rather new empirical orthogonal function (EOF) analysis capable of detecting propagating features in the wind systems. The current study (Part I) was confined to variations within ±10° of the equator.

Results show the two wind systems are strongly coupled at interannual time scales. The coupling is effected through cyclostationary pulsations and longitudinal shifts of the huge surface convergence over Indonesia. The interaction may also he thought of as the spatial expansion/contraction of the wind systems. These changes can be viewed as the transition of the Monsoon/Trade Winds between two preferred climate states. One sub-element of this apparent bimodality in the wind fields is the El Niño phenomenon.

The zonal component of the combined wind fields seems to instigate the large-scale interaction noted above. Perturbations in the u-component are composed of two interesting elements. One is a traveling disturbance which moves from the Indian Ocean eastward into the Pacific, and has many features of an equatorially trapped Kelvin wave. The second is a standing wave pattern which has maxima in the Indian and western Pacific with a node over Indonesia (the Walker cell). Thew perturbations preceed El Niño events and are phase-locked to the seasonal cycle. Their time variation can be viewed as a frequency-modulated process. A quasi-biennial oscillation appears in both wind fields as a standing wave pattern with in-phase maxima off the cast coast of Africa and west cost of South America.

Subsequent papers will describe the interaction of the Monsoon/Trade Winds system in the band ±30° of the equator (Part II) and, in Part III, build a mechanistic, physical picture for the results obtained in Parts I and II.

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