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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

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 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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T. P. Barnett

Abstract

A limited comparison over the Northern Hemisphere oceans has been made between sea surface temperatures obtained from “Marine lkcks,” air temperatures over the ocean obtained from the same decks, and the historical file of hydrographic data. The intercomparison of these data suggest the following conclusions.

1) The SST observations have been contaminated by a systematic conversion from bucket to injection measurements. The bias so introduced may constitute as much as 30 to 50% of the observed change in sea surface temperature since the turn of the century.

2) The same bias effects are apparent in data sets that are alleged to contain bucket measurements of sea surface temperature only.

3) The behavior of the temperature field over the ocean appears to have significant and substantial differences from the behavior of estimated temperature changes over the Northern Hemisphere land masses. It seems clear that a reliable estimate of hemispheric or global temperature cannot be made without including adequate coverage of the ocean regions.

4) Many of the data suggest a shift in mean state of the sea surface temperature field of certain regions. It appears that this change is partially real and partially due to the merging of rather different types of data.

5) All estimates of pentad average temperature since 1900 are attended by relatively large standard deviations. This fact makes definitive discussion of pentad-to-pentad or decade-to-decade changes in hemispheric and global temperature difficult, if not impossible. Different methods of calculating the uncertainty in pentad averages could effect this conclusion.

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T. P. Barnett

Abstract

The large growth of SST anomalies during 1982–83 in the equatorial Pacific could have been predicted 4–5 months in advance via advanced statistical models. The decay of the warmth was not well predicted, a result attributed to the need for local forcing and thermodynamic considerations in the modeling process. “Kelvin-wave” models will likely have this same deficiency. The success of the statistical models indicates that the development of the SST anomalies during 1982–83 was not unusual.

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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

The space/time scales of surface temperature anomalies over much of the Northern Hemisphere have been analyzed for winter and annual average data. The variability over land is 2–6 times larger than over the oceans. This fact and the latitudinal dependence of the land/ocean area ratio can explain many results obtained by other authors. Regions of high variability were found to be “local” in both space and time, thus suggesting that a rather dense spatial network of sampling points is required to adequately estimate changes in hemispheric temperature. The “local” nature of the anomaly field is related to the meridional advection of heat by long waves, a result suggested by van Loon and Williams (1976). If the results of this study hold to first order for the Southern Hemisphere, then it may be concluded that reliable estimates of temperature change in that hemisphere, and hence also for the entire planet, have yet to be made.

The covariance matrix of annual average data had an eigenvector whose components had virtually the same sign over the entire hemisphere. This implies there is a pattern of temperature change that is coherent over nearly the entire hemisphere. The principal variance in this pattern occurred in the oceans and not over land masses.

The variance in the wintertime hemispheric temperature field was essentially constant between 1950–62. Since 1963 the hemispheric variance has experienced large oscillations with an apparent three-year cycle. This remarkable change in variability began before the eruption of Mt. Agung.

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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

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

Five million ship observations have been used to obtain a description of the Pacific trade wind fields in terms of empirical orthogonal functions. The description covers the period 1950–72 in two-month intervals.

The major conclusions are as follows: 1) The core regions of the trade winds are quasi-permanent in position. 2) The winds in these regions experience large (20–40%) interannual fluctuations in strength, thus suggesting similarly large changes in the momentum and kinetic energy of the trade wind system. 3) The area of maximum trades expands and contracts at interannual time scales. 4) The ITCZ and the thermal equator in the ocean are congruent on annual and interannual time scales. 5) The northeast and southeast trades are seasonally out of phase. However, the winds immediately poleward of a given trade system fluctuate in phase with the trade system of the opposite hemisphere. The northeast and southeast trades are weakly coupled at interannual time scales. 6) The maximum interannual variations in the trades occur away from the region of maximum intensity, particularly in the western and southwest Pacific. 7) The trade wind field is closely and interactively coupled to the underlying equatorial ocean. 8) Interannual variations in the sea-level pressure field over the North Pacific lead by about a season associated changes in the zonal component of the trades.

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