Abstract

Long-range sea surface temperature forecasts from two different coupled ocean-atmosphere models of the tropical Pacific are used in conjunction with statistical models relating winter Northern Hemisphere 700-mb height and tropical SST to forecast the former field at a lead time of two seasons in advance. The forecasts show considerable skill over large areas, with a regional distribution of predictive performance that is consistent with the observed contemporaneous relation between the two fields. Comparable skills for lead time of a year or more in advance seem likely.

Abstract

The authors have investigated the interactions of the tropical oceans on interannual timescales by conducting a series of uncoupled atmospheric and oceanic general circulation experiments and hybrid-coupled model simulations. The results illustrate the key role of the El Niño/Southern Oscillation phenomenon in generating interannual variability in all three tropical ocean basins. Sea surface temperature anomalies in the tropical Pacific force SST anomalies of the same sign in the Indian Ocean and SST anomalies of the opposite sign in the Atlantic via a changed atmospheric circulation. However, although air-sea interactions in the Indian and Atlantic Oceans are much weaker than those in the Pacific, they contribute significantly to the variability in these two regions. The role of these air-sea interactions is mainly that of an amplifier by which the ENSO-induced signals are enhanced in the ocean and atmosphere. This process is particularly important in the tropical Atlantic region.

The authors investigated, also, whether ENSO is part of a zonally propagating “wave,” which travels around the globe with a timescale of several years. Consistent with observations, the upper-ocean heat content in the various numerical simulators seems to propagate slowly around the globe. SST anomalies in the Pacific Ocean introduce a global atmospheric response, which in turn forces variations in the other tropical oceans. Since the different oceans exhibit different response characteristics to low-frequency wind changes, the individual tropical ocean responses can add up coincidentally to look like a global wave, and that appears to be the situation. In particular, no evidence is found that the Indian Ocean can significantly affect the ENSO cycle in the Pacific. Finally, the potential for climate forecasts in the Indian and Atlantic Oceans appears to be enhanced if one includes, in a coupled way, remote influences from the Pacific.

Abstract

Long-range P-3 aircraft have been used to occupy two 4000 km long sections from 20°N to 17°S along 150 and 158°W in the central equatorial Pacific. The temperature field along these sections was measured at approximately weekly intervals for three months (November 1977–January 1978). The principal meridional scales of variability derived from this data set suggest highly coherent fluctuations in the upper ocean thermal structure within ±10° of the equator. The region of coherent variability extends across the equator, across boundaries of major current systems and through the ITCZ. The time scale of these fluctuations is of order 2–3 months. Variability in the zonal direction is also coherent, although of smaller amplitude, with space scales of at least 2000 km and time scales of several months. These latter modes of variability are suggestive of propagating disturbances, although that hypothesis could not be proved with the current data set. The observed oceanic variability at 150°W was not closely related with changes in the local wind stress or curl of the wind stress field in the central tropical Pacific, suggesting that a large part of the observed oceanic fluctuations may not have been “locally” forced.

The relation between transports in the North Equatorial Countercurrent derived from the AXBT data and T/S relations agreed well with similar transport estimates obtained from hydrographic observations and current meter arrays. This suggests that the scales of variability we have observed in the temperature field may also apply, in part, to the zonal velocity and transport fields.

Abstract

During the autumn and winter seasons, large amounts of heat are given up to the atmosphere at the subarctic frontal zone off the east coast of Asia. According to Fisher, the Laplacian of this heat flux (∇² Q) is related to increases in the intensity of the relative vorticity in the westerly wind regime. This increase is related to a similar increase in strength of the wind stress curl, which thereby increases the Sverdrup transport of the subarctic and subtropical gyres. The increase in transport in turn intensifies ∇² Q at the subarctic frontal zone via geostrophic adjustment. This coupling of atmospheric relative vorticity, Sverdrup transport, and ∇² Q results in the intensification of the relative vorticity of both fluid media that can be checked only by an instability in either one or the other media. This mutual interaction of the ocean and atmosphere is termed a servomechanism, the natural time scales of which are determined by a mathematical development wherein the vertically integrated vorticity equations of the ocean and atmosphere are coupled by their interaction at the naviface. This coupling leads to a single wave equation for the ocean/atmosphere system, the solutions of which are Rossby waves modulated by exp[(1+i)αt)], where α depends upon the coupling parameters. Normal values of α are found to produce an e-folding increase in the vorticity of the ocean/atmosphere system in less than two months. For anomalously high values of α, the increase in vorticity can be extreme, possibly leading to the formation of a barotropic instability in the atmospheric medium. These theoretical results are illustrated using geophysical data from 1950–60 and are used to explain the events that triggered the unusual ocean/atmospheric vorticity state that existed in the North Pacific between 1956 and 1958.

Abstract

In this paper a decadal climate cycle in the North Atlantic that was derived from an extended-range integration with a coupled ocean–atmosphere general circulation model is described. The decadal mode shares many features with the observed decadal variability in the North Atlantic. The period of the simulated oscillation, however, is somewhat longer than that estimated from observations. While the observations indicate a period of about 12 yr, the coupled model simulation yields a period of about 17 yr. The cyclic nature of the decadal variability implies some inherent predictability at these timescales.

The decadal mode is based on unstable air–sea interactions and must be therefore regarded as an inherently coupled mode. It involves the subtropical gyre and the North Atlantic oscillation. The memory of the coupled system, however, resides in the ocean and is related to horizontal advection and to the oceanic adjustment to low-frequency wind stress curl variations. In particular, it is found that variations in the intensity of the Gulf Stream and its extension are crucial to the oscillation. Although differing in details, the North Atlantic decadal mode and the North Pacific mode described by M. Latif and T. P. Barnett are based on the same fundamental mechanism: a feedback loop between the wind driven subtropical gyre and the extratropical atmospheric circulation.

Abstract

A phenomenon called the Antarctic Circumpolar Wave (ACW), suggested earlier from fragmentary observational evidence, has been simulated realistically in an extended integration of a Max Planck Institute coupled general circulation model. The ACW both in the observations and in the model constitutes a mode of the coupled ocean–atmosphere–sea-ice system that inhabits the high latitudes of the Southern Hemisphere. It is characterized by anomalies of such climate variables as sea surface temperature, sea level pressure, meridional wind, and sea ice that exhibit intricate and evolving spatial phase relations to each other.

The simulated ACW signal in the ocean propagates eastward over most of the high-latitude Southern Ocean, mainly advected along in the Antarctic Circumpolar Current. On average, it completes a circuit entirely around the Southern Ocean but is strongly dissipated in the South Atlantic and in the southern Indian Ocean, just marginally maintaining statistical significance in these areas until it reaches the South Pacific where it is reenergized. In extreme cases, the complete circumpolar propagation is more clear, requiring about 12–16 yr to complete the circuit. This, coupled with the dominant zonal wavenumber 3 pattern of the ACW, results in the local reappearance of energy peaks about every 4–5 yr.

The oceanic component of the mode is forced by the atmosphere via fluxes of heat. The overlying atmosphere establishes patterns of sea level pressure that mainly consist of a standing wave and are associated with the Pacific–South American (PSA) oscillation described in earlier works. The PSA, like its counterpart in the North Pacific, appears to be a natural mode of the high southern latitudes. There is some ENSO-related signal in the ACW forced by anomalous latent heat release associated with precipitation anomalies in the central and western tropical Pacific. However, ENSO-related forcing explains at most 30% of the ACW variance and, generally, much less.

It is hypothesized that the ACW as an entity represents the net result of moving oceanic climate anomalies interacting with a spatially fixed atmospheric forcing pattern. As the SST moves into and out of phase with the resonant background pattern it is selectively amplified or dissipated, an idea supported by several independent analyses. A simplified ocean heat budget model seems to also support this idea.

Abstract

In this study, a hybrid coupled model (HCM) is used to investigate the physics of decadal variability in the North Pacific. This aids in an understanding of the inherent properties of the coupled ocean–atmosphere system in the absence of stochastic forcing by noncoupled variability. It is shown that the HCM simulates a self-sustained decadal oscillation with a period of about 20 yr, similar to that found in both the observations and coupled GCMs.

Sensitivity experiments are carried out to determine the relative importance of wind stresses, net surface heat flux, and freshwater flux on the initiation and maintenance of the decadal oscillation in the North Pacific. It is found that decadal variability is a mode of the coupled system and involves interaction of sea surface temperature, upper-ocean heat content, and wind stress. This interaction is mainly controlled by the wind stress but can be strongly modified by the surface heat flux. The effect of the salinity is relatively small and is not necessary to generate the model decadal oscillation in the North Pacific.

There are some limitations with this study. First, the effect of a stochastic forcing is not included. Second, a weak negative feedback is needed to run the control experiment for a longer time period. These two areas will be addressed in a future investigation.

Abstract

The sensitivity of the global climate system to interannual variability of he Eurasian snow cover has been investigated with numerical models. It was found that heavier than normal Eurasian snow cover in spring leads to a “poor” monsoon over Southeast Asia thereby verifying an idea over 100 years old. The poor monsoon was characterized by reduced rainfall over India and Burma, reduced wind stress over the Indian Ocean, lower than normal temperatures on the Asian land mass and in the overlying atmospheric column, reduced tropical jet, increased soil moisture, and other features associated with poor monsoons. Lighter than normal snow cover led to a “good” monsoon with atmospheric anomalies like those described above but of opposite sign. Remote responses from the snow field perturbation include readjustment of the Northern Hemispheric mass field in midlatitude, an equatorially symmetric response of the tropical geopotential height and temperature field and weak, but significant, perturbations in the surface wind stress and heat flux in the tropical Pacific.

The physics responsible for the regional response involves all elements of both the surface heat budget and heat budget of the full atmospheric column. In essence, the snow, soil and atmospheric moisture all act to keep the land and overlying atmospheric column colder than normal during a heavy snow simulation thus reducing the land–ocean temperature contrast needed to initiate the monsoon. The remote responses are driven by heating anomalies associated with both large scale air-sea interactions and precipitation events.

The model winds from the heavy snow experiment were used to drive an ocean model. The SST field in that model developed a weak El Niño in the equatorial Pacific. A coupled ocean-atmosphere model simulation perturbed only by anomalous Eurasian snow cover was also run and it developed a much stranger El Niño in the Pacific. The coupled system clearly amplified the wind stress anomaly associated with the poor monsoon. These results show the important role of an evolving (not specified) sea surface temperature in numerical experiments and the real climate system. Our general results also demonstrate the importance of land processes in global climate dynamics and their possible role as one of the factors that could trigger ENSO events.

Abstract

The characteristic space–time scales of surface solar radiation fields measured by the 111-instrument MESONET in Oklahoma are estimated after removal of the diurnal cycle. These estimates of “within-day” variability are used to deduce the representativeness of surface solar radiation measurements made at the central ARM measurement site as a function of time-averaging interval. Nomograms of the relation between point measurements and area averages are given for different space–time-averaging intervals. Examples from the nomograms show, for instance, that under conditions of low mean radiation (cloudy days), the central site point measurements are representative of a spatial area the size of a T42 GCM grid box (280 km × 280 km) if one uses hourly averages and is willing to accept a correlation of 0.45 between area average and point measurement. The point data represent a 60 km × 60 km region at a 0.90 correlation level if a 5-min time average is used. The characteristic timescale for the within-day radiation variability was roughly 60 min. Estimates of scale lengths for days when the mean background radiation conditions are high are also given in the nomographs.

Abstract

Two extended integrations of general circulation models (GCMs) are examined to determine the physical processes operating during an ENSO cycle. The first integration is from the Hamburg version of the ECMWF T21 atmospheric model forced with observed global sea surface temperatures (SST) over the period 1970–85. The second integration is from a Max Planck Institut model of the tropical Pacific forced by observed wind stress for the same period. Both integrations produce key observed features of the tropical ocean-atmosphere system during the 1970–85 period.

The atmospheric model results show an eastward propagation of information from the western to eastern Pacific along the equator, although this signal is somewhat weaker than observed. The Laplacian of SST largely drives the surface wind field convergence and hence determines the position of large scale precipitation-condensation heating. This statement is valid only in the near-equatorial zone. Air-sea heat exchange is important in the planetary boundary layer in forcing the wind field convergence but not so important to the main troposphere, which is heated largely by condensation heating. The monopole response seen in the atmosphere above about 500 mb is due to a combination of factors, the most important being adiabatic heating associated with subsidence and tropic-wide variations in precipitation.

The models show the role of air-sea heat exchange in the ocean heat balance in the wave guide is one of dissipation/damping. Total air-sea heat exchange is well represented by a simple Newtonian cooling parameterization in the near-equatorial region. In the wave guide, advection dominates the oceanic heat balance with meridional advection being numerically the most important in all regions except right on the equator. The meridional term is largely explained by local Ekman dynamics that generally overwhelm other processes in the regions of significant wind stress. The principal element in this advection term is the anomalous meridional current acting on the climatological mean meridional SST gradient.

The eastward motion of the anomalies seen in both models is driven primarily by the ocean. The wind stress associated with the SST anomalies forces an equatorial convergence of heat and mass in the ocean. Outside the region of significant wind forcing, the mass source leads to a convergent geostrophic flow, which drives the meridional heat flux and causes warming to the east of the main wind anomaly. West of the main anomaly the wind and geostrophic divergence cause advective cooling. The result is that the main SST anomaly appears to move eastward. Since the direct SST forcing drives the anomalous wind, surface wind convergence, and associated precipitation, these fields are seen also to move eastward.