Anchukaitis, K. J., M. N. Evans, A. Kaplan, E. A. Vaganov, M. K. Hughes, H. D. Grissino-Mayer and M. A. Cane, 2006: Forward modeling of regional scale tree-ring patterns in the southeastern United States and the recent influence of summer drought. Geophysical Research Letters, 33(4): L04705, doi:10.1029/2005GL025050.
We use a mechanistic model of tree-ring formation to simulate regional patterns of climate-tree growth relationships in the southeastern United States. Modeled chronologies are consistent with actual tree-ring data, demonstrating that our simulations have skill in reproducing broad-scale patterns of the proxy's response to climate variability. The model predicts that a decrease in summer precipitation, associated with a weakening Bermuda High, has become an additional control on tree ring growth during recent decades. A nonlinear response of tree growth to climate variability has implications for the calibration of tree-ring records for paleoclimate reconstructions and the prediction of ecosystem responses to climate change.
Biasutti, M. and A. Giannini, 2006: Robust Sahel drying in response to late 20th century forcings. Geophysical Research Letters, 33(11): L11706, doi:10.1029/2006GL026067.
The African Sahel experienced severe drying between the 1950s and the 1980s, with partial recovery since. We compare Sahel rainfall in the 20th century, pre-industrial, and increased greenhouse gases (GHG) simulations produced for the Intergovernmental Panel on Climate Change (IPCC). The simulations forced by 20th century concentrations of aerosol and GHG reproduce (i) a global change in SST akin to that associated with Sahel drought and (ii) a correspondent drying of the Sahel. We conclude that late 20th century Sahel climate was significantly dryer than pre-industrial, and at least 30% of the drying was externally forced. Comparison between 20th century runs and runs forced by GHG alone reveals the key role of reflective aerosols: they force a gradient in SST that excites robust drying in the northern edge of the Atlantic Inter-Tropical Convergence Zone (ITCZ) and in the Sahel.
Biasutti, M., A. H. Sobel and Y. Kushnir, 2006: AGCM precipitation biases in the tropical Atlantic. Journal of Climate, 19(6): 935-958.
Many general Circulation models (GCMs) share similar biases in the representation of the intertropical convergence zone (ITCZ) in the Atlantic, even when they are forced with the time series of the observed sea surface temperature (SST). Specifically, they overestimate precipitation in the Southern Hemisphere in boreal spring and in the Caribbean region in boreal Summer.
Cane, M. A., P. Braconnot, A. Clement, H. Gildor, S. Joussaume, M. Kageyama, M. Khodri, D. Paillard, S. Tett and E. Zorita, 2006: Progress in paleoclimate modeling. Journal of Climate, 19(20): 5031-5057.
This paper briefly surveys areas of paleoclimate modeling notable for recent progress. New ideas, including hypotheses giving a pivotal role to sea ice, have revitalized the low-order models used to simulate the time evolution of glacial cycles through the Pleistocene, a prohibitive length of time for comprehensive general circulation models (GCMs). In a recent breakthrough, however, GCMs have succeeded in simulating the onset of glaciations. This occurs at times (most recently, 115 kyr B.P.) when high northern latitudes are cold enough to maintain a snow cover and tropical latitudes are warm, enhancing the moisture source. More generally, the improvement in models has allowed simulations of key periods such as the Last Glacial Maximum and the mid-Holocene that compare more favorably and in more detail with paleoproxy data. These models now simulate ENSO cycles, and some of them have been shown to reproduce the reduction of ENSO activity observed in the early to middle Holocene. Modeling studies have demonstrated that the reduction is a response to the altered orbital configuration at that time. An urgent challenge for paleoclimate modeling is to explain and to simulate the abrupt changes observed during glacial epochs (i.e., Dansgaard-Oescher cycles, Heinrich events, and the Younger Dryas). Efforts have begun to simulate the last millennium. Over this time the forcing due to orbital variations is less important than the radiance changes due to volcanic eruptions and variations in solar output. Simulations of these natural variations test the models relied on for future climate change projections. They provide better estimates of the internal and naturally forced variability at centennial time scales, elucidating how unusual the recent global temperature trends are.
Cook, B. I., G. B. Bonan and S. Levis, 2006: Soil moisture feedbacks to precipitation in southern Africa. Journal of Climate, 19(17): 4198-4206.
The effects of increased soil moisture on wet season ( October - March) precipitation in southern Africa are investigated using the Community Climate System Model version 3 (CCSM3). In the CTRL case, soil moisture is allowed to interact dynamically with the atmosphere. In the MOIST case, soil moisture is set so that evapotranspiration is not limited by the supply of water. The MOIST scenario actually results in decreased precipitation over the region of perturbed soil moisture, compared to CTRL. The increased soil moisture alters the surface energy balance, resulting in a shift from sensible to latent heating. This manifests in two ways relevant for precipitation processes. First, the shift from sensible to latent heating cools the surface, causing a higher surface pressure, a reduced boundary layer height, and an increased vertical gradient in equivalent potential temperature. These changes are indicative of an increase in atmospheric stability, inhibiting vertical movement of air parcels and decreasing the ability of precipitation to form. Second, the surface changes induce anomalous surface divergence and increased subsidence. This causes a reduction in cloud cover and specific humidity above 700 hPa and results in a net decrease of column-integrated precipitable water, despite the increased surface water flux, indicating a reduction in moisture convergence. Based on this and a previous study, soil moisture may act as a negative feedback to precipitation in southern Africa, helping to buffer the system against any external forcing of precipitation ( e. g., ENSO).
Dominguez, F., P. Kumar, X. Z. Liang and M. F. Ting, 2006: Impact of atmospheric moisture storage on precipitation recycling. Journal of Climate, 19(8): 1513-1530.
Computations of precipitation recycling using analytical models are generally performed under the assumption of negligible change in moisture storage in the atmospheric column. Because the moisture storage term is nonnegligible at smaller time scales, most recycling studies using analytical models are done at monthly or longer time scales. A dynamic precipitation recycling model, which incorporates the change in moisture storage, is developed. If is derived formally from the conservation of mass equation and is presented in a simple and computationally efficient form. This model allows for recycling analysis at a range of temporal scales, from daily to monthly and longer. In comparison to the traditional models that do not include the storage term, the new model presents almost identical spatial and temporal variability. but predicts recycling ratios that are 12%-33% larger at a monthly level.
Evans, M. N., B. K. Reichert, A. Kaplan, K. J. Anchukaitis, E. A. Vaganov, M. K. Hughes and M. A. Cane, 2006: A forward modeling approach to paleoclimatic interpretation of tree-ring data. Journal of Geophysical Research-Biogeosciences, 111(G3): doi:10.1029/2006JG000166.
We investigate the interpretation of tree-ring data using the Vaganov-Shashkin forward model of tree-ring formation. This model is derived from principles of conifer wood growth, and explicitly incorporates a nonlinear daily timescale model of the multivariate environmental controls on tree-ring growth. The model results are shown to be robust with respect to primary moisture and temperature parameter choices. When applied to the simulation of tree-ring widths from North America and Russia from the Mann et al. (1998) and Vaganov et al. (2006) data sets, the forward model produces skill on annual and decadal timescales which is about the same as that achieved using classical dendrochronological statistical modeling techniques. The forward model achieves this without site-by-site tuning as is performed in statistical modeling. The results support the interpretation of this broad-scale network of tree-ring width chronologies primarily as climate proxies for use in statistical paleoclimatic field reconstructions, and point to further applications in climate science.
Gan, J. P., H. Li, E. N. Curchitser and D. B. Haidvogel, 2006: Modeling South China sea circulation: Response to seasonal forcing regimes. Journal of Geophysical Research-Oceans, 111(C6): doi:10.1029/2005JC003298.
A three-dimensional ocean model has been utilized to study circulation and its seasonal variation in the South China Sea (SCS) in response to the forcing of the Asian monsoon and the Kuroshio intrusion. The SCS ocean model has a resolution of approximately 10 km horizontal spacing and 30 vertical levels with a realistic bottom topography. The model is forced with time-dependent wind stress and heat flux from National Center for Environmental Prediction Reanalysis data as well as with lateral fluxes from a Pacific Ocean model of 40 km horizontal resolution. This study reports on the analysis of the mean seasonal circulation and dynamic processes in response to monsoonal wind stress, the Kuroshio intrusion, and other intrinsic forcing processes. It is found that the seasonal circulation in the SCS is mainly driven by the monsoonal wind stress and greatly influenced by the inflow from the Kuroshio intrusion. Strong currents along the continental margin of the SCS form mean basin-wide cyclonic and anticyclonic circulations in the winter and summer, respectively. Multiscale eddies are embedded in the general circulation across the basin. While mainstream of the Kuroshio passes through the Luzon Strait without intruding into the SCS, partial intrusion occurs in the upper 200 m near the shelf margin southwest of Taiwan at times when winter dynamic conditions prevail in the north SCS. The intrusion of the Kuroshio into the SCS also occurs at depths in all seasons, mainly along the continental slope. The coastal current separation to the east off southern Vietnam and the associated eddy formations characterize the circulation in the south SCS. The simulated results compare well with the corresponding observed fields. Dynamical processes involved in the forced flow fields are investigated by examination of the momentum balances. The analyses reveal that the circulation in the SCS is generally dominated by the geostrophic currents. North of the Luzon Strait, positive nonlinearity in the zonal direction is locally intensified, which leads to the formation of centripetal acceleration for the mainstream of the Kuroshio to turn eastward. The Kuroshio intrusion at depths is governed by the ageostrophic flows and highly associated with the net westward pressure gradient force. Coastal jet separation to the east off Vietnam is mainly associated with the local wind stress field and with the shelf topography in the summer and winter, respectively. Sensitivity study reveals that the weakening of the Kuroshio markedly enhances Kuroshio's intrusion and forms an anticyclonic eddy west of the Luzon Strait.
Giulivi, C. F. and A. L. Gordon, 2006: Isopycnal displacements within the Cape Basin thermocline as revealed by the Hydrographic Data Archive. Deep-Sea Research Part I-Oceanographic Research Papers, 53(8): 1285-1300.
The transfer of upper kilometer water from the Indian Ocean into the South Atlantic, the Agulhas leakage, is believed to be accomplished primarily through meso-scale eddy processes. There have been various studies investigating eddies of the "Cape Basin Cauldron" from specific data sets. The hydrographic data archive acquired during the last century within the Cape Basin region of the South Atlantic provides additional insight into the distribution and water mass properties of the Cape Basin eddies. Eddies are identified by mid-thermocline isopycnal depth anomalies relative to the long-term mean. Positive depth anomalies (the reference isopycnal is deeper than the long-term mean isopycnal depth) mark the presence of anticyclonic eddies; negative anomalies mark cyclonic eddies. Numerous eddies are identified in the whole region; the larger isopycnal displacements are attributed to the energetic eddies characteristic of the Cape Basin and indicate that there is a 2:1 anticyclone/cyclone ratio. Smaller displacements of the less energetic features are almost equally split between anticyclones and cyclones (1.4:1 ratio). Potential temperature, salinity and oxygen relationships at thermocline and intermediate levels within each eddy reveal their likely origin. The eddy core water is not solely drawn from Indian Ocean: tropical and subtropical South Atlantic water are also present. Anticyclones and cyclones carrying Agulhas Water properties are identified throughout the Cape Basin. Anticyclones with Agulhas Water characteristics show a predominant northwest dispersal, whereas the cyclones are identified mainly along the western margin of the African continent, possibly related to their origin as shear eddies at the boundary between the Agulhas axis and Africa. Cyclones and anticyclones carrying pure South Atlantic origin water are identified south of 30 degrees S and west of the Walvis Ridge. Tropical Atlantic water at depth is found for cyclones north of the Walvis Ridge, west of 10 degrees E and for stations deeper than 4000 m, and a few anticyclones with the same characteristics are found south of the ridge. (c) 2006 Elsevier Ltd. All rights reserved.
Gorodetskaya, I. V., M. A. Cane, L. B. Tremblay and A. Kaplan, 2006: The effects of sea-ice and land-snow concentrations on planetary albedo from the earth radiation budget experiment. Atmosphere-Ocean, 44(2): 195-205.
The high-latitude ice/snow-albedo feedback is a principal element in many pateoclimate theories and global warming scenarios. The strength of this feedback is determined by the ice/snow effects on the top-of-atmosphere (TOA) albedo, which is also strongly affected by clouds. Using currently available satellite observations, we estimate the radiative effectiveness (RE) of ice and snow with regards to the TOA albedo, which we define as the change in the TOA albedo corresponding to changes of 0% to 100% in the ice or snow cover. The REs of the northern hemisphere (NH) sea ice, land snow, and southern hemisphere (SH) sea ice are found to be 0.22, 0.23 and 0.16, respectively. This means that, for an incident solar flux of about 400 W m(-2) reaching the TOA it? the polar latitudes in summer, local reduction in ice/snow concentrations from 100% to 0% will result in a decrease in reflected short wave radiation of approximately 80 W m(-2). These changes in the TOA albedo are significant, yet smaller than the associated changes in the surface albedo. Comparison of the TOA albedo values with available surface albedo observations helps to identify the role of clouds in the RE of ice/snow. The analysis is based on the whole time-space domain where the sect ice and land snow appear, and reveals a remarkable similarity in the ice and snow RE in the areas with high sea-ice and land-snow cover variability, despite the varying nature of the surface cover, seasonality, and locations. These estimates provide a useful constraint to test current climate models.
He, Z. G., Z. Q. Dong and X. J. Yuan, 2006: Fronts and strong currents of the upper southeast Indian Ocean. Acta Oceanologica Sinica, 25(2): 1-24.
Hydrographic data, ADCP velocity and sea level anomaly derived from the satellite altimeter have been jointly analyzed in the southeast Indian Ocean. Results show the locations and orientations of the major oceanic fronts as well as the characteristics of the currents within these fronts in the area. Double subtropical fronts are observed in the section along 120 degrees E, which conflicts with the frontal structure frequently observed before-the North Subtropical Front (NSTF) and South Subtropical Front (SSTF) merge into a single STF between 110 degrees similar to 115 degrees E. The Subantarctic Front (SAF), influenced by the out-of-phase double eddies, runs across 48 degrees S three times between 120 degrees and 127 degrees E. The surface current within the SAF is strengthened up to 105.4 cm/s by the geostrophic effect of these eddies Furthermore eddies may cause the strong current to split up into two branches within the SAF. The SAF and the primary polar front (PF1) can be identified individually in the ADCP data with a separation distance of about 0.3 degrees at latitude between 140 degrees and 145 degrees E, although they cannot be identified separately in the low-resolution hydrographic data. The different thermohaline characteristics of Circumpolar Deep Water (CDW) and Modified Circumpolar Deep Water (MCDW) result in the formation of Southern Antarctic Circumpolar Current Front (SACCF) in the southeast Indian Ocean. It consistently turns northward along the east flank of the Kerguelen Plateau after it runs through the Princess Elizabeth Trough and turns southward sharply north of 60 degrees S with a little seasonal variations. It is shown that the locations and orientations of the SAF, the primary PF and SACCF in the ACC of the southeast Indian Ocean can be identified more precisely by the current distribution derived from ADCP data than by hydrographic data, because these fronts are usually accompanied by strong currents. However, the locations and orientations of the STF and the secondary PF are more difficult to be identified through current data, since these two fronts are usually not accompanied by any jet. The STF the and the secondary PF are usually confined in the first few hundred meters of the upper ocean and the latter is often determined by the northern terminus of 2 degrees C isothermal.
Herweijer, C., R. Seager and E. R. Cook, 2006: North American droughts of the mid to late nineteenth century: a history, simulation and implication for Mediaeval drought. Holocene, 16(2): 159-171.
Unlike the major droughts of the twentieth century that are readily identified in the instrumental record, similar events in the nineteenth century have to be identified using a combination of proxy data, historical accounts and a sparse collection of early instrumental records. In the USA, three distinct periods of widespread and persistent drought stand out in these records for the latter half of the nineteenth century: 1856-1865, 1870-1877 and 1890-1896. Each of these events is shown to coincide with the existence of an anomalously cool, La Nina-like tropical Pacific. To examine the physical mechanisms behind these droughts two ensembles of simulations with an atmosphere general circulation model (AGCM) were generated: the First forces an AGCM with the observed history of Sea Surface Temperatures (SSTs) everywhere front 1856 to 2001 (the GOGA experiment), the second forces the AGCM only with tropical Pacific SSTs, being coupled to a two-layer entraining mixed layer (M L) ocean elsewhere (the POGA-ML experiment). Owing to a sparsity of instrumental precipitation data at this time, proxy evidence from tree rings is used as verification. A comparison of modelled soil moisture with tree-ring reconstructions of the Palmer Drought Severity Index (PDSI), a proxy for soil moisture, from the North American Drought Atlas is made. Both the POGA-ML and GOGA ensemble means capture the three multi-year droughts of the mid to late nineteenth century, indicating that the droughts were SST forced. The similarity of the POGA-ML and GOGA simulations implies that the component of each drought signal that is forced by the SST is driven ultimately by the La Nina-like tropical Pacific. The global atmosphere-ocean context of each of the mid to late-nineteenth century droughts reveals it zonally and hemispherically symmetric pattern consistent with forcing from the tropics. in addition, Rossby wave propagation from the cooler equatorial Pacific amplifies dry conditions over the USA. Finally, using published coral data for the last millennium to reconstruct a NINO 3.4 history, the modern-day relationship between NINO 3.4 and North American drought is applied to recreate two of the severest Mediaeval 'drought epochs' in the western USA. The large-scale spatial similarity to the Drought Atlas data demonstrates the potential link between a colder eastern equatorial Pacific and the persistent North American droughts of the Mediaeval period.
Hurrell, J. W., M. Visbeck, A. Busalacchi, R. A. Clarke, T. L. Delworth, R. R. Dickson, W. E. Johns, K. P. Koltermann, Y. Kushnir, D. Marshall, C. Mauritzen, M. S. McCartney, A. Piola, C. Reason, G. Reverdin, F. Schott, R. Sutton, I. Wainer and D. Wright, 2006: Atlantic climate variability and predictability: A CLIVAR perspective. Journal of Climate, 19(20): 5100-5121.
Three interrelated climate phenomena are at the center of the Climate Variability and Predictability (CLIVAR) Atlantic research: tropical Atlantic variability (TAV), the North Atlantic Oscillation (NAO), and the Atlantic meridional overturning circulation (MOC). These phenomena produce a myriad of impacts on society and the environment on seasonal, interannual, and longer time scales through variability manifest as coherent fluctuations in ocean and land temperature, rainfall, and extreme events. Improved understanding of this variability is essential for assessing the likely range of future climate fluctuations and the extent to which they may be predictable, as well as understanding the potential impact of human-induced climate change. CLIVAR is addressing these issues through prioritized and integrated plans for short-term and sustained observations, basin-scale reanalysis, and modeling and theoretical investigations of the coupled Atlantic climate system and its links to remote regions. In this paper, a brief review of the state of understanding of Atlantic climate variability and achievements to date is provided. Considerable discussion is given to future challenges related to building and sustaining observing systems, developing synthesis strategies to support understanding and attribution of observed change, understanding sources of predictability, and developing prediction systems in order to meet the scientific objectives of the CLIVAR Atlantic program.
Karspeck, A. R., A. Kaplan and M. A. Cane, 2006: Predictability loss in an intermediate ENSO model due to initial error and atmospheric noise. Journal of Climate, 19(15): 3572-3588.
The seasonal and interannual predictability of ENSO variability in a version of the Zebiak - Cane coupled model is examined in a perturbation experiment. Instead of assuming that the model is "perfect," it is assumed that a set of optimal initial conditions exists for the model. These states, obtained through a nonlinear minimization of the misfit between model trajectories and the observations, initiate model forecasts that correlate well with the observations. Realistic estimates of the observational error magnitudes and covariance structures of sea surface temperatures, zonal wind stress, and thermocline depth are used to generate ensembles of perturbations around these optimal initial states, and the error growth is examined. The error growth in response to subseasonal stochastic wind forcing is presented for comparison.
Kent, E. C. and A. Kaplan, 2006: Toward estimating climatic trends in SST. Part III: Systematic biases. Journal of Atmospheric and Oceanic Technology, 23(3): 487-500.
A method is developed to quantify systematic errors in two types of sea surface temperature (SST) observations: bucket and engine-intake measurements. A simple linear model is proposed where the SST measured using a bucket is cooled or warmed by a fraction of the air-sea temperature difference and the SST measured using an engine intake has a constant bias. The model is applied to collocated nighttime observations made at moderate wind speeds. allowing the effects of solar radiation and strong vertical gradients in the upper ocean to be neglected The analysis is complicated by large random errors in all of the variables used. To estimate coefficients in this model, a novel type of linear regression. where errors in two variables are correlated with each other. is introduced. Because of the uncertainty in a priori estimates of the error covariance matrix, a Bayesian analysis of the regression problem is developed. and maximum likelihood approximations to the posterior distributions of the model parameters are obtained. Results show that the temperature change in bucket SST resulting from the air-sea temperature difference can be detected. The analysis suggests that bucket SST may be in error by a fraction from 0.12 degrees +/- 0.02 degrees to 0.16 degrees +/- 0.02 degrees C of the air-sea temperature difference. When this temperature change of the bucket SST is accounted for. a warm bias in engine-intake SST in the mid- to late 1970s and the 1980s was found to be smaller than that suggested by previous studies, ranging between 0.09 degrees +/- 0.06 degrees and 0.18 degrees +/- 0.05 degrees C. For the early 1990s the model suggests that the engine-intake SSTs may have a cold bias of -0.13 degrees +/- 0.07 degrees C.
Kieke, D., M. Rhein, L. Stramma, W. M. Smethie, D. A. LeBel and W. Zenk, 2006: Changes in the CFC inventories and formation rates of Upper Labrador Sea Water, 1997-2001. Journal of Physical Oceanography, 36(1): 64-86.
Chlorofluorocarbon ( component CFC-11) and hydrographic data from 1997, 1999, and 2001 are presented to track the large-scale spreading of the Upper Labrador Sea Water (ULSW) in the subpolar gyre of the North Atlantic Ocean. ULSW is CFC rich and comparatively low in salinity. It is located on top of the denser "classical" Labrador Sea Water (LSW), defined in the density range sigma(Theta) = 27.68 - 27.74 kg m(-3). It follows spreading pathways similar to LSW and has entered the eastern North Atlantic. Despite data gaps, the CFC-11 inventories of ULSW in the subpolar North Atlantic (40 degrees - 65 degrees N) could be estimated within 11%. The inventory increased from 6.0 +/- 0.6 million moles in 1997 to 8.1 +/- 0.6 million moles in 1999 and to 9.5 +/- 0.6 million moles in 2001. CFC-11 inventory estimates were used to determine ULSW formation rates for different periods. For 1970 - 97, the mean formation rate resulted in 3.2 - 3.3 Sv ( Sv = 10(6) m(3) s(-1)). To obtain this estimate, 5.0 million moles of CFC-11 located in 1997 in the ULSW in the subtropical/tropical Atlantic were added to the inventory of the subpolar North Atlantic. An estimate of the mean combined ULSW/LSW formation rate for the same period gave 7.6 - 8.9 Sv. For the years 1998 - 99, the ULSW formation rate solely based on the subpolar North Atlantic CFC-11 inventories yielded 6.9 - 9.2 Sv. At this time, the lack of classical LSW formation was almost compensated for by the strongly pronounced ULSW formation. Indications are presented that the convection area needed in 1998 - 99 to form this amount of ULSW exceeded the available area in the Labrador Sea. The Irminger Sea might be considered as an additional region favoring ULSW formation. In 2000 - 01, ULSW formation weakened to 3.3 - 4.7 Sv. Time series of layer thickness based on historical data indicate that there exists considerable variability of ULSW and classical LSW formation on decadal scales.
Kunze, E., E. Firing, J. M. Hummon, T. K. Chereskin and A. M. Thurnherr, 2006: Global abyssal mixing inferred from lowered ADCP shear and CTD strain profiles. Journal of Physical Oceanography, 36(8): 1553-1576.
Internal wave - wave interaction theories and observations support a parameterization for the turbulent dissipation rate epsilon and eddy diffusivity K that depends on internal wave shear [V-z(2)] and strain [xi(2)(z)] variances. Its latest incarnation is applied to about 3500 lowered ADCP/CTD profiles from the Indian, Pacific, North Atlantic, and Southern Oceans. Inferred diffusivities K are functions of latitude and depth, ranging from 0.03 x 10(-4) m(2) s(-1) within 2 degrees of the equator to (0.4 - 0.5) x 10(-4) m(2) s(-1) at 50 degrees-70 degrees. Diffusivities K also increase with depth in tropical and subtropical waters. Diffusivities below 4500-m depth exhibit a peak of 0.7 x 10(-4) m(2) s(-1) between 20 degrees and 30 degrees, latitudes where semidiurnal parametric subharmonic instability is expected to be active. Turbulence is highly heterogeneous. Though the bulk of the vertically integrated dissipation integral(epsilon) is contributed from the main pycnocline, hotspots in integral(epsilon) show some correlation with small-scale bottom roughness and near-bottom flow at sites where strong surface tidal dissipation resulting from tide - topography interactions has been implicated. Average vertically integrated dissipation rates are 1.0 mW m(-2), lying closer to the 0.8 mW m(-2) expected for a canonical (Garrett and Munk) internal wave spectrum than the global-averaged deep-ocean surface tide loss of 3.3 mW m(-2).
Kushnir, Y., W. A. Robinson, P. Chang and A. W. Robertson, 2006: The physical basis for predicting Atlantic sector seasonal-to-interannual climate variability. Journal of Climate, 19(23): 5949-5970.
This paper reviews the observational and theoretical basis for the prediction of seasonal-to-interannual (S/I) climate variability in the Atlantic sector. The emphasis is on the large-scale picture rather than on regional details. The paper is divided into two main parts: a discussion of the predictability of the North Atlantic Oscillation (NAO)-the dominant pattern of variability in the North Atlantic-and a review of the tropical Atlantic prediction problem. The remote effects of El Nino are also mentioned as an important factor in Atlantic climate variability. Only a brief discussion is provided on the subject of South Atlantic climate predictability.
Linsley, B. K., A. Kaplan, Y. Gouriou, J. Salinger, P. B. Demenocal, G. M. Wellington and S. S. Howe, 2006: Tracking the extent of the South Pacific Convergence Zone since the early 1600s. Geochemistry Geophysics Geosystems, 7: doi:10.1029/2005GC001115.
[ 1] The South Pacific Convergence Zone (SPCZ) is the largest and most persistent spur of the Intertropical Convergence Zone. At the southeastern edge of the SPCZ near 170 degrees W and 15 degrees - 20 degrees S a surface ocean salinity frontal zone exists that separates fresher Western Pacific Warm Pool water from saltier and cooler waters in the east. This salinity front is known to shift east and west with the phase of the El Niño Southern Oscillation. We have generated subannually resolved and replicated coral oxygen isotopic time series from Fiji ( 17 degrees S, 179 degrees E) and Rarotonga (21.5 degrees S, 160 degrees W) that have recorded interannual displacements of the salinity front over the last 380 years and also indicate that at lower frequencies the decadal mean position of the salinity front, and eastern extent of the SPCZ, has shifted east-west through 10 degrees to 20 degrees of longitude three times during this interval. The most recent and largest shift began in the mid 1800s as the salinity front progressively moved eastward and salinity decreased at both sites. Our results suggest that sea surface salinity at these sites is now at the lowest levels recorded and is evidence for an unprecedented expansion of the SPCZ since the mid 1800s. The expansion of the SPCZ implies a gradual change in the South Pacific to more La Niña-like long-term mean conditions. This observation is consistent with the ocean thermostat mechanism for the Pacific coupled ocean-atmosphere system, whereby exogenous heating of the atmosphere would result in greater warming in the western Pacific and a greater east-west surface temperature gradient.
Massom, R. A., S. E. Stammerjohn, R. C. Smith, M. J. Pook, R. A. Iannuzzi, N. Adams, D. G. Martinson, M. Vernet, W. R. Fraser, L. B. Quetin, R. M. Ross, Y. Massom and H. R. Krouse, 2006: Extreme anomalous atmospheric circulation in the West Antarctic Peninsula region in Austral Spring and Summer 2001/02, and its profound impact on sea ice and biota. Journal of Climate, 19(15): 3544-3571.
Exceptional sea ice conditions occurred in the West Antarctic Peninsula (WAP) region from September 2001 to February 2002, resulting from a strongly positive atmospheric pressure anomaly in the South Atlantic coupled with strong negative anomalies in the Bellingshausen - Amundsen and southwest Weddell Seas. This created a strong and persistent north-northwesterly flow of mild and moist air across the WAP. In situ, satellite, and NCEP - NCAR Reanalysis (NNR) data are used to examine the profound and complex impact on regional sea ice, oceanography, and biota. Extensive sea ice melt, leading to an ocean mixed layer freshening and widespread ice surface flooding, snow - ice formation, and phytoplankton growth, coincided with extreme ice deformation and dynamic thickening. Sea ice dynamics were crucial to the development of an unusually early and rapid ( short) retreat season ( negative ice extent anomaly). Strong winds with a dominant northerly component created an unusually compact marginal ice zone and a major increase in ice thickness by deformation and over-rafting. This led to the atypical persistence of highly compact coastal ice through summer. Ecological effects were both positive and negative, the latter including an impact on the growth rate of larval Antarctic krill and the largest recorded between-season breeding population decrease and lowest reproductive success in a 30-yr Adelie penguin demographic time series. The unusual sea ice and snow cover conditions also contributed to the formation of a major phytoplankton bloom. Unexpectedly, the initial bloom occurred within compact sea ice and could not be detected in Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) ocean color data. This analysis demonstrates that sea ice extent alone is an inadequate descriptor of the regional sea ice state/conditions, from both a climatic and ecological perspective; further information is required on thickness and dynamics/deformation.
Newton, B., L. B. Tremblay, M. A. Cane and P. Schlosser, 2006: A simple model of the Arctic Ocean response to annular atmospheric modes. Journal of Geophysical Research-Oceans, 111(C9): doi:10.1029/2004JC002622.
[ 1] A dynamical mechanism is described that modulates the tilt of the sea-surface height and pycnocline depth between the central Arctic and the continental shelves. A simple analytical model is presented, forced with idealized zonal winds over an idealized, 2-layer, cylinder representing the Arctic Ocean. Ekman transports are linked to sea-surface and pycnocline tilt anomalies and basin-scale circulation in response to an annular wind anomaly. We compare the results to tide gauge data, as well as results from a more realistic numerical simulation and find that the model explains a major fraction of the interannual-to-decade scale sea-surface height anomalies at Arctic coastal tide gauges. The analytical model indicates, for example, that on the order of 10 cm of the observed rise of about 18 cm in coastal Arctic sea-surface height between about 1985 and 1993 was probably a response to increased Westerly winds associated with a strong positive phase of the Northern Annular Mode of atmospheric variability. The pycnocline depth anomaly time series from the model is used to calculate implied changes in the outflow of relatively fresh Polar Water to the North Atlantic. The comparisons indicate that the Ekman transport mechanism is important to changes in the export of buoyancy from the Arctic Ocean on seasonal, interannual, and decadal timescales.
Ou, H. W. and D. Chen, 2006: Wind-induced shear dispersion and genesis of the shelf-break front. Progress in Oceanography, 70(2-4): 313-330.
Through a simple analytical model, we examine the shear dispersion associated with oscillatory winds in an unstratified coastal ocean. As noted previously in the tidal regime, the vertical -integrated (total) horizontal diffusivity has a maximum where the water depth equals the diffusive depth - defined as the reach of the vertical diffusion during one forcing cycle. Due principally to the long synoptic timescale that characterizes the wind forcing, this depth lies over the outer shelf. When combined with effective mixing of the slope water by meso-scale eddies, the total diffusivity exhibits a minimum around the shelf break, thus facilitating frontogenesis. Due again to the long forcing period, the bottom Ekman flow is well developed at the diffusive depth, which would accentuate the gradient enhancement of the front over the inshore water, which however is bounded above by doubling.
Ou, H. W., 2006: Meridional thermal field of a coupled ocean-atmosphere system: a conceptual model. Tellus Series a-Dynamic Meteorology and Oceanography, 58(3): 404-415.
This paper constitutes the author's continuing effort in the construction of a minimal theory of the earth's climate. In an earlier paper published in the Journal of Climate in 2001, this author has derived the global-mean fields of an aquatic planet forced by the solar insulation, which provide the necessary constraints for the present derivation of the meridional thermal field. The model closure invokes maximized entropy production (MEP), a thermodynamic principle widely used in turbulence and climate studies.
Rauscher, S. A., A. Seth, J. H. Qian and S. J. Camargo, 2006: Domain choice in an experimental nested modeling prediction system for South America. Theoretical and Applied Climatology, 86(1-4): 229-246.
The purposes of this paper are to evaluate the new version of the regional model, RegCM3, over South America for two test seasons, and to select a domain for use in an experimental nested prediction system, which incorporates RegCM3 and the European Community-Hamburg (ECHAM) general circulation model (GCM). To evaluate RegCM3, control experiments were completed with RegCM3 driven by both the NCEP/NCAR Reanalysis (NNRP) and ECHAM, using a small control domain (D-CTRL) and integration periods of January-March 1983 (El Niño) and January-March 1985 (La Niña). The new version of the regional model captures the primary circulation and rainfall differences between the two years over tropical and subtropical South America. Both the NNRP-driven and ECHAM-driven RegCM3 improve the simulation of the Atlantic intertropical convergence zone (ITCZ) compared to the GCM. However, there are some simulation errors. Irrespective of the driving fields, weak northeasterlies associated with reduced precipitation are observed over the Amazon. The simulation of the South Atlantic convergence zone is poor due to errors in the boundary condition forcing which appear to be amplified by the regional model.
Smerdon, J. E., H. N. Pollack, V. Cermak, J. W. Enz, M. Kresl, J. Safanda and J. F. Wehmiller, 2006: Daily, seasonal and annual relationships between air and subsurface temperatures. J. Geophys. Res., 111(D07101): doi:10.1029/2004JD005578.
Inversions of borehole temperature profiles that reconstruct past ground surface
Susanto, R. D., T. S. Moore and J. Marra, 2006: Ocean color variability in the Indonesian Seas during the SeaWiFS era. Geochemistry Geophysics Geosystems, 7: Q05021, doi:10.1029/2005GC001009.
More than 6 years of satellite-derived ocean color (SeaWiFS) and 7 years of sea surface temperature (AVHRR) and sea surface wind (ERS1/2, NSCAT, and QuikSCAT) are investigated for the Indonesian Seas. Harmonic analysis and monthly means in ocean color indicate that during the southeast Asia-Australia monsoon southeasterly wind from Australia generates upwelling and brings cooler and nutrient-rich water near the surface, enhancing productivity and increasing ocean color in the Banda Sea and the southern coasts of Jawa (Java)-Sumatra. Conditions are reversed during the northwest monsoon. The northwest wind induces downwelling and produces a weaker biological response in terms of ocean color. Anomalous winds associated with the 1997-1998 El Niño/La Niña events coinciding with the Indian Ocean Dipole (IOD) produced significant departures from the 6-year monthly mean in both magnitude and timing of the seasonal response to the southeast monsoon. Ocean color intensified in the upwelling region along the southern coast of Jawa-Sumatra, and the area of increased amplitude extended westward and prolonged the southeast monsoon period. In addition, localized minimum values of ocean color are observed along the exit pathways of the Indonesian Throughflow.
Thurnherr, A. M., 2006: Diapycnal mixing associated with an overflow in a deep submarine canyon. Deep-Sea Research Part II-Topical Studies in Oceanography, 53(1-2): 194-206.
In order to close the global overturning circulation the buoyancy loss to the atmosphere associated with the formation of deep and bottom water at high latitudes must be balanced by buoyancy gain elsewhere. In case of water that is not in contact with the atmosphere except in its formation region the required buoyancy gain is accomplished primarily by diapycnal mixing. Recent observations suggest that a significant portion of the diapycnal buoyancy fluxes in the abyss is associated with overflows in submarine valleys or canyons, especially on the flanks of slow-spreading mid-ocean ridges. In the present study, hydrographic and velocity data from an overflow across a sill rising 1000 m above the floor of the rift valley of the Mid-Atlantic Ridge near 36 degrees N were used to infer the associated diapycnal mixing. The cross-sill density gradients are characterized by a vertical dipole, with an upper layer of streamwise density increase overlying a lower layer where the density decreases along the flow. This is qualitatively consistent with the effects of diapycnal mixing transferring buoyancy from the upper into the lower layer. Hydrographic and velocity profiles provide evidence for strong mixing associated with the overflow across the sill: in addition to thick layers that are susceptible to shear instability, there are nurnerous density overturns indicating a spatially averaged diffusivity of order 10(-2) m(2) s(-1). Inversions of the density equation reveal that the cross-sill gradients can be accounted for either by adiabatic vertical advection or by vertical eddy diffusion, although the adiabatic solutions are considered energetically implausible because they require upwelling in the layer of cross-sill density increase. Eddy-diffusive solutions, both with and without adiabatic downwelling, are characterized by diffusivity profiles attaining local maxima of order 10(-2) m(2) s(-1) approximate to 250 m above the sill depth. The vertical structure of the inversion-derived diffusivities is consistent with the Thorpe-scale based estimates and with observations from overflows in major ocean passages, which also are often characterized by local diffusivity maxima of order 10(-2) m(2) s(-1) a few 100 m above sill depth. Combining the diffusivity profiles near the sill with budget-derived bulk estimates for 150 km of rift valley implies that; approximate to 50% of the mixing in the rift valley is associated with overflows, while the remainder is caused by other processes, including the breaking of tidally forced internal waves. (c) 2006 Elsevier Ltd. All rights reserved.
Ting, M. F. and H. L. Wang, 2006: The role of the north American topography on the maintenance of the great plains summer low-level jet. Journal of the Atmospheric Sciences, 63(3): 1056-1068.
Summer precipitation over the central United States depends strongly on the strength of the Great Plains low-level jet (LLJ). The Geophysical Fluid Dynamics Laboratory's new generation of the atmospheric general circulation model (GCM) and the linear and nonlinear stationary wave models are used in this study to examine the role of North American topography in maintaining the Great Plains summer mean LLJ and precipitation. Atmospheric GCM experiments were first performed with and without the North American topography and with prescribed climatological sea surface temperatures. Results show that the Great Plains LLJ disappears completely in the experiment when the North American topography is removed, while the summer seasonal mean LLJ is well simulated in the experiment with full earth topography. In the absence of the North American topography, the summer precipitation is significantly reduced over the central United States and increased along the Gulf States and northeast Mexico.
Wang, G. H., D. K. Chen and J. L. Su, 2006: Generation and life cycle of the dipole in the South China Sea summer circulation. Journal of Geophysical Research-Oceans, 111(C6): C06002, doi:10.1029/2005JC003314.
The South China Sea (SCS) summer circulation often has a dipole structure associated with an eastward jet, appearing off central Vietnam. The dipole has an anticyclonic eddy (AE) south of the jet and a cyclonic eddy (CE) north of it. The life cycle of the dipole structure is analyzed using satellite altimetry data and a reduced gravity model. On average the dipole structure begins in June, peaks in strength in August or September, and disappears in October. The dipole evolution lags behind the basin scale wind by about 40 days, and 40 days are exactly what it takes for baroclinic planetary waves to cross the southern SCS. Our results show that the vorticity transports from the nonlinear effect of the western boundary currents are crucial for the generation of the dipole structure. In addition, the strength and direction of the offshore wind jet also play a significant role in determining the magnitudes and the core positions of the two concomitant eddies.
Ziv, B., U. Dayan, Y. Kushnir, C. Roth and Y. Enzel, 2006: Regional and global atmospheric patterns governing rainfall in the southern Levant. International Journal of Climatology, 26(1): 55-73.
This study attempts to find a linkage between the interannal variations of the rainfall measured in 12 stations spread over the northern half of Israel (the southern Levant) and atmospheric circulations ranging from regional to global scale. The analysis was done for the midwinter months, December-February, in which two-thirds of the annual rainfall occurs, during the years 1950-2002. The study is based on composite maps for extremely dry/wet seasons and on maps of correlation between atmospheric variables and the rainfall time series.
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