Baldwin, M. P., P. B. Rhines, H. P. Huang and M. E. McIntyre, 2007: The jet-stream conundrum. Science, 315(5811): 467-468.
Fluid jets occur in Earth's atmosphere and oceans, and on other planets. A new theoretical view of jets has helped reveal why they form and how they interact with the surrounding fluid.
Camargo, S. J., H. L. Li and L. Q. Sun, 2007: Feasibility study for downscaling seasonal tropical cyclone activity using the NCEP regional spectral model. International Journal of Climatology, 27(3): 311-325.
The potential use of the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) for downscaling seasonal tropical cyclone (TC) activity was analyzed here. The NCEP RSM with horizontal resolution of 50 km, was used to downscale the ECHAM4.5 Atmospheric General Circulation Model (AGCM) simulations forced with observed sea surface temperature (SST) over the western North Pacific. An ensemble of ten runs for June-November 1994 and 1998 was studied. The representation of the TCs is much improved compared to the low-resolution forcing AGCM, but the TCs are not as intense as observed ones, as the RSM horizontal resolution is not sufficiently high. The large-scale fields of the RSM are examined and compared to both the AGCM and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. The large-scale fields of RSM characteristics are in general similar to those of the reanalysis. Various properties of the TCs in the RSM are also examined such as first positions, tracks, accumulated cyclone energy (ACE) and duration. While the RSM does not reproduce the higher number of TCs in 1994 than in 1998, other measures of TC activity (ACE, number of cyclone days) in the RSM are higher in 1994 than in 1998. Copyright (C) 2006 Royal Meteorological Society.
Chant, Robert J., Wayne R. Geyer, Robert Houghton, Elias Hunter and James Lerczak, 2007: Estuarine Boundary Layer Mixing Processes: Insights from Dye Experiments. Journal of Physical Oceanography 37(7): 1859–1877, DOI: 10.1175/JPO3088.1.
A series of dye releases in the Hudson River estuary elucidated diapycnal mixing rates and temporal variability over tidal and fortnightly time scales. Dye was injected in the bottom boundary layer for each of four releases during different phases of the tide and of the spring–neap cycle. Diapycnal mixing occurs primarily through entrainment that is driven by shear production in the bottom boundary layer. On flood the dye extended vertically through the bottom mixed layer, and its concentration decreased abruptly near the base of the pycnocline, usually at a height corresponding to a velocity maximum. Boundary layer growth is consistent with a one-dimensional, stress-driven entrainment model. A model was developed for the vertical structure of the vertical eddy viscosity in the flood tide boundary layer that is proportional to u2*/N∞, where u* and N∞ are the bottom friction velocity and buoyancy frequency above the boundary layer. The model also predicts that the buoyancy flux averaged over the bottom boundary layer is equal to 0.06N∞u2* or, based on the structure of the boundary layer equal to 0.1NBLu2*, where NBL is the buoyancy frequency across the flood-tide boundary layer. Estimates of shear production and buoyancy flux indicate that the flux Richardson number in the flood-tide boundary layer is 0.1–0.18, consistent with the model indicating that the flux Richardson number is between 0.1 and 0.14. During ebb, the boundary layer was more stratified, and its vertical extent was not as sharply delineated as in the flood. During neap tide the rate of mixing during ebb was significantly weaker than on flood, owing to reduced bottom stress and stabilization by stratification. As tidal amplitude increased ebb mixing increased and more closely resembled the boundary layer entrainment process observed during the flood. Tidal straining modestly increased the entrainment rate during the flood, and it restratified the boundary layer and inhibited mixing during the ebb.
Chen, D. and M. A. Cane, 2007: El Niño prediction and predictability. Journal of Computational Physics, 227(7): 3625-3640.
El Niño-Southern Oscillation (ENSO) is by far the most energetic, and at present also the most predictable, short-term fluctuation in the Earth's climate system, though the limits of its predictability are still a subject of considerable debate. As a result of over two-decades of intensive observational, theoretical and modeling efforts, ENSO's basic dynamics is now well understood and its prediction has become a routine practice at application centers all over the world. The predictability of ENSO largely stems from the ocean-atmosphere interaction in the tropical Pacific and the low-dimensional nature of this coupled system. Present ENSO forecast models, in spite of their vast differences in complexity, exhibit comparable predictive skills, which seem to have hit a plateau at moderate level. However, mounting evidence suggests that there is still room for improvement. In particular, better model initialization and data assimilation, better simulation of surface heat and freshwater fluxes, and better representation of the relevant processes outside of the tropical Pacific, could all lead to improved ENSO forecasts. (c) 2007 Elsevier Inc. All rights reserved.
Cook, E. R., R. Seager, M. A. Cane and D. W. Stahle, 2007: North American drought: Reconstructions, causes, and consequences. Earth-Science Reviews, 81(1-2): 93-134.
Severe drought is the greatest recurring natural disaster to strike North America. A remarkable network of centuries-long annual tree-ring chronologies has now allowed for the reconstruction of past drought over North America covering the past 1000 or more years in most regions. These reconstructions reveal the occurrence of past "megadroughts" of unprecedented severity and duration, ones that have never been experienced by modem societies in North America. There is strong archaeological evidence for the destabilizing influence of these past droughts on advanced agricultural societies, examples that should resonate today given the increasing vulnerability of modem water-based systems to relatively short-term droughts. Understanding how these megadroughts develop and persist is a timely scientific problem. Very recently, climate models have succeeded in simulating all of the major droughts over North America from the Civil War to the severe 1998-2004 drought in the western U.S. These numerical experiments indicate the dominating importance of tropical Pacific Ocean sea surface temperatures (SSTs) in determining how much precipitation falls over large parts of North America. Of central importance to drought formation is the development of cool "La Niña-like" SSTs in the eastern tropical Pacific region. This development appears to be partially linked to changes in radiative forcing over that region, which affects the Bjerknes feedback mechanism of the ENSO cycle there. Paradoxically, warmer conditions over the tropical Pacific region lead to the development of cool La Niña-like SSTs there, which is drought inducing over North America. Whether or not this process will lead to a greater prevalence of drought in the future as the world warms due to accumulating greenhouse gases is unclear at this time. (c) 2007 Elsevier B.V. All rights reserved.
Ducklow, H. W., K. Baker, D. G. Martinson, L. B. Quetin, R. M. Ross, R. C. Smith, S. E. Stammerjohn, M. Vernet and W. Fraser, 2007: Marine pelagic ecosystems: The West Antarctic Peninsula. Philosophical Transactions of the Royal Society B-Biological Sciences, 362(1477): 67-94.
The marine ecosystem of the West Antarctic Peninsula (WAP) extends from the Bellingshausen Sea to the northern tip of the peninsula and from the mostly glaciated coast across the continental shelf to the shelf break in the west. The glacially sculpted coastline along the peninsula is highly convoluted and characterized by deep embayments that are often interconnected by channels that facilitate transport of heat and nutrients into the shelf domain. The ecosystem is divided into three subregions, the continental slope, shelf and coastal regions, each with unique ocean dynamics, water mass and biological distributions. The WAP shelf lies within the Antarctic Sea Ice Zone (SIZ) and like other SIZs, the WAP system is very productive, supporting large stocks of marine mammals, birds and the Antarctic krill, Euphausia superba. Ecosystem dynamics is dominated by the seasonal and interannual variation in sea ice extent and retreat. The Antarctic Peninsula is one among the most rapidly warming regions on Earth, having experienced a 2 degrees C increase in the annual mean temperature and a 6 degrees C rise in the mean winter temperature since 1950. Delivery of heat from the Antarctic Circumpolar Current has increased significantly in the past decade, sufficient to drive to a 0.6 degrees C warming of the upper 300 m of shelf water. In the past 50 years and continuing in the twenty-first century, the warm, moist maritime climate of the northern WAP has been migrating south, displacing the once dominant cold, dry continental Antarctic climate and causing multi-level responses in the marine ecosystem. Ecosystem responses to the regional warming include increased heat transport, decreased sea ice extent and duration, local declines in ice-dependent Adelie penguins, increase in ice-tolerant gentoo and chinstrap penguins, alterations in phytoplankton and zooplankton community composition and changes in krill recruitment, abundance and availability to predators. The climate/ecological gradients extending along the WAP and the presence of monitoring systems, field stations and long-term research programmes make the region an invaluable observatory of climate change and marine ecosystem response.
Emile-Geay, J., M. Cane, R. Seager, A. Kaplan and P. Almasi, 2007: El Niño as a mediator of the solar influence on climate. Paleoceanography, 22(3): PA3210, doi:10.1029/2006PA001304.
Using a climate model of intermediate complexity, we simulate the response of the El Niño over tildeo-Southern Oscillation (ENSO) system to solar and orbital forcing over the Holocene. Solar forcing is reconstructed from radiocarbon production rate data, using various scaling factors to account for the conflicting estimates of solar irradiance variability. As estimates of the difference since the Maunder Minimum range from 0.05% to 0.5% of the solar "constant,'' we consider these two extreme scenarios, along with the intermediate case of 0.2%. We show that for large or moderate forcings, the low-pass-filtered east-west sea surface temperature gradient along the equator responds almost linearly to irradiance forcing, with a short phase lag (about a decade). Wavelet analysis shows a statistically significant enhancement of the century-to-millennial-scale ENSO variability for even a moderate irradiance forcing. In contrast, the 0.05% case displays no such enhancement. Orbitally driven insolation forcing is found to produce a long-term increase of ENSO variability from the early Holocene onward, in accordance with previous findings. When both forcings are combined, the superposition is approximately linear in the strong scaling case. Overall, the sea surface temperature response is of the magnitude required, and is persistent enough, to induce important climatic perturbations worldwide. The results suggest that ENSO may plausibly have acted as a mediator between the Sun and the Earth's climate. A comparison to key Holocene climate records, from the Northern Hemisphere subtropics and midlatitudes, shows support for this hypothesis.
Farmer, E. C., A. Kaplan, P. B. de Menocal and J. Lynch-Stieglitz, 2007: Corroborating ecological depth preferences of planktonic foraminifera in the tropical Atlantic with the stable oxygen isotope ratios of core top specimens. Paleoceanography, 22(3): doi:10.1029/2006PA001361.
 Past variability in upper ocean thermocline depth is commonly estimated from the abundance of different species of planktonic organisms or the difference in oxygen isotopic composition between two species of planktonic foraminifera, one that lives in the mixed layer and one that lives in or near the thermocline. To test the latter relationships, we measured the oxygen isotopic composition of eight species of planktonic foraminifera (pink and white varieties of Globigerinoides ruber, Globigerinoides sacculifer without the final chamber, Orbulina universa, Pulleniatina obliquiloculata, Globorotalia menardii, Neogloborotalia dutertrei, and Globorotalia tumida) in surface sediment samples from 31 tropical Atlantic deep-sea sediment cores. Bayesian analysis was used to compare measured oxygen isotopic compositions with their predictions based on modern data sets of annual temperatures and oxygen isotopic composition of ocean water in the upper 500 m at the core sites. Posterior probability densities for predictive model parameters were computed. Probability distributions of calcification depth for analyzed species corroborated their ecological preferences inferred from net tow and sediment trap data. Robustness of the habitat signals in core top specimens suggests that reconstructions of the entire upper ocean temperature profiles, not just their thermocline depth or temperature, might be possible.
Gordon, A. L., M. Visbeck and J. C. Comiso, 2007: A possible link between the Weddell Polynya and the Southern Annular Mode. Journal of Climate, 20(11): 2558-2571.
Shortly after the advent of the first imaging passive microwave sensor on board a research satellite an anomalous climate feature was observed within the Weddell Sea. During the years 1974-1976, a 250 x 10(3) km(2) area within the seasonal sea ice cover was virtually free of winter sea ice. This feature, the Weddell Polynya, was created as sea ice formation was inhibited by ocean convection that injected relatively warm deep water into the surface layer. Though smaller, less persistent polynyas associated with topographically induced upwelling at Maud Rise frequently form in the area, there has not been a reoccurrence of the Weddell Polynya since 1976. Archived observations of the surface layer salinity within the Weddell gyre suggest that the Weddell Polynya may have been induced by a prolonged period of negative Southern Annular Mode (SAM). During negative SAM the Weddell Sea experiences colder and drier atmospheric conditions, making for a saltier surface layer with reduced pycnocline stability. This condition enables Maud Rise upwelling to trigger sustained deep-reaching convection associated with the polynya. Since the late 1970s SAM has been close to neutral or in a positive state, resulting in warmer, wetter conditions over the Weddell Sea, forestalling repeat of the Weddell Polynya. A contributing factor to the Weddell Polynya initiation may have been a La Nina condition, which is associated with increased winter sea ice formation in the polynya area. If the surface layer is made sufficiently salty due to a prolonged negative SAM period, perhaps aided by La Nina, then Maud Rise upwelling meets with positive feedback, triggering convection, and a winter persistent Weddell Polynya.
Hall, T. M., T. W. N. Haine, M. Holzer, D. A. LeBel, F. Terenzi and D. W. Waugh, 2007: Ventilation rates estimated from tracers in the presence of mixing. Journal of Physical Oceanography, 37(11): 2599-2611.
The intimate relationship among ventilation, transit-time distributions, and transient tracer budgets is analyzed. To characterize the advective-diffusive transport from the mixed layer to the interior ocean in terms of flux we employ a cumulative ventilation-rate distribution, Phi(tau), defined as the one-way mass flux of water that resides at least time tau in the interior before returning. A one-way (or gross) flux contrasts with the net advective flux, often called the subduction rate, which does not accommodate the effects of mixing, and it contrasts with the formation rate, which depends only on the net effects of advection and diffusive mixing. As tau decreases Phi(tau) increases, encompassing progressively more one-way flux. In general, Phi is a rapidly varying function of tau( it diverges at small tau), and there is no single residence time at which Phi can be evaluated to fully summarize the advective-diffusive flux. To reconcile discrepancies between estimates of formation rates in a recent GCM study, Phi(tau) is used. Then chlorofluorocarbon data are used to bound Phi(tau) for Subtropical Mode Water and Labrador Sea Water in the North Atlantic Ocean. The authors show that the neglect of diffusive mixing leads to spurious behavior, such as apparent time dependence in the formation, even when transport is steady.
Herweijer, C., R. Seager, E. R. Cook and J. Emile-Geay, 2007: North American droughts of the last millennium from a gridded network of tree-ring data. Journal of Climate, 20(7): 1353-1376.
Drought is the most economically expensive recurring natural disaster to strike North America in modern times. Recently available gridded drought reconstructions have been developed for most of North America from a network of drought-sensitive tree-ring chronologies, many of which span the last 1000 yr. These reconstructions enable the authors to Put the famous droughts of the instrumental record (i.e., the 1930s Dust Bowl and the 1950s Southwest droughts) into the context of 1000 yr of natural drought variability oil the continent. We can now, with this remarkable new record, examine the severity, persistence, spatial signatures, and frequencies of drought variability over the past milllenmum. and how these have changed with time.
Huang, H. P., A. Kaplan, E. N. Curchitser and N. A. Maximenko, 2007: The degree of anisotropy for mid-ocean currents from satellite observations and an eddy-permitting model simulation. Journal of Geophysical Research-Oceans, 112(C9): doi:10.1029/2007JC004105.
The degree of anisotropy is calculated for the mid-ocean currents estimated from satellite altimetry and simulated with a numerical model of the Pacific Ocean. A high resolution eddy-permitting model is used for its ability to simulate mid-ocean multiple zonal flows, crucial for the evaluation of the degree of anisotropy. Using a commonly defined parameter of anisotropy, a, that falls between -1 and 1 and equals 0, - 1, and 1 for an isotropic, purely meridional, and purely zonal flow, respectively, it is found that a increases from nearly zero for weekly data to significantly positive values for seasonally and annually averaged data. This tendency of increasing zonal anisotropy with averaging time is true for the surface geostrophic velocity from satellite altimetry and for both surface and deep-ocean velocities from the model simulation. The absolute value of a for the simulated surface currents agree with that derived from satellite observation, reaffirming the reliability of both data sets. In the model simulation, the behavior of a at the surface is very similar to that at 1000 m, indicating a deep structure of the zonally elongated features in the middle of the Pacific Ocean. The implications of these findings are discussed in the context of oceanic eddies, Rossby waves, and zonal jets.
Ihara, C., Y. Kushnir, M. A. Cane and V. H. De la Pena, 2007: Indian summer monsoon rainfall and its link with ENSO and Indian Ocean climate indices. International Journal of Climatology, 27(2): 179-187.
We examine the relationship between the state of the equatorial Indian Ocean, ENSO, and the Indian summer monsoon rainfall using data from 1881 to 1998. The zonal wind anomalies and SST anomaly gradient over the equatorial Indian Ocean are used as indices that represent the condition of the Indian Ocean. Although the index defined by the zonal wind anomalies correlates poorly with Indian summer monsoon rainfall, the linear reconstruction of Indian summer monsoon rainfall on the basis of a multiple regression from the NINO3 and this wind index better specifies the Indian summer monsoon rainfall than the regression with only NINO3. Using contingency tables, we find that the negative association between the categories of Indian summer monsoon rainfall and the wind index is significant during warm years El Niño but not during cold years La Niña. Composite maps of land precipitation also indicate that this relationship is significant during Ell Niño events. We conclude that there is a significant negative association between Indian summer monsoon rainfall and the zonal wind anomalies over the equatorial Indian Ocean during Ell Niño over tildeo events. A similar investigation of the relationship between the SST index and Indian summer monsoon rainfall does not reveal a significant association. Copyright (c) 2006 Royal Meteorological Society
Kieke, D., M. Rhein, L. Stramma, W. M. Smethie, J. L. Bullister and D. A. LeBel, 2007: Changes in the pool of Labrador Sea Water in the subpolar North Atlantic. Geophysical Research Letters, 34(6): -.
Measurements of chlorofluorocarbon inventories during 1997 - 2003 allow the detection and quantification of significant changes in the formation rates of two modes of Labrador Sea Water (LSW): Upper (ULSW) and deep LSW, both here defined in fixed density intervals. Both modes contribute to the cold limb of the Meridional Overturning Circulation. Results reveal that the lighter ULSW formed since the mid-1990s has started to replace the large pool of the deep LSW stored in the western North Atlantic. Formation of deep LSW was absent in 1997 - 2003. Formation of ULSW compensated for this absence during 1998/99 (7.9 Sv), but afterwards significantly declined to 2.5 Sv. The decrease of the overall LSW formation throughout 1997 - 2003 correlates with a declining eastward baroclinic mass transport between the centers of the subpolar and subtropical gyres since 1997, a warming of LSW, and a gradually decreasing North Atlantic Oscillation index after 1999.
Martinson, D. G. and W. C. Pitman, 2007: The Arctic as a trigger for glacial terminations. Climatic Change, 80(3-4): 253-263.
We propose a hypothesis to explain the very abrupt terminations that end most of the glacial episodes. During the last glaciation, the buildup and southerly expansion of large continental ice-sheets in the Northern Hemisphere and extensive cover of sea ice in the N. Pacific and the N. Atlantic imposed a much more zonal climatic circulation system than exists today. We hypothesize that this, in combination with the frigid (dry) polar air led to a significant decrease in freshwater runoff into the Arctic Ocean. In addition the freshwater contribution of the fresher Pacific water was completely eliminated by the emergence of the Bering Strait (sill depth 50 m). As the Arctic freshwater input was depleted, regions of the Arctic Ocean lost surface stability and eventually overturned, bringing warmer deep water to the surface where it melted the overlying sea ice. This upwelled water was quickly cooled and sank as newly formed deep water. For sustained overturn events, such as might have occurred during the peak of very large glacial periods (i.e. the last glacial maximum), the voluminous deep water formed would eventually overflow into the Nordic Seas and North Atlantic necessitating an equally voluminous rate of return flow of warmer surface waters from the North Atlantic thus breaking down the Arctic's zonal isolation, melting the expansive NA sea ice cover and initiating oceanic heating of the atmosphere over the ice-sheets bordering the NA. We suggest that the combined effect of these overturn-induced events in concert with a Milankovitch warming cycle, was sufficient to drive the system to a termination. We elaborate on this proposed sequence of events, using the model for the formation of the Weddell Sea polynya as proposed by Martinson et al. (1981) and various, albeit sparse, data sets from the circum-Arctic region to apply and evaluate this hypothesis to the problem of glacial terminations.
Molnar, P. and M. A. Cane, 2007: Early pliocene (pre-Ice Age) El Niño-like global climate: Which El Niño? Geosphere, 3(5): 337-365.
Paleoceanographic data from sites near the equator in the eastern and western Pacific Ocean show that sea-surface temperatures, and apparently also the depth and temperature distribution in the thermocline, have changed markedly over the past similar to 4m.y., from those resembling an El Niño state before ice sheets formed in the Northern Hemisphere to the present-day marked contrast between the eastern and western tropical Pacific. In addition, differences between late Miocene to early Pliocene (pre-Ice Age) paleoclimates and present-day average climates, particularly in the Western Hemisphere, resemble those associated with teleconnections from El Niño events, consistent with the image of a permanent El Niño state. Agreement is imperfect in that many differences between early Pliocene and present-day climates of parts of Africa, Asia, and Australia do not resemble the anomalies associated with canonical El Niño teleconnections. The teleconnections associated with the largest El Niño event in the past 100 yr, that in 1997-1998, do, however, reveal similar patterns of warming and the same sense, if not magnitude, of precipitation anomalies shown by differences between late Miocene-early Pliocene paleoclimates and present-day mean climates in these regions. If less consistent than those for the 1997-1998 event, temperature and precipitation anomalies correlated with the Pacific Decadal Oscillation also mimic many differences between early Pliocene and present-day climates. These similarities suggest that the sea-surface temperature distribution in the Pacific Ocean before Ice Age time resembled most that of the 1997-1998 El Niño, with the warmest region extending into the easternmost Pacific Ocean, not near the dateline as occurs in most El Niño events. This inference is consistent with equatorial Pacific proxy data indicating that at most a small east-west gradient in sea-surface temperature seems to have existed along the equator in late Miocene to early Pliocene time. Accordingly, such a difference in sea-surface temperatures may account for the large global differences in climate that characterized the earth before ice sheets became frequent visitors to the Northern Hemisphere.
Murtugudde, R., R. Seager and P. Thoppi, 2007: Arabian Sea response to monsoon variations. Paleoceanography, 22(PA4217): doi:10.1029/2007PA001467.
This study aims to quantify the impact of strong monsoons on the mixed layer heat budget in the Arabian Sea by contrasting forced ocean general circulation model simulations with composite strong and weak monsoon winds. Strong (weak) monsoons are defined as years with zonal component of the Somali Jet being greater (smaller) by more than a standard deviation of the long-term mean of the National Centers for Environmental Prediction reanalysis winds. Coastal upwelling is shown to be demonstrably stronger for strong monsoons leading to significant surface cooling, shallower thermoclines, and deeper mixed layers. A coupled ecosystem model shows that surface chlorophyll, primary, and export production are indeed higher for strong monsoons compared to weak monsoons driven by the supply of colder, nutrient-rich waters from greater than 100 m depths. The surprising result is that a strong monsoon results in stronger negative wind stress curl away from the coasts and drives Ekman pumping that results in a deeper thermocline. The weaker stratification and larger turbulent kinetic energy from the winds drive deeper mixed layers leading entrainment cooling with some contribution from the advection of colder upwelled waters from the coastal upwelling regions. Thus the strong monsoons, in fact, enhance oceanic heat uptake indicating that ocean dynamics are cooling the surface and driving the lower atmosphere which has implications for the interpretation of monsoon variability from paleorecords.
Otto, T. D. and A. M. Thurnherr, 2007: Efficient algorithms for finding sills in digital topographic maps. Computers & Geosciences, 33(5): 678-684.
In stratified geophysical flows, the energetically optimal exchange of dense fluid across a topographic barrier generally takes place at the deepest unblocked connection, which is typically a saddle point (sill). The flow at or near a sill is often hydraulically controlled, in which case the sill is called a controlling sill. Oceanographic examples include overflows of newly formed dense water at high latitudes as well as sills in channels connecting major ocean basins, such as the Strait of Gibraltar. Controlling sills are usually associated with strong flows, making them ideal sites for monitoring transport and hydrographic variability. The locations and depths of controlling sills also provide strong constraints for the downstream hydrographic properties below sill depth. Here, two algorithms for finding sills in digital topographic maps are presented. The first approximates the sill height to arbitrary precision in O(k) steps, where k is the number of data points in the map. The second algorithm, which requires O(k log k) steps, additionally returns the sill location. Several tests carried out with realistic problems from physical oceanography reveal that the second algorithm runs faster in practice, even though its worst case behavior is worse. (c) 2006 Elsevier Ltd. All rights reserved.
Ou, H. W., 2007: Hydrological cycle and ocean stratification in a coupled climate system: a theoretical study. Tellus Series a-Dynamic Meteorology and Oceanography, 59(5): 683-694.
As a logical progression of a deductive climate theory, this paper addresses three interconnected climatic features: the humidity profile, the atmospheric water transport and the ocean stratification-taking as given the thermal field previously determined. The theory invokes the maximization of the entropy production, which propels the tropospheric temperature and specific humidity to their updraft values, yielding moist-adiabatic lapse rate and bi-modal relative humidity in the vertical. With the humidity profile known, the atmospheric water transport at mid-latitudes can be linked to the thermal field, which then determines the ocean salinity range. The deduced density ratio across the thermocline however is quite larger than unity, suggesting considerable static stability.
Ou, H. W., 2007: Watermass properties of the Antarctic Slope Front: A simple model. Journal of Physical Oceanography, 37(1): 50-59.
In the Antarctic, dense shelf water is formed in coastal polynyas and is differentiated from the fresher surface water by the wind-induced ice motion that displaces offshore the ice melt from production zones. Where the shelf water discharges into the deep ocean, the Antarctic Slope Front (ASF) is V shaped, separating the shelf and surface waters (referred to as "frontal" waters) from the Circumpolar Deep Water (CDW). To elucidate basic constraints on frontal properties, a minimal model of homogeneous water masses forced by offshore wind and freshwater input in a perpetual winter is considered. With the surface water stirred by-and hence aligned with-the ice cover, there is little leakage of ice or meltwater from the frontal system, so ice production and melt merely redistribute heat and salt between frontal waters. As such, the heat loss to the atmosphere needs to be supplied by entraining CDW, which then necessitates the shelf water discharge on account of the mass balance. Because of the freshwater input, the discharged shelf water may not be saltier than the CDW, which thus may descend the slope to form bottom water only because of its coldness. With both frontal waters cooled to the freezing point, dominant balances are formulated to determine their salinity and exchange rate with the ambient CDW. Although extremely crude, the model derivations are favorably compared with observations, which thus may provide physically based parameterizations for the bottom-water formation that can be incorporated into global models.
Previdi, M. and B. G. Liepert, 2007: Annular modes and Hadley cell expansion under global warming. Geophysical Research Letters, 34(22): -.
Projections of 21st century climate from the latest state-of-the-art climate models consistently call for a poleward expansion of the tropical Hadley cell (HC) and subtropical dry zones (SDZ) in response to increasing levels of atmospheric greenhouse gases. We find that approximately half of the model-simulated HC and SDZ expansion during the next hundred years can be explained by positive trends in the Northern Hemisphere and Southern Hemisphere annular modes (NAM and SAM), implying a close connection between changes in the tropical and extratropical atmospheric circulation. The link between NAM and SAM variability and the SDZ expansion suggests that future changes in the hydrologic cycle are likely to be strongly influenced by atmospheric dynamics.
Previdi, M. and D. E. Veron, 2007: North Atlantic cloud cover response to the North Atlantic oscillation and relationship to surface temperature changes. Journal of Geophysical Research-Atmospheres, 112(D7): -.
 Cloud cover changes over the high-latitude North Atlantic during January associated with the North Atlantic oscillation (NAO) are studied using the Polar MM5 regional climate model. The cloud response to the NAO is marked by an increase in cloudiness during the high phase over much of the North Atlantic east of Greenland and an overall decrease in cloudiness west of Greenland. These cloud changes are accompanied by surface cloud radiative forcing anomalies that are of the same sign as the model-simulated surface air temperature (SAT) anomalies in most areas. NAO-related cloud cover changes in January are therefore likely to represent a positive feedback on the SAT changes induced by anomalous atmospheric advection.
Seager, R., 2007: The Turn of the Century drought across North America: global context, dynamics and past analogues. Journal of Climate, 20: 5527-5552.
The causes and global context of the North American drought between 1998 and 2004 are examined using atmospheric reanalyses and ensembles of atmosphere model simulations variously forced by global SSTs or tropical Pacific SSTs alone. The drought divides into two distinct time intervals. Between 1998 and 2002 it coincided with a persistent La Niña–like state in the tropical Pacific, a cool tropical troposphere, poleward- shifted jet streams, and, in the zonal mean, eddy-driven descent in midlatitudes. During the winters reduced over North America in the climate models was sustained by anomalous subsidence and reductions of moisture convergence by the stationary flow and transient eddies. During the summers reductions of evaporation and mean flow moisture convergence drove the precipitation reduction, while transient eddies acted diffusively to oppose this. During these years the North American drought fitted into a global pattern of circulation and hydroclimate anomalies with noticeable zonal and hemispheric symmetry.
Seager, R., M. F. Ting, I. Held, Y. Kushnir, J. Lu, G. Vecchi, H. P. Huang, N. Harnik, A. Leetmaa, N. C. Lau, C. H. Li, J. Velez(Nakamura) and N. Naik(Henderson), 2007: Model projections of an imminent transition to a more arid climate in southwestern North America. Science, 316(5828): 1181-1184.
How anthropogenic climate change will affect hydroclimate in the arid regions of southwestern North America has implications for the allocation of water resources and the course of regional development. Here we show that there is a broad consensus among climate models that this region will dry in the 21st century and that the transition to a more arid climate should already be under way. If these models are correct, the levels of aridity of the recent multiyear drought or the Dust Bowl and the 1950s droughts will become the new climatology of the American Southwest within a time frame of years to decades.
Seager, R., N. Graham, C. Herweijer, A. L. Gordon, Y. Kushnir and E. Cook, 2007: Blueprints for Medieval hydroclimate. Quaternary Science Reviews, 26(19-21): doi:10.1016/j.quascirev.2007.04.020, pp. 2322-2336.
According to tree ring and other records, a series of severe droughts that lasted for decades afflicted western North America during the Medieval period resulting in a more arid climate than in subsequent centuries. A review of proxy evidence from around the world indicates that North American megadroughts were part of a global pattern of Medieval hydroclimate that was distinct from that of today. In particular, the Medieval hydroclimate was wet in northern South America, dry in mid-latitude South America, dry in eastern Africa but with strong Nile River floods and a strong Indian monsoon. This pattern is similar to that accompanying persistent North American droughts in the instrumental era. This pattern is compared to that associated with familiar climate phenomena. The best fit comes from a persistently La Ni (n) over tildea-like tropical Pacific and the warm phase of the so-called Atlantic Multidecadal Oscillation. A positive North Atlantic Oscillation (NAO) also helps to explain the Medieval hydroclimate pattern. Limited sea surface temperature reconstructions support the contention that the tropical Pacific was cold and the subtropical North Atlantic was warm, ideal conditions for North American drought. Tentative modeling results indicate that a multi-century La Ni (n) over tildea-like state could have arisen as a coupled atmosphere-ocean response to high irradiance and weak volcanism during the Medieval period and that this could in turn have induced a persistently positive NAO state. A La Ni (n) over tildea-like state could also induce a strengthening of the North Atlantic meridional overturning circulation, and hence warming of the North Atlantic Ocean, by (i) the ocean response to the positive NAO and by shifting the southern mid-latitude westerlies poleward which (ii) will increase the salt flux from the Indian Ocean into the South Atlantic and (iii) drive stronger Southern Ocean upwelling. (C) 2007 Elsevier Ltd. All rights reserved.
Seth, A., S. A. Rauscher, S. J. Camargo, J. H. Qian and J. S. Pal, 2007: RegCM3 regional climatologies for South America using reanalysis and ECHAM global model driving fields. Climate Dynamics, 28(5): 461-480.
To enable downscaling of seasonal prediction and climate change scenarios, long-term baseline regional climatologies which employ global model forcing are needed for South America. As a first step in this process, this work examines climatological integrations with a regional climate model using a continental scale domain nested in both reanalysis data and multiple realizations of an atmospheric general circulation model (GCM). The analysis presents an evaluation of the nested model simulated large scale circulation, mean annual cycle and interannual variability which is compared against observational estimates and also with the driving GCM for the Northeast, Amazon, Monsoon and Southeast regions of South America. Results indicate that the regional climate model simulates the annual cycle of precipitation well in the Northeast region and Monsoon regions; it exhibits a dry bias during winter (July-September) in the Southeast, and simulates a semi-annual cycle with a dry bias in summer (December-February) in the Amazon region. There is little difference in the annual cycle between the GCM and renalyses driven simulations, however, substantial differences are seen in the interannual variability. Despite the biases in the annual cycle, the regional model captures much of the interannual variability observed in the Northeast, Southeast and Amazon regions. In the Monsoon region, where remote influences are weak, the regional model improves upon the GCM, though neither show substantial predictability. We conclude that in regions where remote influences are strong and the global model performs well it is difficult for the regional model to improve the large scale climatological features, indeed the regional model may degrade the simulation. Where remote forcing is weak and local processes dominate, there is some potential for the regional model to add value. This, however, will require improvments in physical parameterizations for high resolution tropical simulations.
St Laurent, L. C. and A. M. Thurnherr, 2007: Intense mixing of lower thermocline water on the crest of the Mid-Atlantic Ridge. Nature, 448(7154): 680-683.
Buoyancy exchange between the deep and the upper ocean, which is essential for maintaining global ocean circulation, mainly occurs through turbulent mixing(1,2). This mixing is thought to result primarily from instability of the oceanic internal wave field(3), but internal waves tend to radiate energy away from the regions in which they are generated rather than dissipate it locally as turbulence(4) and the resulting distribution of turbulent mixing remains unknown. Another, more direct, mixing mechanism involves the generation of turbulence as strong flows pass through narrow passages in topography, but the amount of turbulence generated at such locations remains poorly quantified owing to a lack of direct measurements. Here we present observations from the crest of the Mid-Atlantic Ridge in the subtropical North Atlantic Ocean that suggest that passages in rift valleys and ridge-flank canyons provide the most energetic sites for oceanic turbulence. Our measurements show that diffusivities as large as 0.03 m(2) s(-1) characterize the mixing downstream of a sill in a well-stratified boundary layer, with mixing levels remaining of the order of 10(-4) m(2) s(-1) at the base of the main thermocline. These mixing rates are significantly higher than the diffusivities of the order of 10(-5) m(2) s(-1) that characterize much of the global thermocline and the abyssal ocean(5). Our estimates suggest that overflows associated with narrow passages on the Mid-Atlantic Ridge in the North Atlantic Ocean produce as much buoyancy flux as has previously been estimated for the entire Romanche fracture zone(6,7), a large strait in the Mid-Atlantic Ridge that connects the North and South Atlantic basins. This flux is equivalent to the interior mixing that occurs in the entire North Atlantic basin at the depth of the passages, suggesting that turbulence generated in narrow passages on mid-ocean ridges may be important for buoyancy flux at the global scale.
Susanto, R. D., A. L. Gordon and J. Sprintall, 2007: Observations and proxies of the surface layer throughflow in Lombok Strait. Journal of Geophysical Research-Oceans, 112(C3): doi:10.1029/2006JC003790.
Seasonal to interannual variability of the Lombok Strait surface layer transport is investigated. The geostrophic transport within the surface layer is estimated from the cross-channel pressure gradient measured by a pair of shallow pressure gauges positioned on opposing sides of Lombok Strait during 1996-1999. The Ekman transport through Lombok Strait, derived from scatterometer winds, is less than 10% or similar to 0.15 Sv of the estimated surface layer geostrophic transport. Monsoonal forcing is clearly evident in the regional sea surface height anomalies (SSHA) as derived from the satellite altimeter measurements. During the southeast monsoon, relatively low sea level is observed to the south of Lombok Strait, with relatively high sea level to the north; conditions reverse during the northwest monsoon. Estimated transports from the cross-channel pressure gradient, winds, SSHA and thermocline depth anomalies all reveal interannual variability associated with ENSO. Both the thermocline depth anomaly and the SSHA to the south of the East Java coast correlate significantly (r = 0.7) with the Lombok Strait total surface layer throughflow. The difference of SSHA from the south of the East Java coast minus the SSHA north of Lombok shows a higher correlation (r = 0.84). These high correlation values suggest that SSHA and thermocline depth anomalies can be used as proxies for the Lombok Strait surface layer throughflow. Qualitatively, such proxy transports agree with the surface transport inferred from the pressure gauges and Ekman transport in Lombok Strait from 1996 to 1999, and also with direct velocity measurements from current meter data obtained in 1985 and 2004-2005.
Terenzi, F., T. M. Hall, S. Khatiwala, C. B. Rodehacke and D. A. LeBel, 2007: Uptake of natural and anthropogenic carbon by the Labrador Sea. Geophysical Research Letters, 34(6): -.
We apply to Classical Labrador Sea Water (CLSW) the transit-time distribution (TTD) method to estimate the inventory and uptake of anthropogenic carbon dioxide (Cant). A model of TTDs representing bulk-advection and diffusive mixing is constrained with CFC11 data. The constrained TTDs are used to propagate Cant into CLSW, allowing the air-sea disequilibrium to evolve consistently. Cant in the Labrador Sea (LS) surface waters cannot keep pace with increasing atmospheric CO2 and is highly undersaturated. Our best estimate for 2001 is an anthropogenic inventory of 1.0 Gt C and an uptake of 0.02 Gt C/year. By additionally using the constraint of present-day CO2 measurements, we estimate that the preindustrial LS was neutral or a weak source of CO2 to the atmosphere. Our estimates are subject to possible error due to the assumption of steady-state transport and carbon biochemistry.
Wing, A. A., A. H. Sobel and S. J. Camargo, 2007: Relationship between the potential and actual intensities of tropical cyclones on interannual time scales. Geophysical Research Letters, 34(8): L08810, doi:10.1029/2006GL028581.
The thermodynamic theory for the physics of a mature tropical cyclone (TC) tells us that the cyclone's intensity cannot exceed an upper bound, the potential intensity ( PI). This combined with an empirical result due to Emanuel leads to a prediction of average TC intensity change, given the change in PI. The slope of the predicted relationship between percentagewise variations in PI and those in intensity can vary between 0.5 and 1, depending on the mean PI and on what threshold is applied to the intensity data. For the Atlantic and Pacific, typical values are around 0.65 when tropical storms are excluded and 0.8 when they are included. The authors use best track data for the North Atlantic and western North Pacific, combined with PI computed from reanalysis data sets, to test these predictions. The results show that observed interannual variations of maximum TC intensity are consistent with the predictions of PI theory. Modest fractions of the variance in actual intensity are explained by PI variations. Much of the interannual variation in PI experienced by the storms comes from variation in TC tracks, so that the storms in different years are more or less likely to sample regions of high PI, rather than from variations in PI at a fixed location.
The database was updated today.