High-latitude Forcing Research Articles

Sensitivity of the thermohaline circulation to tropical and high latitude freshwater forcing during the last glacial‐interglacial cycle

Recent studies indicate that the tropical freshwater budget of the Atlantic is modulated by changes in El Niño‐Southern Oscillation (ENSO). If anomalies of the Atlantic freshwater balance persist on the order of decades, these might have a large influence on the North Atlantic thermohaline circulation (THC). Here we present a sensitivity study in which we use a model scenario for ENSO behavior during the last 120 kyr to force a simplified model of the THC with corresponding freshwater exchange between the tropical Atlantic and Pacific. If the steady state strength of the North Atlantic THC is similar to the present‐day, its response to the forcing is of the order of 1–3 Sv (1 Sv = 106 m3 s−1). No mode changes of the THC are simulated for reasonable values of the coupling constant between freshwater exchange and ENSO. If, on the other hand, the steady state overturning is significantly weaker, a collapse of the THC occurs due to forcing from the tropics. The modeled THC variation due to tropical freshwater perturbations are compared to those resulting from middle‐ and high‐latitude freshwater forcing due to long‐term growth and decay of Northern Hemisphere ice sheets. It is found that both forcings are of similar amplitude except during the deglaciation when high latitude forcing dominates. A possible out‐of‐phase relationship between deep water formation in the North Pacific and North Atlantic is also explored.

Late Quaternary Climate Changes in Central Africa as Inferred from Terrigenous Input to the Niger Fan

Time series of terrigenous source elements (Al, K, Ti, Zr) from core GeoB4901-8 recovered from the deep-sea fan of the Niger River record variations in riverine sediment discharge over the past 245,000 yr. Although the flux rates of all the elements depend on physical erosion, which is mainly controlled by the extent of vegetation coverage in central Africa, element/Al ratios reflect conditions for chemical weathering in the river basin. Maximum sediment input to the ocean occurs during cold and arid periods, when precipitation intensity and associated freshwater runoff are reduced. High carbonate contents during the same periods indicate that the sediment supply has a positive effect on river-induced marine productivity. In general, variations in the terrestrial signals contain a strong precessional component in tune with changes in low-latitude solar radiation. However, the terrestrial signal lags the insolation signal by several thousand years. K/Al, Ti/Al, and Zr/Al records reveal that African monsoonal precipitation depends on high-latitude forcing. We attribute the shift between insolation cycle and river discharge to the frequently reported nonlinear response of African climate to primary orbital configurations, which may be caused by a complex interaction of the secondary control parameters, such as surface albedo and/or thermohaline circulation.

High resolution Holocene monsoon record from the eastern Arabian Sea

Through stable oxygen and carbon analyses of rapidly accumulating sediment cores from the eastern Arabian Sea, we show that the excess of evaporation over precipitation ( E− P) steadily appears to have decreased during the last ∼10 000 to ∼2000 years, most probably due to an increasing trend in the summer monsoon rainfall, contrary to the land-based paleoclimatic data from this region, which indicate onset of aridity around 4000 years ago. Our results are consistent with the hypothesis that significant spatial variability in the monsoon rainfall observed today was persistent during most of the Holocene. Alternatively, the trend can be seen as an adjustment between two phases, one between ∼10 000 and ∼6000 years ago of increasing precipitation and another between 3500 and 2000 years ago after the arid episode. We also report a significant ∼700 year periodicity, similar to that reported recently from the South China Sea, indicating that the centennial/millennial scale response of the Indian and Chinese monsoons to high latitude forcing may be alike.

Dynamics and predictability of Stommel’s box model. A phase-space perspective with implications for decadal climate variability

The dynamics and predictability of Stommel’s box model of the thermohaline circulation isstudied. This nonlinear model with idealized geometry of the North Atlantic is solved exactly. A phase space analysis of the model reveals that the optimal perturbation affecting long-termclimate variability is provided by high-latitude haline forcing in the Atlantic ocean, althoughthis perturbation has little resemblance with the most unstable mode of the system and theleading EOF. Furthermore, the predictability problem is investigated by means of singularvector analysis and the evolution of the probability distribution function. Uncertainties in theoceanic initial conditions do increase in the phase space of the model. In the stochasticallyforced box model with identical oceanic initial conditions, the climate predictability is examinedfor the damped persistence forecast. We find that the loss of the predictability is related to thedifferent stages of the variance evolution which is also measured by the relative entropy. Ouranalysis shows that the non-normal system matrix of Stommel’s model does affect the dynamicsand predictability of the system which is useful for the interpretation of long-term climatevariability and predictability.

Modeling thermospheric neutral density waves and holes in response to high latitude forcing

Numerical simulations have been performed to model the neutral density response to two substorms in March 1979. Observations of density from the Satellite Electrostatic Triaxial Accelerometer (SETA) during this interval revealed neutral density waves propagating from high latitudes in response to the substorms. For the first substorm, waves were seen on both the day and nightside in the Northern Hemisphere, and for the second, only strong nightside waves were detected. The data also revealed the formation of deep neutral density “holes” at high latitudes. To simulate the response, a coupled thermosphere ionosphere model was driven by an empirical substorm model designed to follow the various phases of the substorms. By tuning the magnitude of the energy injection and, in one case, rotating the spatial distribution of the sources, both the wave features on the day and nightside, and the neutral density holes were reproduced. The neutral density waves were found to be consistent with the passage of atmospheric gravity waves with propagation speeds of 600 to 700 m/s. The neutral density holes are dynamically driven, created by the high velocity neutral winds from ion-drag. During strong geomagnetic activity momentum forcing resulting from ion-drag drives neutral winds of several hundreds of m/s. Inertial forces on the gas parcels cause a divergence in the vortex-like circulation. Backpressure induced by the divergent wind field does not completely balance the flow, and air rises within the vortex core, to maintain continuity, and cools adiabatically to produce a low temperature. Due to the cooling that takes place, in spite of the strong Joule heating that must accompany the ion-drag, the cold vortex core appears as a density hole. The physical model is able to reproduce the magnitude and location of the density holes. The study indicates that physical models, with well-defined magnetospheric inputs, will be able to improve neutral density specification for orbital tracking during period of high geomagnetic activity.

Orbital forcing of freshwater input in the Zaire Fan area—clay mineral evidence from the last 200 kyr

Abstract Late Quaternary sediments recovered in a core from the area of the Zaire Fan, Central Africa, were analyzed for clay mineral composition in order to reconstruct fluctuations in the sediment input and freshwater discharge of the Zaire River. Clay mineral assemblages are dominated by kaolinite and smectite, which both originate mainly from the Zaire River and contain only minor contributions of eolian dust. Smectite crystallinity and chemical character of illites (Fe, Mg- or Al-rich) are used to track sediment input from the Zaire River and assess fluctuations in the freshwater discharge. Both parameters record a high-latitude forcing of river runoff at 100 ka periodicities reflecting glacial aridity and increased runoff during interglacials 1, 5 and 7. This signal is also observed in kaolinite/smectite ratios which represent the extension and intensity of the freshwater plume of the Zaire River. Clay mineral proxies reveal that river discharge and associated sediment input fluctuated in tune with precessional cycles of African monsoon intensity. Increased eolian input of kaolinite-rich dust with intensified northeast trades during glacials flattens the precessional signal in kaolinite/smectite ratios.

The electrodynamics of the low-latitude ionosphere: Recent results and future challenges

Over the last few years, experimental and numerical modeling studies have provided extensive new information on the electrodynamics of the low-latitude ionosphere. Ground-based studies have determined the average seasonal, solar cycle and magnetic effects on the equatorial and low-latitude plasma drifts and thermospheric neutral winds, and their control over the F-region plasma density distribution. Satellite observations have been used, for the first time, to determine the latitudinal and longitudinal dependence of the F-region plasma drifts, and coordinated ground-based measurements have examined the global response of the low-latitude ionosphere to different high latitude forcing conditions. Recent studies have examined the coupling between storm-time thermospheric neutral winds and plasma drifts, and have determined the low-latitude signatures of magnetospheric and ionospheric dynamo electric fields. In this review, we initially examine the present understanding of low-latitude F-region electrodynamics focusing on the large-scale distribution of the ionospheric electric fields and their response to magnetospheric disturbances. We also discuss future initiatives necessary for a more complete understanding of these low-latitude transport processes.

Late Pliocene climate in the Southeast Atlantic: Preliminary results from a multi-disciplinary study of DSDP Site 532

Site 532 on the Walvis Ridge was sampled at 4000- to 800-year intervals from 2.24 to 2.60 Ma, spanning the three large glacial advances of the late Pliocene. An age model was created by correlating the oxygen isotope record to Site 607 with linear interpolations between tie-lines. The resultant age model differs from that in the site reports by more than 800,000 years, due to misidentification of a magnetic boundary. Sedimentation rates varied by an order of magnitude at this site, with minimum accumulation during glacial events. Interglacial intervals were charactrized by high marine production and high summer precipitation on land, while glacials had very low production and arid continental climate. During the large glacial events (Stages 96–100) conditions of low production and continental aridity reached their greatest intensity, but there is no evidence of a permanent mode shift in either marine or terrestrial records. Calcite concentration has a strong variation at obliquity frequencies, with maxima during interglacials, but occasionally shows a large amplitude at precessional frequencies as well, so that high concentrations occur in a few glacial intervals. As a result, color variation is not a reliable guide to glacial-scale cycles at this site. Composition of the phytoplankton assemblage is diverse and highly variable, and we have not been able to distinguish a clear indicator of upwelling-related production. Spectral analysis reveals obliquity and precessional signals in the pollen data, while several diatom records contain combination tones, indicating that these data represent a complicated response to both local and high-latitude forcing.

"Thermospheric dynamics during September 18–19, 1984: 1. Model simulations""

For realistic predictions of global thermospheric dynamics using thermospheric general circulation models (TGCMs), one of the main problems lies in the specification of the three‐dimensional, time‐dependent forcing fields for any given day. The September 18–19, 1984, Equinox Transition Study (ETS) interval was simulated using the National Center for Atmospheric Research TGCM with inputs guided by observations. During the ETS campaign, large quantities of high‐quality data were available from high latitudes, and the high‐latitude forcings for the TGCM could therefore be defined with precision for this period. The importance of upward propagating semidiurnal tides is also emphasized. This is the first realistic time‐dependent simulation to incorporate upward propagating tides. Magnetically quiet conditions prevailed for several days prior to the magnetic storm which began about 0930 UT on September 19, 1984. The storm generated major disturbances in the global circulation, temperature, and composition of the thermosphere. A description of the simulated quiet time thermospheric structure for September 18 is contrasted with the storm time behavior on September 19. Several features of the storm response are described, including the generation of large‐scale equatorward propagating disturbances. Longer‐lived perturbations of the neutral temperature and composition are also discussed. A new feature has been discovered in the model predictions for altitudes around 200 km. This feature comprises a quasi‐fixed four‐cell pattern of high‐ and low‐density regions in the polar cap. The exact location of the feature varies with UT, and the magnitude of the density perturbations depends on the strength of the high‐latitude convection.