Intensity Of Thermohaline Circulation Research Articles

Response of Low Flows of Polish Rivers to Climate Change in 1987–1989

The paper discusses changes in the low-flow regime of rivers in Poland, resulting from climate change that occurred between 1987 and 1989. The low-flow variability of rivers was measured with the use of the number of days with low flows (NDLF) below a threshold value, which was adopted as the 0.1 (10%) percentile (Q10) from the set of daily flows recorded in the multi-annual period 1951–2020 at 140 water gauges on 83 rivers. The analysis of the course of climate change over Poland showed that it was caused by macro-circulation conditions, controlled by changes in the intensity of thermohaline circulation in the North Atlantic (NA THC). Climate change consisted of a sharp increase in sunshine duration and air temperature, and a decrease in relative humidity after 1988. Along with the lack of changes in precipitation totals, characterized by a strong yearly variability, and an increase in field evaporation, it led to noticeable changes in the water balance. As a result, in 1989–2020, there was a significant increase in NDFL detected in about 2/3 of the area of Poland. With the change in the NA THC phase and the macro-circulation conditions, there was also a change in the spatial distribution of areas drained by rivers with increased NDFL. In 1951–1988, these included the eastern parts of Poland, while after the climate change (1989–2020), its western and south-western parts.

Intensywność cyrkulacji termohalinowej na Atlantyku Północnym a susze w Polsce

The work considers the cause of the frequency of droughts occurrence variability in Poland. It was proven, that the frequency of droughts shows statistically significant relationship with intensity of thermohaline circulation (THC) in the North Atlantic. In periods of occurrence of positive phase the THC frequency of droughts’ occurrence in Poland grows up and it is about 3.6 times greater, than in periods of occurrence of negative phase the THC. The sign and the value of coefficient characterizing the THC determines the drought occurrence and its duration time. Changeability of the THC is not however the only factor influencing on droughts’ occurrence. The analyses show, that the probability of the Spring and Summer droughts’ occurrence in a year with a positive THC phase is considerably larger, if in period of Winter preceding the drought, the sign of NAO index will be positive. At present (the year 2019) we are presumably already after the culmination of the positive phase of the THC, lasting from the year 1989. As the long-term course of the THC changeability shows the quasi- periodicity, we can presume, that the strong positive trend of occurrence of droughts, lasting from the 80’ of 20th century, will undergo break down.

Abiotic and biotic responses to Milankovitch-forced megamonsoon and glacial cycles recorded in South China at the end of the Late Paleozoic Ice Age

Abstract At the end of the Late Paleozoic Ice Age (LPIA) from late Early Permian to early Late Permian, the global climate was impacted by a prevailing megamonsoon and Gondwanan deglaciation. To better understand the abiotic and biotic responses to Milankovitch-forced climate changes during this time period, multi-element X-ray fluorescence (XRF) geochemistry analyses were conducted on 948 samples from the late Early−late Middle Permian Maokou Formation at Shangsi, South China. The Fe/Ti, S/Ti, Ba/Ti and Ca time series, which were calibrated with an existing “floating” astronomical time scale (ATS), show the entire suite of Milankovitch rhythms including 405 kyr long eccentricity, 128 and 95 kyr short eccentricity, 33 kyr obliquity and 20 kyr precession. Spectral coherency and cross-phase analysis reveals that chemical weathering (monitored by Fe/Ti) and upwelling (captured by S/Ti and Ba/Ti) are nearly antiphase in the precession band, which suggests a contrast between summer and winter monsoon intensities. Strong obliquity signal in the Ba/Ti series is proposed to derive from changes in thermohaline circulation intensity from glaciation dynamics in southern Gondwana. The abundance of foraminifer, brachiopod and ostracod faunas within the Maokou Formation were mainly controlled by the ~1.1 Myr obliquity modulation cycle. The obliquity-forced high-nutrient and oxygen-depleted conditions generally produced a benthic foraminifer bloom, but threatened the brachiopod and ostracod faunas.

Feature of thermohaline circulation in two-layer conceptual model based on energy constraint

The assertion that the thermohaline circulation (THC) is driven and sustained by mechanical energy has been increasingly accepted. The simplest conceptual model describing the THC is the Stommel two-box model. Given the vertical stratification in the real ocean, layered models were designed and used. In this research, using a two-layer conceptual model based on energy constraint, we studied basic features of thermal-mode and saline-mode circulations. We focused on the effects of freshwater flux and mixing energy on the intensity and multiple equilibrium states of the THC. The results show that more important than affecting the THC intensity, both the decrease of freshwater flux and increase of mixing energy can lead to an “abrupt transition” in the THC from a stable saline to a stable thermal mode, which further develops the THC energy theory.

Response of thermohaline circulation and thermal structure to removal of ice sheets and high atmospheric CO2 concentration

The thermohaline circulation (THC) response to ice sheet removal and quadrupling of atmospheric CO2 in a coupled model and the equilibrium thermal structure are examined. After THC weakening, diffusion of heat and salt to the northern North Atlantic at deep layers increases the temperature and salinity there, in response to CO2‐quadrupling. Resulting convective instability induces the exchange of warmer, saltier water in deep layers and cooler, fresher water near the surface. This contributes to a gradual increase in the THC intensity, culminating in its complete and rapid recovery due to positive haline feedback overcoming negative thermal feedback on the THC. Removal of ice sheets prolongs the overall recovery and strengthens the final THC due to precipitation changes over the northern North Atlantic and Labrador Sea. Bottom water and high‐latitude sea‐surface temperatures are higher without ice sheets, leading to a smaller meridional temperature gradient as indicated by Cenozoic reconstructions.

Neogene history of the Deep Western Boundary Current at Rekohu sediment drift, Southwest Pacific (ODP Site 1124)

ODP Site 1124, located 600 km east of the North Island of New Zealand, records post-middle Oligocene variations in the Pacific Deep Western Boundary Current (DWBC) and New Zealand’s climatic and tectonic evolution. Sediment parameters, such as terrigenous grain size, flux, magnetic fabric, and non-depositional episodes, are used to interpret DWBC intensity and Antarctic climate. Interpretations of DWBC velocities indicate that the Antarctic Circumpolar Current reached modern intensities at ∼23 Ma, as the tectonic seaways expanded, completing the thermal isolation of Antarctica. Periods of more intense bottom water formation are suggested by the presence of hiatuses formed under the DWBC at 22.5–17.6, 16.5–15, and 14–11 Ma. The oldest interval of high current intensity occurs within a climatically warm period during which the intensity of thermohaline circulation around Antarctica increased as a result of recent opening of circum-Antarctic gateways. The younger hiatuses represent glacial periods on Antarctica and major fluctuations in the East Antarctic Ice Sheet, whereas intervals around the hiatuses represent times of relative warmth, but with continued current activity. The period between 11 to 9 Ma is characterized by conditions surrounding a high velocity DWBC around the time of the formation and stabilization of the West Antarctic Ice Sheet. The increased terrigenous input may result from either changing Antarctic conditions or more direct sediment transport from New Zealand. The Pacific DWBC did not exert a major influence on sedimentation at Site 1124 from 9 Ma to the present; the late Miocene to Pleistocene sequence is more influenced by the climatic and tectonic history of New Zealand. Despite the apparent potential for increased sediment supply to this site from changes in sediment channeling, increasing rates of mountain uplift, and volcanic activity, terrigenous fluxes remain low and constant throughout this younger period.

Periodicity of Holocene climatic variations in the Huguangyan Maar Lake

There exist five primary periods of 2 930, 1 140, 490, 250 and 220 a in the Holocene climatic variations in the Huguangyan Maar Lake, according to the energy-spectrum and filter analyses of high-resolution time sequences (10–15 a) of the sediment dry density. The peak values of the three temperature-decreasing periods with the 2 930 a cycle occur at about 7 300, 4 250 and 1 200 Cal. aBP. There are 7–8 temperature-decreasing periods with the 1 140 a cycle, and the climate fluctuation range is largest in the early Holocene, and reduces gradually in the middle and late Holocene. The millennial-scale climatic change in the Holocene may adjust the global water cycle and the thermohaline circulation intensity through the harmonic tones of the earth’s precession cycle, which in turn influences the global climate change.

CO2‐induced change in a coupled ocean‐atmosphere model and its paleoclimatic implications

The climatic effects of very large changes of CO2 concentration in the atmosphere are explored using a general circulation model of the coupled ocean‐atmosphere system. As a simplification the model has an annual mean insolation and a highly idealized geography, A series of climatic equilibria are obtained for cases with ½, 1/√2, 1, 2, 4, and 8 times the present CO2 concentration in the atmosphere. The results from these six numerical experiments indicate the climatic signatures of large CO2 changes in the atmosphere and in the abyssal and surface waters of the ocean. As the CO2 concentration in the model atmosphere increased from 1 to 8 times the normal value, the meridional gradient of surface air temperature decreased, while that of upper tropospheric temperature increased in agreement with the results of earlier CO2 climate sensitivity studies. However, the intensity and latitudinal placement of the atmospheric jet hardly changed. Despite the reduction of meridional temperature gradient, the meridional density gradient of water at the ocean surface changed little because of the increase of thermal expansion coefficient of seawater with increasing temperature. Thus the intensity of thermohaline circulation in the ocean model does not diminish as expected in the range from 1 to 8 times the normal atmospheric CO2 concentration. As was shown in an earlier study, the CO2‐induced changes in the deep sea follow the change of sea surface temperature in high latitudes and thus are much larger than the globally averaged changes of sea surface temperature. The model predicts that the area mean rates of precipitation, evaporation, and runoff increase with increasing CO2 concentration in the atmosphere. The latitudes of the arid zone and the high surface pressure belt in the subtropics are almost constant in the entire range of 1–8 times normal CO2. In general, the climatic signature obtained from the model appears to be consistent with a CO2 hypothesis for the climatic changes in the Cenozoic with the following exception: the tropical sea surface temperature in the model has a small but significant increase with increasing atmospheric CO2 concentration, while tropical sea surface temperature as deduced from the isotopic record appears to have no systematic trend during the Tertiary. It is found that the climate corresponding to one‐half normal CO2 is markedly different from the normal and high‐CO2 cases. Sea ice extends to middle latitudes, and the thermohaline circulation in the model ocean loses its intensity and is largely confined to an area between the sea ice margin and the equator. The poleward heat transport by ocean currents is very small in high latitudes, markedly reducing the surface air temperature there. It is suggested that a similar process, which enhances the positive albedo feedback effect of sea ice, played a key role in reducing surface air temperatures over the North Atlantic during the last glacial maximum.