Tropical/extratropical forcing on wintertime variability of the extratropical temperature and circulation

The secular trends and interannual variability of wintertime temperatures over northern extratropical lands and circulations over the northern hemisphere are examined using the NCEP/NCAR reanalysis from 1961 to 2010. A primitive equation dry atmospheric model, driven by time-averaged forcing in each winter diagnosed from the NCEP reanalysis, is then employed to investigate the influences of tropical and extratropical forcing on the temperature and circulation variability. The model has no topography and the forcing is thus model specific. The dynamic and thermodynamic maintenances of the circulation and temperature anomalies are also diagnosed. Distinct surface temperature trends over 1961–1990 and 1991–2010 are found over most of the extratropical lands. The trend is stronger in the last two decades than that before 1990, particularly over eastern Canadian Arctic, Greenland, and Asia. The exchange of midlatitude and polar air supports the temperature trends. Both the diagnosed extratropical and tropical forcings contribute to the temperature and circulation trends over 1961–1990, while the extratropical forcing dominates tropical forcing for the trends over 1991–2010. The contribution of the tropical forcing to the trends is sensitive to the period considered. The temperature and circulation responses to the diagnosed tropical and extratropical forcings are approximately additive and partially offsetting. Covariances between the interannual surface temperature and 500-hPa geopotential anomalies for the NCEP reanalysis from 1961 to 2010 are dominated by two leading modes associated with the North Atlantic Oscillation (NAO) and Pacific-North American (PNA) teleconnection patterns. The diagnosed extratropical forcing accounts for a significant part of the NAO and PNA associated variability, including the interannual variability of stationary wave anomalies, as well as dynamically and thermodynamically synoptic eddy feedbacks over the North Atlantic and North Pacific. The tropical forcing contributes to the PNA related temperature and circulation variability, but has a small contribution to the NAO associated variability. Additionally, relative contributions of tropical Indian and Pacific forcings are also assessed.     

Orbital forcing in a stochastic resonance model of the Late-Pleistocene climatic variations

Applying a number of simplifying assumptions, we have used a zero-dimensional energy balance climate model (EBM) of the Budyko-Sellers type to investigate the degree to which the climatic variability of the Late Pleistocene can be explained by the mechanism of stochastic resonance. As a logical extension of an earlier version of this model we have included a more complete representation for the orbital forcings to show that a reasonably good agreement with the paleoclimatic record can be obtained. This zerodimensional EBM is appealing because of its simplicity and its ability to reproduce some of the most striking qualitative features of the observed climatic variability with a very limited number of parameters, several of which can be derived from either the observations or general circulation model (GCM) simulations of ice-age conditions.     

Sensitivity of the northern extratropics hydrological cycle to the changing insolation forcing at 126 and 115 ky BP

The orbital configuration at the end of the last interglacial, 115,000 years BP (115 ky BP), was such that the Northern Hemisphere seasonal contrast was decreased when compared to the last interglacial maximum, 126 ky BP. Climatic reconstructions argue for increased latitudinal surface temperature and salinity gradients in the North Atlantic at 115 ky BP compared to 126 ky BP. According to proxy measurements the high-latitude ocean freshening may be explained by enhanced northward atmospheric moisture advection which would have then led to decreased deep convection activity in the northern seas. To evaluate such re-adjustments of the atmospheric circulation to the insolation forcing changes, we have explored the changes in atmospheric energy balance and transport with two AGCM experiments, one for each climate. We show that the northward increase in static heat transport at 115 ky BP to 126 ky BP constitutes a first order response to the changing insolation. It tends to equalise the heat balance of the atmosphere. Despite sea surface temperatures fixed (SSTs) to present-day this feature is strongly amplified by the air–sea heat flux exchanges. By comparing with OAGCM experiments for the same periods, we find that the simulated surface ocean heat flux responses to insolation forcing are similar whether the ocean is allowed to vary or not. The latent heat transport does not undergo the same changes as the dry static one. On an annual basis, it decreases over the high northern latitudes. This is the result of summer modification of moisture sources and transient activity. The latter appears to affect latent heat transport much more than the dry static one. The winter response, however, differs from the summer response which dominates the annual mean. There is an enhanced northward atmospheric moisture advection during winter at 115 ky BP, which is responsible for the freshening of high-latitude ocean during this season. This result seems to confirm the hypothesis inferred from marine data.     

Tropical Indian Ocean subsurface temperature variability and the forcing mechanisms

The first two leading modes of interannual variability of sea surface temperature in the Tropical Indian Ocean (TIO) are governed by El Niño Southern Oscillation and Indian Ocean Dipole (IOD) respectively. TIO subsurface however does not co-vary with the surface. The patterns of the first mode of TIO subsurface temperature variability and their vertical structure are found to closely resemble the patterns of IOD and El Niño co-occurrence years. These co-occurrence years are characterized by a north–south subsurface dipole rather than a conventional IOD forced east–west dipole. This subsurface dipole is forced by wind stress curl anomalies, driven mainly by meridional shear in the zonal wind anomalies. A new subsurface dipole index (SDI) has been defined in this study to quantify the intensity of the north–south dipole mode. The SDI peaks during December to February (DJF), a season after the dipole mode index peaks. It is found that this subsurface north–south dipole is a manifestation of the internal mode of variability of the Indian Ocean forced by IOD but modulated by Pacific forcing. The seasonal evolution of thermocline, subsurface temperature and the corresponding leading modes of variability further support this hypothesis. Positive wind stress curl anomalies in the south and negative wind stress curl anomalies in the north of 5°S force (or intensify) downwelling and upwelling waves respectively during DJF. These waves induce strong subsurface warming in the south and cooling in the north (especially during DJF) and assist the formation and/or maintenance of the north–south subsurface dipole. A thick barrier layer forms in the southern TIO, supporting the long persistence of anomalous subsurface warming. To the best of our knowledge the existence of such north–south subsurface dipole in TIO is being reported for the first time.     

Orbital forcing of Arctic climate: mechanisms of climate response and implications for continental glaciation

Progress in understanding how terrestrial ice volume is linked to Earths orbital configuration has been impeded by the cost of simulating climate system processes relevant to glaciation over orbital time scales (10³–10⁵ years). A compromise is usually made to represent the climate system by models that are averaged over one or more spatial dimensions or by three-dimensional models that are limited to simulating particular snapshots in time. We take advantage of the short equilibration time (10 years) of a climate model consisting of a three-dimensional atmosphere coupled to a simple slab ocean to derive the equilibrium climate response to accelerated variations in Earths orbital configuration over the past 165,000 years. Prominent decreases in ice melt and increases in snowfall are simulated during three time intervals near 26, 73, and 117 thousand years ago (ka) when aphelion was in late spring and obliquity was low. There were also significant decreases in ice melt and increases in snowfall near 97 and 142 ka when eccentricity was relatively large, aphelion was in late spring, and obliquity was high or near its long term mean. These glaciation-friendly time intervals correspond to prominent and secondary phases of terrestrial ice growth seen within the marine ¹⁸O record. Both dynamical and thermal effects contribute to the increases in snowfall during these periods, through increases in storm activity and the fraction of precipitation falling as snow. The majority of the mid- to high latitude response to orbital forcing is organized by the properties of sea ice, through its influence on radiative feedbacks that nearly double the size of the orbital forcing as well as its influence on the seasonal evolution of the latitudinal temperature gradient.     

Water chemical changes along a latitudinal gradient in relation to climate and atmospheric deposition

Evaluating trends over time (nonparametric Mann–Kendall test) for 18 water chemical variables from 79 reference lakes, distributed all over Sweden, during spring since 1984 showed most significant trends for atmospheric deposition driven sulfate (SO₄) concentrations. The decrease in SO₄ concentrations was on average 2.7 times higher at lower (56°N to 59°N) than at higher latitudes (60°N to 68°N). This large difference in the rate of change between lower and higher latitudes could not solely be explained by atmospheric deposition as the rates of change in SO₄ wet deposition differed by a factor of only 1.5 between lower and higher latitudes. Significantly higher rates of change at lower than at higher latitudes are known from the timing of lake ice breakup, a typical climate change indicator. The rates of change in the timing of lake ice breakup differed by a factor of 2.3 between lower and higher latitudes. Other water chemical variables showing significantly higher rates of change at lower than at higher latitudes were water color (a factor of 3.5), calcium (a factor of 2.9), magnesium (a factor of 5.5) and conductivity (a factor of 5.9). The rates of change of all these variables were strongly related to the rates of change in the timing of lake ice breakup along a latitudinal gradient (R ² = 0.41–0.78, p < 0.05), suggesting that climatic changes can accelerate atmospheric driven changes at especially lower latitudes. This acceleration will result in more heterogeneous lake ecosystems along a latitudinal gradient.     

The dynamics of the Indian Ocean sea surface temperature forcing of Sahel drought

Given the pronounced warming in the Indian Ocean sea surface temperature (SST) during the second half of the twentieth century and the empirical relationship between the Indian Ocean SST and Sahel summer precipitation, we investigate the mechanisms underlying this relationship using the GFDL atmospheric model AM2.0 to simulate the equilibrium and transient response to the warming of the Indian Ocean. Equatorial wave dynamics, in particular the westward propagating equatorial Rossby waves, communicates the signal of tropospheric warming and stabilization from the Indian Ocean to the African continent. The stabilization associated with the Rossby wave front acts to suppress the convection. Feedbacks with local precipitation and depletion of moisture amplify the dynamically induced subsidence. While this stabilization mechanism is expected to operate in climate change response, the future prospects for the Sahelian climate under global warming are complicated by the intricate sensitivities to the SSTs from different ocean basins and to the direct radiative forcing of greenhouse gases.     

Multidecadal to decadal variability in the equatorial Indian Ocean subsurface temperature and the forcing mechanisms

Climate Dynamics - The Tropical Indian Ocean (TIO) is seen to exhibit robust warming after the 1950s. Most of the previous studies on the Indian Ocean (IO) surface and subsurface temperature...     

Processes shaping the spatial pattern and seasonality of the surface air temperature response to anthropogenic forcing

In the period 1960–2010, the land surface air temperature (SAT) warmed more rapidly over some regions relative to the global mean. Using a set of time-slice experiments, we highlight how different physical processes shape the regional pattern of SAT warming. The results indicate an essential role of anthropogenic forcing in regional SAT changes from the 1970s to 2000s, and show that both surface–atmosphere interactions and large-scale atmospheric circulation changes can shape regional responses to forcing. Single forcing experiments show that an increase in greenhouse gases can lead to regional changes in land surface warming in winter (DJF) due to snow-albedo feedbacks, and in summer (JJA) due to soil-moisture and cloud feedbacks. Changes in anthropogenic aerosol and precursor (AA) emissions induce large spatial variations in SAT, characterized by warming over western Europe, Eurasia, and Alaska. In western Europe, SAT warming is stronger in JJA than in DJF due to substantial increases in clear sky shortwave radiation over Europe, associated with decreases in local AA emissions since the 1980s. In Alaska, the amplified SAT warming in DJF is due to increased downward longwave radiation, which is related to increased water vapor and cloud cover. In this case, although the model was able to capture the regional pattern of SAT change, and the associated local processes, it did not simulate all processes and anomalies correctly. For the Alaskan warming, the model is seen to achieve the correct regional response in the context of a wider North Pacific anomaly that is not consistent with observations. This demonstrates the importance of model evaluation that goes beyond the target variable in detection and attribution studies.     

Estimating the Asian radon flux density and its latitudinal gradient in winter using ground-based radon observations at Sado Island

Terrestrial radon-222 flux density for the Asian continent, integrated over distances of 4500 km, is estimated in two 20° latitudinal bands centred on 48.8°N and 63.2°N. The evaluation is based on three years of wintertime radon measurements at Sado Island, Japan, together with meteorological and trajectory information. A selection of 18% of observations are suitable for evaluation of an analytical expression for the continental surface flux. Various meteorological assumptions are discussed; it is found that there is a substantial effect of increased complexity of the formulation on the flux estimates obtained. The distribution of spatially integrated radon flux over the Asian landmass is reported for the first time. Expressed as geometric means and 1σ-ranges, estimated fluxes are 14.1 mBq m⁻² s⁻¹ (1σ-range: 18 mBq m⁻² s⁻¹) and 8.4 mBq m⁻² s⁻¹ (1σ-range: 10 mBq m⁻² s⁻¹) for the lower and higher latitude bands. These results constitute an annual minimum in flux densities for these regions, and are higher than previously reported. The existence of a latitudinal gradient in the continental radon source function is confirmed; the present estimate for Asia (−0.39 mBq m⁻² s⁻¹ per degree of latitude) is in agreement with the northern hemisphere terrestrial radon flux gradient proposed previously.     

Sensitivity of Hudson Bay Sea ice and ocean climate to atmospheric temperature forcing

A regional sea-ice?Cocean model was used to investigate the response of sea ice and oceanic heat storage in the Hudson Bay system to a climate-warming scenario. Projections of air temperature (for the years 2041?C2070; effective CO₂ concentration of 707?C950?ppmv) obtained from the Canadian Regional Climate Model (CRCM 4.2.3), driven by the third-generation coupled global climate model (CGCM 3) for lateral atmospheric and land and ocean surface boundaries, were used to drive a single sensitivity experiment with the delta-change approach. The projected change in air temperature varies from 0.8°C (summer) to 10°C (winter), with a mean warming of 3.9°C. The hydrologic forcing in the warmer climate scenario was identical to the one used for the present climate simulation. Under this warmer climate scenario, the sea-ice season is reduced by 7?C9?weeks. The highest change in summer sea-surface temperature, up to 5°C, is found in southeastern Hudson Bay, along the Nunavik coast and in James Bay. In central Hudson Bay, sea-surface temperature increases by over 3°C. Analysis of the heat content stored in the water column revealed an accumulation of additional heat, exceeding 3?MJ?m^?3, trapped along the eastern shore of James and Hudson bays during winter. Despite the stratification due to meltwater and river runoff during summer, the shallow coastal regions demonstrate a higher capacity of heat storage. The maximum volume of dense water produced at the end of winter was halved under the climate-warming perturbation. The maximum volume of sea ice is reduced by 31% (592?km3) while the difference in the maximum cover is only 2.6% (32,350?km²). Overall, the depletion of sea-ice thickness in Hudson Bay follows a southeast?Cnorthwest gradient. Sea-ice thickness in Hudson Strait and Ungava Bay is 50% thinner than in present climate conditions during wintertime. The model indicates that the greatest changes in both sea-ice climate and heat content would occur in southeastern Hudson Bay, James Bay, and Hudson Strait.     

A new assessment of the mid-1970s abrupt atmospheric temperature change in the NCEP/NCAR reanalysis and associated solar forcing implications

Based on the reanalysis data from the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP?CNCAR) and solar radio irradiance (SRI) at 10.7?cm wavelength obtained from the National Oceanic and Atmospheric Administration??s Space Weather Prediction center, the abrupt temperature change in the mid-1970s and its possible association with solar irradiance variability have been investigated. The results show that a discontinuous abrupt change in the mid-1970s in the NCEP?CNCAR reanalysis was observed in the tropical lower and middle stratospheric temperature. The shift in temperature and its timing agrees well with the climate regime shift discovered in the radiosonde observations (HadAT), European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40), and many previous studies and manifests a statistically significant change at the 95% confidence level. A corresponding change of the SRI was identified in the mid 1970s although the statistical t test value is not very high. The running correlation with a 21-year moving time window exhibits a strong positive correlation between the solar cycle and atmospheric temperature in the tropical stratosphere during the period of 1948?C2007. However, the positive correlation was broken at the time of the mid-1970s abrupt change and two peak positive correlation points were observed in 1972 and 1982, respectively.     

The role of atmospheric teleconnection in the subtropical thermal forcing on the equatorial pacific

The equatorial response to subtropical Pacific forcing was studied in a coupled climate model.The forcings in the western,central and eastern subtropical Pacific all caused a significant response in the equatorial thermocline,with comparable magnitudes.This work highlights the key role of air-sea coupling in the subtropical impact on the equatorial thermocline,instead of only the role of the ＂oceanic tunnel＂.The suggested mechanism is that the cyclonic （anticyclonic） circulation in the atmosphere caused by the subtropical surface warming （cooling） can generate an anomalous upwelling （downwelling） in the interior region.At the same time,an anomalous downwelling （upwelling） occurs at the equatorward flank of the forcing,which produces anomalous thermocline warming （cooling）,propagating equatorward and resulting in warming （cooling） in the equatorial thermocline.This is an indirect process that is much faster than the ＂oceanic tunnel＂ mechanism in the subtropical impact on the equator.     

Simulation of climate phase lags in response to precession and obliquity forcing and the role of vegetation

Long (130,000 years) transient simulations with a coupled model of intermediate complexity (CLIMBER-2) have been performed. The main objective of this study is to examine leads and lags in the response to the climate system to separate obliquity and precession-induced insolation changes. Focus is on the role of internal feedbacks in the coupled atmosphere/ocean/sea-ice/vegetation system. No interactive ice sheets were used. The results show that leads and lags occur in response to the African/Asian monsoon, temperatures at high latitudes and the Atlantic thermohaline circulation. For the monsoon, leads and lags of the monthly precipitation with respect to the precession parameter were found, which are strongly modified by vegetation. In contrast, no lag was observed for the annual precipitation. At high latitudes during late winter/early spring a vegetation-induced lag with respect to the precession parameter was found in surface air temperatures. Again, no annual lag was detected. The lag in the monthly surface air temperatures induces a lag in the annual overturning in the Atlantic Ocean by changing the strength of the deep convection. The lag is several thousand years. The obliquity-related forcing does not give rise to lags in the climate system. We conclude that lags in monthly climatic variables, which are due to vegetation feedbacks, can result in an annual lag when a climatic process (like deep water formation) acts as a filter for certain months.     

温度初值确定之合理性与气压梯度力项的计算

本文讨论数值试验中两个基本问题:温度初值确定和气压梯度力的计算。首先比较几种利用静力方程差分解求温度初值方案所引起的误差,提出一种把等压面高度场插值与静力方程微分解结合起来求取等σ面温度初值的方法,结果表明用这种方法确定的温度初值及其水平分布和垂直递减率接近实测值。文章又讨论山脉地区气压梯度力计算问题,表明气压梯度力的计算精度不仅与计算方案有关,而且与温度初值有很大的关系。利用本文提出的方案得到的温度初值来计算气压梯度力,误差达到比较满意的精度。     

On some characteristics of insolation changes in the past and the future

Variations in terrestrial insolation, induced by perturbations of the earth's orbital parameters, are calculated for different geographical latitudes for ±100000 yr and in detail for the modern period between A.D. 1800–2100. The calculations show that short-period insolation variations occur against a background of secular variation, with an amplitude which can be comparable in magnitude to that of the 300-yr secular trend. For comparison we calculate the secular trends of insolation for Milankovich's caloric half-years for the period ±100000 yr with high time resolution. The nature of secular and short-term insolation changes is discussed for different latitudinal circles during future centuries. We conclude that orbitally-induced variations of insolation with periods of 18.6, 11.9, 5.9, 4.0, and 2.7 yr will perturb the radiation regime at the upper atmospheric boundary.