Search results for "northern hemisphere"

Has the Northern Hemisphere been warming or cooling during the boreal winter of the last few decades?

2013

Abstract The Intergovernmental Panel on Climate Change (IPCC) reported a significant rise of the global mean surface temperature over the last 100 years, with a rate of warming over the last few decades almost double that over the last 100 years. The rate of warming is higher at land surfaces over the Northern Hemisphere (NH), and particularly extreme warming is occurring over the Arctic. In spite of the global warming trend, some asymmetries have been observed. In a recent study, a widespread cooling over the NH during the boreal winter season was reported, although other previous studies observed a warming during this season. In this paper we report trends in winter air temperatures over …

Variability and extremes of northern Scandinavian summer temperatures over the past two millennia

2012

article i nfo Palaeoclimatic evidence revealed synchronous temperature variations among Northern Hemisphere regions over the past millennium. The range of these variations (in degrees Celsius) is, however, largely unknown. We here present a 2000-year summer temperature reconstruction from northern Scandinavia and compare this timeseries with existing proxy records to assess the range of reconstructed temperatures at a regional scale. The new reconstruction is based on 578 maximum latewood density profiles from living and sub-fossil Pinus sylvestris samples from northern Sweden and Finland. The record provides evidence for sub- stantial warmth during Roman and Medieval times, larger in exten…

2020

Abstract. A compilation of the published literature on dust content in terrestrial and marine sediment cores was synchronized with pollen data and speleothem growth phases on the Greenland Ice Core Chronology 2005 (GICC05) time axis. Aridity patterns for eight key areas of the global climate system have been reconstructed for the last 60 000 years. These records have different time resolutions and different dating methods, i.e. different types of stratigraphy. Nevertheless, all regions analysed in this study show humid conditions during early Marine Isotope Stage 3 (MIS3) and the early Holocene or deglaciation, but not always at the same time. Such discrepancies have been interpreted as reg…

Changes in the seasonal snow cover of alpine regions and its effect on soil processes: A review

2007

Abstract At its maximum annual development, snow can cover more than half the Northern Hemisphere land area with one-third experiencing seasonal snow cover. The precise conditions that develop during the annual pattern of snowpack development formation have implications for: (i) soil microbiological activity and nutrient transformations; (ii) the capacity of the accumulating snowpack to retain atmospheric derived solutes; (iii) preferential elution and rapid runoff of solutes from the snowpack during periods of thaw; and (iv) leaching of solutes. Long-term records of annual snow accumulation suggest that substantial, regional scale shifts in snowpack characteristics have been occurring. The…

Convergence and divergence in organization of phytoplankton communities under various regimes of physical and biological control

2010

The hypothesis that physical constraints may be as important, if not more important, than biological ones in shaping the structure of phytoplankton assemblage was tested by analyzing longterm (11–29 years) phytoplankton series in eight lakes and nine sites located along a latitudinal gradient in the Northern hemisphere. Phytoplankton biomass was used and similarity of assemblages in same months of the annual data sets was then calculated by subtracting the Bray–Curtis dissimilarity index from 1. The extent of biological and physical forcing was partly based on ‘‘expert evaluation’’: the importance of four physical (light availability, temperature, conductivity, and sediment stirring up) and…

Northern Hemisphere atmospheric pattern enhancing Eastern Mediterranean Transient-type events during the past 1000 years

2021

High-resolution climate model simulations for the last millennium were used to elucidate the main winter Northern Hemisphere atmospheric pattern during enhanced Eastern Mediterranean Transient (EMT-type) events, a situation in which an additional overturning cell is detected in the Mediterranean at the Aegean Sea. The differential upward heat flux between the Aegean Basin and the Gulf of Lion was taken as a proxy of EMT-type events and correlated with winter mean geopotential height at 500 mbar in the Northern Hemisphere (20–90∘ N and 100∘ W–80∘ E). Correlations revealed a pattern similar to the East Atlantic/Western Russian (EA/WR) mode as the main driver of EMT-type events, with the past …

Climate of the Mediterranean

2012

This chapter considers a set of issues related to the synoptic climatology of the Mediterranean region (MR). The main Northern Hemisphere teleconnections affecting the MR and their role on temperature, precipitation, and atmospheric cyclones are described. The characteristics of the cyclones in the MR are presented. The role of teleconnections and atmospheric regimes on temperature and precipitation is discussed. The content includes extremes of temperature, precipitation, wind, and storminess (considering also marine aspects such as waves and storm surges).

Holocene millennial-scale productivity variations in the Sicily Channel (Mediterranean Sea)

2008

[1] The calcareous nannofossil assemblages of Ocean Drilling Program Hole 963D from the central Mediterranean Sea have been investigated to document oceanographic changes in surface waters. The studied site is located in an area sensitive to large-scale atmospheric and climatic systems and to high- and low-latitude climate connection. It is characterized by a high sedimentation rate (the achieved mean sampling resolution is <70 years) that allowed the Sicily Channel environmental changes to be examined in great detail over the last 12 ka BP. We focused on the species Florisphaera profunda that lives in the lower photic zone. Its distribution pattern shows repeated abundance fluctuations of …

Proposal of a simple model for the characterization of aerosols in relation to the dominant air masses

2012

In this work, we present a simple model that allows us to easily obtain the character of the dominant air mass for any location in the Northern Hemisphere at any time. Specifically, in this article the model has been tested at Valencia, Spain, for the years from 2006 to 2009. The classification of the air masses is performed analysing their trajectories over pre-defined source regions in the Northern Hemisphere. Daily trajectories were simulated by the hybrid single-particle Lagrangian integrated trajectory model HYSPLIT and then classified by our model. In order to verify our classification results, those cases identified as African were compared with the Saharan dust intrusions reported f…

The low-latitude monsoon climate during Dansgaard–Oeschger cycles and Heinrich Events

2000

During the last 100,000 years Dansgaard–Oeschger cycles (D/O cycles) and Heinrich Events have been the dominant signal of past climate variability over Greenland and the North Atlantic. The succession of stadials (cold) and interstadials (warm) associated with these cycles has been documented in records from the entire northern hemisphere, South America, New Zealand, Antarctica, the South Atlantic and the Southern Ocean. Evidently, climate forcing in the D/O band affects both hemispheres. The origin and cause of these teleconnected patterns is still unknown, even if a large proportion of the cooling in Europe and northern Asia during Heinrich Events is a meteorological response to cold surf…