Summary of changes and trends

• The annual mean Central England Temperature has increased by about 0.5°C during the 20th Century. The warmest years since records began in 1659 occurred in 1990 and 1999 and the 1990s was the warmest decade, with five of the six warmest years occurring then.
• The 30-year mean of annual mean temperature in Northern Ireland and Scotland increased by between 0.11°C to 0.39°C from 1873-1902 to 1961-1990.
• The average number of storms in October to March at UK stations has increased significantly over the past 50 years or so, with the largest increases in the south. However, the magnitude of storminess at the end of the 20th century was similar to that at the start.
• There is a tendency towards wetter winters in north-east England and drier summers in south-east England.
• The 24-month period ending in March 2001 was the wettest in England and Wales since records of the monthly total precipitation began in 1766. April 2000 to March 2001 was the wettest twelve months on record. There were no statistically significant trends in either annual precipitation or winter precipitation in Northern Ireland for the period from 1931-2000.
• The most extreme change in the NAO since the 1860s has occurred from about 1960 up to the present, with the Winter (December - March average) Index showing an upward trend. There are indications of several earlier years of comparable values over the past 500 years, but the systematic rise in values from the 1960s to the 1990s is unique.

1. Introduction

The three main weather parameters that drive ocean circulation are the wind speed and direction, air/sea heat exchange and evaporation/precipitation. Thus they affect the strength and character of the Atlantic thermohaline circulation, thereby altering the distribution of sea surface temperature and salinity on a broad scale.

On a local scale, the same parameters affect the distribution of temperature and salinity in UK waters. For example, stronger or more frequent westerly winds over the North Atlantic will drive a greater influx of Atlantic water into UK waters and bring more rainfall and warmer air temperatures. Higher rainfall will result in lower salinities in coastal waters due to increased river runoff and this will enhance density driven coastal flows. Warmer air temperatures will warm the shallower areas of UK waters or at least slow their cooling.

Changes in atmospheric pressure and wind speed and direction, particularly during storms, enhance the generation of surge levels, waves and associated currents; thus enhancing coastal erosion, flooding and mixing processes.

Rainfall affects the input of inorganic and organic terrestrial material from the land to the sea via rivers.

Descriptions of the monitoring networks that regularly measure marine weather data are given in Chapter 1, including details of how to access near real-time data.

Click here for a list of links to monitoring networks and data sets.

2. Global atmospheric features - ENSO and the North Atlantic Oscillation

2.1 ENSO

“El Niño” and “La Niña” events are driven by a “see-saw” of atmospheric pressure over the Pacific and Indian Oceans region, known as the Southern Oscillation. The term “ENSO activity” is used to collectively describe the variability of the Southern Oscillation and associated El Niño and La Niña events. During El Niño events, unusually high atmospheric sea level pressure develops in the western tropical Pacific and Indian Ocean regions, and unusually low sea level pressure develops in the southern tropical Pacific. This causes weaker than normal trade winds, allowing warm water to flow easterly across the equatorial Pacific from the Indonesian region. Consequently there is a warming of the upper layers of the sea in the eastern and central equatorial Pacific Ocean, a release of carbon dioxide from the sea and atmospheric warming through the greenhouse effect. During La Niña events, unusually low pressures to the west and unusually high pressures to the east of the International Date Line cause stronger than normal trade winds, inhibiting the easterly flow of warm water across the equatorial Pacific and hence causing anomalously cold sea temperatures, absorption of carbon dioxide from the atmosphere and atmospheric cooling.

The evidence for an influence of ENSO on the North Atlantic and European weather is weak and mostly limited to precipitation variability in parts of the Mediterranean (IPCC, 2001). However, there appears to be a correlation between the frequency of tropical Atlantic storms and ENSO activity, with El Niño and La Niña events inhibiting or enhancing the genesis of storms respectively. The number of hurricanes and tropical storms in the North Atlantic Basin was above average in 2001, a La Niña year, with 15 named storms, five more than the long-term average (WMO, 2001). 2002 started with near neutral ENSO conditions and then an El Niño event developed, and 12 named tropical storms were observed in the North Atlantic, above the average of around 10, but only four developed to hurricane strength – fewer than the average of five to six (WMO, 2002).

2.2 The North Atlantic Oscillation

2.2.1 Introduction

The North Atlantic Oscillation (NAO) is an important influence on the North Atlantic and European weather and climate. It is a “see-saw” in atmospheric sea level pressure between the subtropical high and the polar low-pressure systems, most noticeable during November to April, which drives westerly winds over the North Atlantic.

During the winter season (December-February), the NAO accounts for more than one-third of the total variance in sea level pressure (SLP) over the North Atlantic, and appears with a slight northwest-to-southeast orientation. In the so-called positive phase, higher-than-normal surface pressures south of 55ºN combine with a broad region of anomalously low pressure throughout the Arctic to enhance the climatological meridional pressure gradient. The largest amplitude anomalies occur in the vicinity of Iceland and across the Iberian Peninsula. The positive phase of the NAO is associated with stronger than- average surface westerlies across the middle latitudes of the Atlantic onto Europe.

By spring (March-May), the NAO appears as a north-south dipole with a southern centre of action near the Azores. The amplitude, spatial extent, and the percentage of total SLP variability explained by the NAO reach minimums during the summer (June-August) season, when the centres of action are substantially north and east relative to winter. By autumn (September-November), the NAO takes on more of a southwest-to-northeast orientation, with SLP anomalies in the northern centre of action comparable in amplitude to those during spring.

The basic structure of the NAO arises from the internal, non-linear dynamics of the atmosphere. There is presently no evidence of a causal connection between ENSO and the NAO, and both appear to respond quite independently of one another. However, the NAO is a regional expression of the seesaw of atmospheric pressure in the Northern Hemisphere, between the polar cap and the middle latitudes in both the Atlantic and Pacific Ocean basins, termed the Arctic Oscillation (Ambaum et al. 2001), and therefore there may be teleconnections between it and the ENSO.

2.2.2 The NAO Index

The NAO’s intensity is traditionally defined using a monthly, seasonal or annual Index calculated as the normalized sea level pressure difference between a station characteristic of the subtropical high (Gibraltar or Lisbon or Ponta Delgada, Azores) and one characteristic of the polar low (Akureyri or Stykkisholmur, Iceland). The Azores/Iceland data set produces an Index better representative of the strength of the Atlantic westerly winds during the whole year, but Hurrell (1995) concluded that the Lisbon/Iceland data set better captured NAO-related wintertime variability in sea level pressure over the North Atlantic sector and produced a time series back to 1864.

Click here for a plot of the extended Winter Index of Hurrell (1995) from 1864 onwards. Link to Hurrell’s web page http://www.cgd.ucar.edu/~jhurrell/nao.stat.winter.html .

Jones et al. (1997) subsequently showed that an adequate Winter Index could be obtained using the even longer record from Gibraltar (to 1821). Jones et al. (2003) showed that all of these indices are highly correlated on interannual and longer time scales. Also, the choice of the Iceland station is not critical since the temporal variability over this region is much larger than the spatial variability; e.g. the December-March anomalies in SLP at Stykkisholmur and Akureyri correlate at 0.98 (Hurrell and van Loon, 1997). For continuity with the previous report (IACMST, 2001), we use Jones’ NAO Winter Index. (This version of the Index has also been used in a recent study of wave climate in UK waters, refer to the work of Cotton et al. (1999) in the chapter on Waves.).

For further information on the NAO Index compiled by the Climate Research Unit, click here. Link to UEA web page (http://www.cru.uea.ac.uk/cru/data/nao.htm). The Met Office make predictions of the NAO Winter Index (http://www.metoffice.com/research/seasonal/regional/nao/index.html)

According to Hurrell et al. (2003), a disadvantage of station-based indices is that they are fixed in space, so given the movement of the NAO centres of action through the annual cycle, such indices can only adequately capture NAO variability for parts of the year. Moreover, individual station pressures are significantly affected by small-scale and transient meteorological phenomena not related to the NAO and thus contain noise. An alternative approach is to derive an Index from the principal component (PC) time series of sea level pressure anomalies over the Atlantic, although they can only be computed for parts of the 20th century, depending on the data source.

Click here for a plot on Hurrell’s Winter (December – March) PC-based NAO Index from 1899 onwards. Link to http://www.cgd.ucar.edu/~jhurrell/nao.pc.winter.html .

Changes in the NAO index correspond to large-scale changes in the north-south pressure difference across the north-east Atlantic. A positive, or high, Index indicates a stronger than usual subtropical high-pressure centre and a deeper than normal Icelandic low. The increased pressure difference results in more and stronger winter storms crossing the Atlantic Ocean on a more northerly track, with increased mid-latitude westerly winds over the NE Atlantic and northern Europe. This results in mild and wet winters and unsettled and chilly summers in the UK. A negative, or low, Index indicates a weak subtropical high and a weak Icelandic low pressure. The reduced pressure gradient results in fewer and weaker westerly winds crossing the Atlantic on a more west-east path and more occurrences of easterly winds. Anticyclones can dominate and winters become colder than normal and summers warmer in the UK. High index years are associated with warming in the southern North Atlantic and northwest European shelf seas, and with cooling in the Labrador and Nordic Seas. Low index years generally show the reverse.

2.2.3 NAO trends

Over the full historical record of the NAO, the most extreme change since the 1860s has occurred from about 1960 up to the present, with the Winter (December to March average) Index showing a recent upward trend from the 1960s to the early 1990s, but with high year-to-year variability superimposed. The 1960s were generally low index years, with associated very weak westerly winds; whereas the 1980s and 1990s were generally high index years, with associated very strong westerly winds and relatively mild and wet winters over NW Europe. Long instrumental records and palaeoclimatic reconstructions of the NAO using ice cores and tree ring chronologies indicate several earlier periods of comparable values over the past 500 years. Thus strongly positive values for individual winters occurred during the early decades of the 20th century and for several earlier periods of one or two decades in earlier centuries; but the rise in values from the 1960s to the 1990s does appear unique in the long records (Jones, 2003; Mann and Jones, 2003).

The winter of 1994/95 had one of the most positive values on record, followed in winter 1995/96 by the lowest value on record (see wind data, above). This “flip” was associated with radical changes in European weather: a reversal of the precipitation regime over Europe from more than 150% of the average winter precipitation over most of northern Europe in the winter of 1994/95 to less than 60% of the average in the winter of 1995/96 (ICES, 1999). The index subsequently rose from the extreme low of 1995/96 and the recovery continued during 1999/2000, again became negative during the winter of 2000/01 but positive in both winter 2001/02 and 2002/03. However, whilst the index for 2002/03 suggested weakly positive NAO conditions, the winter sea level pressure anomaly was not dominated by the NAO pattern and conditions in the west were more consistent with conditions associated with a negative NAO pattern (ICES, 2003a).

The recent trend in the NAO is fairly unusual but might nevertheless be part of a natural cycle, and it is uncertain if the dip in the index in the mid 1990s is only part of a decadal oscillation, or if the upward trend of the past few decades has ceased, or perhaps reversed. However, the NAO undergoes long-term cycles with varying periodicity, so any long-term trends are confused by variations on time scales from annual to multi-decadal. However, Gillet et al. (2003) showed that the observed trend in the Winter Index is outside the 95% range of internal variability, indicating that the recent climate change is due in part to external forcing; perhaps from volcanic aerosols, anthropogenic influences on the atmospheric composition or variations in solar activity, all of which can modulate the strength of the winter polar vortex.

The upward trend in the NAO strength during the last several decades has been associated with a stratospheric trend toward much stronger westerly winds encircling the pole and anomalously cold polar temperatures (Thompson et al., 2003). Reductions in stratospheric ozone and increases in GHG concentrations also appear to enhance the meridional temperature gradient in the lower stratosphere, via radiative cooling of the wintertime polar regions. This change implies a stronger polar vortex. It is possible, therefore, that the upward trend in the Winter NAO index in recent decades is associated with trends in either or both of these trace-gases quantities. Gillett et al. (2003) examined 12 coupled ocean-atmosphere models and found that nine showed an increase in the Winter Index in response to increasing GHG levels, leading them to conclude that increasing GHG concentrations have contributed to a strengthening of the North Atlantic surface pressure gradient.

2.2.4 Effects of NAO on MPC parameters

(Some of these effects are considered in more detail in the relevant parameter chapters.)

Most studies of the NAO focus on the winter months, when the atmosphere is most active dynamically and perturbations grow to their largest amplitudes. As a result, the influence of the NAO on surface temperature and precipitation, as well as on ecosystems (see section 6.5), is also greatest at this time of year. But Hurrell et al. (2003) document significant interannual to multi-decadal fluctuations in the summer NAO pattern, including a trend toward persistent anticyclonic flow over northern Europe that has contributed to anomalously warm and dry conditions in recent decades. Moreover, they state that vigorous wintertime NAO can interact with the slower components of the climate system (the ocean, in particular) to leave persistent surface anomalies into the ensuing parts of the year that may significantly influence the evolution of the climate system.

The NAO produces changes in the strength and direction of the westerly wind flow over the North Atlantic and such changes alter the seasonal mean heat and moisture transport between the Atlantic and the neighbouring continents, as well as the intensity and number of storms, their paths, and their weather. Significant changes in ocean surface temperature and heat content, ocean currents and their related heat transport, and sea ice cover in the North Atlantic are also induced by changes in the NAO. Such climatic fluctuations affect agricultural harvests, water management, energy supply and demand, and fisheries yields; the NAO thus has significant impact on a wide range of human activities as well as on marine, freshwater and terrestrial ecosystems (Dickson and Meincke, 2003), (see section 2.2.5, below).

Changes in the mean circulation patterns over the North Atlantic associated with the NAO are accompanied by changes in the intensity and number of storms, their paths, and their weather. During winter, a well-defined storm track connects the North Pacific and North Atlantic basins, with maximum storm activity over the oceans (Hurrell et al., 2003). Generally, positive NAO index winters are associated with a northeastward shift in the Atlantic storm activity with enhanced activity from Newfoundland into northern Europe and a modest decrease in activity to the south. Positive NAO index winters are also typified by more intense and frequent storms in the vicinity of Iceland and the Norwegian Sea.

The NAO and its time dependence appear central to changes in global temperature. Hurrell (1996) showed that much of the local cooling in the northwest Atlantic and the warming across Europe and downstream over Eurasia resulted directly from decadal changes in the North Atlantic atmospheric circulation in the form of the NAO, and that the NAO accounted for 31% of the wintertime interannual variance of Northern Hemisphere extratropical temperatures over the latter half of the 20th century. Moreover, changes in the atmospheric circulation associated with the NAO accounted linearly for much, but not all, of the hemispheric warming through the mid-1990s. However, the warming of the most recent winters is beyond that which can be linearly explained by changes in the NAO. Over 1999-2002, for instance, record warmth was recorded while generally cold conditions prevailed in the tropical Pacific and NAO-related circulation anomalies were weak.

According to Pingree (2002), it is now established that winter NAO indices correlate with rainfall with the positive phase of NAO tending to lead to mild and wet winters over northern Europe.

The NAO controls or modifies three of the main parameters that drive ocean circulation (wind speed, air/sea heat exchange and evaporation/precipitation). Changes in NAO are also reflected in sea surface temperature, e.g. accounting for 40-50% of the variability in winter sea surface temperatures in the southern North Sea (Loewe, 1996). Subsurface ocean observations over the North Atlantic indicate fluctuations that are coherent with the low frequency winter NAO index to depths of 400 m (Curry and McCartney, 2001).

The oceanic response to NAO variability is also evident in changes in the distribution and intensity of winter convective activity in the North Atlantic. The intensity of wintertime convective renewal of intermediate and deep waters in the Labrador Sea and the Greenland-Iceland-Norway Seas, for instance, is not only characterized by large interannual variability, but also by inter-decadal variations that appear to be synchronized with variations in the NAO (Dickson et al., 1996). These changes in turn affect the strength and character of the Atlantic thermohaline circulation and the horizontal flow of the upper ocean, thereby altering the oceanic poleward heat transport and the distribution of sea surface temperature.

There are past occurrences of low salinity anomalies that propagate around the sub polar gyre of the North Atlantic - the most famous example being the Great Salinity Anomaly (GSA) (Dickson et al., 1988). This formed during the extreme negative index phase of the NAO in the late 1960s, when clockwise flow around anomalously high pressure over Greenland fed record amounts of freshwater from the Arctic Ocean through the Fram Strait into the Nordic Seas. From there some of the fresh water passed through the Denmark Strait into the sub polar North Atlantic Ocean gyre. There have been other similar events and statistical analyses have revealed that the generation and termination of these propagating salinity modes are closely connected to a pattern of atmospheric variability strongly resembling the NAO.

Wakelin et al. (2003) have shown that winter-mean (December to March) sea levels and the NAO Index are significantly correlated over much of the northwest European shelf.

The recent upward trend toward more positive NAO index winters has been associated with increased wave heights over the northeast Atlantic and decreased wave heights south of 40°N (Bacon and Carter, 1993; Kushnir et al., 1997). There is a strong link between the NAO and the wave climate to the north and west of Britain, but not to the east (Cotton et al., 1999; Woolf et al., 2002 and 2003).

2.2.5 Effects of NAO on non-MPC parameters

A brief description is given here; see other sector reports for more details.

Changes in the NAO have been associated with a wide range of effects on the marine ecosystem, including changes in the production of plankton and the distribution of different fish species. For example, the northward shift of phytoplankton and zooplankton in the Northeast Atlantic over the last 40 years, and recent visits in UK waters by warm-water fish such as sailfin dory, blue marlin and barracuda, have been linked to the general rise in temperature in the northern hemisphere along with the additional effect of the NAO, which in recent years has brought warmer conditions to the region (Beaugrand et al., 2002; ICES, 2003b).

According to Hurrell et al. (2003), fluctuations in temperature and salinity, vertical mixing, circulation patterns and ice formation induced by variations in the NAO have a demonstrated influence on the marine ecosystem through both direct and indirect pathways. Drinkwater et al. (2003) state that there are three possible pathways by which the NAO affects the marine ecosystem. The first is the effect of NAO-induced temperature changes on metabolic processes such as feeding and growth. Since the NAO can simultaneously warm ocean temperatures in one part of the Atlantic basin and cool them in another, its impact on a single species can vary geographically. An example is the out-of-phase fluctuations in year-class strength of cod between the northeast and northwest Atlantic. More complex pathways may involve several physical and biological steps, e.g. the intense vertical ocean mixing generated by stronger-than-average westerly winds during a positive NAO index winter. This enhanced mixing delays primary production in the spring and leads to less zooplankton (e.g. Fromentin and Planque 1996), which in turn results in less food and eventually lower growth rates for fish. A third pathway occurs when a population is repeatedly affected by a particular environmental situation before the ecological change can be perceived (biological inertia), or when the environmental parameter affecting the population is itself modulated over a number of years (Heath et al., 1999).

3. Global temperature

Click here to see figures of Northern Hemisphere, Southern Hemisphere and Global average near-surface temperature annual anomalies from 1861 to 2003, compiled by the Hadley Centre and the University of East Anglia’s Climate Research Unit from regular measurements of air temperature at land stations and sea surface temperatures measured from ships and buoys. Link to
http://www.metoffice.com/research/hadleycentre/CR_data/Monthly/HadCRUGNS_3plots.gif .

Global surface temperature has increased by about 0.6 ± 0.2ºC since the late 19th Century (IPCC, 2001). The increase in temperature in the 20th century is likely to have been the largest in any century during the last 1,000 years (WMO, 2003). Based on a reconstruction of the global climate from data derived from ice cores, trees’ annual growth rings and other records, Mann and Jones (2003), consider that the Earth appears to have been warmer since 1980 than at any time in the last 18 centuries.

Including 2002, the 10 warmest years since records began in 1860 have all occurred since 1990, with the four warmest years being 1998, 2002, 2001 and 1997 (in descending order). The general increase in atmospheric gases like carbon dioxide, nitrous oxide, ozone etc. is considered to be the major contributor to this global warming, through the greenhouse effect; but one contributory warming factor over the last few decades has been the El Niño events of 1982-83, 1990-95 and 1997-98, with the latter the strongest of the 20th century and contributing to the warmest year, 1998. However, in some years global warming has been offset by cooling due to factors like La Niña events and aerosol emissions from volcanoes. For example, the eruption of Mount Pinatubo in June 1991 was followed by a 0.5ºC decrease in mean global annual temperature and a La Niña event was a cooling factor in 2001, even though that year was the third warmest on record.

4. UK temperature

The Central England Temperature (CET) record is the longest continuous record of measured surface air temperatures in the world and is representative of a triangular central area of the United Kingdom enclosed by Bristol, Manchester and London (Parker et al., 1992). It is compiled from records in a roughly triangular area enclosed by Bristol, Manchester and London; and annual temperature fluctuations in this region are considered to be representative of those in most of the UK. The monthly series began in 1659 and daily records extend back to 1772. During the twentieth century, the annual mean CET has warmed by about 0.5°C. The warmest years since 1659 occurred in 1990 and 1999 and the 1990s was the warmest decade in central England since records began, with five of the six warmest years occurring then. There is a high correlation between the CET record and the NAO; for example, the cold winter of 1995/96 was associated with the lowest value on record of the NAO index.

Click here to see a figure of the CET Annual anomalies from 1772 to 2003. Link to CET record at http://www.metoffice.com/research/hadleycentre/CR_data/Annual/cet.gif .

The Scottish and Northern Ireland Forum for Environmental Research (SNIFFER, 2000) has produced three regional terrestrial indices of temperature – a Northern Ireland Index (data from Armagh), a Scottish Mainland Index (data from Barmier, Dumfries, Edinburgh Royal Botanic Gardens, Paisley and Wick) and a Scottish Islands Index (data from Stornoway and Lerwick). A comparison of 30-year means of annual mean temperature between 1873-1902 and 1961-1990 demonstrated clearly that, although the amount of warming varied, the three indices showed warming of between 0.11°C and 0.39°C - similar to the CET record. Most of this warming was found to result from an increase in the mean minimum temperature, rather than any significant change in the mean maximum temperature.

Temperature records from Lerwick (Figure 1) also show a long-term warming, although values for 2000 and 2001 were lower than those seen in 1998 and 1999 (FRS, 2003).

5. UK Wind

Recent work by the Hadley Centre (2003) has shown that the average number of storms in October to March (as detected by 3-hourly pressure changes) at UK stations has increased significantly over the past 50 years or so. (Pressure changes were used instead of winds because the results are less sensitive to site moves and instrumentation changes.) There is also some evidence that storm frequency has increased over the UK and decreased in the north (Iceland), which is consistent with a southerly movement of the Atlantic storm track. Regional analysis shows that the largest increases occur over the southern UK.

There is poor correlation between the storm rate calculated from the pressure measurement sites and the changes (an upward trend) in the NAO Index, implying that the severe storms over the UK are more related to strong local gradients of pressure than to the large-scale pressure differences over the Atlantic. However, it is likely that the local severe storms are modified by the long-term changes on the large-scale, which are seen in the NAO index (Hadley Centre, 2003).

However it is important to place these results in context. Evidence of storm frequency from daily indices suggest that although it has increased in recent times, the magnitude of storminess at the end of the 20th century was similar to that at the start. This could mean that natural variations in the magnitude of storminess on timescales of several decades or more are responsible for all or part of the trends seen in these new results and that data covering a longer period is needed in order to distinguish a climate change trend from the natural variability (Hadley Centre, 2003).

Figure 1 (above) shows that average wind speeds at Lerwick have been increasing by approximately half a knot every 10 years, but with a good deal of variability from year to year (FRS, 2003).

The change in wind regime over the UK due to the extreme differences in the NAO between winter 1994/95 and winter 1995/96 (see below) is illustrated in figures 9, 10 and 11.

Click here for an animation of annual winter wind rose data from Lerwick, Bidston, and Shoeburyness. Link to animation of wind roses (STEMgis).

6. UK precipitation

Compiled by the Met Office, the monthly time-series of England and Wales total precipitation begins in 1766 and is the longest instrumental series of this kind in the world. It is currently based on weighted averages of daily observations from a network of stations in five regions. The 24-month period ending in March 2001 was the wettest in England and Wales since records began and April 2000 to March 2001 the wettest twelve months (WMO, 2001). There is a tendency towards wetter winters in north-east England and drier summers in south-east England (Alexander and Jones, 2001).

Click here to see the England and Wales Precipitation Annual Totals from 1766 – 2003. Link to E+W Precipitation
(http://www.metoffice.com/research/hadleycentre/CR_data/Annual/HadEWP_act_graph.gif).

Figure 1 (above) shows that rainfall at Lerwick during 2000 and 2001 was lower than in 1998 and 1999, with the greatest amounts since 1961 in the late 1960s (FRS, 2003).

An analysis (SNIFFER, 2000) of area-averaged monthly rainfall records in Northern Ireland for the period from 1931-2000 concluded that there were no statistically significant trends in either annual precipitation or winter precipitation on its own. Summers in Northern Ireland have generally been drier during the past three decades than earlier in the 70-years record, with 1976, 1983 and 1995 being particularly dry years. This has led to an increasing trend in the balance between winter and summer precipitation, measured as proportions of the relatively unvarying (or trend-less) total annual precipitation, i.e. a trend in rainfall towards relatively drier summers and wetter winters.

7. References

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Beaugrand, G., P.C. Reid, F. Ibañez, J.A. Lindley and M. Edwards (2002). Reorganization of North Atlantic Marine Copepod Biodiversity and Climate. Science, 296: 1692-1694.

Cotton, P.D., D.J.T. Carter, T.D. Allan, P.G. Challenor, D. Woolf, J. Wolf, J.C. Hargreaves, R.A. Flather, Li Bin, N. Holden and D. Palmer (1999). Joint Evaluation of Remote Sensing Information for Coastal And Harbour Organisations (JERICHO). Final Report to the British National Space Centre, project no: R3/003. 38 pages. Retrieved 10th September 2003 from the World Wide Web: http://www.satobsys.co.uk/Jericho/webpages/jeripdf.html .

Curry, R. G., and M. S. McCartney (2001). Ocean gyre circulation changes associated with the North Atlantic Oscillation, Journal of Physical Oceanography, 31: 3374–3400.

Dickson, R. R., J. Lazier, J. Meincke, P. Rhines, and J. Swift (1996). Long-term co-ordinated changes in the convective activity of the North Atlantic, Progress in Oceanography, 38: 241–295, 1996.

Dickson, R. R., J. Meincke, S. A. Malmberg, and A. J. Lee (1988). The “Great Salinity Anomaly” in the northern North Atlantic 1968–1982, Progress in Oceanography, 20: 103–151.

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Fromentin, J.M., and B. Planque, 1996. Calanus and environment in the eastern North Atlantic. II. Influence of the North Atlantic Oscillation on C. finmarchicus and C. helgolandicus. Marine Ecology Progress Series, 134: 111-118.

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Hadley Centre (2003). Climate Change, Observations and Predictions. Recent research on climate change science from the Hadley Centre. Brochure available from http://www.metoffice.com/research/hadleycentre/pubs/brochures/2003/global.pdf

Heath, M.R., J. O. Backhaus, K. Richardson, E. McKenzie, D. Slagstad, D. Beare, J. Dunn, A. Gallego, S. Hay, S. Jónasdóttir, D. Hainbucher, H. Madden, J. Mardalijevic and A. Schlacht (1999), Climate fluctuations and the spring invasion of the North Sea by Calanus finmarchicus. Fisheries Oceanography, 8: 163–176.

Hurrell, J.W. (1995). Decadal trends in the North Atlantic Oscillation and relationships to regional temperature and precipitation. Science 269: 676-679.

Hurrell, J.W. (1996). Influence of variations in extratropical wintertime teleconnections on Northern Hemisphere temperature. Geophysical Research Letters 23(6): 665-668.

Hurrell, J.W., Y. Kushnir, G. Ottersen, and M. Visbeck, Editors (2003). The North Atlantic Oscillation: Climate Significance and Environmental Impact. Geophysical Monograph Series, 134, 279pp.

Hurrell, J.W. and H. van Loon (1997). Decadal variations in climate associated with the North Atlantic oscillation. Climate Change, 36: 301-326.

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