Long‐term variability of sea—air heat transfer in the North Atlantic ocean

Self Cite

In order to evaluate long-term climatic changes in wind wave height, visual wave estimates available from the Comprehensive Ocean-Atmosphere Data Set (GOADS) were updated for the period from 1964 to 1993. Analysis of the accuracy of visual estimates shows that observations from merchant ships can be used for the study of climate changes in storminess. Climate changes obtained in significant wave height, computed on the basis of the voluntary observed data, are quite consistent with those shown by the instrumental records at OWS L, Seven Stones Light Vessel and NDBC buoys. The linear trends in significant wave height, as well as in the wind sea and swell heights, were computed for the entire North Atlantic. Significant wave height increases of 10-30 cm/decade over the whole of the North Atlantic, except for the western and central subtropics were found. Changes in the swell height are very consistent with those seen in significant wave height. Nevertheless, wind sea indicates strong upward tendencies only in the central mid-latitudinal North Atlantic and does not show any significant trends in the Northeast Atlantic, where instrumental records of Bacon and Carter report secular changes of about 1% a year. Wind waves of smaller occurrences show significantly negative changes in the Northeast Atlantic; that is in agreement with the wind sea periods changes. Possible mechanisms driving the swell changes with no pronounced increase of the sea height and wind velocity are discussed. Changes in the intensities of intramonthly variability in different synoptic ranges are considered as the major agent of the increasing swell. The conclusion is made that the upward swell changes are driven by the intensification of high frequency synoptic processes.

Section: Introductionmentioning

confidence: 99%

Changes of wind waves in the North Atlantic over the last 30 years

Gulev¹,

Hasse²

1999

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“…Most of the climatic studies, that deal with the large-scale ocean-atmosphere interaction in the North Atlantic (Barnett, 1984;Wallace et al, 1990;Deser and Blackmon, 1993;Kushnir, 1994;Gulev, 1995) and such phenomena as the North Atlantic Oscillation (NAO) (van Loon and Rogers, 1978;Rogers, 1984;Palmer and Sun, 1985;Hurrell, 1995) are based on the analysis of monthly and seasonal means. It is obvious, however, that 2 months (seasons) having the same monthly (seasonal) means can at the same time be characterised by remarkably different intensities of synoptic activity (Zorita et al, 1992;Gulev, 1995). The short-period processes play a leading role in variations of the sea level pressure (SLP) fields over the North Atlantic, and their contribution to the total variance of SLP is up to 75% in this region (Zveryaev and Razoryonova, 1997).…”

Section: Introductionmentioning

confidence: 99%

Decadal and longer changes of the winter sea level pressure fields and related synoptic activity over the North Atlantic

Zveryaev¹

1999

Long-term intensity changes in the winter synoptic activity over the North Atlantic are studied in relation to changes of the winter mean sea level pressure (SLP) fields. Analysis of linear trends has revealed a good agreement between long-term (interdecadal) changes in the intensity of synoptic processes and variations of the winter SLP. On the contrary, no such agreement was found between detrended and low-pass filtered anomalies. There are periods, that are characterised by the enhanced (reduced) synoptic activity attributed to the low (high) index of the North Atlantic Oscillation (NAO). It appears that low-passed anomalies of intramonthly root mean square deviations (RMSD) of SLP are negatively correlated with NAO and East Atlantic (EA) teleconnection patterns over most of the North Atlantic. The low-passed winter SLP anomalies demonstrate both propagating and standing patterns. The latter have a period of about 8 years. While meridional dipole-like structures formed by the winter SLP anomalies are shifted to the west (east) of the North Atlantic, the related anomalies of synoptic activity tend to be located in the eastern (western) part of the region. When decadally averaged, anomalies in the intensity of synoptic activity are strongly linked to the North Atlantic storm track position. The exception is the 1980 -1990 decade, characterised by a very high NAO index. During this decade, enhanced synoptic activity is observed to the south of the North Atlantic storm track.

Changes of wind waves in the North Atlantic over the last 30 years

Gulev

Hasse²

1999

Self Cite

100

In order to evaluate long-term climatic changes in wind wave height, visual wave estimates available from the Comprehensive Ocean-Atmosphere Data Set (GOADS) were updated for the period from 1964 to 1993. Analysis of the accuracy of visual estimates shows that observations from merchant ships can be used for the study of climate changes in storminess. Climate changes obtained in significant wave height, computed on the basis of the voluntary observed data, are quite consistent with those shown by the instrumental records at OWS L, Seven Stones Light Vessel and NDBC buoys. The linear trends in significant wave height, as well as in the wind sea and swell heights, were computed for the entire North Atlantic. Significant wave height increases of 10 -30 cm/decade over the whole of the North Atlantic, except for the western and central subtropics were found. Changes in the swell height are very consistent with those seen in significant wave height. Nevertheless, wind sea indicates strong upward tendencies only in the central mid-latitudinal North Atlantic and does not show any significant trends in the Northeast Atlantic, where instrumental records of Bacon and Carter report secular changes of about 1% a year. Wind waves of smaller occurrences show significantly negative changes in the Northeast Atlantic; that is in agreement with the wind sea periods changes. Possible mechanisms driving the swell changes with no pronounced increase of the sea height and wind velocity are discussed. Changes in the intensities of intramonthly variability in different synoptic ranges are considered as the major agent of the increasing swell. The conclusion is made that the upward swell changes are driven by the intensification of high frequency synoptic processes.