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- Author or Editor: Peter Clark x
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Abstract
Extratropical cyclones with damaging winds can have large socioeconomic impacts when they make landfall. During the last decade, studies have identified a mesoscale transient jet, the sting jet, that descends from the tip of the hooked cloud head toward the top of the boundary layer in the dry intrusion region as a cause of strong surface winds, and especially gusts, in some cyclones. While many case studies have focused on the dynamics and characteristics of these jets, there have been few studies that assess the climatology of the associated cyclones and their importance for wind risk. Here the climatological characteristics of North Atlantic cyclones are determined in terms of the possibility that they had sting jets using a previously published sting-jet precursor diagnostic applied to ERA-Interim data over 32 extended winter seasons from 1979 to 2012. Of the 5447 cyclones tracked, 32% had the precursor (42% in the 22% of cyclones that developed explosively). Precursor storms have a more southerly and zonal storm track than storms without the precursor, and precursor storms tend to be more intense as defined by 850-hPa relative vorticity. This study also shows that precursor storms are the dominant cause of cyclone-related resolved strong wind events over the British Isles for 850-hPa wind speeds exceeding 30 m s−1. Hence, early detection of a sting-jet storm could give advance warning of enhanced wind risk. However, over continental northwestern Europe, precursor cyclone-related windstorm events occur far less often.
Abstract
Extratropical cyclones with damaging winds can have large socioeconomic impacts when they make landfall. During the last decade, studies have identified a mesoscale transient jet, the sting jet, that descends from the tip of the hooked cloud head toward the top of the boundary layer in the dry intrusion region as a cause of strong surface winds, and especially gusts, in some cyclones. While many case studies have focused on the dynamics and characteristics of these jets, there have been few studies that assess the climatology of the associated cyclones and their importance for wind risk. Here the climatological characteristics of North Atlantic cyclones are determined in terms of the possibility that they had sting jets using a previously published sting-jet precursor diagnostic applied to ERA-Interim data over 32 extended winter seasons from 1979 to 2012. Of the 5447 cyclones tracked, 32% had the precursor (42% in the 22% of cyclones that developed explosively). Precursor storms have a more southerly and zonal storm track than storms without the precursor, and precursor storms tend to be more intense as defined by 850-hPa relative vorticity. This study also shows that precursor storms are the dominant cause of cyclone-related resolved strong wind events over the British Isles for 850-hPa wind speeds exceeding 30 m s−1. Hence, early detection of a sting-jet storm could give advance warning of enhanced wind risk. However, over continental northwestern Europe, precursor cyclone-related windstorm events occur far less often.
Abstract
Recent analysis of 38 globally distributed paleoclimatic records covering Marine Isotope Stage 3 (MIS 3) 60–26 ka demonstrated that the two leading empirical orthogonal functions (EOFs) explaining the data are the Greenland ice-core signal (“northern” signal) and the Antarctic ice-core signal (“southern” signal). Here singular spectral analysis (SSA) is used to show that millennial-scale variability of each of these two leading EOFs is characterized by two independent modes. The two modes of each EOF share similar relative distributions of variance, identical spectra, and, where each mode has spectral power, coherency spectra, which are significantly above the null hypothesis level at 95% confidence. The only difference between the modes of the northern and southern signals is that they are phase shifted. The phasing and long response time of the low-frequency mode, combined with its relationship to atmospheric CO2 and sea level, are consistent with coupled changes in the ocean, ice sheets, atmosphere, and carbon cycle, whereas the phasing and short response time of the high-frequency mode are consistent with an atmospheric transmission likely induced by changes in hemispheric sea ice distributions and attendant feedbacks.
Abstract
Recent analysis of 38 globally distributed paleoclimatic records covering Marine Isotope Stage 3 (MIS 3) 60–26 ka demonstrated that the two leading empirical orthogonal functions (EOFs) explaining the data are the Greenland ice-core signal (“northern” signal) and the Antarctic ice-core signal (“southern” signal). Here singular spectral analysis (SSA) is used to show that millennial-scale variability of each of these two leading EOFs is characterized by two independent modes. The two modes of each EOF share similar relative distributions of variance, identical spectra, and, where each mode has spectral power, coherency spectra, which are significantly above the null hypothesis level at 95% confidence. The only difference between the modes of the northern and southern signals is that they are phase shifted. The phasing and long response time of the low-frequency mode, combined with its relationship to atmospheric CO2 and sea level, are consistent with coupled changes in the ocean, ice sheets, atmosphere, and carbon cycle, whereas the phasing and short response time of the high-frequency mode are consistent with an atmospheric transmission likely induced by changes in hemispheric sea ice distributions and attendant feedbacks.
Abstract
The authors examine relationships between Indian Ocean sea surface temperature (SST) variability and the variability of the Indian monsoon, including analysis of potential long-lead predictions of Indian rainfall by regional SST and the influence of ENSO and decadal variability on the stability of the relationships. Using monthly gridded (4° × 4°) SST data from the Global Sea-Ice and Sea Surface Temperature (GISST) dataset that spans 1945–94, the correlation fields between the All-India Rainfall Index (AIRI) and SST fields over the tropical Indian Ocean are calculated. In the boreal fall and winter preceding the summer Indian monsoon, SST throughout the tropical Indian Ocean correlates positively with subsequent monsoon rainfall. Negative correlation occurs between SST and the AIRI in the subsequent autumn in the northern Indian Ocean only. A strong correlation (0.53) is found between the summer AIRI and the preceding December–February Arabian Sea SST. The correlation between the AIRI and the SST to the northwest of Australia for the same period is 0.58. The highest correlation (0.87) for the years following 1977 is found between the AIRI and the central Indian Ocean SST in the preceding September–November, but this relationship is much weaker in earlier years. Based upon these correlations, the authors define Arabian Sea (AS1), northwest Australia (NWA1), and central Indian Ocean (CIO1) SST indexes. The relationships of these indexes to the AIRI and ENSO are examined. The authors find that the high correlation of the AS1 and NWA1 SST indexes with the Indian summer rainfall is largely unaffected by the removal of the ENSO signal, whereas the correlation of the CIO1 index with the AIRI is reduced. The authors examine the interdecadal variability of the relationships between SST and the AIRI and show that the Indian Ocean has undergone significant secular variation associated with a climate shift in 1976. The possible mechanisms underlying the correlation patterns and the implications of the relationship to the biennial nature of the monsoon and predictability are discussed.
Abstract
The authors examine relationships between Indian Ocean sea surface temperature (SST) variability and the variability of the Indian monsoon, including analysis of potential long-lead predictions of Indian rainfall by regional SST and the influence of ENSO and decadal variability on the stability of the relationships. Using monthly gridded (4° × 4°) SST data from the Global Sea-Ice and Sea Surface Temperature (GISST) dataset that spans 1945–94, the correlation fields between the All-India Rainfall Index (AIRI) and SST fields over the tropical Indian Ocean are calculated. In the boreal fall and winter preceding the summer Indian monsoon, SST throughout the tropical Indian Ocean correlates positively with subsequent monsoon rainfall. Negative correlation occurs between SST and the AIRI in the subsequent autumn in the northern Indian Ocean only. A strong correlation (0.53) is found between the summer AIRI and the preceding December–February Arabian Sea SST. The correlation between the AIRI and the SST to the northwest of Australia for the same period is 0.58. The highest correlation (0.87) for the years following 1977 is found between the AIRI and the central Indian Ocean SST in the preceding September–November, but this relationship is much weaker in earlier years. Based upon these correlations, the authors define Arabian Sea (AS1), northwest Australia (NWA1), and central Indian Ocean (CIO1) SST indexes. The relationships of these indexes to the AIRI and ENSO are examined. The authors find that the high correlation of the AS1 and NWA1 SST indexes with the Indian summer rainfall is largely unaffected by the removal of the ENSO signal, whereas the correlation of the CIO1 index with the AIRI is reduced. The authors examine the interdecadal variability of the relationships between SST and the AIRI and show that the Indian Ocean has undergone significant secular variation associated with a climate shift in 1976. The possible mechanisms underlying the correlation patterns and the implications of the relationship to the biennial nature of the monsoon and predictability are discussed.
CORRIGENDUM
Indian Ocean SST and Indian Summer Rainfall: Predictive Relationships and Their Decadal Variability
Abstract
The variance of the rainfall during the October–November–December (OND) “short rain” season along the coast in Kenya and Tanzania correlates strongly with sea surface temperature (SST) in the Indian Ocean between 1950 and 1999. A zonal pattern of positive correlation in the Arabian Sea and negative correlation southwest of Sumatra forms in the summer preceding the rainy season. The positive correlation strengthens in the western Indian Ocean and the negative correlation in the eastern Indian Ocean weakens in the subsequent fall concurrent with the short rain. Reduced OND East African rainfall is associated with the reversed SST pattern. The OND rainfall also correlates strongly with ENSO. The SST–rain correlation pattern breaks down between the years 1983 and 1993, as does the correlation with ENSO. However, between 1994 and 1999 the OND rainfall, ENSO, and the SST zonal mode again return to strong correlation, as in the years preceding 1983.
Abstract
The variance of the rainfall during the October–November–December (OND) “short rain” season along the coast in Kenya and Tanzania correlates strongly with sea surface temperature (SST) in the Indian Ocean between 1950 and 1999. A zonal pattern of positive correlation in the Arabian Sea and negative correlation southwest of Sumatra forms in the summer preceding the rainy season. The positive correlation strengthens in the western Indian Ocean and the negative correlation in the eastern Indian Ocean weakens in the subsequent fall concurrent with the short rain. Reduced OND East African rainfall is associated with the reversed SST pattern. The OND rainfall also correlates strongly with ENSO. The SST–rain correlation pattern breaks down between the years 1983 and 1993, as does the correlation with ENSO. However, between 1994 and 1999 the OND rainfall, ENSO, and the SST zonal mode again return to strong correlation, as in the years preceding 1983.
Abstract
Retreat of the Laurentide Ice Sheet (LIS) following the Last Glacial Maximum 21 000 yr BP affected regional to global climate and accounted for the largest proportion of sea level rise. Although the late Pleistocene LIS retreat chronology is relatively well constrained, its Holocene chronology remains poorly dated, limiting our understanding of its role in Holocene climate change and sea level rise. Here new 10Be cosmogenic exposure ages on glacially deposited boulders are used to date the final disappearance of the Labrador sector of the LIS (LS-LIS). These data suggest that following the deglaciation of the southeastern Hudson Bay coastline at 8.0 ± 0.2 cal ka BP, the southwestern margin of the LS-LIS rapidly retreated ∼600 km in 140 yr and most likely in ∼600 yr at a rate of ∼900 m yr−1, with final deglaciation by 6.8 ± 0.2 10Be ka. The disappearance of the LS-LIS ∼6.8 10Be ka and attendant reduction in freshwater runoff may have induced the formation of Labrador Deep Seawater, while the loss of the high albedo surface may have initiated the Holocene Thermal Maximum in eastern Canada and southern Greenland. Moreover, the rapid melting just prior to ∼6.8 10Be ka indicates that the remnant LIS may be the primary source of a postulated rapid rise in global sea level of ∼5 m that occurred sometime between 7.6 and 6.5 cal ka BP.
Abstract
Retreat of the Laurentide Ice Sheet (LIS) following the Last Glacial Maximum 21 000 yr BP affected regional to global climate and accounted for the largest proportion of sea level rise. Although the late Pleistocene LIS retreat chronology is relatively well constrained, its Holocene chronology remains poorly dated, limiting our understanding of its role in Holocene climate change and sea level rise. Here new 10Be cosmogenic exposure ages on glacially deposited boulders are used to date the final disappearance of the Labrador sector of the LIS (LS-LIS). These data suggest that following the deglaciation of the southeastern Hudson Bay coastline at 8.0 ± 0.2 cal ka BP, the southwestern margin of the LS-LIS rapidly retreated ∼600 km in 140 yr and most likely in ∼600 yr at a rate of ∼900 m yr−1, with final deglaciation by 6.8 ± 0.2 10Be ka. The disappearance of the LS-LIS ∼6.8 10Be ka and attendant reduction in freshwater runoff may have induced the formation of Labrador Deep Seawater, while the loss of the high albedo surface may have initiated the Holocene Thermal Maximum in eastern Canada and southern Greenland. Moreover, the rapid melting just prior to ∼6.8 10Be ka indicates that the remnant LIS may be the primary source of a postulated rapid rise in global sea level of ∼5 m that occurred sometime between 7.6 and 6.5 cal ka BP.
Abstract
In comprehensive and idealized general circulation models, hemispherically asymmetric forcings lead to shifts in the latitude of the intertropical convergence zone (ITCZ). Prior studies using comprehensive GCMs (with complicated parameterizations of radiation, clouds, and convection) suggest that the water vapor feedback tends to amplify the movement of the ITCZ in response to a given hemispherically asymmetric forcing, but this effect has yet to be elucidated in isolation. This study uses an idealized moist model, coupled to a full radiative transfer code, but without clouds, to examine the role of the water vapor feedback in a targeted manner.
In experiments with interactive water vapor and radiation, the ITCZ latitude shifts roughly twice as much off the equator as in cases with the water vapor field seen by the radiation code prescribed to a static hemisperically symmetric control distribution. Using energy flux equator theory for the latitude of the ITCZ, the amplification of the ITCZ shift is attributed primarily to the longwave water vapor absorption associated with the movement of the ITCZ into the warmer hemisphere, further increasing the net column heating asymmetry. Local amplification of the imposed forcing by the shortwave water vapor feedback plays a secondary role. Experiments varying the convective relaxation time, an important parameter in the convection scheme used in the idealized moist model, yield qualitatively similar results, suggesting some degree of robustness to the model physics; however, the sensitivity experiments do not preclude that more extreme modifications to the convection scheme could lead to qualitatively different behavior.
Abstract
In comprehensive and idealized general circulation models, hemispherically asymmetric forcings lead to shifts in the latitude of the intertropical convergence zone (ITCZ). Prior studies using comprehensive GCMs (with complicated parameterizations of radiation, clouds, and convection) suggest that the water vapor feedback tends to amplify the movement of the ITCZ in response to a given hemispherically asymmetric forcing, but this effect has yet to be elucidated in isolation. This study uses an idealized moist model, coupled to a full radiative transfer code, but without clouds, to examine the role of the water vapor feedback in a targeted manner.
In experiments with interactive water vapor and radiation, the ITCZ latitude shifts roughly twice as much off the equator as in cases with the water vapor field seen by the radiation code prescribed to a static hemisperically symmetric control distribution. Using energy flux equator theory for the latitude of the ITCZ, the amplification of the ITCZ shift is attributed primarily to the longwave water vapor absorption associated with the movement of the ITCZ into the warmer hemisphere, further increasing the net column heating asymmetry. Local amplification of the imposed forcing by the shortwave water vapor feedback plays a secondary role. Experiments varying the convective relaxation time, an important parameter in the convection scheme used in the idealized moist model, yield qualitatively similar results, suggesting some degree of robustness to the model physics; however, the sensitivity experiments do not preclude that more extreme modifications to the convection scheme could lead to qualitatively different behavior.
