Aslak Grinsted; J. Moore; S. Jevrejeva

Abstract We use a physically plausible four parameter linear response equation to relate 2,000 years of global temperatures and sea level. We estimate likelihood distributions of equation parameters using Monte Carlo inversion, which then allows visualization of past and future sea level scenarios. The model has good ... [view more]

Abstract

We use a physically plausible four parameter linear response equation to relate 2,000 years of global temperatures and sea level. We estimate likelihood distributions of equation parameters using Monte Carlo inversion, which then allows visualization of past and future sea level scenarios. The model has good predictive power when calibrated on the pre-1990 period and validated against the high rates of sea level rise from the satellite altimetry. Future sea level is projected from intergovernmental panel on climate change (IPCC) temperature scenarios and past sea level from established multi-proxy reconstructions assuming that the established relationship between temperature and sea level holds from 200 to 2100 ad. Over the last 2,000 years minimum sea level (−19 to −26 cm) occurred around 1730 ad, maximum sea level (12–21 cm) around 1150 ad. Sea level 2090–2099 is projected to be 0.9 to 1.3 m for the A1B scenario, with low probability of the rise being within IPCC confidence limits.

Stefan Grab; David Nash

Abstract This study presents the first 19th century cold season climate chronology for the Kingdom of Lesotho in southern Africa. The chronology is constructed using a variety of documentary sources including letters, diaries, reports, monographs and newspaper articles obtained from southern African and British archives. Information relating ... [view more]

Abstract

This study presents the first 19th century cold season climate chronology for the Kingdom of Lesotho in southern Africa. The chronology is constructed using a variety of documentary sources including letters, diaries, reports, monographs and newspaper articles obtained from southern African and British archives. Information relating to cold season weather phenomena during the austral autumn, winter and early spring months were recorded verbatim. Each of the cold seasons from 1833 to 1900 was then classified as “very severe”, “severe” or “normal/mild”, with a confidence rating ranging from low (1) to high (3) awarded against each annual classification. The accuracy of the document-derived chronology was verified against temperature data for Maseru for the period 1893–1900. Excellent correspondence of the document-derived chronology with the Maseru instrumental data and also with other global proxy temperature records for the 19th century is achieved. The results indicate 12 (18% of the total) very severe, 16 (23%) severe and 40 (59%) normal/mild cold seasons between 1833 and 1900. The overall trend is for more severe and snow-rich cold seasons during the early part of the study period (1833–1854) compared with the latter half of the 19th century (with the exception of the 1880s). A reduction in the duration of the frost season by over 20 days during the 19th century is also tentatively identified. Several severe to very severe cold seasons in Lesotho follow after major tropical and SH volcanic eruptions; such years are usually characterized by early frosts, and frequent and heavy snowfalls. The blocking of solar radiation and the enhanced northward displacement of polar fronts that are directly or indirectly associated with volcanic events, may account for many of the most severe Lesotho winters during the 19th century.

Hongmei Li; Aiguo Dai; Tianjun Zhou; Jian Lu

Abstract The East Asian summer monsoon (EASM) circulation and summer rainfall over East China have experienced large decadal changes during the latter half of the 20th century. To investigate the potential causes behind these changes, a series of simulations using the national center for atmospheric research (NCAR)... [view more]

Abstract

The East Asian summer monsoon (EASM) circulation and summer rainfall over East China have experienced large decadal changes during the latter half of the 20th century. To investigate the potential causes behind these changes, a series of simulations using the national center for atmospheric research (NCAR) community atmospheric model version 3 (CAM3) and the geophysical fluid dynamics laboratory (GFDL) atmospheric model version 2.1 (AM2.1) are analyzed. These simulations are forced separately with different historical forcing, namely tropical sea surface temperature (SSTs), global SSTs, greenhouse gases plus aerosols, and a combination of global SSTs and greenhouse gases plus aerosols. This study focuses on the relative roles of these individual forcings in causing the observed monsoon and rainfall changes over East Asia during 1950–2000. The simulations from both models show that the SST forcing, primarily from the Tropics, is able to induce most of the observed weakening of the EASM circulation, while the greenhouse gas plus (direct) aerosol forcing increases the land-sea thermal contrast and thus enhances the EASM circulation. The results suggest that the recent warming in the Tropics, especially the warming associated with the tropical interdecadal variability centered over the central and eastern Pacific, is a primary cause for the weakening of the EASM since the late 1970s. However, a realistic simulation of the relatively small-scale rainfall change pattern over East China remains a challenge for the global models.

Courtenay Strong; Gudrun Magnusdottir

Abstract The relationship between winter sea ice variability and the North Atlantic Oscillation (NAO) is examined for the time period 1860–2300. This study uses model output to extend recently reported observational results to multi-century time scales. Nine ensemble members are used in two Global Climate Models... [view more]

Abstract

The relationship between winter sea ice variability and the North Atlantic Oscillation (NAO) is examined for the time period 1860–2300. This study uses model output to extend recently reported observational results to multi-century time scales. Nine ensemble members are used in two Global Climate Models with forcing evolving from pre-industrial conditions through the so-called A1B scenario in which carbon dioxide stabilizes at 720 ppm by 2100. Throughout, the NAO generates an east-west dipole pattern of sea ice concentration (SIC) anomalies with oppositely signed centers of action over the Labrador and Barents Seas. During the positive polarity of the NAO, SIC increases over the Labrador Sea due to wind-driven equatorward advection of ice, and SIC decreases over the Barents Sea due to wind-driven poleward transport of heat within the mixed layer of the ocean. Although this NAO-driven SIC variability pattern can always be detected, it accounts for a markedly varying fraction of the total sea ice variability depending on the strength of the forced sea ice extent trend. For the first half of the 20th century or 1990 control conditions, the NAO-driven SIC pattern accounts for almost a third of the total SIC variance. In the context of the long term winter sea ice retreat from 1860 to 2300, the NAO-driven SIC pattern is robustly observable, but accounts for only 2% of the total SIC variance. The NAO-driven SIC dipole retreats poleward with the retreating marginal ice zone, and its Barents Sea center of action weakens. Results presented here underscore the idea that the NAO’s influence on Arctic climate is robustly observable, but time dependent in its form and statistical importance.

Virginie Guemas; David Salas-Mélia; Masa Kageyama; Hervé Giordani; Aurore Voldoire; Emilia Sanchez-Gomez

Abstract This study aims at understanding the summer ocean-atmosphere interactions in the North Atlantic European region on intraseasonal timescales. The CNRMOM1d ocean model is forced with ERA40 (ECMWF Re-Analysis) surface fluxes with a 1-h frequency in solar heat flux (6 h for the other forcing... [view more]

Abstract

This study aims at understanding the summer ocean-atmosphere interactions in the North Atlantic European region on intraseasonal timescales. The CNRMOM1d ocean model is forced with ERA40 (ECMWF Re-Analysis) surface fluxes with a 1-h frequency in solar heat flux (6 h for the other forcing fields) over the 1959–2001 period. The model has 124 vertical levels with a vertical resolution of 1 m near the surface and 500 m at the bottom. This ocean forced experiment is used to assess the impact of the North Atlantic weather regimes on the surface ocean. Composites of sea surface temperature (SST) anomalies associated with each weather regime are computed and the mechanisms explaining these anomalies are investigated. Then, the SST anomalies related to each weather regime in the ocean-forced experiment are prescribed to the ARPEGE Atmosphere General Circulation Model. We show that the interaction with the surface ocean induces a positive feedback on the persistence of the Blocking regime, a negative feedback on the persistence of the NAO-regime and favours the transition from the Atlantic Ridge regime to the NAO-regime and from the Atlantic Low regime toward the Blocking regime.

J. Schewe; A. Levermann

Abstract Experiments with the coupled climate model CLIMBER-3α, which contains an oceanic general circulation model, show deep upwelling in the Southern Ocean to be proportional to the surface wind stress in the latitudinal band of Drake Passage. At the same time, the distribution of the Southern ... [view more]

Abstract

Experiments with the coupled climate model CLIMBER-3α, which contains an oceanic general circulation model, show deep upwelling in the Southern Ocean to be proportional to the surface wind stress in the latitudinal band of Drake Passage. At the same time, the distribution of the Southern Ocean upwelling onto the oceanic basins is controlled by buoyancy distribution; the inflow into each basin being proportional to the respective meridional density difference. We observe approximately the same constant of proportionality for all basins, and demonstrate that it can be directly related to the flow geometry. For increased wind stress in the Southern Ocean, the overturning increases both in the Atlantic and the Indo-Pacific basin. For strongly reduced wind stress, the circulation enters a regime where Atlantic overturning is maintained through Pacific upwelling, in order to satisfy the transports set by the density differences. Previous results on surface buoyancy and wind stress forcing, obtained with different models, are reproduced within one model in order to distill a consistent picture. We propose that both Southern Ocean upwelling and meridional density differences set up a system of conditions that determine the global meridional overturning circulation.

D. Pattanaik; Arun Kumar

Abstract The performance of a dynamical seasonal forecast system is evaluated for the prediction of summer monsoon rainfall over the Indian region during June to September (JJAS). The evaluation is based on the National Centre for Environmental Prediction’s (NCEP) climate forecast system (CFS) initialized during ... [view more]

Abstract

The performance of a dynamical seasonal forecast system is evaluated for the prediction of summer monsoon rainfall over the Indian region during June to September (JJAS). The evaluation is based on the National Centre for Environmental Prediction’s (NCEP) climate forecast system (CFS) initialized during March, April and May and integrated for a period of 9 months with a 15 ensemble members for 25 years period from 1981 to 2005. The CFS’s hindcast climatology during JJAS of March (lag-3), April (lag-2) and May (lag-1) initial conditions show mostly an identical pattern of rainfall similar to that of verification climatology with the rainfall maxima (one over the west-coast of India and the other over the head Bay of Bengal region) well simulated. The pattern correlation between verification and forecast climatology over the global tropics and Indian monsoon region (IMR) bounded by 50°E–110°E and 10°S–35°N shows significant correlation coefficient (CCs). The skill of simulation of broad scale monsoon circulation index (Webster and Yang; WY index) is quite good in the CFS with highly significant CC between the observed and predicted by the CFS from the March, April and May forecasts. High skill in forecasting El Nino event is also noted for the CFS March, April and May initial conditions, whereas, the skill of the simulation of Indian Ocean Dipole is poor and is basically due to the poor skill of prediction of sea surface temperature (SST) anomalies over the eastern equatorial Indian Ocean. Over the IMR the skill of monsoon rainfall forecast during JJAS as measured by the spatial Anomaly CC between forecast rainfall anomaly and the observed rainfall anomaly during 1991, 1994, 1997 and 1998 is high (almost of the order of 0.6), whereas, during the year 1982, 1984, 1985, 1987 and 1989 the ACC is only around 0.3. By using lower and upper tropospheric forecast winds during JJAS over the regions of significant CCs as predictors for the All India Summer Monsoon Rainfall (AISMR; only the land stations of India during JJAS), the predicted mean AISMR with March, April and May initial conditions is found to be well correlated with actual AISMR and is found to provide skillful prediction. Thus, the calibrated CFS forecast could be used as a better tool for the real time prediction of AISMR.

Tianjun Zhou; Stefan Brönnimann; Thomas Griesser; Andreas Fischer; Liwei Zou

Abstract The authors present a reconstruction of summer (June–July–August) mean dynamic Indian monsoon index (DIMI) back to 1880 based on a large number of historical surface observation data as well as information from the upper air data. The reconstruction shows a satisfying skill in terms ... [view more]

Abstract

The authors present a reconstruction of summer (June–July–August) mean dynamic Indian monsoon index (DIMI) back to 1880 based on a large number of historical surface observation data as well as information from the upper air data. The reconstruction shows a satisfying skill in terms of both the value of reduction of error and an evaluation against other independent monsoon indices. The skill of reconstruction increases over time with more predictor data (in particular upper-level data) becoming available. A comparison with the observed all Indian summer monsoon rainfall index (AIRI) shows a high consistence in both inter-decadal and inter-annual variability. The reconstruction shows stronger than normal monsoon during the 1880s, 1915–1925 (around 1920) and 1930–1945 (around 1940) as the AIRI. The El Nino/Southern Oscillation (ENSO)—monsoon relationship is reasonably captured in the reconstruction. Powers concentrating within quasi-biennial band stand out in the reconstruction as well as in the AIRI. A comparison of the reconstruction against an atmospheric general circulation model simulation with specified SST and external forcing agents spanning 1901–1999 indicates a slightly higher reproducibility of monsoon circulation than monsoon rainfall in terms of interannual variability. The relationship between the Asian continent warming and the ENSO–monsoon connection is also discussed by using the new dynamic index.

Violeta Toma; Peter Webster

Abstract We examine the mean and transient state of the intertropical convergence zone (ITCZ) by analyzing data and using simple theory. We concentrate on the tropical eastern Pacific Ocean noting that there exists in this region a well-developed mean ITCZ. Furthermore, it is a region where ... [view more]

Abstract

We examine the mean and transient state of the intertropical convergence zone (ITCZ) by analyzing data and using simple theory. We concentrate on the tropical eastern Pacific Ocean noting that there exists in this region a well-developed mean ITCZ. Furthermore, it is a region where there has been considerable discussion in the literature of whether easterly waves develop in situ or propagate westwards from the Atlantic Ocean. The region is typical of tropical regions where there is a strong cross-equatorial pressure gradient (CEPG): mean convection well removed from the equator but located equatorward of the maximum sea-surface temperature (SST) and minimum sea level pressure (MSLP). Further to the west, near the dateline where the CEPG is much smaller, convection is weaker and collocated with SST and MSLP extrema. It is argued that in regions of significant CEPG that the near-equatorial tropical system is inertially unstable and that the rectification of the instability for a given CEPG determines the location and intensity of the climatological ITCZ. Using simple theoretical arguments, we develop an expression for the mean latitude of the ITCZ as a function of the CEPG. We note on a day-by-day basis that the ITCZ is highly transient state with variability occurring on 3–8 day time scales. Transients with amplitudes about half of the mean ITCZ, propagate northwards from the near-equatorial southern hemisphere as anomalous meridional oscillations, eventually amplifying convection in the vicinity of the mean ITCZ. It is argued that in these longitudes of strong CEPG the mean ITCZ is continually inertially unstable with advections of anticylonic vorticity across the equator resulting in the creation of an oscillating divergence–convergence doublet. The low-level convergence produces convection and the resultant vortex tube stretching generates cyclonic vorticity which counteracts the northward advection of anticylonic vorticity. During a cycle, the mid-troposphere heating near 10ºN oscillates between 6 and 12 K/day at the inertial frequency of the latitude of the mean convection. As a result, there exists an anomalous and shallower, oscillating meridional circulation with a magnitude about 50% of the mean ITCZ associated with the stable state following the generation of anticylonic vorticity. Further, it is argued that the instabilities of the ITCZ are directly associated with in situ development of easterly waves which occur with the inertial period of the latitude of the mean ITCZ. The dynamical sequences and the genesis of easterly waves are absent in the regions further to the east where the CEPG is much weaker or absent altogether. In a companion study (Part II), numerical experiments are conducted to test the hypothesis raised in the present study.

Violeta Toma; Peter Webster

Abstract A companion paper (Part I: Toma and Webster 2008), argued that the characteristics of the mean Intertropical Convergence Zone (ITCZ) arise from instabilities associated with the strong cross-equatorial pressure gradient (CEPG) that exists in the eastern Pacific Ocean as a result of the latitudinal... [view more]

Abstract

A companion paper (Part I: Toma and Webster 2008), argued that the characteristics of the mean Intertropical Convergence Zone (ITCZ) arise from instabilities associated with the strong cross-equatorial pressure gradient (CEPG) that exists in the eastern Pacific Ocean as a result of the latitudinal sea-surface temperature (SST) gradient. Furthermore, it was argued that instabilities of the mean ITCZ resulted in the in situ development of easterly waves. Thus, in Part I, it was hypothesized that the mean and transient state of eastern Pacific convection was due to local processes and less so to the advection of waves from the North Atlantic Ocean. To test this hypothesis and, at the same time, consider others such as a possible role of the equatorial and subtropical orography in generating local instabilities, a series of controlled numerical experiments are designed using the WRF regional model. The domain of the model was configured to include the western Atlantic Ocean, the Isthmus of Panama and the eastern Pacific Ocean to 155°W. Lateral boundaries were set at 40°N and 40°S, thus containing the mountains of Central America, the Andes and the Sierra Madre of Mexico. In a series of experiments, analysis products were used as boundary conditions that were successively updated four times per day, set as 10-day running average fields or as running mean monthly fields. Finally, the model was run with topography essentially eliminated over the land areas. Although there are differences between the details of the resultant fields, the location of mean convection and the form of the transients remain the same. It is concluded, in support of the theoretical and diagnostic studies of Part I that orographic forcing or waves generated in the North Atlantic Ocean are not the major causes of the mean and transient nature of disturbances in the eastern Pacific.