Browse

You are looking at 11 - 20 of 117,433 items for

  • All content x
Clear All
Na-Yeon Shin, Jong-Seong Kug, F. S. McCormack, and Neil J. Holbrook

Abstract

Recently, El Niño diversity has been paid much attention because of its different global impacts. However, most studies have focused on a single warm peak in sea surface temperature anomalies (SSTAs), either in the central Pacific or the eastern Pacific Ocean. Here, we demonstrate from observational analyses that several recent El Niño events show double warm peaks in SSTA—called “double-peaked (DP) El Niño”—that have only been observed since 2000. The DP El Niño has two warm centers, which grow concurrently but separately, in both the central and eastern Pacific. In general, the atmospheric and oceanic patterns of the DP El Niño are similar to those of the warm-pool (WP) El Niño from the development phase, such that the central Pacific peak is developed by the zonal advective feedback and reduced wind speed anomalies. However, a distinctive difference exists in the eastern Pacific where the DP El Niño has a second SSTA peak. In addition, the DP El Niño shows more distinctive anomalous precipitation along the Pacific intertropical convergence zone (ITCZ) when compared with the WP El Niño. We demonstrate that the peculiar precipitation anomalies along the Pacific ITCZ play a critical role in enhancing the equatorial westerly wind stress anomalies, which help to develop the eastern SSTA peak by deepening the thermocline in the eastern Pacific.

Restricted access
Lina Boljka, David W. J. Thompson, and Ying Li

Abstract

Baroclinic waves drive both regional variations in weather and large-scale variability in the extratropical general circulation. They generally do not exist in isolation, but rather often form into coherent wave packets that propagate to the east via a mechanism called downstream development. Downstream development has been widely documented and explored. Here we document a novel but also key aspect of baroclinic waves: the downstream suppression of baroclinic activity that occurs in the wake of eastward propagating disturbances. Downstream suppression is apparent not only in the Southern Hemisphere storm track as shown in previous work, but also in the North Pacific and North Atlantic storm tracks. It plays an essential role in driving subseasonal periodicity in extratropical eddy activity in both hemispheres, and gives rise to the observed quiescence of the North Atlantic storm track 1–2 weeks following pronounced eddy activity in the North Pacific sector. It is argued that downstream suppression results from the anomalously low baroclinicity that arises as eastward propagating wave packets convert potential to kinetic energy. In contrast to baroclinic wave packets, which propagate to the east at roughly the group velocity in the upper troposphere, the suppression of baroclinic activity propagates eastward at a slower rate that is comparable to that of the lower to midtropospheric flow. The results have implications for understanding subseasonal variability in the extratropical troposphere of both hemispheres.

Restricted access
Alexandre Tuel, Paul A. O’Gorman, and Elfatih A. B. Eltahir

Abstract

Future climate simulations indicate that the Mediterranean Basin will experience large low-level circulation changes during winter, characterized by a strong anomalous ridge that drives a regional precipitation decline. Previous research highlighted how shifts in stationary wave structure and the atmospheric response to reduced warming of the Mediterranean Sea relative to land could explain the development of this anomalous pressure high. Here, we expand on these results and provide new arguments for why and how the Mediterranean is projected to experience large circulation changes during winter. First, we find that zonal asymmetries in the vertical structure of stationary waves are important to explain the enhanced circulation response in the region and that these asymmetries are related through the external mode to the vertical structure of the mean zonal wind. Second, in winter, the Mediterranean is located just to the north of the Hadley cell edge and consequently is relatively free of large-scale descent; together with low near-surface static stability above the sea, this condition allows the weaker warming trend above the sea to propagate to the low troposphere and trigger a major circulation response. During summer, however, remotely forced descent and strong static stability prevent the cooling anomaly from expanding upward. Most of the intermodel scatter in the projected low-level circulation response in winter is related to the spread in upper-tropospheric dynamical trends. Importantly, because climate models exhibit too much vertical coherence over the Mediterranean, they likely overestimate the sensitivity of Mediterranean near-surface circulation to large-scale dynamical changes.

Restricted access
L. C. Slivinski, G. P. Compo, P. D. Sardeshmukh, J. S. Whitaker, C. McColl, R. J. Allan, P. Brohan, X. Yin, C. A. Smith, L. J. Spencer, R. S. Vose, M. Rohrer, R. P. Conroy, D. C. Schuster, J. J. Kennedy, L. Ashcroft, S. Brönnimann, M. Brunet, D. Camuffo, R. Cornes, T. A. Cram, F. Domínguez-Castro, J. E. Freeman, J. Gergis, E. Hawkins, P. D. Jones, H. Kubota, T. C. Lee, A. M. Lorrey, J. Luterbacher, C. J. Mock, R. K. Przybylak, C. Pudmenzky, V. C. Slonosky, B. Tinz, B. Trewin, X. L. Wang, C. Wilkinson, K. Wood, and P. Wyszyński

Abstract

The performance of a new historical reanalysis, the NOAA–CIRES–DOE Twentieth Century Reanalysis version 3 (20CRv3), is evaluated via comparisons with other reanalyses and independent observations. This dataset provides global, 3-hourly estimates of the atmosphere from 1806 to 2015 by assimilating only surface pressure observations and prescribing sea surface temperature, sea ice concentration, and radiative forcings. Comparisons with independent observations, other reanalyses, and satellite products suggest that 20CRv3 can reliably produce atmospheric estimates on scales ranging from weather events to long-term climatic trends. Not only does 20CRv3 recreate a “best estimate” of the weather, including extreme events, it also provides an estimate of its confidence through the use of an ensemble. Surface pressure statistics suggest that these confidence estimates are reliable. Comparisons with independent upper-air observations in the Northern Hemisphere demonstrate that 20CRv3 has skill throughout the twentieth century. Upper-air fields from 20CRv3 in the late twentieth century and early twenty-first century correlate well with full-input reanalyses, and the correlation is predicted by the confidence fields from 20CRv3. The skill of analyzed 500-hPa geopotential heights from 20CRv3 for 1979–2015 is comparable to that of modern operational 3–4-day forecasts. Finally, 20CRv3 performs well on climate time scales. Long time series and multidecadal averages of mass, circulation, and precipitation fields agree well with modern reanalyses and station- and satellite-based products. 20CRv3 is also able to capture trends in tropospheric-layer temperatures that correlate well with independent products in the twentieth century, placing recent trends in a longer historical context.

Open access
Caroline M. Wainwright, John H. Marsham, David P. Rowell, Declan L. Finney, and Emily Black

Abstract

The East African precipitation seasonal cycle is of significant societal importance, and yet the current generation of coupled global climate models fails to correctly capture this seasonality. The use of convective parameterization schemes is a known source of precipitation bias in such models. Recently, a high-resolution regional model was used to produce the first pan-African climate change simulation that explicitly models convection. Here, this is compared with a corresponding parameterized-convection simulation to explore the effect of the parameterization on representation of East Africa precipitation seasonality. Both models capture current seasonality, although an overestimate in September–October in the parameterized simulation leads to an early bias in the onset of the boreal autumn short rains, associated with higher convective instability and near-surface moist static energy. This bias is removed in the explicit model. Under future climate change both models show the short rains getting later and wetter. For the boreal spring long rains, the explicit convection simulation shows the onset advancing but the parameterized simulation shows little change. Over Uganda and western Kenya both simulations show rainfall increases in the January–February dry season and large increases in boreal summer and autumn rainfall, particularly in the explicit convection model, changing the shape of the seasonal cycle, with potential for pronounced socioeconomic impacts. Interannual variability is similar in both models. Results imply that parameterization of convection may be a source of uncertainty for projections of changes in seasonal timing from global models and that potentially impactful changes in seasonality should be highlighted to users.

Open access
Hyun-Ju Lee, Wonbae Jeon, Woo-Seop Lee, and Hwa Woon Lee

Abstract

This study investigates the spatiotemporal characteristics of human-perceived temperature (HPT) data, which describe the joint effects of temperature and humidity on the human body, and examines the related large-scale atmospheric circulation patterns for the summer season (July–August) in South Korea using trend and composite analyses. The increasing trend of HPT was stronger than that of the maximum, mean, and minimum temperatures during 1981–2018. There was an abrupt change in HPT between 1981–2009 and 2010–18, which is likely caused by the northward upper-level subtropical jet, strengthened downward motion, anomalous anticyclones around South Korea, and increased sea surface temperature over the western North Pacific Ocean, which are related to the enhancement and western expansion of the western North Pacific subtropical high (WNPSH). These results highlight the importance of the activity of the WNPSH in the variability of HPT in South Korea. When the western edge of the WNPSH is located in the northwest, a positive geopotential height anomaly at 500 hPa is centered over South Korea, which is associated with high temperatures and low relative humidity. The southwestern extension of the WNPSH modifies the wind circulation pattern and brings warm and moist air from the West (Yellow) Sea along the ridge line of the WNPSH. Eventually, it leads to extreme HPT, associated with high relative humidity and temperature over South Korea, particularly in the southern part of the country. Therefore, we concluded that monitoring and predicting the location of WNPSH and understanding the mechanism and factors influencing the movement of WNPSH under global warming are necessary for predicting and coping with extreme HPT.

Restricted access
Mian Xu, Wenshou Tian, Jiankai Zhang, Tao Wang, and Kai Qie

Abstract

Using the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) dataset and the Specified Chemistry Whole Atmosphere Community Climate Model (WACCM-SC), the impacts of sea ice reduction in the Barents–Kara Seas (BKS) on the East Asian trough (EAT) in late winter are investigated. Results from both reanalysis data and simulations show that the BKS sea ice reduction leads to a deepened EAT in late winter, especially in February, while the EAT axis tilt is not sensitive to the BKS sea ice reduction. Further analysis shows that the BKS sea ice reduction influences the EAT through the tropospheric and stratospheric pathways. For the tropospheric pathway, the results from a linearized barotropic model and Rossby wave ray tracing model reveal that long Rossby wave trains stimulated by the BKS sea ice loss propagate downstream to the North Pacific, strengthening the EAT. For the stratospheric pathway, the upward planetary waves enhanced by the BKS sea ice reduction shift the subpolar westerlies near the tropopause southward. With the critical lines displaced equatorward, the poleward transient eddies break at lower latitudes, shifting the eddy momentum deposit throughout the troposphere equatorward. Tropospheric westerlies maintained by eddy momentum deposit are also shifted southward, inducing the cyclonic anomalies over the North Pacific and deepening the EAT in late winter. Nudging experiments show that the tropospheric pathway only contributes to around 29.7% of the deepening of the EAT in February induced by the BKS sea ice loss, while the remaining 70.3% is caused by stratosphere–troposphere coupling.

Restricted access
Edward Blanchard-Wrigglesworth, Lettie A. Roach, Aaron Donohoe, and Qinghua Ding

Abstract

Antarctic sea ice extent (SIE) has slightly increased over the satellite observational period (1979 to the present) despite global warming. Several mechanisms have been invoked to explain this trend, such as changes in winds, precipitation, or ocean stratification, yet there is no widespread consensus. Additionally, fully coupled Earth system models run under historic and anthropogenic forcing generally fail to simulate positive SIE trends over this time period. In this work, we quantify the role of winds and Southern Ocean SSTs on sea ice trends and variability with an Earth system model run under historic and anthropogenic forcing that nudges winds over the polar regions and Southern Ocean SSTs north of the sea ice to observations from 1979 to 2018. Simulations with nudged winds alone capture the observed interannual variability in SIE and the observed long-term trends from the early 1990s onward, yet for the longer 1979–2018 period they simulate a negative SIE trend, in part due to faster-than-observed warming at the global and hemispheric scale in the model. Simulations with both nudged winds and SSTs show no significant SIE trends over 1979–2018, in agreement with observations. At the regional scale, simulated sea ice shows higher skill compared to the pan-Antarctic scale both in capturing trends and interannual variability in all nudged simulations. We additionally find negligible impact of the initial conditions in 1979 on long-term trends.

Restricted access
Guomin Wang, Pandora Hope, Eun-Pa Lim, Harry H. Hendon, and Julie M Arblaster

Abstract

When record-breaking climate and weather extremes occur, decision-makers and planners want to know whether they are random natural events with historical levels of reoccurrence or are reflective of an altered frequency or intensity as a result of climate change. This paper describes a method to attribute extreme weather and climate events to observed increases in atmospheric CO2 using an initialized subseasonal to seasonal coupled global climate prediction system. Application of this method provides quantitative estimates of the contribution arising from increases in the level of atmospheric CO2 to individual weather and climate extreme events. Using a coupled subseasonal to seasonal forecast system differs from other methods because it has the merit of being initialized with the observed conditions and subsequently reproducing the observed events and their mechanisms. This can aid understanding when the reforecasts with and without enhanced CO2 are compared and communicated to a general audience. Atmosphere–ocean interactions are accounted for. To illustrate the method, we attribute the record Australian heat event of October 2015. We find that about half of the October 2015 Australia-wide temperature anomaly is due to the increase in atmospheric CO2 since 1960. This method has the potential to provide attribution statements for forecast events within an outlook period (i.e., before they occur). This will allow for informed messaging to be available as required when an extreme event occurs, which is of particular use to weather and climate services.

Restricted access
Nicholas M. Leonardo and Brian A. Colle

Abstract

The largest medium-range (72–120 h) cross-track errors (CTE) of tropical cyclone (TC) forecasts from the Global Ensemble Forecast System (GEFS) over the northern Atlantic Ocean are examined for the 2008–16 seasons. The 38 unique forecasts within the upper quartile of most negative CTEs (i.e., left-of-track bias larger than 250 km by 72 h) do not have a clear common source of steering error, although 12 of the forecasts involve the underprediction of a weak upper-level trough to the west of the TC by 36 h. Meanwhile, at least 18 of the 36 most positive CTEs (right-of-track bias) are associated with TCs embedded in the southwest extent of a subtropical ridge, the strength of which is increasingly underpredicted during the first 24 h of the forecast. Excessive height falls north of the TC are driven by overpredicted divergence aloft, which corresponds to overpredicted TC outer-core convection. The convection is triggered by a 5%–20% overprediction of near-TC moisture and instability in the initial conditions. Weather Research and Forecasting (WRF) Model simulations are run at 36-, 12-, and 4-km grid spacing for select right-of-track cases, using the GEFS for initial and lateral boundary conditions. The 36-km WRF reproduces the same growth of errors as the GEFS because of, in part, sharing the same stability and moisture errors in the initial conditions. Changes in the convective parameterization affect how quickly these errors grow by affecting how much convection spins up. The addition of a 4-km nest with no convective parameterization causes the errors to grow ~20% faster, resulting in an even larger right-of-track error.

Restricted access