Mean Sea Level Pressure Gradient Research Articles

Understanding winter windstorm predictability over Europe

Abstract. Winter windstorms belong to the most damaging meteorological events in the extra-tropics. Their impact on society makes it essential to understand and improve seasonal forecasts of these extreme events. Skilful predictions on a seasonal timescale have been shown in previous studies by investigating hindcasts from various forecast centres. This study aims to explain storm forecast skill based on relevant dynamical factors. Therefore, a number of factors which are known to influence either windstorms directly or their synoptic relevant systems, mid-latitude cyclones, are investigated. These factors are analysed for their relation to windstorm forecast performance based on a reanalysis (ERA5) and the seasonal hindcast of the UK Met Office (Global Seasonal forecasting system version 5, GloSea5). Within GloSea5, relevant dynamical factors are (1) validated with respect to their physical connections to windstorms, (2) investigated with respect to the seasonal forecast skill of the factors themselves, and (3) assessed on the relevance and influence of their forecast performance to and on windstorm forecast skill. Although not all investigated factors reveal a clear and consistent influence on windstorm forecast skill over Europe, core factors like mean sea level pressure gradient, sea surface temperature, equivalent potential temperature and Eady growth rate show consistent results within these three steps: their physical connection is well represented in the model; these factors are skilfully predicted in storm-relevant regions, and, consequently, this skill leads to increased forecast skill of winter windstorms over Europe. This study thus explains existing forecast skill in winter windstorms but also indicates potential for further model developments to improve seasonal winter windstorm predictions.

Assessment of thermocline depth bias in the Seychelles-Chagos Thermocline Ridge of the Southwestern Indian Ocean simulated by the CMIP6 models

The Seychelles-Chagos Thermocline Ridge (SCTR, 5°S-10°S, 50°E-80°E) is a unique open-ocean upwelling region in the southwestern Indian Ocean. Due to the negative wind stress curl between the equatorial westerlies and southeasterly trade winds, SCTR is known as a strong upwelling region with high biological productivity, providing a primary fishing zone for the surrounding countries. Given its importance in shaping the variability of the Indian Ocean climate by understanding the sea-air interaction and its dynamics, the simulation of SCTR is evaluated using outputs from the Coupled Model Intercomparison Project Phase Sixth (CMIP6). Compared to observations, 23 out of 27 CMIP6 models tend to simulate considerably deeper SCTR thermocline depth (defined as the 20°C isotherm depth (D20))– a common bias in climate models. The deep bias is related to the easterly wind bias in the equatorial to southern Indian Ocean, which is prominent in boreal summer and fall. This easterly wind bias produces a weak annual mean Ekman pumping, especially in the boreal fall. Throughout the year, the observed Ekman pumping is positive and is driven by two components: the curl term, is associated with the wind stress curl, leads to upwelling during boreal summer to fall; the beta term, is linked to planetary beta and zonal wind stress, contributes to downwelling during boreal spring to fall. However, the easterly wind bias in the CMIP6 increases both the positive curl and negative beta terms. The beta term bias offsets the curl term bias and reduces the upwelling velocity. Furthermore, the easterly wind bias is likely caused by the reduced east-west sea surface temperature (SST) difference associated with a pronounced warm bias in the western equatorial Indian Ocean, accompanied by the east-west mean sea level pressure gradient over the Indian Ocean. Furthermore, this study finds local wind-induced Ekman pumping to be a more dominant factor in thermocline depth bias than Rossby waves, despite CMIP6 models replicating Rossby wave propagation. This study highlights the importance of the beta term in the Ekman pumping simulation. Thus, reducing the boreal summer-to-fall easterly wind bias over the Indian Ocean region may improve the thermocline depth simulation over the SCTR region.

Climate-induced variability in South Atlantic wave direction over the past three millennia

Through alteration of wave-generating atmospheric systems, global climate changes play a fundamental role in regional wave climate. However, long-term wave-climate cycles and their associated forcing mechanisms remain poorly constrained, in part due to a relative dearth of highly resolved archives. Here we use the morphology of former shorelines preserved in beach-foredune ridges (BFR) within a protected embayment to reconstruct changes in predominant wave directions in the Subtropical South Atlantic during the last ~ 3000 years. These analyses reveal multi-centennial cycles of oscillation in predominant wave direction in accordance with stronger (weaker) South Atlantic mid- to high-latitudes mean sea-level pressure gradient and zonal westerly winds, favouring wave generation zones in higher (lower) latitudes and consequent southerly (easterly) wave components. We identify the Southern Annular Mode as the primary climate driver responsible for these changes. Long-term variations in interhemispheric surface temperature anomalies coexist with oscillations in wave direction, which indicates the influence of temperature-driven atmospheric teleconnections on wave-generation cycles. These results provide a novel geomorphic proxy for paleoenvironmental reconstructions and present new insights into the role of global multi-decadal to multi-centennial climate variability in controlling coastal-ocean wave climate.

The effect of cyclones crossing the Mediterranean region on sea level anomalies on the Mediterranean Sea coast

Abstract. Large positive and negative sea level anomalies on the coast of the Mediterranean Sea are linked to intensity and position of cyclones moving along the Mediterranean storm track with dynamics involving different factors. This analysis is based on a model hindcast and considers nine coastal stations, which are representative of sea level anomalies with different magnitudes and characteristics. When a shallow water fetch is present, the wind around the cyclone centre is the main cause of positive and negative sea level anomalies, depending on its onshore or offshore direction. The inverse barometer effect produces a positive anomaly on the coast near the cyclone pressure minimum and a negative anomaly at the opposite side of the Mediterranean Sea. The latter is caused by the cross-basin mean sea level pressure gradient that is associated with the presence of a cyclone. This often coincides with the presence of an anticyclone above the station, which causes a local negative inverse barometer effect. Further, at some stations, negative sea level anomalies are reinforced by a residual water mass redistribution within the basin, which is associated with a transient response to the atmospheric pressure forcing. Though the link with the presence of a cyclone in the Mediterranean has comparable importance for positive and negative anomalies, the relation between cyclone position and intensity is stronger for the magnitude of positive events. The area of cyclogenesis, track of the central minimum and position at the time of the event differ depending on the location the sea level anomaly occurs and on its sign. The western Mediterranean is the main cyclogenesis area for both positive and negative anomalies overall. Atlantic cyclones mainly produce positive sea level anomalies in the western basin. At the easternmost stations, positive anomalies are caused by cyclogenesis in the eastern Mediterranean. North African cyclogenesis is a major source of positive anomalies on the central African coast and negative anomalies on the eastern Mediterranean and northern Aegean coasts.

Along-Channel Winds in Howe Sound: Climatological Analysis and Case Studies

ABSTRACTThis paper examines along-channel winds within Howe Sound, British Columbia, Canada, that occur from both the interior plateau out toward the coast as outflows and from the coast inland as inflows. First, the relationships between along-channel winds and pressure, temperature, and humidity are explored in Howe Sound–Cheakamus Valley. The pressure gradients between Pam Rocks and Squamish and Pam Rocks and Pemberton have the strongest correlations with outflow strength and that between Pam Rocks and Squamish has the strongest correlation with inflow strength. Outflows (inflows) have lower (higher) temperatures and dew point temperatures, except for the inflows in summer, which have lower dewpoint temperatures than the overall mean. Second, two case studies of outflow events are presented and described during the period of intensive observations prior to and during the Vancouver 2010 Winter Olympics. The January 2010 outflow event is caused by a zone of strong across-barrier mean sea level pressure gradient. The pressure gradient is formed behind an Arctic front that moved southward across Howe Sound. The February 2010 outflow event is caused by an approaching sea level low pressure centre from the Pacific that formed a northeast–southwest mean sea level pressure gradient across southern British Columbia. In the January case, the outflow layer is about 1.5 km deep, while it is shallower in the February case. Only the January outflow case exhibits hydraulic behaviour.

Influence of Climate Variability on Extreme Ocean Surface Wave Heights Assessed from ERA-Interim and ERA-20C

AbstractExtreme ocean surface wave heights significantly affect coastal structures and offshore activities and impact many vulnerable populations of low-lying islands. Therefore, better understanding of ocean wave height variability plays an important role in potentially reducing risk in such regions. In this study, global impacts of natural climate variability such as El Niño–Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and Pacific decadal oscillation (PDO) on extreme significant wave height (SWH) are analyzed using ERA-Interim (1980–2014) and ECMWF twentieth-century reanalysis (ERA-20C; 1952–2010) datasets for December–February (DJF). The nonstationary generalized extreme value (GEV) analysis is used to determine the influence of natural climate variability on DJF maxima of SWH (Hmax), wind speed (Wmax), and mean sea level pressure gradient amplitude (Gmax). The major ENSO influence on Hmax is found over the northeastern North Pacific (NP), with increases during El Niño and decreases during La Niña, and its counter responses are observed in coastal regions of the western NP, which are consistently observed in both Wmax and Gmax responses. The Hmax response to the PDO occurs over similar regions in the NP as those associated with ENSO but with much weaker amplitude. Composite analysis of different ENSO and PDO phase combinations reveals stronger (weaker) influences when both variability modes are of the same (opposite) phase. Furthermore, significant NAO influence on Hmax, Wmax, and Gmax is observed throughout Icelandic and Azores regions in relation to changes in atmospheric circulation patterns. Overall, the response of extreme SWH to natural climate variability modes is consistent with seasonal mean responses.

Diagnosing Santa Ana Winds in Southern California with Synoptic-Scale Analysis

Abstract Santa Ana winds (SAW) are among the most notorious fire-weather conditions in the United States and are implicated in wildfire and wind hazards in Southern California. This study employs large-scale reanalysis data to diagnose SAW through synoptic-scale dynamic and thermodynamic factors using mean sea level pressure gradient and lower-tropospheric temperature advection, respectively. A two-parameter threshold model of these factors exhibits skill in identifying surface-based characteristics of SAW featuring strong offshore winds and extreme fire weather as viewed through the Fosberg fire weather index across Remote Automated Weather Stations in southwestern California. These results suggest that a strong northeastward gradient in mean sea level pressure aligned with strong cold-air advection in the lower troposphere provide a simple, yet effective, means of diagnosing SAW from synoptic-scale reanalysis. This objective method may be useful for medium- to extended-range forecasting when mesoscale model output may not be available, as well as being readily applied retrospectively to better understand connections between SAW and wildfires in Southern California.

Seasonality of floods and their hydrometeorologic characteristics in the island of Crete

The seasonality of the hydrometeorologic characteristics of floods that occurred in Crete during the period 1990–2007 is presented. Hydrological characteristics were analyzed using seasonality indices based on a dataset of 53 daily precipitation stations as well as 15 daily and 7 monthly recording flow stations. The atmospheric circulation conditions during the flood events were examined based on a joint subjective classification and meteorological analysis. The flood event-based seasonality was found to coincide with the seasonality of the daily precipitation maxima of November and December. The seasonality of the three largest long term daily precipitation maxima indicates that 50% of the maximum precipitation events occur from November to January (NDJ period). Analysis showed that the maximum annual stream flows in Crete are lagging by approximately 1 month from annual maximum daily precipitation in the region. The circulation type classification of the flood events showed that most of the weather systems occurring in the Mediterranean and passing over Crete have SW, NW and W direction. For the majority of the events, a common mean sea level pressure gradient field was observed over Europe. This comparison of the seasonality of selected hydrometeorologic characteristics reveals valuable information within the context of flood occurrence.

Summer Sahel-ENSO teleconnection and decadal time scale SST variations

The correlation between Sahel rainfall and El Nino–Southern Oscillation (ENSO) in the northern summer has been varying for the last fifty years. We propose that the existence of periods of weak or strong relationship could result from an interaction with the global decadal scale sea surface temperature (SST) background. The main modes of SST variability have been extracted through a principal component analysis with Varimax rotation. The correlations between a July-September Sahel rainfall index and these SST modes have been computed on a 20-year running window between 1945 and 1993. The correlations with the interannual ENSO-SST mode are negative, not significant in the 1960s during the transition period from the wet climate phasis to the long-running drought in the Sahel, but then were significant since 1976. During the former period, the correlations between the Sahel rainfall index and the other SST modes (expressing mostly on quasi and multi-decadal scales) are the highest, in particular correlations with the tropical Atlantic “dipole”. Correlations between Sahel and Guinea Coast rainfall are also significantly negative. After 1970, the Sahel-Guinea Coast rainfall correlations are no longer significant, and the ENSO-SST mode becomes the only one significantly correlated with Sahel rainfall, especially due to the impact of warm events. The partial correlations between the ENSO-SST mode and the Sahel rainfall index, when the influence of the other SST modes are eliminated, are significant over all the 20-year running periods between 1945 and 1993, suggesting that this summer teleconnection could be modulated by the decadal scale SST background. The NCEP/NCAR reanalyses reproduce accurately the interannual variability of the atmospheric circulation after 1968. In particular a regional West African Monsoon Index (WAMI), combining wind speed anomalies at 925 and 200 hPa, is highly correlated with the July-September Sahel rainfall index. A warm ENSO event is associated both with an eastward mean sea level pressure gradient between the eastern tropical Pacific and the tropical Atlantic and with a northward pressure gradient along the western coast of West Africa. This pattern leads to enhanced trade winds over the tropical Atlantic and to weaker moisture advection over West Africa, consistent with a weaker monsoon system strength and a weaker Southern Hemisphere Hadley circulation. The NCEP/NCAR reanalyses do not reproduce accurately the decadal variability of the atmospheric circulation over West Africa because of artifical biases. Therefore the impact of the decadal scale pattern of the atmospheric circulation has been investigated with atmospheric general circulation model (AGCM) sensitivity experiments, by forcing the ARPEGE-Climat model with different combinations of an El Nino-like SST pattern with the pattern of the main mode of decadal scale SST variability where the hightest weights are located in the Pacific and Indian basins. AGCM outputs show that the decadal scale SST variations weakly affect Sahel rainfall variability but that they do induce an indirect effect on Sahel rainfall by enhancing the impact of the warm ENSO phases after 1980, through an increase in the fill-in of the monsoon trough and a moisture advection deficit over West Africa.

Short-term climatic fluctuations in North Atlantic Oscillation and frequency of cyclonic disturbances over North Indian Ocean and Northwest Pacific

A relationship between mean sea level pressure gradient between Azores High (AH) and Icelandic Low (IL) here after called North Atlantic Oscillation Index (NAOI) and the frequency of cyclonic disturbances over North Indian Ocean is investigated using 98 years of date (1891–1988). The analysis is carried out on monthly, seasonel, annual and decadal scales. Similar studies are also done for North West Pacific ocean cyclonic disturbances. It is noticed that the number of cyclonic disturbances over North Indian Ocean during monsoon season (June–September) as well as on annual scale is significantly correlated with NAOI. However for pre (April–May) and post (October-November-December) monsoon seasons frequency of cyclonic disturbances do not bear similar relationship with NAOI. The study also shows that decadal scale variability of cyclonic disturbances during the summer monsoon over North Indian Ocean has a remarkable correspondence with the decadal variability of NAOI.

Comments on “a synoptic climatological analysis of air quality in the Grand Canyon National Park”

Comments on “a synoptic climatological analysis of air quality in the Grand Canyon National Park”

An investigation of summer sea surface temperature anomalies in the eastern North Pacific Ocean

A study of summer sea surface temperature anomalies in the eastern North Pacific Ocean was undertaken to examine some of the processes that could affect their evolution and which may be important for their prediction. An empirical approach was utilized. The factors considered include sea surface temperature (SST) persistence (due to the relatively large heat capacity of the water), oceanic thermal advection, and wind mixing (which presumably acts through changes in the mixed layer depth of the ocean). The primary variable studied was the summer SST anomaly for each of 25, 5° latitude by 10° longitude boxes in the region 140°W-170°E, 30–55° N. Thirty years of data were used (1947–76). Diagnostic analyses using data from four Ocean Weather Stations (C, D, P and V) were performed prior to the analyses described above in order to examine wind mixing more closely. Only at the OWS's are high frequency (3 hour), measured wind speeds available at the same location as SST observtions for a long period of time. These analyses indicated a statistically significant lag relationship between average April and May monthly wind speed and subsequent summer SST at Station P. The monthly mean SLP gradient was used as an estimate of average monthly wind speed over the 25 box domain where measured winds are not routinely available. In addition, winds derived from daily sea level pressure (SLP) analyses were averaged to form monthly means. At most boxes, no significant lag relationship between summer SST anomalies and spring (April and May) winds was found using either wind data set. The box-averaged data were employed to specify the summer SST from the components of the total horizontal thermal advection (based on the mean and anomalous components of the surface current and SST fields). The anomalous summer surface current advecting the mean summer SST field was found to be the dominant horizontal advective term affecting the local time rate of change of SST, particularly for 40–50° N. Substitution of derived May anomalous currents for those of summer greatly diminished the strength of the relationship. In the predictive portion of the study, skill relative to persistence was marginal based upon dependent sample testing. Thus, in the predictive mode neither wind mixing (as parameterized here) nor oceanic thermal advection added very much information not contained in the initial SST anomaly field. However, persistence alone exhibited considerable skill, particularly in the northeastern portion of the domain. DOI: 10.1111/j.1600-0870.1983.tb00202.x

On the Use of Analogues to Improve Regression Forecasts

Three methods of objectively forecasting maximum temperature using multiple-stepwise screening are described and compared. The first method is the traditional “perfect prog” approach whereby a historical series of upper air predictors are screened against maximum temperature to establish a prediction equation. The second method employs a stratification of the historical data on the basis of mean sea level pressure gradient analogues prior to the regression step. A new prediction equation is required for each forecast. The third method is similar to the second except that random stratification is used instead of analogue stratification. A comparison of the accuracy of the three methods when tested on independent data indicates that the best forecasts are obtained when analogue stratification is used. Application of the analogue stratification method to “model output statistics” forecasts and the statistical correction of prognoses is discussed.