Changes In Sea Level Pressure Research Articles

Joint modes of climate variability across the inter-Americas

Surface temperature (Ts), sea-level pressure (SLP), and zonal wind (U) fields from NCEP/NCAR reanalysis spanning the period 1949–2006 have been jointly analysed by means of principal component analysis to assess the dominant modes of climate variability in the inter-American region, including the eastern Pacific and Atlantic Oceans. Two aspects of these time-series were analysed: the annual cycle and its residual. Seasonal modes 1 and 2 take the form of north-south dipoles of Ts over oceanic and terrestrial environments. Seasonal mode 3 derives from SLP changes over the Amazon with a Caribbean wind response. Temporal fluctuations in the residual (ANOM) fields are resolved by six modes. ANOM1–3 is dominated by SLP and Ts changes across the equator in the Atlantic and east Pacific. The ANOM4 pattern is an isolated wind mode in the NE Atlantic. ANOM5 and 6 are dominated by sea temperature patterns in the South Atlantic and in the path of African easterly waves crossing the tropical Atlantic. The residual time scores are marked by significant warming (Ts) trends and co-located lower pressure. Cycles in the anomaly time scores tend to occur at 2- to 5-year periods. Modes dominated by atmospheric variables exhibit greater high-frequency ‘noise’ than oceanic modes. Predictability is assessed through development of multivariate regression models trained on Caribbean rainfall and related target time-series. The seasonal dipole between North and South America is found to modulate Pacific El Nino-Southern Oscillation and tropical Atlantic influence on Caribbean rainfall. Copyright © 2011 Royal Meteorological Society

Medieval hydroclimate revisited

Figure 2: A) Changes in December-March precipitation (fraction of control simulation climatology; color) and sea level pressure (difference with control simulation climatology; hPa, contours) for a simulation with increased tropical Indian and western Pacific SST; precipitation changes are shown only where the differences exceed the 95% confidence level. B) As in (A), but for December-March temperature (°C, color; values are SST over ocean and 2-meter temperature over land). Colors are shown only where temperature differences exceed the 95% confidence level. Lined contour interval is 0.5°C between 30°N and 30°S and 1°C elsewhere. Global average temperature change has been subtracted. Acknowledgements N.E.G. was supported by funding from grants NA06OAR4310120 and NA08OAR4310732 from the US National Oceanographic and Atmospheric Administration (NOAA) CCDD program. N.E.G. is also grateful to PAGES program and the Oeschger Centre for Climate Research at the University of Bern for their support of a visit to the PAGES office and Oeschger Centre in 2008 during which the research reported here was advanced.

Decline in wintertime air-mass transition frequencies in the USA

Air-mass transitions are an important component of atmospheric circulation, but are dif- ficult to objectively identify and analyze in the context of decadal-scale climate change. Spatial syn- optic classification (SSC) uses surface weather observations of daily dew point temperature range, sea-level pressure range, and wind shift to identify days with a distinct change from one air mass to another ('transition days'). Thus, the SSC offers a unique opportunity to examine the climatology of synoptic-scale transition events. The frequency of the transition type has significantly declined across United States over the second half of the twentieth century, potentially revealing a climatic shift toward fewer abrupt air-mass changes, but the underlying causes have not been well-documented to date. We link the decline in winter transition frequency to decreasing intra-daily moisture and pres- sure variability by examining the largest within-day dew point temperature and sea-level pressure changes. Fewer transitions are being observed in the continental United States because of increasing daily minimum dew point temperatures in the upper Midwest and smaller pressure changes over 24 h periods in the western states. The frequency and intensity of air-mass transitions are closely tied to other components of the climate system; therefore, examination of transition trends serves as a novel framework for exploring climatic change.

The North Pacific Climate Transitions of the Winters of 1976/77 and 1988/89

Abstract This paper examines characteristic changes in North Pacific sea surface temperature (SST) variability during the boreal winter (December–February) for two subperiods (1956–88 and 1977–2009) during which the 1976/77 and the 1988/89 climate transitions occurred. It is found that the Pacific decadal oscillation (PDO)-like SST variability plays a dominant role in the 1976/77 climate transition, while both the North Pacific Gyre Oscillation (NPGO)-like and PDO-like SST variability contribute to the 1988/89 climate transition. Furthermore, the leading mode changes from PDO-like SST variability during the period 1956–88 to NPGO-like SST variability during the period 1977–2009, indicative of an enhancement of NPGO-like SST variability since 1988. Changes in sea level pressure across the 1976/77 climate transition project strongly onto the Aleutian low pressure system. But sea level pressure changes across the 1988/89 climate transition project primarily onto the North Pacific Oscillation, which is associated with remote changes in the Arctic Oscillation over the polar region as well. This contributes to enhancing the NPGO-like SST variability after 1988. The authors also analyze the output from an ensemble of Tropical Ocean and Global Atmosphere (TOGA) experiments in which the observed SSTs are inserted only at grid points in the tropics between 20°S and 20°N. The results indicate that the changes in the North Pacific atmosphere in the 1976/77 climate transition are mostly due to the tropics, whereas those in the 1988/89 climate transition are not.

Understanding the Predictability of East Asian Summer Monsoon from the Reproduction of Land–Sea Thermal Contrast Change in AMIP-Type Simulation

Abstract Previous studies on the predictability of East Asian summer monsoon circulation based on SST-constrained Atmospheric Model Intercomparison Project (AMIP)-type simulations show that this phenomenon is reproduced with lower skill than other monsoon patterns. The authors examine the reason in terms of the predictability of land–sea thermal contrast change. In the observation, a stronger monsoon circulation is dominated by a tropospheric warming over East Asian continent and a cooling over the tropical western Pacific and North Pacific, indicating an enhancement of the summertime “warmer land–colder ocean” mean state. The tropospheric cooling over the tropical western Pacific and North Pacific, and the tropospheric warming over East Asian continent are reproducible in AMIP-type simulations, although there are biases over both the North Pacific and East Asia. The tropospheric temperature responses in the model indicate a reasonable predictability of the meridional land–sea thermal contrast; the zonal land–sea thermal contrast change is also predictable but shows bias over the region north to 25°N in North Pacific. The reproducibility of the meridional thermal contrast is higher than that of the zonal thermal contrast. An examination of the predictability of two commonly used monsoon indices reveals far different skills. The index defined as zonal wind shear between 850 and 200 hPa averaged over East Asia is highly predictable. The skill comes from the predictability of the meridional land–sea thermal contrast. Although the zonal thermal contrast change is mostly predictable except for the biases over the North Pacific, the monsoon index defined as zonal sea level pressure (SLP) difference across the East Asian continent and the North Pacific is unpredictable. The low skill is related to the index definition, which attaches more importance to the land SLP change. The limitation of the index in measuring the land SLP change reduces the model skill. Although regional features of monsoon precipitation changes remain a challenge for current climate models, the predictable land–sea thermal contrast change sheds light on monsoon circulation prediction.

Physical mechanisms of the seasonal, subseasonal, and high‐frequency variability in the seasonal cycle of summer precipitation in Korea

Three distinct physical mechanisms in the seasonal cycle of the 120 day (19 May to 15 September) summer precipitation in Korea (126°E–130°E, 33°N–38°N) were identified using the 1979–2008 observed precipitation records at 61 Korea Meteorological Administration stations. Detailed space‐time structures of the physical mechanisms of precipitation variability were derived using the daily National Center for Environmental Prediction/National Center for Atmospheric Research reanalysis data over Asia (80°E–180°E, 0°–60°N). The seasonal cycle of summertime precipitation in Korea exhibits three principal temporal scales (seasonal, subseasonal, and high‐frequency components) of variability, each with distinct physical mechanisms. The seasonal component represents the variability associated with the evolution of the Asian summer monsoon, specifically the East Asia summer monsoon, governed primarily by large‐scale circulation as a result of changes in sea level pressure contrasts between the Asian continent and the surrounding oceans. The arrival and the duration of a monsoon front primarily shape the seasonal evolution of precipitation in Korea. The bimodal peaks are due to the low‐level circulation change as a result of redistribution of temperature and, subsequently, of sea level pressure during summer. The subseasonal component has characteristic time scales of 10–30 days and is associated with eastward‐moving upper‐level disturbances at ∼40°N. The upper‐level disturbances affect the meridional circulations, resulting in low‐level convergence/divergence not only underneath but also to the south and to the north of the disturbance. From mid‐July to mid‐August, the subseasonal component is more clearly observable, and the period of oscillations is generally shorter, than during early or late summer. The high‐frequency component with time scales of less than 10 days is associated with midlatitude baroclinic Rossby waves; synoptic‐scale variations of upper‐level geopotential height and low‐level moisture convergence are well correlated with the high‐frequency variability of precipitation in Korea. Positive precipitation anomalies over Korea correspond to an upper‐level divergence with anticyclonic wind anomalies caused by baroclinic Rossby waves with axes around ∼40°N. The vertical structures of key physical variables are well explained in the context of baroclinic instability with the geopotential height anomaly field connecting the upper‐level divergence and the lower‐level convergence tilted westward with height and the temperature anomaly field slanting slightly in the opposite direction.

Climatology of winter transition days for the contiguous USA, 1951–2007

In middle and high latitudes, climate change could impact the frequency and characteristics of frontal passages. Although transitions between air masses are significant features of the general circulation that influence human activities and other surface processes, they are much more difficult to objectively identify than single variables like temperature or even extreme events like fires, droughts, and floods. The recently developed Spatial Synoptic Classification (SSC) provides a fairly objective means of identifying frontal passages. In this research, we determine the specific meteorological patterns represented by the SSC’s Transition category, a “catch-all” group that attempts to identify those days that cannot be characterized as a single, homogeneous air mass type. The result is a detailed transition climatology for the continental USA. We identify four subtypes of the Transition category based on intra-day sea level pressure change and dew point temperature change. Across the contiguous USA, most transition days are identified as cold fronts and warm fronts during the winter season. Among the two less common subtypes, transition days in which the dew point temperature and pressure both rise are more frequently observed across the western states, and days in which both variables fall are more frequently observed in coastal regions. The relative frequencies of wintertime warm and cold fronts have changed over the period 1951–2007. Relative cold front frequency has significantly increased in the Northeast and Midwest regions, and warm front frequencies have declined in the Midwest, Rocky Mountain, and Pacific Northwest regions. The overall shift toward cold fronts and away from warm fronts across the northern USA arises from a combination of an enhanced ridge over western North America and a northward shift of storm tracks throughout the mid-latitudes. These results are consistent with projections of climate change associated with elevated greenhouse gas concentrations.

Variability of the Icelandic Low and the Azores High in January and Their Influence on Climatic Conditions in Poland

Abstract The study analysed long-term changes and the variability of the Icelandic Low and the Azores High in January and their influence on thermal, precipitation and nival conditions in Poland in the period 1901-2000. There were no statistically significant (0.05) trends of analysed centres of action in January in the 20th century. However, in the second half of the century the center of the Icelandic Low in January moved northwards and the sea-level pressure in the center of the Azores High increased significantly. The latter change was the reason for the significant increasing difference between the Azores High and the Icelandic Low. This change caused the intensification of western advection over Europe. Changes in sea-level pressure in the Icelandic Low and the longitude of the Azores High in January explain about 40% of temperature and snow cover variability in Poland. The variability of some features of the centres of action describes the changes of precipitation and snow cover in Poland better than the NAO index.

Attribution of anthropogenic influence on seasonal sea level pressure

Previous analyses of sea level pressure (SLP) trends have often focused on boreal winter trends. Here we demonstrate that externally‐forced SLP trends are observed in all four seasons, with simulated and observed decreases in SLP at high latitudes and increases elsewhere. We find that the observed pattern of seasonal mean zonal mean SLP changes is inconsistent with simulated internal variability, and we detect anthropogenic influence independently of natural influence on SLP. When we divide the globe into the mid‐ and high‐latitude regions of both hemispheres and the tropics and subtropics, we find that external influence is only detectable in the low‐latitude region, where models and observations show increasing trends in SLP, and where internal variability is low, and not in the mid‐ and high‐latitude regions of either hemisphere. Low‐latitude increases in SLP, which are significant compared to internal variability, but which have previously received little attention, could have important regional climate impacts.

Environmental change and fishery management in the northern Persian Gulf

Approximately 52% of the 380,000 tons of fish landings in Iran come from the northern Persian Gulf, with a total of 108,000 fishermen and 8900 fishing vessels (2003 data). The fishery in the region is very important for the local economy, but the major problems facing the Iranian fisheries are an uncertain availability of fish and over-fishing. The Persian Gulf is a sensitive marine ecosystem. The Gulf is a partially enclosed sea with an average depth of 35 metres and replacement of waters take some three to five years. Environmental factors have a major effect on fish stocks and fisheries in such a sensitive ecosystem. This study examines the influence of environmental changes in water temperature, rainfall, wind and sea level pressure on fish stocks in the northern Persian Gulf and estimates the maximum sustainable yields for fisheries management. Multivariate data and surplus production bio-economic models with environmental parameters are used to determine optimal harvest and forecasting. The study found that rainfall, water temperature and wind had significant positive effects whereas sea level pressure had a negative effect on the fisheries in the northern Persian Gulf. It is concluded that environmental parameters should be monitored concurrently with the fisheries to allow a greater understanding for fisheries management in the Persian Gulf.

Interferential Impact of ENSO and PDO on Dry and Wet Conditions in the U.S. Great Plains

Abstract The influence of the El Niño–Southern Oscillation (ENSO) and Pacific decadal oscillation (PDO) interference on the dry and wet conditions in the Great Plains of the United States has been examined using monthly observational datasets. It is shown that both ENSO and PDO can generate a similar pattern of atmospheric and oceanic anomalies over the eastern part of the North Pacific and western North America that has significant impact on the climate over the Great Plains. Furthermore, the relationship between ENSO–PDO and climate anomalies in the Great Plains is intensified when ENSO and PDO are in phase (El Niño and warm PDO or La Niña and cold PDO). On average, anomalies over the Great Plains favor wet (dry) conditions when both ENSO and PDO are in the positive (negative) phase. However, when ENSO and PDO are out of phase, the relationship is weakened and the anomalies over the Great Plains tend toward neutral. Without ENSO, PDO alone does not affect the North American climate significantly. The relationship is quite robust for different seasons, with the strongest effects for the months of spring and the weakest effects for the months of autumn, whereas the months of winter and summer fall in between. The seasonality of the relationship may be associated with the seasonal dependence of the anomalies of general circulation and the pattern of mean seasonal cycle in the North Pacific. The contrasting impact of the interference of ENSO and PDO on the climate anomalies in the Great Plains is associated with differences in the ocean–atmosphere anomalies. When ENSO and PDO are in phase, the sea surface temperature (SST) anomalies extend from the equatorial Pacific to the higher latitudes of the North Pacific via the eastern ocean. The distribution of the corresponding anomalies of sea level pressure (SLP) and the wind at 1000 hPa form an ellipse with a southeast–northwest orientation of the long axis because the SST anomalies promote coherent changes in SLP in the central North Pacific. In the upper troposphere, a similar teleconnection pattern with the same sign generated by ENSO and PDO is overlapped and enhanced, which favors anomaly (dry and wet) conditions in the Great Plains. However, when ENSO and PDO are out of phase, the SST anomalies have the same sign in the tropical and central North Pacific, which is opposite to the anomalies near the west coast of North America. The anomalously cyclonic circulation over the North Pacific is weaker in the out-of-phase situation than in the in-phase situation. The distribution of the anomalies of SLP and the wind at 1000 hPa resembles a circle. Meanwhile, in the upper troposphere, ENSO and PDO generate a similar teleconnection pattern with opposite sign, causing cancellation of the anomalous circulation and favoring neutral climate in the Great Plains.

Contribution of regional climate drivers to future winter sea-level changes in the Baltic Sea estimated by statistical methods and simulations of climate models

A statistical downscaling approach is applied to the output of five different global climate model simulations driven by twenty-first century future scenarios of greenhouse gas concentrations. The contribution of sea-level pressure (SLP) and precipitation changes to regional future winter sea-level changes is estimated for four Baltic sea-level stations by establishing statistical relationships between sea level as predictand and large-scale climate fields as predictors. Using SLP as predictor for the central and eastern Baltic Sea level stations, three climate models lead to statistically significant twenty-first century future trends in the range of the order of 1–2 mm/year. Using precipitation as predictor for the stations in the southern Baltic coast all five models lead to statistically significant trends with a range of the order of 0.4 mm/year. These numbers are smaller, but of the order of magnitude as the predicted global sea-level rise.

Impact of different diffusion schemes on simulated rainfall: Land‐ocean contrast

The impact of vertical diffusion schemes on simulated precipitation has been investigated using two diffusion schemes, the Mellor and Yamada level 2 (MY) and the Holtslag and Boville (HB) schemes in the Center for Climate System Research/National Institute for Environmental Studies (CCSR NIES) model. The results indicate the HB scheme reproduced strong vertical mixing over land, especially in summer hemisphere, which has apparent diurnal variation owing to strong vertical mixing in the convective boundary layer. The MY scheme displayed relatively weak vertical mixing over land but strong mixing over the ocean. The MY scheme tends to reproduce strongly the local mixing by wind shear compared to the HB scheme. The variations in the thermodynamic feature were due to the difference in the planetary boundary layer scheme results in the warmer and drier air over land and relatively colder and wetter air over the ocean. Such an apparent contrast over land and ocean leads to a horizontal gradient in temperature and changes the large‐scale circulation. The change in sea level pressure over the western equatorial Pacific, induced by the HB scheme, results in changes in the zonal and meridional pressure gradients between the equator and subtropical region, which is the main driving force of the low‐level divergence. A weakening of low‐level easterly winds owing to changes in the zonal and meridional pressure gradients has a great influence on the rising motion in the western Pacific, together with the low‐level moisture content advected toward the off‐equatorial area.

Scenario changes in the climatology of winter midlatitude cyclone activity over eastern North America and the Northwest Atlantic

The present study explores how midlatitude winter cyclone activity can be modified under warming‐induced conditions due to enhanced greenhouse gas concentrations. We performed simulations with the Canadian Regional Climate Model (CRCM version 3.5) implemented on a domain that covers the Northwest Atlantic and eastern North America. These simulations are driven by control conditions (1975–1994) and high‐CO2 scenario conditions (2040–2059) suggested by the Canadian Climate Centre model, CGCM2 (Second Generation Coupled Global Climate Model), following the IPCC IS92a scenario. Comparisons between model simulations for the control period (1975–1994) and North America Regional analysis (NARR) suggest that both CGCM2 and CRCM reliably reproduce the overall NARR patterns of sea level pressure, tropospheric baroclinicity and Atlantic storm tracks. However, compared to CGCM2 results, CRCM offers an improvement in simulations of the most intense cyclones. Although both models underestimate the track density of intense cyclones, the CGCM2 underestimates are larger than those of CRCM. Under the high‐CO2 climate change scenario, the CRCM and CGCM2 model simulations show similar changes in sea level pressure, surface temperature, and total track density of midlatitude winter cyclones. Although we can see the northwest shift of the dominant Atlantic storm track, it is not statistically significant. Moreover, simulations from both models show a decrease in the total cyclone track density along the Canadian east coast; the decrease is more robust in CRCM simulations than in CGCM2 results. For intense cyclones, CRCM simulations show a slight decrease in the track density, while no such change is found in CGCM2 simulations.

Dimming/brightening over the Iberian Peninsula: Trends in sunshine duration and cloud cover and their relations with atmospheric circulation

This study analyzes the spatial and temporal changes in sunshine duration (SunDu) and total cloud cover (TCC) over the Iberian Peninsula (IP) and four subregions during 1961–2004 using high‐quality, homogenized data sets. The analyses confirm that over most of the IP and in most seasons, SunDu and TCC variations are strongly negatively correlated, with absolute values ∼0.8–0.9. Somewhat weaker correlations (0.5–0.6) are found in the southern portion of the IP in summer. A large discrepancy between the SunDu and TCC records occurs from the 1960s until the early 1980s when the SunDu series shows a decrease that it is not associated with an increase in TCC. This negative trend or “dimming” is even more pronounced after removing the effects of TCC via linear regression. Since the early 1980s, the SunDu and TCC residual SunDu series exhibit an upward trend or “brightening.” In addition to the long‐term dimming and brightening, the volcanic eruptions of El Chichon and Mount Pinatubo are clearly evident in the TCC residual SunDu record. The TCC and SunDu records over the IP are well correlated with sea level pressure (SLP), with above normal TCC and below normal SunDu corresponding to below normal SLP locally in all seasons. The TCC and SunDu related SLP changes over the IP in winter and spring are part of a larger‐scale north‐south dipole pattern that extends over the entire Euro‐Atlantic sector. Other more regional atmospheric circulation patterns, identified from rotated principal component analysis, are also linked to TCC and SunDu variations over the IP. Finally and perhaps surprisingly, the TCC residual SunDu series exhibits a statistically significant relationship with a regional atmospheric circulation pattern during spring, summer, and autumn.

Refinements to Atlantic basin seasonal hurricane prediction from 1 December

Atlantic basin seasonal hurricane predictions have been issued by the Tropical Meteorology Project at Colorado State University since 1984, with early December forecasts being issued every year since early December 1991. These forecasts have yet to show real‐time forecast skill, despite several statistical models that have shown considerable hindcast skill. In an effort to improve both hindcast skill and hopefully real‐time forecast skill, a modified forecast scheme has been developed using data from 1950 to 2007. Predictors were selected based upon how much variance was explained over the 1950–1989 subperiod. These predictors were then required to explain similar amounts of variance over a latter subperiod from 1990 to 2007. Similar amounts of skill were demonstrated for each of the three predictors selected over the 1950–1989 period, the 1990–2007 period, and the full 1950–2007 period. In addition, significant correlations between individual predictors and physical features known to affect hurricanes during the following August–October (i.e., tropical Atlantic wind shear and sea level pressure changes, ENSO phase changes) were obtained. This scheme uses a new methodology where hindcasts were obtained using linear regression and then ranked to generate final hindcast values. Fifty‐four percent of the variance was explained for seasonal Net Tropical Cyclone (NTC) activity over the 1950–2007 period. These hindcasts show considerable differences in landfalling U.S. tropical cyclones, especially for the Florida Peninsula and East Coast. Seven major hurricanes made Florida Peninsula and East Coast landfall during the top 15 largest NTC hindcasts compared with only two major hurricane landfalls in the bottom 15 smallest NTC hindcasts.

Regional climate change experiments over southern South America. II: Climate change scenarios in the late twenty-first century

We present an analysis of climate change over southern South America as simulated by a regional climate model. The regional model MM5 was nested within time-slice global atmospheric model experiments conducted by the HadAM3H model. The simulations cover a 10-year period representing present-day climate (1981–1990) and two future scenarios for the SRESA2 and B2 emission scenarios for the period 2081–2090. There are a few quantitative differences between the two regional scenarios. The simulated changes are larger for the A2 than the B2 scenario, although with few qualitative differences. For the two regional scenarios, the warming in southern Brazil, Paraguay, Bolivia and northeastern Argentina is particularly large in spring. Over the western coast of South America both scenarios project a general decrease in precipitation. Both the A2 and B2 simulations show a general increase in precipitation in northern and central Argentina especially in summer and fall and a general decrease in precipitation in winter and spring. In fall the simulations agree on a general decrease in precipitation in southern Brazil. This reflects changes in the atmospheric circulation during winter and spring. Changes in mean sea level pressure show a cell of increasing pressure centered somewhere in the southern Atlantic Ocean and southern Pacific Ocean, mainly during summer and fall in the Atlantic and in spring in the Pacific. In relation to the pressure distribution in the control run, this indicates a southward extension of the summer mean Atlantic and Pacific subtropical highs.

Are changes of the geomagnetic field intensity related to changes of the tropical Pacific sea-level pressure during the last 50 years?

[1] The influence of solar variability into the lower atmospheric regions has been suggested on different atmospheric parameters in different time scales. However, a plausible mechanism to explain these observations remains unclear. Although it is widely accepted that the climate change over the past 50 years is attributed to human influence, we present the case that local climate change in the tropical Pacific may be due to changes in the Earth's magnetic field strength. The changes in the tropical Pacific circulation have been observed during the last 50 years, and they are attributed to the increase of the global surface temperature. However, a geomagnetic modulation of the net radiative flux in the southern tropical Pacific was recently suggested. Moreover, comparisons of long-term reconstructions of the Northern Hemisphere surface temperature and solar activity proxies indicated that the existence of a geomagnetic signal in climate data would support a direct link between solar variability and their effects on climate. Here we show that in the tropical Pacific the sea-level pressure, which is a component of the Walker circulation, could be related to the magnetospheric, ionospheric, and upper-atmosphere processes which may propagate downward to the lower atmosphere. Furthermore, we show that the changes in sea-level pressure and the Walker circulation are correlated to the westward drift of the magnetic anomaly. We compare the region averaged monthly values of the sea-level pressure in the tropical Pacific with those of the magnetic field intensity near the surface for the last 50 years. We find that the sea-level pressure in the tropical Pacific is increasing as the magnetic field intensity is decreasing. The correlation coefficient of the sea-level pressure 36-month running means versus the magnetic field intensity is 0.96. We anticipate our investigation to be a starting point for a more sophisticated investigation of the coupling between the space weather processes and lower atmosphere and ocean dynamics.

Structural optimisation and input selection of an artificial neural network for river level prediction

Summary Accurate river level prediction, necessary for reliable flood forecasting, is a difficult task due to the complexity and inherent nonlinearity of the catchment hydrological system. Although artificial neural networks (ANNs) offer advantages over mechanistic or conceptual hydrological models for river level prediction, their applicability is limited by the fact that each ANN has to be specifically optimised and trained for a particular prediction problem and suitable input vectors selected. A recently developed novel optimisation algorithm combining properties of simulated annealing and tabu search is used to arrive at an optimal ANN for the prediction of river levels 5 h in advance. The algorithm seeks to minimise the value of a cost function based on the complexity and performance of the ANN. This is done by removing inter-neuron connections and adjusting the weights of the remaining connections. The candidate inputs presented to the algorithm were: current values of river levels at the flood location and two upstream locations; the change in level over the previous 4 h at the flood point, mean sea level pressure (SLP) and the change in SLP over the previous 24 h. The optimisation removed 79% of the network connections and three of the candidate inputs, leaving the current levels at the two upstream locations and at the flood point as the only inputs. The optimised ANN was then trained using the standard backpropagation algorithm. This methodology produces an ANN of greatly reduced complexity albeit with a reduced performance compared to an unoptimised ANN trained with backpropagation only. However, it has the advantage of being generally applicable and represents an improvement over trial and error as a method of ANN structural optimisation and input selection. For this prediction problem, current levels at two upstream locations and at the flood point are the best predictors of the level at the flood point.

The early 1950s regime shift in temperature in Taiwan and East Asia

A summer climate regime shift in temperature in Taiwan and East Asia during the early 1950s was identified in this study. The event was characterized by a cooling land-warming ocean dipole in East Asia and the western North Pacific, marking the decreasing land–sea thermal contrast from the 1940s to the 1950s. The corresponding sea surface temperature (SST) anomaly was characterized by the sign flipping of a north–south dipole in the western North Pacific from north/cool-south/warm to north/warm-south/cool, indicating a weakening north–south SST gradient in the area. The corresponding change in mean sea level pressure was characterized by the rising pressure in continental East Asia and the Philippine Sea, and the falling pressure over the extratropical western North Pacific to the east of Japan. This change was the reflection of a weakening thermal low in the continental East Asia, a weakening monsoon trough in the tropical western North Pacific, a strengthening and southwestward-expanding ridge in the subtropical western North Pacific, and a deepening mid-latitude trough over eastern China and Japan. The phase reversal of the SST anomaly in the western North Pacific exhibited the characteristics of the Pacific Decadal Oscillation (PDO), but lagged behind the phase reversal of the PDO in the extratropical North Pacific by several years. The connection with the PDO is speculated, although the mechanism is not understood.