Sea Level Pressure Variability Research Articles

Simulation of the Northern and Southern Hemisphere Annular Modes by CAMS-CSM

As leading modes of the planetary-scale atmospheric circulation in the extratropics, the Northern Hemisphere (NH) annular mode (NAM) and Southern Hemisphere (SH) annular mode (SAM) are important components of global circulation, and their variabilities substantially impact the climate in mid−high latitudes. A 35-yr (1979−2013) simulation by the climate system model developed at the Chinese Academy of Meteorological Sciences (CAMS-CSM) was carried out based on observed sea surface temperature and sea ice data. The ability of CAMS-CSM in simulating horizontal and vertical structures of the NAM and SAM, relation of the NAM to the East Asian climate, and temporal variability of the SAM is examined and validated against the observational data. The results show that CAMS-CSM captures the zonally symmetric and out-of-phase variations of sea level pressure anomaly between the midlatitudes and polar zones in the extratropics of the NH and SH. The model has also captured the equivalent barotropic structure in tropospheric geopotential height and the meridional shifts of the NH and SH jet systems associated with the NAM and SAM anomalies. Furthermore, the model is able to reflect the variability of northern and southern Ferrel cells corresponding to the NAM and SAM anomalies. The model reproduces the observed relationship of the boreal winter NAM with the East Asian trough and air temperature over East Asia. It also captures the upward trend of the austral summer SAM index during recent decades. However, compared with the observation, the model shows biases in both the intensity and center locations of the NAM’s and SAM’s horizontal and vertical structures. Specifically, it overestimates their intensities.

High-resolution wave climate hindcast around Japan and its spectral representation

A 34-year high-resolution ocean surface wave climate hindcast around Japan is presented, and the wave climate around Japan is examined from a spectral point of view. The spectral wave model is forced by the sea surface wind data obtained from the JRA-55 atmospheric reanalysis. The wave climate hindcast is validated by comparing it with buoy observations around Japan. The correlation coefficient of the significant wave height is 0.9. The correlation coefficient of the mean wave period is 0.8 for the Japan Sea and Pacific side of eastern Japan and 0.7–0.8 for the Pacific side of western Japan. The wave climate is represented by temporal-mean two-dimensional wave spectra. The characteristics of the spectral wave climate are investigated by classifying them into three types. Distinctive characteristics of the mean wave spectra along the Japan Sea are narrower band widths for both the period and direction. The mean wave spectra along the Pacific side of eastern Japan are characterized by swells with a spectral peak propagating from the northeast. The distinctive spectral features corresponding to the Pacific side of western Japan are bi-modal peaks with long-period components typically generated by typhoons. The variability in the monthly mean wave spectra is examined using an empirical orthogonal function (EOF) analysis. In the winter, it is found that the wave height variability (EOF 1st mode) is related to wave direction variability (EOF 2nd mode) at locations 1000 km apart via the sea level pressure variance over the North Pacific. In the summer, the EOF 1st mode corresponding to the Pacific side locations is dominated by the variability in typhoon-generated swells. The spectral wave climate representation provides new insight into the wave climate around Japan with clear relationships between the atmospheric conditions, the wave height, direction, and period.

Wintertime internal climate variability over Eurasia in the CESM large ensemble

Based on outputs of historical runs of 38 members from the Community Earth System Model (CESM) Large Ensemble Project, this study evaluates the capability of the model to simulate the winter climate over the Northern Hemisphere, with an emphasis on Eurasia. The climatology and interannual variability of the wintertime circulation and surface air temperature (SAT) can be well reproduced. The multi-member mean of the long-term trends of sea level pressure (SLP) and SAT resemble the observations, suggesting the capability of CESM to reproduce the forced component of the observed trends. Meanwhile, the trends of SLP and SAT show large diversity among the 38 members, implying the importance of low-frequency variability in the observed trends. Considering the high skill of the CESM to simulate the observed climate, the internal climate variability described by the inter-member spread is further examined. It reveals that the internal climate variability of wintertime SLP over Eurasia can be captured by analyzing the inter-member spread of their climatology, interannual variability, or long-term trend. Moreover, the variations of the inter-member spread are regulated by the same mechanism as that of the observed interannual variability, implying the feasibility of explaining the characteristics and mechanism of the inter-member spread with the corresponding interannual variability in observations.

Revisiting the Northern Mode of East Asian Winter Monsoon Variation and Its Response to Global Warming

This study revisits the northern mode of East Asian winter monsoon (EAWM) variation and investigates its response to global warming based on the ERA dataset and outputs from phase 5 of the Coupled Model Intercomparison Project (CMIP5) models. Results show that the observed variation in East Asian surface air temperature (EAT) is tightly coupled with sea level pressure variation in the expanded Siberian high (SH) region during boreal winter. The first singular value decomposition (SVD) mode of the EAT and SH explains 95% of the squared covariance in observations from 1961 to 2005, which actually represents the northern mode of EAWM variation. Meanwhile, the first SVD mode of the EAT and SH is verified to be equivalent to the first empirical orthogonal function mode (EOF1) of the EAT and SH, respectively. Since the leading mode of the temperature variation is significantly influenced by radiative forcing in a rapidly warming climate, for reliable projection of long-term changes in the northern mode of the EAWM, we further employ the EOF1 mode of the SH to represent the northern mode of EAWM variation. The models can well reproduce this coupling between the EAT and SH in historical simulations. Meanwhile, a robust weakening of the northern mode of the EAWM is found in the RCP4.5 scenario, and with stronger warming in the RCP8.5 scenario, the weakening of the EAWM is more pronounced. It is found that the weakening of the northern mode of the EAWM can contribute 6.7% and 9.4% of the warming trend in northern East Asian temperature under the RCP4.5 and RCP8.5 scenarios, respectively.

Pacific Decadal Oscillation: Tropical Pacific Forcing versus Internal Variability

The Pacific decadal oscillation (PDO) is the leading mode of sea surface temperature (SST) variability over the North Pacific (north of 20°N). Its South Pacific counterpart (south of 20°S) is the South Pacific decadal oscillation (SPDO). The effects of tropical eastern Pacific (TEP) SST forcing and internal atmospheric variability are investigated for both the PDO and SPDO using a 10-member ensemble tropical Pacific pacemaker experiment. Each member is forced by the historical radiative forcing and observed SST anomalies in the TEP region. Outside the TEP region, the ocean and atmosphere are fully coupled and freely evolve. The TEP-forced PDO (54% variance) and SPDO (46% variance) are correlated in time and exhibit a symmetric structure about the equator, driven by the Pacific–North American (PNA) and Pacific–South American teleconnections, respectively. The internal PDO resembles the TEP-forced component but is related to internal Aleutian low (AL) variability associated with the Northern Hemisphere annular mode and PNA pattern. The internal variability is locally enhanced by barotropic energy conversion in the westerly jet exit region around the Aleutians. By contrast, barotropic energy conversion is weak associated with the internal SPDO, resulting in weak geographical preference of sea level pressure variability. Therefore, the internal SPDO differs from the TEP-forced component, featuring SST anomalies along ~60°S in association with the Southern Hemisphere annular mode. The limitations on isolating the internal component from observations are discussed. Specifically, internal PDO variability appears to contribute significantly to the North Pacific regime shift in the 1940s.

Atmospheric Response to Mesoscale Ocean Eddies over the South China Sea

The South China Sea (SCS) is an eddy-active area. Composite analyses based on 438 mesoscale ocean eddies during 2000–2012 revealed the status of the atmospheric boundary layer is influenced remarkably by such eddies. The results showed cold-core cyclonic (warm-core anticyclonic) eddies tend to cool (warm) the overlying atmosphere and cause surface winds to decelerate (accelerate). More than 5% of the total variance of turbulent heat fluxes, surface wind speed and evaporation rate are induced by mesoscale eddies. Furthermore, mesoscale eddies locally affect the columnar water vapor, cloud liquid water, and rain rate. Dynamical analyses indicated that both variations of atmospheric boundary layer stability and sea level pressure are responsible for atmospheric anomalies over mesoscale eddies. To reveal further details about the mechanisms of atmospheric responses to mesoscale eddies, atmospheric manifestations over a pair of cold and warm eddies in the southwestern SCS were simulated. Eddy-induced heat flux anomalies lead to changes in atmospheric stability. Thus, anomalous turbulence kinetic energy and friction velocity arise over the eddy dipole, which reduce (enhance) the vertical momentum transport over the cold (warm) eddy, resulting in the decrease (increase) of sea surface wind. Diagnoses of the model’s momentum balance suggested that wind speed anomalies directly over the eddy dipole are dominated by vertical mixing terms within the atmospheric boundary layer, while wind anomalies on the edges of eddies are produced by atmospheric pressure gradient forces and atmospheric horizontal advection terms.

On the Choice of Ensemble Mean for Estimating the Forced Signal in the Presence of Internal Variability

Abstract In this paper we examine various options for the calculation of the forced signal in climate model simulations, and the impact these choices have on the estimates of internal variability. We find that an ensemble mean of runs from a single climate model [a single model ensemble mean (SMEM)] provides a good estimate of the true forced signal even for models with very few ensemble members. In cases where only a single member is available for a given model, however, the SMEM from other models is in general out-performed by the scaled ensemble mean from all available climate model simulations [the multimodel ensemble mean (MMEM)]. The scaled MMEM may therefore be used as an estimate of the forced signal for observations. The MMEM method, however, leads to increasing errors further into the future, as the different rates of warming in the models causes their trajectories to diverge. We therefore apply the SMEM method to those models with a sufficient number of ensemble members to estimate the change in the amplitude of internal variability under a future forcing scenario. In line with previous results, we find that on average the surface air temperature variability decreases at higher latitudes, particularly over the ocean along the sea ice margins, while variability in precipitation increases on average, particularly at high latitudes. Variability in sea level pressure decreases on average in the Southern Hemisphere, while in the Northern Hemisphere there are regional differences.

An inter-decadal increase in summer sea level pressure over the Mongolian region around the early 1990s

The East Asian summer monsoon is affected by processes in the mid-high latitudes in addition to various tropical and subtropical systems. The present study investigates the summer sea level pressure (SLP) variability over northern East Asia (NEA) and emphasizes the closed active center over the Mongolian region. It is found that the seasonal mean Mongolian SLP (MSLP) anomaly is closely connected with the variability of summertime regional synoptic extra-tropical cyclones on longer time scales. A significant inter-decadal increase in the MSLP around the early 1990s has been detected, which is accompanied by a weakening in the activity of regional extra-tropical cyclones. Recent warming over NEA may have a contribution to the inter-decadal change, which features evidently meridional inhomogeneity around 45°N. The inhomogeneous air temperature anomaly distribution results in decreased vertical wind shear, reduced atmospheric baroclinicity over the Mongolian region, and thus inactive regional cyclones and increased MSLP in the latter decade. The associated temperature anomaly distribution may be partly attributed to regional inhomogeneity in cloud and radiation anomalies, and it is further maintained by two positive feedback mechanisms associated with atmospheric internal processes: one via adiabatic heating and the other via horizontal temperature advection.

Mechanisms of variability in decadal sea-level trends in the Baltic Sea over the 20th century

Abstract. Coastal sea-level trends in the Baltic Sea display decadal-scale variations around a long-term centennial trend. In this study, we analyse the spatial and temporal characteristics of the decadal trend variations and investigate the links between coastal sea-level trends and atmospheric forcing on a decadal timescale. For this analysis, we use monthly means of sea-level and climatic data sets. The sea-level data set is composed of long tide gauge records and gridded sea surface height (SSH) reconstructions. Climatic data sets are composed of sea-level pressure, air temperature, precipitation, evaporation, and climatic variability indices. The analysis indicates that atmospheric forcing is a driving factor of decadal sea-level trends. However, its effect is geographically heterogeneous. This impact is large in the northern and eastern regions of the Baltic Sea. In the southern Baltic Sea area, the impacts of atmospheric circulation on decadal sea-level trends are smaller. To identify the influence of the large-scale factors other than the effect of atmospheric circulation in the same season on Baltic Sea sea-level trends, we filter out the direct signature of atmospheric circulation for each season separately on the Baltic Sea level through a multivariate linear regression model and analyse the residuals of this regression model. These residuals hint at a common underlying factor that coherently drives the decadal sea-level trends in the whole Baltic Sea. We found that this underlying effect is partly a consequence of decadal precipitation trends in the Baltic Sea basin in the previous season. The investigation of the relation between the AMO index and sea-level trends implies that this detected underlying factor is not connected to oceanic forcing driven from the North Atlantic region.

Time evolution of atmospheric parameters and their influence on sea level pressure over the head Bay of Bengal

A zonal dipole in the observed trends of wind speed and significant wave height over the Head Bay of Bengal region was recently reported in the literature attributed due to the variations in sea level pressure (SLP). The SLP in turn is governed by prevailing atmospheric conditions such as local temperature, humidity, rainfall, atmospheric pressure, wind field distribution, formation of tropical cyclones, etc. The present study attempts to investigate the inter-annual variability of atmospheric parameters and its role on the observed zonal dipole trend in sea level pressure, surface wind speed and significant wave height. It reports on the aspects related to linear trend as well as its spatial variability for several atmospheric parameters: air temperature, geopotential height, omega (vertical velocity), and zonal wind, over the head Bay of Bengal, by analyzing National Centers for Environmental Prediction (NCEP) Reanalysis 2 dataset covering a period of 38 years (1979–2016). Significant warming from sea level to 200 mb pressure level and thereafter cooling above has been noticed during all the seasons. Warming within the troposphere exhibits spatial difference between eastern and western side of the domain. This led to fall in lower tropospheric geopotential height and its east–west variability, exhibiting a zonal dipole pattern across the Head Bay. In the upper troposphere, uplift in geopotential height was found as a result of cooling in higher levels (10–100 mb). Variability in omega also substantiated the observed variations in geopotential height. The study also finds weakening in the upper level westerlies and easterlies. Interestingly, a linear trend in lower tropospheric u-wind component also reveals an east–west dipole pattern over the study region. Further, the study corroborates the reported dipole in trends of sea level pressure, wind speed and significant wave height by evaluating the influence of atmospheric variability on these parameters.

The role of South Pacific atmospheric variability in the development of different types of ENSO

AbstractRecent advances in tropical Pacific climate variability have focused on understanding the development of El Niño–Southern Oscillation (ENSO) events, specifically the types or “flavors” of ENSO (i.e., central versus eastern Pacific events). While precursors to ENSO events exist, distinguishing the particular flavor of the expected ENSO event remains unresolved. This study offers a new look at ENSO predictability using South Pacific atmospheric variability during austral winter as an indicator. The positive phase of the leading mode of South Pacific sea level pressure variability, which we term the South Pacific Oscillation (SPO), exhibits a meridional dipole with with a(n) (anti)cyclonic anomaly dominating the subtropics (extratropics/high latitudes). Once energized, the cyclonic anomalies in the subtropical node of the SPO weaken the southeasterly trade winds and promote the charging of the eastern equatorial Pacific Ocean, giving rise to eastern Pacific ENSO events. Indeed, the type of ENSO event can be determined accurately using only the magnitude and phase of the SPO during austral winter as a predictor (17 out of 23 cases). The SPO may also play a role in explaining the asymmetry of warm and cold events. Collectively, our findings present a new perspective on ENSO‐South Pacific interactions that can advance overall understanding of the ENSO system and enhance its predictability across multiple timescales.

Projected Significant Increase in the Number of Extreme Extratropical Cyclones in the Southern Hemisphere

Extratropical cyclones are responsible for much of the extreme weather in the midlatitudes; thus, how these cyclones may change under increasing greenhouse gas forcing is of much general interest. Previous studies have suggested a poleward shift in the location of these cyclones, but how the intensity may change remains uncertain, especially in terms of maximum wind speed. In this study, projected changes in extreme cyclones in the Southern Hemisphere, based on 26 models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5), are presented. Multiple definitions of extreme cyclones have been examined, including intensity exceeding constant thresholds of sea level pressure perturbations, 850-hPa vorticity, and 850-hPa winds, as well as variable thresholds corresponding to a top-5 or top-1 cyclone per winter month in these three parameters and the near-surface winds. Results presented show that CMIP5 models project a significant increase in the frequency of extreme cyclones in all four seasons regardless of the definition, with over 88% of the models projecting an increase. Spatial patterns of increase are also consistent, with the largest increase projected between 45° and 60°S, extending from the South Atlantic across the south Indian Ocean into the Pacific. The projected increases in cyclone statistics are consistent with those in Eulerian statistics, such as sea level pressure (SLP) variance. However, while the projected increase in SLP variance can be linked to increase in the mean available potential energy (MAPE), the increases in cyclone statistics are not well correlated with those in MAPE.

Influence of ENSO on the Pacific decadal oscillation in CMIP models

Emerging decadal climate predictions call for an assessment of decadal climate variability in the Coupled Model Intercomparison Project (CMIP) database. In this paper, we evaluate the influence of El Nino Southern Oscillation (ENSO) on Pacific Decadal Oscillation (PDO) in 10 control simulations from the CMIP3 and 22 from the CMIP5 database. All models overestimate the time lag between ENSO forcing and the PDO response. While half of the models exhibit ENSO-PDO correlation which is close to that in observation (>0.5) when the time lag is accounted for, the rest of the models underestimate this relationship. Models with stronger ENSO-PDO correlation tend to exhibit larger PDO-related signals in the equatorial and south Pacific, highlighting the key role of ENSO teleconnection in setting the inter-hemispheric Pacific pattern of the PDO. The strength of the ENSO-PDO relationship is related to both ENSO amplitude and strength of ENSO teleconnection to the North Pacific sea-level pressure variability in the Aleutian Low region. The shape of the PDO spectrum is consistent with that predicted from a combination of direct ENSO forcing, atmospheric stochastic forcing over the North Pacific and the re-emergence process in 27 models out of 32. Given the essential role of ENSO in shaping the Pacific decadal variability, models displaying realistic ENSO amplitude and teleconnections should be preferentially used to perform decadal prediction experiments.

Sea level pressure variation and major cyclone events in coastal regions of Bangladesh

The study was conducted in 16 coastal district viz. Satkhira, Khulna, Mongla, Khepupara, Barishal, Bhola, Patuakhali, Hatiya, Chandpur, Feni, Swandip, Sitakunda, Chittagong, Kutubdia, Cox’s Bazar and Teknaf of Bangladesh during the period of 1975-2014 to investigate the variation of sea level pressure (SLP) and rainfall. SLP data of 40 years (1975-2014) have been analyzed in this study. The SLP was increasing slightly to the month of September and then a rapid increasing was observed from the month of October to December. Of all the stations, in the month of June and July the lowest SLP was observed in the Khulna region and highest in Teknaf. In the month of December and January the highest and lowest SLP was found in the Month of January. As the fluctuation of SLP was high in the summer season the possibility of cyclone occurrence also rise in this season. The average rainfall is lowest in the station Sitakunda and highest in Swandip. The average rainfall of all district divided the stations into two parts. The rainfall of Barishal, Bhola, Chandpur, Mongla, Sitakunda and Khulna is lower the average of all district. This shows a great instability of weather in these regions. Rest of the stations has the higher average rainfall than the average rainfall of all stations.Progressive Agriculture 27 (3): 285-295, 2016

Modulation of Bjerknes feedback on the decadal variations in ENSO predictability

AbstractClear decadal variations exist in the predictability of the El Niño–Southern Oscillation (ENSO), with the most recent decade having the lowest ENSO predictability in the past six decades. The Bjerknes Feedback (BF) intensity, which dominates the development of ENSO, has been proposed to determine ENSO predictability. Here we demonstrate that decadal variations in BF intensity are largely a result of the sensitivity of the zonal winds to the zonal sea level pressure (SLP) gradient in the equatorial Pacific. Furthermore, the results show that during low‐ENSO predictability decades, zonal wind anomalies over the equatorial Pacific are more linked to SLP variations in the off‐equatorial Pacific, which can then transfer this information into surface temperature and precipitation fields through the BF, suggesting a weakening in the ocean‐atmosphere coupling in the tropical Pacific. This result indicates that more attention should be paid to off‐equatorial processes in the prediction of ENSO.

Variability of Some Meteorological Elements During the Southwest Monsoon Season in Bangladesh

Statistical computation and analysis of the co-efficient of variation of different meteorological parameter such as mean sea level pressure, rainfall, dew-point depression, minimum temperature and maximum temperature over Bangladesh during the onset, on-going and withdrawal phases of southwest monsoon, season of 1981- 2010 was dealt with. The study reveals that the coefficient of variation of the mean sea level (msl) pressure is practically small over Bangladesh during the onset and ongoing phases of southwest monsoon. During the onset phase, the CV of msl pressure is relatively higher and decreases during the on-going phase. The variation of msl pressure increases slightly in the withdrawal phase of monsoon. The ranges of CV of msl pressure are 0.064 - 0.197%, 0.079 - 0.809%, 0.091 - 0.793%, 0.069 - 0.272%, 0.0695 - 0.229% and 0.127 - 0.32% in May, June, July, August, September and October, respectively. The variation of dew-point depression decreases in the eastern part during the onset phase of southwest monsoon and becomes lower all over Bangladesh during the on-going phase of monsoon. It again increases during the withdrawal phase. The maximum co-efficient of variation of rainfall exists over the southeastern part of Bangladesh during the on-going phase of southwest monsoon and decreases in June and July over Bangladesh i.e. during the early on-going phase of monsoon. The co-efficient of variation of minimum and maximum temperatures is found to decrease during the onset and on-going phases of southwest monsoon and increases during the withdrawal of monsoon from Bangladesh.Journal of Bangladesh Academy of Sciences, Vol. 40, No. 2, 155-167, 2016

The influence of winter and summer atmospheric circulation on the variability of temperature and sea ice around Greenland

Most peripheral seas of the Arctic Ocean have seen a pronounced rise in sea surface temperatures in the past century, and this signature of Arctic amplification in proximity to the land suggests that the observed marine and terrestrial changes might be connected to each other. Using in situ observations of temperature from nine coastal meteorological stations around Greenland (GrSTs) and remotely sensed fields of sea ice extent (SIE), we examine the interannual variations of surface air temperature (T2m) and sea level pressure (SLP) anomalies associated with the GrSTs and SIEs surrounding Greenland, specifically within Baffin Bay, the Greenland Sea and Kara-Barents Seas. During winter, the interannual variation in T2m and SLP of the west and south coasts of GrSTs and the Baffin Bay SIE are different from that of the east coast of GrSTs and the SIEs in the Greenland Sea and Kara-Barents Seas. The GrSTs on the west and south coasts of Greenland and the Baffin Bay SIE are associated with the T2m anomalies over Baffin Bay and Davis Strait. The winter SLP patterns associated with these GrSTs and SIEs show positive anomalies over the Arctic and negative anomalies over the North Atlantic with a large-scale atmospheric circulation such as the winter NAO. On the contrary, the east coast of GrSTs and the SIEs in the Greenland Sea and Kara-Barents Seas are correlated with the T2m anomalies over the Greenland Sea and Barents Sea. The surface wind pattern associated with the SIEs in the Greenland Sea and Kara-Barents Seas has a cyclonic circulation in the Greenland Sea and Barents Sea. At the local scale the cyclonic circulation could induce negative SIE anomalies and contribute to increasing open water in the Greenland Sea and Barents Sea. The effect of the loss of sea ice and the heat from the open ocean warming to the atmosphere may influence the GrSTs in the east coast of Greenland. As a result, the T2m pattern associated with the GrSTs in the east coast of Greenland is similar to the pattern of the SIEs in the Greenland Sea and Kara-Barents Seas. During summer, the T2m anomalies associated with all GrSTs and SIEs have positive anomalies over mid-latitudes. The two times series of all GrSTs and SIEs fluctuate quickly and display large trends towards warming temperatures and decreasing SIE. The summer SLP associated with all GrSTs and SIEs are characterised by a seesaw pattern between positive anomalies over the Arctic and negative anomalies over mid-latitudes. The summer SLP anomalies are similar to the summer AO pattern, and it is noteworthy that the summer anticyclonic circulation over the Arctic and Greenland has contributed to the variability and trends in both summer GrSTs and SIEs.

A shift in the upper-ocean temperature trends in the South China Sea since the late 1990s

In this paper, the interdecadal variability of upper-ocean temperature in the South China Sea (SCS) is investigated based on several objectively analyzed data sets and two reanalysis data sets. The trends of the SCS sea surface temperature (SST) have changed from warming to cooling since the late 1990s. A heat budget analysis suggests that the warming of the surface mixed layer during 1984–1999 is primarily attributed to the horizontal heat advection and the decrease of upward long wave radiation, with the net surface heat flux playing a damping role due to the increase of upward latent and sensible heat fluxes. On the other hand, the cooling of the surface mixed layer during 2000–2009 is broadly controlled by net surface heat flux, with the radiation flux playing the dominant role. A possible mechanism is explored that the variation of a sea level pressure (SLP) over the North Pacific Ocean may change the prevailing winds over the SCS, which contributes to the change of the SST in the SCS through the horizontal heat advection and heat fluxes.

Impacts of the tropical trans-basin variability on Australian rainfall

A large-scale mode of sea level pressure variability between the Pacific and Atlantic basins—referred to as the tropical trans-basin variability (TBV)—has shown multi-year predictability beyond that normally associated with El Nino Southern Oscillation (ENSO). Here we examine the relationship between the TBV and regional rainfall, which, if significant, would imply that the TBV predictability could be used to improve rainfall forecasts. We find TBV to be significantly related to rainfall, a relationship that is strongest over Australia during spring. We find Australia to experience drier conditions during the positive phase of TBV and wetter conditions during the negative phase, a pattern similar to that of ENSO. Furthermore, TBV is associated with rainfall responses that are of similar magnitude to ENSO, but there is some degree of independence between TBV and the Pacific modes defined by sea surface temperature indices. The global teleconnections related to the different TBV phases suggest that though many TBV and ENSO events overlap, pure ENSO events are primarily driven by the Pacific whereas pure TBV events are driven by the Atlantic Ocean. An El Nino coupled with a positive TBV event leads to significant drying over the east coast of Australia while the combined negative phase leads to widespread wetting over most of the country, which can be attributed to an enhancement of the anomalous Walker Circulation and the resulting moisture convergence and wind patterns.

Winter temperatures over the Korean Peninsula and East Asia: development of a new index and its application to seasonal forecast

This study proposes a new index for monitoring and predicting winter temperatures of the Korean Peninsula based on the dominant atmospheric winter teleconnection patterns. The utilization of this index is further extended to the East Asian Winter Monsoon (EAWM) index because the new index is found to well represent the main feature of the EAWM circulation. Among the teleconnection patterns, the East Atlantic (EA) and Western Pacific (WP) patterns are found to be most strongly correlated with winter temperatures via their partial association with changes in sea level pressure (SLP) around the Korean Peninsula, i.e., the EA and WP patterns are associated with SLP variation over the Siberian High region and the Kuroshio extension region to the east of Japan, respectively. On the basis of this relationship, the two regions representing the northwest-to-southeast SLP gradients are determined to define the new index. It is found that the new index can represent the Korean winter temperatures consistently well regardless of their considerable decadal changes. When compared with the existing SLP-based EAWM indices, the new index shows the best performance in delineating winter air temperatures, not only in the Korean Peninsula but also in the entire East Asian region. We also assess the prediction skill of the new index with seasonal coupled forecast models of the APEC Climate Center of Korea and its capability to predict winter temperatures. This assessment shows that the new index has potential for operationally predicting and monitoring winter temperatures in Korea and the whole of East Asia.