Positive Arctic Oscillation Research Articles

Influences of atmospheric and oceanic low‑frequency climate fluctuations on European winter surface air temperatures (1870-2010)

We isolated natural atmospheric and oceanic low-frequency climate fluctuations and its linkages with European winter surface air temperature (SAT) anomalies since 1870. Singular spectrum analysis was applied to SAT, North Atlantic sea surface temperature (SST), and the North Atlantic Oscillation (NAO) time series. The evolution of their low-frequency components was then compared with other time-series variability such as the Arctic Oscillation. The North Atlantic SST was found to be out of phase with the Atlantic sea level pressure and European SAT, following a ~65 yr climate fluctuation reminiscent of the Atlantic multi-decadal oscillation. Atlantic positive/negative SST anomalies preceded negative/positive European SAT anomalies by ~6 yr. Moreover, milder European winters occurred when a negative Atlantic multi-decadal phase overlapped a positive Arctic Oscillation phase (i.e. 1905�1915 and 1985�2000). The quasi- decadal oscillation of SST and the NAO were in-phase and led positive/negative European SAT anomalies by approximately 1 yr. The low-frequency winter SAT variability represented 22.3% of the total variance. From the mid-1980s onward, when a rapid change in climate occurred, involv- ing milder European winters, results showed that the NAO and the Arctic Oscillation phases both remained mostly positive. A strong polar vortex dominated during this period, minimizing merid- ional air-mass exchanges during winter. The anthropogenic component of climate change could modify the Atlantic thermohaline circulation, and with it the low-frequency variability of the North Atlantic SST, as well as the Arctic Oscillation variability, due to changes in the stratospheric thermal state, as shown elsewhere. These complex interactions between the Atlantic Ocean and the overlying atmosphere could have global socio-economical impacts in Europe during the 21st century.

Analysis of the effect of atmospheric oscillations on physical condition of pre–reproductive bluefin tuna from the Strait of Gibraltar

The aim of this study was to explore the possible effects of atmospheric oscillations: North Atlantic Oscillation (NAO) and Arctic Oscillation (AO), on the physical condition of bluefin tuna (Thunnus thynnus). We estimated a fitness ratio from 3,501 pairs of length–weight data based on bluefin tuna caught in bait–boat fisheries before the spawning season (January, February and March), for each length class and year. In order to obtain a single fitness ratio (K–mean) per year we determined the average for all length classes. We also evaluated Le Cren’s condition index (KLC). We observed significant positive correlations between the atmospheric oscillations and both physical condition indexes. In the case of K–mean, the AO explained 75% of the observed variability. Regarding KLC, the NAO explained approximately 73% of the observed variability, while the AO explained 70% of the observed variability. The increase in physical conditions of bluefin tuna in association with positive atmospheric oscillations could be mediated by the increase in the prevalence of strong trade winds. We concluded that the increase in the prevalence of strong westerly winds, mediated by a positive AO or NAO, favours the trip from the Atlantic to the Mediterranean by reducing energy costs due to migration and by increasing the supply of nutrients at the surface by the mixing of deep water and surface water in local areas such as the Strait of Gibraltar.

Unstable relationship between spring Arctic Oscillation and East Asian summer monsoon

ABSTRACTThe relationship between spring (March to May) Arctic Oscillation (AO) and East Asian summer monsoon (EASM, May to July) on interannual timescale has a remarkable decadal variation in late 1990s, and their correlation coefficient abruptly turns from +0.77 during 1979–1997 to −0.62 during 1998–2007. In the following summer after a spring positive AO phase, the lower‐troposphere over East Asia (EA) features a cyclonic anomaly before 1997 but an anticyclonic anomaly after 1997, which results from different simultaneous air–sea features imposed by spring AO and distinctive evolution from spring to summer. In pre‐1997, the spring AO‐associated signal is mainly memorized and persists over Pacific, because the spring AO‐associated wave activity prefers the high‐latitude propagation from North Atlantic to Pacific. In contrast, a subtropical wave train from North Atlantic Ocean to Indian Ocean (IO) is evidently enhanced in post‐1997 epoch, and accordingly the IO plays a dominant role in memorizing and extending the influence of spring AO on EASM. This subtropical route of spring AO‐EASM teleconnection is a new finding. The strengthening of the subtropical wave train in post‐1997 epoch could be partly attributed to the strengthening signals of spring AO over North Atlantic Ocean.

The Surface Climate Response to 11-Yr Solar Forcing during Northern Winter: Observational Analyses and Comparisons with GCM Simulations

Abstract The surface climate response to 11-yr solar forcing during northern winter is first reestimated by applying a multiple linear regression (MLR) statistical model to Hadley Centre sea level pressure (SLP) and sea surface temperature (SST) data over the 1880–2009 period. In addition to a significant positive SLP response in the North Pacific found in previous studies, a positive SST response is obtained across the midlatitude North Pacific. Negative but insignificant SLP responses are obtained in the Arctic. The derived SLP response at zero lag therefore resembles a positive phase of the Arctic Oscillation (AO). Evaluation of the SLP and SST responses as a function of phase lag indicates that the response evolves from a negative AO-like mode a few years before solar maximum to a positive AO-like mode at and following solar maximum. For comparison, a similar MLR analysis is applied to model SLP and SST data from a series of simulations using an atmosphere–ocean general circulation model with a well-resolved stratosphere. The simulations differed only in the assumed solar cycle variation of stratospheric ozone. It is found that the simulation that assumed an ozone variation estimated from satellite data produces solar SLP and SST responses that are most consistent with the observational results, especially during a selected centennial period. In particular, a positive SLP response anomaly is obtained in the northeastern Pacific and a corresponding positive SST response anomaly extends across the midlatitude North Pacific. The model response versus phase lag also evolves from a mainly negative AO-like response before solar maximum to a mainly positive AO response at and following solar maximum.

Influence of the Arctic Oscillation on the TC activity around Taiwan

This study discovered that strong positive correlations exist between the frequency of tropical cyclones (TC) during the summer around Taiwan and the Arctic Oscillation (AO) during the preceding March to May period. In positive AO years, during the preceding spring to summer period, anomalous cyclone and anomalous anticyclone were strongly developed at low and middle latitudes, respectively. Because of such a distribution of pressure system, in Taiwan, Korea, and Japan during the positive AO years, anomalous southeasterlies, which play the role of anomalous steering flows in transferring TCs to these regions, were strengthened. On the other hand, in southern China and the Indochina Peninsula during the positive AO years, anomalous northwesterlies, which prevent the transfer of TCs to these regions, were strengthened. Moreover, such a distribution of pressure system strengthening during the positive AO years led TCs to occur, move, and recurve more eastward in the western North Pacific in positive AO years as compared with the negative AO years. Contrarily, during the negative AO years, TCs showed the tendency to pass over the South China Sea from the Philippines and move west toward southern China and the Indochina Peninsula. Eventually, the intensity of TCs in these years was lower than that of TCs in positive AO years due to the topographic effects from a high TC passage frequency in mainland China.

Combined Effects of the North Atlantic Oscillation and the Arctic Oscillation on Sea Surface Temperature in the Alborán Sea

We explored the possible effects of the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) on interannual sea surface temperature (SST) variations in the Alborán Sea, both separately and combined. The probability of observing mean annual SST values higher than average was related to NAO and AO values of the previous year. The effect of NAO on SST was negative, while that of AO was positive. The pure effects of NAO and AO on SST are obscuring each other, due to the positive correlation between them. When decomposing SST, NAO and AO in seasonal values, we found that variation in mean annual SST and mean winter SST was significantly related to the mean autumn NAO of the previous year, while mean summer SST was related to mean autumn AO of the previous year. The one year delay in the effect of the NAO and AO on the SST could be partially related to the amount of accumulated snow, as we found a significant correlation between the total snow in the North Alborán watershed for a year with the annual average SST of the subsequent year. A positive AO implies a colder atmosphere in the Polar Regions, which could favour occasional cold waves over the Iberian Peninsula which, when coupled with precipitations favoured by a negative NAO, may result in snow precipitation. This snow may be accumulated in the high peaks and melt down in spring-summer of the following year, which consequently increases the runoff of freshwater to the sea, which in turn causes a diminution of sea surface salinity and density, and blocks the local upwelling of colder water, resulting in a higher SST.

The combined effects of the ENSO and the Arctic Oscillation on the winter climate anomalies in East Asia

With the warm/cold phases of the El Niño and Southern Oscillation (ENSO) as a background, the impacts of monthly variation in the Arctic Oscillation (AO) on the winter climate anomalies in East Asia are studied with the NCEP/DOE Reanalysis 2 data and the Chinese station data regarding temperature and rainfall. The combined effects of ENSO and the AO indicate that the winter climate anomalies are mainly influenced by the AO in northern China and the ENSO in southern China, when an El Niño couples with a negative AO month or a La Niña couples with a positive AO month. These climate anomalies in China are consistent with the mechanisms proposed in previous studies. However, most of China presents a different pattern of climate anomalies if an El Niño couples with a positive AO month or a La Niña couples with a negative AO month, with the exception of the temperature anomalies in northern China, which are still affected dominantly by the AO. Further analysis suggests that the causes are attributed to the differences in both the stratosphere-troposphere interaction and the extratropics-tropics interaction. In the former cases, zonal symmetric circulation prevails in the winter and the extratropics-tropics interaction is weakened. Thus, the influences of the ENSO and the AO on the East Asian climate mainly present linear combination effects. On the contrary, an annular mode of atmospheric circulation is not favored in the latter cases and the extratropics-tropics interaction is strong. Hence, the combined effects of the ENSO and the AO on the winter climate in East Asia present nonlinear characteristics.

A look at the date of snowmelt and correlations with the Arctic Oscillation

AbstractSpring snow cover across Arctic lands has, on average, retreated ∼5 days earlier since the late 1980s compared to the previous 20 years. However, it appears that since about the late 1980s the date the snowline first retreats north during the spring has changed only slightly: in the last ∼20 years snow has not been disappearing significantly earlier. Snowmelt changes observed since the late 1980s have been step-like, unlike the more continuous downward trend seen in Arctic sea-ice extent. At 70° N, several longitudinal segments (of 10°) show significant (negative) trends, while only two longitudinal segments at 60° N show significant trends, one positive and one negative. These variations appear to be related to variations in the Arctic Oscillation (AO). When the springtime AO is strongly positive, snow melts earlier. When it is strongly negative, snow disappears later in the spring. The winter AO is less straightforward. At higher latitudes (70° N), a positive AO during the winter months is correlated with later snowmelt, but at lower latitudes (50° N and 60° N) a positive wintertime AO is correlated with earlier snowmelt. If the AO during the winter months is negative, the reverse is true. Similar stepwise changes (since the late 1980s) have been noted in sea surface temperatures and in phytoplankton abundance as well as in snow cover.

Two climate factors in May that affect Korean rainfall in September

This study revealed a high positive correlation between rainfall in Korea during September and the trade wind (TW)/Arctic Oscillation (AO) index in May that combines two climate factors, low-level TWs and the AO. This correlation was identified on the basis of the difference in the 850 h Pa streamline analysis between the positive and negative phases selected using the combined TW/AO index. In May, the spatial pattern of the anomalous pressure systems is similar to that in the positive AO phase. These anomalous pressure systems continue in June to August (JJA) and September, but the overall spatial distribution shifts a little to the south. Particularly in September, a huge anomalous anticyclone centered over the southeast seas of Japan strengthens inmost of the western north Pacific region and supplies a large volume of warm and humid air to the region near Korea. This characteristic is confirmed by the facts that during the positive TW/AO phase, the subtropical western north Pacific high (SWNPH) is more developed to the north and that the continuous positioning of the upper troposphere jet over Korea from May to September strengthens the anomalous upward flow, bringing warm and humid air to all layers. These factors contribute to increasing September rainfall in Korea during the positive TW/AO phase. Because the SWNPH develops more to the north in the positive phase, tropical cyclones tend to make landfall in Korea frequently, which also plays a positive role in increasing September rainfall in Korea. The above features are also reflected by the differences in average rainfall between the six years that had the highest May Nino 3.4 indices (El Nino phase) and the six years that had the lowest May Nino 3.4 indices (La Nina phase).

Recent Northern Hemisphere mid-latitude medium-range deterministic forecast skill

ABSTRACTA multi-model archive of global deterministic forecasts and analyses from three operational systems is constructed to analyse recent Northern Hemisphere mid-latitude forecast skill from 2007 to 2012 and its relation to large-scale atmospheric flow anomalies defined by the Arctic Oscillation (AO) index. We find that the anomaly correlation coefficient (ACC) in 120-hr forecasts of 500 hPa geopotential height has similar variability on synoptic, monthly, and seasonal time scales in each of the three forecast systems examined here: the European Centre for Medium-Range Weather Forecasts, the National Centers for Environmental Prediction Global Forecast System, and the U.S. Navy Operational Global Atmospheric Prediction System. The results indicate that forecast skill as measured by the ACC is significantly correlated with the AO index and its transitions between negative and positive phase. Intervals of exceptionally high ACC skill during the 2009–2010 and 2010–2011 winters are associated with periods in which the AO remained in a persistent negative phase pattern. Episodes of low ACC, including so-called ‘forecast skill dropouts’ most frequently occur during transitions between negative and positive AO index and with positive AO index. The root mean square error (RMSE) of 120-hr forecast 500 hPa height is also modulated by the AO index, but to a lesser extent than the ACC. In two recent winters, the RMSE indicates lower 120-hr forecast accuracy during periods with negative AO index, which is opposite to ‘skill’ patterns provided by the ACC. These results suggest that the ACC is not in all situations an optimal metric with which to quantify model forecast skill, since the ACC can be higher when the large-scale atmospheric flow contains strong anomalies even if there is no actual improvement in model forecasts of that atmospheric state.

Possible relationship between North Korean total rainfall and Arctic Oscillation in May

In this study, a negative correlation between the rainfall over North Korea in May and the Arctic Oscillation (AO) in the same month is analyzed. The reasons for rainfall declines in the positive AO phase are as follows: (a) the strengthening of anomalous anticyclone in the Maritime Province of Siberia, (b) the weakening of the subtropical western North Pacific high, and (c) the stabilization across all the atmospheric layers in North Korea. Anomalous anticyclone strengthened in the Maritime Province of Siberia plays a critical role in placing the North Korean region under the influence of anomalous northeasterlies. In addition, the development of the anomalous northeasterlies results in the supply of a large amount of cold and dry air into North Korea. This consequently stabilizes the atmosphere in North Korea. Moreover, the reinforcement of anomalous cold sea surface temperature in the mid-latitude region of East Asia is found to be another reason for the atmospheric stabilization.

Anomalous precipitation signatures of the Arctic Oscillation in the time-variable gravity field by GRACE

The Arctic Oscillation (AO) controls winter climates in the Northern Hemisphere to a large extent. Positive AO brings lower/higher surface temperature and higher/lower precipitation in high/middle latitude regions, and negative AO reverses the situation. In this study, we investigate signals of anomalous precipitations caused by AO using the data of the Gravity Recovery and Climate Experiment (GRACE) satellites. Wintertime mass deviations inferred from GRACE in the high and middle (boundary ∼55N) latitude regions in Eurasia showed highly positive and negative correlations with the AO indices. This possibly reflects the northward and southward shift of the centre of winter precipitation during the positive and negative phases of AO, respectively. Wintertime mass deviations also showed positive and negative correlations with the AO indices in the northern and southern parts of Greenland, respectively, but the boundary was further to the north, say ∼75N. AO redistributes the water mass as much as ∼1000 Gt between high and middle latitude regions in the Northern Hemisphere. Such mass redistribution causes significant surface deformation by loading large enough to be observed by Global Positioning System. This also causes the shift of the Earth's rotation axis especially towards the Greenwich Meridian large enough to be detected with space geodetic techniques. AO signatures are also derived from the empirical orthogonal function (EOF) analysis, as the first leading mode of GRACE data after excluding seasonal, linear and quadratic components. The EOF analysis also demonstrates that though AO is a main contributor to bring anomalous winter precipitation in the Northern Hemisphere as a whole, in North America, however, influence of the El Niño and Southern Oscillation on the winter precipitation anomaly is larger than AO.

Winter temperature extremes in China and their possible causes

AbstractCold and warm temperature extremes predominantly occurring in winter gained much more attention than mean temperatures. On the basis of daily maximum and minimum surface air temperature records at 303 meteorological stations in China, the spatial and temporal distributions of five indices for winter (DJF: December, subsequent January and February) temperature extremes are analysed during 1961–2003. For the majority of stations, the frequency of cold days/nights decreases by − 1.33/− 2.98 and warm days/nights increases by 0.92/2.35 d/decade, respectively. Cold days/nights are significantly negatively correlated with the Arctic Oscillation (AO) index, while warm days/nights are positively correlated with the AO. The diurnal temperature range (DTR) has a declining trend with rate of − 0.25 °C/decade and positive correlation with the AO index. Compared with other regions in China, stations in the northern China have larger trend magnitudes and stronger correlations with the AO index, and the AO can explain more than 50% of winter temperature extreme change in China. Compared with the annual basis, the winter temperature extremes have larger trend magnitudes, which reflect the rapid warming. During strongly positive AO index years, enhanced contrast tropospheric temperature (defined as the average of air temperature vertically integrated between 200 hPa and 1000 hPa based on the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis) between the north of China and the southern China weakens the East Asian winter monsoon which in turn reduces cold outbreaks in the northern and eastern China. The composites of large‐scale atmospheric circulation are consistent with the asymmetrical changes of the geopotential height, zonal and meridional winds at high and mid latitudes at troposphere. Meanwhile, the linkage between the AO and solar activity also modulates the winter temperature extremes, while the mechanism needs to be investigated. Copyright © 2012 Royal Meteorological Society

The Lower-Stratospheric Response to 11-Yr Solar Forcing: Coupling to the Troposphere–Ocean Response

Abstract The origin of the tropical lower-stratospheric response to 11-yr solar forcing and its possible coupling to a troposphere–ocean response is investigated using multiple linear regression (MLR) analyses of stratospheric ozone and temperature data over the 1979–2009 period and tropospheric sea level pressure (SLP) data over the 1880–2009 period. Stratospheric MLR results, comparisons with simulations from a chemistry–climate model, and analyses of decadal variations of meridional eddy heat flux indicate that the tropical lower-stratospheric response is produced mainly by a solar-induced modulation of the Brewer–Dobson circulation (BDC), with a secondary contribution from the Hadley circulation in the lowermost stratosphere. MLR analyses of long-term SLP data confirm previous results indicating a distinct positive response, on average, during the northern winter season in the North Pacific. The mean response in the Northern Hemisphere resembles a positive Arctic Oscillation mode and can also be characterized as “La Niña–like,” implying a reduction of Rossby wave forcing, a weakening of the BDC, and an increase in tropical lower-stratospheric ozone and temperature near solar maxima. However, MLR analyses of different time periods show that the Pacific SLP response is not always present during every cycle; it was most clearly detected mainly during the ~1938–93 period when 11-yr solar variability was especially strong. During the 1979–93 period, the SLP response was strongly present when the lower-stratospheric responses were large. But during the 1994–2009 period, the SLP response was much less significant and the lower-stratospheric responses were weak, supporting the hypothesis that the lower-stratospheric and surface climate responses are dynamically coupled.

A possible cause of the AO polarity reversal from winter to summer in 2010 and its relation to hemispheric extreme summer weather

In 2010, the Northern Hemisphere, in particular Russia and Japan, experienced an abnormally hot summer characterized by record-breaking warm temperatures and associated with a strongly positive Arctic Oscillation (AO), that is, low pressure in the Arctic and high pressure in the midlatitudes. In contrast, the AO index the previous winter and spring (2009/2010) was record-breaking negative. The AO polarity reversal that began in summer 2010 can explain the abnormally hot summer. The winter sea surface temperatures (SST) in the North Atlantic Ocean showed a tripolar anomaly pattern—warm SST anomalies over the tropics and high latitudes and cold SST anomalies over the midlatitudes—under the influence of the negative AO. The warm SST anomalies continued into summer 2010 because of the large oceanic heat capacity. A model simulation strongly suggested that the AO-related summertime North Atlantic oceanic warm temperature anomalies remotely caused blocking highs to form over Europe, which amplified the positive summertime AO. Thus, a possible cause of the AO polarity reversal might be the “memory” of the negative winter AO in the North Atlantic Ocean, suggesting an interseasonal linkage of the AO in which the oceanic memory of a wintertime negative AO induces a positive AO in the following summer. Understanding of this interseasonal linkage may aid in the long-term prediction of such abnormal summer events.

The response of the North Pacific Decadal Variability to strong tropical volcanic eruptions

In this study, the effects of volcanic forcing on North Pacific climate variability, on interannual to decadal time scales, are examined using climate model simulations covering the last 600 years. The model used is the Bergen Climate Model, a fully coupled atmosphere–ocean general circulation model. It is found that natural external forcings, such as tropical strong volcanic eruptions (SVEs) and variations in total solar irradiance, play an important role in regulating North Pacific Decadal Variability (NPDV). In response to tropical SVEs the lower stratospheric pole–to–equator temperature gradient is enhanced. The North polar vortex is strengthened, which forces a significant positive Arctic Oscillation. At the same time, dipole zonal wind anomalies associated with strong polar vortex propagate downward from the lower stratosphere. Through positive feedbacks in the troposphere, the surface westerly winds across the central North Pacific are significantly weakened, and positive sea level pressure anomalies are formed in the North Pacific. This anomalous surface circulation results in changes in the net heat fluxes and the oceanic advection across the North Pacific. As a result of this, warm water converges in the subtropical western North Pacific, where the surface waters in addition are heated by significantly reduced latent and sensible heat fluxes from the ocean. In the eastern and high–latitude North Pacific the ocean loses more heat, and large–scale decreases in sea surface temperatures are found. The overall response of this chain of events is that the North Pacific enters a negative phase of the Pacific decadal oscillation (PDO), and this negative phase of the PDO is maintained for several years. It is thus concluded that the volcanic forcing plays a key role in the phasing of the PDO. The model results furthermore highlight the important role of troposphere–stratosphere coupling, tropical–extratropical teleconnections and extratropical ocean–atmosphere interactions for describing NPDV.

Modulation of the Arctic Oscillation and the East Asian winter climate relationships by the 11-year solar cycle

The modulation of the relationship between the Arctic Oscillation (AO) and the East Asian winter climate by the 11-year solar cycle was investigated. During winters with high solar activity (HS), robust warming appeared in northern Asia in a positive AO phase. This result corresponded to an enhanced anticyclonic flow at 850 hPa over northeastern Asia and a weakened East Asian trough (EAT) at 500 hPa. However, during winters with low solar activity (LS), both the surface warming and the intensities of the anticyclonic flow and the EAT were much less in the presence of a positive AO phase. The possible atmospheric processes for this 11-year solar-cycle modulation may be attributed to the indirect influence that solar activity induces in the structural changes of AO. During HS winters, the sea level pressure oscillation associated with the AO became stronger, with the significant influence of AO extending to East Asia. In the meantime, the AO-related zonal-mean zonal winds tended to extend more into the stratosphere during HS winters, which implies a stronger coupling to the stratosphere. These trends may have led to an enhanced AO phase difference; thus the associated East Asian climate anomalies became larger and more significant. The situation tended to reverse during LS winters. Further analyses revealed that the relationship between the winter AO and surface-climate anomalies in the following spring is also modulated by the 11-year solar cycle, with significant signals appearing only during HS phases. Solar-cycle variation should be taken into consideration when the AO is used to predict winter and spring climate anomalies over East Asia.

Linkage between the Arctic Oscillation and winter extreme precipitation over central-southern China

In this study, we analyzed the relationship between the Arctic Oscillation (AO) and extreme precipitation events over China during boreal winters from 1954 to 2009. The extreme precipitation events are defined as those with daily precipitation above the 80th (or 90th) per- centiles. The AO shows a significantly positive correlation with the frequency of extreme precipi- tation events over China during January to February. Of all 287 stations in China, 238 stations have positive correlation coefficients and 82 stations are positively significant above the 95% con- fidence level. These stations that have significantly positive correlation are mainly located over central-southern China. The correlation between the AO and the average frequency of extreme precipitation events over central-southern China is 0.49, which is significant at the 99% confi- dence level. This correlation is still significant even when the El Nino/Southern Oscillation signal is excluded from the time series. In association with the AO-precipitation extreme linkage, the Middle East jet stream (MEJS) and the southern branch trough (SBT) over the Bay of Bengal covary consistently. A positive AO phase is accompanied by a stronger than normal MEJS and a deepened SBT. The deepened SBT enhances synoptic scale disturbances in vertical motions in the low to middle troposphere over central-southern China. More moisture transport by the deepened SBT and active synoptic scale disturbances in vertical motion over central-southern China would provoke more extreme precipitation events there.

A dynamical fingerprint of tropical Pacific sea surface temperatures on the decadal-scale variability of cool-season Arctic precipitation

[1] The temporal and spatial characteristics of decadal-scale variability in the Northern Hemisphere (NH) cool-season (October–March) Arctic precipitation are identified from both the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and the Global Precipitation Climatology Project (GPCP) precipitation data sets. This decadal variability is shown to be partly connected to the decadal-scale variations in tropical central Pacific sea surface temperatures (SSTs) that are primarily associated with a decadal modulation of the El Niño–Southern Oscillation (ENSO), i.e., transitions between periods favoring typical eastern Pacific warming (EPW) events and periods favoring central Pacific warming (CPW) events. Regression and composite analyses reveal that increases of central Pacific SSTs drive a stationary Rossby wave train that destructively interferes with the wave number-1 component of the extratropical planetary wave. This destructive interference is opposite to the mean effect of typical EPW on the extratropical planetary wave. It leads to suppressed upward propagation of wave energy into the polar stratosphere, a stronger stratospheric polar vortex, and a tendency toward a positive phase of the Arctic Oscillation (AO). The positive AO tendency is synchronized on the decadal scale with a poleward shift of the NH storm tracks, particularly in the North Atlantic. Storm track variations further induce changes in the amount of moisture transported into the Arctic by synoptic eddies. The fluctuations in the eddy moisture transport ultimately contribute to the observed decadal-scale variations in the total Arctic precipitation in the NH cool season.

Possible connection between summer tropical cyclone frequency and spring Arctic Oscillation over East Asia

This study shows that the frequency of summer tropical cyclones (TCs) in the areas of Japan, Korea, and Taiwan (JKT), which are located in the middle latitudes of East Asia, has a positive correlation with the Arctic Oscillation (AO) occurring during the preceding spring, while summer TC frequency in the Philippines (PH), located in the low latitudes, has a negative correlation with the AO of the preceding spring. During a positive AO phase, when the anomalous anticyclone forms over the mid-latitudes of East Asia, other anomalous cyclones develop not only in the high latitudes but also in the low latitudes from the preceding spring to the summer months. With this change, while southeasterlies in the JKT area derived from the mid-latitude anticyclone plays a role in steering TCs toward this area, northwesterlies strengthened in the PH area by the low-latitude cyclone plays a role in preventing TC movement toward this area. In addition, because of this pressure systems developed during this AO phase, TCs occur, move, and recurve in further northeastern part of the western North Pacific than they do during a negative AO phase.