Meridional Oscillation Research Articles

Millennial-scale paleotemperature change in the Japan Sea during Marine Isotope Stage 3: Impact of meridional oscillation of the subpolar front

The Japan Sea is a semi-closed marginal sea that has been significantly affected by fluctuations of the East Asian monsoon. Due to its geographic setting, the influence of the monsoon is significant especially during the last glacial period, when the low sea level stand restricted surface water exchange with adjacent seas. Millennial-scale change in the summer monsoon controlled the input of freshwater and nutrients into the Japan Sea during Marine Isotope Stage (MIS) 3, resulting in the deposition of alternating dark- and light-colored sedimentary layers. However, the relationships between the sediment color alternations and winter sea surface conditions, such as temperature and salinity, are not understood. Here, we provide high-resolution Mg/Ca-based winter sea surface temperature (SST) records during MIS3 from two sediment cores from the southern Japan Sea. The SSTs at both sites showed millennial-scale fluctuations with significantly different amplitudes. The southern site had a larger SST amplitude with warmer in dark- and cooler in light-colored intervals. The SST difference between the two cores was greater in dark-colored intervals. The millennial-scale changes in SST difference are probably attributable to meridional migration of the subpolar front, induced by winter monsoons and inflow of the Tsushima Warm Current, which had a northerly (southerly) position during interstadials (stadials). The results suggest that the millennial-scale winter monsoon variability contributed to sediment color alternation via greater cooling during stadials, which enhanced ventilation, in conjunction with the summer monsoon-induced productivity change. Therefore, both the winter and summer monsoons are key drivers of the oceanographic condition in the Japan Sea.

Soil moisture-constrained East Asian Monsoon meridional patterns over China from observations

As an internal forcing of the earth climate system, soil moisture (SM) significantly influences the water and energy cycle by controlling evapotranspiration and terrestrial solar energy. The current study used observed precipitation, remotely sensed SM, and reanalysis of atmosphere and land parameters to assess the East Asian Monsoon (EAM) precipitation variability due to meridional SM oscillations across China. A generalized linear method, namely coupled manifold technique (CMT) for assessing the reciprocal forcing between two climate fields and numerical simulations are applied to SM and EAM precipitation. We find that the EAM precipitation interannual variability between north and south China significantly correlates with SM meridional oscillation. The CMT results further showed that SM forcing has a significant (99% confidence) influence on the EAM precipitation explaining about 0.40 of the variance ratio in north and south China. The EAM and SM composite analysis show that the wetter (drier) north (south) oscillates the EAM precipitation over the north (south) of China and vice versa due to SM thermal controls. We then used control and sensitivity simulations with SM observations to further validate the findings implying that SM can potentially improve the interannual EAM forecast skills. The model results show that a wetter (drier) north (south) results in negative (positive) sensible heat (latent heat) anomalies that impact the boundary layer and propagate to change the meridional atmospheric heating profile. When positive (negative) SM anomalies exist over northern (southern) China, the zonal easterlies and extratropical westerlies move to north China causing above-normal precipitation that descends into southern China, suppressing subtropical westerlies and precipitation in southern China. On the contrary, a dry (wet) north (south) favors intensified subtropical westerlies and precipitation in southern China. The findings have dire implications for the water and energy cycle of the region in the projected wetting and drying patterns of the north (south).

Extension of Ekman (1905) wind-driven transport theory to the β plane

Abstract. The seminal Ekman (1905) f-plane theory of wind-driven transport at the ocean surface is extended to the β plane by substituting the pseudo-angular momentum for the zonal velocity in the Lagrangian equation. When the β term is added, the equations become nonlinear, which greatly complicates the analysis. Though rotation relates the momentum equations in the zonal and the meridional directions, the transformation to pseudo-angular momentum greatly simplifies the longitudinal dynamics, which yields a clear description of the meridional dynamics in terms of a slow drift compounded by fast oscillations; this can then be applied to describe the motion in the zonal direction. Both analytical expressions and numerical calculations highlight the critical role of the Equator in determining the trajectories of water columns forced by eastward-directed (in the Northern Hemisphere) wind stress even when the water columns are initiated far from the Equator. Our results demonstrate that the averaged motion in the zonal direction depends on the amplitude of the meridional oscillations and is independent of the direction of the wind stress. The zonal drift is determined by a balance between the initial conditions and the magnitude of the wind stress, so it can be as large as the mean meridional motion; i.e., the averaged flow direction is not necessarily perpendicular to the wind direction.

Boreal Summer Intraseasonal Meridional Oscillations Over East Asia and Their Influences on Precipitation

AbstractThe intraseasonal meridional wave trains are the major features of East Asia summer monsoon (EASM), but the complete physical pictures of dynamic pathways from tropical and mid‐high latitudes and their climate impacts have not been fully illustrated. With NCEP‐DOE Reanalysis‐II daily data set, two boreal summer intraseasonal meridional oscillations (ISMOs) are isolated and identified by extended EOF of zonal wind anomalies along the coast of East Asia on 10‐ to 30‐day timescale, that is, ISMO1 and ISMO2. They both demonstrate obvious quasi‐biweekly oscillations with an equivalent barotropic structure in troposphere, and ISMO1 represents the southward propagating wave train with large variances in upper level, which is initiated from the West Europe and primarily forced by the soil moisture anomalies over mid‐high latitudes of Eurasian continent, whereas ISMO2 propagates northward in mid‐lower troposphere from the tropical western North Pacific to East Asia that is closely related to BSISO2. The two ISMOs are capable of explaining a large fraction of the total intraseasonal variances of EASM, wherein, ISMO1 is responsible for a meridional dipole pattern of precipitation anomalies over eastern China, while ISMO2 has major influence in southern China. Generally, ISMO‐caused changes of extreme precipitation probability are in good accordance with the spatial pattern of its related 10‐ to 30‐day intraseasonal precipitation anomalies. Since ISMO1 and ISMO2 can clearly separate the tropical and extratropical‐origin meridional oscillations and further have robust influences on precipitation over East Asia, they can be used as the new monitoring and predicting factors to the intraseasonal variations of EASM and their associated extreme precipitation.

Millennial-Scale Paleotemperature Change in the Japan Sea During Marine Isotope Stage 3: Impact of Meridional Oscillation of the Subpolar Front

Millennial-Scale Paleotemperature Change in the Japan Sea During Marine Isotope Stage 3: Impact of Meridional Oscillation of the Subpolar Front

Meridional oscillation of tropical cyclone activity in the western North Pacific during the past 110 years

A prominent meridional oscillation of tropical cyclone (TC) activity in the western North Pacific (WNP) during the past 110 years is identified in this paper. Based on the best-track TC data, the first three Empirical Orthogonal Functions of TC occurrence frequency generally show a northeast-southwest dipole, with the major loadings over the South China Sea (SCS) and the area near Japan. During the periods 1951–1963 and 1997–2019, TC frequency is generally lower over the SCS but higher near Japan. The opposite is true during 1969–1988. Inclusion of the historical TC data from 1910 gives two periods (1943–1963 and 1997–2019) during which the TC numbers near Japan (NJP) are higher but those over the SCS (NSCS) are lower, and two periods (1926–1942 and 1969–1988) with just the opposite pattern. The main factors responsible for this meridional oscillation include changes in the genesis positions of the TCs and their subsequent tracks associated with the interdecadal variation of the large-scale environmental flow patterns. During the periods in which the NSCS (NJP) is higher (lower), the large-scale steering flow is favorable for TCs moving to the SCS. At lower latitudes, vertical wind shear is weaker and low-level cyclonic flow is stronger, which lead to a southward shift of the genesis positions. The patterns are just the reverse during the periods in which the meridional oscillation is in the opposite phase. The oscillation is also found to be partly related to the interdecadal Pacific oscillation through changes in its low-level flow pattern.

Contribution of low-frequency climatic–oceanic oscillations to streamflow variability in small, coastal rivers of the Sierra Nevada de Santa Marta (Colombia)

Abstract. This study evaluated the influence of low-frequency oscillations, that are linked to large-scale oceanographic–atmospheric processes, on streamflow variability in small tropical coastal mountain rivers of the Sierra Nevada de Santa Marta, Colombia. We used data from six rivers that had > 32 years of complete, continuous monthly streamflow records. This investigation employed spectral analyses to (1) explore temporal characteristics of streamflow variability, (2) estimate the net contribution to the energy spectrum of low-frequency oscillations to streamflow anomalies, and (3) analyze the linkages between streamflow anomalies and large-scale, low-frequency oceanographic–atmospheric processes. Wavelet analyses indicate that the 8–12-year component exhibited a quasi-stationary state, with a peak of maximum power between 1985 and 2005. These oscillations were nearly in phase in all rivers. Maximum power peaks occurred for the Palomino and Rancheria rivers in 1985 and 1995, respectively. The wavelet spectrum highlights a change in river variability patterns between 1995 and 2015, characterized by a shift towards the low-frequency oscillations' domain (8–12 years). The net contribution of these oscillations to the energy spectrum was as high as 51 %, a value much larger than previously thought for rivers in northwestern South America. The simultaneous occurrence of hydrologic oscillations, as well as the increase in the amplitude of the 8–12-year band, defined periods of extremely anomalous wet seasons during 1989–1990, 1998–2002 and 2010–2011, reflecting the role of low-frequency oscillations in modulating streamflow variability in these rivers. Cross-wavelet transform and wavelet coherence revealed high common powers and significant coherences in low-frequency bands (>96 months) between streamflow anomalies and Atlantic Meridional Oscillation (AMO), Pacific Decadal Oscillation (PDO) and the Tropical North Atlantic Index (TNA). These results show the role of large-scale, low-frequency oceanographic–climate processes in modulating the long-term hydrological variability of these rivers.

Meridional oscillation in genesis location of tropical cyclones in the postmonsoon Bay of Bengal

It is found that the average genesis location of tropical cyclones (TCs) in the postmonsoon (October–December) Bay of Bengal (BoB) shows a notable meridional oscillation during 1980–2015. During years when the average genesis location shifts northward (north-years), the average maximum sustained wind speed, the average landfall wind speed and the number of category 4–5 TCs are all larger than those during years when the average genesis location displaces southward (south-years). Genesis potential index analysis shows that changes in relative humidity and vertical wind shear are mainly responsible for the meridional oscillation of the average genesis location of TCs. The changes in relative humidity and wind shear are closely related to changes in atmospheric circulation. Composite analysis reveals that sea surface temperature (SST) anomalies over the equatorial Pacific Ocean show a La Nina-like pattern (El Nino-like pattern) during north-years (south-years) of TC genesis locations. The SST anomalies over the equatorial Pacific Ocean induce a strengthened (weakened) Walker circulation during north-years (south-years) of TC genesis locations, influencing the atmospheric circulation over the tropical Indian Ocean. In addition, a stationary Rossby wave train from the North Pacific to the Iranian Plateau (east of Iranian Plateau) during north-years (south-years) induces changes in atmospheric circulation over the extratropical North Indian Ocean. Together, the tropical and extratropical influences contribute to the north–south patterns of relative humidity and vertical wind shear anomalies in the postmonsoon BoB, which drive the meridional oscillation of TC genesis locations. This study has significant implications for the prediction of TCs and disaster prevention and mitigation over the BoB.

Patterns and drivers of phytoplankton phenology off SW Iberia: A phenoregion based perspective

Phytoplankton patterns, tightly linked to the dynamics of the ocean surface layer and its atmospheric forcing, have major impacts on ecosystem functioning and are valuable indicators of its response to environmental variability and change. Phytoplankton phenology and its underlying drivers are spatially variable, and the study of its patterns, particularly over heterogeneous regions, benefits from a delineation of regions with specific phenological properties, or phenoregions. The area Southwest off the Iberian Peninsula (SWIP, NE Atlantic) integrates a highly complex set of coastal and ocean domains that collectively challenge the understanding of regional phytoplankton phenology and related forcing mechanisms. This study aims to evaluate phytoplankton phenology patterns over the SWIP area, during an 18-year period (September 1997 – August 2015), using an objective, unsupervised partition strategy (Hierarchical Agglomerative Clustering – HAC) based on phenological indices derived from satellite ocean colour data. The partition is then used to describe region-specific phytoplankton phenological patterns related to bloom magnitude, frequency, duration and timing. Region-specific variability patterns in phenological indices and their linkages with environmental determinants, including local ocean physical-chemical variables, hydrodynamic variables and large scale climate indices, were explored using Generalized Additive Models (GAM). HAC analyses identified five coherent phenoregions over SWIP, with distinctive phytoplankton phenological properties: two open ocean and three coastal regions. Over the open ocean, a single, low magnitude and long bloom event per year, was regularly observed. Coastal phenoregions exhibited up to six short bloom events per year, and higher intra-annual and variability. GAM models explained 50–90% of the variance of all phenological indices except bloom initiation timing, and revealed that interannual patterns in phytoplankton phenology and their environmental drivers varied markedly among the five phenoregions. Over the oceanic phenoregions, large-scale climate indices (Eastern Atlantic Pattern, Atlantic Meridional Oscillation), mixed layer depth (MLD) and nitrate concentration preceding primary bloom events were influential predictors, reflecting the relevance of nutrient limitation. For the Coastal-Slope, a relatively more light-limited phenoregion, North Atlantic Oscillation and wind speed were more relevant, and bloom magnitude was also positively influenced by riverine discharge. This variable was a significant predictor of bloom frequency, magnitude and duration over the Riverine-influenced region. Over the Upwelling-influenced region, upwelling intensity and mean annual MLD showed stronger partial effects on phytoplankton phenology. Overall, our phenology-based unsupervised approach produced a biologically-relevant SWIP partition, providing an evaluation of the complexity of interactions between phytoplankton and multiple environmental forcing, particularly over coastal areas.

Land-falling typhoons are controlled by the meridional oscillation of the Kuroshio Extension

Low-frequency variations of typhoon paths are often attributed to changes in the North Pacific subtropical high and monsoon through influences on the steering wind. Evidence indicates however a strong imprint of the Kuroshio on the atmosphere. Here we show that the meridional oscillation of sea surface temperature (SST) front over the Kuroshio Extension east of Japan significantly correlates with the number of land-falling typhoons along East Asia from June to October, accounting for 70% of the low-frequency variance since 1980. We used observations and a simple model to show that when the SST front shifts poleward (equatorward), SST gradient south of the current and westerly tropospheric wind weaken (strengthen), steering more typhoons to veer toward (away from) the East Asian continent. Our analysis also indicates that long-term weakening of SST gradient and westerly wind appears to be concomitant with poleward shifting of the Kuroshio, attributed to global warming in some studies, and suggests the potential for more land-falling typhoons in East Asia in the coming decades.

Hydroclimatology, modes of climatic variability and stream flow, lake and groundwater level variability

Hydroclimatology is an expansive discipline largely concerned with understanding the workings of the hydrological cycle in a climate context. Acknowledging this, and given the burgeoning interest in the relation between climate and water in the context of working towards an improved understanding of the impacts of climatic variability on water resources, this progress report turns its attention to the connection between large-scale modes of climatic variability and hydrological variability in streams, lakes and groundwater. A survey of the recent literature finds that a plethora of teleconnection indices have been employed in the analysis of hydrological variability. Indices representing modes of climatic variability such as El Niño Southern Oscillation, the North Atlantic Oscillation, the Pacific North America pattern, the Pacific Decadal Oscillation and Atlantic Meridional Oscillation dominate the literature on climatic and hydrological variability. While examples of discernible signals of modes of climatic variability in stream flow and lake and groundwater level time series abound, the associations between periodic to quasi-period oscillations in atmospheric/ocean circulation patterns and variability within the terrestrial branch of the hydrological are far from simple, being both monotonic (linear and non-linear) and non-monotonic and also conditional on period of analysis, season and geographic region. While there has been considerable progress over the last five years in revealing the climate mechanisms that underlie the links between climatic and hydrological variability, a bothering feature of the literature is how climatic and hydrological variability is often viewed through a purely statistical lens with little attention given to diagnosing the relationship in terms of atmosphere and ocean physics and dynamics. Consequently, significant progress remains to be made in obtaining a satisfactory hydroclimatological understanding of stream flow, lake and groundwater variability, especially if hydroclimatological knowledge is to be fully integrated into water resource management and planning.

High-magnitude flooding across Britain since AD 1750

Abstract. The last decade has witnessed severe flooding across much of the globe, but have these floods really been exceptional? Globally, relatively few instrumental river flow series extend beyond 50 years, with short records presenting significant challenges in determining flood risk from high-magnitude floods. A perceived increase in extreme floods in recent years has decreased public confidence in conventional flood risk estimates; the results affect society (insurance costs), individuals (personal vulnerability) and companies (e.g. water resource managers). Here, we show how historical records from Britain have improved understanding of high-magnitude floods, by examining past spatial and temporal variability. The findings identify that whilst recent floods are notable, several comparable periods of increased flooding are identifiable historically, with periods of greater frequency (flood-rich periods). Statistically significant relationships between the British flood index, the Atlantic Meridional Oscillation and the North Atlantic Oscillation Index are identified. The use of historical records identifies that the largest floods often transcend single catchments affecting regions and that the current flood-rich period is not unprecedented.

Amplification of the solar signal in the summer monsoon rainband in China by synergistic actions of different dynamical responses

A rainband meridional shift index (RMSI) is defined and used to statistically prove that the East Asian summer monsoon rainband is usually significantly more northward in the early summer of solar maximum years than that of solar minimum years. By applying continuous wavelet transform, cross wavelet transform, and wavelet coherence, it is found that throughout most of the 20th century, the significant decadal oscillations of sunspot number (SSN) and the RMSI are phase-locked and since the 1960s, the SSN has led the RMSI slightly by approximately 1.4 yr. Wind and Eliassen–Palm (EP) flux analysis shows that the decadal meridional oscillation of the June rainband likely results from both a stronger or earlier onset of the tropical monsoon and poleward shift of the subtropical westerly jet in high-solar months of May and June. The dynamical responses of the lower tropical monsoon and the upper subtropical westerly jet to the 11-yr solar cycle transmit bottom-up and top-down solar signals, respectively, and the synergistic actions between the monsoon and the jet likely amplify the solar signal at the northern boundary of the monsoon to some extent.

Variability of fire emissions on interannual to multi-decadal timescales in two Earth System models

Connections between wildfires and modes of variability in climate are sought as a means for predicting fire activity on interannual to multi-decadal timescales. Several fire drivers, such as temperature and local drought index, have been shown to vary on these timescales, and analysis of tree-ring data suggests covariance between fires and climate oscillation indices in some regions. However, the shortness of the satellite record of global fire events limits investigations on larger spatial scales. Here we explore the interplay between climate variability and wildfire emissions with the preindustrial long control numerical experiments and historical ensembles of CESM1 and the NOAA/GFDL ESM2Mb. We find that interannual variability in fires is underpredicted in both Earth System models (ESMs) compared to present day fire emission inventories. Modeled fire emissions respond to the El Niño/southern oscillation (ENSO) and Pacific decadal oscillation (PDO) with increases in southeast Asia and boreal North America emissions, and decreases in southern North America and Sahel emissions, during the ENSO warm phase in both ESMs, and the PDO warm phase in CESM1. Additionally, CESM1 produces decreases in boreal northern hemisphere fire emissions for the warm phase of the Atlantic Meridional Oscillation. Through analysis of the long control simulations, we show that the 20th century trends in both ESMs are statistically significant, meaning that the signal of anthropogenic activity on fire emissions over this time period is detectable above the annual to decadal timescale noise. However, the trends simulated by the two ESMs are of opposite sign (CESM1 decreasing, ESM2Mb increasing), highlighting the need for improved understanding, proxy observations, and modeling to resolve this discrepancy.

Decadal-scale meridional shift of the typhoon recurvature latitude over five decades

In this study, long-term variations of typhoon trajectory patterns were analysed via the Gaussian process regression technique. Since typhoon trajectory patterns affect the risks of typhoon-related hazards, such changes are important when developing plans to reduce those risks. Herein, long-term variations in typical typhoon trajectory patterns were analysed simultaneously with seasonal variations. The results indicate decadal or longer meridional oscillations of typhoon recurvature points that are distinguishable from seasonal variations. Additionally, a correlation between typhoon recurvature latitudes and long-term trends of the El Nino Southern Oscillation (ENSO) after 1990 was observed, even though this correlation appeared to be poor before 1990. Background wind field variations were also found to be consistent with those of the typhoon latitude recurvature after 1990, but the correspondence was, again, poor before 1990. These results suggest that for at least the two decades after 1990, the typhoon recurvature latitude was associated with a long-term oscillation mode, such as the long-term trends of the ENSO. The poor correlation before 1990 might be due to the fact that meridional variations of the typhoon recurvature latitude were primarily related to central Pacific rather than the eastern Pacific warming. However, further investigation will be necessary to attain a conclusive answer.

Contribution of the phase transition of Pacific Decadal Oscillation to the late 1990s' shift in East China summer rainfall

AbstractBased on our previous study, the interdecadal changes in summer rainfall over East China in the late 1990s are further explored here. The increased local rising motion is implicated as the dominant factor of increased rainfall in the lower Huang‐Huai River valley (LHR). Both the observation and numerical experiments using Community Atmosphere Model, version 4 suggest that the negative Pacific Decadal Oscillation (PDO) mode can result in rising anomalies and thus more rainfall in the LHR. The East Asian westerly jet stream (EAWJS) is suggested as a bridge to link the Pacific sea surface temperature anomalies and East Asian summer rainfall. Model results reveal that the negative PDO mode can lead to significant easterly anomalies over East Asia. As a result, the EAWJS is weakened and shifts poleward, which coincides with observed changes in EAWJS after the late 1990s. In addition, weakened and poleward shifted EAWJS can result in an anomalous ascending motion to its south (in the LHR) by modulating the jet‐related secondary meridional‐vertical circulation. Consequently, rainfall increased in the LHR after the late 1990s. Besides, the positive Atlantic Meridional Oscillation can only induce insignificant changes over East Asia and partly counteract the negative PDO effect there.

Hodographs of Slowly Rotating Winds in Midlatitudes

AbstractTrend analysis of hourly wind direction angle recorded at nine meteorological stations across southern Canada (in the 43°–53°N belt) identified wind direction rotation periods in the range of 7–9 days. Rotation persists during the “summertime” season from May to mid-October during 1953–2001. Rotation with a 7.5-day period was also established in the 850-hPa geostrophic summertime wind over the Canadian province of Saskatchewan in 2000. Hodographs built from wind vectors corresponding to consecutive days of the 7.5-day period (summertime average vectors of winds binned into separate days of the period) formed nearly elliptical loops centered around the end point of the net westerly transport vector that ranged from 0.5 to 1.3 m s−1 and was either smaller than or of comparable magnitude to the zonal and meridional oscillations of wind. The observed rotations appear quasi periodic rather than purely periodic, because nearly elliptical loops of relatively large amplitude were present at different hodographs constructed at each location for sampling periods of 6.5, 7.0, 7.5, and 8.0 days. For sampling periods outside the ~6.5–8.0-day range, the rotational magnitude in a hodograph appears diminished because of partial cancellation of wind vectors in corresponding hodograph bins. Quasi-periodical rotation occurs in the clockwise direction.

Thermospheric wind oscillations during the propagation of large-scale traveling ionospheric disturbances

The parameters of meridional thermospheric wind oscillations during the propagation of largescale traveling ionospheric disturbances, obtained based on the nighttime observations in the ionospheric F region performed at the Institute of the Ionosphere (Almaty, 76°55′ E, 43°15′ N) in 2000–2007 using a digital ionosonde, have been analyzed. The processing of ionospheric sounding data made it possible to obtain electron density time variations N(t) at fixed altitudes and variations in the altitudes of the F region maximum (hmF) and bottom (hbotF). During the indicated period, 1166 observation sessions were performed, and 581 sessions were characterized by wave activity. Sessions with a relative amplitude of N(t) variations larger than 25% were selected for analysis. The total number of such sessions was 63. The expression for calculating the meridional wind oscillation amplitudes was obtained based on the measured amplitudes of hbotF oscillations. It was indicated that increased amplitudes of thermospheric wind oscillations are obtained when this expression for hmF is used. The diffusion term, which causes increased hmF oscillation amplitudes as compared to the hbotF oscillation amplitudes, was estimated using the regression expression.

1993/1994년을 기점으로 나타난 한반도 여름철 강수량 변동의 종관기후학적 원인

In this study, an investigation has been carried out to understand 1) temporal variation of rainfall amount in summer over south Korea during the 30-year period of 1979-2008 and 2) the relationship between the variation of rainfall amount and the change of large-scale monsoon circulation around 1993/1994 over East Asia. The analysis of rainfall amount is carried out separately for whole summer (June-August), climatological Changma period of 23 June-23 July, and August to consider variations within summer. To relate the variation of rainfall amount with the change of large-scale circulation, we have considered two 15-year periods of 1979-1993 and 1994-2008. This study has used observations at 58 stations in South Korea and NCEP-NCAR <TEX>$2.5^{\circ}{\times}2.5^{\circ}$</TEX> reanalysis data. The major change in synoptic environment for the Changma period is characterized by the intensified anticyclone over Mongolia during 1994-2008, which results in a weak meridional oscillation of Changma front. As a result, rainfall amount for the Changma period and the frequency of extreme events have significantly increased after 1993/1994. A major change of synoptic environment for August is the significant westward extension of the western Pacific subtropical high, which allows not only more moisture transports but also stronger cyclonic circulation over the Korean peninsula. Rainfall amount for August and frequency of extreme events have also increased after 1993/1994. However, variability of rainfall amount is larger for August than that for the Changma period, with some years showing very dry August (monthly rainfall amount less than 150 mm).

Can oceanic reanalyses be used to assess recent anthropogenic changes and low-frequency internal variability of upper ocean temperature?

A multivariate analysis of the upper ocean thermal structure is used to examine the recent long-term changes and decadal variability in the upper ocean heat content as represented by model-based ocean reanalyses and a model-independent objective analysis. The three variables used are the mean temperature above the 14°C isotherm, its depth and a fixed depth mean temperature (250 m mean temperature). The mean temperature above the 14°C isotherm is a convenient, albeit simple, way to isolate thermodynamical changes by filtering out dynamical changes related to thermocline vertical displacements. The global upper ocean observations and reanalyses exhibit very similar warming trends (0.045°C per decade) over the period 1965–2005, superimposed with marked decadal variability in the 1970s and 1980s. The spatial patterns of the regression between indices (representative of anthropogenic changes and known modes of internal decadal variability), and the three variables associated with the ocean heat content are used as fingerprint to separate out the different contributions. The choice of variables provides information about the local heat absorption, vertical distribution and horizontal redistribution of heat, this latter being suggestive of changes in ocean circulation. The discrepancy between the objective analysis and the reanalyses, as well as the spread among the different reanalyses, are used as a simple estimate of ocean state uncertainties. Two robust findings result from this analysis: (1) the signature of anthropogenic changes is qualitatively different from those of the internal decadal variability associated to the Pacific Interdecadal Oscillation and the Atlantic Meridional Oscillation, and (2) the anthropogenic changes in ocean heat content do not only consist of local heat absorption, but are likely related with changes in the ocean circulation, with a clear shallowing of the tropical thermocline in the Pacific and Indian oceans.