Midlatitude North Pacific Research Articles

Tracing ventilation source of tropical pacific oxygen minimum zones with an adjoint global ocean transport model

To better understand the changes in the ventilation of Oxygen Minimum Zones (OMZs) in the Eastern Tropical North Pacific (ETNP) and South Pacific (ETSP), we use an adjoint global transport model to trace the surface locations of their source waters. The transit times for surface waters to reach the OMZs are obtained for steady-state transport. For the ETNP OMZ, 26% of the water volume makes last contact with the atmosphere in the Equatorial North Pacific with mean transit time of 109 years. Mid-latitude North and South Pacific make comparable volumetric contributions (17% and 18% respectively), but have mean transit times that differ by a factor of 2 (186 and 382 years, respectively). The combined contribution of the Southern Ocean and North Atlantic accounts for 16% of the ETNP OMZ volume. The mean transit time from these remote regions is in excess of 2500 years. For the ETSP OMZ, 29% of water volume originates from the surface of the equatorial South Pacific with a mean transit time of 65 years while 36% originates from the mid-latitude South Pacific with a mean transit-time of 157 years. The relative contribution of the two regions indicates that the meridional ventilation pathway from the mid- latitudes is of equal importance as the ventilation via eastward equatorial current system. The mean transit-times computed here help clarify the timescales with which advective- diffusive processes link changes in surface ocean processes to the OMZs of the Eastern Tropical Pacific.

Influence of the Boreal Summer Intraseasonal Oscillation on Extreme Temperature Events in the Northern Hemisphere

The impact of the boreal summer intraseasonal oscillation (BSISO) on extreme hot and cool events was investigated, by analyzing the observed and reanalysis data for the period from 1983 to 2012. It is found that the frequency of the extreme events in middle and high latitudes is significantly modulated by the BSISO convection in the tropics, with a 3–9-day lag. During phases 1 and 2 when the BSISO positive rainfall anomaly is primarily located over a northwest–southeast oriented belt extending from India to Maritime Continent and a negative rainfall anomaly appears in western North Pacific, the frequency of extreme hot events is 40% more than the frequency of non-extreme hot events. Most noticeable increase appears in midlatitude North Pacific (north of 40°N) and higher-latitude polar region. Two physical mechanisms are primarily responsible for the change of the extreme frequency. First, an upper-tropospheric Rossby wave train (due to the wave energy propagation) is generated in response to a negative heating anomaly over tropical western North Pacific in phases 1 and 2. This wave train consists of a strong high pressure anomaly center northeast of Japan, a weak low pressure anomaly center over Alaska, and a strong high pressure anomaly center over the western coast of United States. Easterly anomalies to the south of the two strong midlatitude high pressure centers weaken the climatological subtropical jet along 40°N, which is accompanied by anomalous subsidence and warming in North Pacific north of 40°N. Second, an enhanced monsoonal heating over South Asia and East Asia sets up a transverse monsoonal overturning circulation, with large-scale ascending (descending) anomalies over tropical Indian (Pacific) Ocean. Both the processes favor more frequent extreme hot events in higher-latitude Northern Hemisphere. An anomalous atmospheric general circulation model is used to confirm the tropical heating effect.

Inverse Cascades of Kinetic Energy as a Source of Intrinsic Variability: A Global OGCM Study

AbstractA seasonally forced 1/12° global ocean/sea ice simulation is used to characterize the spatiotemporal inverse cascade of kinetic energy (KE). Nonlinear scale interactions associated with relative vorticity advection are evaluated using cross-spectral analysis in the frequency–wavenumber domain from sea level anomaly (SLA) time series. This analysis is applied within four eddy-active midlatitude regions having large intrinsic variability spread over a wide range of scales. Over these four regions, mesoscale surface KE is shown to spontaneously cascade toward larger spatial scales—between the deformation scale and the Rhines scale—and longer time scales (possibly exceeding 10 years). Other nonlinear processes might have to be invoked to explain the longer time scales of intrinsic variability, which have a substantial surface imprint at midlatitudes. The analysis of a fully forced 1/12° hindcast shows that low-frequency and synoptic atmospheric forcing barely affects this inverse KE cascade. The inverse cascade is also at work in a 1/4° simulation, albeit with a weaker intensity, consistent with the weaker intrinsic variability found at this coarser resolution. In the midlatitude North Pacific, the spatiotemporal cascade transfers KE from high-frequency frontal Rossby waves (FRWs), probably generated by baroclinic instability, toward the lower-frequency, westward-propagating mesoscale eddy (WME) field. The WMEs provide local gradients of potential vorticity that support these short Doppler-shifted FRWs. FRWs have periods shorter than 2 months and might be subsampled by altimetric observations, perhaps explaining why the temporal inverse cascade deduced from high-resolution models and mapped altimeter products can be quite different. The nature of the nonlinear interactions between FRWs and WMEs remains unclear but might involve wave turbulence processes.

Persistence of Summer Sea Surface Temperature Anomalies in the Midlatitude North Pacific and Its Interdecadal Variability

The present study investigates the persistence of summer sea surface temperature anomalies (SSTAs) in the midlatitude North Pacific and its interdecadal variability. Summer SSTAs can persist for a long time (approximately 8–14 months) around the Kuroshio Extension (KE) region. This long persistence may be strongly related to atmospheric forcing because the mixed layer is too shallow in the summer to be influenced by the anomalies at depths in the ocean. Changes in atmospheric circulation, latent heat flux, and longwave radiation flux all contribute to the long persistence of summer SSTAs. Among these factors, the longwave radiation flux has a dominant influence. The effects of sensible heat flux and shortwave radiation flux anomalies are not significant. The persistence of summer SSTAs displays pronounced interdecadal variability around the KE region, and the variability is very weak during 1950–82 but becomes stronger during 1983–2016. The changes in atmospheric circulation, latent heat flux, and longwave radiation flux are also responsible for this interdecadal variability because their forcings on the summer SSTAs are sustained for much longer after 1982.

ENSO Transition from La Niña to El Niño Drives Prolonged Spring–Summer Drought over North China

Persistent drought is a major meteorological disaster causing vast agricultural damage and long-term regional water crises. Over north China, this type of drought tends to begin in spring and to persist until summer with severe societal impacts. This paper aims to identify the large-scale dynamic drivers of the prolonged spring–summer drought (PSSD) over north China. This analysis has shown that the north China PSSD is associated with a persistent anomalous low-level cyclonic circulation spanning the midlatitude North Pacific from spring to summer with reduced moisture transport to north China, in combination with a tropospheric cooling along the East Asian upper-level westerly jet with dynamically forced anomalous descent above. Seven of the selected eight north China PSSD events occurred when La Niña transitioned to El Niño with a negative North Pacific Oscillation (NPO) phase in the preceding winter. The two key circulation anomalies in spring are set directly by a negative NPO phase generated in the preceding winter. The NPO, in turn, forces an El Niño onset in summer through the so-called seasonal footprinting mechanism. Consequently, sea surface temperature anomalies of El Niño in summer suppress Indian monsoon rainfall, triggering the tropospheric temperature cooling over East Asia through a circumglobal teleconnection along the East Asian upper-level westerly jet. Modeling analysis of the long preindustrial control simulation confirms the above findings. This study demonstrates that ENSO phase transition from La Niña to El Niño is one of the important precursors of north China PSSD.

EnOI‐IAU Initialization Scheme Designed for Decadal Climate Prediction System IAP‐DecPreS

AbstractA decadal climate prediction system named as IAP‐DecPreS was constructed in the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, based on a fully coupled model FGOALS‐s2 and a newly developed initialization scheme, referred to as EnOI‐IAU. In this paper, we introduce the design of the EnOI‐IAU scheme, assess the accuracies of initialization integrations using the EnOI‐IAU and preliminarily evaluate hindcast skill of the IAP‐DecPreS. The EnOI‐IAU scheme integrates two conventional assimilation approaches, ensemble optimal interpolation (EnOI) and incremental analysis update (IAU). The EnOI and IAU were applied to calculate analysis increments and incorporate them into the model, respectively. Three continuous initialization (INIT) runs were conducted for the period of 1950–2015, in which observational sea surface temperature (SST) from the HadISST1.1 and subsurface ocean temperature profiles from the EN4.1.1 data set were assimilated. Then nine‐member 10 year long hindcast runs initiated from the INIT runs were conducted for each year in the period of 1960–2005. The accuracies of the INIT runs are evaluated from the following three aspects: upper 700 m ocean temperature, temporal evolution of SST anomalies, and dominant interdecadal variability modes, Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). Finally, preliminary evaluation of the ensemble mean of the hindcast runs suggests that the IAP‐DecPreS has skill in the prediction of the PDO‐related SST anomalies in the midlatitude North Pacific and AMO‐related SST anomalies in the tropical North Atlantic.

Dependence of Energy Flux from the Wind to Surface Inertial Currents on the Scale of Atmospheric Motions

AbstractAtmospheric features such as translating cold fronts and small lows with horizontal scales of about 100 km are traditionally thought to be most important in exciting near-inertial motions in the ocean. However, recent studies suggest that a significant fraction of energy flux from the wind to surface inertial currents may be supplied by atmospheric systems of larger scales. Here, the dependence of this energy flux on the scale of atmospheric motions is investigated using a high-resolution atmosphere reanalysis product and a slab model. It is found that mesoscale atmospheric systems with scales less than 1000 km are responsible for almost all the energy flux from the wind to near-inertial motions in the midlatitude North Atlantic and North Pacific. Transient atmospheric features with scales of ~100 km contribute significantly to this wind energy flux, but they are not as dominant as traditionally thought. Owing to the nonlinear nature of the stress law, energy flux from mesoscale atmospheric systems depends critically on the existence of the background, larger-scale wind field. Finally, accounting for relative motions in the stress calculation reduces the net wind energy flux to near-inertial motions by about one-fifth. Mesoscale atmospheric systems are found to be responsible for the majority of this relative wind damping effect.

A strengthened impact of November Arctic oscillation on subsequent tropical Pacific sea surface temperature variation since the late-1970s

The connection between November Arctic Oscillation (AO) and the following spring-summer tropical Pacific sea surface temperature (SST) experienced a pronounced enhancement around the late-1970s. The connection was weak before, but strong after the late-1970s. The present study investigates the plausible reasons for the connection change. After the late-1970s, significant anomalous cyclone appears over subtropical North Pacific and pronounced anomalous westerly winds occur over tropical western Pacific in the positive phase of November AO. As such, November AO could exert a significant influence on the subsequent spring-summer tropical Pacific SST variation. By contrast, before the late-1970s, anomalous cyclone and associated anomalous westerly winds to its south are weak and shift northward. Thereby, the connection between November AO and tropical Pacific SST is weak. Further analyses suggest that the interdecadal change in the anomalous cyclone over the subtropical North Pacific may be related to the change in the November AO’s Pacific component and the strength of feedback of synoptic-scale eddy to mean flow. Specifically, after the late-1970s, November AO’s Pacific component displays a zonally elongated structure, which is favorable for the formation of easterly wind anomalies over the mid-latitude North Pacific and related wave-mean flow interaction. In addition, feedback of synoptic eddy to mean flow is enhanced after the late-1970s, which is partly related to the intensification of the North Pacific storm track. This may also contribute to enhancement of the anomalous cyclone over the subtropical Pacific.

Response of theKuroshioExtension path state to near‐term global warming in CMIP5 experiments with MIROC4h

AbstractIn this study, responses of the Kuroshio Extension (KE) path state to near‐term (2006–2035) global warming are investigated using a Kuroshio‐resolving atmosphere‐ocean coupled model. Under the representative concentration pathway 4.5 (RCP4.5) forcing, the KE system is intensified and its path state tends to move northward and becomes more stable. It is suggested that the local anticyclonic wind stress anomalies in the KE region favor the spin‐up of the southern recirculation gyre, and the remote effect induced by the anticyclonic wind stress anomalies over the central and eastern midlatitude North Pacific also contributes to the stabilization of the KE system substantially. The dominant role of wind stress forcing on KE variability under near‐term global warming is further confirmed by adopting a linear 1.5 layer reduced‐gravity model forced by wind stress curl field from the present climate model. It is also found that the main contributing longitudinal band for KE index (KEI) moves westward in response to the warmed climate. This results from the northwestward expansion of the large‐scale sea level pressure (SLP) field.

The Impact of Ocean Surface Currents on Sverdrup Transport in the Midlatitude North Pacific via the Wind Stress Formulation

AbstractA more complete wind stress τn formulation takes into account the ocean surface currents Vo, while the conventional wind stress τc popularly used in ocean circulation models is only a function of 10-m winds V10. An analytical solution is derived for the difference of Sverdrup transport induced by using τn instead of τc. A scaling analysis of the analytical solution indicates a 6% reduction of the Sverdrup transport in the North Pacific (i.e., the Kuroshio transport in the East China Sea) when Ekman velocity dominates the ocean surface currents. Because of the quadratic nature of wind stress, four nonlinear terms contribute equally to this difference: two vorticity torque terms and two speed gradient torque terms. A pair of 12.5-yr (July 2002–14) Hybrid Coordinate Ocean Model simulations that only differ in the wind stress formulation are used to test the analytical solution. The model results (2004–14) confirm that using τn instead of τc reduces the Sverdrup transport in the North Pacific by 8%–17% between 23° and 32°N. The reduction rate of the simulated 11-yr mean Kuroshio transport through the East Taiwan Channel and Tokara Strait is 8.0% (−2.5 Sv; 1 Sv ≡ 106 m3 s−1) and 12.8% (−4.8 Sv), respectively, in good agreement with the Sverdrup transport reduction rate, which is 7.4% (−2.6 Sv) and 15.4% (−6.3 Sv) at the corresponding latitude. The local effect of changing wind stress/wind work and Ekman transport due to the inclusion of Vo in the wind stress formulation is negligible compared to the Kuroshio volume transport change estimated in this study.

Impact of Satellite Data Assimilation in Atmospheric Reanalysis on the Marine Wind and Wave Climate

Abstract This study investigated the impact of assimilating satellite data into atmospheric reanalyses on trends in ocean surface winds and waves. Two experiments were performed using a numerical wave model forced by near-surface winds: one derived from the Japanese 55-year Reanalysis (JRA-55; experiment A) and the other derived from JRA-55 using assimilated conventional observations only (JRA-55C; experiment B). The results showed that the satellite data assimilation reduced upward trends of the annual mean of wave energy flux (WEF) in the midlatitude North Pacific and southern ocean (30°–60°S), south of Australia, from 1959 to 2012. It was also found that the assimilation of scatterometer winds reduced the near-surface wind speed in the midlatitude North Pacific after the mid-1990s, which resulted in the reduced trend in WEF from 1959 to 2012. By contrast, assimilation of the satellite radiances for 1973–94 increased near-surface wind speed in the southern ocean, south of Australia, whereas the assimilation of the scatterometer winds after the mid-1990s reduced wind speed. The latter led to the reduced trend in WEF south of Australia from 1959 to 2012.

Interdecadal variability of early summer monsoon rainfall over South China in association with the Pacific Decadal Oscillation

ABSTRACTThe interdecadal variability of early summer South China (SC) monsoon rainfall (SCMR) is investigated based on several long‐term rainfall data sets from 1926 to 2013. Both correlation and composite analyses suggest a significant out‐of‐phase relationship between interdecadal fluctuations of SCMR and the preceding wintertime Pacific Decadal Oscillation (PDO), with negative (positive) PDO epochs favouring above‐normal (below‐normal) SCMR. The physical mechanism relating the wintertime PDO to the interdecadal variability of SCMR appears to take place via atmospheric and oceanic teleconnections. The below‐normal SCMR is characterized by a meridionally arranged vortex (MAV) pattern across the East Asia–western North Pacific (WNP) sector, with an anomalous cyclone, an anomalous anticyclone and an anomalous cyclone occurring alternately over the South China Sea (SCS)–WNP, Yangtze Basin and Lake Baikal in the early summer. This may respond to the PDO‐related positive sea surface temperature (SST) anomalies locally over the SCS–WNP and remotely over the tropical eastern Pacific, with the latter forcing anomalous Hadley and Walker circulations. SC is located directly under the divergent moisture environment of anomalous northeasterlies between the anomalous cyclone over the SCS–WNP and the anticyclone over the Yangtze Basin. Again, the below‐normal SCMR is preceded by negative (positive) SST anomalies in the midlatitude North Pacific (tropical eastern Pacific) associated with positive PDO in wintertime. Such an anomalous SST pattern subsequently induces a strengthened and northward‐shifted upper‐tropospheric jet in early summer, leading to significant upper‐tropospheric convergence over SC. The out‐of‐phase relationship between SCMR and wintertime PDO as well as related physical processes are validated by coupled model simulations.

Interdecadal modulation of ENSO-related spring rainfall over South China by the Pacific Decadal Oscillation

The interdecadal modulation of the relationship between El Nino–Southern Oscillation (ENSO) and the South China spring rainfall (SCSR) by the Pacific Decadal Oscillation (PDO) is investigated using long-term observational datasets. When ENSO and PDO are in-phase [i.e., El Nino events during warm PDO (EN_WPDO) and La Nina events during cold PDO (LN_CPDO)], the positive correlations between ENSO and SCSR are enhanced significantly, with above-normal (below-normal) SCSR generally following EN_WPDO (LN_CPDO) events. In contrast, the ENSO–SCSR relationship becomes ambiguous when ENSO and PDO are out-of-phase [i.e., El Nino events during cold PDO (EN_CPDO) and La Nina events during warm PDO (LN_WPDO)]. The PDO modulates the ENSO–SCSR relationship through the impact of variations in the lower-tropospheric subtropical anticyclone over the western North Pacific (WNP) and upper-tropospheric westerly jets over East Asia and the midlatitude North Pacific. An EN_WPDO (LN_CPDO) event induces an enhanced subtropical anticyclone (cyclone) over the WNP and intensified (weakened) subtropical westerly jet around the southern Tibetan Plateau due to modification by the PDO-forced anomalous circulation. Thus, South China falls just under the influence of the anomalous lower-tropospheric southwesterlies (northeasterlies) and upper-tropospheric divergent (convergent) environment, leading to above-normal (below-normal) SCSR. In contrast, the SCSR anomalies exhibit no wet or dry preference following EN_CPDO (LN_CPDO) events, because ENSO-induced and PDO-forced circulation anomalies tend to cancel each other out. These modulating effects by the PDO on the ENSO–SCSR relationship and related physical processes are also examined with coupled model simulations.

The interdecadal change of the leading mode of the winter precipitation over China

The interdecadal change of the leading mode of the mean winter precipitation over China has been investigated using observational data for the period from 1960 to 2012. The leading empirical orthogonal function (EOF) mode (EOF1) of the winter precipitation over China displays a mono-sign pattern over southeastern China, accounting for 49.7 % of the total variance in the precipitation. Both the El Nino-Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) can impact EOF1. A positive (negative) EOF1 is accompanied by warm (cold) ENSO events and weak (strong) EAWM, and the latters can cause anomalous southerlies (northerlies) along the coast of southeastern China, accompanied by the transportation of water vapor from the Bay of Bengal and the South China Sea favoring a wet (dry) winter over southeastern China. An abrupt transition of the EOF1 is observed around the mid-1980s. Therefore, the data are divided into two subperiods, i.e., 1960–1987 (P1) and 1988–2009 (P2). Significant differences in the large scale atmospheric circulation and sea surface temperature anomalies associated with EOF1 during these two subperiods are observed. EOF1 is closely related to the mid- to high-latitude atmospheric circulation in P1, while its relationship to the tropics obviously increases during P2. The partial regression analysis results show that the interdecadal change of EOF1 is caused by both the interdecadal changes of the EAWM and ENSO around the mid-1980s. In P1, the lower-level anomalous southerlies along the coastal southeastern China accompanied by water vapor transportation that causes above-average precipitation are related to an anti-cyclonic system centered over the mid-latitude western North Pacific associated with EAWM. In P2, the influence of the EAWM is weaker, and the southerly anomaly over the coastal southeastern China is mainly caused by the anticyclone over Philippines, which is related to the ENSO.

Structure and dynamics of decadal anomalies in the wintertime midlatitude North Pacific ocean–atmosphere system

The structure and dynamics of decadal anomalies in the wintertime midlatitude North Pacific ocean–atmosphere system are examined in this study, using the NCEP/NCAR atmospheric reanalysis, HadISST SST and Simple Ocean Data Assimilation data for 1960–2010. The midlatitude decadal anomalies associated with the Pacific Decadal Oscillation are identified, being characterized by an equivalent barotropic atmospheric low (high) pressure over a cold (warm) oceanic surface. Such a unique configuration of decadal anomalies can be maintained by an unstable ocean–atmosphere interaction mechanism in the midlatitudes, which is hypothesized as follows. Associated with a warm PDO phase, an initial midlatitude surface westerly anomaly accompanied with intensified Aleutian low tends to force a negative SST anomaly by increasing upward surface heat fluxes and driving southward Ekman current anomaly. The SST cooling tends to increase the meridional SST gradient, thus enhancing the subtropical oceanic front. As an adjustment of the atmospheric boundary layer to the enhanced oceanic front, the low-level atmospheric meridional temperature gradient and thus the low-level atmospheric baroclinicity tend to be strengthened, inducing more active transient eddy activities that increase transient eddy vorticity forcing. The vorticity forcing that dominates the total atmospheric forcing tends to produce an equivalent barotropic atmospheric low pressure north of the initial westerly anomaly, intensifying the initial anomalies of the midlatitude surface westerly and Aleutian low. Therefore, it is suggested that the midlatitude ocean–atmosphere interaction can provide a positive feedback mechanism for the development of initial anomaly, in which the oceanic front and the atmospheric transient eddy are the indispensable ingredients. Such a positive ocean–atmosphere feedback mechanism is fundamentally responsible for the observed decadal anomalies in the midlatitude North Pacific ocean–atmosphere system.

134Cs and 137Cs in the North Pacific Ocean derived from the March 2011 TEPCO Fukushima Dai-ichi Nuclear Power Plant accident, Japan. Part one: surface pathway and vertical distributions

Activities of radiocaesium released by the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident were measured by surface sampling at 408 stations and in vertical profiles at 24 stations in the North Pacific Ocean, and time-series samples were collected at two coastal stations. After July 2012, 137Cs activity in the surface water near FNPP1 remained around 1000 Bq m−3, which corresponds to a discharge rate of about 10 GBq day−1. FNPP1-derived radiocaesium spread eastward in surface water across the mid-latitude North Pacific with a speed of 7 km day−1 (8 cm s−1) until March 2012, and of 3 km day−1 (3.5 cm s−1) from March 2012 through August 2014. In June 2012, 134Cs activity reached a maximum of 6.12 ± 0.50 Bq m−3 at a 151-m depth (potential density, σ θ = 25.3 kg m−3) at 29°N, 165°E. This subsurface maximum, which was also observed along 149°E, might reflect the southward transport of FNPP1-derived radiocaesium in association with the formation and subduction of subtropical mode water (STMW). In June 2012 at 34°N–39°N along 165°E, 134Cs activity showed a maximum at around σ θ = 26.3 kg m−3, which corresponds to central mode water (CMW). 134Cs activity was higher in CMW than in any of the surrounding waters, including STMW. These observations also indicate that the most effective pathway by which FNPP1-derived radiocaesium is introduced into the ocean interior on a 1-year time scale is CMW formation and subduction.

Different responses of Sea Surface Temperature in the North Pacific to greenhouse gas and aerosol forcing

The responses of Sea Surface Temperature (SST) to greenhouse gas (GHG) and anthropogenic aerosol in the North Pacific are compared based on the historical single and all-forcing simulations with Geophysical Fluid Dynamics Laboratory Climate Model version 3 (GFDL CM3). During 1860–2005, the effect of GHG forcing on the North Pacific SST is opposite to that of the aerosol forcing. Specifically, the aerosol cooling effect exceeds the GHG warming effect in the Kuroshio Extension (KE) region during 1950–2004 in the CM3 single forcing. The mid-latitude response of ocean circulation to the GHG (aerosol) forcing is to enhance (weaken) the Subtropical Gyre. Then the SST warming (cooling) lies on the zonal band of 40°N because of the increased (reduced) KE warm advection effect in the GHG (aerosol) forcing simulations, and the cooling effect to SST will surpass the warming effect in the KE region in the historical all-forcing simulations. Besides, the positive feedback between cold SST and cloud can also strengthen the aerosol cooling effect in the KE region during boreal summer, when the mixed layer depth is shallow. In the GHG (aerosol) forcing simulations, corresponding to warming (cooling) SST in the KE region, the weakened (enhanced) Aleutian Low appears in the Northeast Pacific. Consequently, the SST responses to all-forcing in the historical simulations are similar to the responses to aerosol forcing in sign and spatial pattern, hence the aerosol effect is quite important to the SST cooling in the mid-latitude North Pacific during the past 55 years.

Multiyear predictability of Northern Hemisphere surface air temperature in the Kiel Climate Model

The multiyear predictability of Northern Hemisphere surface air temperature (SAT) is examined in a multi-millennial control integration of the Kiel Climate Model, a coupled ocean–atmosphere–sea ice general circulation model. A statistical method maximizing average predictability time (APT) is used to identify the most predictable SAT patterns in the model. The two leading APT modes are much localized and the physics are discussed that give rise to the enhanced predictability of SAT in these limited regions. Multiyear SAT predictability exists near the sea ice margin in the North Atlantic and mid-latitude North Pacific sector. Enhanced predictability in the North Atlantic is linked to the Atlantic Multidecadal Oscillation and to the sea ice changes. In the North Pacific, the most predictable SAT pattern is characterized by a zonal band in the western and central mid-latitude Pacific. This pattern is linked to the Pacific Decadal Oscillation, which drives sea surface temperature anomalies. The temperature anomalies subduct into deeper ocean layers and re-emerge at the sea surface during the following winters, providing multiyear memory. Results obtained from the Coupled Model Intercomparison Project Phase 5 ensemble yield similar APT modes. Overall, the results stress the importance of ocean dynamics in enhancing predictability in the atmosphere.

Effect of seasonal change in gas transfer coefficient on air–sea CO2 flux in the western North Pacific

We used an eddy-permitting three-dimensional ocean ecosystem model and applied it in the western North Pacific to understand the seasonal variations and horizontal distributions of the air–sea CO2 flux and difference in the partial pressure between sea water and the atmosphere (∆pCO2). The high-resolution model reproduced the observed zonal belt of strong CO2 uptake in the mid-latitude (30–45°N) western North Pacific including the Kuroshio extension and mixed water regions, which was difficult to show in previous coarse-resolution models. The East Asian winter monsoon, an important phenomenon in the western North Pacific, affects the seasonal CO2 air–sea gas exchange with a high (low) gas transfer coefficient in winter (summer). In the subtropical region, ∆pCO2 is negative in winter and positive in summer as a result of the temperature effect. Combination of seasonal change in gas transfer coefficient with ∆pCO2 suppresses CO2 release in the subtropical region, and vice versa in the subarctic region (i.e., suppresses CO2 uptake). That is, the East Asian winter monsoon in the western North Pacific contributes to the reduction of the annual CO2 flux through the seasonal change in the gas transfer coefficient, leading to an overall annual CO2 uptake in the subtropical region and CO2 release in the subarctic region.

Interannual variability of the Asian subtropical westerly jet in boreal summer and associated with circulation and SST anomalies

The interannual variability of the Asian Subtropical Westerly Jet (ASWJ) in boreal summer is investigated through the diagnosis of 54-year (1960–2013) NCEP/NCAR reanalysis data. The main characteristics of two leading empirical orthogonal function patterns of 200 hPa zonal wind anomalies are the meridional displacement and southwest–northeast tilting of ASWJ. The first leading mode has significant periods of 4.9 years, whereas the second mode has significant periods of 3.6 and 7.7 years, respectively. The two modes exhibit an equivalent barotropic structure, and are associated with a distinctive north–south and east–west dipole rainfall pattern in China, respectively. The positive phase of the first leading mode appears during El Nino developing phase, whereas the positive phase of the second mode occurs during La Nina decaying phase. A mechanism is put forth based on observational analysis and AGCM sensitivity experiments. The positive phase of the first mode is primarily driven by the combined effect of a cold SST anomaly (SSTA) in mid-latitude North Pacific and a warm SSTA in tropical Indian Ocean and Pacific. In response to the SSTA forcing, a zonally oriented north–south tropospheric temperature dipole is induced. While the tropospheric warming in the tropics arises from El Nino like heating, the tropospheric cooling in the mid-latitudes arises possibly from the local SSTA forcing. For the positive phase of the second mode, the upper-tropospheric anticyclonic vorticity anomaly in the east pole arises from local SSTA forcing in North Pacific, whereas the cyclonic anomaly in the west pole results from southeastward Rossby wave energy emanation from North Atlantic to East Asia.