Asymmetry of Salinity Variability in the Tropical Pacific during Interdecadal Pacific Oscillation Phases

It has been recognized that salinity variability in the tropical Pacific is closely related to the Interdecadal Pacific Oscillation(IPO).Here,we use model simulations from 1900 to 2017 to illustrate obvious asymmetries of salinity variability in the tropical Pacific during positive and negative IPO phases.The amplitude of salinity variability in the tropical Pacific during positive IPO phases is larger than that during negative IPO phases,with a more westward shift of a large Sea Surface Salinity(SSS)anomaly along the equator.Salinity budget analyses show that the asymmetry of salinity variability during positive and negative IPO phases is dominated by the difference in the surface forcing associated with the freshwater flux[FWF,precipitation(P)minus evaporation(E)],with a contribution of 40%–50%near the dateline on the equator.Moreover,the relationships between the salinity variability and its budget terms also show differences in their leadlag correlations during positive and negative IPO phases.These differences in salinity variability during different IPO phases produce asymmetric effects on seawater density which can reduce or enhance upper-ocean stratification.Therefore,the salinity effects may modulate the intensity of El Nino-Southern Oscillation(ENSO),resulting in an enhanced(reduced)El Nino but a reduced(enhanced)La Ni?a during positive(negative)IPO phases by 1.6℃psu^(-1)(1.3℃psu^(-1)),respectively.It is suggested that the asymmetry of salinity variability may be related to the recent change in ENSO amplitude associated with the IPO,which can help elucidate ENSO diversity.

Comparison of multiple salinity datasets:upper ocean salinity and stratification in the tropical Pacific during the Argo period

Ocean salinity is an important variable that affects the ocean stratification.We compared the salinity and ocean stratification in the tropical Pacific derived from the Argo(Array for Real-time Geostrophic Oceanography data),EN4(Ensemble 4 analysis),SODA(the Simple Ocean Data Assimilation reanalysis),IAP(Institute of Atmospheric Physics data),and ORAS4(Ocean Reanalysis System 4)over 2005–2017.Results show that the spatial distribution of climatological mean of sea surface salinity(SSS)in all the products is consistent,and the low salinity region showed large deviation and strong dispersion.The Argo has the smallest RMSE and the highest correlation with the ensemble mean,while the IAP shows a high-salinity deviations relative to other datasets.All the products show high positive correlations between the sea surface density(SSD)and SSS with respect to the deviations of climatological mean from ensemble mean,suggesting that the SSD deviation may be mainly influenced by the SSS deviation.In the aspect of the ocean stratification,the mixed layer depth(MLD)climatological mean in the Argo shows the highest correlation with the ensemble mean,followed by EN4,IAP,ORAS4,and SODA.The Argo and EN4 show thicker barrier layer(BL)relative to the ensemble mean while the SODA displays the largest negative deviation in the tropical western Pacific.Furthermore,the EN4,ORAS4,and IAP underestimate the stability in the upper ocean at the depths of 20–140 m,while Argo overestimates ocean stability.The salinity fronts in the western-central equatorial Pacific from Argo,EN4,and ORAS4 are consistent,while those from SODA and IAP show large deviations with a westward position in amplitude of 0°–6°and 0°–10°,respectively.The SSS trend patterns from all the products are consistent in having ensemble mean with high spatial correlations of 0.95–0.97.

Numerical Simulation of the Relationship between the Anomaly of Subtropical High over East Asia and the Convective Activities in the Western Tropical Pacific

In this paper, a close relationship between the intraseasonal variation of subtropical high over East Asia and the convective activities around the South China Sea and the Philippines is analysed from OLR data.This relationship is studied by using the theory of wave propagating in a slowly varying medium and by using a quasi-geoslrophic, linear, spherical model and the IAP-GCM, respectively. The results show that when the SST is warming around the western tropical Pacific or the Philippines, the convective activities are intensified around the Philippines. As a consequence, the subtropical high will be intensified over East Asia. The computed results also show that when the anomaly of convective activities are caused around the Philippines, a teleconnection pattern of circulation anomalies will be caused over South Asia, East Asia and North America.

Evaluation on data assimilation of a global high resolution wave-tide-circulation coupled model using the tropical Pacific TAO buoy observations

In order to evaluate the assimilation results from a global high resolution ocean model, the buoy observations from tropical atmosphere ocean（TAO） during August 2014 to July 2015 are employed. The horizontal resolution of wave-tide-circulation coupled ocean model developed by The First Institute of Oceanography（FIOCOM model） is 0.1°×0.1°, and ensemble adjustment Kalman filter is used to assimilate the sea surface temperature（SST）, sea level anomaly（SLA） and Argo temperature/salinity profiles. The simulation results with and without data assimilation are examined. First, the overall statistic errors of model results are analyzed. The scatter diagrams of model simulations versus observations and corresponding error probability density distribution show that the errors of all the observed variables, including the temperature, isotherm depth of 20°C（D20）, salinity and two horizontal component of velocity are reduced to some extent with a maximum improvement of 54% after assimilation. Second, time-averaged variables are used to investigate the horizontal and vertical structures of the model results. Owing to the data assimilation, the biases of the time-averaged distribution are reduced more than70% for the temperature and D20 especially in the eastern Pacific. The obvious improvement of D20 which represents the upper mixed layer depth indicates that the structure of the temperature after the data assimilation becomes more close to the reality and the vertical structure of the upper ocean becomes more reasonable. At last,the physical processes of time series are compared with observations. The time evolution processes of all variables after the data assimilation are more consistent with the observations. The temperature bias and RMSE of D20 are reduced by 76% and 56% respectively with the data assimilation. More events during this period are also reproduced after the data assimilation. Under the condition of strong 2014/2016 El Ni？o, the Equatorial Undercurrent（EUC） from the TAO is gradually increased during August to November in 2014, and followed by a decreasing process. Since the improvement of the structure in the upper ocean, these events of the EUC can be clearly found in the assimilation results. In conclusion, the data assimilation in this global high resolution model has successfully reduced the model biases and improved the structures of the upper ocean, and the physical processes in reality can be well produced.

Anomalous pattern of ocean heat content during different phases of the solar cycle in the tropical Pacific

Solar radiation is a forcing of the climate system with a quasi-11-year period.As a quasi-period forcing,the influence of the phase of the solar cycle on the ocean system is an interesting topic of study.In this paper,the authors investigate a particular feature,the ocean heat content（OHC）anomaly,in different phases of the total solar irradiance（TSI） cycle.The results show that almost opposite spatial patterns appear in the tropical Pacific during the ascending and declining phases of the TSI cycle.Further analysis reveals the presence of the quasi-decadal（11-year） solar signal in the SST,OHC and surface zonal wind anomaly field over the tropical Pacific with a high level of statistical confidence（95%）.It is noted that the maximum centers of the ocean temperature anomaly are trapped in the upper ocean above the main pycnocline,in which the variations of OHC are related closely with zonal wind and ocean currents.

Interannual variability of mixed layer depth and heat storage of upper layer in the tropical Pacific Ocean

By using the upper layer data(downloaded from the web of the Scripps Institution of Oceanography),the interannual variability of the heat storage of upper layer(from surface to 400 m depth) and the mixed layer depth in the tropical Pacific Ocean are investigated. The abnormal signal of the warm event comes from the central and west Pacific Ocean, whereas it is regarded that the abnormal signal of the warm event comes from the east Pacific Ocean in the popular viewpoint. From the viewpoint on the evolution of the interannual variability of the mixed layer depth and the heat storage of the whole upper layer, the difference between the two types of El Nino is so small that it can be neglected. During these two El Nino/La Nina events(1972/1973 and 1997/1998), other than the case of the heat storage or for the mixed layer depth, the abnormal signal propagates from the central and west Pacific Ocean to the east usually by the path along the equator whereas the abnormal signal propagates from the east to the west by the path northern to the equator. For the interannual variability, the evolution of the mixed layer depth corresponds to that of the heat storage in the upper layer very well. This is quite different from the evolution of seasonality.

Spatial and temporal variability of heat content above the thermocline in the tropical Pacific Ocean

Heat content of the upper layer above the 20℃ isotherm in the tropical Pacific Ocean isestimated by using the sea temperature data set with a resolution 2°latitude×5°longitude (1980-1993) for the water depths (every 10 m) from 0 m to 400 m, and its temporal and spatial variabilities are analyzed. (1) The temporal variability indicates that the total heat in the upper layer of the equatorial Pacific Ocean is characterized by the interannual variability. The time series of the equatorial heat anomaly 5 months lead that of the El Nino index at the best positive lag correlation between the two, and the former 13 months lag behind the latter at their best negative lag correlation. Therefore the equatorial heat content can be used as a better predictor than the El Nino index for a warm or cold event. In addition, it is also found that less heat anomaly in the equator corresponds to the stronger warm events in the period (1980- 1993) and much more heat was accumulated in the 4 years including 1992/1993 ENSO (1989-1993) than the 4 years including 1982/1983 ENSO (1980-1983); (2) The spatial variability indicates that the area with the highest lag correlation among the grids moves in an anti-clockwise circle in the northern tropical Pacific Ocean within 4 years period and in a clockwise circle in the southern tropical Pacific Ocean. This result provides scientific evidence for the quasi - cycle theory of El Nino events.

Simulation of Salinity Variability and the Related Freshwater Flux Forcing in the Tropical Pacific: An Evaluation Using the Beijing Normal University Earth System Model(BNU-ESM)

The climatology and interannual variability of sea surface salinity （SSS） and freshwater flux （FWF） in the equatorial Pacific are analyzed and evaluated using simulations from the Beijing Normal University Earth System Model （BNU-ESM）. The simulated annual climatology and interannual variations of SSS, FWF, mixed layer depth （MLD）, and buoyancy flux agree with those observed in the equatorial Pacific. The relationships among the interannual anomaly fields simulated by BNU-ESM are analyzed to illustrate the climate feedbacks induced by FWF in the tropical Pacific. The largest interannual variations of SSS and FWF are located in the western-central equatorial Pacific. A positive FWF feedback effect on sea surface temperature （SST） in the equatorial Pacific is identified. As a response to El Nino-Southern Oscillation （ENSO）, the interannual variation of FWF induces ocean processes which, in turn, enhance ENSO. During El Nino, a positive FWF anomaly in the western-central Pacific （an indication of increased precipitation rates） acts to enhance a negative salinity anomaly and a negative surface ocean density anomaly, leading to stable stratification in the upper ocean. Hence, the vertical mixing and entrainment of subsurface water into the mixed layer are reduced, and the associated E1 Nino is enhanced. Related to this positive feedback, the simulated FWF bias is clearly reflected in SSS and SST simulations, with a positive FWF perturbation into the ocean corresponding to a low SSS and a small surface ocean density in the western-central equatorial Pacific warm pool.

Review on observational studies of western tropical Pacific Ocean circulation and climate

The Western Tropical Pacific(WTP) Ocean holds the largest area of warm water(>28℃) in the world ocean referred to as the Western Pacific Warm Pool(WPWP),which modulates the regional and global climate through strong atmospheric convection and its variability.The WTP is unique in terms of its complex 3-D ocean circulation system and intensive multiscale variability,making it crucial in the water and energy cycle of the global ocean.Great advances have been made in understanding the complexity of the WTP ocean circulation and associated climate impact by the international scientific community since the 1960 s through field experiments.In this study,we review the evolving insight to the 3-D structure and multi-scale variability of the ocean circulation in the WTP and their climatic impacts based on in-situ ocean observations in the past decades,with emphasis on the achievements since 2000.The challenges and open que stions remaining are reviewed as well as future plan for international study of the WTP ocean circulation and climate.

Oceanic Origin of A Recent La Nin a-Like Trend in the Tropical Pacific

Global ocean temperature has been rising since the late 1970s at a speed unprecedented during the past century of recordkeeping.This accelerated warming has profound impacts not only on the marine ecosystem and oceanic carbon uptake but also on the global water cycle and climate.During this rapid warming period,the tropical Pacific displays a pronounced La Nin a-like trend,characterized by an intensification of west-east SST gradient and of atmospheric zonal overturning circulation,namely the Walker circulation.This La Nin a-like trend differs from the El Nin o-like trend in warm climate projected by most climate models,and cannot be explained by responses of the global water cycle to warm climate.The results of this study indicate that the intensification of the zonal SST gradient and the Walker circulation are associated with recent strengthening of the upper-ocean meridional overturning circulation.

The Tropical Pacific–Indian Ocean Associated Mode Simulated by LICOM2.0

Oceanic general circulation models have become an important tool for the study of marine status and change. This paper reports a numerical simulation carried out using LICOM2.0 and the forcing field from CORE. When compared with SODA reanalysis data and ERSST.v3 b data, the patterns and variability of the tropical Pacific–Indian Ocean associated mode(PIOAM) are reproduced very well in this experiment. This indicates that, when the tropical central–western Indian Ocean and central–eastern Pacific are abnormally warmer/colder, the tropical eastern Indian Ocean and western Pacific are correspondingly colder/warmer. This further confirms that the tropical PIOAM is an important mode that is not only significant in the SST anomaly field, but also more obviously in the subsurface ocean temperature anomaly field. The surface associated mode index(SAMI) and the thermocline(i.e., subsurface) associated mode index(TAMI) calculated using the model output data are both consistent with the values of these indices derived from observation and reanalysis data. However, the model SAMI and TAMI are more closely and synchronously related to each other.

Variability in Latent Heat Flux over the Tropical Pacific in Association with Recent Two ENSO Events

This paper analyzed the variations of latent heat flux (LHF) over the tropical Pacific in the period 1978-1988 by using COADS (Comprehensive Ocean and Atmospheric Data Set). It has been founded that the interannual variabili ty of LHF exhibits strong ENSO signal, with the significant increasing LHF during the recent two warm events, i.e., 1982 / 83 and 1986 / 87 and decreasing LHF in the cold episodes. However the longitudinal distribution of the LHF departures varies from event to event. In the eastern Pacific, the specific humidity difference at air-sea interface (qs -qa) makes a dominant contribution to the interannual variability of LHF ( r = 0.73 ), while in the western Pacific the surface wind speed, W and the qs - qa make nearly equal contribution to that of LHF.

Variabilities of Surface Current in the Tropical Pacific Ocean

The Simple Ocean Data Assimilation (SODA) package is used to better understand the variabilities of surface current transport in the Tropical Pacific Ocean from 1950 to 1999. Seasonal variation, interannual and decadal variability analyses are conducted on the three major surface currents of the Tropical Pacific Ocean: the North Equatorial Current (NEC), the North Equatorial Countercurrent (NECC), and the South Equatorial Current (SEC). The transport of SEC is quite larger than those of NEC and NECC. The SEC has two maximums in February and August. The NEC has a small annual variation. The NECC has a maximum in October and is very weak in March and April. All currents have remarkable interannual and decadal variabilities. The variabilities of the NEC and the SEC relate to the winds over them well, but the relationship between the NECC and the wind over it is not close. Analysis related to El Nio-Southern Oscillation (ENSO) suggests that before El Nio (La Nia) the SEC is weaker (stronger) and the NECC is stronger (weaker), after El Nio (La Nia) the SEC is stronger (weaker) and the SEC is weaker (stronger). There is no notable relationship between the NEC and ENSO.

Topology-based analysis of pelagic food web structure in the central and eastern tropical Pacific Ocean based on longline observer data

The tropical Pacific Ocean supports many productive commercial fisheries.However,few studies of ecosystem structure in the tropical Pacific Ocean have been carried out.In this study,we analyzed the food web structure in the central and eastern tropical Pacific Ocean based on trophic relationships of 35 pelagic species collected by Chinese tuna longline observers from June to November in 2017.Topology indices(node degree,D;centrality indices,BC and CC;topological importance indices,TI^1,TI^3;keystone indices,K,K_t and K_b)and Key-Player algorithms(KPP-1,KPP-2)were used to select key species and construct a simplified food web combined with body size data.The Kendall rank correlation and hierarchical clustering analysis indicated that different topology indices resulted in consistent rankings of key species.Most key species were the same as those selected in other studies in the Pacific Ocean,such as Shortbill spearfish(Tetrapturus angustirostris),Swordfish(Xiphias gladius),Albacore tuna(Thunnus alalunga),cephalopods and scomber.The food web would be separated into many unconnected parts(F=0.632,FD=0.795,R^D=0.957)after the removal of the five key species,indicating the key roles of these species in the food web structure and stability.Body size was considered an influential indicator in constructing the simplified food web.This study can improve our understanding of the food web structure in the tropical Pacific Ocean and provide scientific basis for further ecosystem dynamics studies.

Interannual Thermocline Signals and El Nio-La Nia Turnabout in the Tropical Pacific Ocean

One of the fundamental questions concerning the nature and prediction of the oceanic states in the equatorial eastern Pacific is how the turnabout from a cold water state （La Nino） to a warm water state （El Nino） takes place, and vice versa. Recent studies show that this turnabout is directly linked to the interannual thermocline variations in the tropical Pacific Ocean basin. An index, as an indicator and precursor to describe interannual thermocline variations and the turnabout of oceanic states in our previous paper （Qian and Hu, 2005）, is also used in this study. The index, which shows the maximum subsurface temperature anomaly （MSTA）, is derived from the monthly 21-year （1980-2000） expendable XBT dataset in the present study. Results show that the MSTA can be used as a precursor for the occurrences of E1 Nino （or La Nino） events. The subsequent analyses of the MSTA propagations in the tropical Pacific suggest a one-year potential predictability for E1 Nino and La Nino events by identifying ocean temperature anomalies in the thermocline of the western Pacific Ocean. It also suggests that a closed route cycle with the strongest signal propagation is identified only in the tropical North Pacific Ocean. A positive （or negative） MSTA signal may travel from the western equatorial Pacific to the eastern equatorial Pacific with the strongest signal along the equator. This signal turns northward along the tropical eastern boundary of the basin and then moves westward along the north side of off-equator around 16°N. Finally, the signal returns toward the equator along the western boundary of the basin. The turnabout time from an E1 Nino event to a La Nino event in the eastern equatorial Pacific depends critically on the speed of the signal traveling along the closed route, and it usually needs about 4 years. This finding may help to predict the occurrence of the E1 Nino or La Nino event at least one year in advance.

Tropical Pacific Decadal Oscillation in Subsurface Ocean Temperature

This study utilizes a new monthly-assimilated sea temperature and analyzes trend and decadal oscillations in tropical Pacific 100 200 m subsurface ocean temperature (SOT) from 1945 to 2005 on the basis of the harmonic analysis and Empirical Orthogonal Function (EOF) methods.Significant cooling trends in the SOT in the tropical western Pacific were found over this 60-year period.The first EOF of the SOT in tropical Pacific displays an ENSO-like zonal dipole pattern on decadal time scale,and we considered this pattern in subsurface ocean temperature the tropical Pacific decadal oscillation (TPDO).Our analysis suggests that TPDO is closely correlated with the Pacific decadal oscillation (PDO) in the surface sea temperature (SST).The correlation coefficient between the indices of TPDO and PDO is +0.81 and reaches a maximum of +0.84 when TPDO lags behind PDO by 2 months.Therefore,a change of TPDO is likely related to the variation of PDO.The long-term change in TPDO best explains decadal warming in the tropical eastern Pacific SST and implies potential impact on the weakening of East Asian summer monsoons after the late 1970s.

Seasonal variability of zonal heat advection in the mixed layer of the tropical Pacific

Zonal heat advection （ZHA） plays an important role in the variability of the thermal structure in the tropical Pacific Ocean, especially in the western Pacific warm pool （WPWP）. Using the Simple Ocean Data Assimilation （SODA） Version 2.02/4 for the period 1958-2007, this paper presents a detailed analysis of the climatological and seasonal ZHA in the tropical Pacific Ocean. Climatologically, ZHA shows a zonal- band spatial pattern associated with equatorial currents and contributes to forming the irregular eastern boundary of the WPWP （EBWP）. Seasonal variation of ZHA with a positive peak from February to July is most prominent in the Nifio3.4 region, where the EBWP is located. The physical mechanism of the seasonal cycle in this region is examined. The mean advection of anomalous temperature, anomalous advection of mean temperature and eddy advection account for 31%, 51%, and 18% of the total seasonal variations, respectively. This suggests that seasonal changes of the South Equatorial Current induced by variability of the trade winds are the dominant contributor to the anomalous advection of mean temperature and hence, the seasonality of ZHA. Heat budget analysis shows that ZHA and surface heat flux make comparable contributions to the seasonal heat variation in the Nifio3.4 region, and that ZHA cools the upper ocean throughout the calendar year except in late boreal spring. The connection between ZHA and EBWP is further explored and a statistical relationship between EBWP, ZHA and surface heat flux is established based on least squares fitting.

Altimetric Data Assimilation by EnOI and 3DVAR in a Tropical Pacific Model:Impact on the Simulation of Variability

When altimetric data is assimilated, 3DVAR and Ensemble Optimal Interpolation （EnOI） have different ways of projecting the surface information downward. In 3DVAR, it is achieved by minimizing a cost function relating the temperature, salinity, and sea level. In EnOI, however, the surface information is propagated to other variables via a stationary ensemble. In this study, the differences between the two methods were compared and their impacts on the simulated variability were evaluated in a tropical Pacific model. Sea level anomalies （SLA） from the TOPEX/Poseidon were assimilated using both methods on data from 1997 to 2001 in a coarse resolution model. Results show that the standard deviation of sea level was improved by both methods, but the EnOI was more effective in the central/eastern Pacific. Meanwhile, the SLA evolution was better reproduced with EnOI than with 3DVAR. Correlations of temperature with the reanalysis data increased with EnOI by 0.1 0.2 above 200 m. In the eastern Pacific below 200 m, the correlations also increased by 0.2. However, the correlations decreased with 3DVAR in many areas. Correlations with the independent TAO profiles were also compared at two locations. While the correlations increased by up to 0.2 at some depths with EnOI, 3DVAR generally reduced the correlations by 0.1 0.3. Though both methods were able to reduce the model-data difference in climatological sense, 3DVAR appears to have degraded the simulated variability, especially during E1 Nifio-Southern Oscillation events. For salinity, similar results were found from the correlations. This tendency should be considered in future SLA assimilations, though the comparisons may vary among different model implementations.

NUMERICAL SIMULATION OF INTERANNUAL AND INTERDECADAL VARIABILITY OF SURFACE WIND OVER THE TROPICAL PACIFIC

A global atmospheric general circulation model (L9R15 AGCMs) forced by COADS SST was integrated from 1945 to 1993. Interannual and interdecadal variability of the simulated surface wind over the tropical Pacific was analyzed and shown to agree vey well with observation. Simulation of surface wind over the central-western equatorial Pacific was more successful than that over the eastern Pacific. Zonal propagating feature of interannual variability of the tropical Pacific wind anomalies and its decadal difference were also simulated successfully. The close agreement between simulation and observation on the existence of obvious interdecadal variability of tropical Pacific surface wind attested to the high simulation capability of AGCM.

Development Processes of the Tropical Pacific Meridional Mode

Mechanisms for the spatio-temporal development of the Tropical Pacific Meridional Mode （TPMM） are investigated using a coupled ocean-atmosphere model and observations. In both observations and the model, this meridional mode displays decadal variations and is most pronounced in spring and early summer. The model simulation suggests that once SST anomalies in the subtropical northeastern Pacific are initiated, say by northeasterly trade wind variability, perturbations evolve into a merdional dipole in 2 -3 months. A wind-evaporative-SST feedback causes a southwestward propagation of initial subtropical SST anomalies, while anomalous equatorial upwelling helps form the southern lobe of the meridional dipole. The TPMM development is a fast process （a few months） and depends on the seasonal cycle.