Characteristics of Semi-diurnal Internal Tides in the Western Equatorial Pacific Ocean Around 1°45′S,156°E

The analyses of a data series obtained during TOGA- COARE show the existence of remarkable semi-diurnal intemal tides in the western equatorial Pacific Ocean around 1°45＇S, 156°E. Some characteristic parameters of the internal tides are vertical wavenumber -1.6×10^-3 m^-1, horizontal wavenumber （wavelength） 3.3×10^-2 km^-1 （210 km）, vertical propagation speed -3.8 cm/s and horizontal propagation speed 2.0 m/s. The waveforms propagate downwards slantingly, that is, the wave energy transfers upwards slantingly. Depth-distribution of the＇rotary spectral levels is a saddle-shape. The depths of the trough and the deeper peaks are almost coincident with those of the south boundaries of the South Equatorial Current and the Equatorial Undercurrent, respectively. The mean orientation of the rotary spectral ellipse changes with depth： 30° from north to east at 40 m, and changes into 14° from east to south at 324 m, and generally, it points to northeastward, which indicates ＂that waves come from the southwest.

ROLE OF EQUATORIAL PACIFIC OCEAN SUBSURFACE OCEANIC TEMPERATURE MODE IN ENSO CYCLE

Based on the simple ocean data assimilation(SODA) reanalysis dataset from the University of Maryland and the method of Empirical Orthogonal Functions(EOF),the characteristics of interannual and interdecadal variabilities of the equatorial Pacific subsurface oceanic temperature anomaly(SOTA) are captured.The first and second modes of the equatorial Pacific SOTA in the interannual and interdecadal variations are found respectively and the effect of the second mode on the ENSO cycle is discussed.Results show that the first mode of SOTA's interannual and interdecadal variabilities exhibit a dipole pattern,indicating that the warm and cold temperature anomalies appear simultaneously in the equatorial subsurface Pacific.The second mode shows coherent large-scale temperature anomalies in the equatorial subsurface Pacific,which is a dominant mode in the evolution of ENSO cycle.The temporal series of the second mode has a significant lead correlation with the Ni?o-3.4 index,which can make a precursory prediction signal for ENSO.The function of this prediction factor in SOTA is verified by composite and case analyses.

Dimethyl sulfide in the atmospheric surface layer of the Equatorial Pacific Ocean

This paper reports a case study of atmospheric stability effect on dimethyl sulfide(DMS) concentration in the air. Investigation includes model simulation and field measurements over the Pacific Ocean. DMS concentration in surface sea water and in the air were measured during a research cruise from Hawaii to Tahiti. The diurnal variation of air temperature over the sea surface differed from the diurnal cycle of sea surface temperature because of the high heat capacity of sea water. The diurnal cycle of average DMS concentration in the air was studied in relation to the atmospheric stability parameter and surface heat flux. All these parameters had minima at noon and maxima in the early morning. The correlation coefficient of the air DMS concentration with wind speed (at 15 m high) was 0. 64. The observed concentrations of DMS in the equatorial marine surface layer and their diurnal variability agree well with model simulations. The simulated results indicate that the amplitude of the cycle and the mean concentration of DMS are dependent on the atmospheric stratifications and wind speed.

Effects of horizontal mixing on the upper ocean temperature in the equatorial Pacific Ocean

The influence of horizontal mixing on the thermal structure of the equatorial Pacific Ocean is examined based on a sigma coordinate model. In general, the distributions of the temperature and currents simulated by the sigma coordinate model are very close to the climatology. However, the simulated thermocline along the equator is slightly diffusive so that there is a cold bias above the main thermocline, while there is a warm bias under the main thermocline. Both horizontal diffusivity and viscosity have important effects on the upper thermal structure in the equatorial Pacific Ocean, while their detailed dynamics are different. Horizontal diffusivity affects the thermal structure in the upper ocean mainly through regulating the vertical diffusivity, while the horizontal viscosity does mainly through regulating directly the circulate system. A large horizontal diffusivity or a small horizontal viscosity can be in favor of simulating a more realistically thermal structure in the equatorial Pacific Ocean.

The monsoon and El Nio events during the last glaciation as recorded in core MD98-2182 from the western equatorial Pacific Ocean

Based on an analysis of δ18O and δ13C, the planktonic foraminiferal assemblage, and sedimentary grain size, and the study of multiproxy data such as paleothermocline depth, paleoproductivity, and paleosalinity during the last glaciation in core MD98-2182 from the central equatorial western Pacific warm pool, together with the data from other sites in this region, we found that sedimentary rates, terrestrial sediment grain size distribution, and paleoproductivity were markedly influenced by glacial-interglacial sea level changes in the study area. In late Marine Isotope Stage 3 （MIS3）, which was affected by glaciation and southeast summer monsoon action restricted by precession-modulated insolation, sedimentary rates and the coarse grain content of the terrestrial sediment in the study area were the highest in the past -38 ka BP, and paleoproductivity was higher during late MIS3 than the Last Glacial Maximum. A La Nifia state dominated in the two intervals of late MIS3 and the early Holocene, and the thermocline deepened. Correlated with the strong East Asian winter monsoon in the glacial period, an El Nifio state dominated during MIS2, the thermocline shoaled, and the sea surface salinity （SSS） trended lower from the west to the east owing to the precipitation zone as the locus of atmospheric convection shifted eastward.

Geochemical characteristics of sediments from the COMRA registered pioneer area (CRPA),equatorial northeastern Pacific Ocean

COMRA(China Ocean Mineral Resources Association) registered pioneer area (CRPA) is located in the western part of the area between the Clarion and Clipperton Fraction Zones (CCFZs). Based on distinctive color and bio-disturbing properties, two layers are recognized in the sediment columns from the CRPA: a pale brown layer (Unit A) on the top and a dark for layer (Unit B) in the lower part. Color division between them can be explained as a hiatus. As a resu1t, the two units are different to each other in chemical composition and sediment properties. It is found that contents of P2O5, MnO2, CaO, Na2O in sediments of older Unit B are higher, but SiO2 content is slightly lower. Based on correlation analysis, Mn and Fe in the sediments have different origins. The former is mainly precipitated in authigenic process, whilst the latter is closely related to terrigenous detritus. Therefore, the differences in chemical composition of the sediments are caused by different sedimentation settings.

Seasonal variation of the surface North Equatorial Countercurrent (NECC) in the western Pacific Ocean

The North Equatorial Countercurrent(NECC) is an important zonal fl ow in the upper circulation of the tropical Pacifi c Ocean, which plays a vital role in the heat budget of the western Pacifi c warm pool. Using satellite-derived data of ocean surface currents and sea surface heights(SSHs) from 1992 to 2011, the seasonal variation of the surface NECC in the western tropical Pacifi c Ocean was investigated. It was found that the intensity(INT) and axis position(Y_(CM)) of the surface NECC exhibit strikingly different seasonal fl uctuations in the upstream(128°–136°E) and downstream(145°–160°E) regions. Of the two regions, the seasonal cycle of the upstream NECC shows the greater interannual variability. Its INT and Y CM are greatly infl uenced by variations of the Mindanao Eddy, Mindanao Dome(MD), and equatorial Rossby waves to its south. Both INT and YC M also show semiannual signals induced by the combined effects of equatorial Rossby waves from the Central Pacifi c and local wind forcing in the western Pacifi c Ocean. In the downstream region, the variability of the NECC is affected by SSH anomalies in the MD and the central equatorial Pacifi c Ocean. Those in the MD region are especially important in modulating the Y CM of the downstream NECC. In addition to the SSH-related geostrophic fl ow, zonal Ekman fl ow driven by meridional wind stress also plays a role, having considerable impact on INT variability of the surface NECC. The contrasting features of the variability of the NECC in the upstream and downstream regions refl ect the high complexity of regional ocean dynamics.

The multifractal characters of temperature finestructuresin the equatorial western Pacific Ocean

By using part of CTD data collected at 2°S, 155° E during the fall cruise of TOGA project in 1992, themultifractal characters of temperature finestructures are investigated. The absolute temperature gradients are supposedto be multifractal and their moments are computed by conventional box-counting method. It is found that these moments have power dependence on the box size. This power dependence has two different scaling regimes, called Sregime and I-regime resistively, with different scaling exponents. This is consistent with the combined effects of internal waves and boxing. Accordingly, the generalized fractal dimensions (Renyi dimension) of temperature gradientsare derived. A nonlinear curve of the scaling exponents suggest a possible multifractal approach of the temperatureshear. In fact, both regimes can be approximated by Besicovitch- Cantor model, respectively, by suitably chosenmoduel parameters. A phenomenological model is developed on the basis of this two-regime mechanism. The model iscompared with field data and good agreement is achieved.

Effects of adjusting vertical resolution on the eastern equatorial Pacific cold tongue

The vertical resolution of LICOM1.0 (LASG/IAP Climate System Ocean Model, version 1.0) is adjusted by increasing the level amount within the upper 150 m while keeping the total of levels. It is found that the eastern equatorial Pacific cold tongue is sensitive to the adjustment. Compared with the simulation of the original level scheme, the adjusting yields a more realistic structure of cold tongue extending from the coast of Peru to the equator, as well as a temperature minimum at Costa Rica coast, north of the cold tongue. In the original scheme experiment, the sharp heating by net surface heat flux at the beginning of spin-up leads to a great warm- ing in the eastern equatorial Pacific Ocean. The weak vertical advection due to a too thick mixed layer in the coarse vertical structure also accounts for the warm bias. The fact that most significant improvements of the upper 50 m temperature appear at the region of the thinnest mixed layer indicates the necessity of fine vertical resolution for the eastern equatorial Pacific Ocean. However, the westward extension of equatorial cold tongue, a defect in the original scheme, gets even more serious in the adjusting scheme due to the intensi- fied vertical velocity and hence vertical advection in the central-eastern equatorial Pacific Ocean.

The slanting property of semi-diurnal internal tide propagation in the Pacific Ocean at 1°45′S,156°E

By analyzing a data set collected using a moored instrument array and CTD during TOGA-COARE, it is found that there exist remarkable internal tides in the western equatorial Pacific Ocean around 1°45′S, 156°E, whose horizontal wavenumber （wavelength）, vertical wavenumber, h 156° orizontal propagation speed and vertical propagation speed are 3.3×10^-2 km^-1 （210 km）, - 1.6×10^-3m, 2.0 m/s and -3.8 cm/s, respectively, that is, the waveform propagates downwards slantingly. Moreover, the propagating direction rotates statistically clockwise as the depth increases and its cause is unclear.

Impacts of the SSTs over Equatorial Central–Eastern Pacific and Southeastern Indian Ocean on the Cold and Rainy/Snowy/Icy Weather in Southern China

Low temperature together with snow/freezing rain is disastrous in winter over southern China.Previous studies suggest that this is related to the sea surface temperature(SST)anomalies,especially La Nina conditions,over the equatorial central–eastern Pacific Ocean(EP).In reality,however,La Nina episodes are not always accompanied by rainy/snowy/icy(CRSI)days in southern China,such as the case in winter 2020/2021.Is there any other factor that works jointly with the EP SST to affect the winter CRSI weather in southern China?To address this question,CRSI days are defined and calculated based on station observation data,and the related SST anomalies and atmospheric circulations are examined based on the Hadley Centre SST data and the NCEP/NCAR reanalysis data for winters of1978/1979–2017/2018.The results indicate that the CRSI weather with more CRSI days is featured with both decreased temperature and increased winter precipitation over southern China.The SSTs over both the EP and the southeastern Indian Ocean(SIO)are closely related to the CRSI days in southern China with correlation coefficients of-0.29 and 0.39,significant at the 90%and 95%confidence levels,respectively.The SST over EP affects significantly air temperature,as revealed by previous studies,with cooler EP closely related to the deepened East Asian trough,which benefits stronger East Asian winter monsoon(EAWM)and lower air temperature in southern China.Nevertheless,this paper discovers that the SST over SIO affects precipitation of southern China,with a correlation coefficient of 0.42,significant at the 99%confidence level,with warmer SIO correlated with deepened southern branch trough(SBT)and strengthened western North Pacific anomalous anticyclone(WNPAC),favoring more water vapor convergence and enhanced precipitation in southern China.Given presence of La Ni?a in both winters,compared to the winter of 2020/2021,the winter of 2021/2022 witnessed more CRSI days,perhaps due to the warmer SIO.

THE EFFECTS OF SEA SURFACE TEMPERATURE OVER THE EQUATORIAL WESTERN PACIFIC AND THE INDIAN OCEAN ON THE ASIAN SUMMER MONSOON

In this paper, the p-σ five layer primitive equation model segmented by mountains and physical parameterizations including short wave radiation; long wave radiation; large-scale and convective condensation; heat and moisture transport from surface to the first model level is used. The horizonial resolution is 5° lat. ×5° long. with the integration region from 25°S to 55°N and from 5°W eastward to 175°W. The model was spun up with perpetual June boundary conditions and forcing starting with June zonal mean heights and geostrophic wind field. In order to investigate the effects of SST (sea surface tempefuture) over the equatorial Western Pacific and the Indian Ocean on the Asian summer monsoon, four sets of numerical experments with positive anomalies over the equatorial Western Pacific, and positive and negative anomalies over the Western Indian Ocean, and zonal mean SST (the control case) are performed. The experimental results show that the South Asian low in the lower troposphere and the anticyclone over the South Asia in the uppet troposphere intensified when positive SST anomalies over the equatorial Western Pacific is included. A statistical test method for simulations is proposed. Finally, the influence mechanism of the SST anomalies over the equatorial oceans is discussed. It is worth stressing that the effects of the SST over the equatorial oceans on the Asian summer monsoon can arise as a result of interaction of SST anomalies, atmospheric flow field and heat sources and sinks in the atmosphere.