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.

Skewness of subsurface ocean temperature in the equatorial Pacific based on assimilated data

The skewness of subsurface temperature anomaly in the equatorial Pacific Ocean shows a significant asymmetry between the east and west. A positive temperature skewness appears in the equatorial eastern Pacific, while the temperature skewness in the western and central Pacific is primarily negative. There is also an asymmetry of the temperature skewness above and below the climatological mean therrnocline in the central and western Pacific. A positive skewness appears below the thermocline, but the skewness is negative above the thermocline. The distinctive vertical asymmetry of the temperature skewness is argued to be attributed to the asymmetric temperature response to upward and downward thermocline displacement in the presence of the observed upper-ocean vertical thermal structure. Because of positive （negative） second derivative of temperature with respect to depth below （above） the thermocline, an upward and a downward shift of the thermocline with equal displacement would lead to a negative temperature skewness above the thermocline but a positive skewness below the thermocline. In the far eastern equatorial Pacific, the thermocline is close to the base of the mixed layer, the shape of the upper-ocean vertical temperature profile cannot be kept. Positive skewness appears in both below the thermocline and above the thermocline in the far eastern basin. Over the central and eastern Pacific, the anomalies of the subsurface waters tend to entrain into the surface mixed layer （by climatological mean upwelling） and then affect the SST. Hence, the positive （negative） subsurface skewness in the far eastern （central） Pacific may favor positive （negative） SST skewness, which is consistent with the observational fact that more La Nina （EI Nino） occur in the central （eastern） Pacific. The present result implies a possible new paradigm for EI Nino and La Nina amplitude asymmetry in the eastern Pacific.

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.

Impact of ocean data assimilation on the seasonal forecast of the 2014/15 marine heatwave in the Northeast Pacific Ocean

A remarkable marine heatwave,known as the“Blob”,occurred in the Northeast Pacific Ocean from late 2013 to early 2016,which displayed strong warm anomalies extending from the surface to a depth of 300 m.This study employed two assimilation schemes based on the global Climate Forecast System of Nanjing University of Information Science(NUIST-CFS 1.0)to investigate the impact of ocean data assimilation on the seasonal prediction of this extreme marine heatwave.The sea surface temperature(SST)nudging scheme assimilates SST only,while the deterministic ensemble Kalman filter(EnKF)scheme assimilates observations from the surface to the deep ocean.The latter notably improves the forecasting skill for subsurface temperature anomalies,especially at the depth of 100-300 m(the lower layer),outperforming the SST nudging scheme.It excels in predicting both horizontal and vertical heat transport in the lower layer,contributing to improved forecasts of the lower-layer warming during the Blob.These improvements stem from the assimilation of subsurface observational data,which are important in predicting the upper-ocean conditions.The results suggest that assimilating ocean data with the EnKF scheme significantly enhances the accuracy in predicting subsurface temperature anomalies during the Blob and offers better understanding of its underlying mechanisms.

Subsurface temperature anomalies in the North Pacific Ocean associated with the ENSO cycle

Multi-year Simple Ocean Data Assimilation （SODA） and National Centers for Environmental Prediction （NCEP） datasets were used to investigate the leading patterns of subsurface ocean temperature anomalies （SOTA） and the corresponding atmospheric circulation structure in the Pacific Ocean （20°S-60°N）. In this paper, the evolution of North Pacific SOTA associated with El Nifio-southern oscillation （ENSO）, and their relationship with the overlying zonal/meridional atmospheric circulations were elucidated. The results indicate that： （1） there are two dominant modes for the interannual variability of the North Pacific SOTA. The primary mode is the dipole pattern of the central and western North Pacific SOTA associated with the leading mode of ENSO, and the second mode is the zonal pattern related to the second mode of ENSO. These two modes consist of the temporal-spatial variation of the SOTA in the North Pacific. （2） During the developing phase of the El Nifio event, positive （negative） SOTA appears in the western （central） portion of the North Pacific Ocean. During the mature and decaying phase of the E1 Nifio event, the western positive center and the central negative center continue to be maintained and enhanced. Meanwhile, the position of the western positive center slightly changes, and the central negative center moves eastward slowly. After the El Nifio event vanishes, the positive SOTA disappears, and the central negative SOTA becomes weak and remains in the northeastern Pacific Ocean. The results for La Nifia are generally the opposite. （3） During the El Nifio/La Nifia cycle, formation and evolution of the SOTA, with opposite signs in central and western North Pacific Ocean, resulted from vertical movement of the two northern branches of the Hadley Cell with opposite direction, as well as the positive feedback of the air-sea interaction induced by dynamic processes in the mid-latitudes. The former gives rise to the initial SOTA, and the latter intensifies SOTA. Under the forcing of these two processes, SOTA evolution is formed and sustained during the El Nino/La Nina events. Also discussed herein as background for the ENSO cycle are the possible connections among the West Pacific subtropical high, the strength of the Kuroshio near the East China Sea, the Kuroshio meanders south of Japan, the Aleutian Low, and cold advection in the central North Pacific Ocean.

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.

Verification of an operational ocean circulation-surface wave coupled forecasting system for the China＇s seas

An operational ocean circulation-surface wave coupled forecasting system for the seas off China and adjacent areas（OCFS-C） is developed based on parallelized circulation and wave models. It has been in operation since November 1, 2007. In this paper we comprehensively present the simulation and verification of the system, whose distinguishing feature is that the wave-induced mixing is coupled in the circulation model. In particular, with nested technique the resolution in the China＇s seas has been updated to（1/24）° from the global model with（1/2）°resolution. Besides, daily remote sensing sea surface temperature（SST） data have been assimilated into the model to generate a hot restart field for OCFS-C. Moreover, inter-comparisons between forecasting and independent observational data are performed to evaluate the effectiveness of OCFS-C in upper-ocean quantities predictions, including SST, mixed layer depth（MLD） and subsurface temperature. Except in conventional statistical metrics, non-dimensional skill scores（SS） is also used to evaluate forecast skill. Observations from buoys and Argo profiles are used for lead time and real time validations, which give a large SS value（more than 0.90）. Besides, prediction skill for the seasonal variation of SST is confirmed. Comparisons of subsurface temperatures with Argo profiles data indicate that OCFS-C has low skill in predicting subsurface temperatures between 100 m and 150 m. Nevertheless, inter-comparisons of MLD reveal that the MLD from model is shallower than that from Argo profiles by about 12 m, i.e., OCFS-C is successful and steady in MLD predictions. Validation of 1-d, 2-d and 3-d forecasting SST shows that our operational ocean circulation-surface wave coupled forecasting model has reasonable accuracy in the upper ocean.

Oceanic thermal forcing of North Atlantic tropical cyclones

利用大气与海洋再分析数据等相关资料,本项研究发现,北大西洋强台风(Saffir-Simpson分类中的第4和第5类)的最大强度与亚热带北大西洋的次表层海温呈正相关.由于亚热带北大西洋的次表层海温会影响当地的海表温度,该地区海面产生的水汽通过近地面的反气旋大气环流可被输送到位于热带的台风主要发展区域,进而影响台风的最大强度.与此同时,位于西非北部和南欧的近岸涌升流会影响亚热带北大西洋的次表层海温.因此,强台风的最大强度也被发现与上述两个涌升流区域的海温具有相关性,但是这种相关性主要体现在年代际时间尺度上。

Linkage between the dominant modes in Pacific subsurface ocean temperature and the two type ENSO events

We investigate the variations of subsurface ocean temperature(SOT) based on the monthly-Simple Ocean Data Assimilation(SODA) during 1958-2007,and discuss the linkage between the variations of SOT and the eastern and central Pacific ENSO(EP and CP-ENSO) events.The wavelet analyses suggest that the variation of the EP and CP-ENSO events shows the 2-7 and the 10-15 years oscillation in the tropical sea surface temperature(SST),and coupled with a zonal dipole mode and a tripole mode in the SOT anomalous field reveled by the singular value decomposition(SVD) analysis.During the mature phase of CP-ENSO,the positive center of SOT at the subsurface layer locates in the west of dateline,which results in the increase of SOT in the Ni o4 region and causes the CP-ENSO event.Statistical analysis implies that,the eastern and central Pacific subsurface indices which are defined by the expansion coefficients of the first and third SVD mode for SOT have shown the capabilities in disguising the EP and CP-ENSO events,respectively.In addition,corresponding to the increase of the SOT amplitude on the 10-15 years time scale,we found that the frequency and intensity of CP-El Ni o events has exhibited an upward trend after the 1980s,which suggests that the CP-ENSO event has shown an enhanced impact on the global climate in the past decades.

Occurrence of two types of El Nin?o events and the subsurface ocean temperature anomalies in the equatorial Pacific

The relationships between the evolution of two types of El Ni?o events and the subsurface ocean temperature anomaly(SOTA) in the equatorial Pacific are compared in this study. The results show that both types of El Ni?o are negatively correlated to the SOTA in the equatorial western Pacific, but relationships are different in different phases of El Ni?o. Furthermore, the occurrence of different types of El Ni?o is related to different features of the equatorial thermocline, e.g. its zonal gradient, significant variation area, amplitude and duration of thermocline oscillation. The propagation of SOTA in the equator plays an important role during the evolution of both types of El Ni?o, but shows dramatic differences in intensity, duration and phase reverse of warm SOTA. Moreover, the pathways of SOTA signal are different between these two types of El Ni?o. The dominant pathway in the life cycle of Eastern Pacific(EP)-El Ni?o lies on the equator and to its north, but there is no loop to the south of the equator. In contrast, the dominant pathway in Central Pacific(CP)-El Ni?o is located on the equator and to its south, and the propagation signal of SOTA to the north of the equator is very weak.The relationships between the zonal wind anomalies and the two types of El Ni?o are also preliminarily discussed. It is shown that EP-El Ni?o is more likely to respond to the westerly anomalies over the equatorial central and western Pacific, while CP-El Ni?o is more likely to respond to the westerly anomalies over the equatorial western Pacific and need the cooperation of easterly anomalies over the equatorial eastern Pacific to certain extent.

赤道西太平洋暖池次表层水温变化对热带气旋的影响

分析了西太平洋暖池和南海次表层水温变化对TC的影响,研究结果表明:当赤道西太平洋暖池次表层水温夏半年持续出现正(负)距平时,西北太平洋生成的TC个数比常年偏多(少)是主要现象,这种现象极值年份尤其明显,对南海TC(指南海生成的TC,下同)的个数影响不明显,西北太平洋和南海生成TC的时间比常年提早(推迟)。当南海北部次表层水温夏季和秋季持续出现正(负)距平时,南海TC比常年偏多(少)。TC对南海和东海的影响趋势是一致的,但所受影响的程度东海比南海更为强烈。原因是东海台风波浪比南海高、海面气温比南海低、相对湿度比南海小,故导致SST急速下降。

热带印度洋次表层偶极子模态及其1997/1998年过程诊断分析研究

利用EOF分解及相关统计方法研究了热带印度洋 40 0m以上的次表层海温异常并对1 997/ 1 998年热带印度洋偶极子事件过程进行诊断分析研究。结果表明 :热带印度洋次表层 40 0m以上的部分海温距平最大是在 1 0 0m左右的深度 ,就整个热带印度洋而言 ,自 2 0世纪 80年代以来 ,次表层 60m以上出现了变暖的趋势 ,而 80m以下则出现了降温的趋势。同时在热带印度洋次表层 80m深度存在着比海表更强的偶极子模态。 1 997/ 1 998年发生在印度洋海表东冷西暖型的偶极子事件 ,是东印度洋次表层的海温正距平西传的结果 ,而海温正距平的西传与热带印度洋上东风异常有关 。

赤道西太平洋暖池次表层水温与热带气旋的关系

为探索赤道西太平洋暖池次表层水温异常与热带气旋的关系,用赤道西太平洋暖池和南海SOTA实测资料,对TC的影响做了统计分析。赤道西太平洋暖池SOTA与同步西太平洋TC个数不存在线性相关;赤道西太平洋暖池1月SOTA滞后5～7个月影响西太平洋的TC;赤道西太平洋暖池区1月的SOTA出现正(负)距平值时,当年西北太平洋和南海的TC生成时间比常年提早(推迟)是主要现象,极值年份尤其明显,当年夏季西北太平洋副热带高压偏弱(强),位置偏北(南),西太平洋暖池区上空对流加强(减弱),对台风生成有(不)利,台风生成平均位置偏西(东),TC的个数偏多(少)、偏强(弱),易于出现西行(东北转向)路径为主;南海中北部2月SOTA出现偏暖(冷)年,当年南海TC生成日期偏早(晚)、数量偏多(少)、偏强(弱)是主要现象。赤道西太平洋暖池SOTA对TC影响明显,时间滞后。

太平洋次表层海温异常年际变率的信号通道与ENSO循环

利用SODA海洋同化资料,分析了太平洋次表层海温异常(SOTA)年际信号变异特征与ENSO循环的联系。结果表明,热带太平洋的年际变率表现为以160°W为纵轴的东西向和以6°—8°N为横轴的南北向的跷跷板分布,南太平洋和北太平洋中高纬度海洋的SOTA则与热带西太平洋SOTA同号,但强度较弱,这些变化都与ENSO事件密切相关,是ENSO事件的两个主要模态,具57和44个月显著周期。ENSO循环期间,热带西太平洋SOTA强信号中心沿赤道东传,到达赤道东太平洋后加强并北扩,导致ElNi?o或LaNi?a事件,同时从热带西太平洋有较弱SOTA信号向东北和西南传播,在南、北太平洋中高纬度海域产生弱SOTA;同期位于热带东太平洋反号的SOTA强信号中心沿10°—15°N(平均12°N)西传,至热带西太平洋后加强并南扩,为下次LaNi?a或ElNi?o事件准备条件,同时在北太平洋中高纬度海洋还存在着反号弱的SOTA。如此周而复始,完成ENSO循环。太平洋次表层海温年际变化信号除在赤道及以北的热带太平洋存在一个逆时针方向的传播通道外,同时在热带西太平洋有异常信号向南、北太平洋中高纬度海域传播,并指出ENSO循环期间太平洋次表层海温异常年际变率信号传播的可能通道。

ENSO事件次表层海温的两个模态及其对大气环流的影响

利用SODA海洋同化资料和NCEP再分析大气资料,分析了热带太平洋次表层海温异常(subsurfaceoceantemperatureanomaly,SOTA)与厄尔尼诺与南方涛动(ElNi?o-SouthernOscillation,ENSO)循环的联系,及SOTA对大气环流的影响。回顾传统ENSO研究,指出存在的问题,提出了ENSO影响大气研究的新思路,得到以下结果:(1)以SOTA为基本资料的研究发现, ENSO事件有两个模态,主要出现在冬季的第一模态对冬季及夏季亚洲-北太平洋-北美地区上空中高纬大气环流有重要影响,主要出现在夏季的第二模态对该地区上空夏季热带和副热带大气系统有重要作用。(2)ENSO事件通过与ENSO相联系的热带太平洋海面温度异常(ENSO-relatedseasurface temperatureanomaly,RSSTA)对大气的异常热通量输送,强迫Walker环流和Hadley环流变化,导致热带和北太平洋及周边地区上空大气环流异常,进而影响相关地区冬季和夏季的气候。(3)海表面温度异常(seasurfacetemperatureanomaly,SSTA)包含RSSTA和大气异常导致的海温变化(sea temperature anomaly caused by atmospheric anomaly, STA)两部分, RSSTA是ENSO事件过程中海洋内部热动力结构调整导致的海面温度变化,在海洋对大气的热输送过程中,它随ENSO事件演变不断更新;STA是大气受RSSTA海洋异常加热后导致的大气环流异常对海面温度的影响,在海洋浅表层STA对RSSTA有重大影响。本文最后讨论了ENSO事件期间热带海洋对大气热输送过程,指出ENSO事件通过海洋内部热动力结构调整产生RSSTA,它直接对大气异常加热,导致大气环流和气候异常,局地海气之间负反馈过程产生STA,反过来抑制RSSTA。结果还指出,人们常用的SSTA变率实际上主要由秋冬季节RSSTA主导,丢失了春夏季ENSO信息,用SSTA研究ENSO事件存在局限性,这也可能是ENSO事件春季预报障碍的原因之一。

1997～1998年厄尔尼诺事件的特征、成因及对气候的影响

1997年爆发了20世纪以来最强的一次厄尔尼诺事件。此次ElNin～o事件的发生,与赤道中、西太平洋地区大气季节内振荡(ISO)在1996年冬到1997年春的异常增强有重要关系。西太平洋暖池次表层海温(SOT)正距平沿温跃层东传到赤道东太平洋并向海洋表层扩展是ElNin～o事件爆发的直接原因。1997～1998年的ElNin～o事件爆发后,引起全球大气环流和世界许多地区的气候异常,导致一些国家和地区多雨洪涝,另外一些国家和地区高温少雨和严重干旱。

MJO持续异常对ENSO的影响

MJO在夏季会出现在太平洋持续异常活跃和在印度洋持续异常活跃两种形式,用指数定义了这种异常的强度,并发现这种异常表现与秋冬季节ENSO的出现之间有很高的相关性。当夏季MJO在太平洋持续异常活跃时,经常激发当年秋冬季节发生El Nino事件;当夏季MJO在印度洋持续异常活跃时,经常激发当年秋冬季节发生La Nina事件。对大气环流的分析表明,MJO的持续异常活跃会对整个赤道太平洋上空的环流造成影响,引起风应力的异常。异常风应力激发次表层冷/暖海水向东输送。冬季冷/暖海水在东太平洋次表层堆积,最终导致ENSO的形成。

西北太平洋台风数目与海洋热含量的年代际关系分析

采用Joint Typhoon Warning Center提供的1945—2003年热带气旋数据对西北太平洋地区(5°—30°N,121°E—180°)的台风活动进行统计分析,结果表明西北太平洋的台风活动具有明显的年代际特征。台风数目在1958—1968年表现为高值,在1970—1980年表现为低值,在1985—1995年又表现为高值;并且在1958—1968年这一时段台风的年生成数要多于1985—1995年期间的台风生成数。利用Scripps海洋研究所环境分析中心提供的同期上400m热含量和11个标准层海温数据分析发现,台风活动与海洋热力变化存在一定的联系。就年代际变化而言,海温变化对台风数量的影响并不是主要引导因素。西北太平洋台风变化与中太平洋的热含量相联系,两者表现为正相关。通过赤道垂直温度剖面可以看出,相对于1970—1980年而言,1958—1968年和1985—1995年的中太平洋热含量明显增加。中太平洋的热含量增加在1958—1968年主要体现在温跃层的海温增暖,而在1985—1995年主要体现在海温表层增暖。这意味着在次表层出现的温度暖异常可能是导致在1958—1968这个时间段内台风年生成数多的重要原因。

赤道太平洋次表层海温模态在ENSO循环中的作用

利用美国马里兰大学海洋同化月平均再分析资料(SODA),分离出赤道太平洋次表层海温异常(SOTA)的年际变率和年代际变率,利用经验正交分解(EOF)方法分别得到SOTA年际变率和年代际变率的第一模态和第二模态,重点分析了第二模态在ENSO循环中的作用。结果表明,赤道太平洋年际变率和年代际变率的第一模态为偶极子分布,此分布型是ENSO循环冷暖位相在次表层的同时表现。第二模态以次表层范围较广的海温异常趋势一致分布为显著特征,该模态是ENSO循环演变过程的重要环节。第二模态时间系数与Ni o-3.4指数具有较好的超前相关性,可作为ENSO事件的预测前兆信号,合成和个例分析验证了这一次表层信号的预测指示作用。

热带太平洋地区风场异常和与El Nio有关的年际变化

利用1964~1993年NCEP/NCAR再分析风应力资料和中国科学院大气物理研究所发展的14层热带太平洋环流模式(OGCM),对热带太平洋与El Ni^no有关的年际变化进行了研究。首先,分析了西太平洋暖池次表层海温异常(SOTA)与Ni^no 3区海表温度异常(SSTA)的年际变化关系,发现在El Ni^no事件之前,暖池的次表层海温都有明显正异常出现,它的东传导致了El Ni^no的发生,并且SOTA的传播随纬度变化,沿赤道东传,在赤道外西传。然后,选取了20世纪70年代和80年代两次最强的El Ni^no事件讨论了引起这种机制的可能原因———西风异常的作用。最后,对1964~1993所有的El Ni^no年的风场、次表层海温和海表温度的异常进行了综合分析。