Modulation of Twin Tropical Cyclogenesis by the MJO Westerly Wind Burst during the Onset Period of 1997/98 ENSO

The modulation of twin tropical cyclogenesis in the Indian-western Pacific Oceans by the Madden-Julian Oscillation (MJO) during the onset period of 1997/98 ENSO is explored for the period of September 1996 to June 1997 based on daily OLR, NCEP/NCAR wind vector, and JTWC best track datasets. The MJO westerly wind burst associated with its eastward propagation can result in a series of tropical cyclogeneses in a multi-day interval. Only in the transition seasons are pairs of tropical cyclones observed in both the tropical sectors of the Indian-western Pacific Oceans. Two remarkable twin tropical cyclogeneses probably modulated by the MJO westerly wind burst are found: one is observed in the Indian Ocean in the middle of October 1996, and the other is observed in the Western Pacific Ocean in late May 1997. The twin tropical cyclogenesis in mid-October 1996 is observed when the super cloud cluster separates into two isolated clusters by the enhanced westerly wind, which is accompanied by two independent vortices in the equatorial tropical sectors. The other one, in late-May 1997, however, is characterized by one cyclonic flow that later results in another cyclonic cell in its opposite equatorial sector. Thus, there are two very important conditions for twin cyclogenesis: one is the MJO westerly wind straddling the equator, and the other is the integral super cloud cluster, which later splits into two cloud convective clusters with independent vortices.

The Heat Balance in the Western Equatorial Pacific Warm Pool during the Westerly Wind Bursts: A Case Study

The responses of sea surface temperature (SST) in the western equatorial Pacific warm pool to the westerly wind bursts (WWBs) play an important role in the relationship between WWB and ENSO. By using data collected from eight buoys of TOGA (Tropical Ocean-Global Atmosphere)- COARE (Coupled Ocean-Atmosphere Response Experiment), the heat balances of the upper ocean in the western equatorial Pacific around 0 degrees, 156 degreesE during two WWB events were calculated according to Stevenson and Niiler's (1983) method. In both events, SST increased before and after the WWBs, while decreased within the WWBs. The SST amplitudes approximated to 1 degreesC. Although sometimes the horizontal heat advections may become the biggest term in the heat balance, the variation of SST was dominated by the surface heat flux. On the other aspect, some different features of the two events are also revealed. The two cases have different variation of mixed layer depth. The depth of mixed layer is almost double in the first case (35 in to 70 m), which is caused by Ekman convergence, while only 10m increments due to entrainment in the second one, There are also differences in the currents structure. The different variations of thermal and currents structure in the mixing layers accounted for the different variation of the heat balance during the two events, especially the advection and residue terms. The seasonal variation of SST in this area is also investigated simply. The first WWB event happened just during the seasonal transition. So we considered that it is a normal season transition rather than a so-called anomaly. That also suggested that the seasonal distinction of the WWB is worthy of more attention in the researches of its relationship to ENSO.

Response of western equatorial Pacificto a westerly wind burst

Atmospheric jets-with shear-can induce a vertical oceanic circulation with upwelling and down -welling even over the open ocean in regions where the Coriolis parameter can be regarded as a constant. Winds with nosheard that bloe parallel to the equator can also induce a vertical oceanic circulation with upwelling and downwellingwithin an equatorial radius of deformation. This study concerns the oceanic response to a westerly wind burst, in theform of an atmospheric jet, similar to those that occur over the western equatorial Pacific . It is shown that the shear ofthe wind, if it is within an equatorial radius of deformation, can alter the vertical circulation substantially, especially ifthere are westerly and easterly winds near the equator. A commentary on measurements amde during a westerly windburst over the western equatorial Pacific has been given.

A New Equatorial Oscillation Index for Better Describing ENSO and Westerly Wind Bursts

An Equatorial Oscillation Index(EOI) is defined, based on the zonal gradient of sea surface pressure between the western Pacific and eastern Pacific along the equator, to describe the distribution of wind and pressure within the equatorial Pacific. The EOI has a stronger correlation with the Ni?o3.4 sea surface temperature anomaly(SSTA), as well as with westerly/easterly wind bursts(WWBs/EWBs), showing a superiority over the Southern Oscillation Index(SOI). In general, the EOI is consistent with the SOI, both of which reflect large-scale sea level pressure oscillations. However, when there are inconsistent SSTAs between the equator and subtropical regions, the SOI may contrast with the EOI due to the reverse changes in sea level pressure in the subtropical regions. As a result, the SOI fails to match the pattern of El Ni?o, while the EOI can still match it well. Hence, the EOI can better describe the variability of the Ni?o3.4 SSTA and WWBs/EWBs. The correlation between the SOI and Ni?o3.4 SSTA falls to its minimum in May, due to the large one-month changes of sea level pressure from April to May in the subtropical southern Pacific, which may be related to the spring predictability barrier(SPB). The newly defined EOI may be helpful for monitoring El Ni?o–Southern Oscillation(ENSO) and predicting ENSO.

ON WESTERLY WIND BURSTS IN EQUATORIAL WESTERN PACIFIC BEFORE AND DURING THE ONSET AND INITIAL DEVELOPMENT PHASES OF ENSO

By means of NCEP/NCAR reanalysis dataset,the origins of westerly wind anomalies at low level over equatorial western Pacific Ocean before and during the onset and initial development phase of ENSO are explored.Evidences show that westerly anomalies in the equatorial western Pacific(140—180°E)are characterized by two remarkable enhancements in the spring and summer of the year when El Nine emerges.The enhancements are not only.to some extem.due to the eastward propagation of low-level westerlies in equatorial Indian Ocean.but also predominantly resulting from Ihe intense convergence of the meridional wind from both hemispheres.The latitudinal convergence leads to the local intensification of zonal pressure gradient so as to cause the reinforcement and bursts of westerly wind over warm pool.Besides,by virtue of the effect of earth rotation,the northeasterlies(southeasterlies)from the Northern(Southern)Hemisphere turn into northwesterlies(southwesterlies)progressively in the near-equatorial zone.which directly strengthens the westerly velocity.Comparing the contributions of the meridional wind from both hemispheres to westerly wind bursts,is seems that southeasterlies from the Southern Hemisphere are much stronger and more stable than northwesterlies of Northern Hemisphere.It is evident that the southeasterlies to the east of Australia originate from the southern mid-and high latitudes and are in close association with the Southern Oscillation.

A Comparative Study on the Dominant Factors Responsible for the Weaker-than-expected El Ni?o Event in 2014

Anomalous warming occurred in the equatorial central-eastern Pacific in early May 2014, attracting much attention to the possible occurrence of an extreme E1 Nifio event that year because of its similarity to the situation in early 1997. However, the subsequent variation in sea surface temperature anomalies （SSTAs） during summer 2014 in the tropical Pacific was evidently different to that in 1997, but somewhat similar to the situation of the 1990 aborted E1 Nifio event. Based on NCEP （National Centers for Environmental Prediction） oceanic and atmospheric reanalysis data, the physical processes responsible for the strength of E1 Nifio events are examined by comparing the dominant factors in 2014 in terms of the preceding instability of the coupled ocean-atmosphere system and westerly wind bursts （WWBs） with those in 1997 and 1990, separately. Although the unstable ocean-atmosphere system formed over the tropical Pacific in the preceding winter of 2014, the strength of the preceding instability was relatively weak. Weak oceanic eastward-propagating downwelling Kelvin waves were forced by the weak WWBs over the equatorial western Pacific in March 2014, as in February 1990. The consequent positive upper-oceanic heat content anomalies in the spring of 2014 induced only weak positive SSTAs in the central-eastern Pacific-unfavorable for the subsequent generation of summertime WWB sequences. Moreover, the equatorial western Pacific was not cooled, indicating the absence of positive Bjerknes feedback in early summer 2014. Therefore, the development of E1 Nifio was suspended in summer 2014.

ENSO Indices and Analyses

New ENSO indices were developed and the spatial variability and temporal evolution of ENSO were analyzed based on the new indices and modeling experiments, as well as multiple data resources. The new indices, after being defined, were validated with their good diagnostic characteristics and correlation with wind and SST. In the analysis after the definition and validation of the new indices, ENSO feedbacks from wind, heat fluxes, and precipitation were spatially and temporally examined in order to understand ENSO variability and evolution with some emphasized points such as the interaction among the feedbacks, the role of westerly wind bursts and the transformation between zonal and meridional circulations in an ENSO cycle, and the typical pattern of modern ENSO.

Anomalous eastward displacement of the tropical western Pacific warm pool and warming of the tropical eastern Pacific

Based on the data from the Climate Diagnostics Bulletin, Oceanographic Monthly Summary, UH Sea Level Center and TOGA─COARE IOP, the response of warm pool in the tropical western Pacific and the tropical eastern Pacific SST to the anomalous wind field during 1992/1993 EI Nino has been analyzed. The results show that the eastward transport of warm water of the tropical western Pacific due to the westerly wind burst leads not only to a drop of sea level but also to a raise of thermocline in the tropical western Pacific. Consequently the heat content in upper layer water decreases especially in the thermocline. Contrary to this, the positive anomalies Of heat content and thermocline depth appear in the tropical eastern Pacific. The positive anomalies in the eastern Pacific lag the negative ones in the western Pacific by two months; The anomalous eastward shift of warm pei (28℃isotherm) is a direct response of ocean current to westerly wind anomalies in low-level atmosphere; quantitative calculations show that the thermal advection caused by anomalous ocean current is the main force of anomalous eastward displacement of the warm pool (28℃isotherm) and the one of main causes for anomalous warming of the tropical eastern Pacific.

Evolution of the 2015/16 El Nio and historical perspective since 1979

The 2015/16 El Nio developed from weak warm conditions in late 2014 and NINO3.4 reached 3℃ in November 2015. We describe the characteristics of the evolution of the 2015/16 El Nio using various data sets including SST, surface winds,outgoing longwave radiation and subsurface temperature from an ensemble operational ocean reanalyses, and place this event in the context of historical ENSO events since 1979. One salient feature about the 2015/16 El Nio was a large number of westerly wind bursts and downwelling oceanic Kelvin waves(DWKVs). Four DWKVs were observed in April-November 2015 that initiated and enhanced the eastern-central Pacific warming. Eastward zonal current anomalies associated with DWKVs advected the warm pool water eastward in spring/summer. An upwelling Kelvin wave(UWKV) emerged in early November 2015 leading to a rapid decline of the event. Another outstanding feature was that NINO4 reached a historical high(1.7℃), which was 1℃(0.8℃) higher than that of the 1982/83(1997/98) El Nio . Although NINO3 was comparable to that of the 1982/83 and 1997/98 El Nio , NINO1+2 was much weaker. Consistently, enhanced convection was displaced 20 degree westward, and the maximum D20 anomaly was about 1/3.1/2 of that in 1997 and 1982 near the west coast of South America.