Anomalous Behaviors Of Wyrtki Jets In 2013

The equatorial Indian Ocean is characterized by strong eastward flows in the upper 80-100 m during boreal spring and fall referred to as the Wyrtki Jets(WJ). These jets are driven by westerly wind during the transition seasons between the southwest and northeast monsoon and represent a major conduit for mass and heat transfer between the eastern and western sides of the basin.In-situ measurement of the upper ocean velocity discloses significant abnormal behaviors of WJ in boreal spring and fall season over the tropical Indian Ocean in 2013. Unlike the climatological peak time of the fall WJ in November, the fall WJ in 2013 unexpectedly peaks in December, one month later than its normal condition, and the exceptional spring jet in May is also unusually stronger than the one in fall. Further analysis illustrates that the anomalous changes in the equatorial zonal wind, associated with the strong Intra-Seasonal Oscillation(ISO) events, are most likely the dominant reason for the anomalous WJ activities. The negative ISOs control tropical Indian Ocean over April and November. The active phases of ISOs prevail from late April to mid-May and from late November to mid-December. As a result of positive ISO events, the surface westerly winds near the equator are particularly strong in early May and early December.For better understanding of the anomalous behaviors of WJ in 2013, we use Princeton Ocean Model(POM) driven by NCEP wind field to simulate the WJ. The control run indicate POM can simulate the dominant character of WJ, and we success successfully reproduced the anomalous behavior in No. 1 sensitive run. No.2-No.4 suggest that the seasonal signal in zonal wind plays an important role in WJ over the tropical Indian Ocean.Although previous studies have paid a lot attentions on the WJ issue over the tropical Indian Ocean, the simulated WJ by the numerical models is still an open question and few jobs have been done to evaluate the simulation performance among the state of art coupled models. CMIP5 provides a good chance to assess the model behaviors on WJ as it comes out from the state of art coupled models. This research utilizes the available 24 CMIP5 models to study the preliminary work on WJ simulation issue in order to take a look at the models’ performances over the tropical Indian Ocean and the results show large diversity in the simulated current intensity at seasonal time scale. These coupled models are able to capture the dominant spatial distribution of observed WJ, the central equatorial region. The simulated seasonal variations of WJ are also reproduced quantitatively, though the simulated amplitudes from CMIP5 models are quite spread among CMIP5 models. Compared with the observation, some coupled models are not able to present the evolution of WJ in fall season and the decay phase has been postponed one month later. Further diagnostic illustrates that the simulated surface zonal wind has remarkable impact on the evolution of WJ in fall season over the tropical Indian Ocean.