Fall Wyrtki Jets Research Articles

Impact of chlorophyll concentration on thermodynamics and dynamics in the tropical Indian ocean

It is well known that the physical properties of the ocean impact marine biological activity. Conversely, ocean biology feeds back to affect physical properties through the influence of phytoplankton on the ocean's absorption of incoming shortwave radiation. In this study, we explore this feedback in the tropical Indian Ocean (TIO) throughout the seasonal cycle by comparing solutions to a biophysical ocean model with and without chlorophyll concentrations. Phytoplankton-induced absorption increases near-surface temperature, thereby increasing evaporation and thinning the mixed layer. The resulting evaporation and restratification lower sea level and thereby affect circulation, particularly the summer monsoon current and the fall Wyrtki Jet. The circulation and evaporation changes in turn affect sea surface salinity. Finally, with chlorophyll there is a tendency for the model's sea-surface-temperature and mixed-layer depth-biases to shift closer to their observed values throughout the TIO, with the largest improvements occurring in the Arabian Sea and the Bay of Bengal.

Seasonal variability and long-term trends of the surface and subsurface circulation features in the Equatorial Indian Ocean.

Herein, we have examined the seasonal variability and long-term trends in the recent 35 years (1980-2014) of the wind stresses and circulation features in the Equatorial Indian Ocean (EIO) using reanalysis and observed data. Annual mean flow of the EIO is eastward along the equator, with the strong current in the upper 80 m and weak below that. The prominent surface currents in the EIO are the eastward Wyrtki Jets (WJs) occurring during spring (April-May) and fall (October-November), with active flows within 2° of the equator between 60° E and 90° E. The fall WJs are stronger than the spring WJs. The Equatorial Undercurrent (EUC), the prominent subsurface eastward flow with core within 70-150 m, occurs twice annually-February-April and August-October-with the well-developed flow in March-April. Long-term trends reveal that during 1980-2014, the annual mean eastward surface current in the upper 75 m has increased, primarily due to the rise of WJs forced by the westerly wind stresses during November-December. But the subsurface eastward current (below 75 m) has decreased due to the weakening of winter North Equatorial Current (NEC) and EUC during February-April. Consolidation of the westerly wind stresses during November-April results in the strengthening of the wind-driven surface WJs and weakening of the pressure-driven EUCs during 1980-2014. Intensification of WJs are the direct effects of wind forcing. However, strengthening of westerly stresses also results in weakening of the surface westward NEC and thereby restricts the strong eastward gradient of SSH which is required to build up the essential eastward pressure gradient for the EUC. Since WJs and EUCs modulate the hydrographic structure of the eastern part of the EIO, any change in the strength of these flows, in turn, will influence mostly this oceanic region and thereby the local as well as global climate.

Interannual variability of zonal currents in the equatorial Indian Ocean: respective control of IOD and ENSO

The observational record is too short to confidently differentiate the relative contributions of Indian Ocean Dipole (IOD) and El Nino–Southern Oscillation (ENSO) on the interannual variability of the equatorial current system in the Indian Ocean because of the strong tendency of these two modes to co-occur. In this study, we analyse a five-decade simulation from an ocean general circulation model forced to describe the main interannual variations of surface and subsurface equatorial zonal currents in the Indian Ocean. This simulation is first shown to accurately capture the surface and subsurface zonal current variations in the equatorial region derived from the available observations. Through an EOF analysis on the model outputs, our results further reveals two main modes of equatorial current interannual variability: a dominant mode with largest amplitude in fall largely describing the variability of the fall Wyrtki jet intensity followed a few months later by a secondary mode maximum in winter largely describing the interannual variability of the subsurface currents in that season. Our analysis further confirms that the IOD is largely responsible for the interannual modulation of fall Wyrtki jet intensity by modulating the equatorial wind intensity during that season. The IOD is also responsible for strong subsurface current variations until December, induced by the delayed effect of the IOD wind signal onto the equatorial thermocline tilt. The equatorial current system response to ENSO is weaker and delayed compared to that of the IOD. The remote and delayed impact of ENSO in the IO indeed induces equatorial wind variations in winter that modulate the winter surface current intensity and the spring equatorial undercurrent intensity through its delayed impact on the thermocline tilt.

Anomalous behaviors of Wyrtki Jets in the equatorial Indian Ocean during 2013.

In-situ measurement of the upper ocean velocity discloses significant abnormal behaviors of two Wyrtki Jets (WJs) respectively in boreal spring and fall, over the tropical Indian Ocean in 2013. The two WJs both occurred within upper 130 m depth and persisted more than one month. The exceptional spring jet in May was unusually stronger than its counterpart in fall, which is clearly against the previous understanding. Furthermore, the fall WJ in 2013 unexpectedly peaked in December, one month later than its climatology. Data analysis and numerical experiments illustrate that the anomalous changes in the equatorial zonal wind, associated with the strong intra-seasonal oscillation events, are most likely the primary reason for such anomalous WJs activities.

Volume transports of the Wyrtki jets and their relationship to the Indian Ocean Dipole

AbstractThe 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. These jets are driven by westerly winds during the transition seasons between the southwest and northeast monsoons and represent a major conduit for mass and heat transfer between the eastern and western sides of the basin. Since their discovery over 40 years ago, there have been very few estimates from direct observations of the volume transports associated with these currents. In this paper we describe seasonal‐to‐interannual time scale variations in volume transports based on 5 years of unique measurements from an array of acoustic Doppler current profilers in the central equatorial Indian Ocean. The array was centered at 0°, 80.5°E and spanned latitudes between 2.5°N and 4°S from August 2008 to December 2013. Analysis of these data indicates that the spring jet peaks in May at 14.9±2.9 Sv and the fall jet peaks in November at 19.7±2.4 Sv, around which there are year‐to‐year transport variations of 5–10 Sv. The relationship of the interannual transport variations to zonal wind stress forcing, sea surface temperature, sea surface height, and surface current variations associated with the Indian Ocean Dipole (IOD) are further highlighted. We also illustrate the role of wind‐forced equatorial waves in affecting transport variations of the fall Wyrtki jet during the peak season of the IOD.

Tropical Indian Ocean surface salinity bias in Climate Forecasting System coupled models and the role of upper ocean processes

In the present study sea surface salinity (SSS) biases and seasonal tendency over the Tropical Indian Ocean (TIO) in the coupled models [Climate Forecasting System version 1 (CFSv1) and version 2 (CFSv2)] are examined with respect to observations. Both CFSv1 and CFSv2 overestimate SSS over the TIO throughout the year. CFSv1 displays improper SSS seasonal cycle over the Bay of Bengal (BoB), which is due to weaker model precipitation and improper river runoff especially during summer and fall. Over the southeastern Arabian Sea (AS) weak horizontal advection associated with East Indian coastal current during winter limits the formation of spring fresh water pool. On the other hand, weaker Somali jet during summer results for reduced positive salt tendency in the central and eastern AS. Strong positive precipitation bias in CFSv1 over the region off Somalia during winter, weaker vertical mixing and absence of horizontal salt advection lead to unrealistic barrier layer during winter and spring. The weaker stratification and improper spatial distribution of barrier layer thickness (BLT) in CFSv1 indicate that not only horizontal flux distribution but also vertical salt distribution displays large discrepancies. Absence of fall Wyrtki jet and winter equatorial currents in this model limit the advection of horizontal salt flux to the eastern equatorial Indian Ocean. The associated weaker stratification in eastern equatorial Indian Ocean can lead to deeper mixed layer and negative Sea Surface Temperature (SST) bias, which in turn favor positive Indian Ocean Dipole bias in CFSv1. It is important to note that improper spatial distribution of barrier layer and stratification can alter the air–sea interaction and precipitation in the models. On the other hand CFSv2 could produce the seasonal evolution and spatial distribution of SSS, BLT and stratification better than CFSv1. However CFSv2 displays positive bias in evaporation over the whole domain and negative bias in precipitation over the BoB and equatorial Indian Ocean, resulting net reduction in the fresh water availability. This net reduction in fresh water forcing and the associated weaker stratification lead to deeper (than observed) mixed layer depth and is primarily responsible for the cold SST bias in CFSv2. However overall improvement of mean salinity distribution in CFSv2 is about 30 % and the mean error has reduced by more than 1 psu over the BoB. This improvement is mainly due to better fresh water forcing and model physics. Realistic run off information, better ocean model and high resolution in CFSv2 contributed for the improvement. Further improvement can be achieved by reducing biases in the moisture flux and precipitation.

Biogeochemical variability in the central equatorial Indian Ocean during the monsoon transition

Abstract. In this paper we examine time-series measurements of near-surface chlorophyll concentration from a mooring that was deployed at 80.5°E on the equator in the Indian Ocean in 2010. These data reveal at least six striking spikes in chlorophyll from October through December, at approximately 2-week intervals, that coincide with the development of the fall Wyrtki jets during the transition between the summer and winter monsoons. Concurrent meteorological and in situ physical measurements from the mooring reveal that the chlorophyll pulses are associated with the intensification of eastward winds at the surface and eastward currents in the mixed layer. These observations are inconsistent with upwelling dynamics as they occur in the Atlantic and Pacific oceans, since eastward winds that force Wyrtki jet intensification should drive downwelling. The chlorophyll spikes could be explained by two alternative mechanisms: (1) turbulent entrainment of nutrients and/or chlorophyll from across the base of the mixed layer by wind stirring or Wyrtki jet-induced shear instability or (2) enhanced southward advection of high chlorophyll concentrations into the equatorial zone. The first mechanism is supported by the phasing and amplitude of the relationship between wind stress and chlorophyll, which suggests that the chlorophyll spikes are the result of turbulent entrainment driven by synoptic zonal wind events. The second mechanism is supported by the observation of eastward flows over the Chagos–Laccadive Ridge, generating high chlorophyll to the north of the equator. Occasional southward advection can then produce the chlorophyll spikes that are observed in the mooring record. Wind-forced biweekly mixed Rossby gravity waves are a ubiquitous feature of the ocean circulation in this region, and we examine the possibility that they may play a role in chlorophyll variability. Statistical analyses and results from the OFAM3 (Ocean Forecasting Australia Model, version 3) eddy-resolving model provide support for both mechanisms. However, the model does not reproduce the observed spikes in chlorophyll. Climatological satellite chlorophyll data show that the elevated chlorophyll concentrations in this region are consistently observed year after year and so are reflective of recurring large-scale wind- and circulation-induced productivity enhancement in the central equatorial Indian Ocean.

Impact of tropical cyclones on the intensity and phase propagation of fall Wyrtki jets

Observations and model simulations are used to study the impact of tropical cyclones (TC) on the fall Wyrtki jets (WJ). These strong narrow equatorial currents peak during November and play a vital role in the energy and mass transport in the tropical Indian Ocean (TIO). Maximum number of TCs is observed over TIO during November with longer than normal life span (8–15 days). These TCs enhance equatorial westerly winds (surface) and amplify monthly mean WJs (both at surface and subsurface) by 0.4 ms−1 (anomalies exceed 0.7 ms−1during TC), which is about half of the climatological amplitude. Intensified WJs increase the heat content of eastern TIO and modulate air‐sea interaction. It is also shown that movement of TCs is mainly responsible for the westward phase propagation of WJs, a previously unexplored mechanism. These features are evident in ECCO2 simulations as well.

Impact of prolonged La Niña events on the Indian Ocean with a special emphasis on southwest Tropical Indian Ocean SST

This study examines the mechanisms governing the teleconnections associated with the long-lived La Niña variability in the tropical Indian Ocean (TIO) sea surface temperature (SST) anomalies using observational and reanalysis products. Two long-lived La Niña events (1973 to 1976 and 1998 to 2001) are observed in the recent years, one falling before and the other after the mid 1970's climatic shift. The winter (boreal) and spring (November to April) TIO SST is highly influenced by long-lived La Niña forcing. Climatic shift in mid 1970s contributes to the changes in TIO SST pattern during these two long-lived La Niña events. Surface heat flux variations due to long-lived La Niña contribute to the SST changes except in the southwest TIO. The upwelling favorable local surface wind stress curl and upwelling Rossby waves originating from the east are the dominant mechanisms responsible for the La Niña related winter time SST cooling over the southwest TIO. Long-lived La Niña induced surface wind anomalies enhance the fall Wyrtki Jet in the equatorial Indian Ocean resulting large scale anomalous heat transport. Local SST cooling reduces convection and contributes to the low rainfall over southwest TIO and the northern parts of Madagascar Island.

Intraseasonal “monsoon jets” in the equatorial Indian Ocean

The zonal wind in the equatorial Indian Ocean (EqIO) is westerly almost throughout the year. It has a strong semiannual cycle and drives the spring and fall Wyrtki jets. In addition, high resolution daily satellite winds show “westerly wind bursts” lasting 10–40 days, associated with atmospheric convection in the eastern EqIO. These bursts have the potential to produce intraseasonal eastward equatorial jets in the ocean. Using an ocean model driven by QuikSCAT scatterometer winds, we show that strong westerly bursts associated with summer monsoon intraseasonal oscillations can drive “monsoon jets” in the eastern EqIO, which have been observed recently. Although there are distinct equatorial wind bursts associated with Madden‐Julian oscillations in January–March, they do not produce equatorial jets in the ocean. The role of ocean dynamics in producing the selective response of the ocean is discussed.

Impacts of salinity on the eastern Indian Ocean during the termination of the fall Wyrtki Jet

The impact of salinity and the barrier layer on eastern Indian Ocean dynamics is investigated using the OPA ocean general circulation model for 1979–1993. The study focuses on processes involved in the eastern Indian Ocean circulation during the period of the fall Wyrtki Jet. An exploration of interactions between salinity and ocean dynamics is made using a set of salinity sensitivity experiments. The inclusion of salinity favors a stronger Wyrtki Jet that extends further eastward in two steps. First, a sporadic barrier layer along the jet increases the jet speed by trapping wind momentum in a thinner mixed layer. Second, zonal advection transports the current maximum eastward and participates in the creation of a strong current front at the eastern edge of the jet during the following month. Off Sumatra, the sea surface salinity gradient, bordering the Indian Ocean fresh pool, strengthens when the Wyrtki Jet reaches this region. The pressure gradient associated with the enhanced salinity front then becomes larger than the eastward wind stress forcing in the zonal momentum equation. This initiates a westward surface current opposed to the wind stress. The later penetration of the Wyrtki Jet into the Indian warm and fresh pool modifies the sea level and favors geostrophic westward currents around 4°N. A combination of these mechanisms additionally favors a northward shift of the Wyrtki Jet. All these phenomena affect the zonal displacement of the Indian fresh pool and create SST anomalies of 0.5°C, in a region of high ocean‐atmosphere coupling.