Wind Bursts Research Articles

The Niño-3.4 Prediction Skill of Empirically Adjusted Wind Power

AbstractWind power, defined as the energy received by the ocean from wind, has been identified as a potentially viable precursor of ENSO. The correlation between tropical Pacific wind power anomalies and eastern equatorial Pacific sea surface temperature anomalies can be enhanced over a range of lead times by applying an empirical adjusted framework that accounts for both the underlying climatological state upon which a wind power perturbation acts and the directionality of wind anomalies. Linear regression is used to assess the seasonal prediction skill of adjusted wind power in comparison to unadjusted, as well as the conventional ENSO predictors wind stress and warm water volume. The forecast skill of each regression model is evaluated in a 1800-yr preindustrial climate simulation (CESM-LENS), as well as 23 years of observations. The simulation results show that each predictor’s effectiveness varies considerably with the sample record, providing a measure of the uncertainty involved in evaluating prediction skill based on the short observational record. The influence of climatological biases is however a demonstrable concern for results from the simulated climate system. Despite the short record, the observational analysis indicates that adjusted wind power skill is comparable to the conventional dynamical predictors and notably is significantly more predictable than unadjusted wind power when initialized in the summer. Moreover, the adjusted framework results in a reduction of error when evaluating wind power associated with wind bursts, reinforcing previous findings that the adjusted framework is particularly useful for capturing the ENSO response to westerly wind bursts.

Intermittency in phytoplankton bloom triggered by modulations in vertical stability

Seasonal surface chlorophyll (SChl) blooms are very chaotic in nature, but traditional bloom paradigms have climbed out of these subseasonal variations. Here we highlight the leading order role of wind bursts, by conjoining two decades of satellite SChl with atmospheric reanalysis in the Northwestern Mediterranean Sea. We demonstrate that weekly SChl fluctuations are in phase with weekly changes in wind stress and net heat flux during the intial state of the bloom in winter and early spring, thus expanding the convection shutdown hypothesis of bloom onset to subseasonal timescales. We postulate that the mechanism reflected by this link is intermittency in vertical stability due to short-term episodes of calm weather in winter or to stormy conditions in early spring, leading to short-term variations in light exposure or to events of vertical dilution. This strong intermittency in phytoplankton bloom may probably have important consequences on carbon export and trophic web structure and should not be overlooked.

Generation of westerly wind bursts by forcing outside the tropics

The westerly wind burst (WWB) is an important triggering mechanism of El Niño and typically occurs in the western Pacific Ocean. The Fourier spectrum of the wind field over the western tropical Pacific is characterised by a large variety of peaks distributed from intra-seasonal to decadal time scales, suggesting that WWBs could be a result of nonlinear interactions on these time scales. Using a combination of observations and simulations with 15 coupled models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we demonstrate that the main drivers initiating WWBs are quantifiable physical processes rather than atmospheric stochastic signals. In this study, ensemble empirical mode decomposition (EEMD) from the Holo-Hilbert spectral analysis (HHSA) is used to decompose daily zonal winds over the western equatorial Pacific into seasonal, interannual and decadal components. The seasonal element, with prominent spectral peaks of less than 12 months, is not ENSO related, and we find it to be strongly associated with the East Asian monsoon (EAM) and cross-equatorial flow (CEF) over the Australian monsoon region. The CEF is directly related to the intensity of the Australian subtropical ridge (STR-I). Both the EAM and CEF are essential sources of these high-frequency winds over the western Pacific. In contrast, the interannual wind component is closely related to El Niño occurrences and usually peaks approximately two months prior to a typical El Niño event. Finally, the decadal element merely represents a long-term trend and thus has little to no relation to El Niño. We identified EAM- and CEF-induced westerly wind anomalies in December–January–February (DJF) and September–October–November (SON). However, these anomalies fade in March–April–May (MAM), potentially undermining the usual absence of WWBs in the boreal spring. Similar results are found in CMIP6 historical scenario data.

Evidence of eddy-related deep-ocean current variability in the northeast tropical Pacific Ocean induced by remote gap winds

Abstract. There has been a steady increase in interest in mining of deep-sea minerals in the Clarion–Clipperton Zone (CCZ) in the eastern Pacific Ocean during the last decade. This region is known to be one of the most eddy-rich regions in the world ocean. Typically, mesoscale eddies are generated by intense wind bursts channeled through gaps in the Sierra Madre mountains in Central America. Here, we use a combination of satellite and in situ observations to evaluate the relationship between deep-sea current variability in the region of potential future mining and eddy kinetic energy (EKE) in the vicinity of gap winds. A geometry-based eddy detection algorithm has been applied to altimetry sea surface height data for a period of 24 years, from 1993 to 2016, in order to analyze the main characteristic parameters and the spatiotemporal variability of mesoscale eddies in the northeast tropical Pacific Ocean (NETP). Significant differences between the characteristics of eddies with different polarity (cyclonic vs. anticyclonic) were found. For eddies with lifetimes longer than 1 d, cyclonic polarity is more common than anticyclonic rotation. However, anticyclonic eddies are larger in size, show stronger vorticity, and survive longer in the ocean than cyclonic eddies (often 90 d or more). Besides the polarity of eddies, the location of eddy formation should be taken into consideration when investigating the impacted deep-ocean region as we found eddies originating from the Tehuantepec (TT) gap winds lasting longer in the ocean and traveling farther distances in a different direction compared to eddies produced by the Papagayo (PP) gap winds. Long-lived anticyclonic eddies generated by the TT gap winds are observed to travel distances up to 4500 km offshore, i.e., as far as west of 110∘ W. EKE anomalies observed in the surface of the central ocean at distances of ca. 2500 km from the coast correlate with the seasonal variability of EKE in the region of the TT gap winds with a time lag of 5–6 months. A significant seasonal variability of deep-ocean current velocities at water depths of 4100 m was observed in multiple-year time series data, likely reflecting the energy transfer of the surface EKE generated by the gap winds to the deep ocean. Furthermore, the influence of mesoscale eddies on deep-ocean currents is examined by analyzing the deep-ocean current measurements when an anticyclonic eddy crosses the study region. Our findings suggest that despite the significant modulation of dominant current directions driven by the bottom-reaching eddy, the current magnitude intensification was not strong enough to trigger local sediment resuspension in this region. A better insight into the annual variability of ocean surface mesoscale activity in the CCZ and its effects on deep-ocean current variability can be of great help to mitigate the impact of future potential deep-sea mining activities on the benthic ecosystem. On an interannual scale, a significant relationship between cyclonic eddy characteristics and El Niño–Southern Oscillation (ENSO) was found, whereas a weaker correlation was detected for anticyclonic eddies.

Tidal and Near-Inertial Internal Waves over the Reykjanes Ridge

AbstractInternal waves in the semidiurnal and near-inertial bands are investigated using an array of seven moorings located over the Reykjanes Ridge in a cross-ridge direction (57.6°–59.1°N, 28.5°–33.3°W). Continuous measurements of horizontal velocity and temperature for more than 2 years allow us to estimate the kinetic energy density and the energy fluxes of the waves. We found that there is a remarkable phase locking and linear relationship between the semidiurnal energy density and the tidal energy conversion at the spring–neap cycle. The energy-to-conversion ratio gives replenishment time scales of 4–5 days on the ridge top versus 7–9 days on the flanks. Altogether, these results demonstrate that the bulk of the tidal energy on the ridge comes from near-local sources, with a redistribution of energy from the top to the flanks, which is endorsed by the energy fluxes oriented in the cross-ridge direction. Implications for tidally driven energy dissipation are discussed. The time-averaged near-inertial kinetic energy is smaller than the semidiurnal kinetic energy by a factor of 2–3 but is much more variable in time. It features a strong seasonal cycle with a winter intensification and subseasonal peaks associated with local wind bursts. The ratio of energy to wind work gives replenishment time scales of 13–15 days, which is consistent with the short time scales of observed variability of near-inertial energy. In the upper ocean (1 km), the highest levels of near-inertial energy are preferentially found in anticyclonic structures, with a twofold increase relative to cyclonic structures, illustrating the funneling effect of anticyclones.

Karydakis Flap: A preferred procedure for Sacrococcygeal Pilonidal Sinus Disease (SPNSD).

Objectives: To evaluate patients with SPNSD who underwent Karydakis Flap procedure with reference to operative time, drain removal time, hospital stay, complications and recurrence rate. Study Design: Prospective study. Setting: Surgical Department of Allied Hospital, Faisalabad. Period: September 2009 to January 2018. Material & Methods: A total of 117 patients underwent Karydakis Flap procedure. Data was collected according to the above said parameters .Patients were followed up for 12 months. Results: The operative time was 35.5 minutes. Average Hospital stay was 2 days. Drain removed at median of 8 days.17 patients (14%) developed seroma. Wound Infection occurred in 5 patients (4%). Wound burst opened in 1 patient (0.8%).No recurrence was noted (0%) in any patient during a follow up of 1 year. Conclusion: Karydakis Flap procedure is a relatively simple and easy technique with minimal chances of recurrence.

Ambient vibration measurements in an irregular building

We show how complex, three-dimensional modes of vibration of an irregular building can be identified by ambient vibration measurements. We present two-dimensional cross sections of the three-dimensional mode shapes and describe a measuring strategy for identifying the building properties when the excitation consists of nonstationary microtremors and short and weak wind bursts. We employ cross-correlations of measured accelerations to identify predominance of excitation by microtremors (causing wave propagation from the ground up into the building), or by weak bursts of wind (wave propagation within the building and down towards the foundation). We describe the two lowest frequencies of vibration at 10.5 Hz and at 11.9 Hz and their mode shapes.

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 Niño3.4 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 Niño, while the EOI can still match it well. Hence, the EOI can better describe the variability of the Niño3.4 SSTA and WWBs/EWBs. The correlation between the SOI and Niño3.4 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 Niño-Southern Oscillation (ENSO) and predicting ENSO.

Observations of air–sea heat fluxes in the southwestern Atlantic under high‐frequency ocean and atmospheric perturbations

AbstractTurbulent air–sea heat fluxes were computed from in situ high‐frequency micrometeorological data during two research cruises performed in the southwestern Atlantic Ocean (SWAO) occurring in June 2012 and October 2014. Two different and dynamical areas were covered by the cruises: the Brazil‐Malvinas Confluence (BMC) and the Southern Brazilian Continental Shelf (SBCS). The eddy covariance (EC) method was used to estimate the air–sea sensible‐ and latent‐heat fluxes. This article compares these novel high‐frequency estimates of heat fluxes with bulk parametrizations made at the same location and time from independent measurements taken on board the ships. When comparing the EC and bulk‐estimated time series of sensible‐heat fluxes, we found a good agreement both in their magnitude and variability, with small bias (generally <20 W·m−2) between the datasets from the two study areas in the SWAO. However, the EC and bulk latent‐heat flux comparisons show large biases ranging from 75 W·m−2 to 100 W·m−2 in the SBCS and BMC, respectively. These biases were always associated with short‐term, high‐frequency environmental perturbations occurring either in the atmosphere or in the ocean with the majority related to strong wind burst events and large air–sea temperature gradients. The short period changes in atmospheric conditions were mostly related to the passage of transient synoptic systems over the two study areas. The large air–sea temperature gradients were mostly linked to the surface characteristics of the BMC and SBCS regions, where sharp oceanographic fronts are located. Our results are able to contribute to improving weather and climate simulations of the mid‐ to high latitudes of South America, a region largely influenced by the sea‐surface temperature patterns of the SWAO in combination with the frequent propagation of transient atmospheric systems.

The Role of Westerly Wind Bursts During Different Seasons Versus Ocean Heat Recharge in the Development of Extreme El Niño in Climate Models

AbstractThe mechanisms and predictability of extreme El Niño events, as in 1982, 1997, and 2015, remain unclear. Here we investigate the relative roles of the ocean initial heat recharge and westerly wind bursts (WWBs) that occur during different seasons in the generation of extreme El Niño events in a climate model (CESM1.2). Several sets of 100‐member ensemble simulations, along with CMIP5 and atmospheric reanalysis data, are used. We show that while the probability of extreme El Niño increases progressively with ocean heat recharge, tropical conditions by year‐end are least predictable (the largest ensemble spread) for moderately‐to‐strongly recharged ocean initial states, rather than neutrally charged. Even after strong early‐year WWBs and recharged ocean the occurrence of extreme El Niño has limited predictability. While the early‐year WWBs are still important for strong El Niño, it is to a lesser extent than suggested previously, as mid‐year WWBs play a critical role in El Niño progression.

Magnetic Field Dropouts at Near-Sun Switchback Boundaries: A Superposed Epoch Analysis

Abstract During Parker Solar Probe’s first close encounter with the Sun in early 2018 November, a large number of impulsive rotations in the magnetic field were detected within 50 R s; these also occurred in association with short-lived impulsive solar wind bursts in speed. These impulsive features are now called “switchback” events. We examined a set of these switchbacks where the boundary transition into and out of the switchback was abrupt, with fast B rotations and simultaneous solar wind speed changes occurring on timescales of less than ∼10 s; these thus appear as step function-like changes in the radial component of B and V. Our objective was to search for any diamagnetic effects that might occur especially if the boundaries are associated with quick changes in density (i.e., a steep spatial density gradient at the switchback boundary). We identified 25 switchback entries where the radial component of B, B r, quickly transitioned from large negative to positive values and V r simultaneously abruptly increased (i.e., step-up transitions) and 28 switchback exits where B r quickly transitioned from large positive to negative values and V r simultaneously abruptly decreased (i.e., step-down transitions). We then performed a superposed epoch analysis on each of these sets of events. We found these fast-transitioning events typically had a clear and distinct decrease in the magnetic field magnitude by 7%–8% detected exactly at the boundary. The presence of the dropout suggests there is a diamagnetic current present at the boundary.

Effects of Semistochastic Westerly Wind Bursts on ENSO Predictability

AbstractWesterly wind bursts (WWBs) occurring over the tropical Pacific play an important role on El Niño‐Southern Oscillation (ENSO) dynamics. Currently, climate models have significant biases in their representation of WWBs, which may limit their ability to predict ENSO. In this study, we explore the possibility of improving ENSO prediction by introducing a semistochastic WWBs parameterization scheme into the Community Earth System Model (CESM). Three ensemble hindcasts, namely, the control run with the original CESM, the WWB run with parameterized WWBs, and the Non_WWB run with built‐in WWBs removed, are conducted for the period 1982–2016. We find that CESM with parameterized WWBs enables better ENSO prediction, especially for both amplitude and spatial distribution of eastern Pacific and central Pacific El Niño events. This improvement is related to more realistic representation of WWBs which leads to better prediction of surface wind stress anomalies and thermocline depth anomalies.

Intraseasonal modulation of Wyrtki jet in the eastern Indian Ocean by equatorial waves during spring 2013

A strong spring Wyrtki jet (WJ) presents in May 2013 in the eastern equatorial Indian Ocean. The entire buildup and retreat processes of the spring WJ were well captured by two adjacent Acoustic Doppler Current Profilers mounted on the mooring systems. The observed zonal jet behaved as one intraseasonal event with the significant features of abrupt emergence as well as slow disappearance. Further research illustrate that the pronounced surface westerly wind burst during late-April to mid-May, associated with the active phase of a robust eastward-propagating Madden-Julian oscillation in the tropical Indian Ocean, was the dominant reason for the rapid acceleration of surface WJ. In contrasting, the governing mechanism for the jet termination was equatorial wave dynamics rather than wind forcing. The decomposition analysis of equatorial waves and the corresponding changes in the ocean thermocline demonstrated that strong WJ was produced rapidly by the wind-generated oceanic downwelling equatorial Kelvin wave and was terminated subsequently by the westward-propagating equatorial Rossby wave reflecting from eastern boundaries of the Indian Ocean.

Wind-forced equatorial wave dynamics of the Pacific Ocean during 2014/2015 and 2015/2016 El Niño events

The equatorial wave dynamics of interannual sea level variations between 2014/2015 and 2015/2016 El Nino events are compared using the Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics Climate Ocean Model (LICOM) forced by the National Centers for Environmental Prediction (NCEP) reanalysis Ⅰ wind stress and heat flux during 2000-2015. In addition, the LICOM can reproduce the interannual variability of sea surface temperature anomalies (SSTA) and sea level anomalies (SLA) along the equator over the Pacific Ocean in comparison with the Hadley center and altimetric data well. We extracted the equatorial wave coefficients of LICOM simulation to get the contribution to SLA by multiplying the meridional wave structure. During 2014/2015 El Nino event, upwelling equatorial Kelvin waves from the western boundary in April 2014 reach the eastern Pacific Ocean, which weakened SLA in the eastern Pacific Ocean. However, no upwelling equatorial Kelvin waves from the western boundary of the Pacific Ocean could reach the eastern boundary during the 2015/2016 El Nino event. Linear wave model results also demonstrate that upwelling equatorial Kelvin waves in both 2014/2015 and 2015/2016 from the western boundary can reach the eastern boundary. However, the contribution from stronger westerly anomalies forced downwelling equatorial Kelvin waves overwhelmed that from the upwelling equatorial Kelvin waves from the western boundary in 2015. Therefore, the western boundary reflection and weak westerly wind burst inhibited the growth of the 2014/2015 El Nino event. The disclosed equatorial wave dynamics are important to the simulation and prediction of ENSO events in future studies.

A Numerical Study on the Influences of Sumatra Topography and Synoptic Features on Tropical Cyclone Formation over the Indian Ocean

Abstract Spanning across the equator with a northwest–southeast orientation, the island of Sumatra can exert significant influences on low-level flow. Under northeasterly flow, in particular, lee vortices can form and some of them may subsequently develop into tropical cyclones (TCs) in the Indian Ocean (IO). Building upon the recent work of Fine et al., this study investigates the roles of the Sumatra topography and other common features on the formation of selected cases for analysis and numerical experiments. Four cases in northern IO were selected for analysis and two of them [Nisha (2008) and Ward 2009)] for simulation at a grid size of 4 km. Sensitivity tests without the Sumatra topography were also performed. Our results indicate that during the lee stage, most pre-TC vortices tend to be stronger with a clearer circulation when the topography is present. However, the island’s terrain is a helpful but not a deciding factor in TC formation. Specifically, the vortices in the no-terrain tests also reach TC status, but just at a later time. Some common ingredients contributing to a favorable environment for TC genesis are identified. They include northeasterly winds near northern Sumatra, westerly wind bursts along the equator, and migratory disturbances (TC remnants or Borneo vortices) to provide additional vorticity/moisture from the South China Sea. These factors also appear in most of the 22 vortices in northern IO during October–December in 2008 and 2009. For the sole case (Cleo) examined in southern IO, the deflection of equatorial westerlies into northwesterlies by Sumatra (on the windward side) is also helpful to TC formation.

Diurnal Cycling of Submesoscale Dynamics: Lagrangian Implications in Drifter Observations and Model Simulations of the Northern Gulf of Mexico

AbstractThe diurnal cycling of submesoscale circulations in vorticity, divergence, and strain is investigated using drifter data collected as part of the Lagrangian Submesoscale Experiment (LASER) experiment, which took place in the northern Gulf of Mexico during winter 2016, and ROMS simulations at different resolutions and degree of realism. The first observational evidence of a submesoscale diurnal cycle is presented. The cycling is detected in the LASER data during periods of weak winds, whereas the signal is obscured during strong wind events. Results from ROMS in the most realistic setup and in sensitivity runs with idealized wind patterns demonstrate that wind bursts disrupt the submesoscale diurnal cycle, independently of the time of day at which they happen. The observed and simulated submesoscale diurnal cycle supports the existence of a shift of approximately 1–3 h between the occurrence of divergence and vorticity maxima, broadly in agreement with theoretical predictions. The amplitude of the modeled signal, on the other hand, always underestimates the observed one, suggesting that even a horizontal resolution of 500 m is insufficient to capture the strength of the observed variability in submesoscale circulations. The paper also presents an evaluation of how well the diurnal cycle can be detected as function of the number of Lagrangian particles. If more than 2000 particle triplets are considered, the diurnal cycle is well captured, but for a number of triplets comparable to that of the LASER analysis, the reconstructed diurnal cycling displays high levels of noise both in the model and in the observations.

A Study on Assimilation of CYGNSS Wind Speed Data for Tropical Convection during 2018 January MJO

The National Aeronautics and Space Administration (NASA) Cyclone Global Navigation Satellite System (CYGNSS) mission was launched in December 2016. CYGNSS provides ocean surface wind speed retrieval along specular reflection tracks at an interval resolution of approximately 25 km. With a median revisit time of 2.8 h covering a ±35° latitude, CYGNSS can provide more frequent and accurate measurements of surface wind over the tropical oceans under heavy precipitation, especially within tropical cyclone cores and deep convection regions, than traditional scatterometers. In this study, CYGNSS v2.1 Level 2 wind speed data were assimilated into the Weather Research and Forecasting (WRF) model using the WRF Data Assimilation (WRFDA) system with hybrid 3- and 4-dimensional variational ensemble technology. Case studies were conducted to examine the impact of the CYGNSS data on forecasts of tropical cyclone (TC) Irving and a westerly wind burst (WWB) during the Madden–Julian oscillation (MJO) event over the Indian Ocean in early January 2018. The results indicate a positive impact of the CYGNSS data on the wind field. However, the impact from the CYGNSS data decreases rapidly within 4 h after data assimilation. Also, the influence of CYGNSS data only on precipitation forecast is found to be limited. The assimilation of CYGNSS data was further explored with an additional experiment in which CYGNSS data was combined with Global Precipitation Mission (GPM) Integrated Multi-satellitE Retrievals for GPM (IMERG) hourly precipitation and Advanced Scatterometer (ASCAT) wind vector and were assimilated into the WRF model. A significant positive impact was found on the tropical cyclone intensity and track forecasts. The short-term forecast of wind and precipitation fields were also improved for both TC Irving and the WWB event when the combined satellite data was assimilated.

A Statistical Comparison of the Westerly Wind Bursts between the Positive and Negative Phases of the PDO

To understand the diversity of the El Nino-Southern Oscillation (ENSO) under the background of Pacific decadal oscillation (PDO) during recent decades, characteristics of westerly wind bursts (WWBs) during positive and negative phases of the PDO were analyzed. It is shown that, during the ENSO developing period, the El Nino evolution may be affected by stronger or more frequent WWBs in the positive PDO phase than in the negative PDO phase. The sustained effects of atmospheric dynamics on the equatorial ocean can be indicated by the accumulated WWB strength, which contains most WWB characteristics, including the accumulated days, occurrence frequency, strength, and spatial range of WWBs. The synoptic/climate systems that are directly related to WWBs show a wider spatial distribution in the positive PDO phase than in the negative PDO phase.

Observed Deep‐Reaching Signatures of the Madden‐Julian Oscillation in the Ocean Circulation of the Western Tropical Pacific

AbstractProminent intraseasonal variability (ISV) of ocean currents was detected by two subsurface moorings deployed at 140°E, 2°N, and 4.7°N during 2014–2018 and shown to be largely induced by westerly wind bursts (WWBs) associated with passages of Madden‐Julian oscillations (MJOs). The North Equatorial Countercurrent between 60 and 200 m is enhanced (attenuated) by 6–8 cm/s prior (subsequent) to the WWB. Current anomalies below 300 m are as large as 2–3 cm/s, close in magnitude to the mean velocities of the North Equatorial Subsurface Current and North Subsurface Countercurrent. The ISV in the upper 600 m shows a vertical dipole structure at 2°N and vertically coherent anomalies at 4.7°N. Further analysis combining satellite data reveals local recirculation structures generated by wind‐forced Rossby waves. Ocean model experiments confirm the importance of the MJO's wind forcing in causing these subsurface ocean ISVs.

Anomalous warming of the western equatorial Indian Ocean in 2007: Role of ocean dynamics

In this study, observational data, together with the simulations from a high-resolution numerical ocean model, are used to identify the processes responsible for the anomalous warm sea surface temperature (SST) in the equatorial Indian Ocean during late spring/early summer in 2007. Our analysis suggests that the SST in the western equatorial Indian Ocean (WEIO) remained unusually warm owing to the combined influence of the wind induced and reflected Rossby waves in the equatorial Indian Ocean. Anomalous westerly wind burst in the equatorial Indian Ocean in early April generated an eastward propagating downwelling Kelvin wave, which got reflected from the eastern boundary as a downwelling Rossby wave and propagated to the WEIO. The easterlies appeared during the second fortnight of April generated another downwelling Rossby wave, which also propagated to the WEIO. Thus a couple of downwelling Rossby waves, reflected and direct wind induced, propagated to the WEIO, deepened the thermocline and prohibited cooling in the WEIO during the early months of summer monsoon season. This study demonstrates the importance of the equatorial winds and associated oceanic dynamics in modulating the SST variability in the WEIO.