1Introduction

Interactions between the atmosphere and the ocean are very important in the climate system. The oceans play an important role in the climate system owing in part to their large heat-storage Capacity. Vertical water movement in the ocean changes sea surface temperatures (SST) and vice versa and thus causing changes in MLD. In this paper the main focus is to understand the variability of Ekman pumping representing vertical mixing and mixed layer depth during strong El Nino/La Nina and IOD events. Ekman pumping is represented by τ/(ρf), where τ is the surface wind stress, ρ is seawater density (1025 kg mˉ³), and f is the Coriolis parameter (=2Ωsinθ, with Ω and θ equal to Earth’s angular velocity and latitude, respectively). The surface wind stress τ is calculated using the bulk formulation,

                     τ = [τ_x, τ_y] = ρ_aC_D V [u, v],         (1)

Where, τ_x and τ_y are east-west and north-south components, respectively. The surface wind (nominally at 10 m) is assumed to be parallel to the stress vector, with components [u, v] and magnitude V (= wind speed). ρ_a is the density of surface air, C_D is the drag coefficient. Vertical velocity at the bottom of the Ekman layer (effective depth of frictional influence) from wind stress, W_EK, is named Ekman suction if upward and Ekman pumping if downward as in (Stommel, 1958). Ekman transport is proportional to the wind stress and inversely proportional to the sine of the latitude as discussed by (Sverdrup et al., 1942). Ekman pumping analysis is given as

                     W_EK   = (curl τ)/ (ρf),                 (2)

Fennel (1999) showed theoretically that wind stress curl could have a substantial impact in coastal upwelling. Off Oregon west coast of North America, Ekman suction was a major contributor to the total upward velocity during coastal upwelling as discussed by (Halpern, 1976). Off the west coast of South America, El Nino conditions include a deepening of the coastal thermocline (Strub, et al., 1998). During the 1997-98 El Nino, the thermocline deepened by 75 m near the coast of southern Peru. Wooster and Guillen (1974) posited that the deepened coastal thermocline off Peru during El Nino was a consequence of the Bjerknes (1969) breakthrough explanation about the weakening of the southeasterly trade wind as the fundamental generating mechanism of El Nino. Observations along the equatorial Pacific during the 1982-1983 El Nino confirmed that thermocline deepened when the southeasterly trade winds weakened (Halpern, 1987). However at the coast of Peru during El Nino, τ_alongshore increased (Enfield, 1981; Huyer et al., 1987), which would lift the thermocline. In the present study τ_alongshore is computed as in Halpern (2002) showing increase during 1997-1998 El Nino, and hence emphasizing importance of Ekman pumping in deepening of thermocline. 

The definition of the mixed layer can be based on different physical parameter (e.g., temperature, density, salinity) and may represent averages over different time interval. The theoretical and experimental foundations of the concepts of diurnal and seasonal mixed layers are described in detail by Brainerd and Gregg, (1995). Solar warming occurs only during the daytime, and mixing occurs primary at night time in regions of light winds (Moum, 1989). Thus seasonal and inter-annual variations in wind mixing and surface heating can be expected to cause variations in the SST diurnal cycle, as well as seasonal and inter-annual variations in MLD. Cronin, (2002)studied modulation of mixed layer variability at 0˚, 110˚W in the eastern equatorial Pacific from Tropical Atmosphere Ocean (TAO) mooring during warm and cold phases of El Nino-Southern oscillation (ENSO).In the present study MLD is also analyzed during El Nino/La Nina events. The years 1997-1998, 1982-1983 (El Nino) and 1988-1989 (La Nina) have been considered. Similarly the positive IOD years 1994, 1997 and negative IOD year 1992 are taken into account to examine variability of Ekman pumping and MLD in the Indo-Pacific Ocean.

2. Data Sources

Monthly mean wind data sets are utilized from the European Center for Medium Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim). The data are available with a spatial resolution of 0.5˚ on both longitude and latitude. Monthly data are used to compute wind stress and curl of wind stress. The wind stress curl is computed with constant air density (ρ_a=1.225 kg.mˉ³) and drag coefficient dependent on wind speed at 10-m height (Trenberth et al., 1990). Ekman pumping is calculated from equation (2) using this data. The alongshore wind stress (τ_alongshore) is calculated which equals wind stress magnitude multiplied by cosa,a being the difference in the directions of the wind stress and the 45˚ alignment of the coastline in the south east Pacific region as in Halpern, (2002). Monthly mean Ekman pumping and anomalies of these fields are calculated using 1980-2011 base period.

The Sea surface temperature (SST) data used here is the improved Extended Reconstructed Sea surface temperature version 4 (ERSST V4) (Smith and Reynolds, 2004). This data analyses use monthly and 2˚ spatial resolution. Note that the main conclusion of the present study is not critical to the data period because most El Nino, La Nina and IOD events occur since 1990.

Monthly mean Mixed Layer Depth (MLD) data available at 1˚´1˚ from the European Center for Medium Range Weather Forecasts (ECMWF) is used. MLD analyses are from ocean mixed layer model which uses a non-local K profile parameterization (Large et al., 1994).

Throughout the paper all of the datasets have been monthly averaged. Seasonal cycles are determined over the period 1980–2011, which includes two strong El Nino events, 1982-83 and 1997-98 as well as a strong La Nina event in 1988-89 and two positive IOD events in 1994, 1997 as well as a negative IOD event in 1992. The seasonal cycle is defined as the monthly mean over these 32 years.

3. Results and discussion

3.1 El Nino and La Nina events

Figure 1 displays evolution of SST anomalies during 1997-1998 El Nino events. Positive SST anomalies are seen to develop in the eastern tropical Pacific Ocean from the month of May 1997, which extend to the central Pacific and intensify during subsequent months till January 1998. Afterward anomalies weaken. The Western tropical Pacific has colder SST anomalies. The maximum values of the warm SST are located in the coastal region of the eastern boundary. As the El Nino develops further, the SST pattern is mostly stationary in spite of a slightly westward shift of the maximum SST anomaly with magnitude of 3.5˚C. Figure 2 displays anomalies of (a) along shore wind stress, (b) Ekman pumping, (c) MLD and (d) SST, averaged over 90˚W-88˚W and13˚S-15˚S region (viz. 2˚x2˚ box near the south east Pacific coast), from January 1994 to December 1998. During the mature phase of 1997 El Nino event i.e. July /August 1997 higher τ_alongshore anomalies with magnitude of (~2*10^-2 Nm^-2) along with deeper thermocline are seen in the south eastern tropical Pacific. Ekman suction is replaced by Ekman pumping with magnitude of anomalies -0.9 *10^-6 m/s (figure 2b). 

Weakened trade winds during the El Nino allow warmer water from the western Pacific to surge eastward. This leads to a buildup of warm surface water and a sinking or deepening of the thermocline in the eastern Pacific. The anomalous trade winds flow from west to eastern Pacific in El Nino event, where the Ekman pumping is negative (downwelling). Positive MLD anomalies of the order of 40 m are seen in eastern tropical Pacific Ocean Figure 2(c). In the El Nino event deeper thermocline is present in the eastern tropical Pacific and has negative Ekman pumping which cause mixed layer depth to increase and hence positive MLD anomalies occur. This demonstrates the increase in τ_alongshore field over the coastal waters off Peru during the 1997–1998 El Nino, which should have raised the thermocline. However, Ekman pumping is playing important role in deepening of thermocline. Positive SST anomalies of the order of (~2˚C) are seen during this event from figure 2(d). 

Monthly Ekman pumping anomalies for 5 years covering 1982-83 El Nino event and 1988-1989 La Nina event averaged over the same region as above , are shown in figure 3(a &b). Negative Ekman pumping anomalies (Peak of the order of 0.5 *10^-6 m/s in 1982 August) and positive Ekman pumping anomalies (peak ~ 0.9 *10^-6 m/s in 1988 September) are easily evident from the figure which would cause deepening and shoaling of thermocline respectively. Comparing the two El Nino events amplitude of Ekman pumping is more during 1997 -98 than that during 1982-83 attributing to the stronger former event. MLD anomalies in the eastern Pacific averaged over complete event are more (~20-30m) in the 1997-98 event than in the 1982-83 event (~10-20m) (Figure not shown).Ekman pumping anomalies are of the same order (~0.9 *10^-6 m/s) in the strong El Nino and strong La Nina event. Ekman pumping is replaced with Ekman suction in the eastern tropical Pacific and hence shallow thermocline is present during La Nina.

Figure 4 shows composite of SST anomalies, Ekman pumping anomalies and MLD anomalies in the El Nino and La Nina event. It is clearly seen that during El Nino events positive SST anomalies, negative Ekman pumping anomalies causing downwelling and hence positive MLD anomalies that is deepening of MLD exist in the eastern tropical Pacific Ocean whereas negative SST anomalies, positive Ekman pumping anomalies and negative MLD anomalies exist during La Nina event. In the west Pacific Ocean this becomes opposite. This composite picture also points that the amplitude of Ekman pumping and MLD anomalies remains unaltered in the south east Pacific during El Nino or La Nina event.

3.2 Positive and Negative IOD events

The Indian Ocean Dipole is a coupled ocean and atmosphere phenomenon in the equatorial Indian Ocean that affects the climate of Australia and other countries that surround the Indian Ocean basin (Saji et al., 1999). The strength of IOD event is commonly measured by an index called the Dipole Mode Index (DMI), which is the difference between SST anomalies in the western (50°E to 70°E and 10°S to 10°N) and eastern (90°E to 110°E and 10°S to 0°S) equatorial Indian Ocean. The strong positive IOD events during 1994 and 1997 and the strong negative IOD event of 1992 are considered for computation of Ekman pumping.

Figures 5 shows monthly anomalies of (a) τ_alongshore, (b) Ekman pumping, (c) MLD and (d) SST averaged over the region (97° - 100°E, 2°S - 5°S) in the south east equatorial Indian Ocean, from January 1992 to December 1998 which includes the IOD years 1992, 1994 and 1997. During the negative IOD year 1992, τ_alongshore anomalies in the eastern box are negative (minimum ~ 2.5 *10^-2 Nm^-2 in September 1992) from figure 5(a) showing reduction in along shore wind stress which would deepen the thermocline. Also negative Ekman pumping anomalies (~-9 *10^-5 m/s) are playing role in deepening the thermocline and effectively increasing the mixed layer depth (Fig. 5b &c). Positive MLD anomalies of the order of 20 m are clearly seen in figure 5(c). Positive SST anomalies of the order of (> 0.5˚C) exist due to downwelling in the eastern box (Fig. 5d).

In the positive IOD year 1994, positive τ_alongshore anomalies of the order of 5*10^-2 Nm^-2 are seen in the month of June associated with positive Ekman pumping anomalies of magnitude ~12*10^-5 m/s from figure 5(a &b). The negative MLD anomalies (> 20 m) and negative SST anomalies (> 2˚C) are also evident from figures 5(c &d). Similarly during another positive IOD event in 1997, positive τ_alongshore anomalies (>4*10^-2 Nm^-2), and positive Ekman pumping anomalies (> 6*10^-5 m/s) and negative MLD anomalies (>20 m) and negative SST anomalies (>2.5˚C), are readily seen from the figures 5(a, b, c &d). These results are consistent with the earlier study in which positive wind curl anomaly was found in the eastern tropical Indian Ocean between 0˚-10˚S during the positive phase of the IOD (Saji et al., 1999; Huang and Kinter III, 2002) causing cooler sea surface due to upwelling.

Thus increase in the along shore wind stress causing lifting of thermocline and consequently decreasing MLD due to Ekman suction and cooler SST anomalies due to upwelling is readily seen in the positive IOD events near south east equatorial Indian ocean coast and vice versa is seen in the negative IOD event. Figure 6 depicts composite of SST anomalies, Ekman pumping anomalies and MLD anomalies in the positive and negative IOD years. The eastern box is characterized by cooler SST anomalies during positive IOD event and correspondingly positive Ekman pumping anomalies and negative MLD anomalies whereas western box has negative Ekman pumping anomalies, and positive MLD anomalies in agreement with warmer SST anomalies. During negative IOD event these anomalies change sign as polarity reverses. Comparing magnitudes during positive and negative dipole events cooling in the east box during positive IOD (~ 2˚C) is more than warming during negative IOD event (~1˚C). In both the east box and west box Ekman pumping values are more during positive IOD than that during negative IOD. Also, the magnitude of MLD anomalies is more during positive IOD than during negative IOD event in both eastern and western boxes. In both the IOD events positive and negative, amplitude of deviations in Ekman pumping, MLD and SST is more in eastern box than those in the western box. This is clearly seen from the table I described below.

Further, MLD and Ekman pumping responses during El Nino and IOD phenomena are considered united. Table 1 is gives the amplitude of SST, MLD and Ekman pumping deviations during different El Nino and IOD events considering the average over all months of event occurrence. It is seen that SST deviations are more during El Nino (~2 - 3˚C) than that in IOD event, values being in the range of 0.5 to 1.5˚C. However, magnitude of Ekman pumping anomalies are higher (by one order) during IOD event especially positive one as compared to El Niño/La Nina event. Deviations in the MLD are also higher in case of IOD events. Thus the ocean parameters like MLD and Ekman pumping respond to a greater extent during IOD than during El Nino/La Nina event.

4. Conclusions

In the present study Ekman pumping and MLD variability is examined during El Nino and IOD events in the Indo-Pacific region. In the El Nino years negative Ekman pumping anomalies occur in the eastern tropical Pacific Ocean in accordance with positive MLD anomalies and warmer SSTs with deeper thermocline depth present there. Stronger MLD deviations are found during stronger El Nino event of 1997-98. The alongshore wind stress component increased during this event which would act to lift the thermocline. In spite of this, downwelling occurred and hence importance of Ekman pumping in deepening of thermocline is emphasized. During La Nina event, positive Ekman pumping anomalies and negative MLD anomalies occur in eastern tropical Pacific Ocean. Magnitude of Ekman pumping remains approximately same during El Nino or La Nina in the south east pacific. During positive IOD events eastern box has positive Ekman pumping and negative MLD anomalies, and vice versa for the negative IOD events. More Ekman pumping anomalies are found in the eastern box during strong positive IOD event in 1994. Stronger deviations in Ekman pumping and MLD are seen during IOD event as compared to those in El Nino/La Nina events.

Acknowledgements

The authors wish to thank the Director, I.I.T.M. from providing the facilities. Wind data from ECMWF and MLD & SST data from APDRC website is gratefully acknowledged. Authors are thankful to Dr. Brian Doty for the GrADS software.

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