ENSO and climatic signals across the Indian Ocean Basin in the global context: part I, interannual composite patterns

Abstract

This study focuses on the interplay between mean sea level pressure (MSLP), sea surface temperature (SST), and wind and cloudiness anomalies over the Indian Ocean in seasonal composite sequences prior to, during, and after strong, near-global El Niñ o and La Niñ a episodes. It then examines MSLP and SST anomalies in the 2 -2.5-year quasi-biennial (QB) and 2.5-7-year low-frequency (LF) bands that carry the bulk of the raw ENSO signal. Finally, these fields were examined in conjunction with patterns of correlations between rainfall and joint spatiotemporal empirical orthogonal function (EOF) time series band pass filtered in the QB and LF bands.The seasonal composites indicate that the El Niñ o-1 (La Niñ a-1) pattern tends to display a more robust and coherent (weaker and less organized) structure during the evolution towards the mature stage of the event. The reverse tends to be apparent in the cessation period after the peak phase of an event, when El Niñ o events tend to collapse quite quickly.Climatic variables over the Indian Ocean Basin linked to El Niñ o and La Niñ a events show responses varying from simultaneous, to about one season's lag. In general, SSTs tend to evolve in response to changes in cloud cover and wind strength over both the north and south Indian Ocean. There are also strong indications that the ascending (descending) branch of the Walker circulation is found over the African continent (central Indian Ocean) during La Niñ a phases, and that the opposite configuration occurs in El Niñ o events. These alternations are linked to distinct warm -cool (cool-warm) patterns in the north-south SST dipole over the western Indian Ocean region during the El Niñ o (La Niñ a) events.An examination of MSLP and SST anomaly patterns in the QB and LF bands shows that signals are more consistent during El Niñ o-1 and El Niñ o sequences than they are during La Niñ a-1 and La Niñ a sequences. The QB band has a tendency to display the opposite anomaly patterns to that seen on the LF band during the early stages of event onset, and later stage of event cessation, during both El Niñ o -Southern Oscillation (ENSO) phases. El Niño events tend to be reinforced by signals on both bands up to their mature phase, but are then seen to erode rapidly, as a result of the presence of distinct La Niñ a anomalies on the QB band after their peak phase. During La Niña events, the opposite is observed during their cessation phase.Both QB and LF bands often display SST dipole anomalies that are not clearly evident in the raw composites alone. An eastern Indian Ocean SST dipole shows a tendency to occur during the onset phase of particular El Niño or La Niñ a episodes, especially during the austral autumn -winter (boreal spring -summer) and, when linked to tropical-temperate cloud bands, can influence Australian rainfall patterns.Analyses of seasonal correlations between rainfall and joint MSLP and SST EOF time series on QB and LF bands and their dynamical relationship with MSLP and SST anomalies during El Niñ o and La Niñ ...

A link between low‐frequency mesoscale eddy variability around Madagascar and the large‐scale Indian Ocean variability

“…Reason et al [2000] described a similar wind curl pattern due to ENSO, but with anomalies extending further east into the Indonesian passages. Furthermore, anomalous winds around Indonesia due to both the IOD and ENSO alter the normal Indo-Pacific sea level gradient, which is the driving force of the Indonesian Throughflow (ITF): during El Niño (La Niña) events, a lower (higher) ITF transport is expected, and vice versa for the IOD phases [Meyers, 1996].…”

“…highly energetic eddies) in 1995and of low EKE in 1997and 2000-2001, with an EKE amplitude of 600 -800 cm 2 /s 2 (Figure 5a). Interestingly, this interannual variation is not present in the EKE in northern regions of the MZC.…”

“…[23] The wind curl variability in the subtropical IO is related to both the IOD and ENSO cycles [Reason et al, 2000]. For instance, the wind curl anomalies averaged over the region 10°S-15°S, 70°E -100°E exhibit a significant correlation with the SOI.…”

“…[28] Two periods of high EKE ( , early 1998( ) and low EKE (1997( , 2000( -2001 are observed in the CC region (Figure 5a), which coincide with years over which the flow in the narrows was relatively high and low, respectively (Figure 10d). The EKE in CC shows a significant correlation with the local SSH field (section 3), and with that north of the Channel.…”

The Tropospheric Biennial Oscillation and Indian Monsoon rainfall

“…However, it has been shown that other years display similar patterns to ENSO in this large region [Meehl, 1987;Lau and Wu, 2001]. In fact, the strength of TBO and ENSO signals is closer in magnitude over the Indian Ocean than the equatorial Pacific (Reason et al, 2000). Therefore the TBO must include most ENSO (El Nifio and La Nifia) years and some other years as well.…”