Impact of atmospheric convectively coupled equatorial Kelvin waves on upper ocean variability

Abstract

Convectively coupled Kelvin waves (CCKWs) are atmospheric weather systems that propagate eastward along the equatorial wave guide with phase speeds between 11 and 14 m s À1 . They are an important constituent of the convective envelope of the Madden-Julian oscillation (MJO), for which ocean-atmosphere interactions play a vital role. Hence, ocean-atmosphere interactions within CCKWs may be important for MJO development and prediction and for tropical climate, in general. Although the atmospheric structure of CCKWs has been well studied, their impact on the underlying ocean is unknown. In this paper, the ocean-atmosphere interactions in CCKWs are investigated by a case study from November 2011 during the CINDY/DYNAMO field experiment, using in situ oceanographic measurements from an ocean glider. The analysis is then extended to a 15 year period using precipitation data from the Tropical Rainfall Measuring Mission and surface fluxes from the TropFlux analysis. A methodology is developed to calculate trajectories of CCKWs. CCKW events are strongly controlled by the MJO, with twice as many CCKWs observed during the convectively active phase of the MJO compared to the suppressed phase. Coherent ocean-atmosphere interaction is observed during the passage of a CCKW, which lasts approximately 4 days at any given longitude. Surface wind speed and latent heat flux are enhanced, leading to a transient suppression of the diurnal cycle of sea surface temperature (SST) and a sustained decrease in bulk SST of 0.1°C. Given that a typical composite mean MJO SST anomaly is of the order of 0.3°C, and more than one CCKW can occur during the active phase of a single MJO event, the oceanographic impact of CCKWs is of major importance to the MJO cycle.

Multiple and spin off initiation of atmospheric convectively coupled Kelvin waves

“…We decided to keep the temporal constrain set to 5 days because beyond that period there is no reason to expect that the diurnal cycle is influenced by the preceding wave. In Baranowski et al (2016) we showed that the magnitude of the diurnal cycle is strongest 2 days after a passage of a developed CCKW. Additionally, a predominant period of CCKW occurrence is 5 days, so clear sky, non-convective conditions preceding CCKW development, should persist for up to 5 days.…”

“…The database has been introduced and is described in more detail in Baranowski et al (2016), here we provide just a short summary. The CCKW trajectories are defined by performing a wavenumber-frequency filtering on the TRMM 3B42 precipitation data (Huffman et al 2007) averaged between 2.5S and 2.5N, following methodology of Wheeler and Kiladis (1999).…”

“…The two nearby maxima are considered part of the same trajectory when they occur within 6 h of each other. Such derived trajectory is quality controlled against unfiltered precipitation data to correct potential discrepancies in the location of the beginning and end of each trajectory (Baranowski et al 2016).…”

“…All data are averaged in the equatorial belt (2.5S-2.5N) in order to create surface fluxes consistent with the precipitation data and CCKW trajectories. An estimate of magnitude of the upper ocean temperature diurnal cycle is based on the TropFlux dataset (Praveen Kumar et al 2013) following methodology described by Matthews et al (2014) and Baranowski et al (2016). It uses daily mean values of shortwave radiation flux and wind speed at the ocean surface to estimate the daily mean diurnal SST anomaly.…”

“…A recent study, based on analysis of the CCKW trajectories (Baranowski et al 2016) showed that in the Indian Ocean (50-100 °E) number of CCKW initiations is high (11 ± 3 initiations in each 1° longitude bin) and number of ending trajectories is low (3 ± 2 terminations). That suggests that local environmental conditions are favorable for CCKW initiation over broad area in the Indian Ocean basin.…”