The bimodal oscillating system (BiOS) consists in an oscillation of the Ionian Sea surface structure with period of 12–13 years, which reflects in a near-surface circulation inversion. BiOS regimes are deeply interconnected with the circulation patterns of the Eastern Mediterranean, and it is a dominant process governing water masses formation, air-sea fluxes and bio-geochemical properties, which impacts living organisms. The BiOS has been partially explained as a self-sustained oscillation maintained by the interplay between Adriatic dense water formation and changes in the relative volume of waters of Levantine and Atlantic origin entering the Adriatic; however, attempts have also been made to explain the BiOS in terms of atmospheric-related processes. Despite the intensive research aiming at reproducing this oscillating system, the fundamental question “which is the source of energy necessary to initiate the BiOS?” has, until now, remained unanswered. The scope of this paper is two-fold. First, we document that, since 1993, two periods in the BiOS can be observed: a first one, between 1993 and 2017, during which the BiOS damped up to nearly disappear, with e-folding time of 11 years; and a second one, starting in 2017, during which the BiOS revitalized. Then, we propose here an analytical model that, under a two-layer ocean assumption, shows how it is possible for winds rotating in the same direction to initiate oscillations of the free surface, as result of the competing effects of wind and internal fluid pressure fields. The proposed model forced with wind data could successfully reproduce the characteristic time scales of the BiOS cycle over the period 1993–2019, and is therefore offered as a novel vision explaining the originating mechanism as the basis of its initiation, as well as a fundamental tool to address possible BiOS regimes in future climate scenarios.

A 1-yr-long series of Advanced Very High Resolution Radiometer sea surface temperature (SST) images is used to study the SST distribution in 1992 over the Channel of Sicily. Time- and spatial scales of the SST are studied by estimating the SST autocorrelation function directly from satellite images. SST timescales vary from 1 week to 45 days, with lower values in areas of intense upwelling. Spatial scales of the SST show seasonal variations with values from 80 to 300 km, with higher values during the summer. Oscillations with periods from 11 to 35 days modulate the annual variability of the SST spatial scales. The first four internal radii of deformation are estimated on the basis of climatological density profiles. The first internal radius varies from 7 km during the winter to 14 km during the summer. It is shown that, although differences between horizontal scales of the SST and internal radii of deformation exist, the annual variability of the SST spatial scales can be interpreted in terms of... Abstract A 1-yr-long series of Advanced Very High Resolution Radiometer sea surface temperature (SST) images is used to study the SST distribution in 1992 over the Channel of Sicily. Time- and spatial scales of the SST are studied by estimating the SST autocorrelation function directly from satellite images. SST timescales vary from 1 week to 45 days, with lower values in areas of intense upwelling. Spatial scales of the SST show seasonal variations with values from 80 to 300 km, with higher values during the summer. Oscillations with periods from 11 to 35 days modulate the annual variability of the SST spatial scales. The first four internal radii of deformation are estimated on the basis of climatological density profiles. The first internal radius varies from 7 km during the winter to 14 km during the summer. It is shown that, although differences between horizontal scales of the SST and internal radii of deformation exist, the annual variability of the SST spatial scales can be interpreted in terms of...

El Niño–Southern Oscillation (ENSO) is the Earth’s strongest source of year-to-year climate variability, whose center of action, although being in the tropical Pacific, influences the global climate, impacting also security aspects. ENSO is commonly described as an irregular oscillation of the coupled ocean-atmosphere system, which produces an oscillation in the sea surface temperature between a warm (El Niño) and a cold (La Niña) phase induced by internal waves propagating along the equatorial wave-guide and generated by the convergence between high-frequency westerlies and the quasi-stationary easterlies. Here we show that internal, downwelling Rossby and Kelvin waves are regularly formed beneath the Easterly/Westerly Wind Convergence Zone (EWCZ). When the longitude of EWCZ is located east of 175^oE, the Kelvin waves formed beneath the EWCZ reach the eastern Pacific boundary and determine an El Niño event and, the more the EWCZ elongates eastwards into the central Pacific, the more the observed El Niño event is classified as severe, suggesting that the position of the EWCZ is an important factor to trigger El Niño. Therefore, we study zonal shifts of EWCZ and show that the EWCZ and the deep atmospheric convection displace zonally in phase with each other and with the Southern Oscillation Index, indicating that the longitude of the clash between Easterlies and Westerlies determines the climate state of the Pacific and triggers the El Niño initiation.

The Ionian Sea is a crucial intersection for various water masses in the Mediterranean. Its hydrography and dynamics play a significant role in the seawater budgets and biogeochemistry of the neighboring sub-basins. Multiple theories have been formulated to gain a better understanding of the Ionian dynamics. These theories primarily attribute the variability of the near-surface Ionian circulation to internal processes. Here, we utilize horizontal currents and temperature–salinity profiles from the Copernicus reanalysis to examine the contribution of baroclinic modes to the variability of the basin horizontal circulation. Our findings demonstrate that, although the basin vertical structure is characterized by three layers, the primary patterns of the Ionian circulation can be attributed to the first baroclinic mode. This mode, along with the barotropic mode, accounts for over 85% of the overall variability in the Ionian circulation, suggesting that only one of the three interfaces separating the different water masses in the basin is dynamically active. We estimate the depth of this interface to be about 490 m. Additionally, our analysis shows that more than 90% of the kinetic energy over the water column is localized above this interface, indicating that the deep layer of the Ionian is dynamically nearly inert.

Internal Kelvin Wave (KW) propagation is studied about variations in the sea surface temperature anomaly (SSTA) over the tropical Pacific. Temperature and Salinity (TS) observations have been used to define the vertical structure of the ocean about the propagation properties of KWs. Changes in the vertical structure of the water column determine consistent zonal variations in the wave velocity, with values varying, roughly, from 1.8 to 2.6 m/s. The authors document that KWs are formed regularly at the western boundary of the tropical Pacific, but, in these cases, never overcome the dateline. Occasionally, KWs are generated in the region comprised between 170oE and 170oW, and, on all these occasions, a positive phase of the El Niño Southern Oscillation (El Niño) event is recorded. A model, named Sloping Interface Model (SIM), is proposed to relate changes in the pycnocline depth, associated with transiting KWs, and SST anomaly variations. In the SIM, whose equations are consistent with the Recharge/Discharge paradigm, the ocean is described as a two-layer system and the climatological state, represented by a sloping pycnocline, is maintained by a constant easterly wind stress. Using the SIM and coherently with the Recharge/Discharge paradigm, the authors show that changes in the averaged SSTA over El Niño 3, 3.4 and 4 regions are nearly perfectly correlated to pycnocline displacements due to transiting KWs.