Research on the interannual variability of the great whirl and the related mechanisms

Abstract

Based on AVISO (archiving, validation and interpretation of satellite data in oceanography) data from Whirl (GW) and related mechanisms are studied. It shows that the origin and termination times of the GW, as well as its location and intensity, have significant interannual variability. The GW appeared earliest (latest) in 2004 (2008) and vanished earliest (latest) in 2006 (2001), with the shortest (longest) duration in 2008 (2001). Its center was most southward (northward) in 2007 (1995), while the minimum (maximum) amplitude and area occurred in 2003 and 2002 (1997 and 2007), respectively. The GW was weaker and disappeared earlier with its location tending to be in the southwest in 2003, while in 2005 it was stronger, vanished later and tended to be in northeast. The abnormal years were often not the same among different characters of the GW, and were not all coincident with ENSO (El Niño-Southern Oscillation) or IOD (Indian Ocean Dipole) events, indicating the very complex nature of GW variations. Mechanism investigations shows that the interannual variability of intraseasonal wind stress curl in GW region results in that of the GW. The generation of the GW is coincident with the arrival of Rossby waves at the Somali coast in spring; the intensity of the GW is also influenced by Rossby waves. The termination of the GW corresponds well to the second one of the top two peaks in the baroclinic energy conversion rate in GW region, and the intensity and the position of the GW are also closely related to the top two baroclinic energy conversion rates.

Intrinsic Versus Wind‐Forced Great Whirl Non‐Seasonal Variability

“…The GW is the strongest in July, decays thereafter (Figures 2e, 2f, and 2g) and disappears completely in November (Figure 2h). This description is consistent with most recent studies (e.g., Beal & Donohue, 2013; Cao & Hu, 2015; Melzer et al., 2019; Trott et al., 2018; Wang et al., 2019). The climatological GW is fully developed and stable in its shape and position during the June, July, August period.…”

“…The added value of our study is to document that these non‐seasonal variations are more specifically associated with meridional shifts in the GW (for which we define an index), and that those meridional shifts are accompanied by clear SST and surface Chl signals, and in particular by a modulation of the two cold and productive SST wedges of the Somalia upwelling (e.g., Schott & McCreary, 2001). We also establish that the GW meridional movements have two timescales: a short subseasonal timescale, as pointed out by Beal and Donohue (2013), and a longer “interannual” signal, as underlined by most other authors (Cao & Hu, 2015; Melzer et al., 2019; Trott et al., 2018; Wang et al., 2019). In observations, the GW displays larger southward than northward GW displacements.…”

“…The main take-home message from our study is that there is a large intrinsic variability component to the GW nonseasonal variability, which is even larger than the forced component for the seasonal mean GW position change based on our quantifications. This means a signal-to-noise ratio of less than one, which may explain why none of the previous studies found a clear link between the GW interannual variability and climate or wind signals (Beal & Donohue, 2013;Cao & Hu, 2015;Melzer et al, 2019;Trott et al, 2018;Wang et al, 2019). We can also examine the dipolar trend in sea surface temperature and height over past three decades discussed by Santos et al (2015), attributed to a ∼1.5°southward migration of the GW over the last three decades.…”

“…Decades of oceanographic cruises and the advent of satellite oceanography now allow for a detailed description of the GW lifecycle. The GW is characterized by a sea level high (Figure 1a) and can thus be tracked from satellite data (e.g., Beal & Donohue, 2013; Cao & Hu, 2015; Melzer et al., 2019; Trott et al., 2017; Wang et al., 2019). A closed whirl of currents (Figure 1b) first appears off Somalia on average in April (Figure 2a; Beal & Donohue, 2013), synchronized with the arrival of a Rossby wave radiated from the west coast of India (Shankar & Shetye, 1997).…”

“…Beal and Donohue (2013) noted strong intraseasonal variations in the GW position, and attributed this feature to mutual advection by flanking eddies. Numerous studies have found significant interannual variations in most of the GW characteristics, including onset and decay dates (e.g., Melzer et al., 2019; Wang et al., 2019), size (Trott et al., 2018) and position (e.g., Beal & Donohue, 2013; Cao & Hu, 2015; Melzer et al., 2019; Trott et al., 2018; Wang et al., 2019). None of these studies found a clear link between the GW position and large‐scale climate modes such as the El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), or the intensity of the monsoon.…”