Different controls of tropical cyclone activity in the Eastern Pacific for two types of El Niño

Abstract

The El Niño–Southern Oscillation (ENSO) is known to have different modes of expression, characterized by different dynamics and spatial anomalies patterns: the Eastern Pacific (EP) and Central Pacific (CP) El Niño. The main region of influence of the former is located in the Eastern Pacific, while CP events have a stronger signature of ocean/climate anomalies in the Central West Pacific. This leads to distinctive oceanic and atmospheric signatures that likely have different influences on tropical cyclone (TC) activity in the Eastern Pacific, the second most active region in the world. In this study we investigate the respective role of oceanic and atmospheric conditions on TC formation and intensification in the Eastern Pacific associated with these two flavors of ENSO. We find that the oceanic control, through meridional redistribution of subsurface heat, is the main driver of TC activity during the hurricane season following EP events. In contrast, atmospheric conditions tend to be destructive to TC intensification after those events. The altered atmospheric circulation, in particular the reduction of vertical wind shear and the increase in relative humidity, tends to be more influential in controlling TC activity post CP events. However, unlike for subsurface heat, these changes in atmospheric conditions are not statistically distinct between these two ENSO flavors—although they are consistent across all atmospheric data sets tested. Overall, unlike after EP El Niño events, the hurricane season activity following a CP event is not significantly different from neutral or even La Niña years.

Influence of Oceanic Intraseasonal Kelvin Waves on Eastern Pacific Hurricane Activity

“…Although all observed El Niño events have been quite different (Johnson 2013), the RD mechanism accounts overall for a significant component ('30%) of the yearly variation of TC intensity in the EPac (Jin et al 2014(Jin et al , 2015. This sheds light on the previously overlooked subsurface ocean properties and in particular the equatorial ocean dynamics as a potentially major contributor to hurricane activity in this region (Price 2009;Lin et al 2013;Jin et al 2014Jin et al , 2015Boucharel et al 2016). This is further confirmed by observational and modeling studies that highlight the role of oceanic subsurface characteristics, such as thermocline depth (Balaguru et al 2013) and stratification (Shay and Brewster 2010;Vincent et al 2014) on TC intensification in the EPac.…”

“…To be consistent with Jin et al (2014Jin et al ( , 2015 and Boucharel et al (2016), we use oceanic conditions (both at and below the surface) from the European Centre for Medium-Range Weather Forecasts (ECMWF) Ocean Reanalysis System 3 (ORA-S3) that spans the period 1959-2009(Balmaseda et al 2008. ORA-S3 has been extensively used and validated (e.g., Zhai and Hu 2013;Jin et al 2014;Boucharel et al 2015).…”

“…All ocean products give similar results on their overlapping temporal period, so we present diagnostics performed only over the last 30 years from an average of these datasets (ensemble average taken over their common period) to smooth out mesoscale features and more clearly identify the EKWs. This reanalysis ensemble mean approach has been used for atmospheric products by Peña-Arancibia et al (2013) and Boucharel et al (2016). They point out that the ensemble mean generally outperformed or was nearly as good as any single member.…”

“…The fundamental ENSO recharge-discharge (RD) mechanism (Jin 1997) has recently been discovered as one of the main drivers of TC intensity in the EPac (Jin et al 2014(Jin et al , 2015Boucharel et al 2016). This mechanism operates via the meridional redistribution of ocean subsurface heat following an El Niño event toward the EPac cyclone-active region.…”

“…One particular mode of ENSO expression is the so-called central Pacific (CP) or Modoki El Niño, which exhibits a maximum anomalous warming around the international date line (Ashok et al 2007). The influence of the RD mechanism on TC activity remains only marginal after such events, as they tend to be characterized by a meridional discharge of heat located in the center of the basin and mostly directed to the south of the equator, away from the EPac cyclone-active region (Ren and Jin 2013;Jin et al 2015;Boucharel et al 2016). Although all observed El Niño events have been quite different (Johnson 2013), the RD mechanism accounts overall for a significant component ('30%) of the yearly variation of TC intensity in the EPac (Jin et al 2014(Jin et al , 2015.…”