Tropical cyclones and permanent El Niño in the early Pliocene epoch

Abstract

(Fig. 1a). Similarly, the meridional temperature gradient from the equator to the mid-latitudes was significantly reduced 21 . In fact, in the early Pliocene the SST distribution was virtually flat between the equator and the subtropics, indicating a poleward expansion of the ocean warm pool (Fig. 1b) modelling the large-scale atmospheric circulation with a General Circulation Model, GCM; and (c) using the GCM data to drive a Statistical DownScaling Model, SDSM, which computes synthetic hurricane tracks and intensity (Methods Summary; Methods).The ocean temperature field is the starting point for our analysis, since the most reliable climatic information for the early Pliocene comes as SST data. Following ref.(21), we take the SST distribution in Fig How would these changes in hurricane activity influence the tropical climate? To answer this question, we first need to look at the upper ocean circulation in the tropics (Fig. 3a). This wind-driven circulation connects the regions of subduction off the coasts of California and Chile with the equatorial undercurrent (EUC) and eventually with the eastern equatorial Pacific. The circulation is associated with a shallow ocean meridional overturning, with penetration depths not greater than ~200m, typically referred to as the shallow Subtropical Cells, STC 7,28 .The effect of hurricanes on the ocean can be measured with the annual average Power Dissipation Index (PDI), which approximates the amount of energy per unit area that tropical cyclones generate each year 1 . A significant fraction of this energy is available to mix the upper ocean. In general, the PDI distribution coincides with the areas of strong hurricane activity, both in terms of frequency of occurrence and strength. For the modern climate, the simulated mean PDI exhibits a strong maximum in the western tropical Pacific north of 10 o N (Fig. 3b). Water parcels can travel towards the equator after subduction through two "windows" of the modern PDI distribution in the central Pacific without any interference from tropical storms (also see Fig. S1). In the West Pacific, the parcels travel at the fringe of the PDI maximum, where the mixing can affect the Pacific Subtropical gyre, but not the STC.For the early Pliocene, two bands of high PDI now cover the entire zonal extent of the Pacific (Fig. 3c), allowing interaction between the hurricanes and ocean circulation. Fig. S3). For comparison, we display SST changes in the same model (Fig. 4c) when only CO 2 concentrations are raised. In this case, the temperature increase is moderate (below 1 o C) and almost uniform, indicating a near radiative-equilibrium response.The warming of the equatorial cold tongue in response to increased extra-tropical mixing remains robust in a broad parameter range. Its magnitude depends on the specified diffusivity and the width of the equatorial gap between the two bands of enhanced mixing. The smaller the gap, the greater the warming that is achieved. The gap being fully closed wipes out the zonal SST gradient almost completely ...

Modeled sensitivity of upper thermocline properties to tropical cyclone winds and possible feedbacks on the Hadley circulation

“…The remainder of this heat remains in the tropics resulting in anomalously warm surface temperatures in the equatorial Pacific. This equatorial warming supports the idea that increased tropical ocean mixing may have contributed to maintaining permanent El Niño-like conditions during past warm climates, such as the early Pliocence (∼5 to 3 million years ago) [Brierley et al, 2009;Fedorov et al, 2010].…”

How well do coupled models replicate ocean energetics relevant to ENSO?

“…In principle, some of the differences in the simulated thermocline depths can be explained by different vertical mixing in the tropical ocean (e.g. Brierley et al 2009;Fedorov et al 2010) or extra-tropical effects that can alter the thermocline depth by modifying the ocean heat budget (Boccaletti et al 2004) and meridional density gradient (Fedorov et al 2004;Fedorov 2007). …”

Observational evidence supports the role of tropical cyclones in regulating climate

“…Because tropical cyclone activity is sensitive to ocean temperature, feedbacks may exist in the climate system between tropical cyclones, ocean mixing, and ocean heat content that are not captured in the current generation of climate models. This feedback may be important for understanding past and potential future changes in the Earth's climate system (17)(18)(19).…”