The South Pacific Meridional Mode as a Thermally Drive Source of ENSO Amplitude Modulation and Uncertainty

Figure 5 from "The South Pacific Meridional Mode as a Thermally Driven Source of ENSO Amplitude Modulation and Uncertainty" published by AMS, the American Meteorological Society. Authors: Sarah M. Larson; Kathy V. Pegion; Ben P. Kirtman Journal of Climate, June 2018, Volume 31, Issue 13: 5127–5145. Caption: "September SSTA in °C and wind stress anomalies in N m−2 from the (a) strongest and (c) weakest El Niño event in EnsoWinds. Events are ranked by the September Niño-3 SSTA index, as September exhibits the largest standard deviation of Niño-3. (b),(d) As in (a),(c), but for the EnsoWinds residual SSTA calculated by subtracting the mean 4-yr ENSO cycle from the full SSTA. Wind stress anomalies are obtained from the atmospheric model output and are representative of the winds used in the model formulation of the heat fluxes. This is not the anomalous wind stress the ocean experiences, as it is prescribed per the methodology described in the text, but rather the uncoupled atmospheric response to the SSTA." Pacific Ocean outline drawing showing currents and water temperatures in reds and blues (warm and cold) on a scale. X axis shows latitude (north-south), and Y axis shows longitude (east-west) from 45N to 45S and from 150E to 90W

The South Pacific Meridional Mode as a Thermally Drive…

Abstract  This study seeks to identify thermally driven sources of ENSO amplitude and uncertainty, as they are relatively unexplored compared to wind-driven sources.

Pacific meridional modes are argued to be wind triggers for ENSO events. This study offers an alternative role for the South Pacific meridional mode (SPMM) in ENSO dynamics, not as an ENSO trigger, but as a coincident source of latent heat flux (LHF) forcing of ENSO SSTA that, if correctly (incorrectly) predicted, could reduce (increase) ENSO prediction errors. We utilize a coupled model simulation in which ENSO variability is perfectly periodic and each El Niño experiences identical wind stress forcing. Differences in El Niño amplitude are primarily thermally driven via the SPMM. When El Niño occurs coincidentally with positive phase SPMM, the positive SPMM LHF anomaly counteracts a fraction of the LHF damping of El Niño, allowing for a more intense El Niño. If the SPMM phase is instead negative, the SPMM LHF amplifies the LHF damping of El Niño, reducing the event’s amplitude. Therefore, SPMM LHF anomalies may either constructively or destructively interfere with coincident ENSO events, thus modulating the amplitude of ENSO. The ocean also plays a role, as the thermally forced SSTA is then advected westward by the mean zonal velocity, generating a warming or cooling in the ENSO SSTA tendency in addition to the wind-forced component. Results suggest that in addition to wind-driven sources, there exists a thermally driven piece to ENSO amplitude and uncertainty that is generally overlooked. Links between the SPMM and Pacific decadal variability are discussed.

Read the full article . . .

Sarah M. Larson, Kathy V. Pegion, Ben P. Kirtman. The South Pacific Meridional Mode as a Thermally Drive Source of ENSO Amplitude Modulation and Uncertainty. Published Online 5 June 2018. American Meterological Society Journal of Climate July 2018