The Pacific Meridional Mode and ENSO: a Review
The following summarizes work published in Current Climate Change Reports.
Major Results:
The NPMM is a seasonally evolving mode of coupled climate variability and features several distinct opportunities to influence ENSO.
Since 1950, the boreal spring NPMM skillfully predicts about 15–30% of observed winter ENSO variability. Improving simulated NPMM-ENSO relationships in forecast models may reduce ENSO forecasting error.
Recent studies have begun to explore the influence of anthropogenic climate change on the NPMM-ENSO relationship; however, the results are inconclusive.
The North Pacific Meridional Mode (NPMM) is connected to ENSO through four main physical pathways (Fig. 1). These include:
WES-driven propagation of SST and surface wind anomalies from the subtropics onto the equator during boreal spring.
Trade Wind Charging (TWC) of equatorial subsurface heat content by NPMM-related surface wind stress curl anomalies in boreal winter and early spring.
The reflection of NPMM-forced ocean Rossby waves off the western boundary in boreal summer.
Northward shift of the ITCZ, which generates a large-scale atmospheric circulation response that projects on the equator during boreal summer.
As a result of these pathways, the NPMM accounts for 15-30% of winter ENSO variability in observations.
Pathways 1 and 4 are discussed in more detail below.
Figure 1 Schematic illustration of the different physical mechanisms that connect the NPMM to ENSO.
Figure 2 Lagged regressions of seasonally averaged SST and surface wind anomalies on NPMM SST time series calculated from a Maximum Covariance Analysis.
The NPMM is generated in the wintertime by random atmospheric forcing associated with the North Pacific Oscillation (NPO; Fig. 2d cyclonic circulation).
The NPO weakens the trade winds, reducing evaporative cooling and generating a positive SST “footprint” (Fig. 3a).
In boreal spring, the positive SST anomalies are strong enough to influence the surface wind field, generating cross-equatorial winds that reinforce the meridional SST gradient (Fig 2e). This is a positive Wind-Evaporation-SST (WES) feedback.
WES feedback reinforces the southwest portion of the warmest SST anomalies, while damping them to the northeast. As a result, the surface anomalies preferentially propagate southwestward towards the equatorial dateline (Fig. 3b).
Zonal wind anomalies on the equator generated by this WES-propagation can trigger ocean Kelvin waves that can contribute to the development of ENSO events in the following winter (Fig. 2f-g).
In boreal summer, the NPMM can anomalously shift the ITCZ northward, triggering a so-called Summer Deep Convection (SDC) response. The SDC response involves a large-scale atmospheric circulation response that projects onto the equator, further increasing the likelihood of westerly wind events (Fig. 4a).
Figure 3 Same as Fig. 1, except lagged regressions of surface wind and latent heat flux anomalies (negative = ocean warming).
Figure 4 First Joint EOF of SST (shading) and precipitation (contours; green = wetter) anomalies in the North Pacific using ERA5 reanalysis. Quivers are surface wind anomalies.