Impact of bio-physical feedbacks on the tropical climate in coupled and uncoupled GCMs

Abstract

The bio-physical feedback process between the marine ecosystem and the tropical climate system is investigated using both an ocean circulation model and a fully-coupled ocean-atmosphere circulation model, which interact with a biogeochemical model. We found that the presence of chlorophyll can have significant impact on the characteristics of the El Niño-Southern Oscillation (ENSO), including its amplitude and asymmetry, as well as on the mean state. That is, chlorophyll generally increases mean sea surface temperature (SST) due to the direct biological heating. However, SST in the eastern equatorial Pacific decreases due to the stronger indirect dynamical response to the biological effects outweighing the direct thermal response. It is demonstrated that this biologicallyinduced SST cooling is intensified and conveyed to other tropical-ocean basins when atmosphere-ocean coupling is taken into account. It is also found that the presence of chlorophyll affects the magnitude of ENSO by two different mechanisms; one is an amplifying effect by the mean chlorophyll, which is associated with shoaling of the mean thermocline depth, and the other is a damping effect derived from the interactively-varying chlorophyll coupled with the physical model. The atmosphere-ocean coupling reduces the biologically-induced ENSO amplifying effect through the weakening of atmospheric feedback. Lastly, there is also a biological impact on ENSO which enhances the positive skewness. This skewness change is presumably caused by the phase dependency of thermocline feedback which affects the ENSO magnitude.

Modification of sea surface temperature by chlorophyll concentration in the Atlantic upwelling systems

“…This cooling is in agreement with previous studies performed in the Pacific [ Nakamoto et al ., ; Manizza et al ., ; Lin et al ., , 2008; Frouin et al ., ; Anderson et al ., ; Park et al ., ] but in contradiction with others that show a warming of the coastal and equatorial upwelling systems [ Timmermann and Jin , ; Murtuggude et al ., 2002; Marzeion et al ., ; Wetzzel et al ., 2006; Lengaigne et al ., ]. In the Atlantic, Frouin et al .…”

“…This contrasts with previous studies in other upwelling regions which proposed that dynamical modifications of the upwelling are responsible for the observed temperature anomalies [ Manizza et al . ; Marzeoin et al ., 2005; Nakamoto et al ., ; Loptien et al ., ; Sweeney et al ., ; Lengaigne et al ., ; Lin et al ., , 2008; Park et al ., ].…”

“…Even with a coupled ocean model, conflicting results are obtained when including the biological effect: Park et al . [] shows that SST cooling is amplified in the fully coupled model (ocean‐atmosphere‐biogeochemistry) experiments whereas Marzeion et al . [] or Lengaigne et al .…”

“…These results have been summarized in Park et al . []. They are generally sensitive to the type of ocean model used: some authors show that the surface cooling induced by the biology is amplified in fully coupled models relative to forced ocean models [ Park et al ., ], while other studies show an inversion of the anomaly from cool to warm when using an hybrid or a fully coupled model [ Marzeion et al ., ; Lengaigne et al ., ].…”

“…Most of the studies focused on the Pacific equatorial upwelling region, owing to potential biological feedback on El Niño/La Niña events. Some studies suggest that the differential heating due to chlorophyll leads to a warming of the surface waters of the order of +1°C in the equatorial pacific system [ Timmermann and Jin , ; Murtuggude et al ., 2002; Marzeion et al ., ; Wetzel et al ., ; Lengaigne et al ., ] while other authors found a sea surface cooling [ Nakamoto et al ., ; Manizza et al ., ; Lin et al ., , , ; Anderson et al ., ; Park et al ., ]. These results have been summarized in Park et al .…”